D6.2 - Tactile CMOS-Based Sensors Array for Applications in Robotics and Prosthetics

SENSOR+TEST Conferences 2011
2011-06-07 - 2011-06-09
Proceedings SENSOR 2011
D6 - Medical I
J. Häfner, W. Mokwa - RWTH Aachen University, M. Görtz - Fraunhofer Institut (IMS), Duisburg (Germany)
624 - 627


In order to allow for a dexterous handling of objects, the skin of a human hand is covered with mechanoreceptors, which exhibit a great dynamic range. Not only can they detect the light touch of a feather but also the weight of carrying a heavy object. At the fingertips the spatial resolution, at which touch can be sensed, lies in the millimeter range. Additionally, sensing of other physical properties like temperature is possible. Mimicking these skin properties is of great scientific interest. Some of the promising application areas are those of humanoid robots and prosthetics. Humanoid robots shall one day be able to fulfill a variety of tasks in unstructured environments. Thus, they will have to deal with objects of different shapes, sizes, weights and fragilities. As a consequence, measuring the contact forces at the robot’s hand is necessary. The development of hand prostheses has made huge progress in recent years. Means of interfacing afferent nerves are researched to provide touch feedback for the patient. However, sensors suitable for integration into prostheses are yet to be developed.
Realizing a technical equivalent of the human skin is a challenging task. Assuming a spatial resolution of 5 mm, about 100 force and temperature sensors have to be distributed over the palm. Existing tactile arrays provide the desired resolution but no additional measurands like temperature. In contrast, active CMOS based sensors are more applicable. They can measure a multitude of physical quantities and the integration of analog and digital electronics allows transmitting the measured values over a digital network. Employing the network approach greatly reduces the required wiring effort compared to connecting each sensor individually. Additionally, on chip self tests, redundancy and sleep functions can easily be integrated. If the silicon thickness is reduced below 50 μm the normally brittle material becomes flexible and the sensors can be
applied to curved surfaces.
In this work an array of CMOS capacitive pressure sensors on a rigid substrate has been implemented. The sensors were encapsulated in a silicone that serves as a protection and a pressure transmission layer. First measurements were achieved that show the practicability of using pressure sensors as a tactile sensor array. In order to realize a bendable system, thin flexible polyimide foils were designed. ANSYS simulations were carried out to develop polyimide spring structures that minimize stress coupling between the polyimide foil and a thinned sensor chip. The polyimide foils were fabricated by spin coating and photolithography. Electrical connection is provided by electroplated gold conductors. The sensors were mounted onto the foil via an anisotropic conductive adhesive. After mounting, the sensors were thinned via lapping. It has been shown that it is possible to thin capacitive pressure sensors to a thickness of approximately 50 μm without visible damage to the membranes.