Building a Cheap Tachometer

We need more informaton. What kind/size/speed is the motor? What kind of power does it run on? At this point, I can only offer the advice that Don Lancaster would probably give you......use a PIC (and don't try to get rich by writing books). ;-)

not really enough info, but you could start here

martin

Hi, A small DC motor maybe, i don't know (i don't decide it yet) but would not be very big sized motor. Is there any circuit that would generally be able to measure motor speed? If not, what really caused that? Is there any special attention i must deal regarding each type/size/speed?

This is the background\\purpose of this tachometer :

1. I was developing a motor controller, so i need a speed sensor to control my motor
2. The motor would be a small DC motor, to be easy to control Any suggestion about the motor itself?

A cheap frequency to voltage converter would be nice. That means i must measure the frequency change according to Doppler? What kind of signal that must be transmitted? What if I can't have LM2907, quite hard to find things like that in my country. Is there any alternative. Or could i made frequency to voltage my own?

I really appreciate your posts. Looking forward to hear more.

Lukman

How small? All CPU fans (three wires) have pulse sensors. Can you adapt one of those?

If you need to control it anyway, why not a micro to count the pulse sensor.

Some Wild Ideas: Pimp up an optical mouse? Hook up the motor to a potentiometer. Measure delta ohms/sec. Put a magnet on the shaft and use a hall sensor connected to a timer. Connect the motor to a dynamo. Measure Vout to measure rev/sec. Drill a hole through the shaft. ... Beam a laser through the hole..Measure laser light interruption rate. Put a mirror on the shaft.. Beam a laser at the mirror .Measure laser light interruption rate. Go to scrap yard...Get old tach from an old car for $5.00. Go to automotive store..get new tach. D from BC

There are many, but you need to specify how. For example, RPM can be determined optically, mechanically, electrically even acoustically. Allot depends upon your accuracy requirements, the speed of the motor and how often you need updates. You could count pulses (magnetic, optical, etc.) over a defined time interval, or measure the time between pulses. What works best depends upon your application.

Just the obvious things like hanging a magnet on the shaft of a pager motor probably won't work well etc... ;-)

Is this for something like a robot where measuring the wheel speed/rotations might be easier/better? Can you be more specific about your end goals?

You probably shouldn't mix responses from different people into one reply like this. :-)

You could hack an old CD player, many of the motors have tach pulse outputs.

I am developing a motor controller, to have the optimum response of a motor. Like a PID controller, i will use the motor speed as an indicator of it's response and compare it to a desired response. A descent accuracy would be enough, what i need is that the sampling rate is high enough to catch enough information.

Hi D, from your wild idea i think i would like to try using one of your idea. Optical mouse, and dynamo seems interesting. I will search for infomation on that subject, or if you have some can you give me a pointer?

This would be interesting. I'll try to dig more information on that. Thanks

I think the measurement should be done independently. The computation in the micro is quite heavy.

I think i have an old CD player in my house. I will look for it, i will be a good and cheap alternative.

I like the wild ideas, i never even think of that (mouse, CPU fan, CD player, etc). Great replies. I really appreciate the all of the response and attention. Thanks a lot, it really helps me. Hope to be able to finish my project with this.

Regards,

Lukman

decide first. techniquies that are applicable to brushless DC motors aren't applicable to other types.

it's basically a frequency counter calibrated in per minute (or per minnute/100) instead of per second (if for display)

that depends what you want to do with it, the speed sensors on VCR and floppy drive motors generate several pulses per rotation to allow closer control of the speed

if you don't need great precision that's one way to go.

getting repeatable accurate results from a design using discrete parts reqwuires more skill (more than I have), how much precision do you need?

the two wire fans have them too, they are just missing the third wire (the sensor is needed to switch the coils in the motor)

Bye. Jasen

Put some numbers on those goals, make them measurable: how much is "enough".

Bye. Jasen

A resolver or a quadrature encoder with a large number of pulses per revolution will allow you the possibilty of getting very close to optimum response, but probably not "cheap".

Best regards, Spehro Pefhany

You may also find that getting 1/RPM is an easier thing to measure. If you only need to run over a narrowish range of speeds, this may be a better feedback in your servo. Unlike some of the other measures, you don't have to wait for several rotations to get it.

Once the motor is turning, all you need to do is count how many clock cycles go by as the shaft makes one turn. The trick to not waiting too long when the motor is stopped is to only wait long enough to create the full error signal if the motor is slower than desired.

You can also make an analog ramp that you capture when the shaft completes the rotation. A DAC controls the slope of the ramp. The ADC grabs the voltage when the shaft makes a full turn.

Look up a "Digital pot" if you want a lower cost encoder.

If the motor is turning a gear, you could place a hall device or other thing so that it picks up the teeth going by.

Your attitude could be better, just direct the slob to s.e.b where it belongs. Better yet, take the whole "conversation" there.