Open and Closed loop Control I. Introduction Óbudai Egyetem Dr. Neszveda József

transmitter actuator Structure of a complex control system Process or plant Material flow sensorstransducers actuating units actuating drives controller operator desk Visualization software SCADA (Supervisory control and data acquisition) Mathematical software, like Matlab, are used preparation the model of the process. Control software Standard signals Variables

Engineering jobs instrumentation measurement (temperature, level, pressure, flow, drive, and so on) final element (control valves, drivers) safety philosophy of the process (Safety systems are usually independent from the normal control system) control philosophy of the process process control manufacturing maintenance

Variables and signals Process or plant transducer actuator controller operator desk disturbance variables manipulated variables measured variables a part of these controlled variables action signals or control signals measured signals or detecting signals a part of these feedback signals

Dimensionless technique controlled variable feedback signal action signal manipulated variable A/D conversion control task D/A conversion max min max min 20 mA 4 mA Domain of variability

Most frequently used standard signals On / Off signals: 0 – 24 V DC Continuous signals: 4 – 20 mA 24 V 0 V Upper limit Logical 0, or low 30 V 20 mA 7 V7 V 4 mA Logical 1, or high Further frequently used continuous signals: 0 – 20 mA, 0 – 10 V, 2 – 10 V, (0.4 – 2 Bar) Further frequently used on /off signals: 0 – 110 / 230 V AC, (0 – 4 Bar) Lower limit

Divide into simpler part Which variables of the process need control. All controlled variable is an independent simpler part of the process. * All simpler part of the process needs a control strategy. The safety philosophy of the process. Safety systems are usually independent from the normal control system and the safety considerations are very dependent on the process. More often a part of the safety considerations is to monitor of exceptional conditions or to detect a malfunction of a device or to reach a dangerous level of a variable. The failsafe philosophy can also be applied to the actuator and the transmitter. (power or wiring failure).

Points of view to choose control law Is the controlled variable performance on/off or continuous? Which measured and manipulated variables are required for developing a control law (algorithm). Can one describe the plant between the input manipulated and disturbances variables and the output controlled variable by a precise model. Is this model linear or not, time invariant or not? Economic efficiency points of view.

Control strategies Open loop control One can describe the plant between the manipulated and disturbance variables and the controlled variables by a precise model and so using the required measured variables one can develop a control algorithm. Advantages: This method is so punctual such as the model, the control action doesn’t require an error. There isn’t stability problem. Disadvantages: Sometimes this solution isn’t economical. Closed loop control ( Feedback ) The reference signal represents the required value of the controlled variable. The controlled variable and the reference signal continuously compare and if the detecting and the reference signal are not equal, than an adequate action signal attempt to eliminate the error. Advantages: Sometimes this solution is economical. Disadvantages: The key is to appear an error and needs a transient time to eliminate this error. The controlled variable isn't always punctual. There is stability problem.

Process requirements It means which variables depend on other equipment of process or environment and which variable can be controlled and which variable can be manipulated. Of course one can use a pump to determine the inflow or a valve to determine the outflow. In steady-state the inflow equals outflow. inflow outflow tank level pump valve difference of pressure Controlled variable is the tank level Manipulated variable is the inflow: Requirement of other equipment of process gives a control signal the pump changing the outflow. Manipulated variable is the outflow: Requirement of other equipment of process gives a control signal the valve changing the inflow. Controlled variable is on/off or continuous? If it is on/off, then enough start or stop the pump, or open or closed the valve.

Classification of control strategies In this example we assume: The controlled variable is the tank level. The manipulated variable is the inflow. Requirement of consumers gives a control signal for the pump, which changes the outflow. It is an disturbance of the system. Further disturbance is the difference pressure of control valve. inflow outflow tank level pump valve difference of pressure Open-loop open loop control: It needs to measure the outflow and the difference pressure, to know the relation between the valve position and the actual value of the tank level at the correct value of outflow. Closed-loop on/off feedback control: It needs to measure the tank level. If it is higher than the reference value the valve is closed and when the tank level is lower than the reference value the valve is opened. modulating feedback control: It needs to measure the level of the tank. The difference between the reference and feedback signals determine a continuously action signal for the actuating drive of valve.

