Position Control Loop

Basic structure of position control loop

CONTROL_TIME

The default value is 100 which leads to a proportional gain of 10.

Note

In case the value of Control Time is too short the position control loop will run into instability.

Note

In case the position control loop is not used this parameter must not be set to 0.

Control Time and static following error in case the feed forward of velocity and the integrational part of the postion control loop is not used.

The static following error depends on the axis velocity and can be calculated easily: Control Time multiplied by the axis velocity ( p_error = v * CT ).

In general it should be aimed to reach a high position control loop gain with a short Control Time to achieve a small following error. As the reaction times take account in the possible Control Time of the complete system (parameters of the drive control loops, PLC cycle time as well as the communication fieldbus) should be considered.

As a basic rule the Control Time should be at least four times longer than the reaction time between the output of the Speed Reference and the input of actual position.

When the time Ts and Tt is measured, a control_time of 4*(Ts+Tt) will result in an aperiodic damping of the position control loop. It is important to measure the values from inside the PLC (e.g. Trace) to have the complete reaction times included. Practical values for CONTROL_TIME might be from 50..500ms. The PLC cycle time as well as bus cycle times and mechanical reaction will influence the value.

FF_PERCENTAGE

The default value is 0.

In case a velocity feedforward has to be configured a value of up to 80 is recommended. For larger values than 80 the parameter Horizon needs to be used as the resulted position will overshoot otherwise.

A value of 100 adds a velocity to the Speed Reference output which corresponds exactly to the ongoing Position Reference value.

INTEGRAL_PART

The integral part of the position control loop can be used to eliminate a permantent positioning error, e.g. in case of hanging loads.

The time value can be regarded as the time the integrator needs to sum up the input value to reach the same value for its output.

Note

In case the Integral Part Time is too short the position control loop will run into instability.

HORIZON

A communication delay of the Speed Reference value to the drive system can cause an overshoot during positioning caused by the velocity feedforward gain.

This function will compensate this commiunication delay to prevend an overshoot by time shifting the signals Velocity Feed Forward and Position Reference relatively to each other.

The value of HORIZON can be approximately assumed to be the time delay of the communication delay.

The delay time might be caused by the cycle time of the control loop and by any delay in sending the speed reference, delay in the drive to build up the torque and delay to receive the actual position. To overcome this delay, a HORIZON > 0 might be used. The feed forward reference will be created in advance, while the proportional gain is applied to the original motion profile. The delay is then compensated.

This function should not be used if the feed forward parameter FF_PERCENTAGE is 0.

A value of 0 will deactivate this function, which is the default value.

While this function is used, it will increase the needed PLC calculation time for this axis.

Result with HORIZON=0

Result with HORIZON>0