Programmable Logic Controllers PLCs Definition According to National

Programmable Logic Controllers (PLC’s)

Definition According to National Electrical Manufacturers’ Association a PLC is: A digital electronic device using a programmable memory to store instructions and to implement specific functions such as logic, sequence, counting, timing and arithmetic to control machines and processes through digital or analog input/output modules.

What is a PLC? n A special computer for logic controls

PLC History In the late 1960's PLCs were first introduced Bedford Associates (Bedford, MA) proposed something called a Modular Digital Controller (MODICON) to a major US car manufacturer In the mid 70's the dominant PLC technologies were sequencer statemachines and the bit-slice based CPU. The AMD 2901 and 2903 were quite popular in Modicon and A-B PLCs Communications abilities began to appear in approximately 1973. The first such system was Modicon's Modbus. The PLC could now talk to other PLCs and they could be far away from the actual machine they were controlling The 80's saw an attempt to standardize communications with General Motor's manufacturing automation protocol(MAP The 90's have seen a gradual reduction in the introduction of new protocols

Basic PLC Operation PLCs consist of input modules or points, a Central Processing Unit (CPU), and output modules or points. Pushbuttons (sensors), in this simple example, connected to PLC inputs, can be used to start and stop a motor connected to a PLC through a motor starter (actuator).

PLC ARCHITECTURE Programmable controllers replace most of the relay panel wiring by software programming. It is consists of the following modules: Power supply module CPU module Digital input module Digital output module Analog input module Analog output

PLC COMPONENTS 1. Processor Microprocessor based, may allow arithmetic operations, logic operators, block memory, computer interface, local area network, functions, etc. 2. Memory Measured in words. ROM (Read Only Memory), RAM (Random Access Memory), PROM (Programmable Read Only Memory), EEPROM (Electric Erasable Programmable ROM), EPROM (Erasable Programmable Read Only Memory),

PLC COMPONENTS 3. I/O Modular plug-in periphery AC voltage input and output, DC voltage input and output, Low level analog input, High level analog input and output, Special purpose modules, e. g. , high speed timers, Stepping motor controllers, etc. PID, Motion 4. Power supply AC power 5. Peripheral hand-held programmer (HHP) printer simulator EPROM loader graphics processor network communication interface modular PC

Discrete I/O : -AC Voltage I/O or Discrete Input: Selector switches, Pushbuttons, Photoelectric Eyes, Limit switches, Circuit breakers, proximity switches, Level switch, Pressure switch, Temperature switches, Motor starter contacts, Relays contacts and Thumbwheel switch -DC Voltage I/O Non-locking Normally open Multiple through SPDT locking Normally closed Multiple pole DPST

RELAYS A switch whose operation is activated by an electromagnet is called a "relay". Coil circuit controls many contact points contact Relay coil R 1 coil input Output contact normally open Output contact normally closed R 1

Relays and Schematic Symbols

Logical Control with Relays

Relay Logic In a PLC

Discrete Output Alarms, Control relays, Fans, Lights, Horns, Valves, motor starters, and Solenoids Analog I/O Analog inputs: -Flow sensor, Pressure sensors, Potentiometers, Humidity sensors, Temperature sensors, load cell transducers Analog outputs: Analog meters Analog valves Actuators Chart recorders DC and AC motor drives

Some Special I/O Thermocouple input Low level analog signal, filtered, amplified, and digitized before sending to the processor through I/O bus. Fast input 50 to 100 microsecond pulse signal detection. ASCII I/O Communicates with ASCII devices. Stepper motor output Provide directly control of a stepper motor. Servo interface Control DC servo motor for point-to-point control and axis positioning. PID control The Proportional Integral Derivative is used for closed loop process control. Network module

LADDER DIAGRAM A ladder diagram (also called contact symbology) is a means of graphically representing the logic required in a relay logic system. Rail Rung

LADDER DIAGRAM The left vertical line of a ladder logic diagram represents the power or energized conductor. The output element or instruction represents the neutral or return path of the circuit. The right vertical line, which represents the return path on a hard-wired control line diagram, is omitted. Ladder logic diagrams are read from left-to-right, top-to -bottom. Rungs are sometimes referred to as networks. A network may have several control elements, but only one output coil.

Reading Ladder Logic

A Ladder Logic Example

Statement list A statement list (STL) provides another view of a set of instructions. The operation, what is to be done, is shown on the left. The operand, the item to be operated on by the operation, is shown on the right. The set of instructions in this statement list perform the same task as the ladder diagram.

Function Block Diagrams (FBD) provide another view of a set of instructions. Each function has a name to designate its specific task. Functions are indicated by a rectangle. Inputs are shown on the left-hand side of the rectangle and outputs are shown on the right-hand side.

