The Role of the PLC

In autosystem, the PLC is commonly regarede as the heart of the control system. With a control application program ( store within the PLC mem.) in execution, the PLC consutaly monitors the state of the system through the field input devices feedback signal. It will the base on the program logic to determine the course of action to be carried out at the field output devices.

The PLC may be use to control a simple and repetitive task, or a few of then may be interconnected together with other host controllers or host controllers of host computers through a sort of communication network, in order to intergrate the control of a complete process.

Input devices

Interlligence of an automated system is greatly depending on the ability of a PLC to read in the sign from varius type of automatic sensing and manual input fiel devices

Keypad, push button and togge switches, witch form the basic ma-machine unterface, are types of manual in put device. On the other hand, for detection of workpice, monitoring of moving mechanism, checking on pressure and or liquid level and many others, the PLC will have to tap the signal from the specific automatic sensing devices like proximity switch, limit switch, level sensor, photo sensor and …Type of input signal to the PLC would be of ON/OFF logic or analogue. These input signals are interfaced to PLC through various types of PLC input module.

Output device

An automatic system is incomplete and the PLC system is virtually paralysed without means of interface to the field output devices Some of the most commonly controlled devices are motor, solenolds, relay indicator, buzzers and etc. Through activation of motors and solenoids the PLC can control from a simple pick and place system to a much complex servo positioning system. These type of output devices are the mechanism of an automated system and so its direct effect on the system performance.

However some output devices is only alarm and notting for the man, which is pilot lamp,buzzes. Like input signal interface, signal from output devices are interfaces to the PLC through the wide range of the PLC output module.

Out put module

Output Modules

As with input modules, output modules rarely supply any power, but instead act as

switches. External power supplies are connected to the output card and the card will

switch the power on or off for each output. Typical output voltages are listed below, and

roughly ordered by popularity.

120 Vac

24 Vdc

12-48 Vac

12-48 Vdc

5Vdc (TTL)

230 Vac

These cards typically have 8 to 16 outputs of the same type and can be purchased

with different current ratings. A common choice when purchasing output cards is relays,

transistors or triacs. Relays are the most flexible output devices. They are capable of

switching both AC and DC outputs. But, they are slower (about 10ms switching is typical),

they are bulkier, they cost more, and they will wear out after millions of cycles. Relay

outputs are often called dry contacts. Transistors are limited to DC outputs, and Triacs are

limited to AC outputs. Transistor and triac outputs are called switched outputs.

- Dry contacts - a separate relay is dedicated to each output. This allows mixed

voltages (AC or DC and voltage levels up to the maximum), as well as isolated

outputs to protect other outputs and the PLC. Response times are often greater

than 10ms. This method is the least sensitive to voltage variations and spikes.

- Switched outputs - a voltage is supplied to the PLC card, and the card switches it

to different outputs using solid state circuitry (transistors, triacs, etc.) Triacs are

well suited to AC devices requiring less than 1A. Transistor outputs use NPN or

PNP transistors up to 1A typically. Their response time is well under 1ms.

WARNING - ALWAYS CHECK RATED VOLTAGES AND CURRENTS FOR PLC’s

AND NEVER EXCEED!

Caution is required when building a system with both AC and DC outputs. If AC is

accidentally connected to a DC transistor output it will only be on for the positive half of

the cycle, and appear to be working with a diminished voltage. If DC is connected to an

AC triac output it will turn on and appear to work, but you will not be able to turn it off

without turning off the entire PLC.

ASIDE: A transistor is a semiconductor based device that can act as an adjustable valve.

When switched off it will block current flow in both directions.While switched on it

will allow current flow in one direction only. There is normally a loss of a couple of

volts across the transistor. A triac is like two SCRs (or imagine transistors) connected

together so that current can flow in both directions, which is good for AC current.

