Introduction to Control System

Introduction to Control System

1.1 Definitions

   System : A system is a combination or an arrangement of different physical components which act together as an entire unit to achieve certain objective.

Control system : To control means to regulate, to direct or to command. Hence a control system is an arrangement of different physical elements connected in such a manner so as to regulate, direct or command itself or some other system.

In other meaning that we can describe control system is a group of components that maintains a desired result by manipulating the value of another variable in the system. an example that control system is use such as in electrical control. pneumatic control, hydraulic control.

1.1.1 Description of electrical, pneumatic and hydraulic control

Electrical control

electrical control is a control system of an electrical current that's uses either direct current or alternating current as a source of supply. Electrical control has two fundamental laws of current law and voltage law kirchoff. research, for electrical control system involving resistor, capacitor, inductor and amp.  

 

Basic block diagram of an electrical control system manually.

Basic block diagram of an electrical control system automatically by PLC

Explanation about current law and voltage law kirchoff

• Kirchoff current law (the law of the node) states that the algebraic sum of all current entering and leaving a node is ZERO.
• Kirchoff voltage law (loop law) states at any instant the algebraic sum of the voltages around any loop in a circuit is ZERO.

Example :

• Circuit L-R-C

• Complex Impedence

 

 
Pneumatic control
Pneumatic control is the most efficient way to transmit signals and power, whether liquid or gas fluid. Pneumatic control also is a system that uses compressed air to generate power / energy to do the work. The use of pneumatic system :

• Industrial process (the food, petrochemical and industry the use of robotics)
• Speed and position control systems
• Brake systems, horns and bumps.
• Water spraying systems, elevators and doors automatically.
  
  

This system can be controlled manually and automatically.

• Manually Controlled

• Automatically Controlled

 4 elements of pneumatic control systems :

• Compressed air supply.
• Control valves.
• Connecting tube.
• Transducers.
  

Hydraulic control
Hydraulic control is a system that uses fluid to force / energy to do a job. Used in the automobile industry such as power systems, brake system, cranes, jack cars, satellites and so on. The fluid commonly used in oil. 3 elements hydraulic control system of :

• Cylinders
• Hydraulic fluid supply
• Control valves

 

                                              Block Diagram of an Hydraulic Control

1.2 Identify general terms used in process control
 
This part, we focus about general terms used in process control.

• Input : The signal is fed to the system and is known also as a reference or set point.

• Process : An on going process that consists of controlled movement systematically to produce certain results.

• Feedback : A device used to measure the output signal.

• Controller : A device that controls the process of the system and respond to the error signal to reduce the error to produce the required output.

• Output : Usually consists of variables such as temperature, pressure, velocity, etc.

• Error signal : The difference between the input signal and the feedback signal/output.

Example :

1.3 Basics process of control system


1.3.1 Temperature control  

          example: heating electrical system ( dry paint)


              


The system is start from set point (sp) . the sp is desired value or desired temperature. E.g if we wanted the desired temperature of the heating system to be 280 celcius, the sp would be adjusted to 280 celcius.

Next part of the control system that we will be examine is the process variable (PV). Which is the signal that comes from signal. Eg in the heating system, the pv is the signal from thermocouple which is the sensor for this system. The PV also called as feedback signal and it's present value or actual value of the temperature at the instant the sensor reading takes place. Since the sensor continually reading, PV also continually indicate in change temperature.

Summing junction is the place in control system where the sp is compared to the pv. that means if the sp is 280 celcius and the pv signal indicates the actual temperature is 270 celcius , the difference is 10 celcius. the summing junction is identified as sigma.

the difference between sp and pv is called error. The error can be positive value if  the sp is larger than the pv, or it can be negative value if the sp is smaller than the pv.

the controller will use gain, reset and rate to adjust the output signal in responses to the amount of error. gain, reset and rate are also called proportional, intergal and derivate (PID). the PID value can be adjusted to change the speed of responses for the system. E.g gain value, can be used to make the output change the temperature at a given rate of 1 celcius per minute, or it can be set to provide a change of 3 celcius per minute.



1.3.2 Pneumatic controller

 fig 1.3.2 

In automated industrial processes, it is always essential to keep the process variables such as temperature, flow rate, system pressure, fluid level, etc. at the desired value for safety and economical operation. Consider an example where the flow of water through a pipe has to be kept constant at some predetermined value (Fig. 1.3.2). Let the value of flow to be measured is ‘V' (process variable PV). This flow rate is compared with the required flow value say ‘V 1 ' (set point SP). The difference between these two values is the error which is sent to the controller. If any error exists, the controller adjusts the drive signal to the actuator, informing it to move the valve to give the required flow (zero error). This type of control system is called closed loop control system. It mainly includes a controller, actuator and a measuring device.

The control can be achieved by using control electronics or by pneumatic process control. The pneumatic systems are quite popular because they are safe. In the process industries like refinery and chemical plants, the atmosphere is explosive. Application of electronics based systems may be dangerous in such cases. Since the pneumatic systems use air, there are very scant chances of any fire hazards. Even though electrical actuators are available, but most of the valves employed are driven by pneumatic signals.

1.4 Open-loop and Closed-loop

1.4.1 Open-Loop system

A system in which output is dependent on input but controlling action or input is totally  independent of the output charges in output of the system. refer in figure 1.4 a.

 figure 1.4 a

The good example of an open-loop system is an electric switch. This is because output is light and switch is controller of lamp. Any change in light has no effect on the on-off position of the switch, i.e its controlling action. Some other example are traffic signal, automatic toaster system etc.

1.4.1.1 The advantanges and disadvantanges of open-loop system

Advantages	Disadvantages
1)such system simple in contruction	1) such system are inaccurate & unreliable because accuracy of such system are totally dependent on the accurate precalibration of the controller.
2) very much convenient when output is difficult to measure	2) such systems give inaccurate results if there are variations in the external environment.
3) such system are easy from maintenance point of view.	3) similarly they cannot sense internal disturbances in the system after the controller stages.
4) generally these are not troubled with the problems of stability.	4) to maintain the quality & accuracy, realibration of the controller is necessary time to time.

1.4.2 Closed-loop system

A system in which the controlling action or input is somehow dependent on the output or changes in output is called closed loop system. (for dependence of input on the output, such system uses the feedback property. refer figure 1.4 b for block diagram.

figure 1.4 b

1.4.2.1 The advantanges ang disadvantages of closed-loop system

Advantages	Disadvantages
1)accuracy of such system is always very high because controller modifies and manipulates the actuating signal such that error in the system will be zero.	1)such systems are complicated and time consuming from design point of view and hence costlier.
2) such system senses environmental changes, as well as internal disturbances and accordingly modifies the error.	2) dueto feedback, system tries to correct the error time from time. Tendency to overcorrect the error may cause oscillattions without bound in the system. Hence system has to be designed taking into consideration problems of instability due to feedback. The stability problems are severe and must be taken care of while designing the system.
3) in such system, there is reduced effect of nonlinearities and distortions.
4)bandwidth of such system i.e. operating frequency zone for such system is very high.