Quadrature encoders do not provide absolute position. To use a quadrature encoder in a position application, a known reference position must be established. This position is called the Home position. The process of moving the axis to find the home position is called Homing. As the encoder is rotated it increments or decrements the position from the initial home position.
Each RMC quadrature axis has two inputs that can be used for homing:
The index (Z) input comes from the encoder to indicate that the encoder has rotated a full revolution.
The home (H) input comes from a limit switch or proximity sensor.
Both the Z and H inputs must be active for the RMC to register an active Home. Therefore, if either the Z or H input is not used, the input must be made active either by wiring or inverting the Z or H input in the Config parameter word.
The RMC is commanded to wait for the Home condition by the Arm Home command. That RMC axis is then said to be Armed. Unless the quadrature axis is armed, the Home condition is ignored. If the Auto Home Re-arm configuration bit is set, the axis will automatically be re-armed after homing. Otherwise, the Arm home command must be issued to arm the axis again after homing.
Note: If the Auto Home Re-arm configuration bit is cleared after the Arm Home command has been issued, the axis will still be armed. However, once the axis is homed, it will not be automatically re-armed.
When the axis is armed and there is a falling edge in the Home active, the exact quadrature counts at the time of the falling edge are used as the physical home position. The Actual Position will be changed to reflect the current position with respect to the home position as specified in the Arm Home command. The Command Position and Target Position will be offset by the difference between the new and old Actual Positions.
The Home Status bit in the Status word can be used to help home the system. It can be configured with the Level/Edge bit in the Config word either to be latched on the falling edge of the Home active condition or to match the level of the Home Active condition.
Linear Positioner Homing Hints
When using a quadrature encoder on a linear positioner, the axis must be homed to retain absolute positions. This is easiest if the proximity switch controlling the H input to the axis is placed at one end of the travel; this will be the home position. The Z input is not used (not connected) and should be configured to be always active.
Set the Level/Edge bit in the Config word to Level of H input so that the status bit will be active when the H input is active. Next, arm the axis’s homing by issuing an Arm Home command.
When starting from an unknown position, retract until the Home Status bit is ON. Then, extend until the Home Status bit is OFF and stop. When the Level/Edge bit is set, the falling edge on the Home Status bit indicates the axis has been homed.
If the axis is already retracted, the axis will see the Home Status bit ON immediately and extend as described above. When the axis is shut down, it should be retracted until the Home Status bit is ON. This way the axis need only extend a small amount to be homed on startup.
Copyright (c) 1997-2015 by Delta Computer Systems, Inc.