What Transducers Do

      On CNC equipment, transducers are used to record actual machine motion for feedback to the controller, improving accuracy and preventing error. July 21, 2006

Question
Can anyone explain the operation and use of rotary and linear transducers in CNC control systems?

Forum Responses
(CNC Forum)
From contributor M:
In CNC applications, a transducer measures physical motion, then converts that measurement to an electrical input/output. There are different applications of this technology, but the prevalent one is using the transducer in a closed loop environment. This insures that the actual movement of the CNC machine as commanded by the control, then encoded to the motor for that particular axis is, in effect, actually occurring. If the system was not closed loop and had no means to verify movement, potential problems that could occur would be:

1.) CNC control sends signal to encoder to count the revolutions of the axial motor and sends signal to the axial motor telling it to revolve, resulting in linear axial movement across the machine.

2.) The machine momentarily strikes a rigid portion of the machine or fixturing on the machine, momentarily causing the machine to physically pause its linear movement.

3.) Because the motor is designed to "clutch slip" in this case to prevent damage to the motor itself, the motor shaft at the encoder end keeps revolving and the encoder keeps counting.

4.) At the end of its movement the control thinks the correct revolutions of the motor have been executed, therefore assuming the axis has moved linearly correctly also. However, the actual linear physical movement is inaccurate because of the physical pause which occurred.

If a transducer of some type were added to this scenario, now all of a sudden a signal is sent back to the control verifying that the actual physical movement the control thought had occurred, in fact has not. An error would be generated within the control to prevent further movement until the condition is corrected.



From contributor C:
Talk to me about following errors. What are they and can I adjust my AC servos to get a better result?


From contributor M:
Following error is basically the result of the condition we discussed above. The main method of correcting or minimizing it, within servo driven closed loop systems, is adjustment of the servo/servo amplifiers. This process is commonly called "tuning." The process and technique can vary dramatically depending on machine manufacturer and servo/servo amplifier manufacturer. You will have to consult both of the above to get the specific process for your configuration.

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