DELTA PLC

3.1.1 INTRODUCTION TO PLC

Programmable Logic Controllers (PLCs), also referred to as programmable

controllers, are in the computer family. They are used in commercial and

industrial applications. It was called “Sequence Controller” beforeIt was named

“Programmable Logic Controller (PLC)” by NEMA (National Electrical

Manufacture Association) in 1978 and defined as electronic equipment. The

operation of PLC is in the following:

Step 1. Read the external input signal, such as the status of keypad, sensor,

switch and pulse.

Step 2. Using microprocessor to execute the calculations of logic, sequence,

timer, counter and formula according to the status and the value of the input

signal read in the step 1 and pre-write programs saved inner to get the

Corresponding output signal, such as open or close of relay, operation of

controlled machine or procedure to control automatic machine or procedure of

manufacture. PLC also can be used to maintain and adjust of production

program by editing or modifying the peripheral equipments (personal

Computer/handheld programming panel). The common program language of

PLC is ladder diagram.

There are stronger functions in PLC with the development and application

requirements of electronic technology, such as position control, network and etc.

Output/Input signals are DI (Digital Input), AI (Analog Input), PI (Pulse Input), DO

(Digital Output), AO (Analog Output) and PO (Pulse Output). Thus PLC plays an

important role in the feature industry.

3.1.2 BASIC PLC OPERATION

PLC consists of input modules or points, a central processing unit(CPU) and

output points

Fig 1.BASIC COMPONENTS OF PLC

3.1.3 PARTS OF PLC

3.1.3.1. CPU

The central processing unit (CPU) is a microprocessor system which contains the

system memory and is the PLC’s decision making unit. The CPU monitors the

inputs and makes decisions based on instructions held in the program memory.

The CPU performs relay, counting, timing, data comparison, and sequential

operations

Figure 2. CENTRAL PROCESSING UNIT

3.1.3.2. ANALOG INPUTS

An analog input is an input signal that has a continuous signal. Typical analog

inputs may vary from 0 to 20 milliamps, 4 to 20 milliamps, or 0 to 10 volts. In the

following example, a level transmitter monitors the level of liquid in a tank.

Depending on the level transmitter, the signal to the PLC can either increase or

decrease as the level increases or decreases.

FIG.3ANALOG INPUT

3.1.3.3. DIGITAL INPUTS

A discrete input, also referred to as a digital input, is an input that is either in an

ON or OFF condition.

Fig 4 DIGITAL INPUT

3.1.3.4. DISCRETE OUTPUTS

A discrete output is an output that is either in an ON or OFF condition. Solenoids,

contactor coils, and lamps are examples of actuator devices connected to

discrete outputs. Discrete outputs may also be referred to as digital outputs. In

the following example, a lamp can be turned on or off by the PLC output it is

connected to.

Fig 5. DISCRETE OUTPUTS

3.1.3.5. ANALOG OUTPUTS

An analog output is an output signal that has a continuous signal. The output

may be as simple as a 0-10 VDC level that drives an analog meter. Examples of

analog meter outputs are speed, weight, and temperature. The output signal may

also be used on more complex applications such as a current-to pneumatic

transducer that controls an air-operated flow-control valve.

Fig 6. ANALOG OUTPUT

3.1.2 BASIC INNER EQUIPMENTS OF THE PLC

TABLE 1:- BASIC INNER EQUIPMENTS OF THE PLC

Input

relay

Input relay is the basic storage unit of internal memory that corresponds to external input point (it is the terminal that used to connect to external input switch and receive external input signal). Input signal from external will decide it to display 0 or 1. You couldn’t change the state of input relay by program design or forced ON/OFF via HPP. The contacts (contact a, b) can be used unlimitedly. If there is no input signal, the corresponding input relay could be empty and can’t beused with other functions. Equipment indication method: X0, X1,…X7, X10, X11,…. The symbol of equipment is X and the number uses octal. There are numeric indications of

input point on MPU and expansion unit.Output

relay

Output relay is the basic storage unit of internal memory that corresponds to external output point (it is used to connect to external load). It can be driven by input relay contact, the contact of other internal equipment and itself contact. It uses a normally open contact to connect to external load and other contacts can be used unlimitedly as input contacts. It doesn’t have the corresponding output relay, if need, it can be used as internal relay. Equipment indication: Y0, Y1,…Y7, Y10, Y11,…. . The symbol of equipment is Y and the number uses octal. There are numeric indications of output point on MPU and expansion unit.