Family of Microprocessor based controller Microcontrollers PC-based system Programmable Logic Controller Process controller

Microprocessor circuit arrangement Internal bus (data and control signal) CPUmemorySIO Clock generator PIO D/A, A/D converter The base The special interface circuits galvanic isolation

Microcontroller and typical application field Microcontrollers An arrangement of the basic circuit elements like CPU, RAM or EEPROM memory, SIO,... Application field sequence manufactured devices for examples: household (washing machine), controller (PLC), telecommunication devices (mobile phone) Programmability assembly, special developer software based on QuickC

PC-based system and typical application field PC-based system (depend on factory) Application field control intelligent devices in laboratory (scope, frequency generator, power supply, etc.) data acquisition in factory, process control (industrial computer!) Programmability factory depending graphical language

PLC and typical application field Programmable Logic Controller ready to use for industrial control Application field ready to use for industrial control, for example elevator, packaging machinery, conveyor, and other individual machines or machines lines, or multiple axis drive system control, like metal forming, Cartesian robots, assembly machines, etc. Programmability standard control languages IEC

Process controller and typical application field Process controller ready to use for industrial control and it’s possible the hardware redundancy and integrate the functionality of control and SCADA software. Application field process industry applications (paper, primary metals, food processing, and so on.) batch-type control (chemical-technology, pharmaceutic chemistry, or injection molding machines, and so on.) Programmability standard control languages IEC and SCADA depending of factory

PLC families Control relays 8-20 I/O, limited memory size, only LD programming languages with limited and not standard instructions. Compact PLC I/O, normal memory size, standard IL, LD, FBD programming languages with limited instructions. Modular PLC with compact unit flexible arrangement up to 800 I/O, different memory size, standard IL, LD, FBD programming languages. Modular PLC with rack system Higher I/O number, memory size, CPU frequency, network ability. All standard programming languages are used.

Structure of modular PLC Programming device CPU module Local extension I/O module Network station Remote extension I/O module

Program structure of the user program of PLC Deafault task Periodic task Event task System task Priority Program A1 Program An Program E1 Program En Program Rn Program R1 Program Pn Program P1 Function block 1 Function block N Function N Function x Function 1 Data FB1/1 Data FB1/2 Data FB1/3 The system programs of PLC handle the error, keep the connection with operator, and so on. Function block 2

How the programs use the CPU t t t t D P E S A1A2A3A1 P1 A2A3A1A P E1 P AAA3A1A2A3A1 R1 A3A1A2A3A

Cyclic processing of the default task Write data interrupt Standby PLC selftest Process user program Read data

Reaction (response) time T R = T FI + 2*T C + T FQ + T N +T I T R : reaction time T FI : input filter T C : cycle time (scan time) T FQ : output filter T N : reading, writing data via network T I : executing time of an interrupt program

Open loop On/Off control All variables and signals are on/off. One can use analogue inputs, but it needs compare a value (lower than or higher than), and so the result is on/off. Using the Boolean algebra one always can create a model of the process! Usually cheaper than analogue control, because the switches (level, proximity, and so on) are cheaper than transmitters. Must use closed sequences! Graphical representation a branch of the ladder diagram.

Boolean algebra Karnaugh table, graphical representation, and so on. AND OR EXOR Negation The result: normal (store), Set/Reset, IEC standard are falling and rising edge, Control relays use the impulse relay! LD (load)

Operands Inputs While processing the user program these are special memory Outputs The same as inputs Marker (temporary operands) These are special memory field bit, byte, or word arrangement. One can use the negation. Graphical symbol are normally open or normally closed.

Control relays The main difference between the PLC and the Control Relay are the next: The simplified language used by CR Typically the ladder diagram Not possible the online programming Must stop the program running while changing the user program Limited options to join network In generally only one.

Control relays Limited variation of hardware Limited number of Inputs and Outputs Typically the relay outputs Limited number of Markers Depends on the hardware type Limited number of function blocks Timers, Counters, Clocks. Limited number of functions Analogue and digital comparators, Text blocks.