1、Introduction of Programmable controllersFrom a simple heritage, these remarkable systems have evolved to not only replace electromechanical devices, but to solve an ever-increasing array of control problems in both process and non process industries. By all indications, these microprocessor powered
2、giants will continue to break new ground in the automated factory into the 1990s.HISTORY In the 1960s, electromechanical devices were the order of the day as far as control was concerned. These devices, commonly known as relays, were being used by the thousands to control many sequential-type manufa
3、cturing processes and stand-along machines. Many of these relays were in use in the transportation industry, more specifically, the automotive industry. These relays used hundreds of wires and their interconnections to effect a control solution. The performance of a relay was basically reliable - at
4、 least as a single device. But the common applications for relay panels called for 300 to 500 or more relays, and the reliability and maintenance issues associated with supporting these panels became a very great challenge. Cost became another issue, for in spite of the low cost of the relay itself,
5、 the installed cost of the panel could be quite high. The total cost including purchased parts, wiring, and installation labor, could range from $30$50 per relay. To make matters worse, the constantly changing needs of a process called for recurring modifications of a control panel. With relays, thi
6、s was a costly prospect, as it was accomplished by a major rewiring effort on the panel. In addition these changes were sometimes poorly documented, causing a second-shift maintenance nightmare months later. In light of this, it was not uncommon to discard an entire control panel in favor of a new o
7、ne with the appropriate components wired in a manner suited for the new process. Add to this the unpredictable, and potentially high, cost of maintaining these systems as on high-volume motor vehicle production lines, and it became clear that something was needed to improve the control process to ma
8、ke it more reliable, easier to troubleshoot, and more adaptable to changing control needs. That something, in the late 1960s, was the first programmable controller. This first revolutionary system wan developed as a specific response to the needs of the major automotive manufacturers in the United S
9、tates. These early controllers, or programmable logic controllers (PLC), represented the first systems that 1 could be used on the factory floor, 2 could have there logic changed without extensive rewiring or component changes, and 3 were easy to diagnose and repair when problems occurred. It is int
10、eresting to observe the progress that has been made in the past 15 years in the 1programmable controller area. The pioneer products of the late 1960s must have been confusing and frightening to a great number of people. For example, what happened to the hardwired and electromechanical devices that m
11、aintenance personnel were used to repairing with hand tools? They were replaced with computers disguised as electronics designed to replace relays. Even the programming tools were designed to appear as relay equivalent presentations. We have the opportunity now to examine the promise, in retrospect,
12、 that the programmable controller brought to manufacturing. All programmable controllers consist of the basic functional blocks shown in Fig. 10. 1. Well examine each block to understand the relationship to the control system. First we look at the center, as it is the heart ( or at least the brain )
13、 of the system. It consists of a microprocessor, logic memory for the storage of the actual control logic, storage or variable memory for use with data that will ordinarily change as a function power for the processor and memory. Next comes the I/O block. This function takes the control level signal
14、s for the CPU and converts them to voltage and current levels suitable for connection with factory grade sensors and actuators. The I/O type can range from digital (discrete or on / off), analog (continuously variable), or a variety of special purpose smart I/O which are dedicated to a certain appli
15、cation task. The programmer is shown here, but it is normally used only to initially configure and program a system and is not required for the system to operate. It is also used in troubleshooting a system, and can prove to be a valuable tool in pinpointing the exact cause of a problem. The field d
16、evices shown here represent the various sensors and actuators connected to the I/O. These are the arms, legs, eyes, and ears of the system, including push buttons, limit switches, proximity switches, photo sensors, thermocouples, RTDS, position sensing devices, and bar code reader as input; and pilot lights, display devices, motor starters, DC and AC drives, solenoids, and printers as outputs. No single attempt could cover its rapidly changing scope, but thre
