jaguars - send voltage

[MorocoMan]

hello.
I know that if you replace the PWM by CAN then you can send a constant voltage from the jaguar instade of a percente of the voltage of the battary.
my question is there is another way to do it? and what are the up sides and the down sides of CAN againts PWM?

[Al Skierkiewicz]

Mo,
The Jag doesn't send a constant voltage, it can be commanded to operate in "Voltage Mode" through the CAN bus. In this mode the Jag produces an "average voltage" that simulates a constant voltage so that the motor behaves as if it always is receiving the commanded voltage. In this mode you can also program a "ramp" which will change the average voltage over a specified length of time. I know there are several teams that have used this mode that can comment on it's usefulness.

[FrankJ]

I know referring to the output of the jag as a DC voltage is not technically correct, but lets go with it in the interest of simplicity. The jag documentation refers to voltage output as well.

The PWM & default canbus mode (%VBus) converts the command from -100% to 100% output. The actual voltage out depends on the command signal & battery charge.

One of the CanBus modes is called Voltage. Its input is a voltage & it measures the incoming battery voltage & adjusts the actual command to try to give a constant output voltage independent of the battery charge. Sort of a poor mans close loop control.

We used it on our shooter this year & it seemed to help.

The only to access this mode is through can. Can gives you more controls & simplifies wiring. It is a single point failure that will bring down everything connected with. There are some programming gottyas that stop the whole robot. Done right it can be very reliable.

[EricVanWyk]

You can try to recreate your own Voltage Mode control over PWM. Set your PWM output to (Desired Voltage) / (Measured Battery Voltage) each cycle. The advantage of CAN in this setup is that it measures the voltage more often and more accurately (closer to where it is used).


As Al said, it doesn't actually create that voltage - it just does a good enough approximation of it. For FRC purposes it should be indistinguishable.

The advantage of voltage mode control in general is that it reduces the impact of your battery voltage and current draw on the behavior of the motors. In Percentage control, the voltage / oomph applied to the motor can easily change by +/-40% as conditions change.