Denso Window Motors, Warning - Don't use with Jaguars !
 

Important warning to FRC teams.

During this year's 2010 competition at the DC regional we were struggling to solve an intermittent problem with our arm. The arm is powered with one of the Denso window motors that came in this years KOP. This motor used a Jaguar controller, commanded with a joystick. We initially suspected an electrical problem, but the symptoms were mechanical also.

What happens is that the motor will occasionally simply fail to respond to the controls. When the motor fails to respond, there is a voltage present on the input to the motor. We [well, our students with young ears] could hear a high-pitched sound from the motor as further indication that it was being powered. There was no mechanical interference, yet we found that there was sort of a mechanical hysteresis taking place. If the arm was commanded to go down, but was unresponsive, we could gently push it up with our finger and that would cause it to properly drive down in response to the command. Likewise, if the arm was commanded to go up, but became unresponsive, we could nudge it down and it would respond by driving up as commanded. We tried a spare motor, but if anything it was worse with the replacement motor.

After the competition, we took the arm back to the shop, and were able to re-create the problem by connecting it to another robot. We looked at the output of the Jaguar on a scope both when in this frozen state and not. The Jaguar outputs a pulse-width modulated waveform that toggles between -12V and +12V. The frequency of the PWM waveform is around 15 KHz (hence the audible tone). When the arm entered this frozen state, the output waveform stopped toggling altogether and went to some intermediate voltage with a small amount of 15 KHz superimposed.

This is not a fluke due to a bad Jaguar or a bad motor because we saw it with 2 different motors and 2 different Jaguars. Once we observed the failure in the shop, we substituted a Victor speed controller, and everything then worked perfectly. The Victor has an output waveform that's very different than the Jaguar. Instead of toggling between +12 and -12, it toggles between +12 and zero (one direction), and between -12 and zero for the other direction. More importantly, it operates at around 117 Hz instead of 15 KHz. It is our belief that there is something about the Jaguar that is not unconditionally stable. The particular combination of the Denso motor and the Jaguar seems to be unstable. We are guessing that the impedance presented by the Denso motor at 15 KHz causes the Jaguar output to become unstable. Nudging the motor a little affected the instability allowing it to recover for a bit.

One final observation - we were not able to get the motor to fail by simply commanding it on and off. By using the joystick to vary the speed up and down, forward and backward, it would fail readily. If you are not varying the speed and just want forward / reverse on / off, you [might] be OK.

Note: Both of the Jaguars we tried are the older brown ones. We have not tried this on one of the newer black ones. Our advice is to NOT use a Jaguar to power a Denso window motor. If I get a chance, I may measure the impedance of the motor to see if there is some kind of a resonance near the 15 KHz Jaguar frequency.

This problem can be quite elusive as it behaves as though you have a loose connection or similar.

If I have a chance to check the motor impedance characteristics, I will post a follow-up

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

Wow, I'm glad our four Window motors were powered by spikes. I'd be interested to hear why this problem happens.

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

Interesting. We had a similar problem with our swerve drive, as we used 4 window motors for the steering control. However, we noticed that our battery was going down to about 7 volts when the motors started failing, so we figured that was the problem.

Replacing the battery seemed to solve our issues. I've heard that the cRIO disables all motor outputs when it falls below a certain cutoff voltage. Could that be the case here?

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

We are running 3 Denso's off of 3 Black Jaguars on CAN, and have had absolutely no problems with them.

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

We are using all victors this year because of space constrains and have had the same problems with the window motors. I think that there is a internal circuit over load device in the window motors that is the problem. The problem occurs for us when the motor gets hot.

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

the Denso motors have an internal heat breaker. if the motor gets too hot it will shut itself down for a while to cool down.

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

The thermal breaker trips before the motor gets very hot at all. In cars, the motors are designed to stall when they reach the end of their motion as their controls do not turn them off when they reach the end stops. They could be at stall infinitely, so they will turn off and stay off for a while.

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

All,

As alluded to by several people, the window motors have a positive temperature coefficient (PTC) resistor in series with the motor windings.

The motor was designed for moving a window up and down and the PTC is designed to keep your son from decapitating his little sister. Since a window is not a variable speed device, the PTC is designed to sense a motor stall. The PTC increases its resistance as it gets hot which causes the current to diminish reducing the power of the motor...

While it is possible that the switching frequency of the Jaguar causes more heating than a Victor, it is far more likely that the problem is how it is being used.

When operated at "full on" and "stop", you are likely at a good efficiency point on the motor torque curve. This is why a properly designed system using a spike has no problems.

However, at variable speeds, the Jag (or Vic) accomplishes speed reduction via reducing the voltage. For the same torque at lower speeds, you end up off the optimum efficiency point of the curve and motor heating results.

So the motor heat adds to the heat that the PTC uses to detect a stall and it cuts back the current giving you exactly the symptoms you have described.

Therefore, the problem is not in the Jaguar but in the use of the motor.

Regards,

Mike

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

Mike (and all):

I do not believe this problem has anything to do with the application and/or the PTC in the window motor as evidenced by several things...

First, switching to a Victor has completely eliminated the problem. If it were application / heating, this would be no different with a victor or a Jag. Second, the output of the Jag went brain-dead when the motor was mis-behaving. It was no longer switching as observed on a scope. Finally, we are using the motor on a very intermittant basis. It hardly gets warm to the touch. Yes, the PTC may be intended to react quickly (ie: Junior's arm in the window), but our problems have not occurred after a period of use, but rather after a period where it has been idle (when we first try to deploy the arm after being idle for the whole match).

I can capture a scope plot or-two to illustrate the erroneous output waveform when in it's bad mode.

-Tom

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

Tom,

I just re-read your original post and agree with you. My apologies for going off on a tangent...

You may wish to get a Luminary Micro applications engineer in the loop of your investigation...

Regards,

Mike

Postscript: Let us know what they say...

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

In addition to PTCs automotive motors often have EMC/EMI suppression circuitry intended to keep radios from buzzing etc.

Is it possible that this EMC/EMI suppression circuit that has a resonance with the PWM frequency of the Jaguar?

It could happen that some oscillation is building up due to resonance and the Jaguar is either getting freaked out by it or it is detecting this and shutting down of its own accord.

No data, but a guess at what could be causing this strange behavior.

Joe J.

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

Mike:

I just took a very crude look at the motor impedance. I realize the "small signal" impedance will differ from the dynamic impedance with real loads / drive levels, but this was just a simple look with a signal generator and a scope.

The DC resistance is around 2 ohms. The signal generator test showed a flat 4.5 ohms from around 100 Hz, up through 1 KHz. Above this, the impedance increases kind of linearly. It's about 40 ohms at the 15 KHz switching rate of the Jag. Beyond this, there is what appears to be a parallel resonance with the impedance peaking to 200 ohms at 90 KHz.

I wonder if a "filter capacitor" would help this?

Is Luminary still the right POC, or is it now through TI somehow? It's academic at this point, we are changing to the Victor, but an interesting question nontheless.

