RPM Meter
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From: Dookie on 27 Jul 2010 21:30 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.
From: David Eather on 27 Jul 2010 22:08 On 28/07/2010 11:30 AM, Dookie 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. > > PIC? Shouldn't you use something like a 555 and an analogue meter?
From: Martin Riddle on 27 Jul 2010 22:09 "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. > > 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. > > Muffin fan with one blade painted white, Use a photo detector to catch the pulses. I used a cheap cds cell in the past. Cheers
From: John Larkin on 27 Jul 2010 23:07 On Tue, 27 Jul 2010 18:30:29 -0700 (PDT), Dookie <abstract.dissonance(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
From: Jim Thompson on 27 Jul 2010 23:12
On Tue, 27 Jul 2010 20:07:25 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Tue, 27 Jul 2010 18:30:29 -0700 (PDT), Dookie ><abstract.dissonance(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 > I would comment, but I'll not. Bwahahahaha! ...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 |