Have you ever wondered how a robotic vehicle is controlled?
How do you imagine the doors in hospitals and supermarkets open and close automatically?
Perhaps it’s how DVD players replay the disc that fascinates you.
These operations and more are all possible thanks to the servo motor.
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What is a servo motor?
A servo motor is a kind of rotary motor or actuator that allows for accurate control when it comes to acceleration, angular position, and velocity. Regular motors don’t have this unique ability.
In order to achieve this, servos use a motor guided by a sensor that uses negative feedback mechanisms in order to convert electrical signals to a controlled motion.
Types of servo motors
- AC Servo Motors
This is an AC motor that includes an encoder and a controller for giving closed-loop and feedback. AC Servos have high levels of accuracy and can be precisely controlled for their applications.
This type of servo motor is designed to have better bearing or tolerance. Some can use higher voltages to achieve greater torque.
- DC Servo Motors
This one usually has a separate DC source in winding and armature winding. Control can be achieved by controlling the field current or the armature current.
DC servos are highly accurate and are also quite fast to respond to the stop or start command signals because of their low armature inductive reactance. As a result, they are usually used to provide corrective actions in controlled-velocity drive mechanisms or remote control positioning systems.
- Position Rotation Servo Motors
This is the most common type of servo available. It features an o/p shaft that rotates at least 180 degrees as well as a physical stop. The stop is placed in the gear mechanism in order to guard the rotation sensor and stop the shaft from turning outside its limits.
- Linear Servo Motors
Quite similar to the positional servos, linear servos come with extra gears in order to change their o/p from circular motion to back-and-forth. This type of servo is quite rare, but can sometimes be found in hobby stores where they’re used as actuators for advanced model planes.
- Continuous Rotation Servo Motors
These are also similar to the positional rotation servos, but this type can go in any direction. Its control signal is already understood as the direction and speed of rotation instead of being set in the static position. Its range of potential commands can rotate counterclockwise and clockwise at changing speed depending on the signal.
Construction of the servo motor
A servo motor can be broken down into 5 parts:
- Stator Winding: Also called the field winding, this part winds on the stationary part of the servo.
- Rotor Winding: Also called the armature winding, this part winds on the rotating part of the servo.
- Shaft: This is the armature winding coupled on the iron rod.
- Bearing: Available in 2 types (back or front bearing), this is used for shaft movement.
- Encoder: It features the approximate sensor that determines the revolution per minute and the rotational speed of the motor.
Controlling the Servo Motor
A servo motor is controlled via the control wire, which sends an electrical pulse (maximum pulse, minimum pulse, and a repetition rate) of variable width. This is called Pulse Width Modulation.
Servos can typically turn 90 degrees in either direction for a total of 180 degrees of movement. The PMW sent to the servo determines the shaft position and based on the duration of the pulse sent, the rotor will turn to the desired position.
Servos expect to see a pulse every 20ms and the pulse length determines how far it will turn.
For example, a 1.5ms pulse can make the servo turn to 90 degrees. A pulse longer than 1.5ms can turn the servo toward the 180 degrees position in a clockwise direction. If it’s shorter than 1.5ms, it moves toward the 0-degree position in a counterclockwise direction.
When servos are commanded to move, they’ll move and hold that position. And even if an external force pushes against the motor, it will still hold that position and resist an external force.
Servos won’t hold their position forever, however, so the pulse needs to be repeated in order to instruct the motor to stay in such a position.
Servo motor usage
- Used in robotics to activate movements and position their arms at precise angles
- Used in the automatic doors seen in hospitals, supermarkets, and theaters
- Built into cameras to automatically bring a subject into focus
- Used to stop and move conveyor belts carrying items along stages, i.e. bottling, labeling, packaging, and other types of manufacturing
- Commonly used in the aerospace industry in combination with hydraulic systems in order to maintain fluids
- Used to maintain vehicle speed in the automobile industry
