Basic of Controller & Control Loop

A controller is an instrument that receives an input signal from the transducer or transmitter as measured value (MV), compares this measured value with the desired value (SV), and produces an output signal for the correcting element/ control value.

Types of Controller: Controller can be classified in many different ways.

Based on Auxiliary Power

• a) Pneumatic Controller
• b) Electronic Controller

Based on the Working Principle

• a) On-off controller
• b) Proportional controller
• c) Proportional plus Integral (PI) controller
• d) Proportional plus Derivative (PD) controller
• e) Proportional plus integral plus Derivative (PID) controller

 Proportional (P) Controller:

In the proportional controller, the output variation is proportional to the deviation between the measured value (MV) and the desired value (SV). The figure shown below is a schematic diagram of a pneumatic proportional controller.

At a balanced condition, the resultant of the moments should be zero. So taking a moment around the pivot (Clockwise +ve), we get

a. Pm - a. Ps + b. P_f - b. Po = 0

Or Po =  a/b (Pm - Ps) + P_f

Or Po = k.e + 0.6 bar,     Where P_f = load spring pressure = 0.6 bar

                                          & K = 1/ PB = 100 / (PB in %)

So the output equation can be written as Po = k.e + 0.6 bar

Offset (Closed - Loop)

The Disadvantage of Proportional Controller: Remains offset.

Proportional + Integral (PI) Controller:

In the proportional + Integral (PI) controller, a reset bellows with a variable restrictor is introduced which replaces the load spring. It is capable of making offset zero by integral action.

Features of PI Controller

• Load spring is replaced by reset bellows with a variable restrictor
• Variable restrictor is a needle valve and the dial is graduated in terms of time
• The output of the PI controller, P_o = K.e + K / T_i  ∫ e dt + K_o Bar
• Because of integral action, the output continues to change until there exist deviation
• Ultimately there is no offset

Different Conditions of Reset Restrictor:

• Restrictor fully closed (p controller)
• Restrictor fully open (ON-OFF controller)
• Restrictor partially open (PI controller)

Reset / Integral Time:

Reset or integral time is the time required to change the output of the controller due to integral action which is equal to the same amount of change due to the proportional action.

Proportional + Integral + Derivative Controller (PID):

Features of PI D Controller

• Variable restrictor is introduced with the feedback below
• Restrictor dial is graduated in terms of time
• The output of PID controller, P_o = K.e + k / T_i  ∫  e dt + K T_d. de/dt + K_o  Bar

Typical Response of a PID Controller to Change Measured Value

Control Loop: The arrangement of instruments connected with each other to control a physical variable to the desired value is called a control loop.

Elements of Control Loop:

• Process: a variable that has to be controlled
• Measuring system: measures the physical variable
• Transmitter: The transmitter measured the signal to the controller
• Controller: This generates the control command according to the desired value
• Final control element: Which carried out the control command
• Connecting tubes and pipes: which passes the signals

Types of Control:

1. Feedback Control ( closed-loop control)

2. Feedback Control (open-loop control)

Block Diagram of Feedback Control Loop

Simple Temperature Control Loop:

Temperature can be controlled according to the above control loop. However, there is a fluctuation in the measured value because the temperature-sensing response time is high.

Cascade Control Loop:

In cascade control, two control loops are coupled to each other. The output of one controller is the set point for another controller. The cascade control loop is often called “Mater-Slave” control.

The cascade control loop consists of the following:

• Two measuring systems /transmitters
• These two controllers; one is the master and another is the slave
• One control valve

Block Diagram of Cascade Control Loop

Operation: For a particular set-value of the master controller TIC (PID controller), if the temperature of the hot water increases the output of TIC is feeding as the remote set-point of FIC also increases causing the control valve to close. At the same time, the steam flow will decrease and the salve controller FIC will get a sense through FT. Ultimately the temperature will decrease and control at set value. During the decrease in hot water temperature, the reverse operation will happen.