Characterizing the response of piezoelectric-polymer composite haptic actuators and their interaction with skin

From our cell phones to game controllers, haptics - the use of skin stimulation, often in the form of vibrations, to relay information - has expanded its pervasiveness over the last few decades. Most applications leverage low-cost solutions, such as linear resonant and eccentric rotating mass actuators, which are limited in either their bandwidth or response time. However, some time-sensitive applications that require complex representation of the environment, such as obstacle avoidance and emergency response, pose restrictive requirements for haptic technologies. To this end, our team has recently developed a new type of high-performance haptic actuators, based on composites of piezoelectric materials called macro fiber composites (MFCs). The MFCs are glued on an aluminum back, enclosed by a custom-made case, and put in contact with a hollow cylinder, filled with a dense material. The mass in the cylinder allows the tuning of the frequency response of the actuator, toward increasing the amplitude of the response in the frequency range in which skin is most sensitive (10-250 Hz). In this paper, we put forward a detailed characterization of this new type of haptic actuators. First, we experimentally detail their mechanical and piezoelectric response. We then assess their frequency response while varying the mass in the cylinder. Finally, we study how actuators would interact mechanically with the skin. To this end, we conduct experiments with an actuator in contact with a pre-stretched membrane, whose mechanical properties are within the range of variability of human skin. We measure the frequency response of the actuator while varying the pre-stretch level of the membrane, simulating different skin indentations. Our results demonstrate that this new type of actuators can maintain an amplitude over the skin discrimination threshold over a large bandwidth, while offering low latency due to the fast piezoelectric response times.