A major limitation of piezoelectric translator ͑PZT͒ actuators is their lack of accuracy originated from the hysteresis and creep. Nevertheless the creep phenomenon is an important factor in many applications of PZT actuators, but it has been investigated less frequently in comparison with the displacement hysteresis. In this article, we present a basic creep model with some parameters that have hysteresis properties which make it possible to predict an open loop response of PZT actuators based on these properties.

Background: Stress is associated with activation of the sympathetic nervous system, and can lead to lasting alterations in autonomic function and in extreme cases symptoms of posttraumatic stress disorder (PTSD). Vagal nerve stimulation (VNS) is a potentially useful tool as a modulator of autonomic nervous system function, however currently available implantable devices are limited by cost and inconvenience. Objective: The purpose of this study was to assess the effects of transcutaneous cervical VNS (tcVNS) on autonomic responses to stress. Methods: Using a double-blind approach, we investigated the effects of active or sham tcVNS on peripheral cardiovascular and autonomic responses to stress using wearable sensing devices in 24 healthy human participants with a history of exposure to psychological trauma. Participants were exposed to acute stressors over a three-day period, including personalized scripts of traumatic events, public speech, and mental arithmetic tasks. Results: tcVNS relative to sham applied immediately after traumatic stress resulted in a decrease in sympathetic function and modulated parasympathetic/sympathetic autonomic tone as measured by increased pre-ejection period (PEP) of the heart (a marker of cardiac sympathetic function) of 4.2 ms (95% CI 1.6e6.8 ms, p < 0.01), decreased peripheral sympathetic function as measured by increased photoplethysmogram (PPG) amplitude (decreased vasoconstriction) by 47.9% (1.4e94.5%, p < 0.05), a 9% decrease in respiratory rate (À14.3 to À3.7%, p < 0.01). Similar effects were seen when tcVNS was applied after other stressors and in the absence of a stressor. Conclusion: Wearable sensing modalities are feasible to use in experiments in human participants, and tcVNS modulates cardiovascular and peripheral autonomic responses to stress.

This paper deals with a new tracking control method for piezoelectric actuators. When actuating in an open-loop manner, in order to compensate for the creep effect of the piezoelectric transducer as well as hysteresis, a new concept of `voltage creep' is proposed, which is represented by the two parameters V0 and γv. It is shown that these two parameters have the properties of wiping out and congruency, which make it possible to derive parameter values appropriate to control piezoelectric actuators using the Preisach model. Finally, a tracking control experiment of piezoelectric actuators for an arbitrary desired trajectory is performed giving greatly improved results compared to other open-loop actuating methods.

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