One major difference for a triac is that if it has been switched on so that current flows,

and then switched off, it will not turn off until the current stops flowing. This is fine

with AC current because the current stops and reverses every 1/2 cycle, but this does

not happen with DC current, and so the triac will remain on

A major issue with outputs is mixed power sources. It is good practice to isolate all

power supplies and keep their commons separate, but this is not always feasible. Some

output modules, such as relays, allow each output to have its own common. Other output

cards require that multiple, or all, outputs on each card share the same common. Each output

card will be isolated from the rest, so each common will have to be connected. It is

common for beginners to only connect the common to one card, and forget the other cards

then only one card seems to work!

The output card shown in Figure 3.5 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.

In this example the outputs are connected to a low current light bulb (lamp) and a

relay coil. Consider the circuit through the lamp, starting at the 24Vdc supply. When the

output 07 is on, current can flow in 07 to the COM, thus completing the circuit, and allowing

the light to turn on. If the output is off the current cannot flow, and the light will not

turn on. The output 03 for the relay is connected in a similar way. When the output 03 is

on, current will flow through the relay coil to close the contacts and supply 120Vac to the

motor. Ladder logic for the outputs is shown in the bottom right of the figure. The notation

is for an Allen Bradley PLC-5. The value at the top left of the outputs, O:012, indicates

that the card is an output card, in rack 01, in slot 2 of the rack. To the bottom right of the

outputs is the output number on the card 03 or 07. This card could have many different

A major issue with outputs is mixed power sources. It is good practice to isolate all

power supplies and keep their commons separate, but this is not always feasible. Some

output modules, such as relays, allow each output to have its own common. Other output

cards require that multiple, or all, outputs on each card share the same common. Each output

card will be isolated from the rest, so each common will have to be connected. It is

common for beginners to only connect the common to one card, and forget the other cards

- then only one card seems to work!

The output card shown in Figure 3.5 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.

voltages applied from different sources, but all the power supplies would need a single

shared common.

The circuits in Figure 3.6 had the sequence of power supply, then device, then PLC

card, then power supply. This requires that the output card have a common. Some output

schemes reverse the device and PLC card, thereby replacing the common with a voltage

input. The example in Figure 3.5 is repeated in Figure 3.6 for a voltage supply card.

In this example the positive terminal of the 24Vdc supply is connected to the out-

put card directly.When an output is on power will be supplied to that output. For example,

if output 07 is on then the supply voltage will be output to the lamp. Current will flow

through the lamp and back to the common on the power supply. The operation is very similar

for the relay switching the motor. Notice that the ladder logic (shown in the bottom

right of the figure) is identical to that in Figure 3.5. With this type of output card only one

power supply can be used.

We can also use relay outputs to switch the outputs. The example shown in Figure

3.5 and Figure 3.6 is repeated yet again in Figure 3.7 for relay output.

In this example the 24Vdc supply is connected directly to both relays (note that

this requires 2 connections now, whereas the previous example only required one.) When

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

This layout is more similar to Figure 3.6 with the outputs supplying voltage, but the relays

could also be used to connect outputs to grounds, as in Figure 3.5. When using relay outputs

it is possible to have each output isolated from the next. A relay output card could

have AC and DC outputs beside each other.

(resource : on internet Hugh Jack's book)

Input module

Inputs

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, or cards, with 8 or 16

inputs of the same type on each card. For discussion purposes we will discuss all inputs as

if they have been purchased as cards. The list below shows typical ranges for input voltages,

and is roughly in order of popularity.

PLC input cards rarely supply power, this means that an external power supply is

needed to supply power for the inputs and sensors. The example in Figure 3.2 shows how

to connect an AC input card.

Note: inputs are normally high impedance. This means that they will

use very little current.

In the example there are two inputs, one is a normally open push button, and the

second is a temperature switch, or thermal relay. (NOTE: These symbols are standard and

will be discussed in chapter 24.) Both of the switches are powered by the hot output of the

24Vac power supply - this is like the positive terminal on a DC supply. Power is supplied

to the left side of both of the switches. When the switches are open there is no voltage

passed to the input card. If either of the switches are closed power will be supplied to the

input card. In this case inputs 1 and 3 are used - notice that the inputs start at 0. The input

card compares these voltages to the common. If the input voltage is within a given tolerance

range the inputs will switch on. Ladder logic is shown in the figure for the inputs.

