Tactile sensor system

		Fig.1: Micrograph of the tactile sensor Fig. 2: Recorded pulse waveform

Such anastomotic devices, as proposed in the context of this project, enable cardiac operations through small incisions. However, the small incisions deprive the surgeon of any direct tactile sensation and, thus, the ability to palpate within the body. In a subproject we therefore develop a tactile sensor, that can assist in bypass surgery to locate the coronary arteries and identify them from the veins, as well as locate the stenosis. In conjunction with a calibration from a sphygmomanometer the sensor is also capable of monitoring the pulse waveform non-invasively both in hospital settings as well as in mobile monitoring.

The goal of this work is to study the feasibility of using a post-CMOS-micromachined tactile sensor comprising a membrane capacitor array and integrated readout electronics as a replacement for the lost tactile feedback and for extravascular monitoring of the blood pressure waveform. The use of industrial CMOS technology for sensor fabrication enables the co-integration of addressing and signal conditioning circuitry. Furthermore, it offers a straightforward way to mass-fabrication. The micrograph of the third generation sensor chip is shown in Figure 1.

During operation the membrane deflects under pressure or force. We selected a capacitive read-out scheme to determine the membrane deflection in order to achieve a high sensitivity, yet still maintaining a small overall structure size. We make the membranes movable by a post-fabrication release etch of aluminium layer from the backside of the chip. Further, to fabricate large arrays with tight spacing, we have developed a new processing sequence. It combines the benefits of DRIE (deep reactive ion etching) dry etching method with the ECE (electro chemical etch-stop) method of wet anisotropic silicon etching. We innovated a grid-like masking pattern to allow for the fabrication of micromechanical structures with a large size variation on a multi-purpose CMOS wafer.

We have realized three generations of sensors with integrated readout electronics, including a 2nd order sigma/delta-modulator with necessary voltage and capacitance references and an I2C serial interface to connect the sensor to the out-side world. Digital decimation filter and a USB interface are external on an FPGA. The modular design of the sensor permits a straightforward modification of the system to suit different tactile sensing applications. The sensors are characterised and tested in blood pressure waveform monitoring experiments (Figure 2) using tonometric principle for the extra vascular detection. The integrated microsensor has a 12-bit amplitude resolution and a 500 Hz bandwidth. The readout rate is 1000 samples/s with SNR better than 70 dB. The power consumption is 11.5 mW. The tactile sensor shows a sensitivity of ~7.7 digits/mN between 0 and 60 mN of applied force. The recorded waveforms indicate that the designed sensor is capable of detecting the fine features of the amplitude as well as frequency information content. These results were obtained with the membrane array covered by a PDMS layer. This is necessary to guarantee biocompatibility of the structures.