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Positional Feedback
Means the feedback to the control to maintain the position (In case of digital there could be just one unit acting as both velocity and positional- This device could be either going to the control first and the drive next, and vice versa). This device is used to achieve the repeatability. In case of axis it repeats the position linearly and in case of spindle it reaches the same angle in a circle thereby able to produce a thread.
Resolver:
This is the one of the earlier devices. This is an analog device. It can be checked with a scope. Still, it is hard to tell whether it is OK or not. Best way is to switch it, if it is suspected. One has to keep an eye for the bearing in the unit, which tends to go bad.
Encoder:
This is a digital device, which is used as a positional feedback device. It has a photocell and three receiving cells, one for home position and two for incremental position. This can measured with a scope. Look for two square waves. One has to keep an eye for the bearing in the unit, which tends to go bad.
Pulse Coder:
This is very similar in construction to the Encoder. This is used to provide both velocity and positional feedback. The output from this unit can be configured to go to drive first and control next or vice versa. When there is problem, any one of the three areas, drive, control or pulse coder could be causing it and it is difficult to say which one.
Scale:
Though there are a few kinds of scales available, Linear glass scale is the widely used one. The scale has to be kept clean. It has a reader moving on it linearly. This has a photocell and three receiving cells, exactly as in the case of encoder. There is home pulse every so often. Also there is a unit inside the scale, which can be moved to assign a home position pulse. The output of the head is connected to a Preamp.