Programmable Logic Controller

Every aspect of industry - from power generation to automobile painting to food packaging - uses programmable controllers to expand and enhance production. In this book, you will learn about all aspects of these powerful and versatile tools. This article will introduce you to the basics of programmable controllers - from their operation to their vast range of applications. In it, we will give you an inside look at the design philosophy behind their creation, along with a brief history of their evolution. We will also compare programmablecontrollers to other types of controls to highlight the benefits anddrawbacks of each, as well as pinpoint situations where PLCs work best. When you finish this chapter, you will understand the fundamentals of programmable controllers and be ready to explore the number systemsassociated with them.

Programmable logic controllers, also called programmable controllers or PLCs, are solid-state members of the computer family, using integrated circuits instead of electromechanical devices to implement control functions. They are capable of storing instructions, such as sequencing, timing, counting, arithmetic, data manipulation, and communication, to control industrial machines and processes. Figure below illustrates a conceptual diagram of a PLC application.

PLC conceptual application diagram

Programmable controllers have many definitions. However, PLCs can be thought of in simple terms as industrial computers with specially designed architecture in both their central units (the PLC itself) and their interfacing circuitry to field devices (input/output connections to the real world).

Control engineering has evolved over time. In the past humans were the main method for controlling a system. More recently electricity has been used for control and early electrical control was based on relays. These relays allow power to be switched on and off without a mechanical switch. It is common to use relays to make simple logical control decisions. The development of low cost computer has brought the most recent revolution, the Programmable Logic Controller (PLC). The advent of the PLC began in the 1970s, and has become the most common choice for manufacturing controls.

PLCs have been gaining popularity on the factory floor and will probably remain predominant for some time to come. Most of this is because of the advantages they offer.

• Cost effective for controlling complex systems.
• Flexible and can be reapplied to control other systems quickly and easily.
• Computational abilities allow more sophisticated control.
• Trouble shooting aids make programming easier and reduce downtime.
• Reliable components make these likely to operate for years before failure.

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Introduction to ATMEGA 32

To give you a basic understanding of the microcontroller, the AVR Atmega32 microcontroller is considered to be a computer on a chip. The microcontroller is able to execute a set of instructions in the form of a program. The program language that I will be using for theseprojects is C++. To giv ethe usersof this website the best opportunity to learn, the C++ programs will be explained is great detail.

The really cool thing about microcontrollers is that you have control over all the pins. For a beginner, this can be a difficult concept to understand, especially having no experience with electronics. Don't fret, I will walk you through each tiny detail. Each pin has a special assignment, or can be used as an input or output feature, with a few exceptions, the power pins.

On the left hand side of the chip, looking at it form the top and the little triangle is at the top left, there are 20 pins (this is a 40 pin microcontroller). The first starting from the top left are the PB0-7 pins. That's a total of 8 pins as the index of these pins and most everything in the program starts with an index at 0. This set of pins are called "Port B" and there are 3 other ports labeled from A to D. These ports can be set to receive information and is called INPUT and they can be set to send voltage out in some form called OUTPUT. General power pins to receive the power for the chip called VCC and GND. All but one pin of Port D (PD0-6) is also located on the left side (lower section). PD7 (Pin 7 of Port D) is all alone starting the right hand side of the microcontroller.

Continuing on the right side, and the ending of Port D, Port C continued from the lower corner up. From there on, may favorite pins continue, the analog to digital pins. These pins have the capability to sense the environment with the help of components that feed these pins an analog voltage. Dopn't worry about not understanding analog or even digita at this point, it will be explained in greter detail later. These analog to digidal converter pins compose Port A.

One example of the use of the analong to digital conversion would be, say, sensing the temperature. You can connect a component that converts temperature to a level of voltage called a thermistor to one of the Port A pins and the microcontroller will convert this voltage to a number from 0 to 255 (an 8-bit number - higher resolution is possible at 10-bits). The program that is written and stored into the microcontroller can use this temperature and respond in a specific way. For example, if you have the thermistor against a boiling pot, the microcontroller can respond and provide an output to another pin that beeps, or flashes a light.

Other features of this and other microcontrollers, other than the actual programming is the programming space (where the program is stored in the chip and how much space you have), memory, or space for data and variables that the program will use, and finally, there is a clock built into the chip that counts. The counting can be in many different speeds depending on the speed of the chip and the divisor that is selected for the speed. This is starting to get complicated, so I will back up. The counting can be in seconds, miliseconds, microseconds, or whatever you determine for the program and application that you select.

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Evolution of microcontrollers

The first microprocessor, named the 4004, was introduced by the Intel Corporation in 1971. This was a simple 4-bit device, supported by three other chips to make a computer; the 4001 and 4002 memory chips, and the 4003 shift register. 4004 was initially used in calculators and in simple control applications.
Shortly after the 4004 appeared in the commercial marketplace, many electronic companies realised the power and future prospects of microprocessors and so have heavily invested in this field. Three other general-purpose microprocessors were soon introduced: Rockwell International 4-bit PPS-4, Intel 8-bit 8008 and the National Semiconductor 16-bit IMP-16. These microprocessors were based on PMOS technology and can be classified as the firstgeneration devices.
In the early 1970s, we see the second-generation microprocessors in the marketplace, designed using the NMOS technology. The shift to NMOS technology resulted in higher execution speeds, as well as higher chip densities. During this time, we see 8-bit microprocessors such as the Motorola 6800, Intel 8080 and 8085, the highly popular Zilog Z80, and Motorola 6800 and 6809.

