Experimental Assessment of the Period Calibration Method: A Case Study

In this paper we present an experimental evaluation of the period calibration method (PCM) which was developed in Gerber et al. (1994, 1995) as a systematic design methodology for real-time systems. The objective of this experimental study is to assess design alternatives integrated into the method and their performance implication on resultant systems built via the PCM. Such design alternatives include scheduling jitter, sensor-to-output latency, intertask communication schemes, and system utilization. For this study, we have chosen a computerized numerical control (CNC) machine as our target real-time system, and built a realistic controller and a plant simulator. We show the detailed development process of the CNC controller and report its performance. The performance results were extracted from a controlled series of more than hundred test controllers obtained by varying four test variables. This study unveils several weaknesses of the PCM: (1) the communication scheme built into PCM incurs a large latency though average sensor-to-output latency is one of the most dominating factors in determining control quality; (2) scheduling jitter is taken seriously in PCM though its effect appears only when average sensor-to-output latency is sufficiently small; (3) loop processing periods are not properly optimized for control quality though they are another dominating factor of performance; and (4) transient overloads are not considered at all in PCM, even though they can seriously damage the performance of a system. Based on these results, we propose a new communication scheme and a transient overload handling technique for the improved period calibration method.