Sensing 0-10 RPM of a shaft continuously

[Braeden Hamson]

I feel like I'm really over thinking this. I want to measure the speed of a shaft at low RPMS down to zero to detect when it first turns. I think a hall effect sensor system is a good candidate. If I mount the magnets on the shaft I will be able to detect a change in voltage indicating the shaft is moving. However, I feel like I can't see a solution right in front of me. I can't figure out how to measure the actual RPM. I can't directly measure the slope because it's a sine wave and the slope is always changing. I'm attempting to code this into an Arduino and I'm struggling to understand what I actually need to calculate to get to RPM.

 

[Bluejets]

Look for Arduino pulsein.
I'd tend to use a digital hall effect.
One I commonly use is a module from Ebay KY-003 that uses an Alegro 3144

 

[Minder]

For that low RPM I would go with some Hi resolution disc, I have used these optical versions off Ebay for a couple of $$.
Item # 112453707979

 

[Bluejets]

Or simply use one magnet and a hall effect as noted above on opposite sides of a metal disc with a series of holes around the perimeter or cut outs .

 

[Minder]

Another version that has been used is the retro-reflective sensor, just needs a spot of paint on any moving shaft, as used in the SuperPID motor controller.
up to 25,000 RPM's.

 

[Bluejets]

Another version that has been used is the retro-reflective sensor, just needs a spot of paint on any moving shaft, as used in the SuperPID motor controller.

Ok as long as the sensor (if IR) is shielded from any outside light.

 

[Minder]

Never been a problem so far when setting up something like this http://www.vhipe.com/product-private/SuperPID-Home.htm
The retro-reflective type sensor is fairly closely gapped to the rotating shaft which has the dab of paint or reflective tape etc.

 

[TomB19]

I wish to add there are both optical and hall-effect end stops for 3d printers that can be had for a couple of bucks that would do your job.

Curiously, I have a SuperPID to control a CNC router. I've seen guys at the RC airfield measure prop RPM with a cheap, hand held, optical tachometer in broad daylight.

 

[Bluejets]

I wish to add there are both optical and hall-effect end stops for 3d printers that can be had for a couple of bucks that would do your job.

Curiously, I have a SuperPID to control a CNC router. I've seen guys at the RC airfield measure prop RPM with a cheap, hand held, optical tachometer in broad daylight.

They use an optical transistor and sunlight.
Use it under a fluro indoors and you'll be measuring mains frequency x 2.

 

[TomB19]

I have a cheap one from Harbor Freight and it works fine in my shop to measure the chuck speed of my drill press. The only lighting in the shop is fluorescent. I can post a video if that is helpful.

 

[Minder]

I wish to add there are both optical and hall-effect end stops for 3d printers that can be had for a couple of bucks that would do your job.

Curiously, I have a SuperPID to control a CNC router. I've seen guys at the RC airfield measure prop RPM with a cheap, hand held, optical tachometer in broad daylight.

I can say i have never had a issue with them, I have used slot opto's for motor speed detection for decades and no problems.

 

[crutschow]

I can't figure out how to measure the actual RPM.

To convert the pulses to RPM you can measure the time between pulses with a micro, and then do the math to generate the RPM value.
For example, if you have 10 pulses per revolution from your sensor, and you measure the pulses as having a 0.5 second period, then the shaft speed is 0.5 / (10*60) = 0.83RPM

 

[Minder]

I came upon this thesis some years ago and played around with it for a while.
Might give a few insights.
https://pdfslide.net/reader/f/dc-motor-speed-control-55844d271a765

 

[Bluejets]

I have a cheap one from Harbor Freight and it works fine in my shop to measure the chuck speed of my drill press. The only lighting in the shop is fluorescent. I can post a video if that is helpful.

maybe, maybe not but the fact is, if an optical relying on sunlight for source, it will respond to fluoresent lighting.
It is one way to actually calibrate the optical tacho.

Plenty of examples with Arduino as a tacho using interrupts (better) rather than pulsein in youtube or on Arduino site. Most code already proven.