RPM Meter
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From: John Larkin on 28 Jul 2010 22:04 On Thu, 29 Jul 2010 10:47:08 +1000, Grant <omg(a)grrr.id.au> wrote: >On Wed, 28 Jul 2010 08:41:55 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > >>On Wed, 28 Jul 2010 21:24:48 +1000, Grant <omg(a)grrr.id.au> wrote: >> >>>On Wed, 28 Jul 2010 01:08:54 -0500, "bw" <bwegher(a)hotmail.com> wrote: >>> >>>> >>>>"Dookie" <abstract.dissonance(a)gmail.com> wrote in message >>>>news:29566efc-1fcd-4f2f-b30a-d78cab3d7bc7(a)q35g2000yqn.googlegroups.com... >>>>>I have a contact/photo rpm meter but it is acting funky and would like >>>>> to create a simple meter for some testing. >>>>> >>>>> I've created the pic code and seem to have gotten it to work but need >>>>> some sensor to debug it. The pic code basically counts the time >>>>> between pulses. I do not use the ADC but rather the digital inputs and >>>>> monitor when the pin goes high. The code does have built in dead time >>>>> to prevent multiple countings. >>>>> >>>>> My idea was to stick a magnet on the device I want to measure the >>>>> rpms(flywheel) and use a coil of wire as the sensor. >>>>> >>>>> The problem is that I'm getting much lower voltage than I expected. >>>>> Faraday's law says the voltage on the coil is the number of turns >>>>> times the change in flux. The change in flux depends on the rpm's but >>>>> I'm simply using my hand to move the magnet across the coil. I get >>>>> something like a sinusoid damped(or alternatively the derivative of >>>>> the erf function). I have about 200-300 turns and the coil is getting >>>>> unwieldy. >>> >>>Aircore, or a coil on an E or I section ferrite, bunch of nails? >>>Or a magnet, that's how variable reluctance (also guitar) pickups work. >>>>> >>>>> This is fine but it most I get about 30mV(surely to be much larger >>>>> with the flywheel at full speed). What I'm thinking of doing is simply >>>>> using an opamp or bjt to act as a switch/comparator but the only >>>>> problem I see is that the noise may add extra pulses. >>> >>>I think opamps are easy, dual 8 pin, there's an 8pin amp + comp? >>>>> >>>>> Is there a simple way I can increase the noise floor, say, by >>>>> offseting the ground of the op amp and effectively setting making dc >>>>> negative relative to the opamp. I'm thinking that I could add a >>>>> resistor to the -V of the op amp. I only have a + supply. >>> >>>John L's circuit looks okay for single supply, ac amp -> comparator, >>>dual opamp, maybe CMOS, for 5V single supply to match PIC? >> >> >>My circuit is used mostly with variable-reluctance pickups, basically >>a rotating ferrous gear which passes near a spark-plug looking pickup, >>which is a small rod magnet with a coil around it. >> >>http://prototype.infolytica.com/en/coolstuff/ex0128/VR_Sensor_380.png >> >>http://www.phoenixamerica.com/Images/VR%20Grouping%281z3o73%29.jpg >> >>These give modestly distorted sine wave outputs whose amplitude and >>frequency are both roughly proportional to speed, so benefit from an >>integrator if used over a wide speed range. A real-world VR pickup >>might have output from a few millivolts to 100 volts p-p over a real >>speed range. >> >>I use a similar circuit in my 8-channel VME tach and tach-overspeed >>modules. I've sold maybe 400 of them so far, over 3200 channels, and >>I've has surprisingly few calls from users having setup difficulties. >>The integrator+comparator thing is very forgiving. > >Yes, you're only after the zero crossings, like FM radio, amplitude >doesn't matter much. >> >>If the OP's pickup makes, say, one short pulse per revolution, the >>comparator threshold might be offset to accomodate the more pulse-like >>duty cycle. If his speed range is limited, just an amp and a >>comparator may be all he needs. Or an opto-chopper, hall effect, reed >>relay, or some other switch-type gadget. > >Or use a self biasing comparator, RC on signal to get chop point >reference, it's a common circuit. >> >>A stepper motor makes a neat speed sensor, behaving just like a VR >>pickup. Tons of signal. Rip one out of an old floppy drive or some >>such. The signal conditioner might be just an R-C lowpass, diode >>clamps maybe, and a comparator, the poor man's integrator. > >But some paint + optical, or VR on existing gear teeth is much easier ;) > >A stepper might power LCD tacho display too? Yup. You can get a lot of power out of a stepper. John
