# Optical Rpm Sensing



## Azbrewer (Dec 12, 2015)

I have just about completed the Digital RPM display for my Atlas 6". I just need to get the actual RPM signal. Using an infrared LED transmitter and receiver pair, I just have to interrupt the light path. My thought is drill a small hole in the spindle back gear aligned with the LED’s. Should be about 3mm hole as that is the size of LEDs I have. This gear shouldn’t be hurt by this I hope. Comments or other suggestions?


----------



## dlane (Dec 12, 2015)

I would find another place to put it


----------



## JimDawson (Dec 12, 2015)

Maybe look through the gear teeth?  Would give you more accuracy, more pulses per revolution.  I don't think drilling a hole in the gear would hurt anything, as long as the hole is not too close to the teeth where it might weaken them.


----------



## wa5cab (Dec 12, 2015)

Inside the headstock is a pretty dirty environment for any kind of optical sensor.  Unless you never grease or oil anything in there.  I would recommend a reluctance or an eddy current sensor counting the bull gear teeth.  Plus as Jim indicated, that would give much higher resolution which could be important at very low spindle RPM.


----------



## brino (Dec 12, 2015)

Would the electronics accept a reflective configuration?
You could put the transmitter and receiver on one side and put a dab of white paint on the gear (or fix a piece of reflective tape).

Same idea as one of these:
http://www.amazon.com/Handheld-Digital-Tachometer-Engine-Reflective/dp/B00DK9GK0C
there are several sellers of just the tape on ebay.
(actually if anyone wants to buy the entire tool also see ebay, the price will be lower for the exact same item but it will be on the slow boat from China)

-brino


----------



## wrat (Dec 12, 2015)

wa5cab said:


> I would recommend a reluctance or an eddy current sensor counting the bull gear teeth.


+1 on this ^^^
A reluctor ring a robust setup.  Works for your anti-skid brakes, anyway.
And you could probably pull a ring sensor straight off a car and use it.  Probably.

Wrat


----------



## Azbrewer (Dec 12, 2015)

My intent was to read the spindle directly, don’t have to worry about belt slip or calculate pulleys. I have some Hall Effect sensors I had ordered, they just don't seem to like the way I was using them, got very sporadic output (you get what you pay for). I may try again with them. Using reflective paint/tape might be an option, will research more and maybe test, would make mounting a little easier as won’t need to span gear. I am not worried about dirt/grease because I read the rising signal and it is very forgiving as to how much light is needed. I had thought about reading through the gear teeth but they are not that big and alignment would be tight.


----------



## RJSakowski (Dec 12, 2015)

wa5cab said:


> Inside the headstock is a pretty dirty environment for any kind of optical sensor.  Unless you never grease or oil anything in there.  I would recommend a reluctance or an eddy current sensor counting the bull gear teeth.  Plus as Jim indicated, that would give much higher resolution which could be important at very low spindle RPM.





Azbrewer said:


> My intent was to read the spindle directly, don’t have to worry about belt slip or calculate pulleys. I have some Hall Effect sensors I had ordered, they just don't seem to like the way I was using them, got very sporadic output (you get what you pay for). I may try again with them. Using reflective paint/tape might be an option, will research more and maybe test, would make mounting a little easier as won’t need to span gear. I am not worried about dirt/grease because I read the rising signal and it is very forgiving as to how much light is needed. I had thought about reading through the gear teeth but they are not that big and alignment would be tight.



On my Atlas 6", the gear teeth are Zamak so a reluctance type pickup won't work.  There is a spacer (P/N M6-78) immediately behind the back bearing which could be replaced with a sensor wheel. A new wheel could increased in diameter to about 1.8" which would allow some magnets to be embedded for a hall effect type pickup.  I don't really like using magnets because of the tendency to pick up chips but this location is protected by the real cover plate and could be easily cleaned with the right design.

Bob


----------



## Azbrewer (Dec 12, 2015)

My reasoning for Optical as to not have the magnetic/chip problem.


----------



## RJSakowski (Dec 12, 2015)

Azbrewer said:


> My reasoning for Optical as to not have the magnetic/chip problem.


My concern as well but trying to keep optics clean under the hood could be much worse.  

One, two, or four button magnets set into the aforementioned spacer and covered with a thin plastic sleeve will fairly easy to clean.  A hall effect sensor does not have to be close to the magnet to work so you shouldn't have a problem with fouling the sensor.

Bob


----------



## wawoodman (Dec 12, 2015)

I have one of the handhelds that brino mentioned, and it's pretty convenient. Much cheaper than installing a tach on the lathe, mill, drill press...


