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A control system is a group of mechanical or electronic devices that uses control loops to control other devices or systems. Control systems are automated with the use of computers.
It is a vital part of the automation industry. Some types of control loops are as follows:
- ICS – Industrial Control Systems
- SCADA – Supervisory Control and Data Acquisition
- DCS – Distributed Control Systems
Elements of Control Loops
- Control server
- IED – Intelligent Electronic Device
- PAC – programmable automation controller
- PLC – programmable logic controller
- RTU – remote terminal unit
These elements of control loops are also part of control systems.
Control systems are used in the following areas for improved production, efficiency and safety:
- Boiler controls and power plant
- Chemical plants
- Environmental control
- Food and food processing
- Metal and mines
- Nuclear power plants
- Pharmaceutical manufacturing
- Pulp and paper mills
- Quality control
- Sewage treatment plants
- Sugar refining plants
- Water treatment plants
Control system addresses the following needs:
- To reduce high labor content of manufactured goods
- To handle smaller production runs
- To gain increased accuracy of manufacture
- For sophisticated tests of product during manufacture
Examples of Control System
- Traffic Lights Control System – An order of input signal is implemented to the control system and the output is one of the three lights that will be turned on for a duration of time, while the other two lights will be turned off. The time duration of turning on and off the lights are based on a traffic study at a specific intersection. The input signal directs the output. The traffic lights control system works based on time.
- Automobile Steering Control System – The driver utilises the variance between the actual direction of travel and the desired direction of travel to result in a controlled fine-tuning of steering wheel.
- Aircraft Flight Path Control System using GPS – The demand for air traffic control systems increases as airplane traffic increases at busy airports. Air traffic control systems and collision avoidance systems are being developed using the GPS (Global Positioning System) navigation satellites. GPS lets each aircraft know its exact location in the airspace landing corridor.
Control System Classification
- Continuous Time vs. Discrete Time
- Continuous Time – all signals are continuous time signals.
- Discrete Time – one or more signals are discrete time signals.
- SISO vs MIMO
- SISO (Single Input and Single Output) – have one input and output.
- MIMO (Multiple Inputs and Multiple Outputs) – have more than one input and more than one output.
- Open Loop vs. Closed Loop
- Open Loop – output is not sent back to the input. Control action is not dependent on the desired output. This requires human intervention.
- Closed Loop – output is sent back to the input. Control action is dependent on the desire output. This is fully automatic, no need for human input.
Some loops can be converted from open to closed modes and vice versa. It is manually controlled when open and fully automated when closed.
Differences between Open Loop and Closed Loop Control Systems
|Open Loop Control System||Closed Loop Control System|
|Independent control action||Dependent control action|
|Without feedback path||With feedback path|
|Non-feedback control system||Feedback control system|
|Can be designed easily||Cannot be designed easily|
|Manually controlled||Fully automated|
If the output or a part of the output is sent back to the input and used as part of the system input, then it is called feedback. Feedback helps substantially to improve control system performance.
Types of Feedback
- Positive Feedback – the reference input and feedback output are added.
- Negative Feedback – error between the reference input and system output is reduced.