Title: Fundamental Control Theory
Course: 2016FA-MCO-110-401 > Section: Module 5: Control Theory

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Fundamental Control Theory

Discrete Control

Also known as Digital Control, Discrete Control is the control of systems that involve controlling whether or not system components are either on or off [3]. If something is discrete or digital it means that it is on or off (or at least has a number of states like this that can be counted).

Open Loop

An open loop system does not use feedback in the control process. The current state is the only information used and the output of the process is not monitored. A control signal (input) is turned into action (or output) using a predetermined relationship between the two [4].

Closed Loop

A closed loop system gets its feedback from the variable that is being controlled. The feedback is used when trying to maintain the set point. The sensor monitors the variable, this value is turned into a number appropriate to what is being measured, this is then compared to the set point (the difference between the set point and the variable is known as the error), finally this number is fed back into the system for the error to be minimized and the error counteracted.

Feedback

Feedback is information usually given in the form of error (difference between the sensed variable and the set point), that is used by the control system to maintain the set point.

Process Control

Process control is the name of an engineering discipline that involves the mechanism, algorithms, and architecture for controlling a process. Process control can be described as a mixture of statistics and engineering [5]. It can be thought of as a system that uses computers or controllers to automatically regulate operations or processes [3]. Mission critical operations, especially those in critical manufacturing, use process control.

Some instances where process control is sometimes used:

- Controlling the height of fluid in a tank to prevent its overflow.
- Controlling the temperature of a water stream by controlling the amount of steam added to a heat exchanger [5].

The two main things that most businesses desire from process control are maintaining a process or changing or transitioning a process.

Maintaining

Process control can be used in such a way that it makes operations safer, keeps a process running at maximum efficiency, or keeps a process running with the maximum quality objectives met.

Transitioning

Process control can be used to carry a process from one operational state to another. A change in specifications by a customer, a change in the environment, a budget change, a new environmental regulation, or a newly realized safety concern can cause a need for a transition. [5]

Process control systems can measure all kinds of things. Some of the most common things measured are temperature, flow rate, humidity, pressure, and electrical behavior. Process control systems make use of measuring devices like thermostats[1] in order to make these measurements. Control systems are provided with feedback in one of two ways. Open loop systems use a manual measurement device and provide manual feedback. Closed loop systems use an automated measurement device and provide automated feedback [6]. Process control systems typically are closed loop systems.

Signal Types

Digital

“A term applied to a signal or device that uses binary digits to represent continuous values or discrete states; pertaining to data that is in the form of digits [3].” If something is digital it is quantifiable and discrete.

Analog

“Data representation across a continuous range of signals… is referred to as analog data [7].” The value of the signal directly represents the variable being measured. If something is analog it is continuously variable.

Conversion

The real world is analog by nature. However, things in the computer controlled world are digital by nature. To get information from the real world into a computerized control, one needs A/D and to get from the computerized control world to the real world one needs D/A.

D/A conversion

Digital data can be converted into analog data. You might see the conversion referred to as Digital Analog Conversion (DAC) or as D/A Conversion.

A/D Conversion

Analog data can be converted into digital data. This conversion can be referred to as A/D Conversion or Analog Digital Conversion (ADC).

As a simple example, take a car's cruise control. The rotational speed of the tires on the car are converted from an analog (continuously variable) signal to a digital (discrete and quantifiable) signal (typically a number) via an A/D converter. The car's computer then calculates the appropriate throttle response and generates a number that is sent to a D/A converter to make the necessary analog signal to manipulate the actuator (throttle) to have the car maintain the desired speed (setpoint). This process repeats as fast as the analog input sensor (wheel speed) can acquire new data (sampling rate) and send it to the controller (communication rate). The controller then processes the control algorithm in its code (processor rate) and sends the signal back to the D/A (communication rate, again) which then feeds the analog ouput data to the actuator as fast as the actuator can take it in (sampling rate, again) and the action is applied to the manipulated variable (in this case, the throttle).

Motion Control

Motion control is a hybrid of discrete and process control. Positions and velocities are both controlled by analog (or digital converted to analog) methods. Motion control is often used in assembly, packaging, textile, semiconductor, and printing. It is at the heart of CNC machining and robotics. A typical motion control system consists of the motion controller, a drive or amplifier that turns the signals into an electrical voltage, an actuator (which the voltage is applied to), and feedback sensors [2].

[1] Thermostat – “An automatic device for regulating temperature [36].”

Objectives met:
- C5
- 5.2
- 5.3
- 5.4