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limit switch accuracy
« on: September 22, 2011, 09:09:32 AM »
What type of limit switch is the most accurate?

Thanks

Offline Tweakie.CNC

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Re: limit switch accuracy
« Reply #1 on: September 22, 2011, 11:59:47 AM »
Hi Doc,

Good question - bit like asking "how long is a piece of string ?"   ;D

I suppose it all depends on how much you want to pay and the level of accuracy that you need.

I use ordinary Honeywell Microswitches but Baumer Electric produce a superior range and their mechanical push switches, in the My-Com range, have a repeat accuracy of better than 0.001 mm (constructed a bit like a touch probe).

Tweakie.
Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.
Re: limit switch accuracy
« Reply #2 on: September 22, 2011, 12:14:44 PM »
Thanks,  I think I need to modify the way I am doing the probing to this switch to
get repeatability so I will try that first then go to better switches and see if I see
a difference.  I have been using the switch to check the size of my grinding wheel but
only moving slowly in Y to the switch to trip it and noting the position but I need
to trip the swith and move past then reverse and note the closure point.  It seems
like moving toward the switch is better for this application though.  Otherwise there
may be some other mechanical issues in my system because I am not getting
expected consistency.

Offline Tweakie.CNC

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Re: limit switch accuracy
« Reply #3 on: September 22, 2011, 12:31:32 PM »
The touch probe macro sounds like it should meed your needs then.

Tweakie.
Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.
Re: limit switch accuracy
« Reply #4 on: September 22, 2011, 07:33:21 PM »
I just looked at the speed of homing and I had it at 100% so I cut
it down to 5 % and now I am getting much more reproducible
results so it does not look like the switch was the weak link!

Offline Tweakie.CNC

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Re: limit switch accuracy
« Reply #5 on: September 23, 2011, 02:22:30 AM »
You are nearly there then  ;D

Tweakie.
Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.

Offline BR549

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Re: limit switch accuracy
« Reply #6 on: September 23, 2011, 08:46:24 AM »
Might want to consider the G31 probing routine to do the Wheel diam function. The G31 sets the trip point on the way INTO the shape and is deadly accurate. and repeatable.

Just a thought, (;-) TP

Offline Jeff_Birt

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Re: limit switch accuracy
« Reply #7 on: September 23, 2011, 09:58:13 AM »
I always like questions like this as the accuracy of the system in general is probably more important to know.  

Quote
have a repeat accuracy of better than 0.001 mm

Is that really the manufactures specification?  It is quite deceptive of them. Under what conditions would such a rating be true. That is about 4 hundred-thousandths of an inch. If you breath on it your likely to heat it up enough to change the reading not to mention how the reading will change as the machine itself heats and cools. A speck of dust in around 0.00002" so if the sensor or actuator is not perfectly clean then your reading will be an order of magnitude off (gasp only good to 4 ten-thousandths of an inch).

A typical mechanical switch will vary a bit in the exact position it opens and close due to several factors. The spring loaded parts inside will react slightly differently with temperature changes. The contacts also don't instantly change from open to closed (or closed to open) they 'bounce' a little so which of these bounces is Mach likely to see?

Think of the switch contacts as a resistor, when the contacts are closed you have close to zero ohms of resistance, when open the contacts have nearly infinite resistance. As the contacts start to open the resistance starts to rise every so slightly. Witch a good contact design this resistance change is fairly accurate and expensive contact probes look for this change is in resistance which provides a more repeatable indication that the switch is opening.

Back to the job that the OP is trying to do. You may find it is more accurate to move until you contact the switch and then back off of it really slowly.
Happy machining , Jeff Birt
 

Offline Tweakie.CNC

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Re: limit switch accuracy
« Reply #8 on: September 23, 2011, 10:57:50 AM »
Quote
Is that really the manufactures specification?  It is quite deceptive of them. Under what conditions would such a rating be true. That is about 4 hundred-thousandths of an inch. If you breath on it your likely to heat it up enough to change the reading not to mention how the reading will change as the machine itself heats and cools. A speck of dust in around 0.00002" so if the sensor or actuator is not perfectly clean then your reading will be an order of magnitude off (gasp only good to 4 ten-thousandths of an inch).

A typical mechanical switch will vary a bit in the exact position it opens and close due to several factors. The spring loaded parts inside will react slightly differently with temperature changes. The contacts also don't instantly change from open to closed (or closed to open) they 'bounce' a little so which of these bounces is Mach likely to see?

Think of the switch contacts as a resistor, when the contacts are closed you have close to zero ohms of resistance, when open the contacts have nearly infinite resistance. As the contacts start to open the resistance starts to rise every so slightly. Witch a good contact design this resistance change is fairly accurate and expensive contact probes look for this change is in resistance which provides a more repeatable indication that the switch is opening.

It sure is Jeff  ;)

Although their switches are used with CNC many are used on spacecraft as well as military equipment.

Attached, typical construction of "My-Com" series.

Tweakie.
Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.
Re: limit switch accuracy
« Reply #9 on: September 23, 2011, 07:57:59 PM »
I was reading this post with interest as I am looking for homing switches for a lathe project.  I found the Australian Distributor and asked for a quote on the MY-COM D250/80 which looked like a suitable switch to me for job I wanted it to do. The quote was $748.00 + $25.00 Delivery + another 10% Tax.  To me a little expensive so I will have to keep looking for an alternative. (this is more then I paid for a 1.2 kW servo motor and drive)
Steve