Design And Analysis Of Low Latency Deterministic Network For Control Applications

Control systems over the wireless network have emerged considerably and showed many benefits such as no tiresome work of wiring, simple and economical system commissioning,etc.. The IEEE 802.15.4 standard has shown good characteristics for deterministic networks in beacon-enabled mode by using guaranteed timeslots(GTS), but with minimum cyclic update time for PAN devices not less than 15.36 ms. Moreover, it is unsuitable for hard real-time systems when used in nonbeacon-enabled mode due to the random nature of channel access protocol(CSMA-CA). This research work combines the benefits of these two modes and introduces the concept of GTS in nonbeacon-enabled mode to achieve superframe time less than 15.36 ms. In this dissertation, we first analyze the IEEE 802.15.4standard and then propose a technique for making this standard suitable for low latency deterministic networks(LLDN) for control applications where the cyclic update time of sensors information is not more than 10 ms. The proposed technique is, in fact, a data exchange protocol between PAN network devices employing time division multiple access(TDMA) based channel access that works on slightly modified IEEE 802.15.4 in star topology with optimum bandwidth utilization. Each network device transmits its data frame in assigned timeslot in response to periodic request from a network coordinator. This timeslot duration and inter-timeslot duration are optimized for channel bandwidth utilization and reliable network data exchange. While this TDMA based protocol eliminates the risk of frame collisions to great extent, MAC sublayer tweaking improves the non-determinism of the network and increases its bandwidth efficiency. With this proposed scheme, the superframe duration(and in turn network latency) of less than 10 ms is realized that is impossible with original IEEE 802.15.4 standard. Empirical models, reflecting limitations of the software stack, are proposed to design and develop secured/unsecured communication networks with cheap commercial off-the-shelf radios. Experiments are conducted to evaluate the network performance of proposed models in different network configurations.Speed tracking control experiments with three dc servo motors are also performed to show the efficient application of the proposed scheme for closed-loop feedback control. This workfinds application in factory automation where robotic arms, cranes, conveyor belts etc. are controlled through LLDN.Main results of this research are as follows:? Chapter 5 describes the LLDN mode of IEEE 802.15.4e communication standard.LLDN is explored through simulations for its suitability in low latency deterministicnetworked control system. Both simple and secured networks are discussed. The linksand compromises among LLDN superframe size, timeslot duration and MAC datapayload, with different security levels, are elaborated. Simulation results show thesuitability of simple LLDN in hard real-time applications and reveal the extremelyhigh cost of network security implementation in terms of network latency.? Chapter 6 proposes a scheme for making IEEE 802.15.4 standard suitable for LLDNfor wireless control applications where the cyclic update time of sensors’ informationis not more than 10 ms. This scheme overcomes the limitations imposed by originalstandard on minimum possible superframe length and total number of timeslots inone superframe with advantage of optimum bandwidth utilization. A mathematicalmodel is developed that incorporates the practical firmware limitations like processingtime jitter etc.. This model takes few empirical values and predicts the time delaysafter which each network end device will transmit and the worst-case arrival time ofa data frame by any end device to the PAN coordinator.? Chapter 7 explains an experimental setup and then presents a series of experimentsfor the evaluation of the proposed scheme. For all sets of experiments, Texas In-strument(TI) Smart RF04 EB evaluation boards fitted with CC2530 EM evaluationradio modules are used for all network devices. First, two experiments are conduct-ed to find the mathematical model parameters. The third experiment evaluates theproposed scheme with two metrics: packet delivery ratio and worst-case arrival timeof data packets at the coordinator. The predicted values, calculated by the math-ematical model, are found in conformance with the experimental data. Moreover,100% packet delivery ratio validates the efficacy of the proposed model.? In Chapter 8, the previous mathematical model is tailored to include the networksecurity.? Chapter 9 presents three experiments to verify the proposed mathematical model forsecured networks. The predicted and experimental data are found in conformancewith each other. Moreover, it is verified experimentally that cost of network securityimplementation is extremely high in low latency exacting control applications.? In the last chapter, Chapter 10, we apply the proposed TDMA based IEEE 802.15.4as a wireless link for the speed tracking control problem of DC servo motors. Twosuperframe configurations are proposed for this problem. Experiments are done witheach configuration. Three dc servo motor systems are successfully controlled in eachproposed superframe configuration with acceptable control performance.