Closed loop drive system

  • #1
Ranger Mike
Science Advisor
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Main Question or Discussion Point

Hello, I have a few questions regarding machine tool drive and feed back system.
Say I have a 1 meter long guide way and a carriage that is driven by DC motor. This carriage glides on air bearings.
The drive mechanism has pinch rollers that pinch a drive rod and these rollers are driven by the dc motor.
I have linear positioning scales attached for precise linear positioning.
I have a mechanical switch that will trigger once it makes contact with a fixed mechanical stop at the end of the meter travel.
This signal goes to a dedicated PC where software provides feed back to the drive motor.
How is the processed linear position going to vary versus the true linear position?
Is there has to be a certain signal lag time due to linear position signal being processed ?
Will heat effect the true signal accuracy of the scales?
Will motor drive noise add to error of the system?
If a software program is written to slow the motor prior to contacting the mechanical stop, would this increase signal lag time?
How can I get the most accurate and precise 1000 mm distance movement with this system?

I appreciate any reply as I am not well versed in this discipline.

Thank you
 

Answers and Replies

  • #2
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A DC motor is not typical for precision positioning. A servo motor, or a stepper motor would be more suitable. A stepper motor connected to a worm gear would give the best results.

Many of your other questions relate to the parameters of your system. So we can't give specific answers without numerical data.
 
  • #3
Bandit127
Gold Member
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The error due to latency in the closed loop while the drive is in motion is called the "following error". (You may find those words help in your search for information).

In my experience, PID control of position is commonly implemented and should help the drive stop accurately at the programmed position. Good tuning should also prevent oscillation around the programmed position once the drive has stopped.

I am assuming that backlash is removed by the pinch roller arrangement. Care needs to be taken to ensure that is the case.
 
  • #4
Tom.G
Science Advisor
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An additional complication can occur when power is interrupted, even momentarily, with the carriage at an arbitrary position. For proper behavior, either the scale must show actual position or the carriage must return to a "Home" position.

This Homing activity may not be desirable, or safe, depending on the application.

The solution is that the Scale indicates absolute position, rather than just counting pulses from Home.

Try "linear positioning stage" in Google for more leads. (No link here because if you type it in, Google supplies some hints.)

Typically, for machine tools, a PLC (Programmable Logic Controller) is used. They are much more reliable than the usual PC's, and the higher end PLC's have built-in routines for PID control and plug-in I/O cards for small motor drive.

Cheers,
Tom
 
  • #5
Baluncore
Science Advisor
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I have linear positioning scales attached for precise linear positioning.
That scale will determine the accuracy of the system. What make and model?
You need to compare the thermal expansion coefficient of the scale material with the thermal expansion coefficient of the thing you are measuring. You may need to stabilise the temperature.
 
  • #6
jrmichler
Science Advisor
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How can I get the most accurate and precise 1000 mm distance movement with this system?
We need more information:

1) What is your accuracy requirement; is it millimeters, micrometers, or nanometers? The solutions are different.

2) What is your travel time requirement for a 1000 mm move? Moving 100 kg 1000 mm in 300 milliseconds is very possible, but requires careful engineering. Moving 1 kg 1000 mm in 10 seconds is a lot easier.

3) What is the total moving mass?
 

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