Motor Control Rules

Technical Electrical
1

I have been looking for a while, but I can’t find answers to the couple questions we have.

Firstly, can we control multiple (2) motors from one PWM output on the roboRIO? This wouold either be done with a PWM splitter or by connecting 2 motors to one motor controller/spike.

Also, R57 states that “Every relay module, servo, and PWM motor controller shall be connected to a corresponding port.” We have, in the past, used PWM outputs to control Spikes, but I am getting the impression that this is illegal per R57. Is my logic correct, or can we still use PWM outputs to control Spikes?

2

You can do it with a PWM splitter cable. Only certain motors may be “doubled up” on a single speed controller; see Table 4-4 under <R42>.

Also, R57 states that “Every relay module, servo, and PWM motor controller shall be connected to a corresponding port.” We have, in the past, used PWM outputs to control Spikes, but I am getting the impression that this is illegal per R57. Is my logic correct, or can we still use PWM outputs to control Spikes?

I do not believe you have ever controlled a Spike using a PWM output. The signals are not compatible. Spike relays are controlled using relay outputs.

3

In my interpretation of R59, “both may not be wired simultaneously on the same device,” is that a PWM splitter would be legal.

I do not believe you have ever controlled a Spike using a PWM output. The signals are not compatible. Spike relays are controlled using relay outputs.

Really? I could have sworn we have used something other than relay ports in the past.

4

Once we used a DIO channel for a spike for our compressor, which iirc resulted in the normal compressor code not working and the spike turning off when we told it to go on and vice-versa. 0/5 stars, do not recommend.

5

Spike outputs happen to work on three wires, just like standard PWM cables. However, they are not the same sort of signal inside the wire. PWM is pulse-width modulation; it’s a way of actually carrying a digital signal. The relays use one of the wires as ground and the other two as ground-power to control the two outputs on the relay module.
TL;DR:
Cables are the same, carried signals are not.

6

That must be what I was remembering. Is there a way to get this to work reliably?

7

There’s no way to get it to work legally…

8

Chris,
R42 is the defining rule here which includes table 4-4. You can split PWM control to feed multiple controllers if you wish. I recommend no more than two personally. The table also lists which motors can be paralleled to work with a single controller. There is a down side to that in that the current limit must be split between the two motors. The PWM control used for motor controllers is the same control for servos provided you make the jumper to supply the required voltage.

9

Why?

BTW, does anyone have the specs for the PWM input current draw for the new 2015 motor controllers?

10

Ether, I know there is a limit to how many loads can be placed on the PWM ( I don’t remember it off the top of my head) output but more than two makes it hard to diagnose an issue. Should a wayward metal part crimp one of the PWM wires and short the leads, you end up loosing a lot.