Characterization of Mechanical and Perceptual Properties of the Human To Improve the Design of Wearable Devices

Thesis (Ph.D.)--University of Washington, 2020Although wearable devices are rapidly improving in features and ability, their performance and comfort remain poor when they are coupled to the wearer. Issues such as poor anchoring between the device and the body, inefficient power transfer, and variability in the perception of haptic cues are some examples. Traditionally, these issues at the human device interface (HDI) are addressed by coupling the device to the body with higher pressure, by adding degrees of freedom to the device, and by implementing active feedback control of the device. However, actions taken to mitigate the issues at the HDI give rise to more challenges. For instance, tightening the device to the body increases discomfort due to pressure. Adding measures such as redundant degrees of freedom, and active control makes the device design more complicated, increases power requirements, and ultimately makes the device bulky. In summary, these device-centered approaches are unsuccessful in mitigating issues of performance and comfort for wearable devices. In this dissertation, we focus on understanding the other half of this coupling problem - The Human. We characterize the mechanical and sensory properties of the tissue structures at the HDI, and we demonstrate that measuring these human properties allows us to improve the design of wearable devices to deliver optimized performance and comfort. Experiment 1: By measuring the MCP torque, we discover that anthropometric measures explain differences previously attributed to sex. Experiment 2: Measuring the hand dorsum stiffness distribution enabled the design of a novel HDI with improved comfort and performance. Experiment 3: Characterizing the stiffness and mechanical impedance about the wrist helped quantify the effect of coupling pressure at the HDI. Experiment 4: Characterizing the mechanical impedance and detection threshold led to the discovery that describing the detection threshold in power units accounts for the effects due to coupling pressure and reduces the complexity of haptic display design. Experiment 5: By changing the mechanical impedance of the HDI, we discovered that we can change the detectability of a vibrotactile stimulus. This dissertation contributes to a fundamental enrichment of our knowledge of the mechanical and perceptual properties of the human body aimed at improving the design of wearable devices. The design of the HDI has thus far been a qualitative effort, where parameters like the softness of the HDI and the coupling pressure are adjusted in an iterative fashion. This dissertation quantifies the mechanical and perceptual properties that are necessary for a quantitative design of the HDI, and demonstrates how this information can be used to optimize HDI design. Specifically, this dissertation demonstrates how describing the performance of a haptic display in terms of mechanical power can simplify the design process for a wearable vibrotactile display. It also quantitatively describes the inverse relationship between comfort and performance at the HDI. This work lays the foundation for the design of the human device interfaces of the future. For instance, VR/AR applications require the design of physical interfaces that heighten our experience of realism. To do this we must amplify our abilities to discern subtle haptic stimuli applied to our body, while attenuating parasitic forces that threaten to destroy this experience. In prosthetic devices and exoskeletons, the advent of impedance and stiffness matched surfaces will revolutionize comfort, while heightening the sensory experience by transmitting afferent stimuli from the artificial peripheral device to the body interface. Finally, as actuators and sensors become less stiff and more conformal, the knowledge of the properties of the human body will become critical in optimizing the performance of the wearable device. This work is the beginning of the next revolution in the human sensory experience