-Tom

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

Tom,
Both the Jaguar and Victor switch the same way. There is no -12 volt output. Both simply connect the negative lead of the power supply to one or the other of the output leads as direction command dictates. The switching on the Victors is speced as 120 Hz while the Jags are speced at 15kHz. If you were reading a very small amount of 15kHz energy when the motor was not turning but your finger helping the motor returned output to normal I think you have another problem. I will bet a buck that in both motors you have one shorted winding. Every once in a while, the motor gets on the shorted winding and shorts the output. (or the motor is resting on a good winding next to the shorted winding) Your conveniently placed finger moves the motor armature to the next segment and the motor starts to spin on it's own because the motor is able to rotate past the dead winding. Herein lies the rub, the jags have a current sense that holds down the output in the event of a high current or a short. If the short lasts for a certain length of time, the Jag goes into fault condition and turns the LED to red while disabling the output for 3-4 seconds. You may have also noted that the Jag always has a small pulse added to the output even when at full throttle. This is required because the charge pump used to turn on the FET gates needs to be charged. As Mike and others have discussed, the PTC is in series with the motor but no filtering for RFI is shown in the documents. Additionally, please check that the motor does not have a short to case. If so, you may have a sneak path through other devices (most notably the Crio and camera) that are interfering as well.
As to why the Victors always work, I suspect that you just haven't tried enough tests yet. There is very little reason for a Victor to work in this application when a Jag does not. Although the inductance of some KOP motors is high enough to change the output waveform of the Jag, the impedance/inductance of the window motor should be small in comparison.
The singing your students hear is the laminations in the motor moving at 15kHz.

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

Al:

The Victors and the Jags do not switch the same way... Yes, this is a single +12V system, but when you observe the motor voltage (line to line), it is bipolar because the H-bridge reverses the polarity as you state. What I was referring to is the observation that the Victors are switching between zero and +12 when any forward command is given, and zero and -12 when any reverse command is given. Another way to put it --- When the motor is commanded to stop, a Victor will park it's output at zero volts (does not connect to either forward or reverse). The Jag (as I observed it) was operating with a 50% duty, toggling at 15 KHz between +12 (fwd), and -12 (rev). This was a little surprising to me, but it does yield a net zero volt average. Perhaps there is a setting that can be programmed to make it work differently, but we have never changed anything from the factory defaults.

Regarding the shorted winding theory, the likelyhood of two motors having a shorted winding is not great. Also, you would have to experience what it feels like when "nudging" the motor, but I don't believe we can turn the armature at all. The ratio of the worm gear makes it so this thing just cannot be back-driven from it's output shaft (after all, you don't want your window falling down and having Junior fall out while driving down the road!).

-Tom

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

Tom,
I have ask ...Are you using CAN? What you describe (50% FWD/REV) is an electronic technique for holding a motor stationary. Effectively it is a brake unlike the short that "brake mode" puts across the motor. According to my documentation this mode is only available in the Jaguar with the CAN bus implementation. (New and improved Jaguars may have this built in, I don't have the latest docs for new Jags) It is capable of being generated externally under software control but you would know if you had programmed it in your software. As you describe it, this mode would emit a high frequency squeal from the motor. Are you using feedback from the arm for position?

I agree that two motors having shorted windings is rare but it does happen. Although the window motor has the worm drive, moving an arm is likely to rotate the motor just a little. That is all it would take to get you to cross a segment boundary. I have not disassembled one of these motors in a while so I can't remember how many poles the motor has. Have you measured the free speed? With a shorted winding, it should be vastly lower than the stated free speed, 89 RPM I think.

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

Tom, what you are saying is that power is flowing at a 50% duty cycle throught the motor when it is at a zero speed command?

Why would they have power flowing at zero speed? Have you measured the current? Wouldn't this cause the motor to heat up over time?

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

Yes, the motor is in stall 100% of the time. It is unclear how this might cause the effect described.

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

I would think that the above would likely trip the PTC?

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

Jared,
The PTC changes resistance with temperature internal to the motor. It is usually a rapid change similar to a switch but I have no data on the PTC.

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

Guys:

We are not using CAN. The test bed is our 2009 robot connected to our new arm. There is no feedback of any kind. Just a simple mapping of a joystick position to the Jaguar (or Victor). I have not "measured" the free-running speed, but it "looks" like it's faster than 1 RPS, so I could believe 89 RPM.

The motor does not get hot (or even warm) when idling with no commanded drive. When it is in it's "stuck" state, if we continue to assert drive, then it becomes warm.

I'm pretty sure we do not get the high-pitch sound until we try to assert drive authority (of course, I can't hear it anyway, only the students).

I am not a motor guy, but with PWM control like this, doesn't the motor respond only to the average value of the input? When the input to the motor is 50% fwd, 50% reverse, the average voltage is zero, therefore there is no average current, and hence it does not draw any current or heat up. It will not brake because there is no net current to cause the braking. The impedance (inductance) of the windings will not allow the current to change at anywhere near the rate of the 15 kHz switching. The motor is not "stalled", but rather it has no net current flow even though the voltage is toggling wildly. Well, there may be some dynamic power dissipation 1/2 CV^2 or some such, but certainly nowhere near what you are thinking as "stall current"???

Do I have this all messed up?

You are making me doubt my memory... I need to go back just to double-check what I believe to be an idle condition with the 50% square wave +/- 12V line-to-line !!

-Tom

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

Tom,
It is common for a motor to be commanded to not move and the simple way to do this is alternate FWD/REV at 50% duty cycle each. Although the average current is zero, the instantaneous (and absolute) is stall current. The net effect is the motor does not move in either direction and the motor sings at the switching frequency. You are correct that at 50% duty cycle the inductance plays into current waveform distortion. However, the Victor is capable of the same motion control you describe, as we have done it in the past. Why it would switch one way with Victors and the other way with Jaguars is puzzling. Are you making a change in software when you change controllers? I know from experience using this method with high current motors eats the battery. We used this when a stable platform was needed for shooting.

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

yes, if the PWM frequency is high enough for the given motor inductance.

correct

the algebraic average will be zero. the rms average will be near zero only if the PWM switching frequency is high enough for the given motor inductance.

algebraic current can be zero and yet rms non-zero, in which case the motor will heat up. see above.

correct, depending on the motor inductance. if the inductance is very very low, it will start to draw current even at high switching speeds. most DC motors today are designed with sufficient inductance that 15KHz is plenty fast.

correct. the algebraic average current is zero, and the rms is near zero. the instantaneous current is also near zero.

correct

No.

