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4.3.4. Automatic Home position calibration

This feature is used for detection and placing the movement to the starting position. This option of the controller is designed for simple search of "Home", or "Zero" position by user himself, with no need of any programming skills. The Home position is set relative to one or two external sensors and/or to an external signal.

Automatic Home position calibration feature is configured by user in Device configuration->Home position folder of XILab software (see the Home position settings chapter) and is activated by the button in XiLab main window.

After this feature is activated, the controller rotates the motor to the defined direction with normal speed to quickly locate the stop position. After finding the boundary position controller assigns motor back half a turn and begins rotate the motor to the defined direction, but with another calibration speed. The calibration speed is usually set lower than the normal rotation speed in order to improve the calibration accuracy. There are three ways to determine the completion of the motion, according to user-defined preferences:
  • rotation until the limit switch is reached, the actual configuration of limit switches is used (e.g., polarity, location). For more details please refer to Motion range and limit switches chapter.
  • rotation until a signal from the synchronization input is received, the actual configuration of synchronization input is used. For more details please refer to Synchronization settings chapter. If the synchronization input is switched off by program, the signal from it will never be processed.
  • rotation until a signal from the revolution sensor is received, the actual configuration of revolution sensor is used. For more details please refer to Position control chapter.

Successful completion of home position calibration results in assertion of the STATE_IS_HOMED flag in the controller state structure. If after this position somehow got lost (stop on limit switch, emergency stop while moving, steps loss detection, turns the Alarm mode), the corresponding flag is dropped and it is necessary to re-calibrate the home position.

Note. If command emergency stop or command power shutdown are executed while the engine is stopped then it isn't necessary to re-calibrate the home position and the STATE_IS_HOMED flag is not dropped.

Note. If the completion of the motion should happen at the limit switch, the calibration program is not crashed.

Fine calibration

After the first displacement the controller position is defined. However, some additional rotation may probably get turned on before the movement to Home position is performed. It provides fine Home position calibration that may reach 1/256 of the step for stages with stepper motors or 1 encoder count for stages with DC motors. If the corresponding flag is switched on, the controller rotates the motor to user-defined direction with defined rotation speed until a signal either from the synchronization input or from the revolution sensor is received or until the limit switch is reached, according to user-defined preferences. Using the signal from the sensor on the shaft from motor to reducer and performing the motion with the small speed does make sense. It provides the maximum accuracy. Since first and second movement signals might happen simultaneously a flag is provided, which will start tracking second motion end signal only after motor shaft half-turn. This allows avoiding the signals of the completion of the first and second motions to be in ambiguous sequence. As a result of optional second motion, the calibrating position becomes more accurate.

Note. If the second phase of the motion is used, the first motion usually can be performed at the high speed since it provides just a coarse calibration and the accuracy is not required there. The accuracy will not improve if the second limit switch is used for the second phase of the motion since its physical characteristics are the same as for the first limit switch.

Finally the controller moves the motor to the defined direction with the defined speed at the defined Standoff distance. This is what required to reach the Home position.

Note. The position reached as a result of calibration will slightly depend on the speed of the last motion until the selected sensor responsed. Therefore, don't change the speed parameters for further successful reaching the same position.

The automatic Home position calibration features are described in the Programming guide chapter.
The parameters configuration commands are described in the Communication protocol specification chapter
The position calibration command is described in the Communication protocol specification chapter.

A set_zero script is supplied with XiLab software pack, providing the automatic Home position configuration. This script changes the Standoff setting in Home position folder, making the actual position as the Home one.

How to use the script:
  • place the movement to the desired position
  • launch the script and wait until it's finished.

As a result, the movement will be in the same position and all the following calls of homing function will move it there. Make sure to save the settings to controller's nonvolatile memory.

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