Closed-loop Control Systems

Closed-loop Control

• It uses feedback control to minimize errors within the system.
• "Feedback" differentiates closed-loop systems from open-loop systems.
• Output accuracy depends on the feedback path.
• Feedback control is more common than open-loop or feed forward control due to its accuracy in electronic control systems.
• It reduces a system's sensitivity to external disturbances, providing robust control, compensating changes in the feedback signal.
• Main characteristics include automatically adjusting system input to reduce errors.
• Improves stability of unstable systems and the robustness against external disturbances.
• Enhances or reduces system sensitivity and produces reliable, repeatable performance.
• Closed-loop systems require complexity with one or more feedback paths to provide control.
• Controller gain changes can cause instability and oscillation if too sensitive.

Types of Controllers

• PID (Proportional Integral Differential)
• Microcontroller
• PLC (Programmable Logic Controller)
• DCS (Distributed Control System)
• SCADA (Supervisory Control And Data Acquisition)

PID Controller

• It adjusts a system based on current and intended settings, widely used in industrial control.

P Action

• Requires large gain to improve steady state error, suitable for stable systems, like those with one energy storage.
• Can be used if constant steady state error is acceptable.
• Small steady state errors are acceptable with sensor gives measured value with error or the importance of measured value is not too great.

PD Action

• Derivative mode controls slow process dynamics with large inertia.
• Integral mode makes already slow dynamics even slower.

PI Action

• It is the most used type in industry.
• Used when fast system response is not required.
• Used when substantial noise and disturbances are present during the process.
• Used when there is only one energy storage in the process, either capacitive or inductive.
• Used when there are significant transport delays.

Microcontroller

• It is a computer on a single chip with computer system hardware and software requirements.
• Microcontrollers are inexpensive.
• Microcontrollers can store and run unique programs (small controllers) making them versatile.
• Microcontrollers can perform math and logic functions.
• They are faster, field programmable, flexible, and easy to use.
• Component include a programmable chip in printed circuit with high power components.
• Programs control a plant by receiving and processing inputs through internal circuits.
• It is easily reprogrammed according to modifications or requirements in control actions.

PLC (Programmable Logic Controller)

• PLCs function like computers, receiving inputs and producing outputs based on a stored program.
• PLCs can be used in complex plants but are more complicated than microcontrollers.
• The advent of programmable controllers has significantly changed process control system design.
• They reduce wiring by 80% compared to conventional process control systems.
• PLCs consume less power than relays.
• Diagnostic functions in PLC controllers enable easy error detection.
• Operating sequence changes or application alternatives of a PLC controller is easily done by replacing a program via console or PC software without rewiring (unless extra input/output devices are required).
• PLCs are cheaper than conventional systems, especially when numerous I/O instruments are needed.

DCS (Distributed Control System)

• A distributed control system controls a manufacturing system, any dynamic system where controller elements are not central but distributed.
• Each component subsystem is controlled by one or more controllers.
• The controller system connects by networks for communication and monitoring.
• DCS is used for monitoring and controlling distributed equipment.
• The system comprises Field Control Stations (FCS) featuring I/O modules, a CPU, and a communication bus.
• An operator station serve as a human interface with a monitor.
• Operators can view the process, check alarms, change settings and print reports.
• The Engineering station configures all inputs, outputs, drawings, and monitoring elements on an Operator station monitor.

SCADA (Supervisory Control and Data Acquisition)

• SCADA systems control geographically dispersed assets over thousands of square kilometers.
• Centralized data acquisition and control are critical for system operation.
• Found in water/wastewater, oil/gas, electrical grids, and railway systems.
• The control center provides monitoring and control for field sites over communication networks and alarms.

Analog Sensors

• Analog sensors communicate with a controller by sending an electrical voltage along a wire.
• The controller interprets the voltage as a numeric value for processing.

Digital Sensors

• Digital Sensors also send a voltage.
• However Digital sensors send only zero OR maximum voltage.
• A controller that detects a voltage above a guaranteed Low or below High cannot determine results.

Sensor Features

• Accuracy: The maximum difference between indicated and actual reading. If a sensor reads 100N with ±1% accuracy, the force is between 99N and 101N.
• Resolution: The smallest increment detectable, used for stepped readings.
• Repeatability: The variation in repeated readings of a single condition.
• Linearity: A linear relationship between input phenomenon and output signal, a desirable feature.
• Non-linear relationships complicate the conversion from sensor output to calculated quantity.
• Precision incorporates accuracy, resolution, and repeatability.
• Range defines the natural limits for the sensor.
• Dynamic Response: The regular operating frequency range.
• Sensors have an upper and possibly a lower operation frequency.
• Environmental Considerations: Sensors all have some limitations over factors (temperature, humidity, dirt/oil, corrosives and pressures).
• Cost: Typically goes higher with added precision, but if some sensors are inexpensive, the signal conditioning equipment costs can be significant.