I'm designing a project to measure the RPM of a slowly rotating shaft (around 10–200 RPM). I'm considering using a Hall effect sensor with a small magnet, but I'm unsure which sensor type would provide the most reliable detection at low speeds. Would a latching Hall sensor be a better choice than a unipolar device? Any recommendations or design tips would be appreciated.

 

I have used both types of prox,
Many of both types are also proximity detectors and do not need a magnet, just the pressence of a ferrous metal tooth etc.

#1


#2
Descrete Type.
Both Should
work well for this application.

 

Many of both types are also proximity detectors and do not need a magnet, just the pressence of a ferrous metal tooth etc.

Those are called reluctance sensors, and to the best of my knowledge, they do require a minimum frequency.
Go better for the hall-effect, as they can go down to DC.

 

I use all the above types with DC supplies??
Typical useage, zero position and over-travel on CNC machines.!
BTW, @cwella you can get them with different features, Latch/unlatch etc etc

 

You are correct, for the pre-magnetic-based variable reluctance sensor.

But according to the Wiki article:
" And because the magnitude of the signal developed by the VR sensor is proportional to target speed, it is difficult to design circuitry to accommodate very-low-speed signals. A given VR-sensing system has a definite limit as to how slow the target can move and still develop a usable signal. An alternative but more expensive technology is Hall effect sensor. Hall effect sensors are true zero-rpm sensors and actively supply information even when there's no transmission motion at all."

 

All I can say is I have used the hall or inductive sensor type of units shown for several decades in industrial enviorenments and personal projects with total satisfaction.
Allen-Bradly, Turk, Siemens, Omron, SquareD, et-al.

 

Automation Direct are one cheaper supplier of inductive prox sensors..

 

I use all the above types with DC supplies??
Hall effect all
Typical useage, zero position and over-travel on CNC machines.!
BTW, @cwella you can get them with different features, Latch/unlatch etc etc

Thanks for the info! That makes sense. I agree that the different options like latching/unlatching features make them quite versatile depending on the application. Appreciate you sharing your experience!

 

For low speeds, I would go with a Hall-effect sensor, since they work down to zero speed, not a magnetic reluctance sensor, whose output amplitude is proportional to speed.

 

total cost $1 !!

 

Hall sensors for sure.
https://forum.allaboutcircuits.com/threads/hall-effect-sensor.168217/post-1491480
https://forum.allaboutcircuits.com/...ing-device-magnetic-sensor.121871/post-976600

 

Most CNC machine manufacturers use inductiive prox switches due to metal swarf covering any PMagnet if used.. Making it impracticle.
Inductive use:
https://knowledge.forneyonline.com/verifying-overtravel-limit-protection


Common Types of Proximity Switches

• • Inductive: These switches emit an electromagnetic field and are exclusively used to detect metallic objects. They are the most common type found in factory automation and manufacturing.

 

I've installed hall type devices on grain elevators. These had 4, or more fixed, small target magnets mounted around the circumference of a shaft mounted disc, to give a higher frequency signal count at lower speeds. That signal got processed (maybe divided) before it went into a readout display.

 

@cwella Here is a video of a guy installing No1 type from post #2 on a CNC M/C.
You can get them pretty reasonable on Amazon!.