A specimen-tracking controller for the transverse dynamic force microscope in non-contact mode

This paper presents results from the practical implementation of a specimen tracking controller for the transverse dynamic force microscope (TDFM). Uniquely, in the TDFM, the scanning cantilever is vertically oriented. It can be controlled in the vertical direction by piezo-actuation and the cantilever tip is excited in the horizontal direction at the resonance frequency of the cantilever beam. Once the cantilever tip approaches and interacts with a thin ordered water-layer usually found on any specimen at ambient conditions, the cantilever excitation amplitude changes. The extent of the changes depends on the vertical distance from the specimen surface, i.e. the amplitude level allows the detection of the distance between the cantilever-tip and the sample-substrate. Applying this relative height characteristic, a controller has been designed and implemented. This is based on a specially introduced amplitude detection scheme, a subsequent frequency-response-based system identification, and a resulting controller design. The practical issues in developing this detection and control system are discussed. Experimental results prove that the presented relative height control method for specimen tracking is feasible and reliable.