RPM Meter
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From: Grant on 28 Jul 2010 07:24 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? For that matter you could chain inverters as amp + schmidtt with a 4069UB or another UB (unbuffered, not 'hcxx series) hex inverter. Three inverters in a row with resistor feedback to bias linear and cap signal coupled with input resistor to define gain like a inv amp, and two in series with resistor feedback to make schmidtt, followed by last inverter to sharpen signal for feeding PIC input. Less needed than opamps, higher the supply voltage, better the gain and speed though, it's old technique. CMOS opamps do it better now? >> >> 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. Optical sensor is easy, you might add your own illumination LED (green for ambient (570nm) opto, or match IR LED to filtered IR photodiode (filtered is one in dark case) at 950nm) to increase signal? >> >> 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. Real easy is a brushless motor with tacho output :^) > >Put the coil closer to the magnet. A cheap tachometer chip has been around >forever for that application. >http://www.national.com/mpf/LM/LM2917.html#Overview > That too :) Lotsa ways to skin this cat. Grant.
From: markp on 28 Jul 2010 08:43 > On a sunny day (Tue, 27 Jul 2010 18:30:29 -0700 (PDT)) it happened Dookie > <abstract.dissonance(a)gmail.com> wrote in > <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. >> >>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 still have some LED-photodiode packages that look one way, > put a reflective strip on the flywheel. > I dunno what these things are called in Engish really, > but I am sure you can still buy those : > --------------- > | LED ---------> | > | | | > | Photo diode <--------- | reflective strip > |_______________| > > Another possibility, if you can afford to put a small paddle on the rim, is to use a slot sensor (these have an infrared emitter in one side and a receiver in the other). When the paddle passes through the slot you get a digital signal out. You could put several paddles diametrically opposed around the rim to keep it balenced. This is probably more immune to noise than reflective or magnetic sensing IMO. Mark.
From: delo on 28 Jul 2010 08:29 what about use a hall switch sensor? bye delo
From: Jim Thompson on 28 Jul 2010 10:59 On Wed, 28 Jul 2010 12:06:00 +0100, "markp" <map.nospam(a)f2s.com> wrote: >> >> I choose no longer to descend to the level of the bloviator scum who >> inhabit/dominate this group. >> >> ...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 > >You mean the type of person who slags people off in every 2nd post? Yeah, >you're right. They're pretty scum like. > Did I "slag" you? (Whatever "slag" means?) I suppose I "slag" people who insist on being technically incorrect? I've pressed you for more technical detail. And actually you ignored me when I derived some numbers myself and asked if this was what you had in mind. I have designed automotive ignition systems, anti-skid braking and speed controllers that routinely use VR pickups... most likely before you were born. There are easy and elegant (read as also cheap) solutions to the OP's problems. It would be helpful to have more detail. 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 -- | 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 | Spice is like a sports car... Performance only as good as the person behind the wheel.
From: markp on 28 Jul 2010 11:36
> > 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. |