Episode #18: End-Of-Travel/Home Sensors vs Absolute Encoders

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The Motion Control Show

A common topic when putting together a motion control system and a gantry is whether to use end-of-travel limit switches, home sensors or absolute encoders.  This really depends upon the application.  There's no simple answer to that.  So, let's talk about the pros and cons.  I'm Corey Foster of Valin corporation.  Let's see what we can learn. 

End-of-travel sensors go on each end of the actuator.  They're really there to protect the mechanics so you don’t hit the hard stop over and over again by accident.  That wears the mechanics down.  It could also be used to protect your application; if you have something hanging off it that could hit something or even you. That could be a safety issue. 

Home sensors typically go in the middle between the limit’s sensors.  This gives you a point of reference, a place to start from every time you power up.  It provides that known location, but you must hunt for it every time you cycle of power.  That gets to be kind of exhausting at the time, another option is to use absolute encoders. 

Absolute encoders, there could be rotary ones on the back of the motors, maybe be on the end of the actuator, or linear ones.  In both cases, there's no need to hunt for the home sensor anymore because you start up in a known position based upon that absolute encoder.  This helps you to possibly not need limit sensors depending upon the criticality of your application and your confidence if your programming capabilities.  Rotary absolute encoders are pretty cost effective these days. But, linear absolute encoders are still expensive; a thousand, two thousand dollars per axis maybe. 

The problem with a rotary absolute encoder is what happens when you decouple the motor from the actuator?  That means you need to re-index it, re-find that location where zero is, maybe change a variable in your program.  If you have a maintenance person who's doing that, you may not want them doing that.  It is kind of a hassle and maybe all you are doing is taking it off to check a coupler or troubleshoot something.  Now you have to go in and re-find that starting position because now you've rotated the shaft and now it is not going to be the same when you put it back on.  That's just something to think about when you’re working on your application.

Now, I'm going to have Michael show you something about the limit sensors on our HMR actuators

These actuators can come with internally mounted sensors or externally mounted sensors.  We chose the externally mounted sensors with the quick connectors so that our customer can adjust the limit switches on their system.  The actuators come with a T-slot so that you can adjust and move the limit switches wherever you may need them in your particular application. 

There you have it.  There's no simple answer whether to use limit and home sensors versus absolute encoders.  In fact, I've seen customers use one limit sensor and no home sensor, or two limit sensors and no home sensor.  Or two limit sensors and no home sensor, or one and one, or none, and homing to a hard stop.  I've seen a lot of different combinations depending upon the actual applications. 

If you have any questions or are just looking for some help, we're happy to discuss your application with you.  Reach out to us at (855) 737-4716 or fill out our online form. I'm Corey Foster of Valin Corporation.