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For devices having non-linear contact, load plays a fundamental role. Variations in the characteristics of the load cause change in eigenfrequency and amplitude of the vibration. In most technical applications, this unwanted behaviour is cancelled by the use of control algorithms. However, multiple applications, like bond quality monitoring or chemical and pressure sensors, have found that the load may be characterised by interpreting the change in characteristics of a resonant vibrating device used as a sensor. Surgical resection of tumours is a very difficult task. After localising the tumour by the use of imaging techniques, the resection demands the surgeon to decide where and what to resect based on visual and tactile differentiation of tumour and healthy tissue. Exactness of this process could be enhanced if we can provide the surgeon with a device capable of evaluating mechanical characteristics of the tissue much more accurately than the surgeon himself can do. As the mechanical characteristics of tumour and healthy tissue differ but slightly, the task is to design a system with high sensitivity. Therefore, we have developed a resonant actuator-sensor that allows the differentiation among distinct media that have similar mechanical characteristics to tumour and healthy tissue using a piezoelectric bimorph. The design is based on the detection and evaluation of frequency shift and amplitude variation of the fundamental and higher harmonics using one layer for the resonant excitation of vibration and the other one as the sensing element.

Mechanical characteristics of tumor and healthy tissue in the brain differ but slightly. The task of designing a system that is able to differentiate tissue dignity with high sensitivity is of great importance in neurosurgery. Even when localization of tumor by use of preoperative imaging techniques provides the surgeon with valuable information to decide where and what to resect, the brain shift due to change in pressure during skull opening demands the surgeon to define the limits of the tumor using tactile and visual differentiation. This paper contains a general description of the tactile sensor system based on a piezoelectric bimorph. The main parts of the measurement system are described and the selection of the electrical parameters for tactile differentiation is justified. Results are discussed for a series of measurements at different concentrations in gelatin phantoms.

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