### Author Topic: Continuous servo motor/transfer function question  (Read 2567 times)

0 Members and 1 Guest are viewing this topic.

#### Inside

• Beginner
• Posts: 1
##### Continuous servo motor/transfer function question
« on: March 20, 2012, 10:20:47 PM »
Heyo -- I've tried googling this, but I've yet to find an answer that suits me.

I have some continuous rotation servos where you can set the speed/direction using PWM. I'm trying to incorporate them into a robot arm that has closed loop feedback PID control and I'm trying to model the arm using matlab before I build the thing.

I guess the question is... does the onboard ic basically act as an H-bridge + PWM ic? That is, if the signal PWM is at 50%, then PWM to the motor is 0%? If control PWM at 100%, then motor PWM is 100% in one direction, and if control PWM is 0%, then the motor PWM is 100% in the other direction?

Basically, if the motor constants could somehow be measured then the transfer function for the motor would simply be what is found here? -- http://www.engin.umich.edu/group/ctm/examples/motor2/motor.html

#### Soeren

• Supreme Robot
• Posts: 4,672
##### Re: Continuous servo motor/transfer function question
« Reply #1 on: March 21, 2012, 06:05:19 PM »
Hi,

[...] does the onboard ic basically act as an H-bridge + PWM ic? That is, if the signal PWM is at 50%, then PWM to the motor is 0%? If control PWM at 100%, then motor PWM is 100% in one direction, and if control PWM is 0%, then the motor PWM is 100% in the other direction?
To clear up the stuff a bit... While a lot of people call servo signals PWM, it isn't. Servo signals are PDM (Pulse Duration Modulation), with a pulse duration of 1ms to 2ms and a repetition rate of around 20ms (this varies a good deal more than the pulses between different makes of servos).

PWM cannot be neither 0% or100%, as it is then DC.

If the pulse duration is 1.5ms, the output of the servos internal H-bridge is 0, but each make of servo will have different control algorithms, so you cannot model anything accurately from it.

In a theoretic servo, a pulse of 1ms will result in the max output and so will a pulse of 2ms, only with the opposite polarity.
Regards,
SÃ¸ren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Engineering is based on numbers - not adjectives

• Jr. Member
• Posts: 14