Aryan utilizes a number of joints and links in its face to adjust its gaze direction and move its facial features. The actuator behind each of these joints is an electric motor. The most adequate type of motor for building an expressive face is hobby servo motor.
A hobby servo motor consists of a DC motor, gear train, feedback sensor and a built-in position controller, all in a compact package. Its duty is to translate positioning commands into right signals for position control task with a relatively suitable torque. Unfortunately, no hobby servo motor was available in my country. So I had to construct it from simple DC motors.
Here I explain each of the components of my home-built servomotors. DC motors rotate at high speeds and generate very low torques. So they must be used in conjunction with reduction gears to increase their torque and to decrease their speed.
Here geared motors were so expensive for my budget that I preferred to build them at home too. We connected the shaft of each motor to a worm gear, which itself was connected to a spur gear for obtaining the desired gear ratio. We built spur gears with an old gear-cutting machine.
Worm gear was chosen for its self-locking ability and high reduction ratio. Self-locking allows an idle joint to stay locked without consuming electrical energy. The disadvantage of a worm gear is its high friction that causes waste of motor's power.
In a servo system, a feedback sensor is required to measure the actual output position and generate an error signal by comparing the actual output with the desired output. Using a PI controller, weighted sum of error and its integral are fed back to the motor for error compensation.
We used a potentiometer for measuring absolute position of each motor. A potentiometer is a compact and low-cost package and does not require any initialization for position measurement purpose. Its disadvantage is its low precision. However, it sufficed for our application. Controller design will be described in the next page.
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