05.11.2007
The force standard machines of PTB serve to realize the unit "newton" from so far 0.5 N to 16.5 MN. To extend this measurement range towards smaller forces, a new device was put into operation. Instead of conventional weights, modern electromagnetically compensated load cells are used for traceability. This allows force transducers to be calibrated in the range from 1 mN to 2 N with known measurement uncertainty.
As a service provider for industry, calibration laboratories and other users, the "Solid Mechanics" Division offers the calibration of reference force transducers in accordance with the standards. For this purpose, force standard machines are operated which allow the force to be traced back in the unit "newton" to the International System of Units (SI). With such force transducers, the users measure - among other things - mechanical properties of materials and products with international comparability, which makes an essential contribution to product safety, material efficiency and transparency for trade. To an ever increasing extent, miniaturized mechanical or electromechanical components and end products are used. To be able to offer a corresponding service also in the range of micro-mechanics, a new facility has been established which - after international comparison measurements have been performed - shall serve as a force standard machine.
Basic idea is the use of electromagnetically compensated load cells which are commercially available for mass determination. The force transducer to be calibrated is pressed against the load cell. The counterforce acting on the load cell corresponds exactly to the negative force acting on the force transducer. To generate this force in a defined way, the adjustment displacements must - due to the high mechanical stiffness of the transducer - be regulated with a resolution of better than one nanometer to the load cell signal. To allow this, very high demands are made on the thermal and mechanical stability of the required mounting frame which must be variable and adjustable in height.
In cooperation with PTB's Scientific Instrumentation Department, a corresponding facility has been designed and established (Figure 1).
Figure 1: 3D draft and photo of the new force standard machine
In this facility, an electromagnetically compensated load cell with a maximum load corresponding to 2 newton and 20 million steps of resolution was tested. An extension to load cells with a maximum range from 12 N and 0.2 N is planned. The measurement uncertainty was determined computationally from all known influences. With the load cell used, force steps from 1 mN to 2 N seem to be reasonable, because a measurement uncertainty below 1 µN can be achieved. Figure 2 shows the result of measurements of the characteristic of a force transducer over two weeks and for different adjustment conditions. The error bars show the measurement uncertainty of the measurements for a confidence range of 95%. The dispersion of the measurement values is partly influenced by the properties of the transducer as, for example, its long-term stability, temperature dependence and its sensitivity to the mounting conditions.
Figure 2: Repeated measurement of the characteristic of a 2 newton force transducer in accordance with ISO 376. The error bars show the expanded measurement uncertainty (95%) of these measurements
Contact person:
Jens Illemann, FB 1.2, AG 1.21, E-mail: jens.illemann@ptb.de