Clock synchronization is a necessary component in modern distributed systems, especially Wireless Sensor Networks (WSNs). Despite the great effort and the numerous improvements, the existing synchronization schemes do not yet address the cancellation of propagation delays. Up to a few years ago, this was not perceived as a problem, because the time-stamping precision was a more limiting factor for the accuracy achievable with a synchronization scheme. However, the recent introduction of efficient flooding schemes based on constructive interference has greatly improved the achievable\ud
accuracy, to the point where propagation delays can effectively become the main source of error.\ud
In this paper, we propose a method to estimate and compensate for the network propagation delays. Our proposal does not require to maintain a spanning tree of the network, and exploits constructive interference even to transmit packets whose content are slightly different. To show the validity of the approach, we implemented the propagation delay estimator on top of the FLOPSYNC-2 synchronization scheme. Experimental results prove the feasibility of measuring propagation delays using off-the-shelf microcontrollers and radio transceivers, and show how the proposed solution allows to achieve sub-microsecond\ud
clock synchronization even for networks where propagation delays are significant
In the process control, many PID loops are primarily
devoted to
rejecting load disturbances, and some of them are crucial for the
quality of the overall plant operation. In such a scenario, automatic
tuning is highly desired. However, load disturbance rejection calls
for strong feedback up to quite high frequencies with respect to the
dominant plant dynamics, on which most tuning rules are centered.
As such it is difficult for a rule to yield good and, above all, uniform
results in the face of all the various process structures it can be
confronted with. In this paper, we propose an explicit model-based
PID tuning rule specifically targeted at the problem just evidenced.
The rule minimizes the magnitude of the nominal disturbance-to-output
frequency response, at the same time preventing that magnitude to
exhibit a peak or a plateau around its maximum. This characteristic,
together with tuning the PID derivative filter, leads to sharp disturbance
rejection without incurring in an excessive control sensitivity to
high-frequency measurement noise and mitigates the problems caused
by heterogeneous process dynamics. The proposed approach is assessed
by comparing the rule with selected counterparts, on a literature
benchmark with different process structures. A laboratory experiment
is finally presented to show that our rule can withstand real-world
operating conditions.
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