22.08.2008
In the past, PTB only had the 16.5 MN force standard machine (FSM) at its disposal for the investigation of force transducers with measurement ranges larger than 2 MN. Due to the great demand for calibrations in this upper force range, this caused, time and again, bottlenecks as this machine for forces in tension and forces in compression, which is worldwide unique due to its size and accuracy, was not always able to satisfy the great demand for measurements applied for. Among the great number of orders, the share of transducers up to 5 MN amounted to approx. 80%.
For this reason, PTB has decided to make use of a hydraulic force standard machine taken over from the former ASMW (Amt für Standardisierung, Messwesen und Warenprüfung of the GDR - Office for Standardization, Metrology and Commodities Testing) and to utilize it - after its complete modernization as force standard machine - for forces up to 5 MN. It was aimed at reducing the uncertainty in the force realization of this machine to 0.01%. At the same time, the updated facility shall allow an efficient, automated operation. New concepts for the cascade control and for the method used to link up the machine with the standards have been investigated and realized.
The renewed measurement device (5 MN force standard machine, Figure 1) works in accordance with the hydraulic amplification principle. This means that - first of all - the weight forces of a 50 kN mass stack (step size 500 N) act on a piston-cylinder system on the device's measuring side. By a control procedure, which is new for this machine type, force equilibrium is established between the weight force of the weights and the hydraulically generated force. The control procedure comprises the cascade connection of a control unit for hydraulic pressure, residual force and position. Due to the construction, the machine is a highly instable system - in favour of smaller mechanical uncertainties in the force realization. A sophisticated, computer-aided control unit allows the equilibrium of the forces from the oil pressure on the measuring cylinder and the mass stack to be established with uncertainties smaller than 2·10-5. The oil pressure required for this force equilibrium acts simultaneously on two piston-cylinder systems arranged in parallel on the operating side. Due to the relation of the surfaces of the piston-cylinder systems on the operating side and on the measuring side, the forces are hydraulically amplified by a factor of approx. 100 while the pressure remains the same.
New procedures were also chosen for the mass linkup. In the past, the masses and the transmission ratio were exactly determined. With increasing hydraulic pressure, the piston and the cylinder are, however, elastically deformed, and the transmission ratio changes in orders of magnitude which cannot be neglected for the overall uncertainty of 1·10-4 aimed at. For this purpose, detailed investigations were performed in the past. Calculations showed that linkup of the machine should not only be performed by determination of the mass and the transmission ratio, but that it is better to link it up directly by means of high-precision transfer transducers to the 2 MN force standard machine, which - as deadweight force standard machine – allows uncertainties below 2·10-5 to be achieved. For this purpose, 2 transducers are connected in parallel for the force range above 2 MN. Mechanical superstructures for such a force transducer arrangement - e.g. a large compression plate under which several transducers are mounted – are, for different reasons which are related with additional mechanical components - not precise enough for the linkup of the machine. This is why the possibility realized on the 5 MN force standard machine - i.e. direct, simultaneous installation of both a transducer for forces in tension and a transducer for forces in compression - is made use of without causing additional uncertainties by additionally mounted parts. In the different mounting positions and with rotational turning of the transducers, uncertainty components below 3·10-5 could be detected due to the rotation effect. The transducers had before been calibrated in the 2 MN force standard machine. For this purpose, a force-in-compression transducer up to 3 MN was used, whose interpolation function had been determined in the 2 MN force standard machine and confirmed in the 16.5 MN force standard machine. From the addition of the measurement values of the transducer for forces in compression (3 MN) and the transducer for forces in tension (2 MN ), the machine was linked up in the range from 2 to 5 MN to the high-precision 2 MN force standard machine.
After the completion of last comparisons in the next few weeks, the 5 MN force standard machine will go into measurement service as a national standard up to 5 MN with an uncertainty below 1·10-4. The small uncertainty of the machine's force realization was impressively shown by a comparison measurement with the 2 MN force standard machine in the range from 1 MN to 2 MN in steps of 0.2 MN, using a high-precision 2 MN transfer standard in which the relative deviation between the two machines amounted to maximally 7·10-6. Last but not least, the revised facility offers in the range up to 5 MN twice as much force steps as the 16.5 MN facility.
Figure 1: 5 MN force standard machine with simultaneously mounted transducer for forces in tension and forces in compression
Contact person:
Falk Tegtmeier, FB 1.2, AG 1.21, e-mail: Falk.Tegtmeier@ptb.de