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

Ether,
There is a thread from last year where inductance vs. input waveform for CIM motors was discussed and plotted. As I remember, the CIM motor inductance limits the rise time of the motor controller output below about 60% duty cycle at 15kHz. It is this that adds to the Jaguar's "linearity" at low throttle values. (long rise time contributes to better response by lowering the average current at low throttle levels) With the suspected lower inductance of the Window motor, I would expect at 50% duty cycle, there would be little effect on the rise time. It is hard to use the term RMS in this context since this is not really an AC signal. It is more of a pulsed bi-polar DC signal and is easier to discuss in those terms when using DC motors. Following the definition of stall current as the current flow when a motor is energized but not moving, then in the example, stall current would indeed flow. All of the KOP motors draw stall current in the instant they are commanded to move and continue to do so until moving. What is particularly intriguing is the fact that motors do not heat up.
Tom, when you mentioned 50% duty cycle were you saying the positive pusle is only at 50% and then it remains off for 50% before changing polarity? I expected you meant that 50% of the time it was positive and 50% was reversed. The former would give an average current at half of the latter and may account for different motor heating.

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

With the suspected lower inductance of the Window motor, I would expect at 50% duty cycle, there would be little effect on the rise time.

I would expect that the Window motor has sufficient inductance so that at 50% -12/+12 duty cycle at 15KHz, the RMS current would be rather small and thus very little heating. If I get a chance later tonight, I will try to find the datasheet for the window motor and see if it lists an inductance... and I'll crunch the numbers.


It is hard to use the term RMS in this context since this is not really an AC signal.

It is indeed an AC signal, just not perfectly sinusoidal. I agree that the calculation is more difficult, but RMS can be calculated for any periodic waveform, sinusoidal or not. The point I was making is that the RMS is correlated with I^2R heating, whereas the algebraic average is not.


It is more of a pulsed bi-polar DC signal

I could be wrong, but I'd bet a buck that the current in the motor windings probably looks more like a triangular waveform than a pulsed bi-polar square wave.


Following the definition of stall current as the current flow when a motor is energized but not moving, then in the example, stall current would indeed flow.

Ah, there's the confusion. We have different definitions of stall current. "Stall current" in the motor datasheets refers to the current flowing in the motor windings with the motor stalled against an external torque with a constant specified DC voltage applied. A motor which is receiving 50% -12/+12 15KHz excitation is not "stalled" in this sense; if the motor inductance is high enough and the PWM frequency is high enough, there will be little or no current flow, and thus little or no I2R heating of the motor windings under these conditions.


All of the KOP motors draw stall current in the instant they are commanded to move and continue to do so until moving.

Yes they draw current, but not "stall current" (as used defined above) unless the command is +12V DC.


What is particularly intriguing is the fact that motors do not heat up

They don't heat up because there is little or no RMS current, because the inductance and PWM switching frequency are sufficiently high that the current never has a chance to rise above a very small value.



~

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

Ether,
I don't see any inductance specs on the window motor data sheet.
At full voltage with a 50% FWD and 50% REV, the motor should be drawing stall current except during the period of charge pump refresh in the Jaguar. The RMS voltage would be near stall under these conditions. You are correct, the inductance will affect the full stall current during rise time. With the Jaguar, dependent on the length of the pulse this average current could be less than stall. I have not scoped the window motor connected to a Jaguar. I have looked at window motors connected to Victors. The only limiting factors are the series resistance of the wiring and speed controllers at 120Hz.

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

Al / Ether:

The "idle" waveform I observed from the Jaguar (joystick centered) was a square wave with a ~ 15 KHz frequency, toggling between +12V and -12V. There was no time where it was zero volts. When the joystick was pushed forward, it continued to toggle between +12V and -12V, while the duty increased until it was a constant +12V. When the joystick was pulled back, it continued to toggle between +12V and -12V, while the duty decreased until it was a constant -12V.

With the Victor, it was was literally zero volts when the stick was centered. When the stick was pushed forward, it began to toggle between zero volts and +12 volts at it's 117 Hz rate with varying duty until it was a constant +12V. When the stick was pulled back, it began to toggle between zero volts and -12 volts at it's 117 Hz rate with varying duty until it was a constant -12V.

BTW -- I think the window motor would have substantially higher inductance than a CIM... It is a much lower power motor (finer gauge windings), higher impedances across the board, --> higher inductance.

-Tom

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

Tom,
Where was your scope probe connected? Where was the ground lead? I am leaning towards a smaller motor, and less power leads to smaller windings although higher series resistance (due to smaller gauge wire). Add to that the smaller field structure (smaller core) would also lead to lower inductance. Of course since none of us has measured the inductance or looked at rise times, all of this is just a guess. We do not use Jaguars, so I can't make any measurements on a robot.
I am guessing that the -12 volts is a ghost due to the reference lead of the scope. Are you viewing in AC mode or DC mode?

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

This is what I was about to throw in to this conversation as well, although, I think I would have used the term "coupling mode".
I must admit though, Tom did say it was a square wave and ac coupling will not display a square wave well. There will always be considerable rise and fall spikes that hint at the coupling error. Depending on the scope, this should be true at 15KHz as well.

For this set up, the scope probe should be connected directly to the +M of the Jag with the reference on the -M. The scope should be in DC coupling mode.

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

Can you put a small resistor (tenth of an ohm maybe?) in series with the motor and measure the voltage drop across it on your scope please, and tell us what you find?

I'm guessing you're gonna see a very low amplitude triangle wave when running with the Jags at "zero" command.


~

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

Robot completely isolated from earth ground. Scope was most definitely DC coupled. Bilbo911 has it exactly right. "ground" lead of scope on the M- and probe tip on M+. Measuring the differential voltage across the motor winding. If the scope reference was tied to the robot battery (either + or -), it would not be possible to observe a bipolar signal.

Nope, definitely not a "ghost".

Smaller wires, higher impedances, --> higher inductance to me.

I don't have a CIM in my hands right now, but a quick check of the Denso with a scope and a pulse generator indicates an inductance on the order of 200 uH.

-Tom

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

Aren't we all pathetic !!!

Any normal human being with half a life would have better things to do than debate over the particulars of PWM motor control.

I don't have this stuff in front of me, will have to go to the robot shop. Just to keep the geek-nerds amused, I will obtain a tenth ohm resistor at work tomorrow and run the current test.

If you guys are really nice, I may capture some scope plots of voltage and current and post them.

After Wednesday, we will be preoccupied at the Chesapeake regional, so additional nerd experiments will go on hold.

-Tom

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

You mean, better things as in browsing the web to see what the hell is Paris Hilton doing today? :p

I'll take particulars of PWM motor control over that, any day...

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

Wow. Reading that last post really robbed me of any desire to respond. I had to re-read Ether, Jared and Al s' posts a few times first. I hope that anyone who reads the dire warning on page 1 also reads page 3, so they may understand the source of the misinformation.

There are two major questions.

1) Is the Jaguar switching when given a command of "0"?
I just tested a grey jag on PWM, a grey jag on CAN, a black jag on PWM, and a black jag on CAN. All signs point to no.

2) Why do I see +/-12V on Jaguars, but not on Victors?
The answer has actually been posted several times before: Inductance vs Frequency. Assuming 12V, 200uH and no resistance, a victor changes current by 500 amps during its switch cycle, whereas a Jaguar changes by only 4 amps. This means that, just as Al said, a Victor will ramp all the way to stall or 0 during its switching period. However, a Jaguar will maintain a relatively steady current.

