rotork Modbus, Technical Manual

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Modbus MFU Option Card Installation Manual
56 of 64 Publication PUB091-004-00_0918
inertia of the valve. They will require adjustment for each specific application. In addition, the Motion
Inhibit Timer is used to ensure the actuator does not carry out an excessive number of starts in a
given period.

Deadband The control used for the positioner is proportional only. The MFU will
run the actuator to the desired position and then it stops. As the
actuator and valve combination have some inertia there is a possibility
that the desired position may be overrun and the positioner will then
reverse the direction of travel to make the valve adopt the desired
position. This overshoot and return may continue for a number of
cycles and is known as hunting; the valve and actuator combination
will hunt around the setpoint if the inertia is high and the deadband is
set too low. To prevent this from happening there is a Deadband
setting whereby once the actuator enters the deadband the motor will
be stopped. For example a 5% deadband will cause the motor to be
stopped once the actual position is within 5% of the desired position.
The inertia will then bring the actual position nearer the desired
position.
The deadband is the allowable error around the setpoint. This
determines the minimum step change response of the actuator.

Hysteresis In addition to the deadband a second setting, hysteresis, further
refines the performance of the position controller. The positioner will
run the actuator towards the setpoint DV until the actual position is
within the deadband minus the hysteresis setting. This has the effect
of instructing the actuator to stop when it is nearer the DV. The
actuator will not restart unless it overshoots and runs outside the
deadband or a new command places the new desired position outside
the deadband.
Fig 19: Deadband and Hysteresis settings
The Hysteresis is the amount of movement inside the deadband
permitted before the motor stops.
0% Setpoint DV = 50% 100%
Valve Stroke Position MV = 50%
47% 48% 49% 50% 51% 52% 53%
.

Hysteresis Hysteresis Hysteresis = 1%
Deadband Deadband Deadband = 3%
CLOSED OPEN

Position Minimum Setting

Position Maximum Setting