Resilience of Motors using PID

[cgmv123]

I tend to use the term similar lightly. A window motor should do the job though. Also, since there are left and right hand motors you could gang to of them together doubling the output. You may or may not have to account for the different power levels for left and right hands.

[Daniel_LaFleur]

Originally Posted by cgmv123

I tend to use the term similar lightly. A window motor should do the job though. Also, since there are left and right hand motors you could gang to of them together doubling the output. You may or may not have to account for the different power levels for left and right hands.

You need to be very careful ganging window motors if you leave the locking pins in.

[EricVanWyk]

You may want to consider using the Current Mode control of a CAN enabled Jaguar. This will create less waste heat in your motor, assuming you properly tune the PID parameters.

I'm using the parameters found here, please let me know if I'm looking at the wrong motor.

You should expect to use about 6 amps to develop 6 Nm with this motor. It uses 22 Amps to generate 22.5 Nm at stall, so a rough pass is 6 Nm * ((22 A) / (22.5Nm)) = ~6Amps. The no-load current (which is really just another word "the sum of the parasitic torques in the system") is small enough that I'm willing to ignore it for the time being.

With a "perfect" motor controller, you will apply a perfectly flat 6 amps. The motor's resistance is roughly half an ohm, so your best case is (6 A)^2 * .5 ohms = 18 Watts.

Any imperfections in the motor controller will cause that current to oscillate a bit and become less efficient. For example, lets take one that applies 12 Amps half the time, and 0 amps the other half. Then our heating becomes (12 A) ^2 * .5 ohms * .5 duty = 36 Watts.

Worst case scenario, we'd expect something approaching stall current: (22 A)^2 * .5 ohms * (6/22) duty cycle = 66 Watts. This is 3.6 times the heat output!

The "flatness" of the current is a function of the ratio of the time constants of the motor and the motor controller. The faster the controller is, with respect to the motor, the flatter the current waveform. The flatter the current waveform, the more efficient the use of that current.

I don't know what the time constant of this particular motor is, so I can not calculate where victors and jaguars fall on the continuum. However, the Jaguar's time constant is over a 100 times faster.

Note that for this, I've assumed that the mechanical time constants are WAY slower than all of the electrical time constants. As long as you don't see any twitching, this is likely a safe assumption.

[EricVanWyk]

Ether helpfully pointed out that the first sentence of my previous post makes a few too many leaps. The remainder of the post supports only that higher frequency motor controllers produce less waste heat. This in turn is based on the premise that torque is proportional the algebraic mean of current, but power is proportional to the geometric mean.

The comment on using Current Mode was unsupported, so let me take a whack at that now. If a PID is not well tuned, it can oscillate. These oscillations can create efficiency losses just like having a low frequency motor controller can. A pathologically mis-tuned loop can have efficiency as bad as a Victor.

Using CAN allows the Jaguar to do the PID itself, and runs at 1kHz. Running the PID on the cRIO introduces some communication time, so the loop runs a bit slower.

Using current mode removes several variables from the equations, and makes the effective loop quicker: The loop isn't waiting on anything mechanical, and can therefore respond on an electrical time line.

It is quite possible to achieve the same stability with other control modes, I just find it easier in current mode generally. For your application, CAN Position Mode might be easiest.