KoalaDrive® - Details of operation
One cycle of motion of the KoalaDrive® is shown in this figure. In step 1 of the cycle the upper piezo element contracts and the upper spring goes into sliding friction. The central tube is held stationary by the lower two springs which stay in static friction with the tube. Subsequently, in step 2 the middle spring is moving downwards while the upper and the lower spring stay at their positions. For the upper spring this is realized by a simultaneous contraction of the lower piezo element and a corresponding expansion of the upper one, leaving the upper spring unmoved. Also here a single spring (middle one) is moving, while the other two ones hold the tube fixed. Finally, in step 3 the lower piezo extends and moves the two upper springs up simultaneously. In this case the lower spring goes into sliding friction and the upper two springs move the tube up (static friction). Simplified, the working principle follows the rule: "Two are stronger than one". If two springs move simultaneously, the central tube moves with them. If only one spring moves, the tube is hold stationary by the other two.
This figure shows the voltage pattern applied to both piezo tubes and the resulting motion of the central tube during one cycle as function of time. One single cycle can induce a motion in the range between 100nm up to 5 μm which is ideally suited for coarse approach in scanning probe microscopy. A long stroke, only limited by the length of the tube, and speeds up to 1 mm/s are possible. Most other nanopositioners used for tip-sample approach in scanning probe microscopy use the inertial motion with saw-tooth like signals inducing large accelerations causing vibrations in the system. The operation mode of the KoalaDrive® is quasi static (one cycle can even last several seconds) avoiding large accelerations which lead to a continuous motion without shaking. Avoiding steep slope signals means also less demands for the power supply (no high slew rate needed) and for the cabling (no high currents flow).
KoalaDrive® performance: single step displacement as function of signal amplitude. The data are shown for a 20 mm long and for a 10 mm long KoalaDrive®. The motion of the tube induced by a single cycle (step) as function of the amplitude of the excitation voltage shown. Starting from low amplitudes, there is a threshold voltage above which the motion of the KoalaDrive® starts. For lower voltages the extension of the piezo tubes results in a build up of stress in the system, but only beyond the threshold voltage a transition to sliding friction occurs. For amplitudes larger than this threshold voltage the single step displacement increases linearly with the signal voltage. Due to the smaller piezo constant at low temperatures, the threshold and the slope of the curves decreases for operation at low temperatures. This shows that the KoalaDrive® works at cryogenic temperatures (down to liquid helium temperatures).
More information can be found also in this publication
The KoalaDrive® is licensed by Forschungszentrum Jülich
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