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Author Topic: Accuracy of Homing Devices? Pro's and Con's of Different Sensors  (Read 28472 times)

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Offline mc

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Re: Accuracy of Homing Devices? Pro's and Con's of Different Sensors
« Reply #20 on: October 16, 2012, 05:59:42 PM »
Here's what I used for the X-axis homing on my lathe -

Home Switch by mc_mtb, on Flickr

It's a Sunx PMR44P (RS part no. 480-5231).
I should really build some form of cover for it, but it's not attracted any swarf yet, and it's easily noticeable if it has. Repeatability is excellent, and only part size issues I get is when tips need replaced, or it loses steps due to trying stupid things.
Re: Accuracy of Homing Devices? Pro's and Con's of Different Sensors
« Reply #21 on: October 16, 2012, 07:14:09 PM »
Thanks mc, what kind of electronic wizardry goes between the sensor and your B.O.B.?
Milton from Tennessee ya'll.
Re: Accuracy of Homing Devices? Pro's and Con's of Different Sensors
« Reply #22 on: October 16, 2012, 11:19:15 PM »
being in a position where i want to pull a old fanuc system off my 20 yr old nardini engine lathe, and replace it with a basic stepper system, this thread has been interesting.  with the old fanuc, i could count on homing and then counting on the machine to be exactly where it was supposed to be , machine coord wise. going to steppers with no index pulse, having to rely on the quality and repeatability of the home switches, has been one of my concerns.  even my servo knee mill can err, if the switches get dirty (ie cast iron jobs) and react slowly and differently than the last time they were used. but this is usually easy to see, as the error is often a full motor revolution or a multiple of.  the lathe has never failed me in this manner, the switches are of much higher quality, and are very well shielded.  it is very handy to be able to count on ones work coords on startup, but with no index pulse on the upcoming lathe steppers, i fear being off a thou, or 3, that could be a problem, not easy to see by eye.  i had thought if i mounted way on the back of the lathe, one of these (http://www.edgetechnologyproducts.com/pro-touch-off-gage.html) for the x and one for the z, kept them covered, and mounted a ball on a stem on the cross slide, the x & z machine coords could be easily checked by hand at any time.  perhaps a nicely made sq and // block could be subbed for the gages, then using a pc of .001 shim as a feeler to do the same thing.  checking the mill is easier, pick up a known work coord, or mount a permanent reference on the table somewhere as someone here prev mentioned.  the lathe is a lil harder, but this might work?  your thoughts? and thanks.

Offline mc

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Re: Accuracy of Homing Devices? Pro's and Con's of Different Sensors
« Reply #23 on: October 17, 2012, 11:06:18 AM »
Thanks mc, what kind of electronic wizardry goes between the sensor and your B.O.B.?
Nothing!
It's connected through a BOB that has pull down resistors, so all I needed was a PNP version (you'd need a NPN version if you have a pull-up resistor).
All it needs is a 5V and GND from the BOB, and the output wire (I think I used dark on) is connected to the input pin on the BOB.

They come in various mounts, so you can usually find one that fits with minimal extra mount/bracket work, so check the datasheet and find one that suits your chosen position best.
Re: Accuracy of Homing Devices? Pro's and Con's of Different Sensors
« Reply #24 on: October 17, 2012, 01:07:41 PM »
It's connected through a BOB that has pull down resistors, so all I needed was a PNP version (you'd need a NPN version if you have a pull-up resistor).
Oh yeah, to me that's definitely electronic wizardry.:(

Now you're gonna make me try to decipher my C11 manual again.  I went to the Jethro Bodine School of Cipherin' and Goesintas and have a long way to go to understand what you said.  I went with a C3 for my spindle index sensor (which works great) but definitely want to avoid buying 2 more of them if I can do it without frying something.
Milton from Tennessee ya'll.

Offline mc

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Re: Accuracy of Homing Devices? Pro's and Con's of Different Sensors
« Reply #25 on: October 17, 2012, 01:42:35 PM »
Just looked at the manual for the C11, and it has jumpers to select pull up, or pull down resistor for each input.

Without going into too much detail, the NPN/PNP refers to the type of transistor used to switch the sensor output. A NPN transistor will switch to earth, where as a PNP transistor will switch to supply voltage.
In this application, using an NPN, the input pin must be pulled high, and will then be driven low when the sensor is activated. A PNP requires the input pulled low, and will then drive it high when activated.

The benefit of the C3, is it provides a TTL output, which doesn't require the use of any external pull-up resistors. The TTL actually drives the output high or low, instead of leaving it to 'float' when not activated. This is what the original C3 would do if set to Open Collector mode.

Offline Dan13

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Re: Accuracy of Homing Devices? Pro's and Con's of Different Sensors
« Reply #26 on: October 17, 2012, 02:25:14 PM »
Hi,

I used to make these home switches with the integral photo sensor. Now there is a new version that will soon be available but still have some of these left if that's of interest to you. Repeatability of these is better than 0.003mm and they are fully waterproof.

Dan
Re: Accuracy of Homing Devices? Pro's and Con's of Different Sensors
« Reply #27 on: October 17, 2012, 03:15:21 PM »
Thanks mc, I wish I could have had you looking over my shoulder when I was having C11/KB speed control isolation/hookup issues.  I think the C11's a good value but some sections of the manual drive me nuts.

Thanks Dan your opto switch unit really looks great!  What would a couple cost including shipping from Oz?  Might be more than my tightwad budget can afford but shoot me a PM when you get a minute.
Milton from Tennessee ya'll.
Re: Accuracy of Homing Devices? Pro's and Con's of Different Sensors
« Reply #28 on: October 17, 2012, 03:34:23 PM »
Hi,

I used to make these home switches with the integral photo sensor. Now there is a new version that will soon be available but still have some of these left if that's of interest to you. Repeatability of these is better than 0.003mm and they are fully waterproof.

Dan

Very nice Dan,
   This one looks to be one sided. Do you have any that are 2 sided ?
Also, what is the amount of over-travel ?
and the $

Thanks,
Russ

Offline Hood

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Re: Accuracy of Homing Devices? Pro's and Con's of Different Sensors
« Reply #29 on: October 17, 2012, 03:37:04 PM »
Look good Dan, are they a  similar kind of idea to the ones I posted a pic of?
Are your revised ones going to have the rod out either side so you just need one per axis?
Hood