Automatic DAC Offset Adjustment

Any controller with no I component has automatic DAC offset adjustment as an option. This adjustment is only active when the velocity feedforward of the axis falls below a certain magnitude. This limited activity prevents the DAC offset adjustment from being affected by the dynamic behavior of the axis. If the axis is subject to position control or is moving at a suitably low velocity, an offset velocity is generated by integrating the control velocity. This offset velocity is added to the output. The negative feedback of the position control loop results in a PT1 behavior creating an exponential function.

Offset Adjustment Parameters

Offset filter time: Data type Double. Unit sec. Time constant for the offset adjustment.

Offset limit: Data type Double. A relative control. At a limit the offset is kept constant not going beyond it.

Quite possibly, the behavior of the offset adjustment could be affected at runtime. Suitably, a range of “switches” is available: For instance, at runtime the PLC or another ADS device can modify time constant parameters or feedforward limit parameters.

The offset adjustment could be switched off entirely. Doing so, it is not always possible to avoid a jump in the output voltage. Therefore, a soft deactivation can be achieved with the “fade out” mode. A fade out reduces the adjustment to zero over time while following its own curve. To keep the adjustment steady for a period of time the “hold” mode can be activated. E.g., the “hold” mode is suitable when stopping the power section of a drive temporarily. Performing such a stop it would be impossible for the offset not to run out of control if the offset adjustment was to remain active.