Real-time communications in a computer-controlled workcell

A computer-integrated manufacturing (CIM) system is composed of several workcells, each of which contains robots, numerical-control machines, sensors, and a transport mechanism. The author considers a communication subsystem that is designed to support real-time control and coordination of devices in each CIM cell. The concept of a poll number is proposed to control the access to the intracell bus. The bus access mechanism with the poll number is intended to minimize the probability of real-time messages missing their deadlines. Use of a poll number provides not only for decentralized control of the intracell bus, but also a high degree of flexibility in scheduling messages. The performance of the bus access mechanism with a poll number is analyzed and compared with that of a token bus which is widely used in CIM systems such as MAP (Manufacturing Automation Protocol) networks. The probability of a real-time message missing its deadline in a token bus is found to be much higher than that of the proposed mechanism