Modeling motor control

[Ether]

Originally Posted by kamocat

I'm going to try to get ahold of a CIM with a toughbox and encoder, but I may not get that for a few days.

If you run a CIM with a toughbox, what do you expect to compare that data to? There is no manufacturer's data for this. Still apples and oranges.

I have a globe motor, but the current is too small to measure with a Jaguar.

If you run the CIM w/o the toughbox, the no-load current is too low to measure with any accuracy with the Jag, isn't it?

[kamocat]

The current just needs to be greater than 1A to be measured.

The point of measuring a CIM with a toughbox is to see if it still has these strange current characteristics.

[Ether]

Originally Posted by kamocat

The current just needs to be greater than 1A to be measured.

Per the datasheet, the accuracy of the Jag's current measurement is +/-2 amps for currents less than 8A. The no-load current for the CIM is 2.7 amps.

The point of measuring a CIM with a toughbox is to see if it still has these strange current characteristics.

The point of measuring something for which you have manufacturer's data is to validate your test method. It is possible that the funky data is a problem with your test method.

Just sayin'

[theprgramerdude]

Would it be better to hook up (not really) a magnetic current monitor outside the wires to measure current that way? It might have a more accurate current reading, if you can find a model that can handle such low currents and ignore any interference from the motor.

[kamocat]

I have a old current clamp I'm trying to get working.
I found most of my motors are too small to be measured by the Jaguar.
There also may be some issues with varying Jaguar accuracy as the PWM duty cycle changes.

[Al Skierkiewicz]

Guys,
I think you are documenting the interaction of a few different elements in the way you are making your measurements. You first have the brush spacing/segment frequency to deal with. Then you have the inductance of the motor vs. the switching frequency of the Jaguar. And finally you have the sample rate of the current monitor in the Jaguar.
It is well documented that the higher switching frequency of the Jaguar does limit current at lower PWM values in higher power motors. This is due to the finite rise time of the applied pulse interacting with the motor internal inductance. It is one of the things that make the Jaguar seem more linear than Victors. Obviously this much more pronounced with CIM motors and FP than with the window motors or the Banebots. At small pulse widths and large inductances, the voltage at the motor terminals never reaches the supply voltage.
If the sample rate or time are sufficiently long enough, at lower RPMs, you will undoubtedly read the current through two windings (while the brush covers two segments) causing a higher reading. As the motor achieves higher RPM, the sample rate likely moves to a point where most current is read while the brush is on only one segment and therefore lower.
There is likely to be significant interaction between the current pulse sampled by the series resistor in the Jaguar and ADC used to transmit the current back to the CAN bus.
My suspicion on the Ryobi drill motor curve is likely due to hysteresis in the core of the motor at lower pulse widths and it's design for increased torque at a specific RPM.
BTW, we should also take into account the rather strange, very short, voltage spikes that occur with Jaguars at the pulse edges under certain conditions. These vary with motors and speed.

[kamocat]

In other words, the motor current (as measured by the Jaguar) is useless for determining the torque output?

[Ether]

Originally Posted by kamocat

In other words, the motor current (as measured by the Jaguar) is useless for determining the torque output?

At very low torques (like no-load), probably. Look at the datasheet. The accuracy is +/-2amps for currents <8amps.

That's why you always want to validate your test method against something that is known, before using it to gather data for an unknown.

The Black Jag uses an INA193AID current-sense chip across a 0.001 ohm shunt in the high side of the bridge. The INA193's analog output is sent directly to an ADC pin in the Jag's LM3S2616 microcontroller, without any hardware (RC) filtering.

So... it depends on what the LM3S2616 firmware is doing. Is it doing some sort of digital filtering? Is it timing the ADC samples to remain phase-locked with the bridge switching? Unless someone intimately familiar with the design is willing to comment, the best way to tell is to compare the Jag's current sense values with oscilloscope current waveform traces taken across an external precision shunt, under various test conditions.

[Al Skierkiewicz]

Marshall,
If your desire is perfectly accurate current then the answer is no. If you are trying to model some sort of feedback loop, then the answer is yes. It will take some experimentation and noise filtering but teams did use current feedback through the CAN bus last year. Can you answer how you measured the voltage? The reason I ask is that except for the low pulse width signals, the voltage to the motor is swinging between 0 and power supply volts or ~12 volts. The average voltage can be calculated from the pulse width but may not be accurate. For those that are trying to correlate real world performance to the manufacturer's published data need to remember the the factory testing is performed using a high current, lab grade, low impedance, stable voltage source. The battery, wiring and Jaguar do not come close to factory testing so although one can come close, there is no way to exactly duplicate factory published data.

[Ether]

So as not to highjack this thread, I started a new thread to discuss technical details of the Jaguar current-sense implementation.

[kamocat]

I measured voltage with the Jaguar. (In the November 2010 spec for the CAN interface, there is an option to get the output voltage as a voltage, rather than a percent. I believe this multiplies the output percent by the Vbus. There is no info on this in the MDL-BDC24 datasheet.)
No, I have not compared this to another device.

If I have to calibrate this as well, I'm not sure the Jaguar is a feasible option for what I'm trying to do.

Let me step back a bit and ask how much it will help me to have an accurate measurement of current.
Is output torque still proportional to current, even at reduced voltage?

[Ether]

Originally Posted by kamocat

Is output torque still proportional to current, even at reduced voltage?

Yes and no. Depends on what you mean in this context by current and torque.

The average torque generated by the motor is proportional to the algebraic average current, assuming Kt is constant (which in real life it is not, but for most practical purposes it is assumed to be).

The output torque at the motor shaft is equal to the torque generated by the motor, minus the torque losses within the motor.

[Joe Ross]

What was the voltage source for your test? Were you using the jaguar %VBus mode or the voltage mode? Did you monitor the voltage of the voltage source over time? What was the sample rate (related question, how long did the test run)?

[kamocat]

I'm using an 18AH AGM battery comparable to the KOP Gel-cell.
I'm using %VBus mode, but the voltage jitter in the output is negligible. (It's plotted according to the output voltage, not according to time)
The test takes a little over 1,000*200ms, or a about 3.5 minutes.

For vBus, I looked at it while I was testing, but I didn't record it. It went from approximately 12.7v at the beginning to 12.3v at the end.
Considering I'm plotting them according to the output voltage, this shouldn't be an issue.

The way I have this setup is I'm setting the output, waiting 200ms, and then requesting [current, vBus, speed, vOut] for 10 iterations in a For loop. I'm not measuring the time between those samples.

[Jared Russell]

Over 3.5 minutes of operation, I could imagine motor heating affecting torque output.