If the motor has a load that relates to a current greater than its delta current ((Vin-Vemf)/Ht), it will always be conducting current. While any h-bridge is conducting current, its motor will report a voltage equal to the input voltage (minus losses). So, a Jaguar driving a stalled* denso with more than 4Amps will show a square wave that looks a lot like +12 / -12. It'll actually be more like +12 / ~-11.

A victor will never show this wave form, unless you can stall your motors at above 500A without lighting them on fire.


This is actually a GoodThing(TM), and for many reasons. For a given torque, reducing the ripple current reduces the power dissipation in the motor (rms vs mean, per Ether).


*Remember that back emf is subtracted from this.

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

Al, could you post a link to this documentation please. I can see how 50% FWD/REV would slow down a moving motor, but not hold a motor stationary against a sustained external torque.

Thanks.

~

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

This so called Locked Anti phase drive is much nicer when you are trying to do speed control for reasons I won't go into here but you are all right, in noting that the frequencies have to be high when compared to the time constant of the motor circuit or bad things can happen.

Joe J.

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

The Denso motors have a rubber flex element in them that absorbs shock at the end of the window travel. It also makes them very sloppy if you are trying to use them for precise positioning.

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

Tom,
When you connect the scope in this fashion the reference lead is switching between +12 and common. So yes, you will see a signal go from +12 to -12 volts. In reverse the scope common is now at +12 volts and the probe is 12 volts below it. The Victor should show the same waveform. As to why the Jaguar exhibits this in zero throttle, I can't answer.
At 200uH and about 18 ohms of series resistance the time constant turns out to be 1.1uSec. Giving it 6 time constants to fully charge that is 6.6uSec. A 15kHz square wave has a period of 66uSec or 33uSec high or low for a 50% duty cycle. So yes, there will be a little rounding of the waveform but displaying several (say 5 periods) waveforms horizontally will not really show much effect of the inductance on the waveform rise time.
Ether, I am quoting the Jaguar manual from last year. It lists analog position control as only available under CAN control in Table 2-1. If the motor is given a FWD command and then a reverse command of equal amplitude, the armature will stay in one place given that the switching frequency equals or exceeds the ability of the motor shaft to turn in between pulses. The trade off is excessive current demand. We used this technique in the game of shooting balls into the overhead goal. By locking our wheels in place while shooting, other robots were not capable of moving the robot to throw off our shooter. Using crab, we simply rotated the high friction wheels 90 degrees to our shooting direction and engaged the lock. We could not be pushed far enough to prevent our balls launching into the goal. After ten balls or whatever we were carrying, the lock went off and we drove normally. Remember that the game left us unmolested during parts of the game so it was not needed more than a few seconds each match. Still it was enough to deplete the battery more than I would have liked.

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

Originally Posted by Mr.G:
Tom, what you are saying is that power is flowing at a 50% duty cycle throught the motor when it is at a zero speed command?

No, he is not saying that "power is flowing". He is saying that a 50% duty cycle -12/+12 voltage is present at the motor coils. He is saying that the Jag, at zero command, apparently is providing a 50% voltage duty cycle between +12 and -12 volts (see attached GIF) at the motor. Whether or not this produces any significant current flow in the motor windings is what we are discussing.


Why would they have power flowing at zero speed?

There's not supposed to be. Locked antiphase PWM is supposed to be used with motors that have sufficient inductance so that negligible current flows under these conditions. Someone mentioned that the Denso Window motor inductance is 200uH. If this is true, I would expect to see less than 500ma* RMS flowing through the motor under the voltage conditions mentioned above. Compare this to the Denso's stall current of 18.6 amps.


Have you measured the current?

I have asked Tom to measure the current using a small series resistor. He said he would do it, but hasn't had a chance to yet. Tom: since we are dealing with such a small sense resistor, please be sure to measure the voltage drop across the actual resistor leads, so you don't pick up extra contact resistance from the alligator clips or whatever you use to connect it.


Wouldn't this cause the motor to heat up over time?

Yes, it would, IF there were sufficient current flowing. But apparently there's not, because the the high PWM frequency and the motor's inductance.


~

* the Denso window motors are fairly low-tech motors not specifically designed to be used with PWM. Even though the low inductance draws more current at the zero-command 50% duty cycle condition than would normally be expected, it is still a small enough current not to cause motor heating. The Denso's stall current is listed at 18.6 amps at 12VDC, so its resistance is about 0.65 ohms. 500ma flowing through 0.65 ohms produces only about 0.16 watts of I2R heating, which is why the motor does not heat up.


~

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

Al Skierkiewicz wrote:
The Victor should show the same waveform.

Not necessarily. The Victor may not be operating in locked antiphase. In fact, Tom's scope measurements show that the Victor is not.


...about 18 ohms of series resistance

The data I have for the Denson window motor is 18.6 amps stall current at 12VDC. This would give a DC resistance of 0.65 ohms.


Ether, I am quoting the Jaguar manual from last year. It lists analog position control as only available under CAN control in Table 2-1. If the motor is given a FWD command and then a reverse command of equal amplitude, the armature will stay in one place given that the switching frequency equals or exceeds the ability of the motor shaft to turn in between pulses.


It is not the locked antiphase per se which holds the motor in position, it is the closed-loop servo control.

If an external load causes the motor to move from the commanded position, the closed-loop servo control adjusts the PWM duty cycle away from 50% in order to produce average motor current (and thus motor torque) to counter the load and hold the motor position. It is this non-50% duty cycle which draws current.

A locked antiphase motor controller not operating in a closed-loop position mode and providing a 50% -12/+12V signal at zero command would NOT hold the motor against an external load, because the 50% -12/+12V creates no net torque in the motor.


~

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

18 ohms is the impedance of a 200 uH inductance at 15kHz. I did not consider that this test is running in locked anti phase since no mention of feedback was listed. There does not appear to be any closed loop control. I am just speculating that the Jaguar can perform position control only under CAN. As strange as it seems, the Jaguar does appear to be very confused compared with the Victor in the same application commanded to zero throttle. I know that the Victor has a deadband at zero throttle, I am not aware of the modes of the Jaguar at zero throttle. This very well may be a jitter phenomena either supplied by the Jaguar itself or the code in use.

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

we are using 4 window motor,
2 with black jaguars and two with brown jaguars.
he had no problems till this day, there is another problem in the window motor you should check it.

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

This is not quite true (at least how I understand it)


"Normal" PWM switches between 12V and open circuit. For low duty cycles this means that the motor is effectively open circuit - a.k.a. not dynamically broken. The Victors (and others) have a method to allow the H-bridge to switch to dynamic breaking mode when off. Under this case, the motors are harder to turn because the motors work like a generator when back driven and they generator is shorted so it is trying to drive a heavy load (in effect).

Stick with me...

Another sort of PWM switches between 12V and Dynamic Braking. In this type of PWM there is no need to "switch" to dynamic braking mode because at low duty cycles the motor IS dynamically broken already. This control makes the FETs work harder in some cases (and wastes more energy as a result) but the control is easier in some ways because there is a term in the system equations that automatically adjusts if the speed changes.

