[Ether]

The inverse kinematic calculations for mecanum require three input rate commands (forward/reverse, right/left, clockwise/counterclockwise) in order to compute the required 4 wheel speeds.


When the driver is controlling robot orientation open-loop, these three rate commands come from the driver.


If you want to use the gyro to go to and hold the robot orientation at some desired angle, then the rotation rate command for the inverse kinematic calculations must come from the output of the PID, rather than from the driver.


There are a few different ways to do this.


One way would be for the driver to open-loop rotate the vehicle to the desired orientation, and when it is oriented the way he wants it, he presses a joystick button to "lock in" that angle. By "lock in", I mean that the software reads and saves that angle from the gyro and uses that as the target value for the PID. The software then changes modes and uses the PID output, instead of the driver, to provide the rotation command (to be feed into the inverse kinematic calculation of wheel speeds). This holds the robot at that angular orientation. When the driver wants to change the robot's orientation, he presses another button to release it and give control back to him.


Another way is to program the buttons so that the driver has a small set of discrete robot orientations he can access with one button press. For example, one button for forward, one for 90 degrees to the right, one for 90 degrees to the left (and one to release control back to the driver). The target values for these buttons are hard-coded to the fixed discrete positions to which they correspond.


I suppose another way to do it would be to provide some sort of separate non-spring-loaded rotary device, like a potentiometer or encoder, for the driver to turn to the desired robot orientation angle. The angle from this device would become the target value for the PID. This could be the full-time mode of operation (replacing the joystick rotation z-axis) or a joystick button could be used to toggle between the two modes of control.


You could even just use the joystick z-axis as the target value for the PID (and use the PID output as the rotation rate input to the inverse kinematic calculations), but it seems to me this would be very awkward for the driver, because he would have to hold the twist at the desired angle.


Because of gyro drift, when using the gyro you might also want to provide a button that tells the software to re-zero the gyro at the present orientation. This allows the driver to manually orient the robot at the zero orientation and then press the button to re-zero the gyro.