Eee3420 lecture02 rev2011

1Week © Vocational Training Council, Hong Kong.

│ Lecture 2 │

Programmable Logic Controller (PLC) Hardware

EEC3420 Industrial ControlDepartment of Electrical Engineering


EEE3420 Industrial Control

Learning Objectives

Understand the basic hardware components. Understand the operating principles of PLC.



Background of PLC low cost, compact, versatile units based on the

standard microprocessor architecture used in the control of machines or processes

designed for ease of programming and maintenance replace the old relay logic control systems in

automated manufacturing provide an easy and efficient replacements for the

bulky relay logic controllers



Definition of PLC also known as programmable controller (PC) defined by

the National Electrical Manufacturers Association (NEMA) in 1978 as:

"a digitally operating electronic apparatus which uses a programmable memory for the internal storage of instructions for implementing specific functions, such as logic, sequencing, timing, counting and arithmetic, to control through digital or analog input/output, various types of machines or process".



Advantages of PLC Cost effective Flexible Computational abilities allow more sophisticated

control Trouble shooting aids make programming easier Reliable



Typical applications of PLC Embedded control in standard or custom-designed machines Control of conveyor systems Networked to manage large complex distributed control systems Security alarm monitoring system Elevator control Air-conditioning control panel Water treatment plant Warning system on-board of ocean vessels. Smoke candle control on-board aircraft for air-show. Underground coal-mining machines. Central building vacuum system.



Electromagnetic relays a voltage applied to the input coil,

the resulting current creates a magnetic field

magnetic field pulls a metal switch towards it and the contacts touch, closing the switch

contact that closes when the coil is energized is called Normally Open (NO) contact

contact that is closed when the input coil is not energized is called Normally Closed (NC ) contact



Relay circuit and ladder diagram The first relay on the left is used

as NC, and will allow current to flow until a voltage is applied to the input A.

The second relay is NO and will not allow current to flow until a voltage is applied to the input B.

If current is flowing through the second relay coil then current will flow through the coil in the third relay, and close the switch for output C.

This circuit would normally be drawn in the ladder diagram as shown.



Simulated model of a PLC there are two inputs from

push buttons imagine the inputs as

activating 24V DC relay coils in the PLC

in turn drives an output relay that switches 220 VAC, that will turn on a light



A self-hold circuit

input B will only be on when the output B is on

if B is off, and A is energized, then B will turn on

if B turns on then the input B will turn on, and keep output B on even if input A goes off

after B is turned on the output B will not turn off

Note: When A is pushed, the output B will turn on, and the input B will also turn on and keep B on permanently - until power is removed.

A

B

B

Note: The line on the right is being left off intentionally and is implied in these diagrams.



PLC standards - IEC 61131Developed with the input of vendors, end-users and

academics, IEC 61131 consists of five parts: • General information • Equipment and test requirements • PLC programming languages • User guidelines

• Communications



PLC Programming Languages StandardIEC 61131-3 is the international standard for

programmable controller programming languages.

As such, it specifies the syntax, semantics and display for the following suite of PLC programming languages:

• Ladder diagram (LD) • Sequential Function Charts (SFC) • Function Block Diagram (FBD) • Structured Text (ST) • Instruction List (IL)



Ladder diagram and instruction list



Sequential Function Charts (SFCs)

SFCs have been developed to accommodate the programming of more advanced systems



PLC simulation using Personal Computer

With TRiLOGI’s simulator anyone could write ladder programs and immediately test them using just the keyboard and see the result of the action on screen.



PLC scan cycle

• the control loop is a continuous cycle of the PLC reading inputs, solving the ladder logic, and then changing the outputs

• like any computer this does not happen instantly

Read inputs

PLC program changes outputsby examining inputs Set new outputs

Process changes and PLC pauseswhile it checks its own operation

THECONTROLLOOP

Power turned on



The PLC internal working cycle• PLC accepts the inputs from the

Input Port and stores the data to the Input Image Register

• PLC then executes the ladder logic program with the input image content in a sequential manner, that is, from the left to the right and from the top to the bottom

• upon the execution of a rung in the ladder, the contents in the Component Image Register are updated and at the end of a cycle, the outputs are updated



PLC HardwarePower Supply - This can be built into the PLC or be an

external unit. Common voltage levels required by the PLC (with and without the power supply) are 24Vdc, 120Vac, 220Vac.

CPU (Central Processing Unit) - This is a computer where ladder logic is stored and processed.

