PROGRAMMABLE LOGIC CONTROLLERS PLC’S

•Overview of Technology•PLC Configuration and Selection•Programming PLC’s

• What is a PLC ? • •PLC (Programmable Logic Controller) • •A PLC works by looking at its inputs

and depending on their state, and theuser entered program, turns on/offoutputs.

• •A PLC can be thought of as: • Industrial Computers with specially • designed architecture in both their

central units (the PLC itself) and theirinterfacing circuitry to field devices(input / output connections to the realworld).

CPU•Power Supply•Memory•Input Blocks•Output Blocks•Communications•Expansion

Connections

Self Check

PLC Program SCAN

Update Outputs

Execute Code

Scan Inputs

CPU Module •The Central Processing Unit (CPU) Module is the brain of the PLC. •Primary role to read inputs, execute the control program, update outputs. •The CPU consists of the arithmeticlogic unit (ALU), timing/controlcircuitry, accumulator, scratch padmemory, program counter, addressstack and instruction register.

•A PLC works by continually scanning a program

The memory includes pre-programmed ROM memorycontaining the PLC’s operating system, driverprograms and application programs and the RAMmemory.

•PLC manufacturer offer various types of retentivememory to save user-programs and data while poweris removed, so that the PLC can resume execution ofthe user-written control program as soon as power isrestored.

Input and output (I/O) modules connect the PLC to sensors and actuators. •Provide isolation for the low-voltage, low-current signals that the PLC usesinternally from the higher-powerelectrical circuits required by mostsensors and actuators.

•Wide range of I/O modules available including: digital (logical) I/O modules and analog (continuous) I/O modules.

Inputs come from sensors that translate physical or chemical phenomena into electrical signals.•The simplest form of inputs are digital/discrete in AC/DC.•In smaller PLCs the inputs are normally built in and are

specified when purchasing the PLC. •For larger PLCs the inputs are purchased as modules, orcards, with 8,16, 32, 64, 96 inputs of the same type oneach card.

The list below shows typical ranges for input voltages. 5 Vdc 12 Vdc 24 Vdc 48 Vdc 12 Vac 24 Vac 120 Vac 240 Vac

Output modules rarely supply any power, but instead act as switches. •External power supplies are connected to the output cardand the card will switch the power on or off for eachoutput.

•A common choice when purchasing output cards is relays, transistors or triacs. •Relay are the most flexible output devices. They arecapable of switching both AC and DC outputs. But, theyare slower, cost more, and they will wear out aftermillions of cycles.

The most important considerationwhen selecting relays, or relayoutputs on a PLC, is the ratedcurrent and voltage.

•For transistor outputs or higherdensity output cards relay terminalblocks are available.

–Advantage of individual standard replaceable relays

Typical output voltages are listed below, 5 Vdc 12 Vdc 24 Vdc 48 Vdc 24 Vac 120 Vac 240 Vac

Typical Analogue Input signals are: –Flow sensors–Humidity sensors–Load Cells–Potentiometers–Pressure sensors–Temperature sensors–Vibration

•Analogue Output signals control: –Analogue Valves–Actuators–Chart Resorders–Variable Speed Drives–Analogue Meters

•Typical Analogue Signal Levels –4-20mA–1-5 Vdc–0-10 Vdc–-10 – 10Vdc

Analogue input cards convert continuous signals via a A/D converter into discrete values for the PLC •Analogue output cards convert digital values in then PLC to continuous signals via a D/A converter.•Resolution can be important in choosing an applicable card•Example, for a temperature input of 0 to 100 degrees C

–For 8 bit resolution the value in the PLC is 0 to 255–For 12 bit resolution the value in the PLC is 0 to 4095–For 12.5 bit resolution the value in the PLC is 0 to 6000–For 13 bit resolution the value in the PLC is 0 to 8192–For 16 bit resolution the value in the PLC is 0 to 32768

RF ID•Voice•Gas Flow Calculation•Weigh Cell•Hydraulic Servo•ASCII•Fuzzy Logic•Temperature Sensor•Temperature Control•Heat/Cool Control•Field Bus Cards

–DeviceNet, Profibus etc –Lonworks, BACNet • Fast Response (Interrupt) • PID• Loop Controller• BASIC Cards• RS232 Comm’s• Modbus ASCII/RTU• Ethernet Comm’s

–

• High Speed Counters• Position Control Cards• Per to Per Comm’s

Controller Link– DH+– Modbus Plus

•Sequence –Input–Output–Control

•Logic•Timer and Counters•Comparison•Range Comparison•Data Movement•Data Shift•Step / Step Next•Serial Communications•Text String Processing•File Manipulation

–IF,THEN,ELSE,LOOP Table Processing –LIFO, FIFO

•Increment/Decrement •Conversion –ASCII –Number Systems •Math•Floating Point Math•Statistics•Scaling•PID•PID with Auto tune•Clock / Date•Block Processing

The configuration of PLC refers to the packaging of the components. •Typical configurations are listed below from largest to smallest. –Rack Type : A rack can often be as large as 18” by 30” by 10” –Mini: These are similar in function to PLC racks, but about the half size. Dedicated Backplanes can be used to support thecards OR DIN rail mountable with incorporated I/O bus inmodule.

–Shoebox: A compact, all-in-one unit that has limited expansion capabilities. Lower cost and compactness make these ideal for small applications. DIN rail mountable. –Micro: These units can be as small as a deck of cards. They tend to have fixed quantities of I/O and limited abilities, but costs will be lowest. DIN rail mountable

Micro PLCs: I/O up to 32 pointsSmall PLC: I/O up to 128 pointsMedium PLC: I/O up to 1024 pointsLarge PLC: I/O up to 4096 pointsVery Large: I/O up to 8192 points

Criteria•Number of logical inputs and outputs•Memory•Number of special I/O modules•Expansion Capabilities•Scan Time•Communication•Software•Support•Dollars