What is Servo Motor?- Definition, Working And Types

What is a Servo motor?

A servomotor (or servo motor) is a rotary or linear actuator that allows precise control of angular or linear position, velocity, and acceleration. It consists of a suitable motor coupled to a position feedback sensor. Servo motors are used in applications such as robotics, CNC machinery, or automated manufacturing.

It also requires a relatively sophisticated controller, often a dedicated module designed specifically for use with servo motors. Servo motors are not a specific class of motor, although the term servo motor is often used to refer to a motor suitable for use in a closed-loop control system.

Servo motors are part of a closed-loop control system and consist of several parts, namely a control circuit, a servo motor, a shaft, a potentiometer, a drive gear, an amplifier, and either an encoder or a resolver. A servo motor is a self-contained electrical device that rotates parts of a machine with high efficiency and great precision.

The output shaft of this motor can be moved to a specific angle, position, and speed that a normal motor does not have. The servo motor uses a regular motor and couples it with a position feedback sensor.

The controller is the most important part of the servo motor specially designed and used for this purpose. The servo motor is a closed-loop mechanism that incorporates position feedback to control rotational or linear speed and position.

The motor is controlled with an electrical signal, either analog or digital, that determines the amount of movement that represents the final commanded position for the shaft. A type of encoder serves as a sensor that provides speed and position feedback. This circuit is built directly into the motor housing, which is usually equipped with a gear system.

Mechanism of Servo Motor

A servomotor is a closed-loop servomechanism that uses position feedback to control its movement and final position. The input to its controller is a signal (either analog or digital) that represents the commanded position for the output shaft.

The motor is paired to some sort of position encoder to provide position and speed feedback. In the simplest case, only the position is measured. The measured position of the output is compared to the commanded position, the external input to the controller.

If the output position differs from that required, an error signal is generated which then causes the motor to turn in either direction as needed to bring the output shaft to the appropriate position. As the positions approach, the error signal reduces to zero, and the motor stops.

The very simplest servomotors use position-only sensing via a potentiometer and bang-bang control of their motor; the motor always rotates at full speed (or is stopped). This type of servomotor is not widely used in industrial motion control, but it forms the basis of the simple and cheap servos used for radio-controlled models.

More sophisticated servo motors use optical rotary encoders to measure the speed of the output shaft and a variable-speed drive to control the motor speed. Both of these enhancements, usually in combination with a PID control algorithm, allow the servomotor to be brought to its commanded position more quickly and more precisely, with less overshooting.

How does a servo motor work?

A servo motor is an electromechanical device that produces torque and speed based on the current and voltage supplied. A servo motor operates as part of a closed-loop controller, providing torque and speed as commanded by a servo controllers that uses a feedback device to close the loop.

The feedback device provides information such as current, speed, or position to the servo controller, which adjusts the motor action depending on the commanded parameters.

Servos are controlled by sending a variable-width electrical pulse or pulse width modulation (PWM) over the control cable. There is a minimum heart rate, a maximum heart rate, and a repetition rate. A servo motor can normally only rotate 90 ° in each direction. Which adds up to a total of 180 ° of movement.

The neutral position of the motor is defined as the position where the servo has the same potential rotation in both clockwise and counterclockwise directions. The PWM sent to the motor determines the position of the shaft and is based on the duration of the pulse sent over the control cable; the rotor turns into the desired position.

The servo motor expects a pulse every 20 milliseconds and the length of the pulse determines how far the motor turns. A pulse of 1.5ms, for example, causes the motor to turn to the 90 ° position.

For less than 1.5ms it moves counterclockwise towards the 0 ° position, and longer than 1.5ms rotates the servo clockwise towards the 180 ° position.

When a move command is given to these servos, they will move into position and hold that position. If an external force is pressing against the servo while the servo is holding a position, the servo will resist moving from that position.

The maximum force the servo can exert is called the servo’s torque. Servos won’t hold their position forever, however; The position pulse must be repeated to tell the servo to stay in position.

Types of servo motors

Servomotors come in many sizes and three basic types. The three types include positional rotation, continuous rotation, and linear.

• Positional Rotation Servos rotate 180 degrees. They also have stops in the gearbox to protect the output shaft from over-rotating.
• A continuous rotation servo motor is a servo whose range of motion is not limited. Instead of letting the input signal determine which position the servo should turn. The continuous rotation of the servo relates the input to the speed of the output and the direction. The limitless movement of these motors allows them to move in both CW and CCW directions.
• Linear servos use a rack and pinion mechanism to change their performance. The rack and pinion convert rotary motion into linear motion.

Servo Motor Repair: Step-By-Step Process

Servo motor repair is claimed by many but mastered by few! There are many steps involved in repairing servo motors; the trickiest step is feedback repair and realignment. This small step is the key to accurately repairing the servo motor, without a perfect alignment the rest of the servo motor repair is irrelevant.

