EEET 224 Industrial Automation EEET 224 Industrial Automation by Dr Peter Graszkiewicz. Discrete-State Control.. Basic Input and Output Devices 3-phase power supply N/C N/O contactor terminals fuses contactor auxiliary contacts contactor main contacts N/O electro-mechanical coupling contactor coil contactor M motor N/C Push button with one contact normally open (N/O) or normally closed (N/C) Contactor switching a 3-phase motor EEET 224 Industrial Automation by Dr Peter Graszkiewicz 2
.2 Discrete-State Process Control Example.. Simple Forward-Reverse Motor Control Design a control system that allows the operation of a motor in two directions of rotation. Two relays or contactors are used in the supply circuit of the motor. The control system has two START pushbuttons, one for each direction, and a single STOP pushbutton. 24 V interlock V STOP FORWARD K K2 Each contactor provides different phase sequence of the motor supply seal or latch K REVERSE K2 K K2 Control circuit K M K2 Main (power) circuit Circuit diagram of a solution system using mechanical contacts Detached diagram - components of the same device (contactor) may appear in different locations on the diagram, but are given the same designation (eg. K2). EEET 224 Industrial Automation by Dr Peter Graszkiewicz 3 +DC STOP K FORW. REV. 2 3 C controller 2 3 L K2 +DC Field Input Devices Field Output Devices Circuit diagram of a solution system using electronic controller There are three input variables and two output variables of the controlled process: FORV, REV, STOP, K and K2. All variables are discrete, i.e. can assume only two values: ON or OFF. A particular combination of the values of all variables defines the discrete state of the system, eg.: (FORW=on, REV=off, STOP=on, K=on, K2=off). EEET 224 Industrial Automation by Dr Peter Graszkiewicz 4 2
Not all possible states are permissible; eg. the state (FORW=off, REV=off, STOP=on, K=on, K2=on) would result in a short circuit in the mains supply! There is a prescribed sequence in which the states can occur. For example, the state (FORW=off, REV=off, STOP=on, K=on, K2=off) is always preceded by the state (FORW=on, REV=off, STOP=on, K=on, K2=off). The described control function is provided by the controller. This is the case of discrete-state process control or sequential logic control. EEET 224 Industrial Automation by Dr Peter Graszkiewicz 5.3 Combinational Logic Control If the sequence of states in discrete-state control does not matter, the output variables are logic (Boolean) functions of the input variables. The case is referred to as combinational logic control. A particular set of the input values results immediately in a particular set of the output values. L STOP FORW REV K N K = STOP x FORW x REV REV FORW K2 A combinational logic solution to Example. This solution employs manual switches instead of pushbuttons, and is less practical. EEET 224 Industrial Automation by Dr Peter Graszkiewicz 6 3
.4 Continuos Process Control In the diagram of a process-control loop shown below, a PID controller receives a feedback input signal and produces an output control signal in a. In general, both signals can be considered as analog signals varying in a continuos manner. Consequently this is a case of continuos process control, as opposed to discretestate process control, discussed previously. Liquid in Set point Controller + - Σ Error (E) Process Variable (PV) PID controller Control Variable (CV) Level transducer Fluid level control system Analog valve Liquid out Valve opening is proportional to the control signal EEET 224 Industrial Automation by Dr Peter Graszkiewicz 7.5 Relay Controllers, Electronic Controllers and PLC-s start/stop pushbuttons selector switches temperature switches water level switches t o Controller based on a microprocessor (embedded system) or a rotating camswitch and relays (older designs) M indicators motors heating elements solenoid valves Field Input Devices Controller Field Output Devices Schematic of a washing machine control system Many appliances such as washing machines, dishwashers, heating/cooling systems, etc. operate on the principle of discrete-state process control (ref. section.2). The controller moves the system through a sequence of discrete states (events) to accomplish the objective of the process. EEET 224 Industrial Automation by Dr Peter Graszkiewicz 8 4
The characteristics of the controller in the case as above are: Dedicated (designed for a specific system). Cheap (mass-produced to reduce cost) Pre-programmed (the user cannot change the internal program). Simple Often for domestic use only (may not be suitable for harsh industrial environment and intense use). A universal controller that is designed for easy adoption to a variety of individual industrial applications is known as a Programmable Logic Controller (PLC). EEET 224 Industrial Automation by Dr Peter Graszkiewicz 9 The set of characteristics that makes the PLC different from other controllers. The PLC: is a microprocessor (microcontroller)- based electronic systems, often containing electromechanical relays (mechanical contacts) in the discrete outputs. is designed for industrial applications and environment (screw-type terminals, rail mounting, sturdy housing, modular construction etc.) ; has programmable memory for the storage of instructions (program); performs discrete-state and continuous control functions in response to discrete (digital) and analog input signals; controls machines, equipment, processes, etc. through its outputs - discrete (digital) or analog. can communicate (exchange information, instructions) with other devices connected in a network (PC-s, touch screens, operator panels, other PLC-s, etc.) EEET 224 Industrial Automation by Dr Peter Graszkiewicz 5