Let me explain by waving my arms. Suppose we have 50% 12V and 50% Dynamic Braking and load TTT and speed SSS. Suppose further that we change the load to TTT+DeltaT. The Speed will switch to SSS - DeltaS. But DeltaS will be less than it would have been with "Normal" PWM.

Why? Because during the 50% of the cycle that the motor is being dynamically broken, the motor is experiencing a Negative Torque (because the motor is having to work as a generator against a shorted load -- see above). The faster the motor is spinning the more Negative Torque the motor sees. Everything balances out to a constant speed because the 50% 12V makes up not only for the load TTT but this extra loss due to the Dynamic Braking portion of the PWM (which gives you a clue into why it is less efficient, especially at low duty cycles). NOW... ...the extra torque on the motor (DeltaT) slowed the motor down but by slowing the motor down, it also reduced the amount of breaking torque that the motor experienced during the dynamic braking portion of the PWM cycle.... SO.... ...(drum roll please)... using this type of PWM makes the motor less sensitive to changes in torque -- and this is without the CODE changing anything. The system itself is less sensitive to torque changes.

NOW... ...allow us to go to Locked Anti Phase PWM.

Same story as 12V and Dynamic Braking ONLY DOUBLE (in a sense).

At any given PWM point, there is a balance in torque that is based on speed. The heuristic arguments are the same as the 12V/Dynamic Braking case only the net effect is such that the system is even less sensitive to torque changes (an actual fact in this type of drive the current curve looks kind of like the a saw tooth -- look here for a reasonable discussion of motor drives and lots of pretty pictures)

So... ...there are good things that come from using locked anti phase PWM... but as we see, there are negatives as well.

Joe J.

P.S. I am not a EE, I only play one on our FIRST team... ...so take the above with a grain of salt. JJ

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

Al Skierkiewicz wrote:
18 ohms is the impedance of a 200 uH inductance at 15kHz.

You said "200uH and about 18 ohms of series resistance" so I assumed you meant 18 ohms resistance. If the 200uH number is correct, and the 18.6 amps stall current spec is correct, then 50% -12/+12V PWM at 15KHz will be fast enough to limit the effective RMS current to about 500ma, which is about 2.5% of the rated stall current. That would explain why the motor doesn't burn up. If someone else wants to check those numbers I welcome it.



I did not consider that this test is running in locked anti phase since no mention of feedback was listed.

You are correct that Tom did not indicate that he was commanding the Jag via CAN in closed-loop position control. However, the waveform he described for the Jag sounds like locked antiphase. The waveform he described for the Victor does not.


I am just speculating that the Jaguar can perform position control only under CAN.

No need to speculate. The Jag will do the loop closure for you only when commanded by CAN. With PWM you can only command a motor voltage. Of course, you could use PWM and close the loop in the cRIO, but probably not as effectively.


As strange as it seems, the Jaguar does appear to be very confused compared with the Victor in the same application commanded to zero throttle.

If you are talking about the waveforms that Tom reported, then I don't see the confusion. The Jag just seems to be using a different PWM switching method than the Victor. There's more than one way to skin a cat (or do PWM).


I know that the Victor has a deadband at zero throttle, I am not aware of the modes of the Jaguar at zero throttle.

One of the main benefits of locked antiphase PWM is the elimination of deadband at zero. So maybe the Jag uses this method even when receiving PWM commands.


~

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

Hi Joe,

I agree (mostly) with everything you said, but nothing you said contradicts the portion of my post that you quoted.

Let me explain.

Yes, locked antiphase provides dynamic braking, but it does NOT provide any static braking (ie holding torque) when the command is zero (and the duty cycle is 50%).

A motor being commanded with 50% -12/+12V is generating no net torque. If you apply an external load to such a motor, it will move. Now, when it moves, it will move more slowly than it would otherwise, because as it moves the dynamic braking torque appears and slows the movement down. But since the dynamic braking torque is essentially a damping torque which is a function of speed, it cannot hold the motor in position against a sustained external load. Apply an external load, and the motor will accelerate until the speed it attains creates a dynamic braking torque which balances the applied external load. Then it will keep moving at that speed until something else stops it. (note: This paragraph assumes no closed-loop position control).

If you want to hold a motor in position against a sustained external load, then the motor must generate torque to balance the external load. This is what a closed-loop position servo does - it changes the PWM duty cycle as required to balance the external load and maintain the desired position.



~

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

EricVanWyk has reported that the Jag's do NOT toggle between +12/-12 when idling. As such, I am doubting my original observations, and perhaps I was observing the behavior only while loaded / in-motion. I had not considered the possibility that reverse emf could produce a waveform that goes bipolar even if the Jag output is unipolar.

I will try to have a look again tonight, armed with the 0.1 ohm resistor. I am planning to watch both the voltage and current on 2 traces, and will strive to place the scope probe monitoring current directly across the resistor.

This thread has gotten MUCH deeper than the fidelity of my initial observations. I need to take more care to be sure I have not mis-reported something.

-Tom

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

My point is that in effect, it does make the robot harder to push out of the way than would be the case without having the locked anti phase PWM. In order to move the robot you have spin the motors at some speed. For a given amount of disturbance and a given amount of time, the system that has a locked anti phase PWM will be pushed out of position less than one that has normal PWM.

Yes, a feedback loop would help (and if implemented well, a help a LOT) but this does provide some added protection with little or no extra work.

By the way, one of the great things about having either 12V/Dynamic Brake or Locked Anti Phase type PWM is that they react at the PWM rate (in this case 15kHz) rather than at the control loop rate (typically <50Hz on a FIRST robot - at least that was default the rate on the old IFI system, I am not up to speed on the cRIO's). A lot can happen in 20ms -- especially when trying to control a big robot arm.

Joe J.

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

Well then forget everything I said :-)

Just kidding. There is such a thing as -12/+12V PWM, whether or not the Jags are doing this. All the discussions pertaining to that are still valid (like the fact that little or no current flows when 50% -12/+12 PWM voltage is commanded, as long as the PWM frequency and the motor inductance are high enough... and the whole dynamic braking discussion).


~

~

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

Thought experiment:

Imagine two robots, completely identical in every way, except robot A uses +12/-12 PWM and Robot B uses +12/0 (and 0/+12 for reverse of course).

The two robots are face-to-face in a pushing match, at full throttle.

What happens?

Answer: nothing. Nobody moves. Both bots have the exact same 100% 12VDC being applied to their motors. Their motors are producing the same torque, and their bots are pushing with the same force.


Now imagine the same scenario, except both bots at exactly half throttle. Robot B will have a 50% 12V / 50% coast duty cycle. I'm not sure what Robot A would have, but I'll guess it would be something like 75% +12V / 25% -12V. It is not at all clear to me which bot would win this shoving match.


Now imagine Robot A is at ZERO throttle (50% +12V / 50% -12V), and Robot B tries to push it. Hold that thought for a moment.

Turn the tables, and imagine Robot B is at ZERO throttle (100% coast) and Robot B tries to push it.

