What is a control system?
There are 3 types of Control System which is :
i. Pneumatic System
ii. Hydraulic System
iii. Electrical System
Pneumatic control system is a system which uses air that in compress to generate force / energy to carry out work. Pneumatic system many in find out in system industrial like food industry,petrochemical and industry which uses robotics. Generally pneumatic system need:
- Air supply is compressed
- Control valve
- Tube link
- Transducer induce
Hydraulic control system is a system which uses fluid to generate force / energy to carry out work. Hydraulic system many in use in automobile industry like power system, system brake, crane, jack car,satellite and so on. Fluid that commonly used is oil. Generally hydraulic system need :
- Supply fluid is Hydraulic
- Control valve
- Cylinder
Electric control system is a control system which uses electric current whether direct current (AT) or current shuttle as supply source.Generally Electric Control System requires:
- Electricity supply (AT) or (DC)
- Element entry (switch, sensor, valve,electronic component)
- Element product (Motor, light and so on)
- Connector cable
Term used in process control
In this section we will give focus on terms that commonly used in process control.
- System : An interconnection of elements and devices for a desired purpose.
- Process : The device, plant, or system under control.
- Feedback control system : Feedback control system is a system where product always compared with entry set point and difference between value made foundation for control.
- Servo control : Servo control is a feedback control system where the product is position, velocity or acceleration.
- Dynamic variable : Dynamic variable is physical any parameter that variable the value spontaneously or through external influences.
- Error signal : Difference between entry signal and feedback signal/product.
- Process compound variable : process compound variable is process which contains more than one variable.
- Controller : Device which control process certain system and acts against error signal to reduce error to product like is wanted.
- Manipulated variables: These input variables are adjustable dynamically to keep the controlled variables at their set- points.
- Regulatory control: The set point is fixed at a constant value. When any disturbance enters the system, the manipulated variable is adjusted to drive the controlled variable back to its fixed set-point.
- Measurement: Refers to the conversion of the process variable into an analog or digital signal that can be use by the control system. The device that performs the initial measurements called a sensor or instrument.
Basic process of control element
process level control
- A level transmitter (LT), a level controller (LC), and a control valve (LV) are used to control the liquid level in a process tank.
- The purpose is to maintain the liquid level at some prescribed height (H) above the bottom of the tank.It is assumed that the rate of flow into the tank is random.
- the level transmitter is a device that measures the fluid level in the tank and converts it into a useful measurement signal, which is sent to a level controller.
- The level controller evaluates the measurement, compares it wiyh a desired set point (SP), and produces a series of corrective action that are sent to the control valve.
- The valve controls the flow of fluid in the outlet pipe to maintain a level in the tank.
process temperature control
the process of measurement, evaluation and control elements
Measurement
- To control a dynamic variable in a process, must have information about the entiry or variable itself.This information is obtained by measuring the variable.
- Measurement refers to the conversion of the process variable into an analog or gigital signal that can be used by the control system.
- The device that performs the initial measurement is called a sensor or instrument.
- Typically measurements are pressure, level, temperature, flow , position and speed.
- The result of any measurement is the conversion of a dynamic variable into some proportional information that is required by the other elements in the process control loop or sequences
Evaluation
- In the evaluation step of the process control sequences, the measurement value is examined, compared with the desired value or set point, and the amount of corrective action needed to maintain proper control is determined.
- A device called a controller can be pneumatic, electronic or mechanical device mounted in a control panel or on the process equipment.
- It can also be part of a computer control.
- System, in which case the control function is performed by software.
Control element
- The control element in a control loop is the device that exerts a direct influence on the process or manufacturing sequence.
- This final control element accepts an input from the controller and transforms it into some proportional operation that is performed on the process.
- in most cases, this final control element will be a control valve that adjusts the flow of fluid in a process.
- Devices such as electrical motors, pumps, and dampers also used as control elements.
Type of Control Systems
1. Open Loop system
• no feedback
• difficult to control output with accuracy
• a common example of an open- loop control system is an electric toaster in he kitchen.
Open loop block diagram
A type of control system that uses only an input signal to actuate an output. There is no automatic feedback to adjust the process, so adjustments must be made manually by the operator.
2. Closed Loop system
- must have feedback
- must have sensor on output
- almost always negative feedback
Closed loop block diagram
Systems that utilize feedback are called closed-loop control systems. The feedback is used to make decisions about changes to the control signal that drives the plant. An open-loop control system doesn't have or doesn't use feedback.
Differences between Open Loop and Closed Loop
Automatic Control System
An automatic control system is a preset closed-loop control system that requires no operator action. This assumes the process remains in the normal range for the control system. An automatic control system has two process variables associated with it: a controlled variable and a manipulated variable.
A controlled variable is the process variable that is maintained at a specified value or within a specified range.In the previous example, the storage tank level is the controlled variable.
A manipulated variable is the process variable that is acted on by the control system to maintain the controlled variable at the specified value or within the specified range. In the previous example, the flow rate of the water supplied to the tank is the manipulated variable.
Functions of Automatic Control
In any automatic control system, the four basic functions that occur are:
- Measurement
- Comparison
- Computation
- Correction
Advantages
- Accuracy: They are more accurate than open loop system due to their complex construction. They are equally accurate and are not disturbed in the presence of non-linearities.
- Noise reduction ability: Since they are composed of a feedback mechanism, so they clear out the errors between input and output signals, and hence remain unaffected to the external noise sources.
Disadvantages :
- Construction: They are relatively more complex in construction and hence it adds up to the cost making it costlier than open loop system.
- Stability: It is less stable than open loop system but this disadvantage can be striked off since we can make the sensitivity of the system very small so as to make the system as stable as possible
Cascade System
Cascade control system contain integrated sets of control loops.
The needed of a cascade control system
Cascade control system is multiple-loop system where the primary variable is controlled by adjusting the set point of a related secondary variable controller. The secondary variable then affects the primary variable though the process.
Feedback control
- Corrective action taken regardless disturbance source
- Corrective action not taken until after the output has deviated from the set point.
- Problematic for processes with large time constants and/or long time delays.
Feed forward control
- Corrective action can be taken before the output has deviated from the set point.
- Disturbance must be explicitly measured.
- Typically requires a process model.
