Basic process in Control System (level control in tank)

Non-adjustable on/off level control

Description
Non-adjustable on/off level control uses a conductivity probe connected to an electronic controller. The probe typically has three or four tips, each of which is cut to length during installation to achieve the required switching or alarm level (see Figure).

When the tip of the probe is immersed in liquid it uses the relatively high conductivity of the water to complete an electrical circuit via the tank metalwork and the controller.
When the water level drops below the tip, the circuit resistance increases considerably, indicating to the controller that the tip is not immersed in the liquid.
In the case of a simple 'pumping in' system with on/off level control:

- The valve is opened when the tank water level falls below the end of a tip.
- The valve is closed when the water level rises to contact another tip.
- Other tips may be used to activate low or high alarms.

Advantage
A simple but accurate and relatively inexpensive method of level control.

Applications:
The system can be used for liquids with conductivities of 1 µS/cm or more, and is suitable for condensate tanks, feedwater tanks and process vats or vessels. Where the conductivity falls below this level it is recommended that capacitance based level controls are used.

Point to note:

If the tank is constructed from a non-conductive material, the electrical circuit may be achieved via another probe tip.

                                                      General arrangement of a non-adjustable on/off level control system for a tank

General term that used in process control

Analog Signal
Analog signals are like voltage or electric current signal, representing temperature, pressure, level etc. Usually the electrical current signal is of magnitude 4-20 mA where 4 mA is the minimum point of span and 20 mA is the maximum point of span.

Analog to Digital Converting, A-D Converting
Electronic hardware converts analog signal like voltage, electric current, temperature, or pressure into digital data a computer can process and interpret.

Auto Mode
In auto mode the output is calculated by the controller using the error signal - the difference between set point and the process variable.

Closed Loop
Controller in automatic mode.

Cascade
Two or more controllers working together. The output of the master controller is the set point for the "slave" controller.

Controller Output - CO
Output signal from the controller.

DDE Windows Dynamic Data Exchange
A standard Microsoft operating system method for communicating between applications. Replaced by OLE for process control - OPC.

Dead Band
The range through which an input can be varied without initiating a response.

Dead Time
Dead time is the amount of time it takes for the process variable to start changing after changing output as a control valve, variable frequency drive etc.

Derivative - D
The derivative - D - part of a PID controller. With derivative action the controller output is proportional to the rate of change of the process variable or process error.

Delay
A term commonly used in stead of dead time.

Deviation
Any departure from a desired or expected process value.

Digital Signal
A discrete value at which an action is performed. A digital signal is a binary signal with two distinct states - 1 or 0, often used as an on - off indication.

Digital Control System - DCS
Digital Control System - DCS refers to larger digital control systems.

Discrete Logic
Refers to digital "on - off" logic.

Discrete I/O
On or off signals sent or received to the field.

Dominant Lag Process
Most processes consist of both dead time and lag. If the lag time is larger than the dead time, the process is a dominant lag process. Most process plant loops are dominant lag types. This includes most temperature, level, flow and pressure loops.

Error
In the control loop the error = set point - process value.

Gain
Gain = 100 / Proportional Band. More gain in the controller gives a faster loop response and a more oscillatory (unstable) process.

Gain in the process is defined as the change in input divided by the change in output. A process with high gain will react more to the controller output changing.

Gain Margin
The difference in the logarithms of the amplitude ratios at the frequency where the combined phase angle is 180 degrees lag is the gain margin.

Hysteresis
The signal change before the output unit (valve or similar) moves.

Input/Output - I/O
Electronic hardware where the field devices are wired.

Integral Action - I
The integral part of the PID controller. With integral action, the controller output is proportional to the amount and duration of the error signal. If there is more integral action, the controller output will change more when error is present.

Load Upset
An upset to the process not from changing the set-point.

Lag Time
Lag time is the amount of time after the dead time that the process variable takes to move 63.3% of its final value after a step change in valve position.

Measurement
Measurement is the same as the process value.

Manual Mode
In manual mode the output is set manual.

Mode
The controller can be set in auto, manual, or remote mode.

Man Machine Interface - MMI
Refers to the software that the process operator operates the process with.

Output
Output of the controller.

Overshoot
The amount a process exceed the set point during a change in the system load or change in the set point.

PID Controller
Controller including Proportional, Integrating and Derivative controller functions. Cfr. ANSI/IEE Standard 100-1977.

Process Value - PV
The actual value in the control loop, temperature, pressure, flow, composition, pH, etc

Programmable Logic Controller - PLC
Controllers replacing relay logic, usually with PID controllers.

Process Variable - PV
The actual value in the control loop, temperature, pressure, flow, composition, pH, etc. See Process Value.

Proportional Band - P
With proportional band the controller output is proportional to the error or a change in process variable. Proportional Band = 100/Gain

Rate
Same as the derivative or "D" part of PID controllers.

Register
A data storage location in a PLC.

Regulator
A controller changing the a output variable to move the process variable back to the set point

Repeatability
The variation in outputs for the same change of input.

Reset
Same as the integral or "I" part of PID controllers.

Reset Windup
Integral action continuing to change the controller output value after the actual output reaches a physical limit.

Response Time
The rate of interrogating a transmitter.

Sample Interval
The rate at which a controller samples the process variable and calculates a new output.

Set Point
The set point is the desired value of the process variable.

Time Constant
Same as lag time.

Transmitter
A transmitter sense the actual value of a system and transforms the value to a standardized signal - 4-20 mA is common for analog signals - as input for the control system.

