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Note: As soon as the SET button is pressed, the HCP position and signal are stored in non-volatile memory, and are retained even if the positioner is de-energised before leaving Setup Mode.
4.9 Reversing Clockwise/ Anticlockwise Sense of the Positioner.
The LCP and HCP settings described in the previous section can be used with complete success to reverse the sense of the positioner, using settings such as the reverse settings listed in Table 1. However, this will not change the direction of movement on loss of signal – see the next section for how to set this as desired.
4.10 Changing the Direction of Movement on Loss of Signal.
On being de-energised with air supply still fully pressurised (i.e. on disconnection of the signal, or when the signal falls below 3.5mA), the positioner moves the actuator in a direction set by the de-energised position of the servo valve. This direction of movement is independent of the LCP and HCP settings described in the sections above.
Movement on loss of air pressure is unpredictable for double acting positioners – if position needs to be held on loss of pressure, then it is necessary to order an external pressure sensing lockout valve assembly, and if a definite end of travel position is required on loss of pressure, then it is necessary to order a spring return unit.
On double acting actuators the unit is factory set to move clockwise (as viewed from the positioner lid) on loss of signal.
On spring return actuators the unit is factory set to move in the spring return direction on loss of air, or on loss of both air and signal, but it is set to move clockwise (as viewed from the positioner lid) irrespective of spring direction on loss of signal only.
To change the direction of movement on loss of signal, carry out the following steps:
(1) Disconnect air supply and signal.
(2) Undo screw holding down changeover block (see Figure 6).
(3) Rotate changeover block 90 degrees, taking care to keep rubber gasket lined up with holes in body.
(4) Re-tighten screw holding down changeover block making sure it remains in alignment with the body.
(5) Undo outer two angle pot wire terminal screws, swap blue and yellow wires over to reverse sense of pot connection, and re-tighten screws.
(6) Re-connect air supply and signal, and carry out Resetting Feedback Pot procedure (see next section).
4.11 Resetting the Feedback Pot.
This procedure is necessary only if the pot has been moved from its factory setting, or if the direction of movement on loss of signal has been changed as instructed in the previous section.
The positioner circuit needs the feedback pot to be positioned at the correct angle, such that its wiper reads the right voltage when the actuator is against its downscale endstop – i.e. against the endstop it reaches when the air supply is connected, but the signal is disconnected. The circuit itself can identify this voltage when it is powered up in Setup Mode with the POT parameter selected – when the pot is adjusted correctly, the POT LED changes from lighting continuously to blinking.
To set the pot, first move the actuator to the downscale endstop by connecting the air supply and disconnecting the signal. When it is in this position, hold it there by fitting a keeper plate if the unit has a spring return set to fail against the opposite stop, or by simply disconnecting the air supply if the unit is double acting or has a spring return set to fail against this stop. Re- connect the signal, and set to any current between 4 and 20mA. Select Setup Mode, and press the UP button until the POT LED lights continuously. Unclamp the pot clamp screws, taking care not to unscrew them too far such that they disengage from the nuts in the carrier plate. Rotate the pot until the POT LED changes from continuous to blinking illumination. Re-tighten the clamp screws, and reconnect the air supply. If the unit needed a keeper plate, drive it downscale by disconnecting the signal, and then remove the keeper plate with the air supply on.