Chief Delphi - Denso Window Motors, Warning

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- - Denso Window Motors, Warning - Don't use with Jaguars ! (http://www.chiefdelphi.com/forums/showthread.php?t=83973)

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

Quote:

Originally Posted by Ether (Post 934430)

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.

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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 !

Quote:

Originally Posted by diviney (Post 934431)

EricVanWyk has reported that the Jag's do NOT toggle between +12/-12 when idling. As such, I am doubting my original observations

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).

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

Quote:

Originally Posted by Joe Johnson (Post 934437)

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... the system that has a locked anti phase PWM will be pushed out of position less than one that has normal PWM.

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).

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

Quote:

Originally Posted by Joe Johnson (Post 934437)

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.

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

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

Quote:

Originally Posted by Ether (Post 934515)

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

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

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

Quote:

Originally Posted by EricVanWyk (Post 934546)

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

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

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

Quote:

Originally Posted by Ether (Post 934505)

Thought experiment:

<snip>
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.

<snip>

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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 !

Quote:

Originally Posted by Joe Johnson (Post 934564)

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 ;-)

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

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

Quote:

Originally Posted by Ether (Post 934558)

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

Quote:

Originally Posted by billbo911 (Post 934596)

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

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 !

Quote:

Originally Posted by Kingofl337 (Post 934575)

I'd like to note that it seems this thread has lost focus on if there is a problem or not

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??

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

Quote:

Originally Posted by Kingofl337 (Post 934575)

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.

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

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

Quote:

Originally Posted by Kingofl337 (Post 934575)

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

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.

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