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Design and realization of the high-precision weighing systems as the gravimetric references in
PTB's national water flow standard
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2012 Meas. Sci. Technol. 23 074020
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IOP PUBLISHING MEASUREMENT SCIENCE AND TECHNOLOGY
Meas. Sci. Technol. 23 (2012) 074020 (12pp) doi:10.1088/0957-0233/23/7/074020
Design and realization of thehigh-precision weighing systems as thegravimetric references in PTB’s nationalwater flow standardRainer Engel1, Karlheinz Beyer2 and Hans-Joachim Baade3
1 Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig, Germany2 Muller-BBM GmbH, Schwieberdinger Str. 62, 70435 Stuttgart, Germany3 Droge Baade Drescher, Matthias-Claudius-Str. 5, 38239 Salzgitter, Germany
E-mail: [email protected], [email protected] and [email protected]
Received 14 November 2011, in final form 9 May 2012Published 11 June 2012Online at stacks.iop.org/MST/23/074020
AbstractPTB’s ‘Hydrodynamic Test Field’, which represents a high-accuracy water flow calibrationfacility, serves as the national primary standard for liquid flow measurands. As the corereference device of this flow facility, a gravimetric standard has been incorporated, whichcomprises three special-design weighing systems: 300 kg, 3 tons and 30 tons. Thesegravimetric references were realized as a combination of a strain-gauge-based and anelectromagnetic-force-compensation load-cell-based balance, each. Special emphasis had tobe placed upon the dynamics design of the whole weighing system, due to the highmeasurement resolution and the dynamic behavior of the weighing systems, which aredynamically affected by mechanical vibrations caused by environmental impacts, flowmachinery operation, flow noise in the pipework and induced wave motions in the weightanks. Taking into account all the above boundary conditions, the design work for thegravimetric reference resulted in a concrete foundation ‘rock’ of some 300 tons that rests on anumber of vibration isolators. In addition to these passively operating vibration isolators, thevibration damping effect is enhanced by applying an electronic level regulation device.
Keywords: liquid flow standard, liquid flow calibration, gravimetric reference standard,precision weighing system, vibration isolation, mechatronics system
(Some figures may appear in colour only in the online journal)
1. Introduction—gravimetric-standard liquid flowcalibration
In liquid flow calibration, within the scope of the possiblemeasurement principles, the gravimetric-reference-based flowstandard facilities have proven that they provide the bestaccuracy levels in meter calibration [1]. That was one of themain reasons for the decision in principle to have a suitablesystem design when PTB’s ‘Hydrodynamic Test Field’ wasin the planning stage at the beginning of the 1990s. Thisfacility represents a high-precision water flow standard facility,which is now the primary national standard for liquid flow
measurands [2]: volumetric and mass flow rate, respectively,and total flow measurement, i.e. the quantity of fluid (volumeor mass) having passed a flowmeter [1].
In fluid flow calibration and measurement, four possibletypes of measurands occur.
• (Average) Volume flow rate:
qV = V = VREF
TMEAS= mREF
ρWater × TMEAS. (1.1)
• (Average) Mass flow rate:
qm = m = mREF
TMEAS. (1.2)
0957-0233/12/074020+12$33.00 1 © 2012 IOP Publishing Ltd Printed in the UK & the USA
Meas. Sci. Technol. 23 (2012) 074020 R Engel et al
∑
Prüfling
Pumpe
M
Messstrecke
Hoch-behälter
Vorratsbehälter
UmschaltklappeRegelventil
T
WaageW
Flow diverter
Constant-head tank
Meter under test
Calibration line
Control valve
Pump
Storage tank
Balance
Connecting pipe
Mass
Figure 1. Gravimetric liquid flow calibration facility [1].
• Total(ized) volume measurement:(volume passed measurement)
VM =∫ TMEAS
0V (t) dt = qV · TMEAS. (1.3)
• Total(ized) mass measurement:(mass passed measurement)
mM =∫ TMEAS
0m(t) dt = qm · TMEAS, (1.4)
with the quantities defined as follows: mREF (kg) is thereference mass (collected water mass, determined byweighing), TMEAS (s) is the measurement time (periodof water collection), VREF (m3) is the reference volume(volumetric equivalent of collected water mass) and ρWater
(kg m−3) is the water density.
From the above measurand definition formulae, themeasurement procedures can be derived, i.e. which have tobe determined in the fluid flow calibration process.
2. The measurement process and environmentalimpacts
As already mentioned, the measurement process characteris-tics, i.e. the operation of a gravimetric liquid flow calibrationfacility, can be described, in principle, from equations (1.1)through (1.4).
