Delivery the common sense of the time to synchronize measurement instruments co-operating into the DMS

Abstract

This research is devoted to investigate the coordination in the time domain of the operations executed by the Measurement Instrument (MI) connected to the nodes of the Distributed Measurement System (DMS) by Hardware Interface (HI). On the basis of the synchronization procedures of the node clocks of the DMS presented in the literature, the HIs can work on a synchronized modality. Nevertheless, the hardware and software architecture of the path involved in the communication HI-MI can randomly time delay the commands. Often in the DMS the PC is used as HI. In this case two different aspects are taken into account: (i) the characterization of the hardware connection PCMI offering the minimum time delay, and (ii) the criteria to modify and to set up the software to reduce the random time delay occurring in the processing steps performed into the PC. The results of the research performed highlights that the main cause of random time delay is the concurrency of the processes running in the PC. In particular, the delay depends on: (i) number of concurrent processes, (ii) their priority, (iii) behavior of the kernel managing the concurrency. In order to overcome the delay caused, the HI based on the Programmable Logic Device (PLD) is taken into account and proposed in the place of the PC. From the analysis of the operations executed on the PLD, operating conditions are shown, making random variation of the synchronization time delay of the MI to the node clock. In order to detect the causes of the random variation, the polling cycle is taken into account and analyzed. From this analysis the model of the uncertainty affecting the synchronization time delay is pointed out in order to evaluate the effects of each cause affecting the trigger check. The result of this evaluation furnishes: (i) information to point out the adequate strategy to reduce the random time variation of the detection of the trigger condition, and (ii) requirements to completely avoid the polling cycle. The experimental tests performed by using implemented embedded HI assess the efficacy of the presented HI to achieve sub-microsecond synchronization accuracy, according to the standard IEEE 1588. VIII In the research the problem of the stand alone MIs is also taken into account. In some cases, indeed, synchronized measurement procedures must be performed in places where the node of the DMS are not reachable, or not convenient to reach, by wired or wireless connections. For this reason it is not possible to use standard synchronization protocol to synchronize the MIs. In these cases the use of the Personal Digital Assistant (PDA) is proposed to physically bring the common sense of time to the stand alone MI. In order to achieve synchronization accuracy in the order of sub-microsecond, it is presented the conjunct use of the PDA and the proper embedded HI. This solution meets the advantages of both the devices. The PDA permits the advantages of the high level programming languages in order to interface the MI and collect the data. The embedded HI guarantees high synchronization accuracy on the basis of the deterministic behavior of the proposed hardware architecture. Experimental tests confirm the suitability of the proposed solution to the requirements of the standard IEEE 1588