Visual assessment of a real-time system design: a case study on a CNC controller

We describe our experiments on a real-time system design, focusing on design alternatives such as scheduling jitter, sensor-to-output latency, intertask communication schemes and the system utilization. The prime objective of these experiments was to evaluate a real-time design produced using the period calibration method (Gerber et al., 1995) and thus identify the limitations of the method. We chose a computerized numerical control (CNC) machine as our target real-time system and built a realistic controller and a plant simulator. Our results were extracted from a controlled series of more than a hundred test controllers obtained by varying four test variables. This study unveils many interesting facts: average sensor-to-output latency is one of the most dominating factors in determining control quality; the effect of scheduling jitter appears only when the average sensor-to-output latency is sufficiently small; and loop processing periods are another dominating factor of performance. Based on these results, we propose a new communication scheme and a new objective function for the period calibration method.