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A Historical Background
The Hydramatic 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
- Survive in an industrial environment
- Be easily programmed and maintained by plant engineers and technicians
- Be reusable.
The Conceptual Design of PLC
The first programmable controllers were more or less just relay replacers. 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
Today’s Programmable Controllers
Many technological advances in the programmable controller industry continue today.
- Faster scan times are being achieved using new, advanced micro-processor and electronic technology.
- High-density input/output (I/O) systems provide space-efficient interfaces at low cost.
- Intelligent, microprocessor-based I/O interfaces have expanded distributed processing. Typical interfaces include PID (proportional-integralderivative),Network, CAN bus, field bus, ASCII communication ,positioning, host computer, and language modules (e.g., BASIC , Pascal).
- 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.
