Purpose of Homing
The zero position is set by a program reset or by detecting the home limit during a homing routine, after which the current position value is cleared to zero to indicate the origin. All positions are referenced from this origin. When defining the zero point, determine the positive and negative motion directions and the motor's actual direction of motion.
Homing Scenarios
- Origin search is performed when the origin has not yet been established.
- Origin return is used when the origin has already been established and the system must return to it. On first power-up the origin is established; if subsequent motion moves away from that origin, use origin return to go back.
- Encoders on servo motors are generally of two types: absolute encoders and incremental encoders. Absolute encoders retain position across power cycles (as long as their backup power is maintained) and typically do not require homing. Incremental encoders lose multi-turn position information on power loss and therefore require a homing operation.
Initial Positioning at Servo Startup
Servo Positioning Principle
- Servo systems do not allow the motor to rotate without preparation. For permanent-magnet servo motors, the rotor can be accurately positioned at any electrical angle; after a short sequence of positioning attempts, the rotor aligns with the stator field. During operation, the Z pulse from an optical encoder is used to correct feedback pulse alignment.
- In vector control, if the output current vector does not align with the rotor axis, the rotor will rotate until it aligns with the stator current vector. This principle is used to detect the rotor initial position. The Z pulse is used as the reset signal, so the angular offset between the Z pulse location and the stator A-phase axis must be known, and this offset depends on the encoder mounting.
- Because of encoder mounting tolerances, the Z pulse location often does not coincide with the stator A-phase axis. Zero offset compensation is required, either by hardware or software. Hardware compensation rotates the encoder plate so the Z pulse aligns with the stator A-phase axis. Software compensation detects the angular offset between the Z pulse and the A-phase axis and corrects it in software.
Role of Initial Positioning
Detection and initial positioning of the rotor (or magnetic field) are essential for servo operation. Only after the actual initial rotor position is detected can the control system operate normally. If the initial rotor position cannot be calculated accurately, starting torque will be reduced, significant vibration may occur, the driver may generate alarms, and temporary reverse rotation is possible. A reliable initial rotor position detector, such as a rotary encoder, is necessary for normal servo startup. After initial positioning, the servo control system can begin returning to the home position.
Initial Positioning Without Hall Sensors
- Without Hall sensors, startup requires zero-crossing current detection, which causes vibration during startup to determine rotor position and motion direction.
- For rotary motors, initial positioning is performed by detecting the encoder Z-phase (index) signal. If the Z-phase signal is not found within the vibration range during initial positioning, a homing-failure alarm is generated and rotor position must be adjusted and reset.
- For linear motors, startup vibration is used to detect the actual position and direction, after which the motor moves in a defined direction to search for the home position.
Initial Positioning With Hall Sensors
Motors equipped with Hall sensors can detect rotor position and start smoothly. Such motors can start at zero speed to search for the home position.
Homing Methods
Basic Homing Methods
- Move in a predetermined direction until the origin is found. (Suitable for rotary tables and linear motors.)
- Move in a predetermined direction and use one end limit as the origin. (Center device, squeegee style). PLC programs and motion control cards often offer this homing mode.
- Move in a predetermined direction to a limit switch, then reverse and return to find the origin. (Common for linear motors.)
- Directly search for the encoder Z-phase (index) during homing; when the Z-phase is detected, decelerate and stop immediately. (Rotary motors with home = 0.)
- After finding the initial zero point, move a fixed distance in one direction determined by the HOME offset value. (Rotary motors where HOME is not zero.)
Additional Homing Strategies
- Combined approaches: fast seek to a point, then creep, reverse, detect the Z-phase, and apply offsets. Different controller libraries encapsulate various homing routines; some include one step, others combine multiple steps. Specific routines can be implemented in custom code when required.
- For the third method using a PLC: start a motion toward one direction without stopping at the origin until a limit switch at that end is reached. Then reverse the output signal and issue a homing command to reliably find the origin.
- For incremental-encoder servo motors, an external sensor is required during homing. When the external sensor detects the homing marker, the servo switches from high-speed homing to low-speed homing. When the motor reaches the falling edge detected by the external sensor, the servo rotates to the encoder Z-phase output point. (Typical for linear motion control.)