The principal setup of a gravimetric facility is shown infigure 1. It comprises the following essential components:
• flow generation system: storage tank and electronicallyactuated speed-controlled liquid pump system (optionally,a constant-head tank can be incorporated for flowstabilization purposes);
• calibration line (location where the meters under test areplaced during calibration);
• diverter-operated gravimetric subsystem for standingstart-and-finish calibration measurement [1]: flow diverterand weighing system.
Table 1. Plant characteristics of PTB’s high-accuracy flowcalibration facility [2].
Plant item Specification Parameters
Calibration mode Flying start-and- Gravim. standards, staticfinish mode [1] weighing operation
Flow rates Min: 0.3 m3 h−1 300 kg balance– 3000 kg balanceMax: 2100 m3 h−1 30 000 kg balance
Expanded ± 0.02%measurementuncertainty
The measurement process of flowmeter calibration can bedescribed as follows.
After having established steady-state conditions, i.e.stable flow rate, temperature, and as well pressure, the diverteris actuated in order to direct the fluid stream from thebypass circulation toward the weigh tank. When the levelor the mass, respectively, in the weigh tank has reacheda predefined magnitude (e.g. the maximum payload of therespective weighing system), the diverter is actuated again toredirect the fluid stream to the bypass circulation position. Thewater collection period TMEAS, in combination with dedicatedcollected water mass mREF, determines the prior-definedflow rate measurands. Coincidently, TMEAS characterizesthe period of time during which the weighing system isexcited dynamically. The mass determination itself is a staticmeasurement procedure, of which the dynamic excitation dueto the diverter-operated water collection is one of the disturbingimpacts (see figure 4).
It should be mentioned here that PTB’s ‘HydrodynamicTest Field’ comprises three special-design weighing systems:300 kg, 3 tons and 30 tons, in order to provide optimumcalibration accuracy within reasonable measurement times.A generalized overview of the characteristics of PTB’s high-accuracy water flow facility is presented in table 1.
Both figures 2 and 3 visualize the more or less greatdimensions of the calibration facility. It is worth mentioningthat the overflow weir, which may be utilized as a functionalcomponent of the flow generation system under certaincircumstances, is located in a tower building, at a height of35 m (figure 2).
Figure 3 provides an idea of the size of the three weighingsystems and how they are situated in the calibration plant.
In a liquid flow calibration facility, the operation of a high-precision weighing system is significantly affected by severalimpact quantities, which are superimposed on the originatingfunction of such a device to provide an accurate reading of themass applied to the balance’s signal input. These additionalinput quantities and the dedicated source-and-effect chains aresymbolically depicted in figure 4 [4]. Their physical effectsoriginate from the impulse of the fluid inflow to the weigh tank(1), the generated sloshing in the tank (2), the loss of collectedwater mass [4] due to evaporation (3) and the fluid motion inthe facility’s pipework (4), respectively. The input quantities(1) through (3) enter the measurement device ‘weigh scale’ viathe ‘normal’ measurement channel, symbolically representedby the transfer function FM(p). All the other external disturbing
2
Meas. Sci. Technol. 23 (2012) 074020 R Engel et al
Balance foundation
Storage tank
EngelC
Pumps
Weighing systemswith diverters
Pipe prover
Constant head tank
Meters under test
Figure 2. Cutaway view of PTB’s water flow calibration plant.
Figure 3. View of the calibration hall along the fluid’s flow direction(calibration lines with the weighing systems in the background).
Figure 4. Impact quantities on the weighing system. (1) Fluidstream impact force. (2) Wave-induced forces (sloshing).(3) Decrease of mass due to evaporation. (4) Flow noise caused byfluid motion and machinery operation.
force signals (4), which result from both the impulses due tothe fluid motion in the pipework and the machinery operation(electric drives and pumps), enter the weighing system via
the transfer function FZ(p). This signal path relies upon themechanical transfer properties of the building constructionmaterial, the soil under the foundation of the weighing systemand, finally, the foundation itself.
The appropriate design of the foundation body, onwhich the three weighing systems were located, and theimplementation of a vibration isolation system comprisingpassively acting isolator elements, in combination withelectronic self-tuning controllers, constituted an essentialtask of the design and planning of the whole water flowcalibration facility, as the gravimetric systems represent thecore components, whose reliable operation is the preconditionthat the calibration facility can be operated at a measurementuncertainty level as low as 0.02% [2, 10].
As boundary conditions for the design of the weighingsystem’s foundation and the necessary vibration isolation, thefollowing information must be made available.
• The magnitude and the time function of the system’sexcitation caused by the water flow impact during theliquid collection period (see section 3 below).