Introduction to Control System

1.0       THE BASIC CONCEPT OF CONTROL SYSTEM

Automatic control has played a vital role in the advance of engineering and science. In addition to its extreme importance in space-vehicle system, missile-guidance systems, air-craft-autopiloting systems, robotic systems and the likes, automatic control has become an important and integral part of modern manufacturing and industrial processes. For example, automatic control is essential in the numerical of machine tools in the manufacturing industries.

The controlled  variable is the quantity or condition that is measured and controlled. The manipulated  variable is the quantity or condition that is varied by the controller so as to affect the value of the controlled variable. Normally, the controlled variable is the output of the system. Control  means measuring the value of the controlled variable of the system and applying the manipulated variable to the system to correct or limit deviation of the measured value from a desired value.

In studying control engineering, we first need to define additional terms that are necessary to describe control systems, such as plants, disturbances, processes, feedback control systems and etc. Then a description of closed-loop and open-loop control systems  and their advantages and disadvantages will be given in the following sections.

1.1       IDENTIFYING THE DEFINITION AND TERMINOLOGY RELATED TO CONTROL SYSTEM

The various definitions of the system variables and components are as mentioned below:

            1.1.1   Pneumatic Control Systems

            The working medium in a pneumatic control systems uses a compressible fluid, such as air because it may be exhausted to the atmosphere at the end of the device’s work cycle, thus eliminating the need for return lines.

1.1.2     Hydraulic Control Systems

Hydraulic control systems is the study of incompressible liquids, and hydraulic devices use an incompressible fluid, such as oil, for their working medium. Liquid level systems consisting of storage tanks and connecting pipes are a class of hydraulic systems whose driving force is due to relative difference in the liquid heights in the tanks.

1.1.3     Reference Input

It is the actual signal input to the control system.

1.1.4     Process

Any operation to be controlled. For example, chemical, economic, and biological processes.

1.1.5     Feedback Element

It is the unit which provides the means for feeding back to the output quantity in order to compare it with the reference input.

            1.1.6   Disturbances

           

            A disturbance is a signal that tends to adversely affect the value of the output of a system. If the disturbance is generated within the system, it is called internal, while an external disturbance is generated outside the system and is an input.

1.2       EXPLANATION OF CONTROL SYSTEM TYPES

A control system may consists of a number of components. In order to show the functions performed by each component, in control engineering, we commonly use a diagram called the block diagram.

A block diagram of a system is a pictorial representation of the functions performed by each component and of the flow of signals. In a block diagram all system variables are linked to each other through functional blocks. The functional block or simply block is a symbol for the mathematical operation on the input signal to the block that produces the output.

Figure 1.1 shows an element of the block diagram. Such arrows are referred to as signals.

                                                          

Figure 1.1      Element of a block diagram

(Source : Katsuhiko Ogata (1990), Modern Control Engineering)

The advantages of the block diagram representation of a system lies in the fact that it is easy to form the overall block diagram for the entire system by merely connecting the blocks of the components according to the signal flow and that it is possible to evaluate the contribution of each component to the overall performance of the system.

                        Summing Point

Referring to Figure 1.2, a circle with a cross is the symbol that indicates a summing operation. The plus or minus sign at each arrowhead indicates whether that signal is to be added or subtracted.

                   

       

           

Figure 1.2      Summing point

(Source : Katsuhiko Ogata (1990), Modern Control Engineering)

Branch point

            A branch point is a point from which the signal from a block goes concurrently to other blocks or summing points.

Control systems are classified into  two general categories:

                        ☺        open-loop system

                        ☺        closed-loop system

The distinction is determined by the control action, which is that quantity responsible for activating the system to produce the output. 

1.2.1   Open-loop Control System

           

An open-loop control system is one in which the control action is independent of the output. Figure 1.3 shows the block diagram of an open-loop control system (basic system) and Figure 1.4 shows the block diagram of an open-loop control system (automobile driving system).

                        

 Figure 1.3      An open-loop control systems (basic system)

(Source : S.P. Eugene Xarier & Joseph Cyril Babu.J (1999), Principles of Control System)

 

Figure 1.4      An open-loop control systems (Automobile driving system)

(Source : S.P. Eugene Xarier & Joseph Cyril Babu.J (1999), Principles of Control System)

1.2.2   Closed-loop Control System

            Closed-loop control systems are more commonly called feedback control systems. Feedback is the characteristic of closed-loop control systems which distinguishes them from open-loop systems. It is the property of closed-loop control systems which permits the output to be compared with the input of the system so that the appropriate control action may be formed as a function of the output and input. In general, feedback is said to exist in a system when closed sequence of cause-and-effect relation exists between system variables.

Figure 1.5      General block diagram of an automatic control system

(Source : Katsuhiko Ogata (1990), Modern Control Engineering

1.2.3     Comparison between open-loop and closed-loop control systems

Open-loop Control System

The important features of open-loop control systems are :

i.              Their ability to perform accurately is determined by their calibration, which simply implies, to establish the input-output relation to obtain a desired system accuracy.

ii.            They are not generally troubled with problems of instability.

Closed-loop Control System

The important features of feedback are :

i.              Reduced effects of  nonlinearities and distortion

ii.            Increased accuracy

iii.           Increased bandwidth

iv.           Reduced sensitivity of the ratio of the output to input to variations in system characteristics.

v.            Tendency towards oscillation or instability.