Lower Price Optical Rev Counter Accuracy

[WindyParticipant @windy30762]

[WindyParticipant @windy30762]

I have bought a new £26 optical rev counter and would like to check the accuracy of it at high rpm and how can it be checked in a home workshop.

It has been checked against various electric motor nameplate speeds and the readings are as followers, these motors are not under load.
Examples 2,800rpm motor, minimum optical reading 2,993rpm, nearly 200revs out.

My mechanical counter reads 2850rpm
On a quality 25,000-rpm motor minimum optical reading was 30,000 over 5,000 revs out.

An article that I read on speed testing of electric motors says a strobe should be used?
I require to check a speed range around 70,000 rpm the optical goes up to 99,000rpm.

A visual slow  speed test was done on my lathe i.e. Slowest back gear was selected and a mark placed on the chuck backplate then rpm counted against a stopwatch for a minute.

The lathe speed plate says 50rpm, my stopwatch test gave just over 62rpm, the optical rev counter read 62.4rpm and a mechanical rev counter said 60rpm

My mechanical counter only goes to 20,000rpm.

At slow speeds the optical counter is reasonably accurate but when the revs go above about 2000rpm there are large discrepancies compared to the mechanical counter.

Any suggestions or are these lower priced optical rev counters unreliable at high speeds.

Another identical rev counter was supplied and it reads the same.

 

Windy

Edited By Windy on 22/02/2010 17:00:45

Edited By Windy on 22/02/2010 17:01:53

Edited By Windy on 22/02/2010 17:05:24

Edited By Windy on 22/02/2010 17:11:16

[KWILParticipant @kwil]

My strobe covers up to 10000rpm only, RS price over £300!!  Talk to somebody from Gas Turbine Builders Assoc, this is more their speed range.

[Les Jones 1Participant @lesjones1]

Hi Windy,

                   I think your optical tachometer is more likely to be correct than the speed marked on a motor or a mechanical tachometer. With no load on an induction motor that is marked 2800  RPM its speed will be close to synchronous speed which is 3000 RPM. A motor rated at 25000 RPM will be a series motor. The speed of these varies greatly with load. I have combined optical and direct drive tachometer which is probable similar to yours. The readings from this agree with a home made tachometer that was originally tested using an audio signal generator and frequency counter. The optical tachometer will almost certainly use a crystal as a time reference. Even the cheapest crystal will have an accuracy measured in tens of parts per million. (Even 1000 parts per million is 0.1% accuracy)

Les.

[Les Jones 1Participant @lesjones1]

Hi Windy,

                   I think your optical tachometer is more likely to be correct than the speed marked on a motor or a mechanical tachometer. With no load on an induction motor that is marked 2800  RPM its speed will be close to synchronous speed which is 3000 RPM. A motor rated at 25000 RPM will be a series motor. The speed of these varies greatly with load. I have combined optical and direct drive tachometer which is probable similar to yours. The readings from this agree with a home made tachometer that was originally tested using an audio signal generator and frequency counter. The optical tachometer will almost certainly use a crystal as a time reference. Even the cheapest crystal will have an accuracy measured in tens of parts per million. (Even 1000 parts per million is 0.1% accuracy)

Les.

[Les Jones 1Participant @lesjones1]

Hi Windy,

                   I think your optical tachometer is more likely to be correct than the speed marked on a motor or a mechanical tachometer. With no load on an induction motor that is marked 2800  RPM its speed will be close to synchronous speed which is 3000 RPM. A motor rated at 25000 RPM will be a series motor. The speed of these varies greatly with load. I have combined optical and direct drive tachometer which is probable similar to yours. The readings from this agree with a home made tachometer that was originally tested using an audio signal generator and frequency counter. The optical tachometer will almost certainly use a crystal as a time reference. Even the cheapest crystal will have an accuracy measured in tens of parts per million. (Even 1000 parts per million is 0.1% accuracy)

Les.

[Martin WParticipant @martinw]

Windy

 

My guess is that the optical rev counter will be more accurate that a mechanical one, even the cheap ones should measure against crystal oscillator circuit which is inherently accurate and the mechanical ones tend to be analogue with things like hairsprings, magnets and aluminium discs.

 

However at very high speeds it will depend on the sensor response but again I would be surprised if the elctronic unit couldn’t handle 70,000 rpm. This is only about 1667Hz (cycles/per sec  in old money) which is very low frequency for electronics these days.

 

I think your own tests at low revs indicate that the electronic unit is more accurate than the mechanical unit and it reflects the sort of error you are getting between the two systems when measured against a stop watch.

 

Cheers

 

Martin W

[mgjParticipant @mgj]

Windy – I think the others are right .

 

A cheap and cheerful test is against the old car ignition type strobe if you can get one.That will use 50hz as its datum, and i think the electricity people are obliged to supply at 50hz with very little leeway – like none in a practical sense!

 

So one of those would give you a reference at reasonable cost.

 

And again. Within reason do you need to be able to measure absolute revs, or do you want a comparison between modifications. 

[Les Jones 1Participant @lesjones1]

Apologies for the multiple posts. My broadband connection dropped out (Which it does quite often at busy times.) and I thought the post was not sent.

Les.

[wheeltapperParticipant @wheeltapper]

Hi

silly question I know, but did you get a strip of stick on reflective tape to fix on the shaft / chuck whatever.

the strips that came with mine were quite wide, as wide as the laser spot in fact.

 

what kind of marker are you using?

