RoboteQ AX2550/2850 user manual PID tuning in Position Mode

Closed Loop Position Mode

PID tuning in Position Mode

As discussed above, three parameters - Proportional Gain, Integral Gain and Differential Gain - can be adjusted to tune the position control algorithm. The ultimate goal in a well tuned PID is a motor that reaches the desired position quickly without overshoot or oscilla- tion.

Because many mechanical parameters, such as motor power, gear ratio, load and inertia are difficult to model, tuning the PID is essentially a manual process that takes experimen- tation.

The Roborun PC utility makes this experimentation easy by providing a screen for changing the Proportional, Integral and Differential gains and another screen for running and monitor- ing the motors.

When tuning the motor, first start with the Integral Gain at zero, increasing the Proportional Gain until the motor overshoots and oscillates. Then add Differential gain until there is no more overshoot. If the overshoot persists, reduce the Proportional Gain. Add a minimal amount of Integral Gain. You may further fine tune the PID by varying the gains from these positions.

To set the Proportional Gain, which is the most important parameter, use the Roborun util- ity to observe the three following values;

•Command Value

•Actual Position

•Applied Power

With the Integral Gain set to 0, the Applied Power should be:

Applied Power = (Command Value - Actual Position) * Proportional Gain

Experiment first with the motor electrically or mechanically disconnected an verify that the controller is measuring the correct position and is applying the expected amount of power to the motor depending on the command you give it.

Verify that when the Command Value equals the Actual Position, the Applied Power equals to zero. Note that the Applied Power value is shown without the sign in the PC utility.

In case where the load moved by the motor is not fixed, you will need to tune the PD with the minimum expected load and tune it again with the maximum expected load. Then try to find values that will work in both conditions. If the disparity between minimal and maxi- mal possible loads is large, it may not be possible to find satisfactory tuning values.

Note that the AX2500/2850 uses one set of Proportional, Integral and Differential Gains for both motors and therefore assumes that similar motors, mechanical assemblies and loads are present at each channel.

122 AX2500/2850 Motor Controller User’s Manual Version 1.7. February 1, 2005