Controller Principle

CHAPTER 2: CONTROLLER PRINCIPLE

What is Controller?

A controller, in a computing context, is a hardware device or a software program that manages or directs the flow of data between two entities. In computing, controllers may be cards, microchips or separate hardware devices for the control of a peripheral device. In a general sense, a controller can be thought of as something or someone that interfaces between two systems and manages communications between them.

Here are a few examples of controllers:

A graphics card is an integrated circuit card in a computer or, in some cases, a monitor that provides digital-to-analog conversion, video RAM, and a video controller so that data can be sent to a computer's display.

Figure: An Example of Controller Block Diagram 

Type of Controller

• ON /OFF Controller
• Analog Controller

Two Position Controller(on/off) 

An On /Off controller is the simplest form of temperature control device. The output from the device is either on or off, with no middle state. An On /Off controller will switch the output only when the temperature crosses the set-point. For heating control, the output is on when the temperature is below the set-point, and off above set-point. This differential requires that the temperature exceed set-point by a certain amount before the output will turn off or on again. On /Off differential prevents the output from “chattering” or making fast, continual switches if the cycling above and below the set-point occurs very rapidly. On-off control is usually used where a precise control is not necessary, in systems which cannot handle having the energy turned on and off frequently, where the mass of the system is so great that temperatures change extremely slowly, or for a temperature alarm. One special type of on-off control used for alarm is a limit controller. This controller uses a latching relay, which must be manually reset, and is used to shut down a process when a certain temperature is reached.

This is a non linear controller which is very simple and it does not need any design. The on /off Controller is defined as:

u(t) =  U max if e(t) > 0
          U min  if e(t) < 0

Where: e(t) = r(t) - y(t) is the tracking error and u(t) is the applied control system.

Two-position control compares the value of an analog or variable input with instructions and generates a digital (two-position) output. The instructions involve the definition of an upper and lower limit. The output changes its value as the input crosses these limit values. There are no standards for defining these limits. The most common terminology used is setpoint and differential. The setpoint indicates the point where the output pulls-in, energizes or is true. The output changes back or drops-out after the input value crosses through the value equal to the difference between the setpoint and the differential.

Two-position control can be used for simple control loops (temperature control) or limit control (freezestats, outside air temperature limits). The analog value can be any measured variable including temperature, relative humidity, pressure, current and liquid levels.

Time can also be the input to a two-position control response. This control response functions like a time clock with pins. The output pulls-in when the time is in the defined on time and drops out during the defined off time.

Analog Controller

An analog controller is a device which implements the controller modes described in this chapter, using analog signals to represents the loop parameters. The analog signal may be in the form of an electric current or a pneumatic air pressure. The controller accepts a measurement expressed in terms of one of these signals, calculates an output for the mode being used, and outputs an analog signal of the same type. Sins the controller does solve equations, we think of it as an analog computer. The controller must be able to add, subtract, multiply, integrate, and find derivatives. It does this working with analog voltages or pressures. 

Introduction to Control System

Control system is an interconnection of components forming a system configuration that will provide a desired system response . The basic for analysis of a system is the foundation provided by linear system theory , which assumes a cause effect relationship for the components of a system . Therefore a component or process to be controlled can be represented by block diagrams Figure 1 & Figure 2 .

Figure 1. An example of open loop block diagram 

Figure 2 . An example of closed loop

There are two type control system, namely logic gate and feedback control or linear. These control system will determine the type of loop that is created as shown in figure 1 & 2 .

These control system uses mainly 3 types of system .
Which are :  

• ELECTRICAL CONTROL SYSTEM ( AC / DC )

• PNEUMATIC CONTROL SYSTEM

• HYDRAULIC CONTROL SYSTEM

Using these basic three control systems , a sophisticated automation control systems can be created.

