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Old November 25th, 2021, 02:30 PM   #1
kckku
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Servo axes: Setup limit sensors + soft limits

Do people typically use both end limit sensors and configure soft limits for linear servo axes? I thought this was standard practice until one of our machine builders told us they don't install limit sensors if soft limits are already programmed. Similarly they don't configure soft limits if the customer requests limit sensors. I am asking for both so they must think I am out of my mind.
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Old November 25th, 2021, 02:57 PM   #2
PLCDontUQuitOnMe
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Quote:
Originally Posted by kckku View Post
Do people typically use both end limit sensors and configure soft limits for linear servo axes? I thought this was standard practice until one of our machine builders told us they don't install limit sensors if soft limits are already programmed. Similarly they don't configure soft limits if the customer requests limit sensors. I am asking for both so they must think I am out of my mind.

Depends on the risk of over travelling I guess. I would prefer both, a soft limit that should in theory be enough, and a physical limit switch to protect you for the day when someone modifies the soft limit and doesn't tell anyone.


If there's enough risk to machine/personnel in the event of overtravel, then I would spec both, soft limit and hardware limit switch. The soft limits should stop the servo first, with the hard limits placed out further.

Last edited by PLCDontUQuitOnMe; November 25th, 2021 at 03:02 PM.
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Old November 25th, 2021, 03:10 PM   #3
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I've always done both and also ensure the program is set up in a way to easily recover from overtravel via manual jog. I hate to see operators and/or techs manually pushing an axis around to get off an overtravel.
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Old November 25th, 2021, 04:51 PM   #4
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+Both
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Old November 25th, 2021, 05:16 PM   #5
Peter Nachtwey
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Typically on hydraulic systems there are not end of travel sensors. This hasn't been a problem. On motor systems we provide the capability of using end of travel sensors but I don't know how many people actually use both. Our software required the software limits to be set as this could affect how to ramp down when ramping down close the limits.
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Old November 26th, 2021, 09:05 AM   #6
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Unless referencing is done to a hard stop, there should be at least one limit switch to establish position, then soft limits can be used. I typically view limit switches as machine limits, and software limits as application limits.
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Old November 26th, 2021, 10:02 AM   #7
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Unless referencing is done to a hard stop, there should be at least one limit switch to establish position, then soft limits can be used. I typically view limit switches as machine limits, and software limits as application limits.
Thank you all for your feedback.

I went around our plant and checked how our systems are currently configured. Most of our existing machines actually just have limit switches and no software limits. We didn't know enough to ask questions to our machine builder to say one way or another.

Recently we had to replace a servo motor on one of our axis and it was a pain. The homing method by default is wherever you set the home bit to the servo drive. Therefore you are just suppose to drive the axis to the spot that you want and then it becomes your home position.

This method is ok but it screws up all of your taught positions if you don't have a mechanical pin or reference point. Nobody knew this so we had to reteach all of the axis points.

Now we are going to start to ask our machine builders to use one of the end limit sensors to reference for homing. The motor will drive pass one of the end limit sensors, move in the other direction, and then take the Z-pulse of the motor as the home position. we haven't tried this yet but the manual says this is possible and I think we won't have to reteach the axis points if we have to replace the servo motor again.

This sounds about right? We will also ask our machine builders to use both the end limit sensors and software (application) limits.
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Old November 26th, 2021, 10:19 AM   #8
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Originally Posted by kckku View Post
Thank you all for your feedback.


Now we are going to start to ask our machine builders to use one of the end limit sensors to reference for homing. The motor will drive pass one of the end limit sensors, move in the other direction, and then take the Z-pulse of the motor as the home position. we haven't tried this yet but the manual says this is possible and I think we won't have to reteach the axis points if we have to replace the servo motor again.

This sounds about right? We will also ask our machine builders to use both the end limit sensors and software (application) limits.

