CONTROL LOOP DIAGRAMS
Process Controls
Summary
The important information in this chapter is summarized below.
Control Loop Diagrams Summary
A controlled system is the system or process through which a particular quantity
or condition is controlled.
Control elements are components needed to generate the appropriate control
signal applied to the plant. These elements are also called the "controller."
Feedback elements are components needed to identify the functional relationship
between the feedback signal and the controlled output.
Reference point is an external signal applied to the summing point of the control
system to cause the plant to produce a specified action.
Controlled output is the quantity or condition of the plant which is controlled.
This signal represents the controlled variable.
Feedback signal is a function of the output signal. It is sent to the summing
point and algebraically added to the reference input signal to obtain the actuating
signal.
The actuating signal represents the control action of the control loop and is equal
to the algebraic sum of the reference input signal and feedback signal. This is
also called the "error signal."
The manipulated variable is the variable of the process acted upon to maintain
the plant output (controlled variable) at the desired value.
A disturbance is an undesirable input signal that upsets the value of the
controlled output of the plant.
Process time lags are affected by capacitance, which is the ability of a process
to store energy; resistance, the part of the process that opposes the transfer of
energy between capacities; and transportation time, the time required to carry a
change in a process variable from one point to another in the process. This time
lag is not just a slowing down of a change, but rather the actual time delay
during which no change occurs.
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