Baixe Number Systems Terminology e outras Notas de estudo em PDF para Tecnologia Industrial, somente na Docsity!
Table of Contents
- Introduction ..............................................................................
- PLCs .........................................................................................
- Number Systems ......................................................................
- Terminology
- Basic Requirements ................................................................
- S7-200 Micro PLCs .................................................................
- Connecting External Devices ..................................................
- Programming a PLC ................................................................
- Discrete Inputs/Outputs .........................................................
- Analog Inputs and Outputs .....................................................
- Timers .....................................................................................
- Counters
- High-Speed Instructions .........................................................
- Specialized Expansion Modules .............................................
- Review Answers .....................................................................
- Final Exam ..............................................................................
Introduction
Welcome to another course in the STEP series, S iemens T echnical E ducation P rogram, designed to prepare our distributors to sell Siemens Energy & Automation products more effectively. This course covers Basics of PLCs and related products.
Upon completion of Basics of PLCs you should be able to:
- Identify the major components of a PLC and describe their functions
- Convert numbers from decimal to binary, BCD, and hexadecimal
- Identify typical discrete and analog inputs and outputs
- Read a basic ladder logic diagram and statement list
- Identify operational differences between different S7- models
- Identify the proper manual to refer to for programming or installation of an S7-200 PLC
- Connect a simple discrete input and output to an S7-
- Select the proper expansion module for analog inputs and outputs
- Describe the operation of timers and counters
PLCs
Programmable Logic Controllers (PLCs), also referred to as programmable controllers, are in the computer family. They are used in commercial and industrial applications. A PLC monitors inputs, makes decisions based on its program, and controls outputs to automate a process or machine. This course is meant to supply you with basic information on the functions and configurations of PLCs.
Photo Sensors
Lights Pumps
Pushbutton Switches
Drive Motors
Basic PLC Operation PLCs consist of input modules or points, a Central Processing Unit (CPU), and output modules or points. An input accepts a variety of digital or analog signals from various field devices (sensors) and converts them into a logic signal that can be used by the CPU. The CPU makes decisions and executes control instructions based on program instructions in memory. Output modules convert control instructions from the CPU into a digital or analog signal that can be used to control various field devices (actuators). A programming device is used to input the desired instructions. These instructions determine what the PLC will do for a specific input. An operator interface device allows process information to be displayed and new control parameters to be entered.
Pushbuttons (sensors), in this simple example, connected to PLC inputs, can be used to start and stop a motor connected to a PLC through a motor starter (actuator).
PLC
Start/Stop Pushbuttons (Sensors)
Motor Starter (Actuator)
Motor
Input
Output
Siemens PLCs Siemens makes several PLC product lines in the SIMATIC® S family. They are: S7-200, S7-300, and S7-400.
S7-200 The S7-200 is referred to as a micro PLC because of its small size. The S7-200 has a brick design which means that the power supply and I/O are on-board. The S7-200 can be used on smaller, stand-alone applications such as elevators, car washes, or mixing machines. It can also be used on more complex industrial applications such as bottling and packaging machines.
S7-300 and S7-400 The S7-300 and S7-400 PLCs are used in more complex applications that support a greater number of I/O points. Both PLCs are modular and expandable. The power supply and I/O consist of separate modules connected to the CPU. Choosing either the S7-300 or S7-400 depends on the complexity of the task and possible future expansion. Your Siemens sales representative can provide you with additional information on any of the Siemens PLCs.
Number Systems
Since a PLC is a computer, it stores information in the form of On or Off conditions (1 or 0), referred to as binary digits (bits). Sometimes binary digits are used individually and sometimes they are used to represent numerical values.
Decimal System Various number systems are used by PLCs. All number systems have the same three characteristics: digits, base, weight. The decimal system, which is commonly used in everyday life, has the following characteristics:
Ten digits 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 Base 10 Weights 1, 10, 100, 1000, ...
Binary System The binary system is used by programmable controllers. The binary system has the following characteristics:
Two digits 0, 1 Base 2 Weights Powers of base 2 (1, 2, 4, 8, 16, ...)
In the binary system 1s and 0s are arranged into columns. Each column is weighted. The first column has a binary weight of 2 0
. This is equivalent to a decimal 1. This is referred to as the least significant bit. The binary weight is doubled with each succeeding column. The next column, for example, has a weight of 2 1 , which is equivalent to a decimal 2. The decimal value is doubled in each successive column. The number in the far left hand column is referred to as the most significant bit. In this example, the most significant bit has a binary weight of 2 7 . This is equivalent to a decimal 128.
