Optical Rpm Sensing

[Azbrewer]

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]

I would find another place to put it

 

[JimDawson]

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]

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]

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]

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]

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]

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.

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]

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

 

[RJSakowski]

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