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Measuring forces in the mN range

Very small forces are measured and generated ever more frequently in various technical areas of application. Examples of this are tactile measuring probes in precision engineering and actuators in micro-assembly as well as applications in medical products, such as in insulin pumps and functional catheters. PTB has developed and put into operation a new standard measuring device based on electromagnetically compensated load cells for the force range from 1 mN to 2 N.

The essential components of the new measuring device are an electromagnetically compensated load cell as well as a piezoelectric actuator, which simultaneously applies the test loads to the load cell assembled above it as well as to the force transducer to be calibrated which is assembled below it. The test piece is thus operated in its normal position. To avoid additional transverse forces the action of forces on the test piece takes place via a horizontal decoupling; furthermore the transducer is positioned so it can be adjusted according to the piezo actuator. To be able to generate the test loads in a defi ned way, the adjustment travel of the piezo actuator has to be regulated with a resolution of better than 1 nm to the load cell signal, because of the high mechanical stiff ness of the sensor. Very high demands on the thermal and mechanical stability of the components necessary for the measuring device result from this, in particular of the mounting frame which is variable and adjustable in height. Up to now, an electromagnetically compensated load cell with a maximum range of 2 N and a resolution of 0.1 ìN has been used in the new measuring device. An extension to load cells with a maximum range of 12 N or 0.2 N is planned. With the load cell used, force steps from 2 N down to 1 mN seem possible, as a measurement uncertainty below 1 ìN has been achieved. The uncertainty is thereby in part itself infl uenced by the characteristics of the transducer, such as for example its long-term stability, its temperature dependence and its sensitivity to the mounting conditions.

Through the new standard measuring device, the force scale of PTB has been extended to small forces of up to 1 mN. Suitable force sensors can now be calibrated with uncertainties in the ìN range. For the measurement of even lower forces in the nN range with pN sensitivity, which are, for example, relevant in scanning probe microscopy or also in Continued on page 3 molecular bonds, an even more sensitive measuring device is currently being set up in PTB, based on another principle of measurement.

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