Logic 0, Logic 1 Programmable controllers can only understand a signal that is On or Off (present or not present). The binary system is a system in which there are only two numbers, 1 and 0. Binary 1 indicates that a signal is present, or the switch is On. Binary 0 indicates that the signal is not present, or the switch is Off.
BCD Binary-Coded Decimal (BCD) are decimal numbers where each digit is represented by a four-bit binary number. BCD is commonly used with input and output devices. A thumbwheel switch is one example of an input device that uses BCD. The binary numbers are broken into groups of four bits, each group representing a decimal equivalent. A four-digit thumbwheel switch, like the one shown here, would control 16 (4 x 4) PLC inputs.
Hexadecimal Hexadecimal is another system used in PLCs. The hexadecimal system has the following characteristics:
16 digits 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F Base 16 Weights Powers of base 16 (1, 16, 256, 4096 ...)
The ten digits of the decimal system are used for the first ten digits of the hexadecimal system. The first six letters of the alphabet are used for the remaining six digits.
A = 10 D = 13 B = 11 E = 14 C = 12 F = 15
The hexadecimal system is used in PLCs because it allows the status of a large number of binary bits to be represented in a small space such as on a computer screen or programming device display. Each hexadecimal digit represents the exact status of four binary bits. To convert a decimal number to a hexadecimal number the decimal number is divided by the base of 16. To convert decimal 28, for example, to hexadecimal:
Decimal 28 divided by 16 is 1 with a remainder of 12. Twelve is equivalent to C in hexadecimal. The hexadecimal equivalent of decimal 28 is 1C.
The decimal value of a hexadecimal number is obtained by multiplying the individual hexadecimal digits by the base 16 weight and then adding the results. In the following example the hexadecimal number 2B is converted to its decimal equivalent of 43.
0 = 1 16 1 = 16 B = 11
Review 1
- Identify the following:
- The binary number system has a base ____________.
- The hexadecimal number system has a base ____________.
- Convert a decimal 10 to the following:
Binary ____________
BCD ____________
Hexadecimal ____________
Terminology
The language of PLCs consists of a commonly used set of terms; many of which are unique to PLCs. In order to understand the ideas and concepts of PLCs, an understanding of these terms is necessary.
Sensor A sensor is a device that converts a physical condition into an electrical signal for use by the PLC. Sensors are connected to the input of a PLC. A pushbutton is one example of a sensor that is connected to the PLC input. An electrical signal is sent from the pushbutton to the PLC indicating the condition (open/ closed) of the pushbutton contacts.
Actuators Actuators convert an electrical signal from the PLC into a physical condition. Actuators are connected to the PLC output. A motor starter is one example of an actuator that is connected to the PLC output. Depending on the output PLC signal the motor starter will either start or stop the motor.
Motor Starter (Actuator)
PLC Output 1 Motor
Analog Inputs An analog input is a continuous, variable 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.
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.
Analog Outputs An analog output is a continuous, variable 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.
CPU The central processor unit (CPU) is a microprocessor system that contains the system memory and is the PLC 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.
In the example program shown example I0.0, I0.1 and Q0. represent the first instruction combination. If inputs I0.0 and I0.1 are energized, output relay Q0.0 energizes. The inputs could be switches, pushbuttons, or contact closures. I0.4, I0.5, and Q1.1 represent the second instruction combination. If either input I0.4 or I0.5 are energized, output relay Q0.1 energizes.
Statement list A statement list (STL) provides another view of a set of instructions. The operation, what is to be done, is shown on the left. The operand, the item to be operated on by the operation, is shown on the right. A comparison between the statement list shown below, and the ladder logic shown on the previous page, reveals a similar structure. The set of instructions in this statement list perform the same task as the ladder diagram.
Function Block Diagrams Function Block Diagrams (FBD) provide another view of a set of instructions. Each function has a name to designate its specific task. Functions are indicated by a rectangle. Inputs are shown on the left-hand side of the rectangle and outputs are shown on the right-hand side. The function block diagram shown below performs the same function as shown by the ladder diagram and statement list.
PLC Scan The PLC program is executed as part of a repetitive process referred to as a scan. A PLC scan starts with the CPU reading the status of inputs. The application program is executed using the status of the inputs. Once the program is completed, the CPU performs internal diagnostics and communication tasks. The scan cycle ends by updating the outputs, then starts over. The cycle time depends on the size of the program, the number of I/Os, and the amount of communication required.
Software Software is any information in a form that a computer or PLC can use. Software includes the instructions or programs that direct hardware.
Hardware Hardware is the actual equipment. The PLC, the programming device, and the connecting cable are examples of hardware.
