The following error of the servo motor means the difference between the received command and the actual position, namely :
Following Error = Command position – Actual position
Generally, in the case where the servo gain is well adjusted, the following error characteristics are :
- When the motor stops : the error is almost 0
- When doing acceleration and deceleration : the error is larger
- When running at constant speed : medium error
The figure obtained by positioning with trapezoidal speed profile is as follows: When the motor speed is 0 (the both side), the tracking error is also close to 0. When the speed is not 0, the error becomes larger !
[Figure 1] Following error during servo operation
If it is a point-to-point application, the motion program will wait for the positioning completion signal after the servo is stopped to perform subsequent actions, so the following error does not cause a problem because there is almost no error after the servo stopped. However, in the case of electronic cams, it is necessary to minimize the following error, otherwise the phase between master and slave will be deviated, thereby reducing the overall accuracy ! For example, the film feeding shaft and the cutter shaft of the packaging machine must be operated without a phase lag from the master(feeding) shaft.
To reduce the following error, you can usually increase the gain of the servo but may have an adverse effect on stability ! In applications such as electronic cams, the machine is often driven by belts and chains so that the rigidity is weak that it is not easy to increase the servo gain! Therefore, it is necessary to make command compensation and gain adjustment simultaneously to have an opportunity to achieve.