Servo refers to an error sensing feedback control which is used to correct the performance of a system. Servo or RCServo 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.
The purpose of this information is to give an overview of how servos operate and how to
communicate with them.
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Servos are controlled by sending them a pulse of variable width. The control wire is
used to send this pulse. The parameters for this pulse are that it has a minimum pulse, a
maximum pulse, and a repetition rate. Given the rotation constraints of the servo, neutral
is defined to be the position where the servo has exactly the same amount of potential r
otation in the clockwise direction as it does in the counter clockwise direction. It is
important to note that different servos will have different constraints on their rotation
but they all have a neutral position, and that position is always around 1.5 milliseconds (ms).
The angle is determined by the duration of a pulse that is applied to the control wire.
This is called Pulse width Modulation. The servo expects to see a pulse every 20 ms.
The length of the pulse will determine how far the motor turns. For example, a
1.5 ms pulse will make the motor turn to the 90 degree position (neutral position).
When these servos are commanded to move they will move to the position and
hold that position. If an external force pushes against the servo while the servo is
holding a position, the servo will resist from moving out of that position. The maximum
amount of force the servo can exert is the torque rating of the servo. Servos will not
hold their position forever though; the position pulse must be repeated to instruct the
servo to stay in position.
When a pulse is sent to a servo that is less than 1.5 ms the servo rotates to a position and
holds its output shaft some number of degrees counterclockwise from the neutral point.
When the pulse is wider than 1.5 ms the opposite occurs. The minimal width and the
maximum width of pulse that will command the servo to turn to a valid position are
functions of each servo. Different brands, and even different servos of the same brand,
will have different maximum and minimums. Generally the minimum pulse will be about 1 ms
wide and the maximum pulse will be 2 ms wide.
Another parameter that varies from servo to servo is the turn rate. This is the time it takes
servo to change from one position to another. The worst case turning time is when the servo is
at the minimum rotation and it is commanded to go to maximum rotation. This can take several
seconds on very high torque servos.
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.
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.
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.
Figure 9 - Typical dc servo motor system with either encoder or resolver feedback. Some older servo motor systems use a tachometer and encoder for feedback.
Servo motors are used in closed loop control systems in which work is the control variable, Figure 9. The digital servo motor controller directs operation of the servo motor by sending velocity command signals to the amplifier, which drives the servo motor. An integral feedback device (resolver) or devices (encoder and tachometer) are either incorporated within the servo motor or are remotely mounted, often on the load itself. These provide the servo motor's position and velocity feedback that the controller compares to its programmed motion profile and uses to alter its velocity signal. Servo motors feature a motion profile, which is a set of instructions programmed into the controller that defines the servo motor operation in terms of time, position, and velocity. The ability of the servo motor to adjust to differences between the motion profile and feedback signals depends greatly upon the type of controls and servo motors used. See the servo motors Control and Sensors Product section.
Three basic types of servo motors are used in modern servosystems: ac servo motors, based on induction motor designs; dc servo motors, based on dc motor designs; and ac brushless servo motors, based on synchronous motor designs.