Two-encoder concept: Possible cause Error Speed comparison
Question: What effect does a dead time in the actual value of the load encoder have on the speed comparison in Extended Safety?
Answer:
In Extended Safety, a speed difference is formed from the motor encoder speed and the load encoder speed. If this difference exceeds the Tolerance speed comparison threshold, the SSM: Error speed comparison is signaled.
Before the comparison, a possible noise of the speed difference is minimized by PT1 filtering.
A dead time in the load encoder position results in a difference being formed between speeds that were sampled at different times. A current motor encoder speed is therefore compared with a load encoder speed,
which corresponded to the actual load velocity x ms ago.
The effect is a speed-dependent difference between the two speeds.
The figure below shows the speed deviation filtered from two SSI linear encoders of different types.
The oscilloscope recording is stitched together from two individual recordings with the same motion and kinematics.
With encoder type A (channel 1; red) a dead time of 12-14 ms is effective in the position detection. This results in a maximum speed deviation filtered = 20 rpm (at approx. n_motor = 4000 rpm).
For encoder type B (channel 2; blue ), approx. half the dead time of encoder type A is effective.
Note: The signal Speed deviation filtered cannot be oscillated with the current firmware version of the Extended Safety. Here, an internal interface was only used for the test.
