PTB-News 3/2008 (3.7 MB) PTB-News 3/2008, deutsche Ausgabe, Dezember 2008In research and development, the precise measurement of forces all around the nano-Newton range is gaining in importance. The PTB has therefore developed the prototype of a standard measuring facility for the force range from 10 µN to 1 pN – based on a disk pendulum with electrostatic force compensation – and has put it into operation.
Due to the increasing application of synthetic microparts in industry, which scratch easily during tactile measurement with too great a contact force, new requirements arise for the corresponding measuring devices, such as tracing systems or scanning force microscopes. Their tactile forces must be adjusted ever more accurately and reliably. Also in other fields of research there are comparable requirements for force measurement down to a pico-Newton: for instance in biology and medicine in the investigation of the elastic properties of individual cells, in chemistry in the measurement of molecule-bonding forces, or in microelectronics in the determination of the material properties of micro- and nano-electromechanical systems. The latter have increasingly found their way into everyday products such as mobile phones, MP3 players, PC peripherals and passenger cars.
In order to measure such small forces, a prototype system has been successfully developed and tested at PTB. Its measuring principle is based on a disk pendulum which is deflected by the force to be measured. The deflection is compensated electrostatically with the aid of external capacitor electrodes; the voltage to be applied is measured. Added to this electrostatic force compensation is the electrostatic stiffness reduction: By reducing the pendulum‘s own stiffness from 0.13 N/m to 0.007 N/m, the sensitivity of the system is increased. In order to compensate for disturbing vibrations and thermic drifts, a second, similar reference system is installed in addition to the actual measuring system.
First measurements in air yielded a standard deviation of 160 pN for a long-term measurement over 3 hours using a low-pass filter of 0.02 Hz. As initial force measurement at the sensitivity limit of the prototype system, the force was determined which the light pressure of a He-Ne laser exerts with a power of 7 mW. The measured value of 38 pN is only 9 pN smaller than the force calculated from the optical power of the laser and the reflection factor of the disk pendulum.
The new system supplements the PTB force standard measurement facility which has recently been put into operation for the traceable calibration of forces in the mN range (PTB news 08.2) which is based on another measuring principle. In future, the new system is still to be optimized for the measurement of pN forces, for which, however, its sensitivity must be further improved. Theoretical analyses give rise to expectations of an attainable force resolution of 1 pN. The essential challenge here consists in the production of measuring and reference disk pendulum systems as identical as possible, with surfaces as ideal as possible (flatnesses in the range of 100 nm). Another focus of research is the calibration of suitable force transfer standards which can then be used in industry for the calibration of smallest forces.