Page 41 - Instrumentation and Measurement

continues to increase as long as there is an error and derivative mode where the control signal is
proportional to the rate at which the error is changing.

The term control unit or controller is often used for the combination of the comparison element, i.e.
the error detector, and the control law implementation element. Therefore, controllers are devices
that monitor signals from transducers and take the necessary action to keep the process within
specified limits according to a predefined program by activating and controlling the necessary
actuators.

3. Correction element

The correction element or, as it is often called, the final control element, produces a change in the
process which aims to correct or change the controlled condition. The term actuator is used for the
element of a correction unit that provides the power to carry out the control action. Examples of
correction elements are directional control valves which are used to switch the direction of flow of a
fluid and so control the movement of an actuator such as the movement of a piston in a cylinder.
Another example is an electric motor where a signal is used to control the speed of rotation of the
motor shaft.

4. Process

The process is the system in which there is a variable that is being controlled, e.g. it might be a room
in a house with the variable of its temperature being controlled.

5. Measurement element

The measurement element produces a signal related to the variable condition of the process that is
being controlled. For example, it might be a temperature sensor with suitable signal processing.

The following are terms used to describe the various paths through the system taken by signals:

1. Feedback path

Feedback is a means whereby a signal related to the actual condition being achieved is fed back
to modify the input signal to a process. The feedback is said to be negative when the signal which
is fed back subtracts from the input value. It is negative feedback that is required to control a
system. Positive feedback occurs when the signal fed back adds to the input signal.

2. Forward path

The term forward path is used for the path from the error signal to the output. In Figure 3.10, these
forward path elements are the control law element, the correction element and the process element.

The term process control is often used to describe the control of variables, e.g. liquid level or the
flow of fluids, associated with a process in order to maintain them at some value. Note also that the
term regulator is sometimes used for a control system for maintaining a plant output constant in the
presence of external disturbances. Hence the term regulator is sometimes applied to the correction
unit.

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