Motor in the IVth quadrant

technik3k · November 3, 2017, 2:30am

nenik:

On the electrical side, things are quite simple. The motor is pretty much a RPM-dependent voltage source in series with internal resistance. This is where the stall current comes from - zero back-EMF, thus the stall current is battery voltage (minus the driver losses/PWM) divided by the internal resistance.

The motor driver fully open in opposite direction is quite simple too, with 2 conducting FETs in the bridge, it is pretty much a low-impedance (the combined bridge resistance is in tens of milliohms, less than that of the battery) voltage source. So, with motor being forcefully turned full speed in the opposite direction, the back-EMF of 7V gets added to the driver’s 7V, and this 14V pushes current through the winding resistance of <2ohm, thus over 7A. Current is what causes torque and unless the magnetic circuit gets saturated, pretty much linearly.

I don’t know what I was thinking before. Now, that you have explained this so clearly - it became very obvious! With FETs resistance close to zero - the circuit is pretty much the main battery and motor’s EMF source in series supplying voltage to motor’s internal resistance, thus almost doubling stall current at the beginning.

However, I have some questions about your results:


266 , 3.330, 3062+34   , 127
267 , 3.340, 3096+33   , 127
268 , 3.350, 3129+31   , 127
269 , 3.360, 3160+32   , 127
270 , 3.370, 3192+34   , 127
--------------------------------------------
271 , 3.380, 3226+17   , -127
272 , 3.390, 3243+46   , -127
273 , 3.400, 3289+24   , -127
274 , 3.410, 3313+15   , -127
275 , 3.420, 3328+6   , -127
276 , 3.430, 3334+1   , -127
277 , 3.440, 3335+0   , -127
278 , 3.450, 3335+0  , -127
279 , 3.460, 3335+0  , -127
280 , 3.470, 3336+1  , -127
281 , 3.480, 3336+0  , -127
282 , 3.490, 3336+0 , -127
283 , 3.500, 3336+0 , -127
284 , 3.510, 3333-3 , -127
285 , 3.520, 3314-19 , -127
286 , 3.530, 3290-24 , -127
287 , 3.540, 3274-16 , -127
288 , 3.550, 3251-23   , -127
289 , 3.560, 3228-23   , -127
290 , 3.570, 3205-24   , -127

What was your exact setup? I assume you plugged motor into MC29 and not 2-wire port on Cortex. Did you measure angle with quad encoder or IME?
Do you have an explanation for samples 271 and 272 or they look to you like instrumentation error due to some sort of delay in receiving sensor value?
It took about 60 msec to fully decelerate, but probably less time, assuming delay in sending command to motor controller. Then there was about 60 msec of no apparent movement, then 50 msec of acceleration but to a slower velocity.

It looks to me like it was picking gear slop when the readings were around zero, and then smaller steady velocity was achieved as a result of the reduced PTC conductivity. Did it achieve faster speed (around -34) later?

If you are going to conduct more experiments - you may want to log both IME and quad encoder values. The the difference between those readings could give additional insights into how motor’s internal gears behave.

Also, here is a great thread that describes RobotC firmware timing in respect to tasks and how it relates to sensor input (an important topic that still gets me confused). You will probably, find it very interesting: