Class 3 control system components

ICE401: PROCESS INSTRUMENTATION
AND CONTROL
Class 3: Control System Components
Dr. S. Meenatchisundaram
Email: meenasundar@gmail.com
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Jan – May 2015

Control System Components:
A control system is comprised of the following components:
1. Primary elements (or sensors/transmitters)
2. Controllers
3. Final control elements (usually control valves)
4. Processes
Look at the surge tank level control
as given in Figure 3.1.

Figure 2.1 illustrates a level control system and its components.
The level in the tank is read by a level sensor device, which transmits
the information on to the controller.
The controller compares the level reading with the desired level or set
point and then computes a corrective action.
The controller output adjusts the control valve, referred to as the ﬁnal
control element.
The valve percent opening has been adjusted to correct for any
deviations from the set point.

Figure 3.2 is an information ﬂow diagram that corresponds to the
physical process ﬂow diagram in Figure 3.1.
The information is transmitted between the different control
system elements as either pneumatic, electronic or digital signals.
These signals often use a live zero.
Typical levels are 20–100 kPa (or 3–15 psi) for pneumatic signals,
a 4–20 mA current loop that is often converted to 1–5 V for analog
electronic signals and binary digits or bits for digital signals.

Figure 3.3. Alternative, classical form of the single input/single output
feedback control loop:

The simplest and most widely used method of process control is
the feedback control loop shown in last slide.
In the feedback control loop we take a measurement of the
process variable (indicated by CV, or controlled variable, in Figure
3.3) we care about (Transmitter in Figure 3.3 means ‘measuring
device’) and this value is compared to a set point (SP) to create an
error, or departure from aim.
In Figure 3.3, OP indicates the operating point around which this
calculation takes place. This error is used to drive the corrective
action of the FCE via the controller. Note that the output of the
controller ‘manipulates’ the mass and energy into the system via
the FCE.

Thus, the property that the controller manipulates is referred to as
the manipulated value, or MV.
The action of the controller may be aggressive or sluggish; it
depends on the internal equations of the controller (sometimes
called the control algorithm or control law) and the tuning that is
used.
In order to successfully control a process, it is important to select
both the right process variable and the right manipulated value.
The process variable is typically a temperature, pressure, ﬂow,
composition or level and is typically a variable that (a) is important
to the product quality or the process operation and (b) is
responsive to changes in the selected manipulated value.

Primary Elements:
• Primary elements, also known as sensors / transmitters, are the
instruments used to measure variables in a process.
• These sensor types can be broadly classified into groups including
the following:
1. Pressure and level
2. Temperature
3. Flow rate and total flow
4. Quality or analysis instruments (e.g. electrolytic conductivity, pH,
pION, moisture, oxidation–reduction potential, gas analysers {O2 ,
CO2 , H2} , thermal conductivity, GLC, heat of reaction, calorific
value)
5. Transducers (working with the above or as individual units)

Final Control Elements:
• Pneumatic, or air-operated, diaphragm control valves are the most
common final control element in process control applications.
• They are used to regulate the flow of material or energy into a
system.
• Variable speed pumps are also possible but are often costly as
motor control is expensive, are less efficient, break down more
often and maintain maximum pressure if they fail.
• Electric valves are seen, but only for large applications above 25
cm pipe/valve diameters.
• Variable electric power control elements such as rheostats are
used in small applications such as laboratory water bath
temperature control.

Final Control Elements – Control Valves:
1. Valve Components
2. Control Valve Sizing
3. Valve Positioners
4. Inherent Valve Characteristics
5. Valve Selection, Rangeability and Pressure Drop

Controller Types:
1. ON / OFF
2. P, PI, PD, & PID Control Algorithms
3. Neuro Fuzzy Control Algorithms
4. Advanced PID Algorithms
5. Nonlinear Control Algorithms, etc.

Assignment Questions:
1.1 Draw an alternate single loop feedback control for the surge tank
shown in figure 3.1. Also comment on the merits / demerits of
new loop structure with the existing one shown in figure 3.1.

Class 3 control system components

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