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Servo refers to an error sensing feedback control which is used to correct the performance of a system. Servo or RC Servo Motors are DC motors equipped with a servo mechanism for precise control of angular position. The RC servo motors usually have a rotation limit from 90° to 180°. Some servos also have rotation limit of 360° or more. But servos do not rotate continually. Their rotation is restricted in between the fixed angles.
Where are Servos used?
The Servos are used for precision positioning. They are used in robotic arms and legs, sensor scanners and in RC toys like RC helicopter, airplanes and cars.
Servo Motor manufacturers
There are four major manufacturers of servo motors: Futaba, Hitec, Airtronics and JR radios. Futaba and Hitec servos have nowadays dominated the market. Their servos are same except some interfacing differences like the wire colors, connector type, spline etc.
Servo Motor wiring and plugs
The Servo Motors come with three wires or leads. Two of these wires are to provide ground and positive supply to the servo DC motor. The third wire is for the control signal. These wires of a servo motor are color coded. The red wire is the DC supply lead and must be connected to a DC voltage supply in the range of 4.8 V to 6V. The black wire is to provide ground. The color for the third wire (to provide control signal) varies for different manufacturers. It can be yellow (in case of Hitec), white (in case of Futaba), brown etc.
Futaba provides a J-type plug with an extra flange for proper connection of the servo. Hitec has an S-type connector. A Futaba connector can be used with a Hitec servo by clipping of the extra flange. Also a Hitec connector can be used with a Futaba servo just by filing off the extra width so that it fits in well.
Hitec splines have 24 teeth while Futaba splines are of 25 teeth. Therefore splines made for one servo type cannot be used with another. Spline is the place where a servo arm is connected. It is analogous to the shaft of a common DC motor.
Unlike DC motors, reversing the ground and positive supply connections does not change the direction (of rotation) of a servo. This may, in fact, damage the servo motor. That is why it is important to properly account for the order of wires in a servo motor.
Servo Control
The servo motor can be moved to a desired angular position by sending PWM (pulse width modulated) signals on the control wire. The servo understands the language of pulse position modulation. A pulse of width varying from 1 millisecond to 2 milliseconds in a repeated time frame is sent to the servo for around 50 times in a second. The width of the pulse determines the angular position.
For example, a pulse of 1 millisecond moves the servo towards 0°, while a 2 milliseconds wide pulse would take it to 180°. The pulse width for in between angular positions can be interpolated accordingly. Thus a pulse of width 1.5 milliseconds will shift the servo to 90°.
It must be noted that these values are only the approximations. The actual behavior of the servos differs based on their manufacturer.
A sequence of such pulses (50 in one second) is required to be passed to the servo to sustain a particular angular position. When the servo receives a pulse, it can retain the corresponding angular position for next 20 milliseconds. So a pulse in every 20 millisecond time frame must be fed to the servo.
Inside a Servo Motor
A servo motor mainly consists of a DC motor, gear system, a position sensor which is mostly a potentiometer, and control electronics.
The DC motor is connected with a gear mechanism which provides feedback to a position sensor which is mostly a potentiometer. From the gear box, the output of the motor is delivered via servo spline to the servo arm. The potentiometer changes position corresponding to the current position of the motor. So the change in resistance produces an equivalent change in voltage from the potentiometer. A pulse width modulated signal is fed through the control wire. The pulse width is converted into an equivalent voltage that is compared with that of signal from the potentiometer in an error amplifier.
The difference signal is amplified and provided to the DC motor. So the signal applied to the DC servo motor is a damping wave which diminishes as the desired position is attained by the motor.
When the difference between the desired position as indicated by the pulse train and current position is large, motor moves fast. When the same difference is less, the motor moves slow.
The required pulse train for controlling the servo motor can be generated by a timer IC such as 555 or a microcontroller can be programmed to generate the required waveform. Refer Servo Motor interfacing with 8051 microcontroller and Servo control using AVR ATmega16.
Power supply for Servo
The servo requires a DC supply of 4.8 V to 6 V. For a specific servo, its voltage rating is given as one of its specification by the manufacturer. The DC supply can be given through a battery or a regulator. The battery voltage must be closer to the operating voltage of the servo. This will reduce the wastage of power as thermal radiation. A switched regulator can be used as the supply for better power efficiency.
Selection of a Servo
The typical specifications of servo motors are torque, speed, weight, dimensions, motor type and bearing type. The motor type can be of 3 poles or 5 poles. The pole refers to the permanent magnets that are attached with the electromagnets. 5 pole servos are better than 3 pole motor because they provide better torque.
The servos are manufactured with different torque and speed ratings. The torque is the force applied by the motor to drive the servo arm. Speed is the measure that gives the estimate that how fast the servo attains a position. A manufacturer may compromise torque over speed or speed over torque in different models. The servos with better torque must be preferred.
The weight and dimensions are directly proportional to the torque. Obviously, the servo having more torque will also have larger dimensions and weight. The selection of a servo can be made according to the torque and speed requirements of the application. The weight and dimension may also play a vital role in optimizing the selection such as when a servo is needed for making an RC airplane or helicopter.
The website of the manufacturers can be seen to obtain details about different models of the servos. Also their product catalogue can be referred to. Some manufacturers like Futaba also provide online calculator for the selection of a servo.
Interference and Noise Signal
The PWM signal is given to the servo by the control wire. The noise or interference signals from the surrounding electronics or other servos can cause positional errors. To eliminate this problem the control signals are supplied after amplification. This will suppress the noise and interference signals.
Servo Modification for full Rotation
One may want to use the servo for his robot applications and desire to move the servo continually. This is possible by a little modification. The servo gear box has a mechanical stop which avoids the servo to make full rotation. File off the mechanical stop(s) so that the gear box is free to make a complete rotation.
But this is not the only sufficient thing. The servo works on a feedback mechanism. So the pot of the servo must be first moved to the centre position. This can be done by sending medium pulses to the servo by a microcontroller. Then fix the gears attached to the pot shaft with glue. This will keep an impression to the control electronics of the servo that the current position is the middle point. So the servo would then move with respect to the middle position and not to the current position.
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