Using the Z pulse is more accurate, but it will throw off your machine positions if the motor is changed, or in any way if it is decoupled, moved and recoupled.
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Old November 27th, 2021, 09:59 AM   #9
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Quote:
Originally Posted by kckku View Post
I went around our plant and checked how our systems are currently configured. Most of our existing machines actually just have limit switches and no software limits. We didn't know enough to ask questions to our machine builder to say one way or another.
So the motor can run full speed until hitting a limit switch and still run into something just behind the limit switch. Hopefully there is enough distance between the limit switch and any obstruction.





Quote:
Recently we had to replace a servo motor on one of our axis and it was a pain. The homing method by default is wherever you set the home bit to the servo drive. Therefore you are just suppose to drive the axis to the spot that you want and then it becomes your home position.
That is not a good way to 'home' a servo. Usually you want to move slowly in open loop until you hit the retract limit switch then issue a command that arms the z encoder pulse and loads a position automatically when the z pulse is detected. A problem occurs is the motor is already behind the limit switch. Then retracting will hit an obstruction. This is why the motion must be slow enough not to damage anything. Also, the controller must be able to detect it is obstructed and reverse direction.




Quote:

Now we are going to start to ask our machine builders to use one of the end limit sensors to reference for homing. The motor will drive pass one of the end limit sensors, move in the other direction, and then take the Z-pulse of the motor as the home position. we haven't tried this yet but the manual says this is possible and I think we won't have to reteach the axis points if we have to replace the servo motor again.
Yes, and you pointed something I didn't mention. Detecting the z pulse should always be done going the same direction after detecting the limit switch.





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This sounds about right?
Yes
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Old November 27th, 2021, 06:52 PM   #10
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Just make sure your machine builder knows how limit switches work. I can't tell you how many machines I have worked on where the axis can get on the back side of a limit switch. You should never be able to move through a limit switch. It should trip, and stay tripped, until a hard stop is reached.
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Old November 27th, 2021, 06:58 PM   #11
GaryS
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Always use both hard limits and soft limits
what are you going to do when the encoder fails the axis will crash without the hard limit
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Old December 1st, 2021, 08:34 AM   #12
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Another vote for both.

Software limits based off the application requirements. Hardware limit switches as the backup. Never want to have a drive going full speed into a hard stop. Been there, seen that unfortunately.
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Old December 1st, 2021, 09:53 AM   #13
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Quote:
Originally Posted by sparky66 View Post
Using the Z pulse is more accurate, but it will throw off your machine positions if the motor is changed, or in any way if it is decoupled, moved and recoupled.
If we change the motor then we should run an origin search again with the servo drive. I wrote a short instruction for our technicians to do this (and for me to remember how to do it). It's hard to remember everything if you don't do it every day.

For setting the home position after a motor change, they are supposed to move the axis to somewhere in the middle between the two limit switches. Preferably closer to the limit switch that is used for homing. Once they issue a command for homing then the axis should run slowly to the limit switch, go over it, and then run in the reverse direction until the motor sees the z-pulse. The servo drive will then take this position as the zero position.

We haven't tried this yet but from reading the manual for the drive, this is how it 'should' work.
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Old December 1st, 2021, 09:54 AM   #14
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Originally Posted by Smeltz View Post
Another vote for both.

Software limits based off the application requirements. Hardware limit switches as the backup. Never want to have a drive going full speed into a hard stop. Been there, seen that unfortunately.
That doesn't sound fun at all.
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Old December 1st, 2021, 09:59 AM   #15
kckku
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Originally Posted by sparky66 View Post
Just make sure your machine builder knows how limit switches work. I can't tell you how many machines I have worked on where the axis can get on the back side of a limit switch. You should never be able to move through a limit switch. It should trip, and stay tripped, until a hard stop is reached.
We also got confused a few months ago when one of our limit switches failed. We didn't realize the limit switch is normally on. The switch turns OFF when the carrier on the axis trips the sensor.

One of the sensors died so the sensor signal was always off and the drive was constantly complaining that the limit sensor was tripped. Physically we can see that the carrier was nowhere near the limit switch so that was a bit of confusion for our guys.
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