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