1. Home
  2. Electronics Design
  3. RPM Meter

RPM Meter

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.

Reply to
Dookie
Loading thread data ...

PIC? Shouldn't you use something like a 555 and an analogue meter?

Reply to
David Eather

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

Reply to
Martin Riddle

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

Reply to
John Larkin

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
Reply to
Jim Thompson

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
Reply to
Jim Thompson

Put the coil closer to the magnet. A cheap tachometer chip has been around forever for that application.

formatting link

Reply to
bw

You mean the type of person who slags people off in every 2nd post? Yeah, you're right. They're pretty scum like.

Reply to
markp

On a sunny day (Tue, 27 Jul 2010 18:30:29 -0700 (PDT)) it happened Dookie wrote in :

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

Reply to
Jan Panteltje

On a sunny day (Wed, 28 Jul 2010 12:08:10 +1000) it happened David Eather wrote in :

NO

Reply to
Jan Panteltje

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.

I think opamps are easy, dual 8 pin, there's an 8pin amp + comp?

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?

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?

Real easy is a brushless motor with tacho output :^)

That too :) Lotsa ways to skin this cat.

Grant.

Reply to
Grant

what about use a hall switch sensor?

bye delo

Reply to
delo

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.

Reply to
markp

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.
Reply to
Jim Thompson

I rest my case.

Reply to
markp

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.

formatting link

formatting link

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

Reply to
John Larkin

He spends so much time obsessing about me, I wonder if he gets any work done. This is very weird.

John

Reply to
John Larkin

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.

Reply to
George Herold

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

formatting link
| 1962 | SED Has Crumbled to Below SEB Status Populated Only by Bloviators and Pompous PhD's

Reply to
Jim Thompson

On a sunny day (Wed, 28 Jul 2010 11:09:17 GMT) it happened Jan Panteltje wrote in :

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

Reply to
Jan Panteltje

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.