PTB News 3/2004 (149 kB) PTB News 3/2004, English edition, Issue January 2005The increasing miniaturization in micro-engineering and nanotechnology calls for dimensional measurement techniques with traceable contact forces in the mN range and lower. PTB has built a micro-force-measurement facility for the 1 mN to 5 N range. Via a modified compensation balance the new set up links up with the mass scale.
The measurement system uses (1.) a piezo-electrically adjusted fine displacement to create a minute force and (2.) a modified electromagnetic compensation balance to trace it back. The force transducer to be investigated is positioned between the balance and the fine displacement assembly. By varying the piezo-electrically induced displacement different load progressions can be generated and traced. The transducer output is compared with the calibrated balance signal.
The entire displacement possibly provided piezo-electrically is 100 µm. In order to make the most use of that range several modifications were carried out on the balance. They were aimed to minimize the load-dependent lowering of the contact position. Due to the finite rigidity of the lever mechanism in the balance an introduced force of 5 N induces a lowering of some 4.5 µm as determined interferometrically.
Measurement operation runs force-controlled on the signal of the force transducer to be measured. Deviations from linearity between the recorder values, the created and measured force values of the measuring device lie within 65 digitalisation steps of the transducer amplifier. Possible interactive influences between transducer and measuring device are collected by means of a rotating table, – rotating around its own force axis – and taken into account by averaging. The relative measurement deviation from the calibration of the transducers in the 200-N force standard facility at PTB in the range of 1 N to 5 N lies below 5 · 10–4.
The modular set-up of the measuring facility will make it possible to extend the scale into the µN-force range by applying more precise laboratory compensation balances.