limit switches vs encoder

  • Thread starter Guillermo Herdez
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Thread Starter

Guillermo Herdez

What advantages or disadvantages do I get using one or another?

I´m having trouble cause the device isn´t stopping at the exact point. I think that the limit switches are the cause. Any of you believe that i can achieve accuracy with an encoder?


James Ingraham

What are you using to move this thing? A motor starter and a motor will never get you terribly precise positioning whether you use an encoder or not. A variable frequency drive would do a little better, but you'll still have this problem where you get close and then lower the speed and creep into position. Then, unless you've got a brake on the motor, you could still have some drift. A stepper motor with an encoder should be able to get you damn close to where you're going. Vector drives can often take an encoder input and do positioning just like a servo.

Closed-loop servos, of course, require an encoder (or resolver) feedback and can be accurate to thousandths of an inch.

Of course, if you're using air you'll never get anywhere close no matter what you do. Hydraulics may be a little better, but not a lot. (There are such things as hydraulic servos, but they are fairly rare and specialized beasts.)

In general, you can be more accurate with an encoder than you can with a limit switch. But if whatever you are using to drive your load can't stop on a dime or has a lot of drift, it's completely irrelevant what feedback you use.

As for failure, my experience is that a mechanical limit switch is vastly more likely to fail than an encoder. We tend to use proximity switches instead of mechanical limit switches, and these hold up pretty well. On a machine with a tremendous amount of vibration, the glass plate on an encoder may well be a more likely fail point than a prox.

-James Ingraham
Sage Automation, Inc.

Hakan Ozevin

I think you are having a problem because of the changes in the inertia of the machine. A machine will stop in different times depending on the inertia on it. If this is the case, consider the followings:

An accurate measurement does not mean too much, unless you use the benefits of that measurement in your control system. In other words, whatever accuracy you get from your feedback, it should be followed by the system in control.

I suggest the following:
Place another limit switch before you final limit switch to lead the machine to a preset minimum speed. You will need a VSD if your machine is directly led from a motor or a proportional valve, if it is hydraulic.
Another thing to consider is the program's scan time- if the motion is fast in relation to the scan time, you'll have problems with positioning. Immediate inputs and outputs, combined with a selectable scan interrupt, will allow you to control the amount of time between output table updates.
In my project, I reset the encoder when the limit switch is hit. It is very straightforward.