The impact of bit stuffing on the real-time performance of a distributed control system

The bit-stuffing mechanism utilised by CAN causes the message transmission time to become (in part) a complex function of the contents of the data fields. This variation in transmission times makes it difficult to predict the precise behaviour of real-time systems implemented using CAN. Previous work in our laboratory has led to the development of a software-based compensation method which significantly reduces the impact of CAN bit stuffing on message transmission times. In the present paper, we focus on the impact of bit stuffing on a system implemented using a “SharedClock” scheduling method. We use a detailed Hardware-in-the-Loop (HIL) testbed to explore the behaviour of an Adaptive Cruise Control (ACC) system for use in a passenger car. Through the use of the testbed, we present quantitative results which demonstrate the impact of variations in the message transmission times on the performance of the ACC system. We go on to demonstrate the improvement in performance which results when the previously-mentioned compensation technique is employed. Finally, the memory and CPU resources required to implement this compensation are discussed.

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