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R. Kumme, O. Mack, B. Bill, Ch. Gossweiler, H. R. Haab
DYNAMIC PROPERTIES AND INVESTIGATIONS OF PIEZOELECTRIC FORCE MEASURING DEVICES

Piezoelectric measurement chains are widely used in many applications of dynamic force measurements, since these sensors offer a wide useable frequency range due to their high stiffness. Typical applications of piezoelectric force sensors are described in this paper. In general, the transducers are calibrated by quasistatic or continuous calibration methods. This contribution describes the dynamic properties of piezoelectric force measuring devices and methods to determine the dynamic response of the piezoelectric force measuring chain.

Hans W. Werner, Emil Harreus
FORCE TRACEABILITY MEASUREMENTS TO ACHIEVE 0.005 % BEST MEASUREMENT CAPABILITY

The increasing accuracy of force transducers and their calibration had two major effects: the chance to better extract the capability of a reference standard machine (compared to a formerly mixed evaluation of machine plus transducer) and on the other hand the need to build up reference standard machines capable to calibrate those high precision transducers. Using the experience of designing force transducers, of constructing calibration machines and of calibrating force within the German Calibration Service (DKD) since 1977 HBM decided to take the challenge. Two new dead-weight force reference standard machines of 2.5 kN and 25 kN have been built up and their best measurement capability of significantly better than 0.01 % has been verified by transfer transducers from HBM´s product range.

Dieter Kenzler, Jörg Oblasser, Andreas Subaric-Leitis, Christian Ullner
UNCERTAINTY IN TORQUE CALIBRATION USING VERTICAL TORQUE AXIS ARRANGEMENT AND SYMMETRICAL TWO FORCE MEASUREMENT

Calibration machines up to a torque range of 20 kN·m need extensive design if the force is performed by dead weights. A machine based on two force reference standards and a vertical torque axis may serve to avoid these specific problems. The precision force transducers work at a lever arm of precision length. The test piece is mounted in vertical arrangement between a hydraulically operating rotary-actuator and the torque measuring system. Such a calibration machine allows additionally performing easily continuous calibration. Regarding an overall relative uncertainty not greater than 2·10-4 there are some significant problems. The estimation of the uncertainty has been proved by comparative tests with the national torque standard by the PTB according to the guidelines EA-10/14 and EA-2/03.

Andre Zander, Rolf Kumme
ANALYSIS OF THE DYNAMIC TRANSMISSION BEHAVIOUR OF PIEZOELECTRIC FILM SENSORS

The goal of this analysis was to obtain knowledge about the dynamic transmission behaviour of piezoelectric film sensors based on Polyvinylidenefluoride (PVDF). Therefore a crossvalidation to the well-known behaviour of strain gauges based on constantan foils was conducted. The test equipment was composed of a shaker and a test piece with affixed sensors. The test piece was stressed through the connection to a load mass. The film sensors based on PVDF were mounted to the test piece in such a way that the load direction was parallel to the longitudinal axis of the PVDF-material. In this way it was guaranteed that the highest piezo strain constant d31 was mainly used. The output signals of the sensors were measured and the frequency response of the piezo film sensors in relation to the strain gauges were calculated. The results indicated that for frequencies above approx. 250 Hz the piezo film sensor frequency response modulus was a constant value with respect to the strain gauges and that for this frequency range there is the possibility to determine a specific transmission value of the used piezo film sensors similar to the “k-factor” of strain gauges. Furthermore, the high pass filter characteristic of the chosen charge amplifier could be identified.

Andreas Brüge
FAST TORQUE CALIBRATIONS USING CONTINUOUS PROCEDURES

Unlike step procedures, continuous procedures permit calibrations to be carried out within a very short time. In continuous calibrations the fast change in torque and thus also in the signals give rise to specific problems. This paper gives advice on how to limit the influence of the most important effects on the measurement uncertainty. A simulation method is introduced, which allows the contribution of the reference creep to the measurement uncertainty to be calculated.

