RoboteQ AX500 manual Control Loop Description

Closed Loop Speed Mode

To set the potentiometer, use the Roborun utility to run the motors at the desired maxi- mum speed while in Open Loop mode (no speed feedback). While the tachometer is spin- ning, adjust the potentiometer until the analog speed value read is reaching 126.

Note: The maximum desired speed should be lower than the maximum speed that the motors can spin at maximum power and no load. This will ensure that the controller will be able to eventually reach the desired speed under most load conditions.

Important Warning:

It is critically important that the tachometer and its wiring be extremely robust. If the tachometer reports an erroneous voltage or no voltage at all, the controller will con- sider that the motor has not reached the desired speed value and will gradually increase the applied power to the motor to 100% with no way of stopping it until power is cut off or the Emergency Stop is activated.

Control Loop Description

The AX500 performs the Closed Loop Speed mode using a full featured Proportional, Inte- gral and Differential (PID) algorithm. This technique has a long history of usage in control systems and works on performing adjustments to the Power Output based on the differ- ence measured between the desired speed (set by the user) and the actual position (cap- tured by the tachometer).

Figure 45 shows a representation of the PID algorithm. Every 16 milliseconds, the control- ler measures the actual motor speed and subtracts it from the desired position to compute the speed error.

The resulting error value is then multiplied by a user selectable Proportional Gain. The resulting value becomes one of the components used to command the motor. The effect of this part of the algorithm is to apply power to the motor that is proportional with the dif- ference between the current and desired speed: when far apart, high power is applied, with the power being gradually reduced as the motor moves to the desired speed.

A higher Proportional Gain will cause the algorithm to apply a higher level of power for a given measured error thus making the motor react more quickly to changes in commands and/or motor load.

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 speed value or the measured speed value. The value of that change is then multiplied by a user selectable Differential Gain and added to the out- put. 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 speed value. The differential component will also greatly help dampen any overshoot and oscillation.

The Integral component of the algorithm perform a sum of the error over time. This compo- nent helps the controller reach and maintain the exact desired speed when the error is reaching zero (i.e. measured speed is near to, or at the desired value).

76 AX500 Motor Controller User’s Manual Version 1.9b. June 1, 2007