DC motor


I’ve purchased the 1093 Micro DC motor.


I am trying to model a DC motor system for PID control of the motor’s speed, and in order to do that I need the following parameters:

  1. (J) moment of inertia of the rotor
    2 .(b) motor viscous friction constant
  2. (Ke) electromotive force constant
    4.(Kt) motor torque constant
  3. ® electric resistance
  4. (L) electric inductance

Can I assume that the electric resistance is 6V/120m- the specs for a free run?
Is the moment of inertia 9 oz-in(0.6 kg-cm) ?

Thank you,

No to both. You have to measure the moment of inertia of the rotor, which is a physical quantity unrelated to the motor torque. The electrical resistance is approximately equal to the input voltage divided by the stall current at that voltage.

Thanks for the reply Jim but what about the other
parameters. Any ideas?

They all need to be measured. Otherwise, the modelling will produce useless numbers.

If you are interested in determining the PID constants for a given situation, there are standard approaches to do this empirically. Google “PID tuning” to learn more.

I understand that I need to measure the parameters.

My question is to the Pololu employees, maybe one of them already measured them.
The DC motor I chose is the same as the one used in the 3pi robot.

I assume that when they designed the PID program they measured these parameters.


Hello, Alex.

We have not measured those parameters for the 30:1 HP micro metal gearmotors; however, you can approximate numbers 3, 4, and 5 using the information listed in the “Specifications tab” of the gearmotor’s product page.

  1. The electromotive force constant (Ke) can be approximated by dividing the free-run speed (at the rated voltage) by the rated voltage (or vice versa, depending on how your constant Ke is defined).
  2. To approximate the motor torque constant (Kt), you can divide the stall torque by the stall current.
  3. As Jim Remington said, the electrical resistance ® of the motor can be approximated by dividing the rated voltage by the stall current (at the rated voltage).

The rest of the motor parameters you ask for will have to be measured. The PID control for the 3pi robot was adjusted using empirical tuning, not a simulation. Also, please note that the 3pi uses the low power variant of the 30:1 micro metal gearmotors, not the high power ones you are asking about.


  1. Can you please elaborate on the empirical tuning you had used in the PID control of the 3pi? More specifically, how do you tune the PID control?

  2. I am trying to build a line following robot. If I use the same motors (low power) you had used in the 3pi and the same gains for the PID control, would I be able to control the speed the same way you did?

Why not spend a bit of time with your friend Google? There are lots of tutorials on PID tuning. Every robot is different, so what works for the 3pi most likely won’t work well for your robot.

As Jim pointed out, you can find many tutorials for PID tuning by doing a Google search. There are also several good tips and suggestions for PID tuning here in our forum. The second half of this post on PID tuning also has some information that might help you get started.

To answer your second question, if you have a similar configuration and use the same motors as the 3pi, your PID constants might be similar; however, as Jim said, every robot is different.