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Author Topic: Accuracy of Homing Devices? Pro's and Con's of Different Sensors  (Read 30021 times)

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After a lot of searching, and much reading, I have found many different opinions and suggestions on the types of sensors/methods used for homing.  I need to outfit a mill and lathe and am looking for the best accuracy and repeatability while maintaining robustness.  Figuring that there will be metal swarf, coolant and oil potentially reaching these sensors.  From what I've read electrical/magnetic switches such as inductive proximity, and hall effect sensors, can shift register due to temperature and electrical interference in the region of a few .001's over the course of a few hours.  On the plus side there are no mechanical part to wear out and it easy to get IP65/67 version's.  On the other hand you have a micro-switch which does not suffer from the previously mentioned climate shifts and electrical interference dilemma, but have mechanical mechanisms that can wear out and fail, that and it's hard to come by waterproof versions cheaply.  

So if you guy's wouldn't mind listing what systems you are using, and/or have tried and scrapped for something better, and what level of accuracy/repeatability you experience.  Is the position shift from some types of sensors only critical if you have to perform manual tool changes over the course of a long cycle, or are using setup plates for repeat parts w/o re-zeroing on the workpiece?  Also, in regards to proximity switches if you use them can you list the model# and the sensing distance.

Which is most accurate and repeatable, and has the least worry of false triggers of the following types of systems:

1) Proximity (inductive)
2) Micro Switch
3) Laser
4) Hall Effect
5) Other...

Thanks.
« Last Edit: April 30, 2011, 08:29:37 PM by Biermann »

Offline Jeff_Birt

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Re: Accuracy of Homing Devices? Pro's and Con's of Different Sensors
« Reply #1 on: April 30, 2011, 11:25:00 PM »
No matter what type of sensor or switch you use it will not be super accurate all by itself. For really accurate homing it is best to use a combination of a home sensor/switch and an index pulse on the axis motor itself. The home sensor/switch and the index pulse are seen at the same time the controller considers this 'home'. Since the index pulse is based on the angle of the axis motor itself it is very accurate.

Having said all of that you have to stop and ask yourself if having a very accurate homing sensor is even needed for your method of operation. A home sensor only sets the zero for the machine coordinates, it lets the machine get a bearing on where each axis is physically when you start up. For most operations you bolt a part/fixture to the mill table and then you zero the machine to some feature of the part/fixture (local coordinates), you can even have multiple fixtures and multiple local coordinate systems set up. There will be a relationship between the mechanical and these local coordinate systems (an offset), but since you will likely always zero to the part or fixture each time the accuracy of the home sensor is not important.
Happy machining , Jeff Birt
 
Re: Accuracy of Homing Devices? Pro's and Con's of Different Sensors
« Reply #2 on: July 16, 2012, 06:28:04 AM »
I was looking at this homing issues and wanted a simple working solution, I did not care if it was not 100% automatic. The only accurate method seems to need the use of encoders or some sort of sensor on the motor shafts and axis switches. This looks expensive for a small china cnc and I don't have enough inputs anyway.

My solution was to use the MachBlue screen and probe. There is a centering function that allows you to center your tool to a circle. So I just fixed a piece of machined pipe to a corner of my cnc table outside of the usable area, and fixed an arm to the spindle plate to reach this pipe with a 1/8" shank from a broken endmill soldered to it. The pipe has an plug soldered inside midway through so all I have to do is center the shank to this pipe, probe the z axis at this position and set my machine coordinates to zero. You have to jog manually to the pipe so it might be slow, but it is very accurate.

Offline Hood

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Re: Accuracy of Homing Devices? Pro's and Con's of Different Sensors
« Reply #3 on: July 16, 2012, 07:22:32 AM »
I used to use optical sensons that were within a housing and a flag was held on a sprung rod that stuck out either side of the housing. I originally bought them from a place in the USA but they sold up and as far as I know the people that took over dont do them. They are easy enough to make up and I had made a few for other machines. They were very accurate and I once did tests by repeatedly homing and watching a glass scale DRO and only once or twice out of 100 or so homings (from different distances) did the DRO read other than 0 and even then it was only 0.005mm

I now however use the switch/ encoder index homing as my servo drives are capable of it.

