OT - DC Motor as Tacho - smoothing - what capacitor

Somewhat off topic but wondered if anyone could give me a place to start. Last year a customer wanted a very rough and ready and cheap speed indication for a machine and they needed it quick. I had a permanent magnet DC motor available which I used as a tacho, and fed the output to a Lascar Panel Pilot M set up as a voltmeter with graph:

http://www.amazon.co.uk/gp/product/B00AUCGD2W?psc=1&redirect=true&ref_=oh_aui_detailpage_o03_s00

http://www.panelpilot.com/panelpilotm

It is working remarkably well but they are getting an occasional speed glitch and want to add smoothing, which I am guessing would be a capacitor and maybe a resistor. Can anybody give me a clue what size and type of capacitor to use? The motor can run forward or reverse so I guess that electrolytic is no good.

Really need to know the ripple frequency of the signal.
But, a variable resistor should help by trial and error.
The cutoff frequency of a low pass filter is 1/(2*pi*R*C)
For 1uF and 1k ohm = 160Hz
For 1uF and 10k ohm = 16Hz
Anything far above these frequencies will be blocked, but anything close you will still see.
A 10k variable resistor will give you a range of 16Hz @ 10k to 160kHz @ 1ohm. Note in the last 10% (from 1000 ohms to 1 ohm) the cutoff frequency is greatly affected (160Hz @ 1000ohms to 160kHz @ 1ohm). This may help you find the R value that works.

If your ouput voltage is within 100V use this 1uF 100V Bipolar electrolytic capacitor http://www.jaycar.com.au/Sight-&-So...100V-Non-Polarised-Electrolytic-Type/p/RY6901

BryanG said:

It is working remarkably well

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for how many month ?

osmanmom said:

for how many month ?

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BryanG said:

Last year a customer

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at least 11

Hi HJTRBO

Thanks for the quick reply.

If your output voltage is within 100V

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Motor is 24VDC at its maximum speed, so we are well within the 100V

Bipolar electrolytic capacitor

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Hadn't heard of bipolar electrolytic capacitors, learn something new everyday.

Really need to know the ripple frequency of the signal.
But, a variable resistor should help by trial and error.
The cutoff frequency of a low pass filter is 1/(2*pi*R*C)
For 1uF and 1k ohm = 160Hz
For 1uF and 10k ohm = 16Hz

Click to expand...

More like 1/2 or 1/4 Hz. I will probably send them a few capacitors and they can stick them in parallel till they get the smoothing they require. Can they just stick them across the motor/tacho leads. Normally I know you have RC circuits but the motor will have resistance anyway.

for how many month ?

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I know that this isn't high tech engineering. I would have liked a PLC, HMI and an encoder to monitor and control the machine properly, but instead they have a guy watching the output from the tacho and adjusting the speed of the machine. So long as it is near enough, it is good enough. The machine is run off car batteries and lives underground.

For 1/2Hz I would try 1uF and 10M ohm. This gives you a filter time constant of 10 seconds (Time for output voltage to rise to 63.2% of input voltage). If that's too sluggish lower the R value.
1uF and 100k = 0.1s
1uF and 1M = 1s
1uF and 10M = 10s
1uF and 100M = 100s

Edit: I made a quick spreadsheet for you to have a play with.

maybe something similar with this,

www.autometer.com/media/2650-1549.pdf

Brilliant stuff, thank you.

The noise is from the brushes. There isn't a frequency or time constant. Put a 1uF ceramic cap directly across the motor leads. Capacitor link.
More info.