RPM Meter
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From: Jim Thompson on 28 Jul 2010 00:04 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. > If your E-mail address is valid I will E-mail you some suggestions. 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
From: bw on 28 Jul 2010 02:08 "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. 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
From: markp on 28 Jul 2010 07:06 > > 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.
From: Jan Panteltje on 28 Jul 2010 07:09 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 |_______________|
From: Jan Panteltje on 28 Jul 2010 07:09
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 |