Calculating Shaft RPM Using a Proximity Switch

First, you have to understand basic geometry. Then you have to understand how this one of thousands of different proximity switches detects proximity. Now comes the hard part, one must think how to accurately place the proximity switch near this spinning shaft so that the motion will trigger this switch. Often the teeth of a gear or an eccentric cog on the shaft will provide the mechanical signal for the switch but only the OP knows what is available. Only once these things are known can a method to calculate anything be found.

Now do your homework.

Cool genius!

You don't calculate; you count.

There are number of ways to do it.

Measure the period T between two consecutive sensed pulses at high resolution (using a gated fast clock) and then numerically compute the frequency as 1/T or 1/(n*T) if there are n teeth for sensing.

60/T or 60/(n*T) will be RPM measured in just one pulse period.

Other simpler slow method has already been discussed by Tornado.

If you are aware of phase lock loop then another method gives quick average RPM on display in same way you read mains frequency accurately and quickly.

I don't think you can do this with a prox. switch if your shaft is round and has no feature to trigger the switch.

Your count would always be 1.

Is there more to this?

#1 For round shaft or wheel simply paint white and black bands on it and use optical reflective proximity sensors.

#2 For magnetic pick up sensor stick soft magnetic tap cut pieces.

#3 For capacitive sensor put paches of different dielectric material.

#4 If you are damn lazzy then use ultrasonic doppler frequency shift for shaft speed measurement and then hire someone to compute RPM for you.

#5 If nothing works for you then buy a tachometer and follow instruction #1

#6 Inductive transformer radiating free energy method - I am leaving this one to intelligent people to try.

Are you still left over with some question then write to me. I have all the free time of this world to solve such problems at zero fee.

With all due respect, Shyam, the OP asked how to do it, not what type of proximity switch to use.

And while you have responded to me here, I'll assume that your remarks are directed toward OP.

I hope that you will agree that there's no practical way to measure the speed of a smooth, round, featureless shaft.

You could do it with a microphone (or a transducer) and a frequency counter, or analyzer...

lyn: You are right.

Perhaps that transducer idea is good one. A pressure sensor can sense the drag pressure changes and mass flow sensor can sense the drag mass. I think these still be dependent on surface friction.

And, we have no idea what specific type of switch/sensor the OP has in mind, or in hand, nor any information about the shaft itself.

humm, that is true. Just some switch, that is all. It looks like a button for a coat. Sometime I assume that the people here who pop in are of some engineering background.

IMHO the key to our conundrum is that the OP identifies the transducer solely as a nebulous "proximity switch". Proximity is an implied non-contact linear positional measurement. Rotation Per Minute measurement on the other hand is a first order derivation of an angular measurement. Additionally there are many different proximity measurement techniques that could be used, from hall effect probes to optical sensors with a vast array of precision from crude luminosity sensors to sub-micron laser interferometers. So unless the OP bothers to explain to us the geometry between sensor and rotating shaft, only wild ass guesses can be proffered. The very naivete of this question posed by the OP makes me wonder if the OP can identify the critical attributes of the mechanics let alone conveying them to us.

In contrast I am certain that if either you (Shyam), Lyn, myself or many others here at CR4 had this proximity detector and spinning shaft in front of ourselves we could quickly identify a design approach and mathematic formula that will produce higher precision measurements than needed.

redfred:

OP has asked for a formula and postings #2, #4 were suggested for computational formula. Anything else is extra.

yn: Other than OP proximity switch there are some direct use methods.

Optical encoder (non-absolute type producing number of pulses per revolution PPR) can also be used for very high accuracy. They must be attached to the shaft.

I looked at my optical mouse which senses motion on table, paper, pad etc senses motion but it can't be used under open loop sensing. Sometime it may sense and sometime it may not as it looks for deformities that are not all that even.

Optical encoder was my first thought too.

We used to use these (360 and 1,000 PPR) to sense engine crankshaft position for automotive active noise cancellation systems we developed in the late 1990s. (That was an interesting job that lasted 5 years)

This is all speculation till we hear from the OP.

You have got to give us more information if you want a reasonable answer.

Are you talking about using a proximity switch to "see"/detect the exposed portion of something like a Woodruff key?

Where are you sending the output of the proximity switch: PLC, DCS, frequency counter.....

What range of RPM, 20 or 50,000? The range matters.

simply, your proxy switch will need to sense maybe a tab that is attached on the shaft then count the number of times it pass per second , minute or day depending on the speed of the shaft.Depending on other factors you can add more tabs to increase accuracy .

eltech: "maybe a tab that is attached ", looks like that he has no such attachments.

I have a feeling his question was suppose to be :How does a proximity switch calculate shaft RPM?

then tell him that it will not do any such thing at all !

allen godwin, where are you?

What is the technique used in non-contact optical tachometers for measuring shaft rpm?.