Inside a Servo

There are different parts in a servo which together makes this worderful device.

Spline: This is an interface between servo and the external world. Generally mechanical levers known as “servo horns” or “servo arms” are connected to spline (a.k.a drive shaft or output shaft).

Motor: Motor inside a servo is a normally a high speed three pole ferrite DC motor. Few high speed servos use five poles coreless motors (rotor constructed without an iron core) and heavy duty servos use heavy duty coreless motors.

Gearset: Gearset in a servo is connected to the motor which reduces speed and increases torque of connected drive shaft. Torque and speed are two important characteristics while selecting a servo. Torque is the maximum power a servo can produce, and speed is the time it takes to move the output shaft from one angle to another. This time taken is known as transit time of servo and usually measured per 60°; i.e. the transit time is calculated on the amount of time a servo takes to move output shaft 60° from existing position.

Servo gears are made of different materials and must be selected depending on the cost, durability, and wear resistance.

  1. Nylon (generally referred as plastic) gears: Most hobby servos use nylon gears for Gearset. They are inexpensive, light, and run smoothly with almost no wear factor. But on the negative side, they are weak. Add a little weight to your robot and you hear a crackling sound of servo gears… they just break.

  2. Metal gears: As the name says, these are metal gears. They boast tremendous strength and can sustain weights. The biggest drawback is that metal gears wear quickly and also lose accuracy.

  3. Karbonite gears: These are reinforced plastic composite materials offering 5 times more strength than nylon gears and better wear resistance. Servos with Karbonite gears are expensive, but are here to stay due to their strength, durability and wear resistance.

Control Circuit (or control board): This is the controller circuit which controls the operation of all the mechanics and electronics inside a servo. It can also be referred as the “brain” of a Servo. Controller circuit gets input from user and controls a DC motor such that the motor shaft will move to a certain position.

This control board also has an H-bridge circuitry to control the direction of rotation of motor shaft.

Potentiometer (a.k.a pot or variable resistor): In most servos, the shaft of pot is connected to drive shaft of servo. Every time the servo shaft moves, so does the pot shaft. There are few indirect drive servos where output shaft is not directly dependant on the pot, but on a bush or bearing. Most small (mini and micro) servos are direct drive servos due to space constraint.

Normally, pot is the first thing to fail. If you ever want to throw away a non-working servo, consider replacing the pot, or at least modify the servo for full rotation and replace pot with resistors to get it back to life.

Connecting wires: Any typical servo has three wires coming out. Two wires are connected to power source and the third wire is for signal input. The wiring scheme and wire color depends on manufactures and in most cases are not the same.

For controlling a servo, most manufacturers use the same closed loop design. Servos are controlled by sending signals (pulses) via the signal wire connected to control circuit. The signal gives a position to servo to rotate to, and the motor (and the gear system) starts rotating. When the pot connected to a gear system rotates, changes its resistance which is monitored by the control board. Once resistance in pot reaches an appropriate value, control circuit concludes that the position of the servo shaft is correct and motor stops running.

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