Calibration of force vector transducers with smallest uncertainties

With the precision force transducers used so far, the force is measured in an axis predefined by the transducer construction. In the last few years, however, the demand for force transducers working with similar precision in all spatial axes (force vector transducers) has increased. In addition, torques shall be measured in the different axes. Now, corresponding transducers and - at PTB - corresponding calibration procedures shall be developed within the scope of a research project with a partner from industry.

Today, a large number of constructions of force and torque transducers with the most different uncertainties and class classifications are offered on the market. A common feature of these transducers is, however, that in all cases the force or torque is measured only as scalar dimension of the vectorial quantity along a predefined axis. In many cases of application, however, a precise statement of the forces and torques in space is required. In aircraft development, for example, a measuring system is required which measures - in wind channel tests - the forces, and also the torques which result from the buoyancy force and the air resistance of the model in several directions of force. Both the inquiries submitted to PTB and the great interest of the project partner show that the transducers at present offered by industry, which have been derived from conventional designs, do not meet all customer requirements with respect to the measurement uncertainty or that they are too large as complex system.

This is why PTB, together with its industrial partner GTM GmbH, has started the "PRO INNO II“ project "Realization and measurement of forces by means of vectorial force sensors“, funded by the Arbeitsgemeinschaft industrieller Forschungsvereinigungen (AIF, Consortium of Industrial Research Associations). It is planned to develop a transducer and a procedure by which all spatial components of the force vector can be determined with very small measurement uncertainty and the link-up to the national standards of PTB can be realized.

In the present course of the project, a prototype of the new transducer has been developed and manufactured. It consists of a precision force transducer for the measurement of the scalar force component, which - to determine the axis of force application - is mounted on a so-called vector sensor. A complex mechanical system in the interior of the vector sensor allows the individual directional fractions to be precisely measured. The low, almost linear cross-talk can easily be compensated by means of a measurement computer. At present, the prototype is being used to develop and investigate new calibration procedures. The aim is to meet the required class 00 in accordance with DIN EN ISO 376 and to link the vector sensor up with the national force and torque standards.

In the ideal case, the force and torque standard measuring machines generate forces and torques only in a predefined direction. To generate force and torque vectors in different directions for calibration, special auxiliary devices for the realization of variable loading directions are required. For this purpose, the calibration artefact shown in Figure 1 has been developed.

Figure 1: Force vector sensor mounted in the ball calibration artefact device

The set-up consists of ball-shaped bearings in which the mounted transducer can be turned. Due to the different fitting positions, this allows different directions of force to be realized relative to the transducer axis. Bending moments are realized with the calibration artefact by off-centre mounting of the force standard measuring device. The concept of the calibration artefact was investigated in a great number of numerical simulations. For this purpose, the influences of the production tolerances were determined and taken into account in the manufacturing process.

At first, the borings for accommodation of the transducers on the ball section body caused problems. The elongation field which, as a result, had become inhomogeneous, led to an irregular application of force into the vector sensor. By an optimised design, this effect could be reduced to a negligible influence. The ball calibration artefact is constructed in such a way that the centre of the vector sensor - as reference point of the force vector during the rotation - always remains in the centre. For this purpose, a special ball head was constructed as point of load application at the top.

In addition, the vector sensor shall be investigated in the 100 kN force standard measuring device with the aid of an additional - but less sophisticated - calibration facility by application of known transverse forces and bending moments. For this purpose, a corresponding load frame with different loading possibilities was manufactured for this measuring device. On the transducer, a special load application beam is mounted to which different transverse forces and bending moments are applied via a load frame to be mounted on the 100 kN force standard measuring device. These can be generated by a small scale pan or by means of a drive controlled by a precision force transducer. Figure 2 shows a computer representation of this set-up.

Figure 2: Load application frames with devices for force generation

The load frame consists of four aluminium profile bars. On a displaceable cross bridge, different devices for high-precision generation of forces can be mounted and exactly aligned on a bar system. On the left of Figure 2, a mechanical system for force generation by means of a spindle and a precision transducer can be seen. By means of a manual drive or a controlled electrical drive, the spindle is displaced in such a way that a force acts on the load application piece. The force as manipulated variable is measured with a precision force transducer. On its right side, a deflection system optimized with respect to the friction can be seen, to which a small direct loading frame is attached. Due to the gravity, the suspension with a known mass generates a weight force. The two procedures are compared with each other and with respect to their uncertainties.This second calibration possibility is aimed at checking the actually simpler ball calibration artefact which is to be used in the subsequent customer calibration and at achieving a small uncertainty by addition of further statistic supporting points in the prototype calibration.

Against this background, the vector sensor is also investigated in a third facility. In the last few years, a multi-component reference measuring device has for the first time been installed and put into operation at PTB (Figure 3). Although this measuring device does not reach the accuracies of the PTB standard measuring devices in force and torque generation, it has, however, the advantage that the behaviour of the sensor can be investigated without additional uncertainty contributions as additional mounting processes during the loading with different force and torque vectors are avoided.

Figure 3: Vector sensor mounted in the multi-component reference measuring device

The measuring computer converts the single measuring signals of the vector sensor into the six possible, spatial force and torque components automatically. It also allows the force vector to be transformed from the reference point in the centre of the transducer to any spatial point desired.

The project performed at PTB is also aimed at establishing an uncertainty model for the complete measuring chain of the vector sensor, into which the uncertainty of the mathematical model will also be included for evaluation. To keep the uncertainty of the calibration as small as possible, the three calibration procedures shall be used for mutual verification. The investigation of these procedures is a multifaceted challenge. Many problems cannot be solved satisfactorily with the available solutions. Compared to conventional force calibrations, considerably higher demands are made, for example, on the exact alignment.

Potential disturbing components may cause problems in the measuring devices even if they are very small (i.e. transverse forces). The transducer detects, for example, smallest oscillations of the mass stacks. For this purpose, mathematical solutions are being developed which are to eliminate this uncertainty contribution. Simple damping is not suited if an optically invisible axial oscillation of the 10 t mass stacks (1.5 m in diameter) with 0.05 Hz is formed around the load button axis.

The investigation of the calibration procedures and the mathematical description of the uncertainty model are made in interaction with the development of the sensor as such by the manufacturer. In this context, numerical simulations are also performed at PTB. The results enter continuously into the development of new prototypes. It is planned to make an efficient sensor, which can be calibrated as transfer standard at PTB, available to the user by the end of the year 2008 .

Contact person:

Sara Lietz, Department 1.2, WG 1.21, e-mail: Sara.Lietz@ptb.de
Falk Tegtmeier, Department 1.2, WG 1.21, e-mail: Falk.Tegtmeier@ptb.de