what are servo motors

7/31/2019 What Are Servo Motors

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What are Servo Motors?Servo refers to an error sensing feedback control which is used to correct theperformance of a system. Servo or RC Servo Motors are DC motors equipped with aservo mechanism for precise control of angular position. The RC servo motors usuallyhave 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 thefixed 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 manufacturersThere are four major manufacturers of servo motors: Futaba, Hitec, Airtronics and JRradios. Futaba and Hitec servos have nowadays dominated the market. Their servos

are same except some interfacing differences like the wire colors, connector type, splineetc.

Servo Motor wiring and plugsThe Servo Motors come with three wires or leads. Two of these wires are to provideground and positive supply to the servo DC motor. The third wire is for the controlsignal. These wires of a servo motor are color coded. The red wire is the DC supplylead and must be connected to a DC voltage supply in the range of 4.8 V to 6V. Theblack 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 byclipping of the extra flange. Also a Hitec connector can be used with a Futaba servo justby filing off the extra width so that it fits in well. Hitec splines have 24 teeth while Futaba splines are of 25 teeth. Therefore splinesmade for one servo type cannot be used with another. Spline is the place where a servoarm is connected. It is analogous to the shaft of a common DC motor.

Unlike DC motors, reversing the ground and positive supply connections does notchange 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.


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Servo ControlThe servo motor can be moved to a desired angular position by sending PWM (pulsewidth modulated) signals on the control wire. The servo understands the language ofpulse position modulation. A pulse of width varying from 1 millisecond to 2 millisecondsin 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 2milliseconds wide pulse would take it to 180. The pulse width for in between angularpositions can be interpolated accordingly. Thus a pulse of width 1.5 milliseconds willshift the servo to 90.It must be noted that these values are only the approximations. The actual behavior ofthe 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 thecorresponding angular position for next 20 milliseconds. So a pulse in every 20millisecond time frame must be fed to the servo

The DC motor is connected with a gear mechanism which provides feedback to aposition sensor which is mostly a potentiometer. From the gear box, the output of themotor is delivered via servo spline to the servo arm. The potentiometer changesposition corresponding to the current position of the motor. So the change in resistance


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produces an equivalent change in voltage from the potentiometer. A pulse widthmodulated signal is fed through the control wire. The pulse width is converted into anequivalent voltage that is compared with that of signal from the potentiometer in an erroramplifier.

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 isattained by the motor.

When the difference between the desired position as indicated by the pulse train andcurrent 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 ICsuch as 555 or a microcontroller can be programmed to generate the requiredwaveform. Refer Servo Motor interfacing with 8051 microcontroller and Servo controlusing AVR ATmega16.Power supply for ServoThe servo requires a DC supply of 4.8 V to 6 V. For a specific servo, its voltage rating isgiven as one of its specification by the manufacturer. The DC supply can be giventhrough 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. Aswitched regulator can be used as the supply for better power efficiency.Selection of a ServoThe typical specifications of servo motors are torque, speed, weight, dimensions, motortype and bearing type. The motor type can be of 3 poles or 5 poles. The pole refers tothe permanent magnets that are attached with the electromagnets. 5 pole servos arebetter than 3 pole motor because they provide better torque.
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The servos are manufactured with different torque and speed ratings. The torque is theforce applied by the motor to drive the servo arm. Speed is the measure that gives theestimate that how fast the servo attains a position. A manufacturer may compromisetorque 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 servohaving more torque will also have larger dimensions and weight. The selection of aservo 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 aswhen a servo is needed for making an RC airplane or helicopter.

what are servo motors

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