 

Roy

[Les Jones 1Participant @lesjones1]

Hi Roy,

              The width of the tape does not matter so long as it is not so narrow that at very high speeds the width of the pulse produced is too short to be seen. (Or it covers such a small percentage of the area seen by the detector that it does not reflect much of the light back)

The same reasoning would apply if the marker covered almost all of the cercumference leaving a very small area unobscured. I find that the type of tape used is not critical. If the object is shiny I would use black insulating tape. If the object was dark I would use a light coloured tape.

Les.

 

 

[wheeltapperParticipant @wheeltapper]

Hi

thats exactly why I asked him what kind of marker he is using, if its too narrow at the high speeds he is using there may not be enough reflection.

 

Roy

[WindyParticipant @windy30762]

Thanks all for your comments it seems the cheap optical one comes out tops.

Have tried reflective tape, white paint and black tape on a bright background as suggested, the readings on the 2800rpm motor where all very close to 3000rpm.

Will borrow my mates old ignition strobe and do a comparison.

Windy 

 

Edited By Windy on 22/02/2010 23:58:27

[Martin WParticipant @martinw]

Hi

 

A way of checking the tacho at the high end is to increase the number of counting marks on the shaft. Put one on and note indicated speed reading and then add another and repeat and so on. While it will not give a calibration for the tacho it will, providing enough marks can be put on the shaft or encoder disc, give an idea of where the tacho starts to lose its ability read the shaft speed. On the high speed motor, 25000rpm unit you have, three distinct marks should give you a reading about 90,000 rpm; this assumes that the measurement you made earlier of 30,000 rpm is about correct.

 

As I said it won’t calibrate it but should give an idea if its up to the task.

 

 Taking up Meyrick’s point regarding the mains frequency. It is possible to strobe against the mains especially if you have a standard fluorescent lamp in the workshop. If so set your machine to 1000 rpm and then looking at the chuck see if you can see a set of ghost jaws revolving slowly, if not move the speed control until they appear; hopefully. If you can see them then adjust the speed control until they appear to be stationary, to all intents the chuck should now be revolving at 1000. Now measure the rpm of the chuck using the optical tacho and with luck it should be in this area.

 

Ideally this should be done at 3000rpm but that’s not an option so using each jaw effectively reduces this by a factor of three to 1000 rpm. The above assumes that you have a three jaw chuck fitted and that you are on standard UK mains power.

 

 

Hope this helps

 

Martin W

[Frank DolmanParticipant @frankdolman72357]

     Something wrong surely.  Have you forgotten the flashes from

   the -ve half cycles?  Anyway a 3jaw will appear stationary @

   2000 rpm under 50 Hz lighting

[Les Jones 1Participant @lesjones1]

I agree with Frank. Although the mains frequency is 50 HZ a normal fluorescent lamp (NOT A LOW ENERGY ONE) flickers at 100 HZ. A three jaw chuck rotating at 1000 RPM would appear to have six jaws. Overnight I had the idea of trying to blank off the illuminating beam from my optical tachometer so that pointing it at a fluorescent lamp would enable it to see the 100 HZ pulses of light. On looking at the tachometer I found that both the source and detector were behind a lens so there was no way to block the light. I thought I would try it anyway. It worked reasonably well at about two feet away from an 11 watt desk lamp or about three feet from an 8 foot tube.The reading should be 6000 RPM for exactly 50 HZ mains. At different times I had reading between 5992 and 6013 RPM (6013 reading would indicate a mains frequency of 50.11HZ)
On the subject of the ignition strobe it will not flash at 50 HZ unless it is being triggered from an engine running at 1500 RPM (Assuming a 4 cylinder 4 stroke.) To use it without being triggered by an engine some way would have to be found to generate a trigger signal. This could be as simple as a squaring circuit fed from a low voltage transformer or as complex as a precision signal generator. I do not think it worth the effort when a fluorescent light works well enough.
Les.

Edited By David Clark 1 on 23/02/2010 10:51:43

[Martin WParticipant @martinw]

Hi Frank & Les

 

I am glad you spotted my deliberate mistake. It was late at night and I claim that I was not thinking clearly. Of course you are right a standard fluoro will flash at 100Hz and not at 50Hz. Retired and brain dead that’s me. But with Les’s help it seems that there is an easy way to calibrate, well fairly accurately, an optical tacho without too much effort which was the aim of my post.

 

I am fairly certain that the instantaneous mains frequency will vary depending on time of day but over a 24hr period is very accurate, hence bog standard mains clocks are relatively accurate. Therefore the readings Les has got look very good and reflect what can be expected when making spot measurements.

 

Les you realise that now everybody that has an optical tacho and access to these forums will be looking at the mains frequency and complaining when they feel it has drifted too far from 50Hz.

 

Good spot and great solution.

 

Cheers

 

Martin W

[Frank DolmanParticipant @frankdolman72357]

     I think you are being too pessimistic Martin.  They have to click a spare

   genny onto the grid without all the winding up that they used to do.  Life

   is too short for that nowadays.  By the standards of we agricultural

   mechanicals, the 50Hz is very good indeed. 

[Martin WParticipant @martinw]

Hi Frank

 

Yes you are right about the 50Hz, afraid I had my wrong head on and was thinking in ‘lectronics’ terms of accuracy . Tomorrow, all being well, my new mill arrives and I can then start getting to grips with things.

 

Cheers

 

Martin W

[Les Jones 1Participant @lesjones1]

Hi all,

           For Windy (And others that may be interested.) here is a way to see the current mains frequency on the UK grid.

Mains frequency display 

The odd thing is the analogue display always seems to show less than 50 HZ while the digital value varies slightly above and below. My cheap optical tachometer seems to agree pretty well with this display.

Les.

[Frank DolmanParticipant @frankdolman72357]

     Well done Les!  That’s put that one to bed.

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