Allen-Bradley Programmable Logic Controller installed in a cabinet with other electrical equipment EEET 224 Industrial Automation by Dr Peter Graszkiewicz Examples of small PLC-s: Micrologix 2 (Allen Bradley) SIMATIC (SIEMENS) EH-5 (HITACHI) EEET 224 Industrial Automation by Dr Peter Graszkiewicz 2 6
.6 PLC Block diagram Power Supply (eg. 24 VDC, VAC) Other Devices (Programmers, PC, PLC-s, Displays, etc.) Communication link (direct connection or via a network) Power Supply (eg. 24 V, 5Hz) PLC Input Field Devices (Discrete) Digital Input Modules Processor Digital Output Modules Output Field Devices (Disc.) Input Field Devices (Analog) Analog Input Modules Battery Backup Memory Fixed Alterable CPU Analog Output Modules Power Supply Output Field Devices (Analog) 24 V 5 Hz The field devices can be connected directly to the PLC or communicate with it over a network. EEET 224 Industrial Automation by Dr Peter Graszkiewicz 3 CPU: The Central Processing Unit contains: Processor: A microprocessor, eg. 6- bit one, that controls the operation of the PLC. It executes the program and provides the timing. It receives, processes and sends digital data only. Therefore, the input discrete signals are represented as or, and the input analog signals are represented as a combination of -s and -s, i.e. as a binary number. Memory: The fixed memory (ROM) contains the program set by the manufacturer. The alterable memory (RAM) contains information that can be downloaded, altered and erased by the user. It can loose information when the power supply is lost. Power Supply: It converts the AC power supply voltage (24 V, 5 Hz) to a low DC voltage (eg. 5 V) required by the CPU, and often to other low DC voltages (eg.24 V) to be used in the input/output circuits. EEET 224 Industrial Automation by Dr Peter Graszkiewicz 4 7
Input Modules: They provide electric isolation between the input field circuits and the CPU. A Digital Input Module contains interfacing electronic circuits that convert the input discrete signals (eg. V, 5 Hz) to the voltage acceptable by the CPU (5 VDC). An Analog Input Module contains Analog-to-Digital Converters. Screw terminals are provided for connecting the wires of the field circuits. Output Modules: A Digital Output Module contains interfacing circuits that provide isolation between the CPU and the output field devices operating at a different voltage (eg. 24 V, 5 Hz). The interfacing circuits can be entirely solid-state (semiconductor) or use electromechanical relays. An Analog Output Module contains Digital-to-Analog Converters. Screw terminals are provided for connecting the wires of the field circuits. EEET 224 Industrial Automation by Dr Peter Graszkiewicz 5 Input Field Devices: Examples of discrete input devices are pushbuttons, manual switches, limit switches, proximity sensors, temperature switches, etc. Examples of analog input devices are potentiometers, tachometers and analog instruments. Output Field Devices: Examples of discrete output devices are coils of electromagnetic relays and contactors, solenoids of hydraulic valves, pilot lights. Analog output devices can be DC servomotors or any equipment controlled by analog signals. EEET 224 Industrial Automation by Dr Peter Graszkiewicz 6 8
2. Basic Ladder Logic Programming 2. Normally Open Contact Graphical symbols used for the Normally Open Contact instruction are: <address> <address> SIEMENS and general use The instruction works like a normally open contact of a relay. If the coil of the relay is energised (Signal State at the coil = ), the contact closes and allows a current (power) to flow through the contact (Result = ). AB (Allen-Bradley) coil contact EEET 224 Industrial Automation by Dr Peter Graszkiewicz 7 The Normally Open Contact instruction checks the Signal State at the Specified Address (SSASA) and produces a Result according to the following table: SSASA Result The address indicates the location where the data to which the instruction applies is stored in the PLC memory. The Result is used for further processing. The address indicates the location of one bit. EEET 224 Industrial Automation by Dr Peter Graszkiewicz 8 9
2.2 Normally closed contact The instruction works like a N/C (normally closed) contact of a relay. If the coil of the relay is energised (Signal State at the coil address = ), the contact opens and current (power) cannot flow through the contact (Result = ). Exemplary address ( I is for input) I. I: SIEMENS AB The instruction checks the SSASA and produces a Result as follows: SSASA Result EEET 224 Industrial Automation by Dr Peter Graszkiewicz 9 2.3 Output coil The output coil instruction assigns the results produced by a connection of contact instructions to the address of the coil. <address> <designation> Relay coil The instruction works like a coil in a relay circuit. The coil is energised, i.e. it s signal state is, only if a current flows through the coil due to a combination of closed contacts in the coil circuit. EEET 224 Industrial Automation by Dr Peter Graszkiewicz 2
