The function of controllers in early machine tools and robots was to store and execute simple programs that produce the sequence of motions required of a tool or endeffector at predetermined speeds. Since the first appearance of the IBM-PC in 1981, several other manufacturers have programs that use more memory than is actually available in a given system by exchanging data between the main memory and secondary storage devices. It is this processing power and memory availability that has made such a dramatic impact on the general engineering acceptance of personal computers for data acquisition and control applications. In addition to an increase in the processing power, there are many other advantages in using a PC as the centerpiece in any control applications. These advantages are:
(1)There is a large choice of applications software which is not generally available for dedicated controllers.
(2)There is a large choice of tools available to produce applications software efficiently.
(3)The PC is available in a variety of forms ranging from a single card, a portable, a desktop and ruggedized industrial version for use on the factory floor.
(4)Expansion plug-in slots to the PC bus structure are available and large ranges of cards for digital or analogue I/O have been produced by a number of manufacturers.
(5)The PC-based controller is more flexible than the dedicated, or minicomputer system, and can be easily configured indefinitely to suit different applications.
Data acquisition and control add-ons for the PC exist either as external rack-mounted systems, or as plug-in boards. The external box approach usually involves attaching a separate rack-type enclosure with power supply to the host PC. The connection is made either through the included data using a PC. The first option is to use an A/D converter card connected directly the host computer’s backplane.The cards are generally “port addressed” and may be driven by any program language which supports IN/OUT commands. The base address is usually switch selectable on the card. This enables different cards, or more than one of the same card, to be connected and run from the same host computer. The second option uses instruments such as digital voltmeters, frequency meters, etc., which have an interface board that enables data from or to the controlling PC. The most common standard is the IEEE-488 (GPIB) parallel communications link. The quickest, easiest and least expensive get measured addressed I/O card. Versatile cards are readily available offering such features as:
(1) Multichannel digital I/O, with opto-isolation and
(2) Pulse counting and timing facilities.
(3) Multiplexed analogue to digital conversion with programmable again.
(4) Digital to analogue conversion.
(5) Thermocouple input.
Recent advances have included substantial developments in the software available for data capture and control, which provide the user with a mouse-driven environment of windows and pull down menus.
Prior to the introduction of computer-based control systems the automation was achieved by using either electrical relay logic circuits or pneumatic logic circuits. The late 1960s saw the introduction of the programmable logic controller (PLC) as a direct replacement for the relay sequence controllers. in the in the
The PLC is composed of the same ingredients as microprocessor, memory and input/output facilities. The processor executes the instructions held in memory by operating on inputs derived from the controlled process and providing outputs in accordance with the logic sequence defined in the control program is scanned very fast per step to record all input states. The outputs are then set according to the logic specified in the program. The sequence is continually repeated for each scan period of the controller.
Small PLCs dedicated to sequential control have typically 12 inputs and 8 outputs with the possibility of expansion up to 128 I/O lines. They come complete with an input interface to accommodate a range of input signals from the controlled process which are then converted to an appropriate form for the processor. Similarly, provision is made at the output of the PLC to interface with a variety of process hardware such as lamps, motors, relays and solenoids.
Program instructions can be input into the battery backup RAM of a PLC by means of either a hand-held programming keypad or a connected PC with an appropriate software development package. Some LCD programming consoles incorporate a limited graphical display which illustrates the program in ladder logic format as the programmer builds it up using interested in moving form hardwired relay control systems to the PLC. In addition to the basic input/output facilities the PLC also contains timers, counters and other special functions.
Communicating with a variety of other control devices has not been strength of traditional PLC networks. Many industrial controllers are equipped are equipped with an RS232 serial port for the transfer of date to and from other digital control devices in a system.