1、PLCs -Past, Present and Future Everyone knows theres only one constant in the technology world, and thats change. This is especially evident in the evolution of Programmable Logic Controllers (PLC) and their varied applications. From their introduction more than 30 years ago, PLCs have become the co
2、rnerstone of hundreds of thousands of control systems in a wide range of industries. At heart, the PLC is an industrialized computer programmed with highly specialized languages, and it continues to benefit from technological advances in the computer and information technology worlds. The most promi
3、nent of which is miniaturization and communications. The Shrinking PLC When the PLC was first introduced, its size was a major improvement - relative to the hundreds of hard-wired relays and timers it replaced. A typical unit housing a CPU and I/O was roughly the size of a 19 television set. Through
4、 the 1980s and early 1990s, modular PLCs continued to shrink in footprint while increasing in capabilities and performance (see Diagram 1 for typical modular PLC configuration). In recent years, smaller PLCs have been introduced in the nano and micro classes that offer features previously found only
5、 in larger PLCs. This has made specifying a larger PLC just for additional features or performance, and not increased I/O count, unnecessary, as even those in the nano class are capable of Ethernet communication, motion control, on-board PID with autotune, remote connectivity and more. PLCs are also
6、 now well-equipped to replace stand-alone process controllers in many applications, due to their ability to perform functions of motion control, data acquisition, RTU (remote telemetry unit) and even some integrated HMI (human machine interface) functions. Previously, these functions often required
7、their own purpose-built controllers and software, plus a separate PLC for the discrete control and interlocking. The Great Communicator Possibly the most significant change in recent years lies in the communications arena. In the 1970s Modicon introduction of Modbus communications protocol allowed P
8、LCs to communicate over standard cabling. This translates to an ability to place PLCs in closer proximity to real world devices and communicate back to other system controls in a main panel. In the past 30 years we have seen literally hundreds of proprietary and standard protocols developed, each wi
9、th their own unique advantages.Todays PLCs have to be data compilers and information gateways. They have to interface with bar code scanners and printers, as well as temperature and analog sensors. They need multiple protocol support to be able to connect with other devices in the process. And furth
10、ermore, they need all these capabilities while remaining cost-effective and simple to program. Another primary development that has literally revolutionized the way PLCs are programmed, communicate with each other and interface with PCs for HMI, SCADA or DCS applications, came from the computing wor
11、ld. Use of Ethernet communications on the plant floor has doubled in the past five years. While serial communications remain popular and reliable, Ethernet is fast becoming the communications media of choice with advantages that simply cant be ignored, such as: * Network speed. * Ease of use when it
12、 comes to the setup and wiring. * Availability of off-the-shelf networking components. * Built-in communications setups. Integrated Motion Control Another responsibility the PLC has been tasked with is motion control. From simple open-loop to multi-axis applications, the trend has been to integrate
13、this feature into PLC hardware and software. There are many applications that require accurate control at a fast pace, but not exact precision at blazing speeds. These are applications where the stand-alone PLC works well. Many nano and micro PLCs are available with high-speed counting capabilities
14、and high-frequency pulse outputs built into the controller, making them a viable solution for open-loop control. The one caveat is that the controller does not know the position of the output device during the control sequence. On the other hand, its main advantage is cost. Even simple motion contro
15、l had previously required an expensive option module, and at times was restricted to more sophisticated control platforms in order to meet system requirements. More sophisticated motion applications require higher-precision positioning hardware and software, and many PLCs offer high-speed option mod
16、ules that interface with servo drives. Most drives today can accept traditional commands from host (PLC or PC) controls, or provide their own internal motion control. The trend here is to integrate the motion control configuration into the logic controller programming software package. Programming Languages