The third generation of microprocessors were based on HMOS technology, which resulted in higher speeds and, more importantly, higher chip densities. During 1978, we see the 16-bitmicroprocessors such as the Intel 8086, Motorola 68 000 and Zilog Z8000. The 8086 microprocessorwas so successful that it was used in early PC designs (called PC XT).

The fourth generation of microprocessors appeared around the 1980s and the technology was based on HCMOS. During this generation we see the introduction of 32-bit devices into the marketplace. Intel introduced the highly popular 32-bit microprocessors 80 386, 80 486, and the Pentium family; and Motorola introduced the 68 020 family. The Intel processors have been used heavily in early PC designs. In parallel to the development of 32-bit microprocessors, we see the introduction of early single chip computers (later named microcontrollers) into the marketplace.

The Intel 8048 was the first microcontroller, followed by the highly popular 8051 series. The 8051 device has been so popular that it is still in use today. This device was a true single chip computer, containing a CPU, data memory and erasable program memories, I/O module, timer/counter, interrupt logic, clock logic, and serial communications module, such as the Universal Synchronous Asynchronous Receiver Transmitter (USART). After the success of the 8051, we see many other companies offering microcontrollers. Today, some of the most popular general-purpose low-cost 8-bit microcontrollers are Microchip PIC series, Atmel AVR series, Motorola HC11 series, and 8051 and its derivatives.

The fifth and the current generation of microcontrollers are now based on 16-bit and 32-bitarchitectures (e.g. PIC32 series). It is interesting to note that currently the 8-bit microcontrollers are still popular and much more in demand. This is because of their simple architectures, low cost, low power requirements, and the availability of the vast number of hardware and software development tools. The power offered by the high-end 8-bit microcontrollers (e.g. the PIC18F series) are enough for most medium to high-speed applications, except perhaps in special cases of digital signal processing where much higher throughput is generally required.

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Microcontroller

A microcontroller (sometimes abbreviated µC, uC or MCU) is a small computer on a single integrated circuit containing a processor core, memory, and programmable input/output peripherals. Program memory in the form of NOR flash or OTP ROM is also often included on chip, as well as a typically small amount of RAM. Microcontrollers are designed for embedded applications, in contrast to the microprocessors used in personal computers or other general purpose applications.

Microcontrollers are used in automatically controlled products and devices, such as automobile engine control systems, implantable medical devices, remote controls, office machines, appliances, power tools, toys and other embedded systems. By reducing the size and cost compared to a design that uses a separate microprocessor, memory, and input/output devices, microcontrollers make it economical to digitally control even more devices and processes. Mixed signal microcontrollers are common, integrating analog components needed to control non-digital electronic systems.

Some microcontrollers may use four-bit words and operate at clock rate frequencies as low as 4 kHz, for low power consumption (single-digit milliwatts or microwatts). They will generally have the ability to retain functionality while waiting for an event such as a button press or other interrupt; power consumption while sleeping (CPU clock and most peripherals off) may be just nanowatts, making many of them well suited for long lasting battery applications. Other microcontrollers may serve performance-critical roles, where they may need to act more like a digital signal processor (DSP), with higher clock speeds and power consumption.

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Single board computer

A single board computer (SBC) is a type of computer with all of its components built onto a single circuit board. The size of an SBC can vary from about the size of a credit card to that of a video game console. They are often incorporated into larger devices such as automatic teller machines, industrial and medical equipment, or robotic devices. Since the mid 2000s, inexpensive single board computers have been used by educators and hobbyists.

Desktop and even laptop personal computers (PCs) generally have separate components connected to a central circuit board through cables or buses. A single board computer packs all of its necessary components, including the microprocessor, memory, and storage, onto a single circuit board. Many SBCs are built to be PC-compatible and use the same processors, memory, and graphics chips as standard PCs. Other units include different types of hardware and some feature a microcontroller, a specialized processor with built-in input/output functions. Some SBCs are expandable or partially reconfigurable, while others are stuck with what they shipped with.

The size of a single board computer can vary widely, but most are far smaller than a typical PC. The earliest such devices, introduced in the late 1970s and early 1980s, were usually found in educational or development computers, and were quite large. Since then, the trend has been towards smaller SBCs, ranging from a little less than the size of a credit card to about the size of Blu-Ray® player. They can come in both standard and nonstandard sizes, and a few are even built to be the same size as a normal PC expansion card or memory module.

Single board computers are commonly housed inside a larger device or product, thereby providing additional intelligence or controlling the functions of machinery or equipment. Automatic teller machines, cash registers, touch screen kiosks, and many other machines and devices often house an embedded single board computer. They are also used in industrial computers and automation equipment, robotics, medical devices, and many other fields. Due to the number of possible uses, SBCs come in a variety of configurations, and many manufacturers build machines tailored to a specific need or industry application.

By the mid 2000s, the cost of computer components had dropped enough to bring the single board computer within reach of the hobbyist community. Several companies now specialize in low-cost yet versatile SBCs for use in amateur electronics and computing projects. These devices may be used on their own to introduce students to computer programming or as part of a larger platform like a robot or interactive art display.

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