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