PC-Based Automated Control System for Jordan Northern Grain Silo ()
1. Introduction
Programmable logic controller (PLC) has long been a major technology of automation in the industrial control market. It has evolved from the need for a control system that can be easily reprogrammed as changes occur or as new products are developed. The principle of PLC is based on the concept of a sequential control process. The operation of a specific device is dependent on a sequence of operations and conditions of other devices. The control process is reprogrammable or reconfigurable using a microprocessor-based electronic device. PLCs have been perceived faster and more reliable than running control software on an embedded personal computer (PC). The benefits of the PC based control system, however, are significant. In addition to control operations, they share data and can combine operator interface, data filing, programming, diagnosis, and monitoring and control functions into one platform.
The PC-based control system has recently gained popularity in the industrial automation market [1-6]. Microsoft’s Windows operating system is a leading choice for many developers of software-based industrial controllers. Windows supports a broad base of applications and simplifies data sharing. Developing Windows/PCbased control systems with real-time, deterministic performance is still a challenge [7,8]. Software-based control logic needs to perform a control operation at precisely the time requested. Within the operating requirements of typical PC operating systems, this is pretty difficult. PCs typically process multiple control and data intensive functions while transferring and displaying data through a graphical user interface. Combining those functions with real-time control has not been possible. The best approach is an integrated system of PLC and PC. The PLC is either a board that can be plugged into the PC, or a separate device that can be connected to the PC via I/O modules. The PLC portion handles direct control of a process while the PC portion handles all data intensive functions.
The northern grain silo of Jordan is in the process of moving towards a PC-based control environment to meet expanding market and administrative requirements in an era of information technology. A grain silo is a storage media of grain. Huge amounts of grain are reserved to meet daily market needs in the area. The silo consists of a number of large cylindrical storage cells. Each cell is about 50 meters high and 10 meters radius. There exists a complex control system to manage the storage and distribution processes of the grain. There are two different control systems for different sets of grain cells. One is the manual control system that uses hardwired relay system. The system is operated manually through a control panel in the control room. The panel mainly consists of switches and pushbuttons to remotely operate motors, conveyers, lifters, etc. The panel also consists of light indicators to monitor the system operation. The second control system is the PLC automated system. The PLC has many advantages. The control process is easily reconfigurable as changes occur. The system does not allow the operator to manually operate the system devices, thus a wrong selection in the operation sequence is not possible. The only disadvantage is the complexity involved, a fact that requires highly skilled personal. The two systems are totally decoupled. Recently, considerable efforts are being devoted to couple the two systems and to develop programming, monitoring and diagnosis interfaces that require less skilled personal.
This paper presents a design framework of a PC-based automated control system for the Northern Grain Silo of Jordan. The system connects to a PLC (programmable logic controller) device, and combines operator interface, PLC programming and monitoring functions into one platform. A simulation package is developed using Visual C++ under Microsoft developer studio. The package generates a graphical user interface for PLC programming and provides real-time animations of the Grain Silo operation. The package runs a small-scale grain silo operating on a basis of PLC automation. It shows graphics of storage cells, conveyer belts, operation sequencing, and grain distribution. The system motors, conveyers, grain cells, switches, and sensors are connected to the PLC through input/output interfacing modules. The package provides validation and testing platforms for future implementation of our design. We use Object oriented Programming (OOP) for design and implementation. The OOP furnishes designers with powerful features to model complex data and applications, [9]. Abstraction, modularity, polymorphism, encapsulation, exception handling, fault tolerance and many other adept features greatly enhance performance and allow easy integration with existing proprietary software. The remainder of the paper is organized as follows: Section 2 presents the proposed grain silo automated control system. Section 3 presents software implementation. Testing results are discussed in Section 4. Finally, the main conclusions are given in Section 5.
2. Grain Silo Automated Control System
An overview of the operation and information flow of the proposed grain silo automated control system is shown in
Figure 1. The system handles direct control through the PLC and collects real-time data for monitoring, fault diagnosis and management planning. The operator issues a production plan (i.e. scheduled grain-in/out, grain cell operation, etc.). The plan is decoded into a PLC program, then transferred to the control server (CS). The international IEC 1131.3 specification for PLC programming languages defines four programming standards: ladder-logic diagrams, function-block diagrams, instruction list and structured text [10]. We use ladder-logic diagram for PLC programming. The CS downloads the decoded plan into the PLC. The PLC implements direct system controls through several input/output modules. The PLC and data sensors gather information on the system status. The CS then uses this information for monitoring, diagnosis, and data analysis. Different PC terminals connect to the CS through a BUS control topology. The information flow can be an Intranet or the Internet. The Internet covers a broad range of users and gives access to remote management personal from the ministry of industry and commerce.