Tachometer design

Posted by Les Jones 1 on 18/01/2010 11:16:12:

 I'm not sure about that either - all I can tell from the photo is that it looks like a Maxim IC, and that it has 40 pins. Maxim have certainly made a few devices that would qualify for this task, but from looking at the circuit it's certainly not crystal controlled, however you look at it.

 

I have to say that I'm still interested in your design though - especially if it can be easily converted to work with different numbers of pulses/rev. With the Chinese counters, I was contemplating using a Phase Locked Loop with a divider around it to convert a single pulse/rev into something that could be displayed directly - in other words multiply the input frequency by 60. I can't see why that wouldn't work, but compared to your approach, it's probably a bit of a kludge.

 

So I'd still say that if you can come up with a design that's easily adaptable to work with all sorts of different stimuli, easy to construct, etc, etc, you are still onto a winner - even if MEW doesn't want to publish it for a while. I want about five of them, for a start! 

Hi Lawrie,

                     The idea of this design is to avoid mounting an encoder disk on the machine.

Using it in one pulse per revolution mode only involves putting a reflective marker on a convenient part of the spindle and fixing a reflective opto sensor close to it. (I just glued a piece of baking foil onto a pulley on my drill press.) Using it in its multi pulse per rev mode just involves placing a hall effect gear sensor near to an existing gear on the shaft.

By the way I got caught out using an encoder taken from an HP500C printer. Testing it on the bench I thought it gave 500 pulses per revolution. When I mounted it on the end of the leadscrew on my mill I found it was 504 pulses per revolution. This was to use it to drive a digital readout. Being a strange number of pulses per revolution made it useless for the purpose.

 

One thing I forgot to comment on in my last message suggesting contacting Dave  Clark.

 I thought he may have posted a reply with his views. He may be waiting to see what other readers comments are first.

Les.

Hi There

I do read the posts but theyare automatically

emailed to me so they are interspersed with other threads.

 

Basicically, are we talking about a digital Tacho that counts

each individual revolution directly rather than say 60 pulses per rev?

 

If so, that would make a good article.

regards David

 

Oh for a smell chequer

 

 

Hi David,

                  It does basically measure the time of one revolution and calculate the number of RPM from that. Using it in that mode is suitable when the simplest way to sense rotation is to stick a reflective marker to the shaft. If you then extend this idea and think of two equally spaced markers then it would then read twice the actual speed. So if it is made so that it can divide the reading by two it will display the correct speed. Then extend this idea to many reflective markers but instead of sticking on markers use a hall effect sensor to sense teeth on an existing gear on the machine. So if the reading is divided by the number of teeth on the gear the correct number of RPM is displayed. There will be a single push switch to configure it for the number of pulses per revolution. The way this would be used is to hold the button pressed when power is applied. This puts the unit into setup mode.

When the button is released the number of pulses per rev the unit is configured for is displayed. (Value of 1 when the unit is first programmed.) If the button is now pressed again this number increments  every  0.7 seconds (Approx.) (Wrapping round to 1 after 99) The button is released when the desired value is reached. To indicate the new value has not yet been saved the top bar of the left hand digit is illuminated. after about five seconds this new value is saved in non volatile memory and the illuminated bar goes out. Power is then removed. When power is re applied the unit comes up in normal mode and the new value of pulses per revolution (Teeth on the gear.) is used in the calculation. I have chosen to allow for betweenone and ninety nine pulses per revolution but this could be changed up to easily up to 255 if required. Using the unit for high speeds and a large number of teeth on the gear reduces the accuracy. For example at speeds near 10000 rpm with a 99 tooth gear the error would be nearly one percent. I think this would be good enough for machine tools. Also I think that worst case situation is unlikely to occur.

