Alicat EPCD Series Manual de instrucciones - Página 9
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Imprimir páginaThe formula for performing linear interpolation is as follows:
Value = (desired setpoint) X 64000 / (full Scale)
For example, when changing the setpoint on a 100 PSIA full scale controller to
35 PSIA, the following value should be entered:
22400 = (35 PSIA) X 64000 / (100 PSIA)
Sending a Setpoint to a Controller with Negative and Positive Range
via RS-232 / RS-485
For a controller with a negative and positive range, the setpoint command may
be sent in either of the above two methods with the following changes:
Using Method 1, a negative setpoint may be set using a negative sign in the
command. For example, AS-4.54<CR> changes the setpoint for units "A" to
-4.54 in the current device units.
Using Method 2, values between 0 and 64000 are acceptable which correspond
linearly to the minimum and maximum pressure, respectively. The formula to
perform linear interpolation for a negative and positive ranged controller is as follows:
Value = (desired setpoint + minimum pressure) X 64000 / (maximum pressure -
minimum pressure)
For example, when changing the setpoint on a -10 PSIG to 10 PSIG ranged
controller to -3 PSIG, the following value should be entered.
20800 = (-3 PSIG + -10 PSIG) X 64000 / 10 PSIG- (-10 PSIG)
PID Tuning your EPC and EPCD over RS-232/RS-485:
The EPC and EPCD series of pressure controllers have an integrated PID loop
that measures the pressure, compares it with the setpoint, and adjusts the valve
accordingly at 1000 times per second. This PID tuning governs the control speed,
control stability, overshoot, and general performance of the EPC or EPCD. If
you encounter issues with valve stability, oscillation or speed, fine tuning these
parameters may resolve the problem. Alicat controllers allow you to adjust the
Proportional, Integral and Differential terms of the PID control loop.
Knowing how to change PID tuning, and knowing how to do it correctly and attain
the results you desire are two very different things. The following section covers
the theory/operational characteristics of the PID parameters:
The PD algorithm is the PID algorithm used on most Alicat controllers. It is divided
into two segments:
The first compares the process value to the setpoint to generate a proportional
error. The proportional error is multiplied by the 'P' gain, with the result added to
the output valve drive.
The second operates on the present process value minus the process value during
the immediately previous evaluation cycle.
This 'velocity' term in multiplied by the 'D' gain, with the result subtracted
from the output valve drive. The above additions to and subtractions from the
output drive register are carried over from process cycle to process cycle, thus
performing the integration function automatically.
Increasing the 'P' gain will promote the tendency of the system to overshoot,
ring, or oscillate. Increasing the 'D' gain will reduce the tendency of the system to
overshoot.
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