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In this drawing, the dashed line represents the controller. The setpoint signal,
which is input to the controller, represents the desired level of response (the
desired pressure in this case). The output, which goes to the "system under
control", is the actuating signal to the process (the amount that each valve is
opened in this case). The feedback signal represents the measured response of
the system (using, for example, a 4-20 mA pressure sensor). Thus, the function
of the controller is to analyze the setpoint and the feedback to produce the output
signal. This is done in the following manner.
Refer to the Figure 20, ignoring for the moment, the lower block containing the
term Kd. We see that the feedback signal is subtracted from the setpoint signal
to form the error signal. The error thus represents the deviation of the actual
system response from the desired response. The output is then produced by
summing two terms. The first term is the product of the error and the constant
Kp (the proportional constant). Thus the larger the error the greater the output
is. The second term is formed by multiplying the integral of the error times the
constant Ki (the integral constant). The effect of the integral is to accumulate the
error over time. The effect of even small errors will eventually cause the output
to increase over time until the system responds and the error is eliminated.
We return now to the effect of the Kd block in the feedback path. This block
multiplies the derivative of the feedback times the constant Kd (the derivative
constant). The derivative is sensitive to rapidly changing signals. Its primary
function in the control algorithm is to suppress oscillations.
"Tuning" a controller is the process of selecting the optimal Kp, Ki, and Kd
constants to yield the "best" response. The "best" response depends on what is
most important for a given application and often becomes a compromise
between response time and stability.
5.2 Control Modes
The ER3000 provides four modes of operation.
These modes depend on
whether the feedback signal is derived from the controller's internal pressure
sensor, a user supplied external sensor (such as a 4-20 mA pressure transducer)
or a combination of the two. The four modes are described in the following
sections.
5.2.1 Internal Feedback Mode
The internal feedback mode uses the ER3000's temperature compensated
internal sensor to control the output pressure in the 0-100 psig range.
5.2.2 External Feedback Mode
The external feedback mode uses a user supplied external transducer to control
the process pressure. This mode is typically used when using the ER3000 to
load the dome of a mechanical air actuated regulator or control valve. Control of
5. Operation 29
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