Hi

 

The Schmitt trigger in the circuit supplied by Les is a quite a good solution as it uses a dedicated comparator IC with a low component count. The resistor between the output and 5V is required  as a pull up as the IC does not have one built in; this gives the designer more flexibility in the IC application. The rest of the component count is minimal while providing  an adjustable input level threshold Schmitt trigger circuit. My only concern is that the LM393 is a 'Dual Comparator' and the other half of the IC is not considered which leaves the inputs and output free. This can cause instability/oscillation in this half of the IC which has the potential to cause interference which could affect the accuracy of the overall circuit.

 

Cheers

 

 

Martin W

Hi Martin and Steve,

                                    The circuit in the picture is only the basics. I think in practice I would put a 5 volt regulator on the board. Also a sense resistor will be required for the hall effect gear tooth sensor that I will probably suggest using. (Allegro ATS642LSHN) This is a two wire device so the voltage across a resistor in series with it has to be sensed. I can not find a source of the thee wire version which would be easier to use. To order the three wire version from Farnell you have to pay the shipping cost from the USA which is high. I have chosen this family of devices as they are very tolerant of their spacing from the gear. As this project is planned for people without access to test equipment I did not want anything that is difficult to set up. I have not yet tested this gear tooth sensor. (I used a hall effect sensor that required a separate magnet on my mill.) One possible use for the spare comparator in the LM393 is to drive an LED so the user can see the led going on and off as the shaft is slowly rotated. On the original unit shown in the picture I used the other comparator as an an alternative input circuit with automatic threshold adjustment. The input signal goes through a high value resistor to the reference input of the comparator with a large value capacitor to ground. (A low pass filter.) Provided the input signal has about a 50% duty cycle the reference input sits at the average value of the input signal.

I have avoided suggesting using variable reluctance transducers  as their output falls off at low speed. I did use one on my lathe but it required a fair bit of playing round with amplifying its signal and converting its output to clean pulses.  I now need to start designing the PCB And test the gear tooth sensor.  The software seems to be working OK so if anyone wants to play with it let me know and I will send you the source code and the hex file to program a PIC

I hope we are not offending too many people talking electronics on this forum.

Posted by Les Jones 1 on 19/01/2010 16:50:49:

 

I rather get the impression that if there's a good, usable result that's cheap to build and flexible, and that they ultimately have access to it, they will forgive us!

 

It's also clear that you've given the transducer issue a good deal of thought and experimentation, and what you are proposing seems eminently sensible.

 

Yes, it would be great to try it out - I think that I can borrow a PICkit 2 programmer... or I might even buy one - I was contemplating that anyway.

 

 

Might be worth noting that the Mach 3 CNC software incorporates a tacho which is needed for screwcutting - it needs to know how fast the h/s is rotating to get the right feed rate; and also where the spindle is to start the cut in the right place.  This means the timing requirement is pretty tight.  It only needs one pulse per rev, so obviously takes the reciprocal of this time to get the speed.  So I'd think this approach is maybe the way to go, and it only needs a single slot sensor disc or a reflective mark on the spindle.  I made a simple sensor using an opto interrupter out of an old printer and an ali disc with one slot on the back of the spindle and it works a charm.  No need for any schmitt trigger or anything, just a 4k7 pull-up on the phototransistor collector.

 

Interestingly, it showed that the speeds on my Super 7 were all slightly faster in reality than shown on the plate on the lathe - 233 rpm instead of 210 for example.  Not significant for straight turning but sure b*******d the thread pitch until I realised what was happening!

 

John.

HI Gentlemen.

 

Have you considered the LM392? this has a comparitor and op-amp in an 8 pin pack. Many moons ago I used the pack as a Variable Reluctance transducer conditioner. The scheme is to use the opamp as a limiter in front of the schmidt comparitor. With mid level biasing through both amps and negative feedback from the schmidt to bias the op-amp you have very clean schmidt action action at almost the zero crosssing points.

 

The schmidt + preamp combo is exactly the same concept in most scope "Auto Trigger"circuits, with no input it will output a low frequency squarewave. But with the faintest sniff it will quickly produce a nice 1:1 squarewave.  A transistor driven from the schmidt could light a led, with the led flashing slowly there is no input, at half brilliance things are normal.