Comparing the above two scenarios, it is obvious that Robot A is harder to push than Robot B, because Robot A will have dynamic braking which will create a resisting force (proportional to the speed at which it is being pushed).



~

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

While we're on this subject, does anyone know what is the iteration rate at which the Jags execute their position-control algorithm ?


~

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

(I believe) It is calculated once per switching cycle, so ~15kHz.

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

What is the source for that information please. That seems awfully fast to be doing closed-loop position control.


~

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

This will not happen. Drivers actually spend 80-90% of their time with the joysticks slammed at one end of travel or the other -- and 100% of the time during a pushing match ;-)

Joe J.

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

Yeah, I know. I just put it in there for completeness.


~

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

I have linked to the spec of 15khz update rate for PID.

I'd like to note that it seems this thread has lost focus on if there is a problem or not and what would the solution would be.

Team 40 is using 4 window motors to steer our all wheel steer robot. We have noticed the stalling issue which causes the motor to stop moving but keep overheating. Our fix was to monitor the wheel and disable the PID control for 1 second if it stalled. Once the PID is re-enabled the motor behaves normally.

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

We are having a similar problem with our 3-wheel swerve drive. I think a large part of it is due to the gear ratio (1:1) on the window motors and mechanical friction on the modules causing the window motors to overheat. However, sometimes when the robot is rebooted (after allowing a lot of time to cool down, i.e. the motor is cool to the touch) the window motor will not move anyways. If we disable and re-enable it will come back and start working. Is this similiar to what you are saying?

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

I think it is time for Toyota to issue yet another recall. This time for window motors.::safety::





Wait, did I just say that out loud?

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

I concur, if by "recall" you mean try another set of motors.

Something seems whacked about those motors -- the Jaguars are acting differently than the Victors do, but I suppose that the Jaguars are cutting out for a current fault issue, in which case, you probably don't want to use them anyway.

Joe J.

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

Actually the lengthy technical discussion here was an attempt to zero in on the cause of the problem.

Go back and read my post about the RMS current flowing when a motor controller using locked antiphase PWM designed for a higher-inductance motor is used to control a low-inductance motor like the Denso window motor. This causes the zero-command current to be higher than expected (500ma).

Even though this is low enough not to cause motor heating, the Jag MAY be sensing it and deciding that the motor has a short, and taking some sort of protective action.

I'm not saying that's what's happening. It's just a working hypothesis.

Does anyone have datasheets for the Jag which have more detail about PWM modes and acceptable motor inductance and quiescent current??


~

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

Overheating... This sounds like a different problem than what the OP described. Tom, what say you?


~

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

I see Table 5 and Table 6, which list the 15KHz measurement rate for certain parameters (not including encoder). Was there more?

The PID loop iteration rate is not listed in either table.

~

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

The following is just an observational opinion.

1) We know the Victors work well with the window motors and have for years.
2) The window motors work well with the Spike relays.
3) The Jag is a new(ish) animal to us and we don't have quite enough experience or information to make a solid determination about them, yet.

The simple, and I really mean simple, solution is to use either a Victor or Spike to control them. (I know, duhhh!!)

Now, if I'm not mistaken, using the Jag on CAN, don't we have a lot more data available to us coming back from the Jag? (Sorry, I haven't even touched CAN with them yet.) If so, could that information give us a lot more insight into what is at the root of the observed problem?

I know it is competition season right now, but I bet the CD community can figure this one out quickly. Ra, Ra!! (The sound of rustling pom-poms in the background.)

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

NI has Greg McKaskle (impressively knowledgeable and helpful and available) to help with cRIO and LabVIEW issues.

Doesn't TI (or Luminary Micro?) have an applications engineer "assigned" to FIRST to assist with issues like this?


~

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

Close. The PID loop is evaluated at a rate of 1 kHz.

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

What is your source for this information please.

Thank you.


~

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

http://www.youtube.com/watch?v=tf_QyFlbA8w


~

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

Please allow me to clarify a few things... I spent some more time in the shop this evening, and I would like to correct a few things I said, and emphasize a few others.

1. The Jag's do NOT produce a +/-12 square wave with an idle command. They behave just like a Victor (albeit at the significantly higher switching rate). EricVanWyk corroborated the fact that I had incorrectly reported this earlier.

3. The Jaguar does NOT seem to exhibit any sort of instability. Also an erroneous notion on my part. It is well-behaved across the full range of control with the Denso.

4. There IS still a problem with the Jag driving a Denso motor (and not with a Victor).

5. The problem is NOT due to overheating and / or the PTC. It IS very easy to trip the PTC resistor, but the failure we see is quite easily separated from the PTC overheating, particularly when observing the current.

I have instrumented both a Jag and a Victor and monitored both the voltage and current waveforms with a scope. I will attach some plots below for reference. The waveforms do look differrent. The Victor current waveform is a series of impulses, while the Jag (because of its higher frequency) is much smoother and essentially constant.


The failure mechanism:

The Denso motor fails to start reliably even when 100% drive is applied (and the PTC is not tripped). The voltage waveform is correct, and the current is high. It just acts like it's stalled even though there is no mechannical load. We can easily get this to happen with just the motor (nothing connected to the output shaft).

The motor does not start even if you try to "help" it move in the commanded direction.

When the motor is "stuck" like this, you can coerce it to start by turning it in the OPPOSITE direction. A slight nudge in the opposite direction makes it suddenly free up and turn in the commanded direction. It is as though there is some sort of backlash in the worm gear, and shifting this backlash causes a release of tension, freeing the motor to turn in the opposite direction.

We can NOT get the motor to fail in this way with a Victor powering it. There was some mild evidence of sticking, but it always freed up with a command authority much less than maximum (maybe 25%) wheras the Jag could not start even with maximum command authority, happily drawing > 10 amps (until the PTC tripped after many seconds).

With the Victor, the motor growls as it begins to start. With the Jag, it's a high pitched squeal. Could it be that the growling vibrates the worm gear enabling it to move the backlash just enough to start when the high-frequency of the Jag cannot?

Scope traces of Voltage and Current... Jag and Victor.

Scope Traces -- Trace1 = Voltage, 10V/div. Trace #2 = Current. 100 mV/Amp

Plot #1 = Jaguar at ~50% negative, Current -5A
Plot #2 = Jaguar at ~95% positive, Current +5A

Plot #3 = Victor at ~10% positive, Current pulses to 7A
Plot #4 = Victor at ~20% positive, Current pulses to 8A. Note distortion of voltage waveform (reverse emf?)
Plot #5 = Victor at ~99% positive, Current now almost constant 1-2A

Bottom line. I still believe there is an issue with the Denso motors powered by a Jag. The issue may be largely mechanical, but it's the combination of the two that seems to be problematic. We will be replacing the Jag with a Victor. Also note, as previously discussed, this motor does heat up quite quickly, and the PTC does trip easily. I don't think it is good for heavy continuous use whether you have a Victor or a Jag.

-Tom

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

He's a TI employee working on the Jaguar.

