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INDUSTRIAL CONTROL HANDBOOK - 0.5 INTERFACING AND SIGNAL CONDITIONING

There was quite a long delay before digital computer control of manufacturing processes became widely implemented. Lack of standardization was the problem.

NC equipment showed up as the first real application of digital control. The suppliers of NC equipment built totally enclosed systems, evolving away from analog control toward digital control. With little need for interconnection of the NC equipment to other computer controlled devices, the suppliers did not have to worry about lack of standards in communication. Early NC equipment read punched tape programs as they ran. Even the paper tape punchers were supplied by the NC equipment supplier.

Meanwhile, large computer manufacturers, such as IBM and DEC, concentrated on interconnecting their own proprietary equipment to their own proprietary office peripherals. (Interconnection capability was poor even there.)

Three advances eventually opened the door for automated manufacturing by allowing easier interfacing of controllers, sensors and actuators. One advance was the development of the programmable controller, then called a "PC," now called a "PLC" (programmable logic controller). PLCs contain digital computers. It was a major step from sequencing automation with rotating cams or with series of electrical relay switches, to using microprocessor-based PLC sequencers. With microprocessors, the sequencers could be programmed to follow different sequences under different conditions.

The physical structure of a PLC, as shown in figure 0.8, is as important a feature as its computerized innards. The central component, called the CPU, contains the digital computer and plugs into a bus or a rack. Other PLC modules can be plugged into the same bus. Optional interface modules are available for just about any type of sensor or actuator. The PLC user buys only the modules needed, and thus avoids having to worry about compatibility between sensors, actuators and the PLC. Most PLCs offer communication modules now, so that the PLC can exchange data with at least other PLCs of the same make. Figure 0.9 shows a PLC as it might be connected for a position-control application: reading digital input sensors, controlling AC motors, and exchanging information with the operator.

Another advance which made automation possible was the development of the robot. A variation on NC equipment, the robot is a self-enclosed system of actuators, sensors and controller. Compatibility of robot components is the robot manufacturer's problem. A robot includes built-in programs allowing the user to "teach" positions to the arm, and to play-back moves. Robot programming languages are similar to other computer programming languages, like BASIC. Even the early robots allowed connection of certain types of external sensors and actuators, so complete work cells could be built around, and controlled by, the robot. Modern robots usually include communication ports, so that robots can exchange information with other computerized equipment with similar communication ports.

The third advance was the introduction, by IBM, of the personal computer (PC). (IBM's use of the name "PC" forced the suppliers of programmable controllers to start calling their "PC"s by another name. hence the "PLC.") IBM's PC included a feature then called open architecture. What this meant was that inside the computer box was a computer on a single "mother" circuit-board and several slots on this "motherboard." Each slot is a standard connector into a standard bus (set of conductors controlled by the computer). This architecture was called "open" because IBM made information available so that other manufacturers could design circuit boards that could be plugged into the bus. IBM also provided information on the operation of the motherboard so that others could write IBM PC programs to use the new circuit boards. IBM undertook to avoid changes to the motherboard that would obsolete the important bus and motherboard standards. Boards could be designed and software written to interface the IBM PC to sensors, actuators, NC equipment, PLCs, robots, or other computers, without IBM having to do the design or programming. Coupled with IBM's perceived dependability, this "open architecture" provided the standard that was missing. Now computerization of the factory floor could proceed.

Fig. 0.8 (a) A programmable controller; (b) installation of I/O module on bus unit.

Fig. 0.8 (a) A programmable controller; (b) installation of I/O module on bus unit. (Photographs by permission, Westinghouse Electric Corporation/ Electrical Components Division, Pittsburgh, Pennsylvania.)

 

0.5.1 Interfacing of Controllers to Controllers

Standards for what form the signals should take for communication between computers are still largely missing. The PLC, robot, and computer manufacturers have each developed their own standards, hut one supplier's equipment can't communicate very easily with another's.

Most suppliers do, at least, build their standards around a very basic set of standards which dates back several decades to the days of teletype machines: the RS 232 standard. Because of the acceptance of RS 232, a determined user can usually write controller programs which exchange simple messages with each other.

The International Standards Organization (ISO) is working to develop a common communication standard, known as the OSI (Open Systems Interconnectivity) model. Several commercial computer networks are already available, many using the agreed-on parts of the OSI model. Manufacturer's Automation Protocol (MAP) and Technical and Office Protocol (TOP) are the best-known of these.

Despite the recognition that common standards are needed to be recognized, the immediate need for communications is leading to the growth of immediately available proprietary standards.

Fig. 0.9 A PLC in a position control application

Fig. 0.9 A PLC in a position control application. (Illustration by permission, OMRON Canada Inc., Scarborough, Ontario, Canada.)

Several large PLC manufacturers sell proprietary local area networks and actively encourage others to join them in using those standards. Simultaneous growth of proprietary office local area network suppliers means that interconnecting the plant to the manufacturing office is becoming another problem area. One promising aspect of the growth of giants in the local area network field is that the giants recognize the need for easy-to-use interfaces between their systems, and have the money to develop them.

 

0.5.2 Interfacing of Controllers to Other Components

One area in which development of standards is not as great a problem is the connection of sensors and actuators to controllers. This area is a less severe problem because electrical actuators have been available for so long that several standards are effectively in force.

The 24 volt DC solenoid-actuated valve is an example. 240 volt three phase AC is another standard, dictated by power supply utility companies. Sensor suppliers, more recent arrivals, have simply adopted those standards most appropriate to their target market.

Lack of a single standard is still an inconvenience. Signal conditioning is often required so that incompatible components and controllers can be interconnected. The size of the problem can be reduced by the user by selecting components with similar power requirements and control signal characteristics, if possible. Another option is using a PLC as the controller, and selecting a PLC which offers I/O modules for all the different sensors and actuators to be used.

The user could also consider employing popular "open architecture" computers that can be retrofitted with interfacing circuit cards available from several sources; however, programming the control program to use those interface cards requires some skill. Another alternative is buying or building signal conditioning interface circuits to do signal modification such as:

  • cleaning noisy electrical signals
  • isolating high power signals from low power signals " amplification (or de-amplification) of current or voltage levels
  • converting analog signals to/from digital numbers
  • converting DC signals to/from AC signals
  • converting electrical signals to/from non-electrical signals

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