 

Commercial VR transducers normally have high inductance- many turns- and are often intended to work into a resistive load, above the XL=R frequency, the 6dB/octave rise is countered by the low pass action so the output is almost constant above the crossover RPM.

 

Finally, if Henry Maudsley, John Harrison, Charles Babbage, Armstrong or Whitworth had electronic devices available then they would have used them to great effect. Engineering is about making better machines using whatever methods are available, if it is hydraulic, ropes or chains,steam,gas, sea waves, nuclear or  electrons it is still engineering and of interest to those who love engineering. 

 

 

Hi There

No problem with electronics.

Ok on this thread but for other projects, start a separate electronics thread.

regards David

 

Hi Steve,

                 I have sent a message to you on this forums messaging system with my email address. If you reply with an email address I will send the files to you.

Les.

Hi John,

                 I do not know anything about Mach 3 software but it sounds like it is working a bit like the "Electronic Leadscrew" project. You should find it using Google etc.

I agree that slotted opto devices work very well without any signal conditioning. My lathe (A Chester DB10G also runs a little faster than the indicated speed.

Les.

Les,

 

I have pm'd you with a request.

 

What compiler did you use to generate the hex file?

 

Thanks for sharing

 

regards

 

John

Hi Alan,

                 I did use a variable reluctance transducer  on my lathe (Made from a coil removed from a low energy light bulb.) with a dual op amp to process the signals but it was not very reliable at very low speeds ( Less than about 50 RPM) I still think the hall effect sensor will be more of a "bolt on and work first time solution.)

Les.

Hi Les Jones 1,

 

Do your tachometers need a relatively constant motor speed to work or could they be used as an ICE rev counter?

 

Peter.

Hi Peter,

                  Sorry but I do not know what an "ICE rev counter " is. At speeds that give over about two pules per second there is averaging over a period of between half a second and a second. This may sound strange but to smooth out readings counts of the number of 0.4 us pulses from one of the timer modules in the PIC are counted until their number exceeds about 0.5 seconds worth. (500000/0.4 = 1250000 )  then the next tine a pulse occurs from the sensor on the spindle the number of  0.4 us pulses from the start of the timing period is   divided by the spindle pulses. This gives the average number of 0.4 us pulses per spindle pulse. This is to give a display update every half second where possible. If the pulses from the spindle have a time of more than half a second between them (120 RPM for one pulse per rev) then this is the update rate. I decided that if this time is more than 3 seconds (20 RPM with one pulse per rev) the display should show zero rather than display the last reading until the next pulse was seen.

I do not know if this answers your question.

Hi Les,

 

Sorry for my abbreviation ICE is internal combustion engine.  As these motors vary their speed I wondered if your tachometers would be able to display revs in a readable way.

 

Peter.

Hi Les,

 

Good luck!

 

Personally I would rather see relevant electronic projects than reams of g-code or lone descriptions of how to use computer packages that are about 3-d modelling, not design or production.

 

I have made a couple of very similar units, but I used an AVR - so it must be better than one made using a PIC eh?

 

It may be of interest that I timed the delay between pulses and took the reciprocal to get rpm, rather than counting pulses. I assume that's how you cope with the need to regularly update the display at low speeds. The one bit of cleverness was detecting sudden decelaration caused by back emf braking to spot the difference between a slow-down (which is more gradual) and a (rapid) stop.

 

I use a hall effect switch, it gives a much cleaner logic-level signal than the 'ratiometric' sensors.

 

I waved some details of this and a DRO at an earlier editor of ME, but he felt it was outside the publication's range, due to the pcb issues and the need to have someone programme the micros (I would happily have done this for little more than cost).

 

These days the PCB design as a suitable file to print out on transparency to make PCBs or (even on paper for the iron on method) could be posted here, as could code for programming micros. I would guess that AVR (or even PIC) based projects would be within the ability of 50% of MEW/ME readers - but I doubt that proportion of us could build a gold medal winner

 

Neil