Controls and Instrumentations

ENGR 363

What are the major parts of a plc?

The CPU, I/O, power supply, and programming device

Architecture

Refers to PLC hardware or software

Open architecture

Allows the system to be connected easily to devices and programs made by other manufacturers

Closed architecture

A system whose design is proprietary making it difficult to connect to other systems

Fixed I/O

Small PLC that comes in one package with not separate removable units. The processor and I/O are packaged together

Modular I/O

A PLC that is divided by compartments into which separate modules can be plugged

Power supply

Provides DC power to all modules that plug into the rack

CPU or processor

A microprocessor for implementing the logic and controlling the communications among the modules

PLC scan cycle

Reads inputs -\> executes program -\> diagnostics & communication -\> updates outputs

I/O (Input/Output) system

Forms the interface by which field devices are connected to the controller.

I/O (Input/Output) system

Forms the interface by which field devices are connected to the controller

Programming device

Used to enter the desired program into the memory of the processor

Personal computer (PC)

The most commonly used programming device

Program

A series of instructions that directs the PLC to execute actions

Relay ladder logic

The standard programming language, is based on electromagnetic relay control

Address

Where an input or output device is conncected

PC vs PLC

PLC is designed to operate in the industrial environment with wide ranges of ambient temperature and humidity

Human machine interface (HMI)

Software that allows the user to monitor and control the process

Programmable Automation Controllers (PACs)

Combine PLC ruggedness with PCfunctionality

Nano

Smallest size with less than 15 I/O

Micro

15-128 I/O

Medium

128-512 I/O

Large

Over 512 I/O

Single ended application

One PLC controlling one process

Multitask application

One PLC controlling several processes

Control management application

One PLC controlling several others

Memory

Part of a PLC controller that stores data, instructions, and the control program

Memory size

Expressed in K values

1K =

1024

Amount of memory required depends on

•Number of I/O points used •Size of control program •Data-collecting requirements •Supervisory functions required •Future expansion

Input interface modules

accept signals from the machine or process devices and convert them into signals that can be used by the controller

Output interface modules

convert controller signals into external signals used to control the machine or process

chassis

hardware assembly that houses I/O modules, processor modules, and power supplies

logical rack

an addressable unit consisting of 128 input points and 128 output points

remote rack

A rack that is located away from the processor module

address

a label or number that indicates where a certain piece of information is located in a PLC's memory

Rack/slot-based addressing schemes

used with Allen-Bradley PLC-5 and SLC 500 controllers

Tag-based addressing

used with Allen-Bradley ControlLogix controllers

Combination I/Omodules

can have both input and output connections in the same physical module

points

refers to the number of inputs or outputs available

discrete I/Ointerface module

connects field input devices of the ON/OFF nature

discrete I/O module is powered by

some field supplied voltage source

Backplane power

Used to power the electronics that reside on the I/O module circuit board

optical isolator

used to provide electrical isolation between the power and logic circuitry

Interposing relay

used to control a larger load or motor starter

Output modules can be purchased with

Transistor, triac, or relay output

If the module is a current-sourcing module then theinput or output device must be a

current-sinking device

If the module is a current-sinking module,then the input or output device must be a

current-sourcing device

The two basic types of analog input modules are

voltage sensing and current sensing

Bipolar input modules

Accept signals that swing between a negative and positive value

Unipolar input modules

Accept an input signal that varies in the positive direction only

Resolution refers to

the smallest change in input signal value that can be sensed

The main element of the analogoutput module is an

digital-to-analog (D/A) converter

analog output module

converts digital signal into a proportional voltage or current to control an analog field device

High-speed counter modules

used to count pulses from sensors, encoders, and switches that operate at very high speeds

TTL module

allows the transmitting and receiving of TTL (Transistor-Transistor Logic) signals.

thumbwheel module

allows the use of thumbwheel switches for feeding information to the PLC to be used in the control program

encoder-countermodule

allows the user to read the signal from an encoder on areal-time basis and stores this information so it can be read later by the processor

stepper-motor module

provides pulse trains to a stepper-motor translator, which enables control of a stepper motor