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

That sure fits the facts (the new facts :-).

The pulsing current produced by the Vic knocks it loose, whereas the smooth current produced by the Jag does not.

Sounds to me like the motors are defective; not commutating properly. Could be a marginal design or a bad lot.


~

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

See, now that's what I'm talking about!

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

Tom,
The DC shift between pulses is normal for a motor that is moving with the bridge turned off (open). You are reading generated open circuit voltage of the motor moving. The low throttle value on the Victor exhibits similar, but lower voltage level because the motor isn't turning as fast. It appears that Jaguar is shorting or otherwise providing a low impedance in between pulses. Is the jaguar in "brake" mode while the Victor is in "coast"? I know that the Victor will switch to brake (both low sides of the H bridge turned on.) when the throttle is zero +/- the deadband. According to the manual, the Jaguar only turns off one side of the bridge with PWM, while leaving the opposite side FETs turned on. That may be what is making the Jaguar waveform look cleaner but still doesn't explain why the two are so different when not supplying current to the motor. Although the small zero volt pulse in the Victor waveform may be the second leg of the H bridge still turned on for a short time.

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

Al, is this true regardless of the Vic's brake/coast jumper setting ?


~

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

No, I am sorry. This is only in brake mode. In coast, all FETs are turned off in the Victor.

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

I was wondering if anybody has resolved this problem further. Our team used 4 Denso motors for our 4 wheel independent steering. We kind of suspected we had this problem from the beginning. We had some programming and mechanical issues that masked it. Now that we have solve the programming and mechanical issues it is very apparent that we are plagued by this problem. It is very intermittent and random. The steering motors have very low load on them and do not get warm unless we run for extended time. (10 - 12 minutes). On Monday night we plan to replace the Jags with Victors. Is this our only option? Do the victors solve the problem? This is puzzling problem. Unlike the other post we can not get the motors to run again unless we power down and reboot. Affects us for the duration of the match. Any other ideas or comments?

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

In software, try setting the output to 0 and then back again. Does this fix it?

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

For what it's worth, the anecdotal evidence I've heard with Jags and Window motors... Problems arise when set to "brake", but go away when set to "coast". I skimmed the thread to look for something like this and couldn't find it, and I have no idea why.

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

Since it's been a while since there has been any activity on this thread, and there were over 70 posts, I took the liberty of excerpting some of the observations and problem descriptions that have been posted. The number before the right paren is the post number.

Considering DavidGitz's post#55, it would be most interesting to get an answer to EricVanWyck's question in post#74

Also, has anyone experienced this problem with BLACK Jags and window motors? See post#4.

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

When this happens we are running competition code and can not set the PWM to 0. We can switch directions and this has no effect. In the beginning of the season, I pulled the pwm cable and reinserted. This should have caused a time out and set to zero. Put the cable back in and nothing. Power cycle solves problem. Have not tried pulling breaker. We can go many matches and not have it happen. Happened to a replaced window motor. Seems to happen with any window motor. This is very irritating because now now that we have the swerve dialed in the robots great except for this last issue. We are using tan jags. Because there is PID involved switching to victors is not the best solution but it's the only thing we know to try. Any other ideas?

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

Do you have a black Jag you could try? See post#4 in this thread.


~

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

Gary,
I had received no further reports of issues with Window motors. However, I reviewed the waveforms Tom posted on the previous page. If his captured waveforms are correctly identified, there would seem to be a typical inductance current waveform on the Victor pictures at less than full throttle while the Jaguar waveforms show no sign of inductive current which they should at 15kHz. Since there is no reactive component to the Jaguar waveforms, is it possible that the temperature protection has some fairly high impedance at 15 kHz?

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

We changed the tan jags to black jags with coast last night and it still happens. If I catch it as soon as the motor locks and pull the PWM cable and reinsert the motor functions. With the black jags once a motor locks it heats up faster with the black jags. < than a minute till the PTC resistance goes high. It heated slower with tan jags. We will install victors Wednesday night and see if victors are the answer. I took apart a spare window motor and there are 2 chokes and 2 caps in addition to the PTC. We could short the PTC now that it is off season but will do the victors to test. If the victors work then there has to be some weird resonance setting up between the filters, PTC and the jags frequency causing the PTC to heat. If the victors work then the jags will always be suspect with emi filtered and PTC protected motors. Maybe this is one reason IFI never went above 2000 HZ switching frequency.

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

all we had was a winch connected to a regular andymark gear box.

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

Gary,
The chokes and caps are for brush noise RFI suppression and should not pose and significant impedance at 15kHz. The core of the chokes would need to be really big to be effective at that low a frequency. Is there any markings on the PTC?

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

Great post Gary. Thanks for the data. This really helps narrow down the problem... assuming everything works fine when you use the Victors.

Al: I know you have lots of documents. Do you by any chance happen to have a circuit diagram of the chokes and caps in the Denso? I'd like to see how they are arranged.

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

The first problem you are dealing with is a mechanical one. The window motor interface to the gearbox pinion has an anti-backdrive mechanism that locks the shaft. if there is intermittent back pressure on the gearbox output shaft it locks easier. This was probably to keep someone from prying the window down because the worm gear pitch isn't steep enough to negate backdrive.

If you are driving a load and the load doesn't keep up with the motor, it sets the mechanism and locks. if nothing detects the lockup and stops the output within a few seconds the motor protection takes over and you're done until it cools down. If you remove the output soon enough and then drive in the same direction again, the mechanism will unlock assuming the load will move. Driving in the opposite direction does not unlock the mechanism unless the load relieves on it's own.

I believe the theory that the motor shaft vibration caused by the Victors at 120hz aids in unlocking the mechanism while the Jaguars smoother output doesn't.

Just some of the frustrations of mating devices and technologies that were never designed to go together but enabling kids to keep after it until something works will hopefully pay off down the road of developing the next generation of stuff.

To test these theories, you might try driving the system into a load that is controllable (like pushing back and pulling on the steered module) and see what happens.

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

John,

Is it fairly simple, and non-destructive, to separate the motor and gearbox in order to see this?

If not, did you guys take any pictures you could post ?

Thanks.

~

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

Just remove the phillips head screws that hold the motor on.
The motor shaft will have a three-lobe coupling on it that interfaces with the gearbox. I believe the gearbox side is metal. There is a brown plastic ring around the metal that contains locking pins. When the brown ring and metal coupling are misaligned they lock. as long as the pieces are rotating together they turn freely. I think the rings are retained by a thin metal plate.

I'm reciting this from memory as I haven't looked in one for three months so there may be inaccuracies but I think if you play with it you will see how it works. Turning the output shaft locks the coupling. turning the coupling with the motor drives the locking ring with it to keep it from locking.

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

I have not found any schematics in my files. Typically, chokes are wired in series with each brush and capacitors are wired across each electrical input wire with the other end tied to the motor case. In this configuration (looking from the brushes) the two components form an L/C low pass filter that filters out RFI before it leaves the motor.

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

Thanks to everyone on this thread who has contributed to the body of knowledge here on CD!

