1.2:Histeorical background

The Hydromantic Division of the General Motors Corporation specified the design criteria for the first programmable controller in 1968. Their primary

goal was to eliminate the high costs associated with inflexible, relay controlled

systems. The specifications required a solid-state system with computer flexibility able to

1) Survive in an industrial environment

2) Be easily programmed and maintained by plant engineers and technicians

3) Be reusable. Such a control system would reduce machine downtime and provide expandability for the future. Some of the initial specifications included the following:

• The new control system had to be price competitive with the use of relay

systems.

• The system had to be capable of sustaining an industrial environment.

• The input and output interfaces had to be easily replaceable.

• The controller had to be designed in modular form, so that subassemblies could be removed easily for replacement or repair.

• The control system needed the capability to pass data collection to a central

systems.

• The system had to be reusable.

• The method used to program the controller had to be simple, so that it could be

easily understood by plant personnel.

1.2.1:The conceptual design of the plc:

The first programmable controllers were more or less just relays replacement .

Their primary function was to perform the sequential operations that were previously implemented with relays. These operations included ON/OFF control of machines and processes that required repetitive operations, such as transfer lines and grinding and boring machines. However, these programmable controllers were a vast improvement over relays. They were easily installed, used considerably less space and energy, had diagnostic indicators that aided troubleshooting, and unlike relays, were reusable if a project was scrapped .

Programmable controllers can be considered new comers when they are compared to their elder predecessors in traditional control equipment technology, such as old hardwired relay systems, analog instrumentation, and other types of early solid-state logic. Although PLC functions, such as speed of operation, types of interfaces, and data-processing capabilities, have improved throughout the years, their specifications still hold to the designers’ original intentions they are simple to use and maintain.

1.2.2:TODAY’S programmable controllers:

Many technological advances in the programmable controller industry continue today. These advances not only affect programmable controller design, but also the philosophical approach to control system architecture.

Changes include both hardware (physical components) and software (control program) upgrades. The following list describes some recent PLC hardware enhancements:

• Faster scan times are being achieved using new, advanced microprocessor and electronic technology.

• Small, low-cost PLCs (see Figure 1-2), which can replace four to ten relays, now have more power than their predecessor, the simple relay replacer.

• High-density input/output (I/O) systems provide space-efficient interfaces at low cost.

• Mechanical design improvements have included rugged input/output enclosures and input/output systems that have made the terminal an integral unit.

• Special interfaces have allowed certain devices to be connected directly to the controller. Typical interfaces include thermocouples, strain gauges, and fast-response inputs.

• Peripheral equipment has improved operator interface techniques, and system documentation is now a standard part of the system.

Figure 1-2. Small PLC with built-in

I/O and detachable, handheld programming unit.

All of these hardware enhancements have led to the development of programmable controller families. These families consist of a product line that ranges from very small “microcontrollers,” with as few as 10 I/O points, to very large and sophisticated PLCs, with as many as 8,000 I/O points and 128,000 words of memory. These family members, using common I/O systems and programming peripherals, can interface to a local communication network.

The family concept is an important cost-saving development for users.

Like hardware advances, software advances, such as the ones listed below, have led to more powerful PLCs:

• PLCs have incorporated object-oriented programming tools and multiple languages .

• Small PLCs have been provided with powerful instructions, which extend the area of application for these small controllers.

• High-level languages, such as BASIC and C, have been implemented in some controllers’ modules to provide greater programming flexibility when communicating with peripheral devices and manipulating data.

• Advanced functional block instructions have been implemented for ladder diagram instruction sets to provide enhanced software capability using simple programming commands.

• Diagnostics and fault detection have been expanded from simple system diagnostics, which diagnose controller malfunctions, to include machine diagnostics, which diagnose failures or malfunctions of the controlled machine or process.

• Floating-point math has made it possible to perform complex calculations in control applications that require gauging, balancing, and statistical computation.

• Data handling and manipulation instructions have been improved and simplified to accommodate complex control and data acquisition applications that involve storage, tracking, and retrieval of large amounts of data.

Programmable controllers are now mature control systems offering many more capabilities than were ever anticipated. They are capable of communicating with other control systems, providing production reports, scheduling production, and diagnosing their own failures and those of the machine or process. These enhancements have made programmable controllers important contributors in meeting today’s demands for higher quality and productivity. Despite the fact that programmable controllers have become much more sophisticated, they still retain the simplicity and ease of operation that was intended in their original design.

1.2.3:Programmable controllers and the future :

The future of programmable controllers relies not only on the continuation of new product developments, but also on the integration of PLCs with other control and factory management equipment. PLCs are being incorporated, through networks, into computer-integrated manufacturing (CIM) systems, combining their power and resources with numerical controls, robots, CAD/ CAM systems, personal computers, management information systems, and hierarchical computer-based systems. There is no doubt that programmable controllers will play a substantial role in the factory of the future.

New advances in PLC technology include features such as better operator interfaces, graphic user interfaces (GUIs), and more human-oriented man/ machine interface (such as voice modules). They also include the development of interfaces that allow communication with equipment, hardware, and software that supports artificial intelligence, such as fuzzy logic I/O systems.

Software advances provide better connections between different types of equipment, using communication standards through widely used networks.

New PLC instructions are developed out of the need to add intelligence to a controller. Knowledge-based and process learning–type instructions may be introduced to enhance the capabilities of a system.

The user’s concept of the flexible manufacturing system (FMS) will determine the control philosophy of the future. The future will almost certainly continue to cast programmable controllers as an important player in the factory. Control strategies will be distributed with “intelligence” instead of being centralized. Super PLCs will be used in applications requiring complex calculations, network communication, and supervision of smaller PLCs and machine controllers.