0
\$\begingroup\$

I think I'm missing something on the PID theory.

I have a motor rotating at X rpm, controlled by a pwm output (range 1 - 1000). Let's say it rotates at 20 rpm when pwm is halfway (500).

I need to control the PWM to keep the motor rotating at a set speed.

How should I setup the PID?

Please tell me where I'm wrong (assuming the motor rotates at 18 rpm instead of 20):

error = setpoint - current_value = 20 - 18 = 2

Only by considering the proportinal part, for sake of simplicity, in this case the output should be:

out = Kp * error = Kp * 2

in order to even move the motor, I should use a Kp of about 250... In my understanding the K values should be very little ones..

thanks

\$\endgroup\$
4
  • 1
    \$\begingroup\$ one of the mistakes is that output is summation .. it does (increase or decrease) based on the error * kp ... but you consider it here as (output = error * kp) directly \$\endgroup\$
    – Hasan alattar
    Oct 21, 2020 at 11:43
  • \$\begingroup\$ "in order to even move the motor, I should use a Kp of about 250" Does this mean getting the motor to start from standstill? That leads to a Kp that is far too large once the motor is running. You probably need a much smaller Kp in operation, and increase it when the motor stops : a classic problem controlling a nonlinear "plant" with a linear controller. With the best value of Kp there will still be some finite error; that's what the I term is for. \$\endgroup\$
    – user_1818839
    Oct 21, 2020 at 12:01
  • \$\begingroup\$ @Hasanalattar now I get it. thank you, really useful. \$\endgroup\$
    – Val
    Oct 21, 2020 at 12:06
  • \$\begingroup\$ @BrianDrummond you are right, but I don't go from 0 to set speed. I first have a linear ramp up, then the PID kicks in \$\endgroup\$
    – Val
    Oct 21, 2020 at 12:07

0

Reset to default

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.