Resilience of Motors using PID

[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.

[theprgramerdude]

Originally Posted by Daniel_LaFleur

A motor thats moving generates a Reverse voltage. In brake mode(you are correct) the controller shorts the leads of the motor, allowing the current generated by the motor to flow backwards back into that motor ... driving it the opposite direction and slowing the motor.

A motor thats not turning generates no reverse voltage, and thus cannot stop (or hold in place) the motor.

Fixed. And the last statement isn't entirely accurate.

[Ether]

Originally Posted by Daniel_LaFleur

A motor thats not turning generates no current, and thus cannot stop (or hold in place) the motor.

Originally Posted by theprgramerdude

the last statement isn't entirely accurate.

@ prgramerdude: Please elaborate.

[theprgramerdude]

Originally Posted by Ether

@ prgramerdude: Please elaborate.

The ideal motor equations don't always hold up in the real world. Every motor in the FRC context has some internal friction that opposes rotation. Thus, while a non-spinning motor may be able to prevent another from moving in certain cases, it could also have enough friction, and thus torque, to prevent another motor from moving.

High gearing stages on said non-moving motor would amplify this effect (Have you ever tried spinning a CIM connected to a tough box via the tough box output shaft?). If the motor happens to be non-moving, like it was stated in the original post, then the static friction from such a motor could prevent movement. It also could easily slow down a running motor, even with open leads, although the how-much depends on context.

Although, I do agree with the original's rough intent: the back-EMF from a motor shorting into itself DOES NOT apply to a non-moving motor (no EMF overall), and so brake/coast mode does not apply. The "motor", though, isn't just an electrical energy transducer.

[Ether]

The context of the sentence to which you were responding was about motor-generated current, so it wasn't clear you were referring to friction instead.

Ok, so let's talk about friction.

Take a toughbox with CIM attached. With the motor leads unshorted, measure the breakaway torque with an appropriately-sized torque wrench. Now short the leads and repeat the same test.

If you see any difference, it's probably due to cogging torque in the motor, not motor friction. The lion's share of the friction is in the gearbox, not the motor.

Your conclusion is correct though. A motor with high cogging torque, connected to a high-ratio gearbox, may be capable of supporting a static load.

[BHOP]

I would recommend taking the load off the motor using a mechanical device like others have said.

I'm not sure which locking pins they are talking about in the window motor, but I do know they utilize a worm "drive".

You can read more about them on wikipedia (search worm drive), but if the window motor won't give you enough power to move the device using to hold your shooter(not hold it in place) then buy your own worm and worm gear(mcmaster,etc) and make your own worm drive with a more powerful motor.

"Worm gear configurations in which the gear can not drive the worm are said to be self-locking. Whether a worm and gear will be self-locking depends on the lead angle, the pressure angle, and the coefficient of friction; however, it is approximately correct to say that a worm and gear will be self-locking if the tangent of the lead angle is less than the coefficient of friction."

This is pretty much a super high gear ratio, but typically you will see self locking effects from a worm drive device meaning you DON'T have to provide a constant torque to hold your device in place.

[theprgramerdude]

Originally Posted by Ether

The context of the sentence to which you were responding was about motor-generated current, so it wasn't clear you were referring to friction instead.

Ok, so let's talk about friction.

Take a toughbox with CIM attached. With the motor leads unshorted, measure the breakaway torque with an appropriately-sized torque wrench. Now short the leads and repeat the same test.

If you see any difference, it's probably due to cogging torque in the motor, not motor friction. The lion's share of the friction is in the gearbox, not the motor.

Your conclusion is correct though. A motor with high cogging torque, connected to a high-ratio gearbox, may be capable of supporting a static load.

Thats actually what I was primarily thinking of, but I had no idea about the proper term, and so I grouped that effect in with the "motor" friction due to bearings, etc, as both end up achieving the same effect in such a test.

[cgmv123]

Originally Posted by BHOP

I would recommend taking the load off the motor using a mechanical device like others have said.

I'm not sure which locking pins they are talking about in the window motor, but I do know they utilize a worm "drive".

You can read more about them on wikipedia (search worm drive), but if the window motor won't give you enough power to move the device using to hold your shooter(not hold it in place) then buy your own worm and worm gear(mcmaster,etc) and make your own worm drive with a more powerful motor.

"Worm gear configurations in which the gear can not drive the worm are said to be self-locking. Whether a worm and gear will be self-locking depends on the lead angle, the pressure angle, and the coefficient of friction; however, it is approximately correct to say that a worm and gear will be self-locking if the tangent of the lead angle is less than the coefficient of friction."

This is pretty much a super high gear ratio, but typically you will see self locking effects from a worm drive device meaning you DON'T have to provide a constant torque to hold your device in place.

The locking pins are separate from the worm drive. The locking pins prevent the motor from turning unless electricity is applied to the motor, which makes the clear winner in my view for the original poster's design.

More on locking pins: http://wiki.team1640.com/index.php?t...r_Locking_Pins Removing the pins is legal, but for a non-backdriving design, it is essential to leave them in.