How to know if the feedback device is working properly

How to know if the feedback device is working properly

Problem

I have a feedback device connected to the drive and I want to check that it is working properly. I want to check no external noise is being coupled in the feedback lines. What can I do to check this?

Steps

  • The first step that needs to be done is to check that the wiring of the whole system is correct in order to minimize EMI susceptibility of the feedback signals. This could include actions like:
    • Ensure proper Protective Earth connection between the drive PE terminals, the motor chassis, power supply PE and the earth plane/system chassis. These type of connections are better explained in the Protective earth section of the manuals of the drives.
    • Add chokes to the motor phases in order to minimize EMI that can affect sensitive signals such as the ones of the feedback. How to wire these chokes along with more recommendations can be found in the Motor and shunt braking resistor section of the manual.
    • Keep the power and signal cables as separate from each other as possible.
    • Ensure that the feedback cables are exactly wired like it is explained in the Feedback connections section of the manual.
    • Shield the signal cables and connect the shield to the same PE point of the system.
  • Once the wiring part is covered and properly implemented, almost (if not all) the feedback signals should be clean and its translation into position inside MotionLab should be properly read without noise when enabling the power stage of the drive. In order to check this, one could do the simple test of enabling the power stage of the motor while plotting the position actual value in the Scope window. Make sure that you have selected the right feedback as Position sensor in the Actuator window or otherwise you might be getting the wrong results. In addition to this, you can use Open loop mode with no target voltage/frequency in order to ensure that the motor will not move so any position variation has to be related to noise. Therefore, for the test to be successful, the position actual value should remain constant and not change like it is shown in the following picture:

  • If you get similar results like the one that we have just shown then you can conclude that your feedback device is working properly.
For high-resolution sensors such as SSI encoders with 16+ single-turn bits, you might see small oscillations in the position value in the range of 1-5 counts (it might depend on the case, could be higher) which should not be attributed to noise, these oscillations exist due to such high resolution of the feedback device and should not give problems later when configuring the system.

Possible problems encountered

When you do the test that we just explained and don't get the exact results, you might be experiencing one of the following cases:
  • After the motor is enabled, the position read is always 0 counts. This is a bad result, it indicates that either the feedback device is broken, there is a problem within the wiring and connections or the feedback interface of the drive is broken. In order to identify which case exactly is happening, a couple quick tests can be done:
    • Replace the drive with one of the same model and repeat the test. If you get the same results, then it is very likely that the problem comes either from the feedback device or the wiring. If it can be ensured (through some connectivity/conductivity tests) that the wiring is good (no loose connections or short circuits), then it is definitely the case that the feedback device is broken. On the other hand, if by changing the drive you get to solve the problem, then you can be certain that the feedback interface of the first drive is broken and therefore an RMA process needs to be started since the drive is not operative.
    • Replace the motor+feedback setup with another one of the same models and repeat the test. If you get the same results, then it is likely that the problems come from the feedback interface of the drive and that an RMA process needs to be started in order to repair the broken interface. On the other hand, if you solve the problem once changing the setup, then it can be confirmed that the problem comes from the feedback device or the wiring. Just as before, if the wiring is ensured to be correct, the cause of the problem is then narrowed to the feedback device only which is broken.
  • After the motor is enabled, there is still some residual noise in the position readings. Different actions can be taken in order to address this:
    • Improve the wiring even more with the same instructions that are explained in the manuals.
    • Certain feedback devices such as the incremental or sin-cos encoder, do have a noise filter implemented in the firmware that can help minimize this noise action in the position readings. By default, this Noise filter is set to "Disabled" but one could start to select a different maximum encoder frequency in order to increase the immunity in front of signal noise. Therefore, the lower the maximum encoder frequency, the higher the immunity to signal noise. However, be also aware that the lower the maximum encoder frequency, the lower the maximum achievable speed of the motor becomes (there is always a compromise between the 2). This noise filter setting can be seen in the following picture:


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