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Handy method of measuring RPM (revolutions per minute)

2002/11/08

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After sourcing alternative gearhead motors for my first robot, I needed to confirm to myself that the manufacturers stated specifications were correct. Apart from learning from the testing process, it also reassured me that the selected motors were suitable alternatives.

In the robotics book I was reading, the author wrote, "Professional handheld laser tachometers can be purchased for as low a price as $100" (US$). He also showed a photo of a tachometer he had built with an MCU (microcontroller unit) and an LCD display etc.

I decided to look for an alternative, cheaper and, at this early stage in my robotics/electronics hobby, less challenging method of measuring RPM!

After searching the net and considering buying a tachometer kit from one site, I finally came across this very useful webpage Spindle and Surface Speed Measurement by Tony Jeffree of Manchester, England.

I followed the section entitled "A Simple Digital RPM Sensor" with a few minor changes.

I used the following items/parts to measure the RPM of my gearhead motor:

Fig.2 Photo taken w/o flash
Fig.1 Measuring RPM

These photos show the handy method of RPM measurement in 'action' ;-)

In Fig.1 I used my camera's flash so it appears that the encoding wheel isn't spinning! Without using the flash (Fig.2) makes the spinning more obvious :)

You could build the circuit on a solderless breadboard to reduce the 'spaghetti effect' but it's still a good idea to power the motor with a separate battery to obtain a more accurate measurement.

Fig.3 Amended RPM circuit

The differences between my Fig.3 circuit and the "Figure 5: Digital RPM Sensor Circuit" on Tony's webpage are:

  1. I didn't have a photoreflector (chip combining an infrared LED and a phototransistor) so I used a white (visible light) LED D1 and a photoresistor R3 instead
  2. Since I used a white LED, I omitted Tony's red power-on LED and it's resistor
  3. I also didn't use the DIN plug connectors. Instead I used jumper leads to connect my circuit directly to the battery and my multimeter's test leads

After sticking the paper encoding wheel to the top of the gearhead motor's shaft with double-sided sticky tape, I switched on my multimeter in frequency mode and connected one battery to the circuit and the other to the motor.

I then positioned the white LED and photoresistor so that they were just above the spinning encoding wheel. This resulted in the multimeter displaying a reading which fluctuated between 14 & 15 Hz.

A frequency reading was gained due to the photoresistor varying the voltage, as the light it detected from the white LED reflected unequally off of the alternating black and white coloured segments on the spinning encoding wheel!

Hertz is a measurement of frequency, in per second cycles.

To calculate the RPM of the gearhead motor I first multiplied 14Hz by 60 (for minutes) then divided by 8 (there are eight black and white segment 'cycles' on the 16 segment encoding disk). I did the same for 15Hz, which then gave me a range of 105-112.5 RPM.

The manufacturer's specification stated that the motor had a speed of 72 RPM when operated at 6V. That equates to 108 RPM at 9V (72 divided by 6 then multiplied by 9).

So there you have it! As long as you already have a multimeter which can measure frequency (which is quite likely) and have or can salvage a few components, you're laughing :-)

Finally, three factors worth considering when you want to measure RPM are, (1.) the Hertz range of your multimeter, (2.) the approximate RPM of the item you want to measure and (3.) the battery voltage.

  1. According to the manufacturer of my multimeter, it can measure frequencies as low as 10Hz. Therefore, using a half black, half white (1 cycle) encoding wheel, the lowest speed I can measure is 600 RPM. An encoding wheel with 12 segments (6 cycles) is the lowest number of segments I could have used to measure my gearhead motor's RPM, with my multimeter.
  2. The relatively slow responsiveness of a photoresistor preclude it from measuring high RPMs. Therefore it's preferable to use an infrared LED and a phototransistor, or a photoreflector as was used by Tony.
  3. Measure the voltage of the battery before connecting it to the motor so that you can correctly calculate the motor's RPM.

I hope you find this RPM measuring method as useful as I did,

KD



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