Guide on Designing a Control System

1 . Establish control goals 

2. Identify the variables to be control 

3. Write the specifications for the variables 

4. Establish the system configuration and identify the actuator

5. Obtain a model of the process and the sensor 

6. Describe a controller and select key parameters to be adjusted 

7. Optimize the parameters and analyze the performance

Types of Systems

HYDRAULIC CONTROL SYSTEM

A hydraulic control is a drive or transmission system that uses pressurized hydraulic to drive hydraulic machinery. The term hydrostatic refers to the transfer of energy from flow and pressure, not from the kinetic energy of the flow. Generally to create a simple hydraulic system , it would require

•  Hydraulic Pump
• A Fluid Tank / Reservoir
• Control Valve
• Cylinder 

 ELECTRICAL CONTROL SYSTEMS

 Electric control system is a control system which uses electric current whether direct current (DC) or alternate current as a supply source. Generally a simple electrical system can be created using 

• A power supply 
• Actuators
• Control logic / "Brain"

 PNEUMATIC CONTROL SYSTEMS

Pneumatics were often utilized to transmit information and control using pressure.This system which uses air that in compress to generate force / energy to carry out work.Generally pneumatic system need 

• air compressor supply
• air cylinder
• valve
• tranducer
• solenoid valve
• pressure gages 

General Terms Used in Process Control 

1. Input Variable –

This variable shows the effect of the surroundings on the process. It normally refers to those  factors that influence the process.

An example of this would be the flow rate of the steam through a heat exchanger that would change the amount of energy put into the process.

2. Output variable -

Also known as the control variable . These are the variables that are process
outputs that effect the surroundings.

An example of this would be the amount of CO2 gas that comes out of a combustion reaction.

                               

3 . Damping -

The progressive reduction or suppression of the oscillation of a system.

4 . Process -

The collective functions performed in and by the equipment in which the variable(s)
is (are) to be controlled.

5 . Range-

The region between the limits within which a quantity is measured, received, or
transmitted, expressed by stating the lower and upper range values.

An example would be the difference between upper limit value and lower limit value of a temperature sensor to either start or stop a process

6 . Dead Time- 
The amount of time it takes for a process to start changing after a disturbance in the system.

Basic Process Control System

LEVEL CONTROL IN A TANK 

 The liquid level control sytem is mainly consists of a tank, which is filled with liquid.The liquid sensor(transducer) is placed in a liquid tank, which senses the level of the liquid.That sensor is connected to the transmitter.Again the transmitter is connected to the controller, where the signal is to be controlled and compared with the set point value.The controller is connected to the control valve, which controls the flow of liquid.

TEMPERATURE 

  This is a simple temperature control system that is mainly consisting of two liquid pipelines which are  filled two difference temperature. The main pipeline is filled with cold temperature liquid  and having a sub hot temperature liquid pipeline attached . The temperature sensor(transducer) is placed directly in contact to the output valve ,  which then senses the initial liquid temperature .The sensor is connected to the transmitter which is connected to the controller, where the signal is to be controlled and compared with the desired temperature .The controller is connected to the control valve, which controls the flow of hot liquid into the main pipeline , will then adjust the amount of hot liquid needed to enter the main liquid line that will increase the temperature of the water to the desired temperature .

Open And Close Loop Systems

OPEN LOOP SYSTEMS

Open loop control is by far the more simple of the two types of control theory. In open loop control, 

 there is some sort of input signal (digital or analog), which then passes through amplifiers to produce the proper output, and is then passed out of the system. Open loop controls have no feedback and require the input to return to zero before the output will return to zero. also called a non-feedback controller, is a type of controller that computes its input into a system using only the current current state and its model of the system.

CLOSED LOOP SYSTEMS

In closed loop control, the system is self adjusting. Data does not flow one way, it may pass back from a specific amplifier (such as velocity or position) to the start of the control system, telling it to adjust itself accordingly. Many physical systems are closed loop control at the lowest level since the data about velocity and current position modify the output (also position) at consistent rate.