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DIGITAL CONTROL SYSTEMS. Muhammad Aatif Shahbaz. AGENDA. Overview of Control System Types of Control System Digital Control System Automation Examples. What is a Control System?.
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DIGITAL CONTROL SYSTEMS Muhammad Aatif Shahbaz
AGENDA • Overview of Control System • Types of Control System • Digital Control System • Automation Examples
What is a Control System? • A control systemis a device, or set of devices to manage, command, direct or regulate the behavior of other device(s) or system(s). • Types of Control System • Open Loop • Feedback • Feed-forward
Open Loop System • An open-loop, also called a non-feedback, is a type of system that computes its input into a system using only the current state and its model of the system. • The drawback of open-loop control is that it requires perfect knowledge of the system (i.e. one knows exactly what inputs to give in order to get the desired output), and it assumes there are no disturbances to the system.
Feedback System • The input to a feedback controller is the same as what it is trying to control - the controlled variable is "fed back" into the controller. • Feedback system settles slowly, usually results in intermediate periods where the controlled variable is not at the desired set-point • Also called Closed loop system
Room Heating System • Consider following scenario, a thermostat is use to put heater On and Off after monitoring the temperature of the room. • Suppose, door of the room left opened. • System remains in intermediate state, until it reaches the lower bound temperature. Environment (Door, Windows) Temperature
Feed-forward System • Feed-forward is a kind of closed loop system which reacts to changes in its environment, usually to maintain some desired state of the system • Feed-forward control can avoid the slowness of feedback control. With feed-forward control, the disturbances are measured and accounted for before they have time to affect the system.
Combined Approach • Feed-forward provides the changes in environment as they occurred to the system. Hence act faster against the disturbances. • Combined approach will provide better solution to this problem. Environment (Door, Windows) Temperature
Digital Control System • Digital control is a branch of control theory that uses digital computers to act as system controllers. Depending on the requirements, a digital control system can take the form of a microcontroller to an ASIC to a standard desktop computer. • Since a digital computer is a discrete system, the Laplace transform is replaced with the z-transform. • A digital controller is usually cascaded with the process in a feedback system. The rest of the system can either be digital or analog.
Why use DCS? • Drawbacks of AnalogControl System • Passive components have aging problem • Circuit parameters and operating points vary with temperature • The control knobs (variable resistors) are vulnerable to dust and oxidation • All of these affect the performance and service life of an Analog controller. • A digital controller is typically constructed as algorithm running in a microcontroller, an ASIC, or a flexible hardware platform such as FPGA. The parameters are kept in system memory and can be adjusted conveniently and accurately by a user.
Types of Digital Controllers • The most simple is on-off controller like the thermostat that just turns the heat on if the temperature falls below a certain value and off it exceeds a certain value. • Another simple type is linear controller which uses linear negative feedback to produce a control signal. PIDcontroller is one of that kind. It is still widely used in industries because of its simplicity. • The user just installs a controller and adjusts 3 gains to get the best achievable performance.
Automation Examples • Automobile steering • A light seeking system • Auto-pilot system • Machine positioning • Robotic welding • Motor speed control • Video games system
