DISTRIBUTED SEQUENTIAL CONTROL IN AUTOMATION USING LONWORKS

Abstract

Conventional sequential control system usually consists of a central controller ( for example. PLC) which has 1/0 modules as the interface to external input and output devices. such as solenoid operated valves. actuators, sensors, etc. This point-to-point wiring connections usually result in messy wiring layout and it can become complex as the number of devices increases. In recent year, most PLC systems have incorporated fieldbus (distributed system applying to low level devices) capability which consists of smart 1/0 devices, for example. sensors, actuators, etc. which have embedded microprocessor chip in them. All the devices are connected by communication medium (multi-drop) which facilitate data and messages transfer between the PLC and the devices. The purpose of this approach is to reduce the wiring cost and increase the reliability for the system. This research was aimed to develop a new approach for sequential control implementation using distributed system. Linear pneumatic cylinder was used as the actuator device to demonstrate various concepts of the proposed distributed sequential control system. The thesis has listed two target subsystems of sequential control. One of which is the sequential control of cylinders without input from independent discrete sensors. The other is the sequential control of cylinders with inputs from independent sensors. Various conventional approaches for such subsystems were also introduced.

Before proceeding to design the distributed system, an overview of distributed system architecture and certain concepts or the object-oriented development were studied. Based on this, various approaches or implementing sequential control using distributed system were explored and a modular approach based on peer-to-peer architecture was chosen for further development. In this peer-to-peer distributed control approach, the cylinder nodes (each controlling a cylinder system) are linked by a communication medium and capable of communicating with one another to achieve the overall sequential control task without the need of a central controller or master-slave architecture. Each node consists of a pre-programmed control module which contains local 1/0 interface and handles the communication task. Three other types or nodes were also designed (the sensor, alarm and the user interface nodes). The system was then implemented using the LonWorks network system to achieve the target sequential control subsystems. An industrial autD1nation application was simulated based on a workbench developed. In the peer-to-peer distributed system, the sequential control of the target systems of the research can be implemented by simply binding (logically connecting) the network variables of the nodes without the need of programming any of the nodes. It is also easy to modify the system by changing the affected binding (logical connection). The end-user just needs to understand the functions of the objects to implement the system. Additional features like "Step through" mode were also built in to assist the installation task and to help certain fault finding in the system. The proposed system has inherent concurrency properties and thus it can be applied to concurrent sequential control problems. However, clue to the token passing approach, the networking platform needs to have a reliable communication ability.