Class 3 control system components

ICE401: PROCESS INSTRUMENTATION

AND CONTROL

Class 3: Control System Components

Dr. S. Meenatchisundaram

Email: [email protected]

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 theinstruments used to measure variables in a process.

• These sensor types can be broadly classified into groups includingthe 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 mostcommon final control element in process control applications.

• They are used to regulate the flow of material or energy into asystem.

• Variable speed pumps are also possible but are often costly asmotor control is expensive, are less efficient, break down moreoften and maintain maximum pressure if they fail.

• Electric valves are seen, but only for large applications above 25cm pipe/valve diameters.

• Variable electric power control elements such as rheostats areused in small applications such as laboratory water bathtemperature 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.



Engineering

Class 3 control system components