Hi Techies,
I have been away from FRC for a while a I need a kick start with using the Jaguar. Our team’s app will need highly accurate speed control for a ball shooter.
Thanks,
There are a number of encoders to choose from for your speed sensor, namely the USDigital optical encoder and the magnetic encoders in last year’s KoP.
Legal motors are listed in [R48], but I think it’s a safe bet to say that you won’t be using the Vex motor for your shooter.
Keep [R61] in mind though, because that specifically disallows the use of Jaguars’ closed feedback loops.
Interesting. I wonder if they will change this like last year.
“As long as the CAN bus is wired legally so that the heartbeat from the cRIO is maintained, the closed loop control features of the Jaguar motor controller may be used. (That is, commands originating from the cRIO to configure, enable, and specify an operating point for all Jaguar closed loop modes fit the intent of <R49>.)
January 21, 2010”
How does R61 Disallow it. The blue box below R52 says otherwise.
Agreed, seems explicitly allowed to me.
I am looking for some clarification regarding this part of [R61]:
A. The Jaguar must receive signals via either a PWM cable -OR- a CAN-bus connection. Both may not be used simultaneously.
Does this only apply to “command signals” that control the output of the jaguar? I might want to control the jag via PWM but monitor output voltage and current via CAN at the same time. Is this legal?
You can do whatever you want for testing. These rules only apply for a competing robot. That said, monitoring via CAN should be legal, but it’s lawyering in my opinion.
Would be? Should be? Could be? All three of those are grey area responses. You should post your question in the FRC Q&A Forum to get a legal explicit ruling from the GDC.
Exactly, I’m not the GDC, therefore I can’t give an official ruling. I can only offer up my interpretation accompanied by a qualifier.
Rule 61 seems pretty clear to me. You can use either Can-Bus or PWM, but not both on the same Jaguar.
Another thought I had was to wire in both the PWM and the CAN. Then only use the CAN bus to control the robot. The PWM would be there as a failover drive system. I have some concerns about the CAN network being reliable annd want to be able to switch to PWM driving if we detect trouble. Not trying to bend the rules, just want to know what’s legal. I will ask my question in the rule QA. Thanks
That would not be legal. There is no way of knowing for sure that only one is controlling the robot.
I don’t see any way on the Q&A to ask a question. I can only search…
Regarding speed sensors: An encoder is a good fit for this task. However, keep in mind how fast your wheel will be spinning.
Let’s say your shooter is powered by a CIM and a 1:1 ratio. You will see ~5000rpm speed on the shooter wheel.
5000rpm is 83.3 revolutions per second. If you use the kit of parts encoders (250 counts per rev), that is more than 20,000 counts per second. In theory, the FPGA can count upwards of 39,000 counts per second. So this arrangement would work if you are doing speed control on the cRIO, but if you wanted to go to, say 10,000rpm on the shooter wheel, you would want a lower resolution encoder. I am not sure about the capabilities of a CAN Jaguar when it comes to maximum encoder rate.
In 2006, we made our own encoder by using an IR reflective sensor (like the ones Banner or Rockwell makes) with strips of reflective tape on the shooter wheel to trigger the counting. This method would of course work in 2012, as well.
You will need the team account information from your team to post directly to ask a question. See the Q&A - How to Guide for more details.
R61 B says if you are using PWM then you cannot have anything in the other ports which includes canbus
R61 A also would prohibit the PWM connection if you are using canbus since there is no way for you to turnoff the PWM signal.
I expect if you tried to do it the Jag would default to one ignoring the other completely.
The Jag firmware 101 (another thread) seems to have fixed an issue with the canbus. A trashy signal use to take the Jag offline until a power reset. It now will try to reset itself.
I have downloaded the specs for the Black Jaguar to research exactly that question. The spec sheet claims 1 million transitions per second. So the 250 counts per revolution is 4 transitions per line, so 1000 transitions per second. That means that it should handle 1,000 revolutions per second, or 60,000 RPM.
My problem is getting a feedback signal to the Jaguar. Currently I have a left over magnetic encoder, but so far the Jaguar has refused to display a speed or position signal from the magnetic encoder output, which means the PID just ramps up until the Jaguar limits out.
Mike
What is the interface of the magnetic encoder? Most I’m familiar with are an analog interface that isn’t compatible with the jaguar.
That would explain the problem. The magnetic encoder I am using generates 2 analog signals that are in quadrature. I thought that the Jaguar could handle that from some of the documentation, but perhaps not. I also tried using the digital Index pulse only, but so fat have not gotten that to work.
The 5/16 inch bore digital encoder should arrive today, and I will try again with that.
Mike
I completely agree.
Now I wonder if you can run some Jags on CAN and other jags on PWM, on the same robot. The rules appear to not prevent this, so long as one OR the other is used, never both.
You definitely can’t use both PWM and CAN on a single Jaguar. I’m not sure it would even work.
Encoders: This Threadhas more info on the same topic.