What is a servo motor?
Servo motors are rotary or linear actuators that allow precise control of angular velocity or linear position, velocity, and acceleration. They are used in applications such as robotics, CNC machinery, and automated manufacturing.
Servo motors consist of a suitable electric motor coupled to a sensor for position feedback. They also require a relatively complex controller, usually a dedicated module designed specifically for use with servo motors.
Servo motors are not a specific class of motors. Any type of electric motor can be used in a closed-loop control system to create a servo motor. However, the term “servo motor” is generally used to refer to electric motors that are specifically designed for use in closed-loop control systems.
Servo mechanism
Servo motors are closed-loop devices, meaning that they use feedback to control their movement. The feedback signal comes from a position encoder, which measures the current position of the output shaft. The servo controller compares the current position to the desired position (commanded position) and generates an error signal. The error signal is used to control the speed and direction of the motor until the desired position is reached.
The simplest servo motors use a potentiometer to measure position and a bang-bang control scheme. In a bang-bang control scheme, the motor is either running at full speed in one direction or the other. This type of servo motor is not very accurate, but it is simple and inexpensive.
More complex servo motors use an optical rotary encoder to measure position and a variable speed drive to control the speed of the motor. This allows the servo motor to reach its desired position more quickly and accurately without overshoot.
Here is a simplified explanation of how a servo motor works:
- The servo controller receives a signal (analog or digital) that represents the desired position of the output shaft.
- The servo controller compares the desired position to the current position of the output shaft.
- The servo controller generates an error signal based on the difference between the desired position and the current position.
- The servo controller uses the error signal to control the speed and direction of the motor.
- The motor turns until the desired position is reached.
- Once the desired position is reached, the servo controller stops the motor.
Servo motors and stepper motors
Servo motors and stepper motors are two types of electric motors that are often used in industrial and hobbyist applications. Both types of motors have their own advantages and disadvantages, so it is important to choose the right motor for the job.
Stepper motors have a built-in output step and therefore an inherent ability to control position. This generally allows them to be used as open loop position controls without any feedback encoder, as their drive signal specifies the number of steps of rotational motion. However, for this to work, the controller needs to “know” the position of the stepper motor to start up. So, on power up, the controller will have to start the stepper motor and rotate it to a known position, say until the end limit switch is activated. This can be observed when an inkjet printer is turned on; the controller will move the inkjet printer’s carriage to the left and right to establish the final position.
Servo motors do not have this limitation. Regardless of the initial position when powered on, the servo motor will immediately rotate to any angle indicated by the controller. This is because servo motors use a feedback loop to control their position. The servo motor has an encoder that measures its current position, and the controller compares this to the desired position. The controller then adjusts the power to the motor to move it to the desired position.
The lack of feedback from a stepper motor limits its performance because a stepper motor can only drive loads within its load capacity. Otherwise, loss of steps under the load may result in positioning errors, and the system may have to be restarted or recalibrated. Encoders and controllers for servo motors are an additional cost, but they optimize overall system performance (optimal in terms of speed, power, and accuracy) relative to the capacity of the base motor. In large systems, where powerful motors account for an increasing proportion of the system cost, servo motors have the advantage.
Closed-loop stepper motors have become increasingly popular in recent years. They are like servo motors but have some differences in their software control to obtain smooth motion. The main advantage of closed-loop stepper motors is their relatively low cost. There is also no need to tune the PID controller on a closed loop stepper system.
Many applications such as laser cutters are available in two ranges: a low-priced range using stepper motors and a high-performance range using servo motors.
Encoder
Servo motors use encoders to provide position feedback to the controller. The encoder measures the current position of the motor shaft, and the controller uses this information to control the speed and direction of the motor.
There are two main types of servo motor encoders: absolute and incremental.
- Absolute encoders can determine their position on power up. This is because they have a unique code for each position. Absolute encoders are more complex and expensive than incremental encoders, but they are more accurate.
- Incremental encoders do not have a unique code for each position. Instead, they generate a pulse train for each revolution of the shaft. The controller uses the number of pulses to determine the position of the motor. Incremental encoders are simpler and cheaper than absolute encoders, but they are less accurate.
Some servo motors use a separate external linear encoder instead of a rotary encoder. This is because linear encoders can be more accurate than rotary encoders, especially for long travel distances.
Motor
Servo motors can be made from different types of electric motors, including brushed DC motors, brushless DC motors, and AC induction motors. The type of motor used depends on the specific application requirements, such as speed, torque, and accuracy.
Small industrial servo motors are typically electronically commutated brushless motors. These motors are more efficient and have a longer lifespan than brushed DC motors.
Large industrial servo motors typically use AC induction motors. AC induction motors are very powerful and can provide high torque at low speeds. However, they are not as efficient as brushless DC motors.
To achieve excellent performance in a compact package, a brushless AC motor with a permanent magnetic field can be used. These motors are often used in high-performance servo motor applications, such as robotics and CNC machines.
Servo motor drive modules are used to control the speed and direction of the servo motor. These modules are typically based on three-phase MOSFET or IGBT H-bridges.
Servo motor drive modules accept a single direction and pulse count (distance of rotation) as input. They may also include over-temperature monitoring, over-torque, and stall detection features.
The type of encoder, the gear reduction ratio, and the dynamics of the entire system are application-specific. Therefore, it is more difficult to produce the entire servo controller as an off-the-shelf module. The servo controller is usually implemented as part of the main controller.
Control
Most modern servo motors are designed and supplied with dedicated controller modules from the same manufacturer. This is because the servo motor and controller need to be tuned to each other to achieve the best performance.
However, servo motor controllers can also be developed around microcontrollers to reduce costs for high-volume applications. This approach is more complex and requires more expertise, but it can be cost-effective for large production runs.