2.4 Contacts in series Examples of two contact instructions connected in series: A B C A B C2 One rung (network) in a Ladder Diagram A B C3 If contacts are in series, the overall result is produced according to the AND Truth Table applied to the result of all individual contact instructions in the connection. EEET 224 Industrial Automation by Dr Peter Graszkiewicz 2 For two contacts (contact instructions) in series: Instruction associated with address A produces: contact closed contact open Instruction associated with address B produces: contact closed contact closed Result assigned to address C: coil energised coil not energised The above Truth Table can be obtained by checking, if power can flow to the coil. EEET 224 Industrial Automation by Dr Peter Graszkiewicz 22
The logic of contact and coil connections can be represented using Boolean algebra expressions and the instruction addresses as signal designations. Accordingly, a Ladder Diagram has a Gate Diagram equivalent. A B C C = A x B A & B C A B C A B C = A x B & C EEET 224 Industrial Automation by Dr Peter Graszkiewicz 23 2.5 Contacts in parallel Examples of two contact instruction in parallel: E25. A24. I. Q4. E25. I. If contact instructions are in parallel, the overall result is produced according the OR Truth Table applied to the results of all individual contact instructions in the parallel connection. I. I. Q4. EEET 224 Industrial Automation by Dr Peter Graszkiewicz 24 2
2 2 3 A Boolean expression and a gate equivalent for the above Ladder Diagram with parallel contacts: ( +2 + 3) = 2 2 3 2 EEET 224 Industrial Automation by Dr Peter Graszkiewicz 25 2.6 Basic rules of Ladder Diagrams Every ladder network (rung) must end with a coil or a box at the right end. We assume that power flows from the left busbar to the right one. Branches that could cause a power flow from right to left are prohibited. Branches that cause a short circuit are not allowed. Multiple coils in parallel are possible. All contacts must run horizontally. EEET 224 Industrial Automation by Dr Peter Graszkiewicz 26 3
Example. I. I. I.2 Q3. Q3. I.3 Q3.2 Q3. = (I. x I.) + I.2 Q3. = Q3. I. I. & Q3. Q3. I.2 EEET 224 Industrial Automation by Dr Peter Graszkiewicz 27 2.7 Program scanning A PLC operates in cycles, each consisting of: () Input scan (read), (2) Program scan (execute), (3) Output scan (write). READ EXECUTE WRITE. The signal states at the input terminals are read and stored. 2. The ladder diagram program is executed from left to right and from top to bottom. The results of instructions are stored. In particular, the output signal status is generated and stored. 3. The output signal state data is transferred to the output modules. The complete program scan is of the order of a few milliseconds. EEET 224 Industrial Automation by Dr Peter Graszkiewicz 28 4
Inputs Outputs 2.8 Simple control systems Example 2.: Design a control system in which a small -V motor is switched on/off using two pushbuttons operating at 24 V. 24 V START STOP PLC 2 2 A coil of a relay or a contactor if the PLC output cannot handle the motor current EEET 224 Industrial Automation by Dr Peter Graszkiewicz 29 M V 3 Output cct 3 Input cct power supply power supply C (DC or AC) L (DC or AC) Diagram of the PLC external connections (hardware solution to Example 2.) (It shows the physical connections of the input and output devices to the PLC. The input/output module number and the terminal number determine the address of the connected device. The assumed PLC design is that the relays inside the output modules connect internally the output terminals with the common.) Operation of the PLC input & output circuits. Specific addresses of specific syntax are allocated to the PLC input and output terminals by the PLC manufacturer (consult the manual). If a voltage of nominal value (eg. 24 V) is applied between an input terminal and the common (C), the Signal State (SS) stored at the terminal address is. Otherwise, it is. The input SS value is updated every input scan. Many PLC output modules operate as follows: If the SS stored at an output terminal address is, the output terminal is connected inside the PLC to the output common terminal (L) through the closed contact of an electromechanical relay or a solid state switch in ON state. If the SS is, the output terminal is disconnected from the common. EEET 224 Industrial Automation by Dr Peter Graszkiewicz 3 5
I = input (SIEMENS) Terminal number Q = output (SIEMENS) Terminal number I. I. Q4. Module number Q4. PLC Ladder Diagram program (software solution to Example 2.) Note that the contact related to the STOP pushbutton are N/O in the PLC program, and N/C in the relay diagram! EEET 224 Industrial Automation by Dr Peter Graszkiewicz 3 Example 2.2 (same as in Section.2) Design a control system that allows the operation of a motor in two directions of rotation. Two relays or contactors are used in the supply circuit of the motor. The control system has two START pushbuttons, one for each direction, and a single STOP pushbutton. Two pilot lights indicate the direction of rotation. +DC STOP PLC K FORW. REV. 2 3 C 2 3 L H K2 H2 +DC PLC External Connections Diagram (hardware solution to Ex. 2.) EEET 224 Industrial Automation by Dr Peter Graszkiewicz 32 6
I. I. Frw Q.2 Q. Motor Forward On STOP Q. Q. Lamp Forward I. I.2 Rev STOP Q.2 Q. Q.2 Q.3 Motor Reverse On Lamp Reverse You can either use absolute addresses (e.g. I.) or enter your own symbols (e.g. STOP) that you have assigned to the absolute addresses. The programming software allows the user to enter a title and comments for each rung (network) of the program. These entries are optional and not essential to the program execution. PLC LD Program (software solution to Ex. 2.) EEET 224 Industrial Automation by Dr Peter Graszkiewicz 33 7