Standard PLC scan cycle

PLC Scan The PLC program is executed as part of a repetitive process referred to as a scan. A PLC scan starts with the CPU reading the status of inputs. The application program is executed using the status of the inputs. Once the program is completed, the CPU performs internal diagnostics and communication tasks. The scan cycle ends by updating the outputs, then starts over. The cycle time depends on the size of the program, the number of I/Os, and the amount of communication required.

Software is any information in a form that a computer or PLC can use. Software includes the instructions or programs that direct hardware. Hardware is the actual equipment. The PLC, the programming device, and the connecting cable are examples of hardware.

Basic Requirements In order to create or change a program, the following items are needed: • PLC • Programming Device • Programming Software • Connector Cable

Programming Devices The program is created in a programming device (PG) and then transferred to the PLC. The program for the S 7 -200 can be created using a dedicated Siemens SIMATIC S 7 programming device, such as a PG 720 (not shown) or PG 740, if STEP 7 Micro/WIN software is installed.

A personal computer (PC), with STEP 7 Micro/WIN installed, can also be used as a programming device with the S 7 -200.

Software A software program is required in order to tell the PLC what instructions it must follow. Programming software is typically PLC specific. A software package for one PLC, or one family of PLCs, such as the S 7 family, would not be useful on other PLCs. The S 7 -200 uses a Windows based software program called STEP 7 -Micro/WIN 32. The PG 720 and PG 740 have STEP 7 software pre-installed. Micro/WIN 32 is installed on a personal computer in a similar manner to any other computer software.

Connector Cables PPI (Point-to-Point Interface) Connector cables are required to transfer data from the (Point-to-Point Interface) programming device to the PLC. Communication can only take place when the two devices speak the same language or protocol. Communication between a Siemens programming device and the S 7 -200 is referred to as PPI protocol (pointto- point interface). An appropriate cable is required for a programming device such as a PG 720 or PG 740. The S 7 -200 uses a 9 -pin, D-connector. This is a straight-through serial device that is compatible with Siemens programming devices (MPI port) and is a standard connector for other serial interfaces.

Connector Cables PPI A special cable is needed when a personal computer is used as a programming device. Two versions of this cable are available. One version, called an RS-232/PPI Multi-Master Cable, connects a personal computer’s RS-232 interface to the PLC’s RS-485 connector. The other version, called a USB/PPI Multi-Master Cable, connects a personal computer’s USB interface to the PLC’s RS-485 connector.

S 7 -200 Micro PLCs The S 7 -200 Micro PLC is the smallest member of the SIMATIC S 7 family of programmable controllers. The central processing unit (CPU) is internal to the PLC. Inputs and outputs (I/O) are the system control points. Inputs monitor field devices, such as switches and sensors. Outputs control other devices, such as motors and pumps. The programming port is the connection to the programming device.

S 7 -200 Models There are five S 7 -200 CPU types: CPU 221, CPU 222, CPU 224 XP, and CPU 226 and two power supply configurations for each type.

S 7 -200 Features The S 7 -200 family includes a wide variety of CPUs and features. This variety provides a range of features to aid in designing a cost-effective automation solution. The following table provides a summary of the major features, many of which will be covered in this course.

Mode Switch and Analog Adjustment When the mode switch is in the RUN position the CPU is in the run mode and executing the program. When the mode switch is in the STOP position the CPU is stopped. When the mode switch is in the TERM position the programming device can select the operating mode. The analog adjustment is used to increase or decrease values stored in special memory. These values can be used to update the value of a timer or counter, or can be used to set limits.

Optional Cartridge The S 7 -200 supports an optional memory cartridge that provides a portable EEPROM storage for your program. The cartridge can be used to copy a program from one S 7 -200 PLC to a like S 7 -200 PLC.

Expansion Modules The S 7 -200 PLCs are expandable. Expansion modules contain additional inputs and outputs. These are connected to the base unit using a ribbon connector. The ribbon connector is protected by a cover on the base unit. Sideby-side mounting completely encloses and protects the ribbon connector.

The ribbon connector is protected by a cover on the base unit. Sideby-side mounting completely encloses and protects the ribbon connector.

External Power Supply An S 7 -200 AC/DC/Relay would be connected to a 120 or 230 VAC power supply.

I/O Numbering S 7 -200 inputs and outputs are labeled at the wiring terminations and next to the status indicators. These alphanumeric symbols identify the I/O address to which a device is connected. This address is used by the CPU to determine which input is present and which output needs to be turned on or off. I designates a discrete input and Q designates a discrete output. The first number identifies the byte, the second number identifies the bit. Input I 0. 0, for example, is byte 0, bit 0.