----------



## francist (Dec 12, 2015)

Quite some years ago I made a reflective-type tachometer using a visible light LED and photo resistor. Considering it was a handheld unit and I was trying to check speeds of a wind turbine whirling around in my backyard it worked surprisingly well. In a fixed scenario it would lots easier to get the focal lengths correct. Foil HVAC tape makes for really good reflector material.

-frank

Edit- meant photo transistor, not resistor.


----------



## John Hasler (Dec 12, 2015)

For a reflective optical transducer you want one of these:

http://www.digikey.com/product-sear...ical-sensors-reflective-analog-output/1967052

For magnetic gear tooth sensing you want one of these:

http://www.digikey.com/product-search/en?FV=fff4001e,fff80480&k=gear+tooth&mnonly=0&newproducts=0&ColumnSort=0&page=1&stock=1&pbfree=0&rohs=0&quantity=1&ptm=0&fid=0&pageSize=25


----------



## wa5cab (Dec 12, 2015)

I wrote "reluctance" but I was more specifically thinking of eddy current, which will work with any conductive material, including Zamak.


----------



## Azbrewer (Dec 12, 2015)

I have never had any dealings with eddy current but what I have seen today it looks promising. More research is in order.


----------



## John Hasler (Dec 12, 2015)

wa5cab said:


> I wrote "reluctance" but I was more specifically thinking of eddy current, which will work with any conductive material, including Zamak.


Aren't those a bit pricey?


----------



## wrat (Dec 12, 2015)

John Hasler said:


> Aren't those a bit pricey?


Take one off of any car wheel in the last 10 years.  Antilock brakes all use reluctor rings that are some kind of pot metal on the back of the disc brake rotor.

Wrat


----------



## John Hasler (Dec 12, 2015)

wrat said:


> Take one off of any car wheel in the last 10 years.  Antilock brakes all use reluctor rings that are some kind of pot metal on the back of the disc brake rotor.
> 
> Wrat


Far as I know those are all Hall-effect or reluctance, not eddy current.


----------



## Azbrewer (Dec 12, 2015)

As I am using an Arduino I have to stay with logic level components(5 volt).


----------



## wa5cab (Dec 12, 2015)

It only takes one transistor and three resistors to make a level changer.  However, I am not certain about the I/O requirements of the ABS sensors.  They are certainly cheap enough.  And definitely sturdy enough.


----------



## JimDawson (Dec 12, 2015)

I think most of the automotive sensors are TTL.  It might take a little research to confirm.


----------



## RJSakowski (Dec 12, 2015)

wa5cab said:


> It only takes one transistor and three resistors to make a level changer.  However, I am not certain about the I/O requirements of the ABS sensors.  They are certainly cheap enough.  And definitely sturdy enough.


  The sensor from my Dodge minivan is a Hall effect device.  I had just connected 12 volts to the power lead and checked the output lead with a scope.  As  I recall, there was no level shifting.


----------



## mattthemuppet2 (Dec 15, 2015)

sounds like a fun project, but a cheap eBay tach and a magnet on the bearing spacer under the gear cover works just fine for me. I haven't even made a proper mount for the pickup, just glued a magnet onto it to hold it to the headstock. Works as well as I can tell (from memory it displayed the correct speeds for each pulley as detailed in the manual) and no chips so far as the gear cover is usually closed. I think it was $13 or so. I have another one I'll be putting on the drill press when I get the motor conversion done.


----------



## Azbrewer (Dec 23, 2015)

Quick update, I had just glanced at the gears and thought there was enough room to do as I planned. After more careful measuring I don't believe there is enough gear above the bearing housing to fit an optical sensor . However I finally got the Hall Effect sensor to read correctly and will use it. I plan on epoxying a small neodymium magnet to the face of the gear and if that fails I will inset the magnet. Now that family have started to arrive all work has come to a stop until after the new year begins.


----------



## The Liberal Arts Garage (Dec 25, 2015)

wa5cab said:


> Inside the headstock is a pretty dirty environment for any kind of optical sensor.  Unless you never grease or oil anything in there.  I would recommend a reluctance or an eddy current sensor counting the bull gear teeth.  Plus as Jim indicated, that would give much higher resolution which could be important at very low spindle RPM.


 
    Real  cheap 3M retro-reflective tape


----------



## Nitmare67 (Dec 31, 2015)

I personally would have kept it much more simple and just ran a tach generator off the back of the spindle. I doubt you will have goodluck with a retroflective photo eye trying to count the bull gear at 2k rpm. The solid state electronis wont cycle fast enough and you will just end up with a constant on signal. The optical route isnt going to work will with 2 pulse per revolution. You near much more resolution that that. I would want at least 50 pulse per revolution for low RPM accuracy.

