Actuator Input Output Signals
Publication PUB089-005-00_0618 55 of 116
Index 52 and 53 Deadband and Hysteresis
When using position control by sending a value to the Actuator Position DV set-point, there are a
number of parameters used to tune the position controller and reduce the possibility of damage to the
actuator. These two parameters are set to prevent hunting around the set-point due to high 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 FF CP card will run the actuator to
within the deadband of 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 set-point if the inertia is high.
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 set-point.
• 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 set-point 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. The Hysteresis is the amount of movement inside the
deadband permitted before the motor stops.
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