[zbrozek]

Originally Posted by Al Skierkiewicz

Some of the items you mention are due to the power supply you are using. The motor testing needs a very high current, low impedance supply to obtain accurate data.

I should have posted a waveform of the input voltage to the Jaguar, but it was rock steady. My power supply is a QPX600DP and I was using remote sensing to regulate the voltage at the device under test. If anything, I'll wager my input supply was lower impedance than most teams' batteries. What it doesn't do, however, is deal nicely with sinking current. It's a source, after all, not a battery. That only manifested when braking.

Originally Posted by Al Skierkiewicz

It is not possible to measure motor current directly as the polarity at the output of the device changes. While possible with multiple INA193, it raises the parts count per device. Measuring input current is affected by device current demands but these are low compared to the motor current. Jaguars rarely reboot in the field although they do fault on low input voltage.

There are plenty of parts that can do bidirectional current and have a sufficient negative common mode range; they're just not the INA193. I agree, logic draw is insignificant compared to the motor draw. Despite that, TI put the shunt between the H-bridge and the bulk bus capacitor. If you're going to do a bus high-side measurement, that's the right place to do it both because it isn't affected by logic consumption and because it isn't smoothed by the bulk capacitor.

Originally Posted by Al Skierkiewicz

You do not mention what throttle value you are running the motor at but it appears to be something much less than 50% duty cycle. So what comes into play is the brush spacing and the motor rpm. So current draw is reflected by the number of commutator segments that are in contact with brush at any one time. This is common for brushed motors.

Good question; I was running 10-50% of Vin while in voltage control mode and 1-20A in current control mode. I was, however, loading the output of the motor with a brake. The torque ripple was evident at all throttles and loads, and its frequency proportional to the rotational rate of the motor. I fully believe that it was due to the brushes making and breaking contact, its magnitude is really what surprised me. As an aside, it surprises me that FRC is using brushed motors given the incredible availability of brushless ones.

Originally Posted by Al Skierkiewicz

If your power supply is really sagging during high current pulses, then I would expect the output of the INA193 to also reflect that. It is internally referenced to the power supply input pin. It might be interesting to add a trace of the power input voltage with the other waveforms. The triangle waveform is a result of the inductance of the motor windings interacting with the current input.

I don't know enough about brushed motors to have good intuition for their 2-port network properties as a function of rotor position. Of course the motor current is a triangle wave, I'm just trying to reconcile the output of the two current sensors - one a magnetic scope probe, the other a shunt with an amplifier. They're not measuring between the same two circuit nodes, so it's not mind-blowing that their output isn't the same. My best guess right now is that the output of the INA193 is mostly just wrong because the amplifier is picking up a lot of interference.