# Can I get some motor parameter for item #4758

Hello, I’m Kim from Korea.

I want researching to control system with Kalman Filter using dc motor.
So I chose the Pololu DC motor(Pololu - 10:1 Metal Gearmotor 37Dx65L mm 12V with 64 CPR Encoder (Helical Pinion)) for my research.
Next, I designed the control system with PID and it worked just as I wanted it to.

However, the Kalman filter did not work when the estimation model of the motor was used.

I searched and found information about resistance, coefficient of friction B, etc. (like this: Calculating motor parameters?), but I couldn’t find how to calculate or measure inductance L.

Q. May I know the inductance L value of the motor? Or is there a way to measure without an lcr meter?

Hello, Kim.

The inductance of our 12V 37D gearmotors is about 2.3 mH.

- Patrick

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Dear PatrickM.

I’m sorry, but I have more question.

Now, I’m trying to calculation for moment of inertia. So, I tried to calculate the moment of inertia of the cylinder (shaft) using the formula J = 1/4 * M * D^2 [kg*m^2].
where M is the weight of the cylinder and D is the diameter.

1. Is using these formulas the right approach?
2. If the approach is correct, is the 22mm shaft on my motor correct?
3. Lastly, do I have to disassemble the gearbox to find out the weight of the shaft?

The formula you suggested is the approach I would recommend, though keep in mind that this will only give you a low-side approximation of the moment of inertia since just calculating the moment of inertia for the output shaft will not account for the inertia of the gears, the motor armature, or the extended shaft with the encoder disc. If you want to try to get a better estimate, you might try carefully removing the gearbox and the gearbox cover so you could measure the thickness and diameter of the gears with calipers.

The output shaft actually extends well into the gearbox before it changes shape, so a better value for the length would be 14mm plus the gearbox length given in the motor’s dimension diagram. So, for the 10:1 gearmotor, where the gearbox length is 20mm, the length of the output shaft is about 34mm.

Instead of trying to remove the output shaft and weigh it, I suggest estimating its mass by calculating its volume and using the density of stainless steel. You could use the same approach with the gears if you decide to try adding those to your estimate.

- Patrick

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