CHAPTER 2: CONTROLLER PRINCIPLE
What is Controller?
A controller, in a computing context, is a hardware device or a software program that manages or directs the flow of data between two entities. In computing, controllers may be cards, microchips or separate hardware devices for the control of a peripheral device. In a general sense, a controller can be thought of as something or someone that interfaces between two systems and manages communications between them.
Here are a few examples of controllers:
A graphics card is an integrated circuit card in a computer or, in some cases, a monitor that provides digital-to-analog conversion, video RAM, and a video controller so that data can be sent to a computer's display.
Figure: An Example of Controller Block Diagram
Type of Controller
- ON /OFF Controller
- Analog Controller
Two Position Controller(on/off)
An On /Off controller is the simplest form of temperature control device. The output from the device is either on or off, with no middle state. An On /Off controller will switch the output only when the temperature crosses the set-point. For heating control, the output is on when the temperature is below the set-point, and off above set-point. This differential requires that the temperature exceed set-point by a certain amount before the output will turn off or on again. On /Off differential prevents the output from “chattering” or making fast, continual switches if the cycling above and below the set-point occurs very rapidly. On-off control is usually used where a precise control is not necessary, in systems which cannot handle having the energy turned on and off frequently, where the mass of the system is so great that temperatures change extremely slowly, or for a temperature alarm. One special type of on-off control used for alarm is a limit controller. This controller uses a latching relay, which must be manually reset, and is used to shut down a process when a certain temperature is reached.
This is a non linear controller which is very simple and it does not need any design. The on /off Controller is defined as:
u(t) = U max if e(t) > 0
U min if e(t) < 0
Where: e(t) = r(t) - y(t) is the tracking error and u(t) is the applied control system.
Two-position control compares the value of an analog or variable input with instructions and generates a digital (two-position) output. The instructions involve the definition of an upper and lower limit. The output changes its value as the input crosses these limit values. There are no standards for defining these limits. The most common terminology used is setpoint and differential. The setpoint indicates the point where the output pulls-in, energizes or is true. The output changes back or drops-out after the input value crosses through the value equal to the difference between the setpoint and the differential.
U min if e(t) < 0
Where: e(t) = r(t) - y(t) is the tracking error and u(t) is the applied control system.
Two-position control compares the value of an analog or variable input with instructions and generates a digital (two-position) output. The instructions involve the definition of an upper and lower limit. The output changes its value as the input crosses these limit values. There are no standards for defining these limits. The most common terminology used is setpoint and differential. The setpoint indicates the point where the output pulls-in, energizes or is true. The output changes back or drops-out after the input value crosses through the value equal to the difference between the setpoint and the differential.
Two-position control can be used for simple control loops (temperature control) or limit control (freezestats, outside air temperature limits). The analog value can be any measured variable including temperature, relative humidity, pressure, current and liquid levels.
Time can also be the input to a two-position control response. This control response functions like a time clock with pins. The output pulls-in when the time is in the defined on time and drops out during the defined off time.
An analog controller is a device which implements the controller modes described in this chapter, using analog signals to represents the loop parameters. The analog signal may be in the form of an electric current or a pneumatic air pressure. The controller accepts a measurement expressed in terms of one of these signals, calculates an output for the mode being used, and outputs an analog signal of the same type. Sins the controller does solve equations, we think of it as an analog computer. The controller must be able to add, subtract, multiply, integrate, and find derivatives. It does this working with analog voltages or pressures.