RPM Meter
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From: John Larkin on 28 Jul 2010 11:41 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. 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. 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. John
From: John Larkin on 28 Jul 2010 11:44 On Wed, 28 Jul 2010 16:36:00 +0100, "markp" <map.nospam(a)f2s.com> wrote: >> >> John "The Bloviator" Larkin's post is a slap-it-on-paper-quick without >> thinking "solution". Use at your peril. He'll never label it with >> values. If he did, his "solution" could be checked. >> >> ...Jim Thompson > >I rest my case. > He spends so much time obsessing about me, I wonder if he gets any work done. This is very weird. John
From: George Herold on 28 Jul 2010 12:27 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. George H.
From: Jim Thompson on 28 Jul 2010 13:00 On Wed, 28 Jul 2010 16:36:00 +0100, "markp" <map.nospam(a)f2s.com> wrote: >> >> John "The Bloviator" Larkin's post is a slap-it-on-paper-quick without >> thinking "solution". Use at your peril. He'll never label it with >> values. If he did, his "solution" could be checked. >> >> ...Jim Thompson > >I rest my case. > Ask him for values that will work in your application. I rest MY case :-) ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | SED Has Crumbled to Below SEB Status Populated Only by Bloviators and Pompous PhD's
From: Jan Panteltje on 28 Jul 2010 13:30
On a sunny day (Wed, 28 Jul 2010 11:09:17 GMT) it happened Jan Panteltje <pNaonStpealmtje(a)yahoo.com> wrote in <i2p350$kid$1(a)news.albasani.net>: >>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. Or you could find an old tape recorder playback head, and run it into an integrator / level comparator as J.L. suggest. Here I use one as AC current sensor: ftp://panteltje.com/pub/play_back_head_current_sensor_img_1153.jpg |