Fieldbus based integrated communication and control systems-architectural implications

Fieldbusses are often used in control applications with feedback loops using the fieldbus for input and output operations. Intuitively, it is felt that communication delays may degrade the achievable feedback performance. In this paper, we use the modified synchronous model to calculate the different constraints imposed by fieldbusses on periodic distributed applications when predictable communications are required. We then apply these results to two different control architectures, feedback and feedforward systems. It appears that feedback systems with sensor to actuator delay below the sampling period duration lead to an inefficient use of processing resources and heavily constrain the transfers on the fieldbus. We present two possible implementations of feedforward and feedback systems which, under the same load conditions on the fieldbus, exhibit a much better use of resources. We also show that the message transfer laxity may always be kept large enough by proper selection of the implementation. Finally, these results are illustrated in the architecture of a computer numerical controller for machine-tools.