L. Stenner, J. Andrae, R. Kumme, J. Mack
EXPERIENCE WITH A NEW CLASS OF FORCE TRANSFER STANDARDS

International force standard machines provide ever increasing accuracy. As a result the requirements for the force transducers used for international comparison measurements are also rising. The development of new force transducers shows that it is possible to reduce their measurement uncertainties. This paper describes the measuring bodies of these force transducers and the necessary selection method. The results of measurements obtained in force calibration machines and in force standard machines are presented.

Roman Schwartz, Andreas Lindau
THE NEW GRAVITY ZONE CONCEPT IN EUROPE FOR WEIGHING INSTRUMENTS UNDER LEGAL CONTROL

A new European gravity zone concept has been developed by WELMEC, the European Cooperation in Legal Metrology. This concept is independent of political or administrative borders and is accepted by all WELMEC member states regardless of existing national regulations concerning gravity zones. The new concept is based on gravity zones defined in terms of the geographical latitude φ and the height above sea level h, on a standardised gravity formula used for the adjustment of a weighing instrument to a reference gravity value, and on a uniform criterion for calculating the maximum permissible variation Δg/g within an individual gravity zone. The concept is of advantage especially for weighing instruments of class III. It offers an option that can, but must not be chosen by a manufacturer.

Andreas Subaric-Leitis, Christian Wöhry, Bernhard Seiffert, Christian Ullner
CHARACTERIZATION OF THE DYNAMIC BEHAVIOUR OF MATERIAL TESTING MACHINES

Material test results may depend strongly on the dynamic properties of the used testing machines. Unique methods and parameters for characterisation and validation of force or displacement controlled testing machines are still missing. In the presented paper a study of the control behaviour of a spindle driven uniaxial machine is carried out using a piezoelectric translator for generating the disturbance variable. The typical responses of the systematically optimized machine are compared to simulations of simplified controller systems. Some parameters concerning the time behaviour are discussed in terms of a systematic characterisation of the machine's dynamic properties and the experimental set up used.

Boris Katz
A NEW WAY TO MINIMIZE UNCERTAINTY IN CALIBRATION PROCESS OF FORCE TESTING MACHINES

The results of the calibration of load cells (force transducers) according to ISO 376-1999 are described in table of values where force F (kN) is a function of electric signal R (mV/V) and by analytical approximation of the calibration curve. The analytical approximation is usually expressed by polynom of 2nd or 3rd degree. The measurement of force in the calibration of testing machines by means of digital unit device DMP-40 (HBM) suppose application of linear interpolation between two adjacent values of signal R. By means of mathematical calculations it is theoretically proven in which cases the linear interpolation is undesirable and leads to additional interpolation error, and increasing of the value of uncertainty measurement in accordance. A few studies and experiments of the issue confirm the theoretical conclusions: in some cases minimizing interpolation error and uncertainty measurement in measuring force F as a function of reading the signal R, can be achieved by approximation polynom instead of linear interpolation between two adjacent values. For this purpose QCC Hazorea has developed program called “MABA-2000,” which can solve this problem.

K. Baethke, M. Faulstich, G. Haucke, A. Dájer Espeleta
INVESTIGATION OF A NEW 1 MN FORCE CALIBRATION MACHINE OF TWIN CYLINDER DESIGN

Three new force calibration machines covering the range from 1 kN to 1 MN has been designed and constructed at DM-SIC in Bogotá, Columbia. The 1 MN force calibration machine (fcm) is a reference force calibration machine of new design. A special flexible sealed twin piston cylinder system with rotating cylinders has been used for the generation of hydraulic forces to reduce the friction, oil losses and pressure drops in the hydraulic system. Time optimization and a high accuracy of force control could be achieved by a three-step combination of rapid preliminary positioning, a PI controller algorithm and, finally, a changeover to PID controller characteristics. The relative deviations between the measurements carried out at PTB and the final measurements performed in the 1 MN reference force calibration machine show that the fcm fulfils the predetermined overall specifications limits of relative uncertainty < 2·10-4 in the whole working range from 40 kN to 1 MN.

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