Today, measuring the quality of a lubricant requires quite an effort: It is necessary to take a sample of the liquid, send it to a laboratory and wait for the results. If large machines stand still over days for this reason, it becomes rather expensive. "Therefore our goal is developing a micro sensor that can measure oil density and viscosity directly in the machine - users thus are informed at any time about the ageing status of the lubricant", says professor Ulrich Schmid from the institute for sensor and actuator systems of the Vienna technical university. For this project, his team maintains a tight collaboration with the excellence centre of tribology ACT2 in Wiener Neustadt.
The core element of the sensor is a tiny cantilever of silicon which is coated by aluminium nitride; the micro structure measures 2500µ x 1300µ. The resonance and vibration properties of the cantilever depend greatly of the liquid surrounding it; these vibrations can be measured very precisely.
The measurement principle is similar to the one used in atomic force microscopes. In both cases, the vibration of a microstructure provides the measurement result. While however in an atomic force microscope the vibrations are measured with a laser beam, such a principle would be much too complex for testing lubricants. Therefore, the Austrian researchers developed a purely electronic solution. The aluminium nitride coating has a piezo electric behaviour; it can be excited by adding electric alternating voltages. If he cantilever vibrates and experiences deformations, free charge carriers are generated at its surface, and its conductivity changes. Vibration generation as well as readout is done electronically.
The decisive idea was the development of a specific electronic circuit: The measurement signal is fed back to the input signal. This feedback loop enables measuring the resonance properties of the cantilever very precisely. It was necessary to integrate a very effective noise cancelling to selectively amplify only the desired signal.