Here it uses Allen Bradley notation for PLC-5 racks. At the top is the location of the input

card I:013 which indicates that the card is an Input card in rack 01 in slot 3. The input

number on the card is shown below the contact as 01 and 03.

Many beginners become confused about where connections are needed in the circuit

above. The key word to remember is circuit, which means that there is a full loop that

the voltage must be able to follow. In Figure 3.2 we can start following the circuit (loop) at

the power supply. The path goes through the switches, through the input card, and back to

the power supply where it flows back through to the start. In a full PLC implementation

there will be many circuits that must each be complete.

A second important concept is the common. Here the neutral on the power supply

is the common, or reference voltage. In effect we have chosen this to be our 0V reference,

and all other voltages are measured relative to it. If we had a second power supply, we

would also need to connect the neutral so that both neutrals would be connected to the

same common. Often common and ground will be confused. The common is a reference,

or datum voltage that is used for 0V, but the ground is used to prevent shocks and damage

to equipment. The ground is connected under a building to a metal pipe or grid in the

ground. This is connected to the electrical system of a building, to the power outlets,

where the metal cases of electrical equipment are connected. When power flows through

the ground it is bad. Unfortunately many engineers, and manufacturers mix up ground and

common. It is very common to find a power supply with the ground and common mislabeled

Remember - Don’t mix up the ground and common. Don’t connect them together if the

common of your device is connected to a common on another device.

One final concept that tends to trap beginners is that each input card is isolated.

This means that if you have connected a common to only one card, then the other cards are

not connected. When this happens the other cards will not work properly. You must connect

a common for each of the output cards.

There are many trade-offs when deciding which type of input cards to use.

• 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 1/60sec 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.

In put and Out put on PLCS

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: logical or continuous.

Consider the example of a light bulb. If it can only be turned on or off, it is logical control.

If the light can be dimmed to different levels, it is continuous. Continuous values seem

more intuitive, but logical values are preferred because they allow more certainty, and

simplify control. As a result most controls applications (and PLCs) use logical inputs and

outputs for most applications. Hence, we will discuss logical I/O and leave continuous I/O

for later.

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.

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.

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.

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.

Sourcing and sinking inputs are also popular. This output method dictates that a device

does not supply any power. Instead, the device only switches current on or off, like a simple

switch.

Sinking -When active the output allows current to flow to a common ground. This

is best selected when different voltages are supplied.

Sourcing - When active, current flows from a supply, through the output device

and to ground. This method is best used when all devices use a single supply

voltage.

This is also referred to as NPN (sinking) and PNP (sourcing). PNP is more popular.

This will be covered in more detail in the chapter on sensors.
(resource : Hugh Jack's ebook: thanks)

Base Introduction PLC

Many PLC configurations are available, even from a single vendor. But, in each of

these there are common components and concepts. The most essential components are:

Power 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.

Indicator lights - These indicate the status of the PLC including power on, program

running, and a fault. These are essential when diagnosing problems.

The configuration of the PLC refers to the packaging of the components. Typical

configurations are listed below from largest to smallest as shown in Figure 3.1.

Rack - A rack is often large (up to 18” by 30” by 10”) and can hold multiple cards.

When necessary, multiple racks can be connected together. These tend to be the

highest cost, but also the most flexible and easy to maintain.

Mini - These are similar in function to PLC racks, but about half the size.

Shoebox - A compact, all-in-one unit (about the size of a shoebox) that has limited

expansion capabilities. Lower cost, and compactness make these ideal for small

applications.

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 the lowest.

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

allows the PLC to be connected to sensors and other PLCs across a network.

thanks and follow ebook of Hugh Jack

PLC FX Training sorf ware

PLC Programming Methods.pdf