Servo motors and robotics

Introduction to Servos :

The servo motor is a specialized motor that may be commanded to turn to a specific position. The servo motor consists of a DC motor, a gear reduction unit, a shaft position sensor, and an electronic circuit that controls the motor’s operation. The next figure shows a typical servo motor used or RC cars or small mobile robots.

closed-loop servos :

Unmodified servos are closed loop in the sense that there is a feedback mechanism that allows the servo to move to a specific angle. Servo motors use a standard three-wire interface consisting of a power, ground, and control line. The power supply is typically 5-6 volts. The control line expects a periodic control pulse (pulse train) no faster than 20 milliseconds. This means that you should not send pulses faster than 20 ms. Pulses slower than 20 ms will work, but you will not get the optimal response out of the servo.


Pulses that are far too slow may cause your robot to seem awkward and jerky. Rotation position of the servo depends on the pulse with (time the pulse is high or near +5V). This is called Pulse Coded Modulation (PCM) or Servo Pulse Width Modulation (Servo PWM). The following figure shows how the closed-loop servos work.

Depending on the manufacturer, the range of the pulse width to drive the servo varies. In general, typical values are in the ranges: 0.9 ms to 1.5 ms to move to the left, 1.5 ms in the center, 1.5 ms to 2.1 ms to move to the right.

open-loop servos :


Modified servos are modified so that they can turn freely forever. There is no way of determining how far they have traveled, or making them only to turn to a certain angle since the feedback mechanism has been removed.
One main advantage of this type of servos is that it can be used to drive a robot wheel continuously while controlling both speed and direction. Open-loop servos are similar to closed-loop in that they turn one direction or the other depending on the pulse that are given.
Open-loop servos expect a periodic control pulse of 20 ms just as close-loop servos. The main difference is that the feedback mechanism stopping the servo from “moving to far” has been removed
The speed of the servo depends on how much more or how much less the pulse with is compared to a “STOP” pulse. Here the STOP pulse stops the servo from moving rather that moving to the CENTER position. By varying the pulse widths, you can vary the speed. The following figure shows how the open-loop servos work.


If a servo is driving the wheel of a mobile robot, you can measure the rotational speed or get the manufacturer rated rotational speed. Using this rotational speed and the diameter of the wheel, you can calculate the distance the robot travels
Distance S = π ω D
where
S is the distance traveled in one minute (cm)
D is diameter of the wheel (cm)
ω is the angular velocity of the servo measured in Revolutions Per Minute (RPM)

All the best ..!