发布时间 : 星期一 文章(完整版)基于PLC的锅炉汽包水位控制系统设计毕业设计更新完毕开始阅读63dd7553cd7931b765ce0508763231126edb77b7
applications worked closely with control vendors to develop new products. They requested the ability to combine the advanced software capabilities of the PC with the reliability of the PLC. These lead users increase in the world-wide demand for PC components, many semiconductor vendors began to redesign their products for industrial applications. Control vendors today are incorporating industrial versions of floating point processors, DRAM, solid-state storage devices such as Compact Flash, and fast Ethernet chipsets into industrial control products. This enables vendors to develop more powerful software with the flexibility and usability of PC-based control systems that can run on real-time operating systems for reliability.
The resulting new controllers, designed to address the “20 percent” applications, combine the best PLC features with the best PC features. Industry analysts at ARC named these devices programmable automation controllers, or PACs. In their “Programmable Logic Controllers Worldwide Outlook” study, ARC identified five main PAC characteristics.
These criteria characterize the functionality of the controller by defining the software capabilities:
1. “Multi-domain functionality, At least 2 of logic, motion, PID control, drives, and process on a single platform.” Except for some variations in IO to address specific protocols like SERCOS; logic, motion, process, and PID are simply a function of the software. For instance, motion control is a software control loop which reads digital inputs from a quadrature encoder, performs analog control loops, and outputs an analog signal to control a
drive.
2. “Single multi-discipline development platform incorporating common tagging and a single database for access to all parameters and functions.” Because PACs are designed for more advanced applications such as multi-domain designs, they require more advanced software. In order to make system design efficient, the software must be a single integrated software package instead of disparate software tools which are not engineered to seamless work together.
3. “Software tools that allow the design by process flow across several machines or process units, together with IEC user guidance, and data management.” Another component that simplifies system design is engineer’s concept of the process into code that actually controls the machine.
4. “Open, modular architectures that mirror industry applications from machine layouts in factories to unit operations in process plants.” Because all industrial applications require significant customization, the pick and choose the appropriate components. The software must enable the engineer to add and remove modules to design the required system.
5. “Employ de-facto standards for network interfaces, languages, etc., such as TCPIP, OPC & XML, and SQL queries.” Communication with enterprise networks is critical for modern control systems. Although PACs include an Ethernet port, the software for communication is the key to trouble-free integration with the rest of the plant.
Two Approaches to Software
While software is the key difference between PACs and PLC, vendors vary in their approach to providing the advanced software. They typically start with their existing control software and work to add the functionality, reliability, and ease-of-use required to program PACs. Generally, this creates two camps of PAC software providers: those with a background in PLC control and those with a background in PC control.
Software Based On PLC Philosophy
Traditional PLC software vendors start with a reliable and easy-to-use scanning architecture and work to add new functionality. PLC software follows a general model of scanning inputs, running control code, updating outputs, and performing of the control code because the input cycles, output cycles, and . With much of the work done by the vendor, this strict control architecture makes it easier and faster to create control systems. The rigidity of these systems also eliminates the need for the control engineer to completely understand the low-level operation of the PLC to create reliable programs. However, the rigid scanning architecture which is the main strength of the PLC can also make it inflexible. Most PLC vendors create PAC software by adding into the existing scanner architecture new functionality such as Ethernet communication, motion control, and advanced algorithms. However, they typically maintain the familiar look and feel of PLC programming and the inherent strengths in logic and control. The result is PAC off ware generally designed to fit specific types of applications such as logic, motion, and PID, but is less flexible for custom applications such as communication, data logging, or
custom control algorithms.
Software Based On PC Philosophy
Traditional PC software vendors start with a very flexible general-purpose programming language, which provides in-depth access to the inner workings of the create the scanner structure normally provided to the PLC programmer, they are not inherent to PC-based control software. This makes the PC software extremely flexible and well suited for complex applications that require advanced structures, programming techniques, or system level control but more difficult for simple applications. The first step for these vendors is to provide reliability and determinism, which are often not available in a general-purpose operating system such as Windows. This is accomplished through real-time operating systems (RTOS) such as Phar Lap from Ardence(formerly Venturcom) or VxWorks from Wind River. These RTOSs provide the capability to control all aspects of the control system, from the IO read and write rates to the priority of individual threads spawned on the controller. These vendors then add abstractions and IO readwrite structures to make it simpler for engineers to build reliable control applications. The result is flexible software suited for custom control, data logging, and communication but lacking the familiar PLC programming architectures, making application development more demanding. National Instruments manufactures a family of PAC deployment platforms that runLabVIEW software. LabVIEW is the de facto standard for test and measurement software. Its intuitive graphical programming style, similar to flow charts, provides the