Inputs Outputs Freeport Mode The programming port has a mode called freeport mode. Freeport mode allows connectivity to various intelligent sensing devices such as a bar code reader.

Printer Freeport mode can also be used to connect to a non-SIMATIC printer. Interconnection It is possible to use one programming device to address multiple S 7 -200 devices on the same communication cable. A total of 31 units can be interconnected without a repeater.

Symbols Contacts Coils represent relays that are energized when power flows to them. When a coil is energized, it causes a corresponding output to turn on by changing the state of the status bit controlling that output to 1. That same output status bit may be used to control normally open and normally closed contacts elsewhere in the program. Boxes represent various instructions or functions that are executed when power flows to the box. Typical box functions are timers, counters, and math operations.

Entering Elements An AND Operation

PLC HARD WIRING DIAGRAM External switches with stored program ladder diagram and output devices can be presented using The shown PLC hard wired diagrams A Input 01 B Output PLC 01 02 20 11 12 02 20 03 20 11 External switches Stored program C

• • PLC Programming Device Programming Software Connector Cable

Input Card Example

Output Card Example (Transistors)

Output Card Example (Relays)

Some Design Issues • • • DC voltages are usually lower, and therefore safer (e. g. , 24 V). DC inputs are very fast, AC inputs require a longer on-time. DC voltages can be connected to more electrical systems. AC signals are more immune to noise than DC. AC power is easier and less expensive to supply to equipment. AC signals are very common. Relays Contactor - Special relays for switching large current loads. Motor Starter - Basically a contactor in series with an overload relay to cut off when too much current is drawn. Arc Suppression - when any relay is opened or closed an arc will jump. This becomes a major problem with large relays. On relays switching AC this problem can be overcome by opening the relay when the voltage goes to zero (while crossing between negative and positive). When switching DC loads this problem can be minimized by blowing pressurized gas across during opening to suppress the arc formation. AC coils - If a normal relay coil is driven by AC power the contacts will vibrate open and close at the frequency of the AC power. This problem is overcome by adding a shading pole to the relay.

Problem: You are planning a project that will be controlled by a PLC. Before ordering parts you decide to plan the basic wiring and select appropriate input and output cards. The devices that we will use for inputs are 2 contact switches, a push button and a thermal switch. The output will be for a 24 Vdc solenoid valve, a 110 Vac light bulb, and a 220 Vac 50 HP motor. Sketch the basic wiring below including PLC cards.

Wiring Symbols

Input Connections AC AC input module allows interfacing of input devices which provide AC output. Converter is used to covert incoming AC voltage to a DC logic signal to be used by the processor DC The Dc input module allows interfacing of input devices which provide a DC output voltage. The range of input voltage for DC input module varies between 5 VDC and 30 VDC TTL input interfaces allow the controller to accept signals from TTL compatible devices including solid state controls and sensing instrumentation. TTL inputs are used for interfacing with some 5 VDC level control devices and several types of photoelectric sensors

Output Connections AC AC output module allows interfacing of output devices which provide AC input. DC DC output module interface is used to control DC loads by switching On and Off. TTL The TTL output interface allows the controller to drive output devices that are TTL compatible such as seven segment LED display

DC Inputs The difference between the two types is whether the load (in our case, the plc is the load) is switched to ground or positive voltage. An NPN type sensor has the load switched to ground whereas a PNP device has the load switched to positive voltage.

DC Inputs Inside the sensor, the transistor is just acting as a switch. The sensors internal circuit tells the output transistor to turn on when a target is present. The transistor then closes the circuit between the 2 connections shown above. (V+ and plc input).

AC Inputs AC input modules are less common these days than dc input modules sensors typically have transistor outputs

Relay Outputs A relay is non-polarized and typically it can switch either AC or DC. Here the common is connected to one end of our power supply and the other end of the supply is connected to the load. The other half of our load gets connected to the actual plc output you have designated within your ladder program.

Transistor Outputs Shown above is how we typically connect our output device to the transistor output. Please note that this is an NPN type transistor. If it were a PNP type, the common terminal would most likely be connected to V+ and V- would connect to one end of our load. Note that since this is a DC type output we must always observe proper polarity for the output. One end of the load is connected directly to V+ as shown above.

Typical I/O connections with PLC: The shown diagram is a typical PLC with input and output module to control pneumatic actuator

Advantages of PLCs: n n n n n Increase flexibility, Faster implementation of changes and correction, Lower cost, Increased visual observations, Increased operation speed, Increased reliability and maintainability, Increased security, Reprogramming capability, Elimination of wiring.

Summary This sequence covered the following topics 1 -PLC’S definition 2 -PLC’s Components 3 -PLC’S advantages 5 -PLC’S I/O connections n