To perform the perfect alignment a repair company must use a very expensive alignment system. It is due to this cost and the lack of an ability to repair the electronics of the feedback device that many repair companies do not repair servo motors.

We would like to invite you to watch our servo motor repair process in the video below. This is how we can accurately repair, realign and fully load test your servo motors for a perfect repair every time!

Step-1 Initial Evaluation

During an evaluation, a visual inspection is carried out to check parts such as the shaft, keyway, end bells, clamps, and connectors. After the inspection, a surge voltage comparison test or a short test is carried out to check whether the stator needs to be rewound.

Next, an insulation resistance test often referred to as the Megger test, is performed at each phase to ensure that the insulation is not broken.

The next test in the evaluation is the phase balance test, which uses an RMS meter to ensure the windings are balanced between phases; The brake is also checked here if the servomotor has one.

Step-2 Disassembly

First, the backplate is removed, followed by the encoder and encoder housing, while the cabling is carefully removed. Then the end bells are removed and the rotor is pulled from the stator; here the rotor and shaft are visually inspected.

Next, the bearings and the bearing housing, as well as the brake, are removed.

Step-3 Cleaning

An alkaline washer is used as it is better for your motor than hand cleaning or pressure washing.  All of our motor parts are washed this way.

Step-4 Bearing Change

It is important to change every bearing every time as they are often the cause of failure in a motor. We only use high-quality bearings that meet or exceed all manufacturer’s specifications.  Once the bearings are changed the motor is reassembled.

Step-5 Final Testing

To ensure the motor is fully repaired, Next, a memory test is performed to ensure the realignment of the feedback device.  After the repair is fully tested and verified the motor is painted and is ready to send back to you, ready to use!

Advantages of Servo Motor

Servo Motor advantages are:

• High output power relative to motor size and weight.
• The encoder determines accuracy and resolution.
• High efficiency. It can approach 90% at light loads.
• High torque to inertia ratio. Servo Motors can rapidly accelerate loads.
• Has 2-3 times more continuous power for short periods.
• Has 5-10 times more rated torque for short periods.
• Servo motors achieve high speed at high torque values.
• Quiet at high speeds.
• Encoder utilization provides higher accuracy and resolution with closed-loop control.

Disadvantages of Servo Motor

The top Servo Motor disadvantages are:

• Servos Motors requires tuning to stabilize the feedback loop.
• Servo Motor will become unpredictable when something breaks. So, safety circuits are required.
• A complex controller requires an encoder and electronic support.
• Peak torque is limited to a 1% duty cycle. Servo Motors can be damaged by sustained overload.
• Gearboxes are often required to deliver power at higher speeds.
• Higher overall system cost and the installation cost of a Servo Motor system may be higher than that of a stepper motor due to the requirement for feedback components.

Common Industrial Applications for Servo Motors

Servo motors are small and efficient but critical for use in applications requiring precise position control. The servo motor is controlled by a signal (data) better known as a pulse-width modulator (PWM). Here are several of the more common servo motor applications in use today.

• Robotics: A servo motor at every “joint” of a robot is used to actuate movements, giving the robot arm its precise angle.
• Conveyor Belts: Servo motors move, stop, and start conveyor belts carrying products along to various stages, for example, in product packaging/bottling, and labeling.
• Camera Auto Focus: A highly precise servo motor built into the camera corrects a camera’s lens to sharpen out-of-focus images.
• Robotic Vehicle: Commonly used in military applications and bomb detonation, servo motors control the wheels of the robotic vehicle. And generating enough torque to move, stop, and start the vehicle smoothly as well as control its speed.
• Solar Tracking System: Servo motors adjust the angle of solar panels throughout the day so that each panel continues to face the sun, harnessing maximum energy from sunup to sundown.
• Metal Cutting & Metal Forming Machines: Servo motors provide precise motion control for milling machines, lathes, grinding, centering, punching, pressing, and bending in metal fabrication for such items as jar lids to automotive wheels.
• Antenna Positioning: Servo motors are used on both the azimuth and elevation drive axis of antennas and telescopes such as those used by the National Radio Astronomy Observatory (NRAO).
• Woodworking/CNC: Servo motors control woodturning mechanisms (lathes) that shape table legs and stair spindles. For example, as well as augering and drilling the holes necessary for assembling those products later in the process.
• Textiles: Servo motors control industrial spinning and weaving machines, looms, and knitting machines that produce textiles such as carpeting and fabrics as well as wearable items such as socks, caps, gloves, and mittens.
• Printing Presses/Printers: Servo motors stop and start the print heads precisely on the page as well as move paper along to print multiple rows of text or graphics in exact lines, whether it’s a newspaper, a magazine, or an annual report.
• Automatic Door Openers: Supermarkets and hospital entrances are prime examples of automated door openers controlled by servo motors. Whether the signal to open is via a push plate beside the door for handicapped access or by a radio transmitter positioned overhead.

The world would be a much different place without servo motors. Whether they’re used in industrial manufacturing or in commercial applications, they make our lives better, and easier.

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