# PMDC Motor Speed Measurement

Discussion in 'General Electronics Chat' started by viju, Apr 12, 2009.

1. ### viju Thread Starter Senior Member

Sep 28, 2008
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I wanted to measure the speed of a PMDC motor by reading the current ripple.I surfed through the internet and got this circuit.The drop aross the shunt is fed to the low pass filter and amplifier. The current ripple is filtered and amplified at x10 gain.This circuit is for 250 ~ 300 HZ( This particular motor has 12 pulses / revolution and at 1000 rpm current ripple frequency is 200hZ).Then the filtered output is converted into square pulses which can be read as rpm.

How can I convert this circuit to suit my appplication which has 7 pulses per revolution and a frequency of 850Hz?

I request the members help.

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2. ### beenthere Retired Moderator

Apr 20, 2004
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How do you obtain the 200 Hz figure? How can the current pulses be at a lower frequency than the actual motor RPM? If there are 12 current pulses per revolution, shouldn't that be 12,000 Hz?

As far as the input filtering goes, adjusting the RC times to suit the new frequency should be enough. If those frequencies are really accurate, then the capacitors need to be reduced in capacity by the ratio of 200/850.

3. ### thatoneguy AAC Fanatic!

Feb 19, 2009
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1,000 rpm x 12 ppr = 12,000 ppm. Divide by 60 for pulses per second/Hz = 200Hz

Apr 20, 2004
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5. ### viju Thread Starter Senior Member

Sep 28, 2008
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The circuit which I got from the internet says multi pole low pass filter.Which part of circuit plays as filter? Or its just a simple RC filter at the input?Pls suggest me which values to be changed?

6. ### thatoneguy AAC Fanatic!

Feb 19, 2009
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A multiple pole filter usually refers to > 12dB per decade or octave, which requires a combination of inductors, capacitors, and resistors.

One example is a Butterworth Filter: Butterworth Filter Calculator

ETA: If using PWM speed control, here is a Microchip App Note For RPM measurement with formulas at the end.

7. ### viju Thread Starter Senior Member

Sep 28, 2008
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No ,I am not using PWM inverter.It's just a regulated power supply fed to the motor.Could someone please explain how the cutoff frequency is calculated in the filter circuit above?

8. ### viju Thread Starter Senior Member

Sep 28, 2008
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I have redrawn( only the input section) the earlier attached ciruit. Is it the lowpass filter connected in a cascading fashion? If so what will be the resultant cut off frequency? Pls suggest me.

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9. ### thatoneguy AAC Fanatic!

Feb 19, 2009
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You would want to have the corner frequency of the lowpass filter around 500Hz, or slightly over the maximum anticipated RPM.

Can you post the schematic as a PNG image attachment?

10. ### viju Thread Starter Senior Member

Sep 28, 2008
120
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Area of interest is encircled.Idea behind in building my circuit is to measure PMDC motor's speed. Rightnow I am measuring the speed ( which is around 6500 rpm) through optic sensor.Sensor input is connected to high speed input of PLC.I came across a circuit which I have attached. I need to extract the pulses from the shunt connected to the motor.My rpm could be around 6500 and pulse frequency would be around 850 Hz.I am confused only at the low pass filter circuit and feeding it to gain amplifier.

Kindly suggest me members.

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Sep 28, 2008
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12. ### viju Thread Starter Senior Member

Sep 28, 2008
120
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What happens if two low pass filters are connected cascadically as in the circuit I attached earlier?Could someone guide me please.

13. ### thatoneguy AAC Fanatic!

Feb 19, 2009
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Sorry, this didn't show up on "New Posts" until today...

If the 2nd "pole" of the filter is designed correctly, which, from a glance, yours is, makes the cutoff much sharper.

e.g. Roughly speaking: If the cutoff is 200Hz, the signal would be -12dB at 400Hz with only one filter, and -24dB at 400Hz with both filters. Actually, the attenuation will be greater using the active filters you've shown.

Instead of the Bode Plot appearing as ¯\, it would look more like ¯|

14. ### viju Thread Starter Senior Member

Sep 28, 2008
120
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Thankyou for your suggestion.Let me try on this to extract pulses from the shunt connected to the motor.

15. ### viju Thread Starter Senior Member

Sep 28, 2008
120
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I have just built the circuit and tired with my software. It works fine gives a squarewave output as desired.Votage levels are just for checking the scheme.Also I tired with different frequency combinations.I designed the low pass filter at 1.2KZ.

Will this work in real application? Have I selected the right components?

Guide me.

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16. ### viju Thread Starter Senior Member

Sep 28, 2008
120
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I have not got result with the circuits.Could somebody help me by suggesting the way to measure the rpm of a PMDC motor through the current pulses?

By theory I hope it is possible. SHUNT-->LOW PASS FILTER-->AMPLIFIER-->SQUAREWAVE GENERATOR-->SYSTEM TO READ PULSES.