• Periodic force excitation due to the water sloshing motionin the weigh tank which is induced while the fluid flow isdirected into the tank (see section 6).
• Time function of the water mass loss due to evaporation:this time function is not relevant for the dynamic designof the balance foundation and its vibration isolationelements. It is an issue that determines the accuracy ofthe steady-state conditions of the weighing process [4].
• Magnitude and frequency spectrum of the noisy vibrationsresulting from the fluid motion in the pipework and othermechanical sources.
In order to obtain realistic data that describe the abovenoise effects, experimental investigations were performedon a small-sized model of the calibration facility underconstruction at that time [6]. Some representative results ofthose investigations are shown in the diagrams of figure 5.
3
Meas. Sci. Technol. 23 (2012) 074020 R Engel et al
(a)
(b)
(c)
Figure 5. Spectra of disturbing mechanical vibrations due tomachinery operation and fluid motion (investigated in a small-sizedtest rig). (a) No flow, i.e. background noise. (b) Flow rate ≈10 m3 h−1. (c) Flow rate ≈ 20 m3 h−1.
The investigations comprised measurements of thespectrum of mechanical vibrations that were emitted by a waterflow fed into a vessel, with the flow rate being varied as a testparameter (flow rates #1 and #2: in diagrams (b) and (c) infigure 5).
The lowest significant frequency that occurred in thespectra at different fluid flow rates amounted to approx. 15 Hz.This was an essential figure which had to be applied to thedesign of the balance foundation’s vibration isolation system.
In reference to the frequency spectrum at zero flowrate, depicted in figure 5(a), it is worth mentioning that thisabscissa reveals a higher resolution than those of the other twodiagrams.
As described in section 3, electromagnetic-force-compensation (EFC) load cells are implemented in PTB’swater flow standard facility [2] as core elements determiningthe accuracy of liquid measurements and the uncertainty of themeasurement results.
Load cells (force metering devices) based on the principleof EFC have proven to provide the highest accuracy possible,compared with other force sensing elements. This is the reasonwhy load cells of this operation principle were chosen to beapplied in the water flow facility as a gravimetric standard.
However, as a matter of fact, this principle—besides providing the highest accuracy—reveals a particularcharacteristic in its operation: sensitivity to external vibrationsentering the force compensation control loop via its sensinginput (signals (1) through (3) in figure 4) or ‘disturbance’ input(signal (4)). As proven by both analytical and experimentalinvestigations [3], the harmonics of spectral mechanicalexcitations are, due to nonlinear cause-and-effect relations inthe EFC load cell, transposed in such a way that each excitationfrequency in a disturbing spectrum causes harmonics of higherfrequencies and, additionally, a zero frequency signal. Thelatter cannot be separated from the direct component of thewanted signal of force metering by any method of signalfiltering!
This effect was also observed by the authors in a small-sized liquid flow calibration rig during the design stage ofPTB’s water flow facility, when they were looking for atechnically and economically optimized solution. This issuewas the subject of further investigations.
Figure 6 depicts the effects caused by mechanicalvibrations (both pump drive and fluid flow induced) in awater flow calibration rig in which no special vibrationisolation provisions had been made. Those investigations thatwere made as preparations for the design of the new high-accuracy water flow standard facility at PTB had proven theineffectiveness of plain signal filtering methods applied onthe output signal of an EMC load cell. As mentioned above,this is due to the nonlinear cause-and-effect relations withinthe EMC load cell that transform harmonic mechanical inputspartly into a direct signal. As seen in figure 6, signal filteringsuch as moving averaging (or any other) techniques is notcapable of correcting this erroneous effect.
It can be recognized that regardless of how manydata samples are involved in the moving averaging interval(figure 6, curves #2 and #3: tested by averaging over 50 and 100values, respectively), the application of a moving average filter[12], in this specific case, cannot solve the problem of vibrationisolation. The mechanical vibrations must be prevented fromentering the EMC load cell, which means the utilization of avibration-isolated foundation on which the weigh scales haveto be placed.
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Meas. Sci. Technol. 23 (2012) 074020 R Engel et al
Figure 6. Response of a 1500 kg EFC-based weigh scale in a waterflow calibration rig to pump shutdown operation (weigh scale wasnot equipped with vibration isolation). (1) Balance reading (rawdata): 5 samples per second. (2) Balance reading: moving averageover 50 values. (3) Balance reading: moving average over100 values.
3. Dynamic characteristics of the weighing system
The principal setup of the biggest weighing system (30 tons) ofthe calibration facility is shown in figure 7 [10]. It is composedof two separate weighing subsystems that are equipped withindividual signal outputs:
• strain-gauge-based force metering system,• EFC load cell.
