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Note: As soon as the UP or DOWN button is pressed, the new DAMP value is stored in non-volatile memory, and is retained even if the positioner is de-energised before leaving Adjust and Setup Modes.
The positioner is active while adjusting the DAMP setting, and continues to follow the input signal. This allows the signal to be waggled to optimise the response while still adjusting the DAMP setting. Try stepping the signal suddenly so that the positioner approaches setpoint at full speed having travelled 45 degrees or more, and adjust the DAMP setting to just avoid overshoot here. Try this travelling up and downscale. When the right setting has been found, press the SET button again to leave Adjust Mode – the DAMP LED will now light continuously.
4.8 Low Current Point and High Current Point Settings. (Replaces Zero and Span Adjustment).
The positioner characteristic can be accurately calibrated by setting two points, the Low Current Point (LCP) and the High Current Point (HCP). Each point is defined by a position and an input current. These points can lie anywhere in the range of travel of the actuator, and in the input signal range of 4 to 20 mA. The only other restriction is that the input current at the HCP must exceed the input current at the LCP by at least 5mA. To obtain the best accuracy, it is necessary to select the two points as far apart as possible. In both linear and non-linear operation this allows forward acting, reverse acting, and forward or reverse acting split range characteristics to be implemented. Table 1 below shows some illustrative examples.
Table 1. Various Sets of LCP and HCP Settings Table
To set LCP, select the LOW parameter in Setup Mode by pressing the UP button until the LOW LED lights continuously. Press the SET button to select Adjust Mode, and the LOW LED will blink. Adjust the input signal current within the range 4-20mA to the value you want to define for the Low Current Point – note that changes in input current within the 4-20mA band do not result in positioner movement at this stage – the positioner holds the actuator at the angle where it was before Adjust Mode was selected, irrespective of input current. Press the UP or DOWN buttons to move the positioner to the angle you want to define for the Low Current Point – if you hold the button down continuously, the positioner will traverse the actuator steadily, or if you give short blips it will move approximately 1/40 degree (one fortieth) for each blip, to allow precise edging into exactly the required position. When the actuator is in exactly the position you want, and the signal is adjusted to exactly the value you want (this can be readjusted after the position has been set if you want), press the SET button again to record the signal and position as the Low Current Point, whereupon the actuator will make a small leap and then settle back to the LCP again, the positioner will return to Setup Mode, and the LOW LED will light continuously. The positioner will now respond to its input signal again.
Note: As soon as the SET button is pressed, the LCP position and signal are stored in non-volatile memory, and are retained even if the positioner is de-energised before leaving Setup Mode.
To set HCP, select the HIGH parameter in Setup Mode by pressing the UP button until the HIGH LED lights continuously. Press the SET button to select Adjust Mode, and the HIGH LED will blink. Adjust the input signal current within the range 4-20mA to the value you want to define for the High Current Point – note that changes in input current within the 4-20mA band do not result in positioner movement at this stage – the positioner holds the actuator at the angle where it was before Adjust Mode was selected,
irrespective of input current. Press the UP or DOWN buttons to move the positioner to the angle you want to define for the High Current Point – if you hold the button down continuously, the positioner will traverse the actuator steadily, or if you give short blips it will move approximately 1/40 degree (one fortieth) for each blip, to allow precise edging into exactly the required position. When the actuator is in exactly the position you want, and the signal is adjusted to exactly the value you want (this can be readjusted after the position has been set if you want), press the SET button again to record the signal and position as the High Current Point, whereupon the actuator will make a small leap and then settle back to the HCP again, the positioner will return to Setup Mode, and the HIGH LED will light continuously. The positioner will now respond to its input signal again.