Internal

relay

The internal relay doesn’t connect directly to outside. It is an auxiliary relay in PLC. Its function is the same as the auxiliary relay in electric control circuit. Each auxiliary relay has the corresponding basic unit. It can be driven by the contact of input relay, output relay or other internal equipment. Its contacts can be used unlimitedly. Internal auxiliary relay can’t output directly, it should output with output point. Equipment indication: M0, M1,…, M4, M5. The symbol of equipment is M and the number uses decimal number system.

STEP DVP PLC provides input method for controlling program of step actions. It is very easy to write control program by using the conversion of control step S of command STL. If there is no step program in the program, step point S could be used as internal relay M or alarm point. Equipment indication: S0, S1,…S1023. The symbol of equipment is S and the number uses decimal.

Counter Counter is used to count. It needs to set counter before using counter (i.e. the pulse of counter). There are coil, contacts and storage unit of counter in counter. When coil is form OFF to ON, that means input a pulse in counter and the counter should add 1. There are 16-bit, 32-bit and high-speed counter for user to use. Equipment indication: C0, C1,…,C255. The symbol of equipment is C and the number uses decimal.

Data

register

PLC needs to handle data and operation when controlling each order, timer value and counter value. The data register is used to store data or parameters. It stores 16-bit binary number, i.e. a word, in each register. It uses two continuous number of data register to store double words

Equipment indication: D0, D1,…,D9,999. The symbol of equipment is D and the number uses decimal..

File

register

The file register can be used to store data or parameter when the register that PLC needs is not enough during handling data and parameter. It can store 16-bit binary number, i.e. a word, in each file register. It uses two continuous number of file register to handle double word. There are 1600 file registers for SA/SX/SC series and 10000 file registers for EH series. There is not the real equipment number for file register, thus it needs to execute READ/WRITE of file Register via commands API148 MEMR, API149 MEMW or the peripheral equipment HPP and WPLSoft. Equipment indication: K0~K9,999. There is no equipment symbol and uses decimal number for number.

Index

register

Index register E and F are 16-bit data register just the same as general data register. It can be wrote and read freely and has the function of index indication to use for character device, bit device and constants. Equipment indication: E0~E7, F0~F7. The symbols of equipment are E, F and the number uses decimal.

3.1.5 PLC SCAN

Fig7. PLC SCAN CYCLE

3.1.5.1 READING INPUT – Reads input and updates process input. The input can be either in analog or digital form

3.1.5.2. EXECUTE PROGRAM – executes user program once. The program is in the form of ladder logic diagram

3.1.5.3. CHECKS COMMUNICATION – Takes care of the system processes (such as communication with other PLC’s)

3.1.5.4. UPDATES OUTPUT – The PLC updates the output according to the execution of the program

3.1.6 THE WORKING PRINCIPLE OF LADDER DIAGRAM

Ladder diagram is an automatic control diagram language that developed

during World War II. At first, it just has basic components, such as A contact

(normally open), B contact (normally close), output coil, timer, counter and etc.

(The power panel is made up of these basic components) It has more functions,

differential contact, latched coil and the application commands, add, minus,

multiply and divide calculation, that traditional power panel can’t make since PLC

is developed. The working principles of the traditional Ladder Diagram and the

PLC Ladder Diagram are similar to each other; the only difference is that the

symbols for the traditional ladder diagram are expressed in the format that are

close to its original substance, while those for the PLC ladder diagram employ

the symbols that are more explicit when being used in computers or data sheets.

In the Ladder Diagram Logics, it could be divided into the Combination Logics

and the Sequential Logics, and is described as follows:

3.1.6.1. Combination Logics:

The following example is the combination logics that show in traditional diagram

and PLC ladder diagram separately.

Fig8.Traditional ladder diagram

Fig9.PLC ladder diagram

3.1.6.2. Sequential logics:

The sequential logics are a type of circuit that possesses the “Draw-Back”

structure, which is to draw back the circuit’s output result and has it serve as the

input condition. Thus, under the same input condition, different output results will

be generated in accordance with previous conditions and motions with different

orders.

The following example is the sequential logics that show in traditional diagram and PLC ladder diagram separately

Table 1:BASIC INSTRUCTIONS OF DELTA PLC

.

Traditional Ladder Diagram

PIC MICRO CONTROLLER