We have an RS395 motor from Banebots this year. It’s tiny. It draws 0.5A no-load, and 15A stalled. At peak power it’s about 1/10 of a CIM.
So the question is, how do you power it?
If you use a Spike Relay, you can us a 5A fuse to keep the motor from burning out.
What if you use a Victor or a Jaguar? Then your protection is none whatsoever.
It seems that we don’t have a variable-speed controller SMALL enough for this motor. I suppose if we could put ATC fuses in the PD board (and I mean FUSES, not auto-resetting breakers), then we could adequately protect it.
All electrical loads (motors, actuators, compressors) must be supplied by an approved power regulating device (speed controller, relay module, or Digital Sidecar PWM port) that is controlled by the cRIO-FRC on the ROBOT.
A. Each CIM motor and Fisher-Price motor must be connected to one and only one approved speed controller. These motors must not be connected to relay modules.
D. Each other electrical load (motor or actuator) must be supplied by one and only one approved speed controller, or one and only one relay module.
Only CIMs and FPs have to have a speed controller. All other motors (i.e. RS-395) can be controlled by a relay.
* All active PD Board branch circuits shall be protected from overload with an appropriate value auto resetting Snap Action circuit breaker (from the KOP or identical equivalent).
B. Each relay module branch circuit must be protected with one and only one 20-amp circuit breaker on the PD Board. No other electrical load can be connected to the breaker supplying this circuit.
*
This does say that each relay must have a 20A breaker on the PD, but nothing about on the relay itself.
Keep in mind that the fuses used are sized to protect the wiring more than they are the motor. You can easily cook an FP or RS540 using allowable wiring setups.
The best way to protect any KOP motor from heat damage is to avoid stalling it.
If connected to a Jaguar on a canbus network I believe it is possible to use the Jag’s built in current sensing ability to limit output in real time. The software implementation might be a little hairy, as in currently non-existent, but probably doable. Take that with a grain of salt, because I’ve never seen it done and what you should be able to do with Jaguars and what you can do are sometimes different. Or it’s just not documented.
Alternatively, stand alone current sensors could be used to provide feedback to the control system. The control system could then limit output of a Victor (or Jaguar I suppose) or turn off a relay to protect a motor once a predefined current limit has been passed.
All in all, the breakers and fuses are there to protect the wiring, not the motors. It’s best practice to protect motors through mechanical, or controls methods.
Marshall,
The best answer comes with another question. Do you need to change speed or are you happy with just changing direction? The 395 is not required to have a speed controller but can be driven by one if you wish.
Unlike previous years, this year there is a mechanism to use other speed controllers. See <R48> and the blue box underneath it, as added by team update 6.
Yeah, I noticed that exception. I don’t suppose FIRST would approve a schematic.
Joe, by that logic FIRST would not permit replacing blown fuse. Auto-resetting breakers provide a different degree of protection than fuses, and so it makes sense to restrict that replacement, for a limited application where it is necessary. I see nowhere that it is their intent to prevent a smaller fuse being used. (They allow 12 gauge wire to be used with a 20A breaker, though you are only REQUIRED to use 18 gauge or larger.)
Marshall,
Either the Jaguar or Victor can be used with this motor if you need variable speed. If you choose to feed the speed controller with #18 wire, then you can use a 20 amp breaker in the PD. At the low currents of this motor, I don’t think the current sense on a Jaguar will give the accuracy you want through a CAN buss connection.
<R40> All active PD Board branch circuits shall be wired with appropriately sized wire:
Application Minimum wire size
40A circuit 12 AWG (2.052mm)
30A circuit 14 AWG (1.628mm)
20A circuit 18 AWG (1.024mm)
I’m curious Al- what are the limitations of the Jaguars current sensing ability? I’m vaguely acquainted with standalone Hall effect current sensor IC’s, and I know they can be fairly sensitive and have wicked fast response times. Do the Jaguars use something different or just operate on a different scale then the tiny applications I’m accustomed to?
The Jaguars use a resistor in the power leads to measure the current drawn from the power distribution. As the current increases the voltage drop across the resistor rises. The sense circuit has to be scaled so that it can read the current from a CIM motor at 133 amps and accurately predict over current trip points at ~90 amps for two seconds. Measuring differences in the sub 10 amp range may not give the data accuracy you might want. At less than 8 amps, the measurement spec is +/- 1 amp and above 8 amps it is +/- 2 amps for the Black Jag.