If your not looking for a perfectly accurate reading and you just want to know the ballpark RPM say +or - 5 % then you can do that without any type of instrumentation. All you need is a VFD upgrade. Run the motor control set up as sensorless vector and you will have very good accuracy for motor RPM right off the VFD. Just figure out what your conversion factory is for each belt combo ( though you wont be switching pulleys because vector drives make peak torque from 0 RPM to basically redline). You could just run the middle of the field pulley combo and run the drive motor at more than 60Hz when you want spindle speeds over 2k rpm and you could also run at 20Hz and still have all the torque you need. Basically all you need is a vector or vfd specific motor and a drive cable of vector contorl. Most of the cheapy drives you see are V/hz control but spend a few buck more and you will accomplish your goals for for a speed output and also have variable speed and torque control.


----------



## Azbrewer (Dec 31, 2015)

Nitmare67, where would you suggest I place this tach generator on the spindle? I have a MK2 and there is no shaft beyond the Direct Drive coupling to attach anything to. I have placed a magnet to the Spindle back gear to read spindle speed whether or not the back gears are in use. This is the only place I see to directly read the spindle. I had thought about reading the motor pulley and with software convert the reading but both being possible, and being simple were not going to happen. As to low RPM accuracy, testing shows I get a stable enough reading at 17 RPM and a 3100 RPM reading from the drive motor. I am using a treadmill motor/controller as a cost saving solution, the price of a VFD motor alone was more than I paid for the lathe. My original problem was not getting a stable signal from the Chinese hall effect sensor (you get what you pay for), however I was using the wrong datasheet. After a slight redesign have changed back the hall effect sensor. Now to build the sensor mount.


----------



## Nitmare67 (Dec 31, 2015)

I didnt realize you were using an MK2. Sorry I was up all night working on my own lathe and running off a total of 1 hour sleep from the night before lol. Dang kids decided to wake up at 5am so I didnt get my normal 4 hours minimum. My thoughts were pretty foggy as I tried to complete that post last night!

I thought about this a little bit now though and i would agree on not doing a VFD on an MK2. The Hall effect sensor is a great idea. Another low cost sensor you might could use is a cam position sensor that comes in any ford 4.6/5.4 from 96 to current. Those are driven off 12-24 volts and will switch extremely fast... must faster than you will ever need to in your application. 

For the sake of keeping thinking simple you could also consider just usings some sort of capacitve prox sensor and glue a small plastic square onto the back side of the chuck and have the prox switch sitting back behind the lathe out of the way.  I would have to get more familiar with the MK2 to offer much more help. I can help you run down which control devices are good and bad though if you need help with that.  Its too bad you cant pick up the speed on the back side of the spindle.. the gear side on a regular altlas 12". it would be easy to machine a press fit end piecd to go into the spinde and the tach could be mounted onto the cover so that it engages when the door is shut and this would keep it out of your way. Or drive the tack off same same pitch gear on the spindle gear and have it offset.

I will try to find some MK2 pictures so I can better help.


----------



## Azbrewer (Jan 1, 2016)

Nitmare67, I appreciate any help I can get. I am using an Arduino so 5 volt logic levels for me. I have the display built and only waiting for my son to mill the hole in  the enclosure for the LED display. Just today I built the sensor using the hall effect sensor embedded in some pvc sheet. I have the magnet on the gear just need time to mount the sensor in headstock. Try this link for pictures of what I am dealing with http://www.lathes.co.uk/atlas6inch/page7.html


----------



## VSAncona (Jan 6, 2016)

Please don't take this as an attack because I don't mean it that way. But what is it you are doing with your lathe that requires you to know the exact RPM?


----------



## Azbrewer (Jan 6, 2016)

I am doing nothing other than playing. I started using Arduinos and if I can build something useful with them then I do. And it looks cool, when I first got the variable speed control sorted out I only had a rheostat with numbers marked out for an approx RPM. Worked but there was no cool factor, look at my forum pic to see the control before the digital display is installed. Currently I am slowly gathering the parts for a DRO using a TI Launchpad and an old android phone. Do I need it, no. Do I want build it, yes.


----------



## VSAncona (Jan 6, 2016)

Ah, I get that. It does sound cool. And that is reason enough. I was just curious if you were doing something where you needed to know the exact rpm. Good luck with your project.


----------



## Azbrewer (Feb 6, 2016)

RPM Display/Start-Stop Control is finished, in the  avatar pic the RPM numbers are a bit washed from the camera. Bright and very readable in person. Buttons light up when relevant, Green to run and Red to stop.


----------



## The Liberal Arts Garage (Feb 6, 2016)

I, a certified GOF, feel you all are having  too much fun discussing beautiful
theories; sharpen a couple of tools and back to work.........BLJHB


----------

