speed limiting

I have set up the AX3500 with optical encoder input and all is well. I am having trouble understanding parts of the manual in the sense of limiting the speed. Using the closed loop control, how do I set the max RPMs of the motors in the controller such that I can limit the top speed?

The speed cannot be limited in position mode. The controller will try to reach the desired destination as fast as possible, limited only by the PID gains.

Is there any way to limit the speed then using the controller?

The only way would be to use a different main battery voltage.

The controller will always try to go the the desired position as fast as possible.

Is the only way to drop the main voltage? As I am inexperienced with controllers, my first guess would be to calibrate the joystick to not go to its limits (less than 127 or 5v), or adding a small resistor between the joystick and the controller to drop the joystick voltage feed to the controller. Would dropping the analog joystick voltage tell the controller to go slower? Would you say this will work, or fry a component?

In position mode, adding a resistor voltage to the joystick would only limit the min and max position (i.e. you would not be able to reach +127 to -127).

This will not change how fast the controller would try to reach the desired value.

What would work, however, is to limit the rate of change of the joystick command. This can be done using the acceleration parameter. You will need v1.7b of the firmware.
You can make the position move from center to min or max as slow as 2seconds. Since this is probably slower than what your motor can do, in effect the motor will never see a voltage higher than a few volts.

I think I may have not been clear with my question, sorry about that. Thanks for your help thus far though.

I wasn't concerned with acceleration per se, but the maximum speed of the cart. In case anyone is reading these forums, my current solution is adding a limiter plate on the joystick such that the controller will never "see" the maximum joystick limits. Adding a resistor won't work because although it will create an effective voltage drop across the board, it will also drop the neutral position voltage and the cart will go in reverse if the joystick is centered. We tested the joystick today and found the vehicle's speed is proportional to the distance from the neutral position of the joystick. I would prefer not to do this if there is a way to program the rpm limits on the controller

I am concerned with the manual of the AX3500 though. On page 123 under "Mode Description" it states:"In this mode,an analog or digital speed sensor measures the actual motor speed and compares it to the desired speed. If the speed changes because of load..."

This is exactly what we want the controller to do, but we can't figure out how to tell the controller what our max desired speed is. This implies you can program a desired speed. This seems like a fundamental function a speed controller should be able to do.

Also page 127 has a block diagram with a desired speed function to compare the actual speed.

We looked at this manual before we bought the controller and expected to easily govern the max speed of the design. Did I phrase my question incorrectly earlier or is the manual incorrect? Thanks again Cosma.

I missunderstood your operating mode to be in "Position Closed Loop". I now understand that you are operating in "Speed Closed Loop", using optical encoders for feedback. Sorry for the confusion, please disregards my earliest answers.

Limiting the top speed is actually quite easy to do. All you have to do is to play with the "Time Base" parameter in the Encoder Tab in Roborun.

Simply enter your encoder's PPR (Pulses Per Revolution). Do not confuse with the CPR (Counts Per Revolution), which is 4 x the PPR (one count for each of the transitions of each channel).

Then select a time base value. You will see the Max RPM value to change accordingly. The higher the Time Base value, the lower the Max RPM. Note that the Time Base value cannot be larger than 64.

Max RPM is the speed that the motor will reach when the Max command value of 127 is applied.

Thanks for your help. That was extremely helpful