Overview
Servo motors are core power components in industrial automation. With precise position, speed, and torque control, they are widely used in data centers, robots, medical equipment, and other applications. Positioning methods—absolute positioning and relative positioning—have distinct characteristics and suit different scenarios.
Absolute positioning: precise reference point
Absolute positioning requires the motor controller to know the motor's current position accurately, then drive the motor to a specified position via control signals. This approach relies mainly on two technologies: encoders and absolute displacement sensors.
Encoders are key devices for detecting motor position. Using optical gratings, magnetic tracks, or Hall sensors, encoders convert motor rotation into digital or analog pulse signals and provide precise position information to the controller. The achievable accuracy depends on the encoder type and resolution, so encoders are essential where very high positioning accuracy is required, for example on assembly robots in automated production lines.
Absolute displacement sensors provide a more direct method of position detection by measuring displacement on the motor shaft and transmitting that value to the controller. A key advantage is that position can be read even when the motor has not moved, which is useful in some specialized applications. However, absolute displacement sensors are generally more expensive and can be limited in high-speed motion applications.
The main benefit of absolute positioning is that it determines position without needing a reference search or repeated initialization. This is particularly important when synchronizing multiple servo motors, since each motor's position information is accurate and consistent. Limitations include potentially complex and time-consuming initialization and dependence on encoder or sensor accuracy.
Relative positioning: flexible displacement control
Relative positioning specifies motion relative to the current position. The controller issues commands to move the servo a given distance or angle from its present position.
Relative positioning is simple and flexible. It avoids complex initialization and only requires issuing the appropriate displacement commands. This makes it suitable for applications that do not require absolute accuracy, such as line-following robots that adjust their position based on sensor feedback to stay on a predefined path.
Relative positioning also fits applications with cyclic repetitive motion, for example a robotic arm that performs back-and-forth movements on an assembly line. Using relative moves, the arm can reliably perform repeated motions to handle workpieces.
However, relative positioning has limitations. Without absolute position feedback, cumulative errors can accumulate, so periodic position calibration is often necessary. Relative positioning is also not appropriate for applications that require high-precision absolute positioning, for example precise grasping and placement in robotic manipulation.
Choosing the right method
Absolute and relative positioning each have advantages and suitable use cases. Selection should be based on application requirements. For high-precision absolute control, such as assembly robots and synchronized multi-axis systems, absolute positioning is generally preferred. For applications that do not require absolute position confirmation, such as line-following vehicles or simple reciprocal motions, relative positioning is often more appropriate. In practice, systems may combine both methods to meet complex motion-control requirements.