BASIC or ASCII module

runs user written BASIC and C programs

BCD-output module

enables a PLC to operate devices that require BCD-coded signals such as seven-segment displays

proportional-integral-derivative (PID) module

used in process control applications that incorporate PID algorithms

Motion and position control modules

used in applications involving accurate high-speed machining and packaging operations

Serial communications modules

used to establish point-to-point connections with other intelligent devices for the exchange of data

Nominal Input Voltage

Specifies the magnitude (e.g., 5V, 24 V, 230 V) and type (AC or DC) of user-supplied voltage that a module is designed to accept

Input Threshold Voltages

Specifies the minimum ON-state voltage at which logic 1 is recognized and the maximum OFF-state voltage at which logic 0 is recognized

Nominal Current Per Input

Specifies the minimum input current that the discrete input devices must be capable of driving to operate the input circuit

Ambient Temperature Rating

Specifies what the maximum temperature of the air surrounding the I/O modules should be for best operating conditions

Input ON/OFF Delay (response time)

Specifies the maximum time duration required by an input module's circuitry to recognize that a field device has switched ON(input ON-delay) or switched OFF (input OFF-delay)

Output Voltage

This AC or DC value specifies the magnitude (e.g., 5 V, 115 V, 230 V) and type (AC or DC) of user-supplied voltage at which a discrete output module is designed to operate

Output Current

Specifies the maximum current that a single output and the module as a whole can safely carry under load (at rated voltage)

Inrush Current

Specifies the maximum inrush current and duration (e.g., 20 A for 0.1 s) for which an output circuit can exceed its maximum continuous current rating

Short Circuit Protection

Specifies whether the particular output module's design has individual protection for each circuit or if fuse protection is provided for groups (e.g., 4 or 8) of outputs

Leakage Current

This value specifies the amount of current still conducting through an output circuit even after the output has been turned off

Electrical Isolation

The specification for electrical isolation, typically 1500 or 2500volts AC, rates the module's capacity for sustaining an excessive voltage at its input or output terminals

Points Per Module

This specification defines the number of field inputs or outputs that can be connected to a single module

Backplane Current Draw

This value indicates the amount of current the module requires from the backplane. The sum of the backplane current drawn for all modules in a chassis is used to select the appropriate chassis power supply rating

Channels Per Module

Whereas individual circuits on discrete I/O modules are specified as points per module, circuits on analog I/O modules are specified as channels per module

Input Current/Voltage Range(s)

These are the voltage or current signal ranges that an analog input module is designed to accept

Output Current/Voltage Range(s)

This specification defines the current or voltage signal ranges that a particular analog output module is designed to output under program control

Input Protection

Analog input circuits are usually- protected against accidentally connecting a voltage that exceeds the specified input voltage range

Resolution

The resolution specifies how accurately an analog value can be represented digitally. This will determine the smallest measurable unit of current or voltage change that can be detected

Input Impedance and Capacitance

For analog I/Os, these values must be matched to the external device connected to the module. Typical ratings are in the Megohm and Picofarad range

Common-Mode Rejection

Noise is generally caused by electromagnetic interference, radio frequency interference, and ground loops

input status files or tables

Sections of memory used to store the status of inputs

output status files or tables

Sections of memory used to store the status of outputs

Nonvolatile memory

memory that has the ability to retain stored information when power is removed accidentally or intentionally

Volatile memory

memory that will lose its stored information if all operating power is lost or removed

Nonvolatile Read Only Memory (ROM)

stores programs, and data that cannot be changed after the memory chip has been manufactured

Random Access Memory (RAM)

designed sothat information can be written into or readfrom the memory

Erasable Programmable Read-Only Memory(EPROM)

memory designed so thatdata stored in themcan be read, but noteasily alteredwithout specialequipment

Electrically erasable programmable read-onlymemory (EEPROM)

a nonvolatile memorythat offers the same programming flexibility asdoes RAM

programming terminal device

needed to enter,modify, and troubleshoot the PLC program

radix or base

determinesthe total number of different symbols or digitsused by that system

Output status file

made up of single bits grouped into16-bit words

One 16-bit output file word

reserved for each slot inthe chassis

"0" indicatesthat the number is

positive

"1" indicates

negative

method of expressing a negative number

complement of a binary number

2's complement

the binarynumber thatresults when1 is added to the1's complement.