From: George Herold on 28 Jul 2010 23:05 On Jul 28, 9:05 pm, "markp" <map.nos...(a)f2s.com> wrote: > "Grant" <o...(a)grrr.id.au> wrote in message > > news:rsj156hcg8q6banbsmtn29q1q6c928g09r(a)4ax.com... > > > > > > > On Wed, 28 Jul 2010 09:27:08 -0700 (PDT), George Herold > > <gher...(a)teachspin.com> wrote: > > >>On Jul 27, 9:30 pm, Dookie <abstract.dissona...(a)gmail.com> wrote: > >>> I have a contact/photo rpm meter but it is acting funky and would like > >>> to create a simple meter for some testing. > > >>> I've created the pic code and seem to have gotten it to work but need > >>> some sensor to debug it. The pic code basically counts the time > >>> between pulses. I do not use the ADC but rather the digital inputs and > >>> monitor when the pin goes high. The code does have built in dead time > >>> to prevent multiple countings. > > >>> My idea was to stick a magnet on the device I want to measure the > >>> rpms(flywheel) and use a coil of wire as the sensor. > > >>> The problem is that I'm getting much lower voltage than I expected. > >>> Faraday's law says the voltage on the coil is the number of turns > >>> times the change in flux. The change in flux depends on the rpm's but > >>> I'm simply using my hand to move the magnet across the coil. I get > >>> something like a sinusoid damped(or alternatively the derivative of > >>> the erf function). I have about 200-300 turns and the coil is getting > >>> unwieldy. > > >>> This is fine but it most I get about 30mV(surely to be much larger > >>> with the flywheel at full speed). What I'm thinking of doing is simply > >>> using an opamp or bjt to act as a switch/comparator but the only > >>> problem I see is that the noise may add extra pulses. > > >>> Is there a simple way I can increase the noise floor, say, by > >>> offseting the ground of the op amp and effectively setting making dc > >>> negative relative to the opamp. I'm thinking that I could add a > >>> resistor to the -V of the op amp. I only have a + supply. > > >>> Or is there any better way that doesn't involve to much work? It > >>> doesn't have to be completely perfect as I am taking an average for > >>> the rpm's and can through out outliers. The main thing is to get the > >>> input to the pic to act digitally. > > >>> I know there are a god awefull number of ways to do this but something > >>> simple with minimum and common parts is what I'm looking for. > > >>I made a bicycle speedometer for myself many years ago. I used a > >>magnetic reed relay.. But I couldn't find it listed at digikey and I > >>suspect I have the wrong name. The relay closed when a magnet was > >>near. Very simple if you just want a digital pulse... Perhaps someone > >>will know the correct name of this sensor. > > > Reed switch, couple bike 'computers' I've had use the same concept. > > > Grant. > > That would work for low revs, but may have difficulty at high revs due to > the mechanical nature of the contacts (they would have inertia, and would > probably get mechanically stressed after a while). I'm unsure of the OPs max > rpm requirements. > > Mark.- Hide quoted text - > > - Show quoted text - Yeah, I had to put some sort of de-bouncer on the input. This was my final design project for an instrument design course in college. (early 80's) I had some clock made from a 555 that worked OK, but drifted with temperature. I might have had some carbon composite resistor in there... not knowing any better at the time. George H.
From: Jan Panteltje on 29 Jul 2010 07:55 On a sunny day (Thu, 29 Jul 2010 09:48:04 +1000) it happened David Eather <eather(a)tpg.com.au> wrote in <OoydnecLgt0rXM3RnZ2dnUVZ_gKdnZ2d(a)giganews.com>: >On 28/07/2010 9:09 PM, Jan Panteltje wrote: >> On a sunny day (Wed, 28 Jul 2010 12:08:10 +1000) it happened David Eather >> <eather(a)tpg.com.au> wrote in<RuidnQS0OouWDNLRnZ2dnUVZ_t2dnZ2d(a)giganews.com>: >> >> >>> PIC? Shouldn't you use something like a 555 and an analogue meter? >> NO > >But a 555 is the highest example of the chip designer's art. It sure is a clever chip, and you can make stable oscillators with it with a frequency that does not depend on the supply voltage much. However, in this case, if you generate a fast duration pulse to drive an analog meter, you still need a voltage stabiliser, as you need a constant amplitude. The PIC driving a LCD however, has an internal oscillator that has an accuracy of +-2% from 0�C to 85�C, and +-5% from -40�C to +125�C, this no longer needs the regulator, other then a simple zener perhaps to make 5 or 3.3 V, and no calibration needed. Better display possibilities, added calculation possibility for other things, like long time average, possibility of setting alarms, possibility of loop control. All in one PIC chip mind you.