I/O (Input/Output) - A number of input/output terminals must be provided so that the PLC can monitor the process and initiate actions.



PLC Hardware

Indicator lights - These indicate the status of the PLC including power on, program running, and a fault. These are essential when diagnosing problems.

Software - A software based PLC requires a computer with an interface card



INPUTS and OUTPUTS of PLC• Inputs to, and outputs from, a PLC are necessary

to monitor and control a process. Both inputs and outputs can be categorized into two basic types: digital or continuous.

• Outputs to actuators allow a PLC to cause something to happen in a process. A short list of popular actuators is given below in order of relative popularity



INPUTS and OUTPUTS of PLC• Inputs come from sensors that translate physical

phenomena into electrical signals. Typical examples of sensors are listed below in relative order of popularity.

• Proximity Switches - use inductance, capacitance or light to detect an object logically.

• Switches - mechanical mechanisms will open or close electrical contacts for a logical signal.



INPUTS and OUTPUTS of PLC• Potentiometer - measures angular positions

continuously, using resistance.• LVDT (linear variable differential transformer) -

measures linear displacement continuously using magnetic coupling.

• Inputs for a PLC come in a few basic varieties, the simplest are AC and DC inputs.



INPUTS and OUTPUTS of PLC• Outputs to actuators allow a PLC to cause

something to happen in a process. A short list of popular actuators is given below in order of relative popularity.

• Solenoid Valves - logical outputs that can switch a hydraulic or pneumatic flow.

• Lights - logical outputs that can often be powered directly from PLC output boards.



INPUTS and OUTPUTS of PLC• Motor Starters - motors often draw a large amount

of current when started, so they require motor starters, which are basically large relays.

• Servo Motors - a continuous output from the PLC can command a variable speed or position.

• Outputs from PLCs are often relays, but they can also be solid state electronics such as transistors for DC outputs or Triacs for AC outputs.

• Continuous outputs require special output cards with digital to analog converters.



Input connection• some PLCs

require external DC supply

• some PLCs have built-in 24 VDC supply



INPUTS of PLC• PLC inputs must

converts a variety of logic levels to the TTL +5V dc logic levels used on the data bus

• can be done with circuits similar to those shown.



PLC Input Trade-offs• DC voltages are usually lower, and therefore safer (i.e., 12-24V)• DC inputs are very fast, AC inputs require a longer on-time. For

example, a 60Hz wave may require up to 1/60s for reasonable recognition

• DC voltages can be connected to larger variety of electrical systems

• AC signals are more immune to noise than DC, so they are suited to long distances, and noisy (magnetic) environments

• AC power is easier and less expensive to supply to equipment• AC signals are very common in many existing automation devices



OUTPUTS of PLC• PLC must convert the

+5V dc dc logic levels on the PLC data bus to external voltage levels

• can be done with circuits similar to those shown, where it may be transistor output, or solid state relay (triac) output, or simply relay output



PLC output with sinking current• the output card

shown in Figure on the right is an example of a 24Vdc output card that has a shared common

• this type of output card would typically use transistors for the outputs



PLC output with sourcing current• the circuit shown in

Figure on the right has the sequence of power supply, then device, then PLC outputs, then power supply

• this requires that the outputs have a common



PLC with relay output • in Figure shown on

the right the 24VDC supply is connected directly to both relays

• when an output is activated the output switches on and power is delivered to the output devices



PLC Wiring

• Wiring Diagram for a Car-washing Machine



Double coil designation• double or dual coiling is not a

recommended practice. • using multiple output coils of

the same device can cause the program operation to become unreliable

• the coil operation designated last is the effective coil



Input durations

• the ON or OFF duration of the PLC inputs must be longer than the operation cycle time of the PLC

• taking a 10 ms (standard input filter) response delay into account, the ON/OFF duration must be longer than 20 ms if the operation cycle (scan time) is 10 ms

• in the example above, input pulses of more than 25 Hz (1/(20ms ON+20ms OFF) = 1/40ms) cannot be sensed



Summary of PLC Hardware

• PLC input modules convert the AC or DC inputs to the suitable format to be detected by the logic of the PLC.

• Outputs of PLC are transistors (DC), triacs (AC) or relays (AC and DC).

• Input and output addresses are a function of the card location and input bit number.

• The input module usually shares a common ground and so does the output module of a PLC



PLC Hardware

End of Lecture 2

RevisionMitsubishi PLC FX series hardware handbook.Allen Bradley PLC handbook