DIGITAL CONTROL SYSTEM

Digital control systems are a type of control system that uses digital signals to control or regulate a process or system. Digital control systems are characterized by their ability to process discrete digital signals which takes on only a finite number of values. Digital control uses discrete time signals, which are sampled, quantized and processed by a digital computer or microcontroller. Continuously physical signals are converted via analog to digital converter (ADC) , processed algorithmically and then converted back via digital to analog converter (DAC) to drive actuators.

The main types of digital control systems are; digital proportional integral derivative (PID), digital programmable logic controllers (PLC), digital computer control system and digital state space control systems.

Digital PID control system uses a digital PID algorithm to control or regulate the output. Digital PID controller implements the classic proportional integral derivative algorithm using sampled data instead of continuous analog circuits. Example of digital PID control system includes; room temperature control, drone altitude hold, 3Dprinter hot end temperature, cruise control in cars etc.

Digital programmable logic controller (PLC) uses a digital PLC to control output. Digital programmable logic controller refers hardware/firmware based logic control as implemented by PLCs. The core control concept is Boolean logic. Example of digital PLC includes; conveyor belt sequencing, elevator control system, traffic light controller, bottling plant safety interlock etc.

Digital computer control system uses a digital computer to control or regulate the output. Digital computer control is the general architecture where a computer, microcontroller, DSP or industrial PCs closes the control lop. Example of digital computer control includes; automotive engine control unit (ECU), CNC machine control, HVAC building management control system, washing machine cycle control etc.

Digital state space control system uses a digital state space algorithm to regulate or control the output. Digital state space control uses state variables to describe system dynamics in discrete time. It is well suited for MIMO (multi input multi output) systems. Examples of digital state space control system includes; space craft altitude control, inverted pendulum on a cart, self-balancing robot, power grid frequency/voltage control, autonomous vehicle path tracking etc.

The advantages of digital control systems are; digital control systems can provide high accuracy and precision control which is essential in applications where precise control is required. Digital control system can be easily modified and reconfigured which makes them highly flexible. Digital control system can be controlled remotely using digital communication networks which makes them ideal for applications where remote control is required. Digital control systems can be easily reprogrammed with built in diagnostics, data logging with remote monitoring software incorporated. Digital control systems can implement complex algorithms impossible with analog systems. Digital control systems are easily integrated with networks, IoT, SCADA, cloud systems etc. digital control systems are immune to component aging and drift though ADC/DAC still introduce errors in their conversion.

The disadvantages of digital control systems are; digital control systems can be complex and require sophisticated software and hardware. Digital control systems can be expensive especially for high performance applications. Digital control system signal sampling introduces delay and quantization errors. Digital control system requires ADC to DAC conversion and vice-versa, which adds cost, complexity and potential noise from conversion. Digital control system is prone to aliasing risk if sampling rate is too low. Digital control systems are prone to software bugs, cyber-attacks and firmware vulnerabilities. Digital control systems require enormous power in their operation. Digital control system requires specialized monitoring and maintenance.

The application of digital control systems are as follows; digital control systems find application in industrial control systems as PLC, DCS systems in manufacturing plants, refineries, automotive engine control, autonomous driving stacks, aerospace control systems as fly by wire systems, satellite altitude control, autopilots, robotics motion controls in manipulator arms, legged robots, consumer electronics as smart thermostats, washing machines, HVAC systems, power systems such as smart grid control, inverters, renewable energy management such as solar and wind turbines, medical devices such as infusion pumps, ventilators, pacemakers, as embedded digital control.

The future of digital control system is based on the advances and development of the following technologies; AI and ML integration in digital control systems will enhance their use in complex models requiring algorithms to regulate and control robots, autonomous vehicles, and cloud based digital control systems with lower latency for critical or extreme robotic and autonomous systems. Since digital controlled systems are prone to cyber-attacks, securing them against these attacks are prime consideration in future advances and development in digital control system research. Real time virtual models or digital twins paired with digital controllers will be used for predictive maintenance and optimization before physical failures occur. Digital control systems algorithms incorporated in MPC and adaptive control with AI will enable fully autonomous systems that will adjust control strategy in real time without human tuning. In the longer term, quantum and neuro-morphic computing may enable ultra-fast digital control computations for extremely high speed or high dimensional systems. However digital control flexibility and integration with AI/data infrastructure make it clear the long term direction which is analog digital hybrids for high speeds as well as safety critical roles is the way to go.

 

SOURCES:

• Digital control engineering: Analysis and design by M. Sami Fadali and Antonio Visioli.
• Discrete time control systems by Katsutuko Ogata.
• Computer controlled systems: Theory and design by Karl J. Astrom and Bjorn Wittenmark.
• Digital control system: Analysis and design by Charles L. Philips and H. Troy Nagle.
• Digital control systems by B.C Kuo.