Quote:
Originally Posted by MikeE
When using quadrature encoders directly connected to the Jaguars, we noticed that the speed returned over CAN is always positive, whichever direction the wheel rotates. We see this both via our Java code and using the BDC-COMM utility.
This causes difficulty running closed loop since the signs of the PID values would depend on the wheel direction, and I'd rather not change PID values on the fly to avoid possible instability.
The most likely explanation is that we made a wiring error and are only getting a single A or B channel from all of our wheel encoders - I won't be able to check until tomorrow - but I'd like to confirm that the Jaguar firmware is working correctly.
So has anyone successfully seen a negative speed returned over CAN from a Jaguar?
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Currently you cannot get direction from the returned speed data. You can, however, configure your Jaguar to use the encoder for position measurement and monitor the position when running in speed control mode. You can determine the direction of rotation by whether or not the position is incrementing or decrementing.
In BDC-COMM you would go momentarily into position mode and click the encoder radio button. Then you would go back into speed mode and run it as normal. You should now see the position value updating.
In Labview/Java/C++ you don't have to ever leave speed mode, you will just configure the position measurement off of the encoder.
*Once you set your PID values for, lets say, forward rotation, they will act the same in reverse rotation. This is because of how speed control is implemented in the Jaguar.
The current implementation of speed control prevents cases where the output of the PID function differs in direction from the desired speed. Say your robot was driving down a steep hill. The PID is trying to maintain a certain speed, but gravity is causing the robot to accelerate. There might be a point where the output of the PID function tells your motors to start driving in reverse, aka bad for the drivetrain.
Setting the output to zero and braking, for example, would be the better option. So, the speed control firmware sets output to zero whenever this "opposite sign" situation occurs.
Hope this helps,
David
Edit: *My above explanation holds true only if you're using the Jaguar in speed control mode. It looks like you are doing your own control loop, so you have two options: hope that the motor is turning the same direction you're telling it to, or get actual direction from monitoring the position.