History

History
        

     Early machines were controlled by means of mechanical devices like cams, gears, levers etc. As the complexity grew, it was needed to have more sophisticated control systems. These systems were developed to operate with electrical power and contained wired relays and switches/contacts as control elements. In 1960s and 1970s electromechanical relays, timers, counters, and sequencers were the industrial standard. Many control panels contained hundreds of electromechanical devices and, in some cases, a mile or more wires.
    These control elements were wired as required to provide control logic necessary for the particular type of machine operation. Hardwired relays and switch logic was cumbersome and time consuming to modify. Wiring had to be removed and replaced to implement new control scheme. One more negative aspect of this electromechanical control was low reliability in contrast to extremely high maintenance cost to keep this machine operative.

Manual Control

Hardware Control

Logic Control

     A new more versatile means of controlling this equipment and modifying the control circuitry was needed. Instead of achieving the desired control or automation through physical wiring of control devices, in a new controller it was achieved through a program or software and it was referred to as programmable controller, abbreviated as PC. To differentiate it from a personal computer, also abbreviated as PC, the new controller is now referred to as programmable logic controller – PLC.
     The first commercially successful PLC was developed in 1969 by Bedford Associates (Bedford, MA) MODICON for General Motors hydromantic division. Through the late 1970s, improvements were made in PLC programs to make them somewhat user friendly. In 1972, with the introduction of the microprocessor chip, the computer power for all kinds of automation systems increased and lowered the computing cost. Robotics, automation devices and computers of all types, including the PLC consequently underwent many improvements. PLC programs written in high level language become more understandable to more people and PLCs became more affordable.