From: langwadt on 29 Jul 2010 12:42 On 28 Jul., 05:07, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > On Tue, 27 Jul 2010 18:30:29 -0700 (PDT), Dookie > > > > <abstract.dissona...(a)gmail.com> wrote: > >I have a contact/photo rpm meter but it is acting funky and would like > >to create a simple meter for some testing. > > >I've created the pic code and seem to have gotten it to work but need > >some sensor to debug it. The pic code basically counts the time > >between pulses. I do not use the ADC but rather the digital inputs and > >monitor when the pin goes high. The code does have built in dead time > >to prevent multiple countings. > > >My idea was to stick a magnet on the device I want to measure the > >rpms(flywheel) and use a coil of wire as the sensor. > > >The problem is that I'm getting much lower voltage than I expected. > >Faraday's law says the voltage on the coil is the number of turns > >times the change in flux. The change in flux depends on the rpm's but > >I'm simply using my hand to move the magnet across the coil. I get > >something like a sinusoid damped(or alternatively the derivative of > >the erf function). I have about 200-300 turns and the coil is getting > >unwieldy. > > >This is fine but it most I get about 30mV(surely to be much larger > >with the flywheel at full speed). What I'm thinking of doing is simply > >using an opamp or bjt to act as a switch/comparator but the only > >problem I see is that the noise may add extra pulses. > > >Is there a simple way I can increase the noise floor, say, by > >offseting the ground of the op amp and effectively setting making dc > >negative relative to the opamp. I'm thinking that I could add a > >resistor to the -V of the op amp. I only have a + supply. > > >Or is there any better way that doesn't involve to much work? It > >doesn't have to be completely perfect as I am taking an average for > >the rpm's and can through out outliers. The main thing is to get the > >input to the pic to act digitally. > > >I know there are a god awefull number of ways to do this but something > >simple with minimum and common parts is what I'm looking for. > > Your pickup is going to behave like a classic variable-reluctance > speed sensor. The signal amplitude and frequency will go up together > as the speed increases. The classic signal conditioner is an > integrator of sorts... > > ftp://jjlarkin.lmi.net/VR_integrator.JPG > > which flattens out the speed:voltage curve. > > This can be made to work nicely over a 1000:1 speed range. I use that > circuit in my VME tach modules. > > John you might not like it but I've seen this works with no changes for the crank sensor in pretty much all cars. hct14 VR_in >--10u---+--22K--+-|>o----> out | 1n | _|_ | 22K +--10K--+ | | | +---1K--+-o<|---+ | hct14 | 47u 10n _|_ _|_ -Lasse
From: John Larkin on 29 Jul 2010 13:22
On Thu, 29 Jul 2010 09:42:42 -0700 (PDT), "langwadt(a)fonz.dk" <langwadt(a)fonz.dk> wrote: >On 28 Jul., 05:07, John Larkin ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> On Tue, 27 Jul 2010 18:30:29 -0700 (PDT), Dookie >> >> >> >> <abstract.dissona...(a)gmail.com> wrote: >> >I have a contact/photo rpm meter but it is acting funky and would like >> >to create a simple meter for some testing. >> >> >I've created the pic code and seem to have gotten it to work but need >> >some sensor to debug it. The pic code basically counts the time >> >between pulses. I do not use the ADC but rather the digital inputs and >> >monitor when the pin goes high. The code does have built in dead time >> >to prevent multiple countings. >> >> >My idea was to stick a magnet on the device I want to measure the >> >rpms(flywheel) and use a coil of wire as the sensor. >> >> >The problem is that I'm getting much lower voltage than I expected. >> >Faraday's law says the voltage on the coil is the number of turns >> >times the change in flux. The change in flux depends on the rpm's but >> >I'm simply using my hand to move the magnet across the coil. I get >> >something like a sinusoid damped(or alternatively the derivative of >> >the erf function). I have about 200-300 turns and the coil is getting >> >unwieldy. >> >> >This is fine but it most I get about 30mV(surely to be much larger >> >with the flywheel at full speed). What I'm thinking of doing is simply >> >using an opamp or bjt to act as a switch/comparator but the only >> >problem I see is that the noise may add extra pulses. >> >> >Is there a simple way I can increase the noise floor, say, by >> >offseting the ground of the op amp and effectively setting making dc >> >negative relative to the opamp. I'm thinking that I could add a >> >resistor to the -V of the op amp. I only have a + supply. >> >> >Or is there any better way that doesn't involve to much work? It >> >doesn't have to be completely perfect as I am taking an average for >> >the rpm's and can through out outliers. The main thing is to get the >> >input to the pic to act digitally. >> >> >I know there are a god awefull number of ways to do this but something >> >simple with minimum and common parts is what I'm looking for. >> >> Your pickup is going to behave like a classic variable-reluctance >> speed sensor. The signal amplitude and frequency will go up together >> as the speed increases. The classic signal conditioner is an >> integrator of sorts... >> >> ftp://jjlarkin.lmi.net/VR_integrator.JPG >> >> which flattens out the speed:voltage curve. >> >> This can be made to work nicely over a 1000:1 speed range. I use that >> circuit in my VME tach modules. >> >> John > >you might not like it but I've seen this works with no changes for the >crank sensor in pretty much all cars. > > hct14 >VR_in >--10u---+--22K--+-|>o----> out > | 1n > | _|_ > | > 22K +--10K--+ > | | | > +---1K--+-o<|---+ > | hct14 | > 47u 10n > _|_ _|_ > > That is a very cool way to set the threshold bias for the upper schmitt. And add a little time dithering, if you ever need it! The 22K+1n is only a 22us time constant. How narrow do the VR pulses get at high revs? I guess it's mostly a deglitcher and doesn't actually integrate much. John |