Hood
Re: Accuracy of Homing Devices? Pro's and Con's of Different Sensors
« Reply #4 on: October 15, 2012, 06:54:33 PM »
No matter what type of sensor or switch you use it will not be super accurate all by itself. For really accurate homing it is best to use a combination of a home sensor/switch and an index pulse on the axis motor itself. The home sensor/switch and the index pulse are seen at the same time the controller considers this 'home'. Since the index pulse is based on the angle of the axis motor itself it is very accurate.
I recently stumbled across this powerful statement and was wondering how one accomplishes "an index pulse on the axis motor itself" and if it's possible on a regular stepper motor without a shaft encoder.  I'm intrigued by this concept and would like to learn more.

Anyone have further info on the matter or links?

Thanks!
Milton from Tennessee ya'll.

Offline Chaoticone

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Re: Accuracy of Homing Devices? Pro's and Con's of Different Sensors
« Reply #5 on: October 15, 2012, 07:12:58 PM »
No matter what type of sensor or switch you use it will not be super accurate all by itself. For really accurate homing it is best to use a combination of a home sensor/switch and an index pulse on the axis motor itself. The home sensor/switch and the index pulse are seen at the same time the controller considers this 'home'. Since the index pulse is based on the angle of the axis motor itself it is very accurate.
I recently stumbled across this powerful statement and was wondering how one accomplishes "an index pulse on the axis motor itself" and if it's possible on a regular stepper motor without a shaft encoder.  I'm intrigued by this concept and would like to learn more.

Anyone have further info on the matter or links?

Thanks!

This thread may help explain that and give an alternate idea on "finer homing" by mechanical means.

http://www.machsupport.com/forum/index.php/topic,22610.msg157871.html#msg157871

Brett
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Offline RICH

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Re: Accuracy of Homing Devices? Pro's and Con's of Different Sensors
« Reply #6 on: October 15, 2012, 08:08:19 PM »
Quote
you have to stop and ask yourself if having a very accurate homing sensor is even needed


I don't have any on my machines. If I couldn't get accurately back to a location without switches then switches are not going to help me. Hope that makes some sense .....

RICH

Offline BR549

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Re: Accuracy of Homing Devices? Pro's and Con's of Different Sensors
« Reply #7 on: October 15, 2012, 09:22:46 PM »
There are times when accurate homing is usefull and MOST of the time it doesn't mean a thing to the average user. Most of the time only when you are using dedicated fixturing does it come into play.

Just a thought, (;-) TP

Offline Jeff_Birt

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Re: Accuracy of Homing Devices? Pro's and Con's of Different Sensors
« Reply #8 on: October 15, 2012, 09:49:55 PM »
No matter what type of sensor or switch you use it will not be super accurate all by itself. For really accurate homing it is best to use a combination of a home sensor/switch and an index pulse on the axis motor itself. The home sensor/switch and the index pulse are seen at the same time the controller considers this 'home'. Since the index pulse is based on the angle of the axis motor itself it is very accurate.
I recently stumbled across this powerful statement and was wondering how one accomplishes "an index pulse on the axis motor itself" and if it's possible on a regular stepper motor without a shaft encoder.  I'm intrigued by this concept and would like to learn more.

Anyone have further info on the matter or links?

Thanks!

You must have an encoder on the motor for the described homing process to work. As Rich and Terry have said though on a typical machine tool really accurate homing does not mean a whole lot. Think of how you use a mill, you clamp a piece of stock on the table, indicate to the edges of the stock to set your part zero and then start machining. Having a super accurate home has no bearing on where your stock is. If you have a machine set up to run only one part, and if you never crash into the fixturing or otherwise disturb it than you might be able to home and then offset to your fixture. I bet most guys would still indicate to the fixture though to make sure nothing had happened to throw things off.
Happy machining , Jeff Birt
 
Re: Accuracy of Homing Devices? Pro's and Con's of Different Sensors
« Reply #9 on: October 15, 2012, 11:01:47 PM »
Thanks very much for the input gents.:)

I'm actually working with a lathe instead of a mill and was hoping to find a way to consistently & very accurately set at least the X-axis zero point so I wouldn't have to use standard methods to indicate to the work.  A known refererence to the spindle centerline would be useful methinks.

What I was thinking was to have a regular mechanical switch to identify the initial rough home point and then have a slotted disc/optical sensor on the ballscrew.  Is it possible to configure Mach to recognize the rough point, slow way down then count a couple turns of the disc and stop/zero at the edge of the slot?  Seems to me that would produce a consistent zero point, no?
Milton from Tennessee ya'll.