The document describes the basic elements of process control systems, including measurement, evaluation, and control elements. It provides examples of using a level transmitter, level controller, and control valve to regulate liquid level in a tank. It also discusses using temperature sensors and controllers to regulate process temperature. Block diagrams are presented to illustrate how these elements work together in a control loop. The purposes of open loop and closed loop control systems are also overviewed.

1. BASIC PROCESS
CONTROL
SYSTEM

2. ELEMENTS OF PROCESS CONTROL
Pocess level control

3. PROCESS LEVEL CONTROL
 Illustrates the essential elements of a process.
 A level transmitter (LT), a level controller (LC), and a
control valve (LV) are used to control the liquid level in a process
tank.
 The purpose is to maintain the liquid level at some prescribed
height (H) above the
bottom of the tank. It is assumed that the rate of flow into the
tank is random.
 The level transmitter is a device that measures the fluid level in
the tank and
converts it into a useful measurement signal, which is sent to a
level controller.
 The level controller evaluates the measurement, compares it
with a desired set
point (SP), and produces a series of corrective actions that are
sent to the control
valve.
 The valve controls the flow of fluid in the outlet pipe to maintain
a level in the tank.

4. ELEMENTS OF PROCESS CONTROL
Pocess temperature control

5. MEASUREMENT
 To control a dynamic variable in a process,must have
information about the entity or variable itself. This
information is obtained by measuring the variable.
 Measurement refers to the conversion of the process
variable into an analog or digital signal that can be
used by the control system.
 The device that performs the initial measurement is
called a sensor or instrument.
 Typical measurements are pressure, level,
temperature, flow, position, and speed.
 The result of any measurement is the conversion of a
dynamic variable into some proportional information
that is required by the other elements in the process
control loop or sequence.

6. EVALUATION
 In the evaluation step of the process control
sequence, the measurement value is examined,
compared with the desired value or set point, and
the amount of corrective action needed to
maintain proper control is determined.
 A device called a controller performs this
evaluation. The controller can be a pneumatic,
electronic, or mechanical device mounted in a
control panel or on the process equipment.
 It can also be part of a computer control
 system, in which case the control function is
performed by software.

7. CONTROL ELEMENT
 The control element in a control loop is the
device that exerts a direct influence on the
process or manufacturing sequence.
 This final control element accepts an input from
the controller and transforms it into some
proportional operation that is performed on the
process.
 In most cases, this final control element will be a
control valve that adjusts the flow of fluid in a
process.
 Devices such as electrical motors, pumps, and
dampers are also used as control elements.

8. BLOCK DIAGRAM OF BASIC
ELEMENTS
 A process-control system to regulate flow and the
corresponding block diagram.
 The use of current and pressure transmission signals.

9. THE PURPOSE AND USE OF OPEN LOOP
o An open-loop control system is controlled directly, and only, by an input signal.
o The basic units of this type consist only of an amplifier and a motor.
o The amplifier receives a low-level input signal and amplifies it enough to drive the
motor to perform the desired job.
o The output of the amplifier is proportional to the amplitude of the input signal.
o The phase (ac system) and polarity (dc system) of the input signal determines the
direction that the motor shaft will turn.
o After amplification, the input signal is fed to the motor, which moves the output
shaft (load) in the direction that corresponds with the input signal.
o The motor will not stop driving the output shaft until the input signal is reduced to
zero or removed.

10. THE PURPOSE AND USE OF CLOSED LOOP
o A closed-loop control system utilizes an additional measure of the actual output to compare the
actual output with the desired output response.
o A feedback control system is a control system that tends to maintain a prescribed relationship of
one system variable to another by comparing functions of these variables and using
the difference as a means of control.
o A feedback control system often uses a function of a prescribed relationship between the output
and reference input to control the process.
o Often the difference between the output of the process under control and the reference input is
amplified and used to control the process so that the difference is continually reduced.
o The feedback concept has been the foundation for control system analysis and design.
o The introduction of feedback enables us to control a desired output and can improve accuracy, but
it requires attention to the issue of stability of response.