finding transfer function for DC geared motor

[faisaldin]

I have a DC Geared Motor.Its just a motor without any related specifications (no Data Sheet) and i dont know its model number. now i have to find out its transfer function how can it be carried out??????????????

 

[Skogsgurra]

Of course it can be done. But I doubt if it is necessary (if not for school work).

It depends a lot what size the motor is. If it is a small one, say a few tens of watts, then a simple power stage fed from a signal generator can be used. If your transfer function includes shaft speed or position, you also need a tacho generator or an encoder.

For larger motors, talking kW and tens or hundreds of them, you need the corresponding drive - and a grid to supply it. Do you have that?

You will probably also need a current transducer and, perhaps, an isolation amplifier for the armature voltage.

Then connect signals to either a DNA, a recorder or a scope and make a measurement across the frequency range of interest. Using a Bode plot is usually the easiest way to get the different gains and break frequencies out of the measurements.

Your question is somewhat surprising. If you have the actual motor, the nameplate should also be available. Is there no manufacturer, no model number, no data available on the nameplate?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[faisaldin]

Its a small DC geared motor and surewly a small power stage is required. The problem is that i have no model no. no manufacturers plate attached to it. Now how can i find its transfer function for using it in position control system.
Ofcourse Bode plots is a useful tool for finding transfer function but remember that its a DC motor how can i apply different frequency signals to find corner frequencies and gain.

 

[Skogsgurra]

You just modulate the power source with different frequencies and check the response. Not much to it really. It is being done in schools in servo courses and not complicated at all.

Another question is: Do you really need to know the transfer function for this specific motor? With the limited experience you seem to have, a cut and try approach works just as well.

Still another question: If this is for a procuct, how comes that you cannot get data for your motor? The manufacturer should be able to provide you with all the data you need.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[sreid]

1)Measure the winding Resistance and Inductance. This gives you the L/R time constant (the fast electrical time constant, the second pole).

Put an electric Tach on the ouput shaft. Hit the motor with some voltage. Where the RPM hits 63% of the final RPM, that's (approximately) the Mechanical Time Constant ( the low frequency pole). Approximately because the acceleration time is affected by the friction in the gearbox. Measure the shaft friction and you can do some math and improve the low frequency pole estimate.