RoboteQ AX2850 user manual PID tuning in Position Mode

Closed Loop Position Mode

A higher Proportional Gain will cause the algorithm to apply a higher level of power for a given measured error, thus making the motor move quicker. Because of inertia, however, a faster moving motor will have more difficulty stopping when it reaches its desired position. It will therefore overshoot and possibly oscillate around that end position.

Proportional

Gain

x

Desired Position

-

Analog Position	A/D
Sensor	Measured Position

or

Optical Encoder

E= Error

dE

dt

Integral

Gain

dE

dt

xΣ Output

x

Differential

Gain

FIGURE 61. PID algorithm used in Position mode

The Differential component of the algorithm computes the changes to the error from one 16 ms time period to the next. This change will be a relatively large number every time an abrupt change occurs on the desired position value or the measured position value. The value of that change is then multiplied by a user-selectable Differential Gain and added to the output. The effect of this part of the algorithm is to give a boost of extra power when starting the motor due to changes to the desired position value. The differential component will also help dampen any overshoot and oscillation.

The Integral component of the algorithm performs a sum of the error over time. In the posi- tion mode, this component helps the controller reach and maintain the exact desired posi- tion when the error would otherwise be too small to energize the motor using the Proportional component alone. Only a very small amount of Integral Gain is typically required in this 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 experimenta- tion.

94 AX2550 Motor Controller User’s Manual Version 1.9b. June 1, 2007