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Author Topic: Accuracy of Homing Devices? Pro's and Con's of Different Sensors  (Read 30003 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 #50 on: November 09, 2012, 07:39:22 PM »
It should increase accuracy, however whether it's noticeable when you take into account other factors such as any backlash/flex/temperature change, is another matter.

Out of curiosity, I'll run through some theoretical figures for the sensor I use, which has a worst case repeatability of 0.03mm.
Now direct on axis, as per my lathe, that's a potential error of 0.03mm, or 0.06mm diameter on a lathe X-axis.

Now I'll assume a lathe with a 5mm pitch screw, and the sensor mounted at 25mm from center.
The total circumference is 157mm (I'm rounding for simplicity hear!), of which 0.03mm is 0.019% of rotational travel.
Now as 1 turn is 5mm, we simply take 0.019% of 5mm, which gives us possible repeatability of 0.00095mm, or 0.002mm on a lathes X-axis.
Even if the sensor is mounted at 12.5mm from centre, repeatability is theoretically 0.002mm.

And having just run through those figures, I'm adding that when I redo my x-axis!
Off course the sensor Dickeybird is using, is of unknown spec, so his figures may vary.

Offline Fastest1

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Accuracy of Homing Devices? Pro's and Con's of Different Sensors
« Reply #51 on: November 10, 2012, 07:50:54 AM »
I hope the explanation helped Hood as it just confused me more. You are going to use the micro and the opto together? Since Mach backs off the switch after the trigger I just don't see the benefit. You make a statement in an above post "a normal opto homing routine would read the axis directly". It knows the direction and IF no steps were lost, it knows where it is. I am in the process of connecting 3 OPB94051W for my switches. I expect repeatability to be better than .0005. Homing speed will have as much effect as anything. You will have to leave the homing speed constant or the overtravel will be different for each speed.
I want to die in my sleep like my grandfather, not like the passengers in the car! :-)
Re: Accuracy of Homing Devices? Pro's and Con's of Different Sensors
« Reply #52 on: November 10, 2012, 09:59:37 AM »
Sorry for the communications gap...not one of my strongest areas.

The opto I chose has onboard electronics that outputs a voltage when it's triggered by the disc slot.  It pulses once per rev of the stepper motor when the axis is moving but the signal doesn't get through to the B.O.B. until the NO microswitch closes near the end of the axis' travel.  At that time the pulse triggers Mach to do its normal homing stop & reversal (I hope.)

I may be wrong but (hopefully) should find out today.  I'm off of work and have all day to play in the shop.  Unfortunately one tiny little wrong move in the shower this morning triggered a back spasm.  I'm waiting for breakfast and a pain pill to stabilize so I can go out to the shop and ignore the "Do not operate machinery" warning on the pill bottle.  Some people have dinner & a movie; I have breakfast and a pain pill. ;D

(I know, I know...I'm fully aware of the dangers of pain pill usage.  I only have to do this every couple months or so.)
Milton from Tennessee ya'll.

Offline Hood

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Re: Accuracy of Homing Devices? Pro's and Con's of Different Sensors
« Reply #53 on: November 10, 2012, 12:22:24 PM »
mc that sounds a rather poor repeatability you have there, maybe they are just giving  a large figure to be on the safe side. Would be interesting if you could test it out, I suspect you will get much better than the quoted number.
The Optek OPB917B that I used doesnt have any figures for repeatability in the data sheet but I and at least one other person (Bertho Bowman sp?) tested this and it is very small. I tested on a stepper driven Bridgeport and used glass scales and a DRO to test, Bertho, I think, used a rig with a micrometer to test. Repeatabilty for me was under 0.005mm (smallest measurement I had) and Bertho had repeatability of less than  0.001mm.
Now my thoughts on asking about where the benefit would be were based on this and also the fact you are both using stepper based systems. Theoretical resolution of your stepper system is (assume you have 10microstep drives)  1/800 = 0.00125mm but as microsteps are not accurate then really we can only say you are 0 +- 0.00125mm. In conclusion I would say there is no benefit to using the opto in the way mentioned as repeatability of the opto is better than the resolution you can expect.
Hood

Edit, thought you had 2:1 gearing but reading again it seems not so your theoretical resolution is double the above numbers, ie 0 +- 0.0025.
« Last Edit: November 10, 2012, 12:34:43 PM by Hood »

Offline Dan13

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Re: Accuracy of Homing Devices? Pro's and Con's of Different Sensors
« Reply #54 on: November 10, 2012, 12:58:40 PM »
The secret with photo sensors' repeatability is their aperture size - the smaller the better.

Dan

Offline Hood

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Re: Accuracy of Homing Devices? Pro's and Con's of Different Sensors
« Reply #55 on: November 10, 2012, 01:31:31 PM »
Dan that is supposedly the case but the OPB917B is not particularly small or at least I dont think it is.
Hood

Offline Fastest1

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Accuracy of Homing Devices? Pro's and Con's of Different Sensors
« Reply #56 on: November 10, 2012, 01:48:08 PM »
Interesting to say the least. I now at least understand how you are thinking it will work. Not doubting. Btw, you got your point across fine, it is my ability to understand it that is the problem. I am still trying to figure out why it would be better. Theoretically the coupler could come loose at the same time as hitting the micro switch therefore triggering the opto circuit mounted on the stepper. The encoder ring would turn and indicate the correct position but would not have moved the table, right? I know the circumstances above are a little far fetched but the combo of the pain pills and breakfast has me thinking that way! I don't abuse that stuff but if something hurts, I got some in the medicine cabinet.
I want to die in my sleep like my grandfather, not like the passengers in the car! :-)

Offline Fastest1

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Accuracy of Homing Devices? Pro's and Con's of Different Sensors
« Reply #57 on: November 10, 2012, 01:49:03 PM »
I was kidding about the pain pills.
I want to die in my sleep like my grandfather, not like the passengers in the car! :-)

Offline mc

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Re: Accuracy of Homing Devices? Pro's and Con's of Different Sensors
« Reply #58 on: November 10, 2012, 07:34:57 PM »
Hood, certainly when I tested mine, I couldn't measure any noticeable variation with a 0.01mm dial gauge, however the issue with these sensors is they can be affected by temperature and ambient light. There's certainly no harm in doing your best to try and improve accuracy.

I'm looking to upgrade mine to a closed loop stepper system (debating between Leadshine HBS, or dynomotion), hence the question on your thread a couple weeks back about weight, and I'd like to imporve homing accuracy at the same time.

Offline Hood

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Re: Accuracy of Homing Devices? Pro's and Con's of Different Sensors
« Reply #59 on: November 11, 2012, 04:46:24 AM »
Stepper systems have a big benefit of ease of setup and easier tuning but you have to be satisfied with their performance constraints.
Personally I would not look at steppers for a lathe or mill  whether they are traditional or closed loop as they all suffer from the same thing, speed increases and torque drops away rapidly.
Cost is obviously a big factor in people choosing a stepper system but I suspect as you move towards these closed loop stepper systems the price gap narrows quite a bit.

Regarding temp differences with the optos I think Bertho did tests with regards to temp and found it not to be of concern, but take that with a pinch of salt as it is just from my memory, I will have to see if I can find his article.
Hood