I know there are a number of white papers and PPP available on motor selection, examining power curves, etc.

Has anyone posted a comparative table evaluating the KOP motors in a qualitative sense? In other words, 1-5 stars for use in various applications, such as drive, winch, steering, etc? I know it would change from year to year and need updating, but a general guide would be useful to rookie (and some veteran) teams to start the selection process.

We need to have the students more involved in these decisions, and lifting the veil of mystery would be helpful.

This doesn't stop them from doing the actual calculations to see if they can lift 3 robots with a window motor!!

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

Ether - Thanks for compiling all those. It sparked a thought.

Theory:
Some Denso motors brushes are failing in such a way that too many windings can be active at the same time. This may be due to a winding-winding short, a brush deformation, or simple poor design. This state is limited to a limited portion of the rotary travel.

Direct Result:
While in this state, several things occur:
* Current increases: More windings means more mhos.
* Torque constant greatly decreases: Windings fight, reducing effectiveness. Imagine a shaft with a keyway in it: This is the extremely over simplified diagram of the torque constant vs angle that I'm imagining.*

Matching Symptoms:
* The motor will tend to lock into the keyway, given a load that is between the depth of the bore and the outer radius.
* Applying extra torque can pop it out.
* If the bore is deep enough (and it could be very deep), "smooth" current won't do anything.
* "Rough" current has the advantage of being little bursts of torque. This has the possibility of pushing a little harder momentarily (duty cycle <100). It also has the advantage of giving up, letting it roll the wrong way (out of the bore) and then charging forward over the valley.


Does this match up with other folks thoughts?

* Sorry for the weird bore analogy. Better wording of it would be appreciated.

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

We removed a window motor in the beginning of the season. I disassembled and inspected the motor. there was no visible wear or signs of a mechanical problem. That was before we realized that all window motors were having very random and intermittent lock ups. We can go for hours of practice and not have a lock up. The randomness makes this hard to track down. When I took the window motor apart on Monday night, I did not see any locking pins. I thought that that was just a flexible shaft coupler. Tonight we will install victors and test a new autonomous mode. We are also going to a competition this weekend and should get more data. One thing I noticed and do not want to state as fact is that when the lock happens the jag stayed solid red even when an opposite direction input was applied. I want to consult others before this is stated as true. If so this is a really weird one. If we can resolve this then we can consider the 1st season 4 wheel independent steering and drive swerve system a success.

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

JDNovak thank you for your post. When I took them apart last time I didn't go far enough and remove the black retaining plate. The locking pins and collar are right there. Tonight we disassembled the window motors and removed the pins. We where able to do this without removing the steering motors from the robot. I did one then the students did the other 3. It is not very difficult. Forget the electrical theories. This is a pure mechanical issue. We ran the robot with the modified steering motors tonight with Jags and have not seen any problems. We are going to an off season competition this weekend and will give it a good test. We took pictures and will create a page on our website to view the evil pins. The problem goes beyond latch up. At low speed and under load the pins cause considerable friction. Other teams noted having hot window motors when driving 2 swerve modules. I bet that with the pins removed their problem would not be as critical. Thanks everyone for your input on this problem. Going forward if this solves the problem FIRST needs to be made aware it. Will pin removal be legal if we get the window motors next year? What is the best notification path for FIRST on this matter.

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

Gdeaver

If it isn't too hard, can you post a couple pictures of the process for those of us who will inevitably end up following your footsteps?

Thanks!

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

Since the GDC (Game Design Committee) has authority to interpret and change the rules, I would think that posting to the official FIRST Q&A forum would be a good way.

Access to the Q&A forum is restricted to one designated person per team. This person has a username and password for access. See this link:

http://usfirst.org/roboticsprograms/...nt.aspx?id=456

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

You might also consider mentioning this on one or more of the official FRC surveys linked here: http://usfirst.org/roboticsprograms/....aspx?id=17127

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

The best thing to start with would be to try to take some pictures, post them here, and explain what's involved. FIRST has been reluctant to allow teams to detach or modify the gearboxes on the window motors (since forever—some of the older window motors weren't so convenient to disassemble, as you probably recall), so it would be great if you could demonstrate that it's no big deal.

Then, we can ask FIRST to have a look (either via the Q&A, frcteams@usfirst.org, or personally to the appropriate GDC members).

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

Gary,
Follow the leads to the Q&A. The GDC has allowed some modifications in the past when it was presented with a benefit to teams to make the modification. Locking pawls in the drill motor transmissions and wires on the Bosch motor are two that jumps to mind.
So did you test with Victors before modifying the motors? That would give us a little more data to actually see if there is some interaction.

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

The steering motors continued to work fine last night. We did not switch to Victors. We took several pictures and put them on a web page. Remember, the window motors were not removed from the robot so the picture quality is not perfect.
Here is the link.
http://wiki.team1640.com/index.php?t...r_Locking_Pins

This is a simple procedure. The window motors allow FIRST to provide a motor and gear box at a economical cost. I think they should continue to be in the KOP as long as First approves the removal of the evil locking pins.

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

Quick update for teams planning on removing their locking pins:
1640 competed at Bridgewater Raritan Battle Royale on Saturday with the locking pins out as above (using black Jags), but still suffered from intermittent steering lock-up. I'm not saying removing them is a bad idea (or a good one for that matter, I have no idea as of yet), but unfortunately it hasn't solved the problem.

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

Nice write up. A few questions.

Does the lock ring have a maximum torque limit? If one continues to add load force does the ring just wedge tighter or will it eventually give?

When the drive shaft engages the locking pin cage, would the command required to move depend strongly upon the direction? This could be estimated by using a JAG step command with increasing amplitudes and seeing what command will break the lock in both directions.

A comment on this diviney statement: When the maximum JAG command authority was applied, I assume it was from a Joystick moved by hand. If so, this would lessen the impulse applied by the motor upon the lock. There just doesn't seem to be any reason that a JAG with a proper waveform input could not duplicate the Victor capabilities unless there is an internal JAG command rate limit that I'm not aware of. With a 100% PWM step input, both the JAG and Victor will reach 10 amps around 2 ms. To reverse and create a matching shutoff like the Victor at .2 duty, the JAG could react within a ms (JAG control iteration rate). It seems plausible that a startup lock breaker pulse could be synthesized for the JAG.

Re: Denso Window Motors, Warning - Don't use with Jaguars !
 

The Victor's output PWM frequency is 150Hz. The Jag's output PWM frequency is 15000Hz. The far-lower PWM output frequency of the Victor produces a very "noisy" current waveform in the motor coils compared to the smooth current waveform produced by the Jag's much higher frequency output PWM.

It is possible that the 150Hz "noise" created by the Victor is responsible for preventing the lockup problem. Perhaps 150Hz is close to the resonant frequency of the rotor.

The only way to simulate the Victor's 150Hz output PWM frequency with the Jag would be to create a periodic task running at least 300Hz in the cRIO (to simulate 50% duty cycle at 150Hz). I don't think this is practical.


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