A special auxiliary device which is part of each weighingsubsystem is the integrated system of mass calibration. Thisbalance calibration subsystem is an essential component whichcontributes to achieving flow calibrations at the claimedmeasurement uncertainty of 0.02%. Besides these staticfunctions of balance calibration, the sets of calibration weightsare part of the mass of the whole gravimetric system, and thusthey cause an impact on the dynamic behavior of the weighingsystem.
Figure 8 shows the dynamic response signals both ofthe strain-gauge load cell and the EFC load cell during thewater collection measurement process that is actuated by theflow diverting device (see also [9]). This diverter activity isindicated by curve #3 in (a).
Figure 8(a) presents the time response signals of the twoabove-mentioned separate load cell systems over the wholeduration of the tank filling process. The signals show that thetank filling represents a dynamic excitation of the weighingsystem that is a superposition of a step function (see curve #2in (b)) and a ramp-like response function.
The curves in figure 8(c) reveal that the balance representsa damped oscillating mechanical system.
4. Environmental conditions—need for uncoupling
Vibration-sensitive research and production facilities need tobe designed in terms of the possible impacts due to external
and internal vibration sources. External sources, in general,should be quantified by vibration measurements under relevantconditions. Here, environmental vibration measurements withthe simulation of impacts due to local traffic were carried outfor the planning of a new PTB research building (New OpticsLaboratory). The simulation considered rough winter surfaceconditions and could be regarded as a worst case. Results fromthese measurements were taken as input for the design of thehydrodynamic test facilities (figure 9).
Based on elastic soil properties, the behavior ofa quasi-rigid foundation, directly supported on the soil,was analyzed mainly for pitching and vertical motionswhich were found between 10 ( ± 1.5) and 16 ( ± 2.5)Hz. It must be expected that a directly supportedfoundation would show rigid body resonances at theseeigenfrequencies, but with only a low amplification due tohigh rates of geometric damping (half space radiation ofvibration energy).
More impacts were expected from in-house activities.Vibrations emitted from pumps and other aggregates were tobe minimized to unimportance through high-quality vibrationisolation by means of spring-supported gravity masses.The impact of the inflow of water into the lower basin,however, could not be isolated. In particular for low waterconditions, it had been assumed that approximately 5% ofthe impulse of the input flow could be emitted as a coloredrandom noise into the soil in an unfavorable frequency range(third octave).
The elastic properties of internal balance componentshad already been analyzed [5, 7] through a finite elementmodel (see also figure 10). By means of the FEM modelin figure 10, those eigenfrequencies were determined whichcharacterize the critical frequency ranges for the weighingsystem’s operation. First eigenfrequencies were detected at6.5 Hz and at 17 Hz. The corresponding modes were onlyweakly damped at rates of approx. 2%. The contribution ofthese resonances to weighing errors had been analyzed only inthe case of a nonlinear mechanical-electrical coupling of loopcontrols at a much higher frequency of more than 1000 Hz [3].In view of maximum bearable measurement errors of 5 × 10−5,a conservative guideline for the design of an appropriatefoundation required that vibrations in the weighing systemstructure should essentially not exceed 10−4 m s−2. The worstcase responses—already as global vibrations without anyinternal resonance—ranged up to 7 × 10–4 m s−2, considerablyexceeding the above limit. Therefore, vibration isolation of theweighing system was required.
5. Vibration isolation system
A first design for passive vibration isolation consisted in aheavy concrete foundation elastically mounted on the baseslab with a vertical translatory eigenfrequency of 2 Hz. Theidea of such a mounting is the isolation I of the supporteddevice versus disturbances with an effectiveness in higherfrequencies of
I ∼ η2 (5.1)
5
Meas. Sci. Technol. 23 (2012) 074020 R Engel et al
Balance foundation Pneumatic vibrationisolation modules
Mass loadingmechanism(electrically driven)
Referenceweights (30 tons)
Lever balancewith electrodynamicforce compensation cel
Suspension ofcalibration weights
Strain-gaugeload cell(s)
Weighing tank
Figure 7. 30 ton weighing system with integrated calibration system (side view, principle).
for quasi-stationary force impacts and
I ∼ η/ζ (5.2)
for base excitation, η being the ratio of excitation to isolationfrequency and ζ the support’s damping ratio. The lowerthe spring frequency is, the better the isolation for higherfrequencies at the cost of possible low frequency resonances.With inexpensive steel springs, a vertical translatory isolationfrequency fV of down to approximately 2 Hz can be achieved.Steel springs are robust but have disadvantages in being onlyvery weakly damped and they therefore often require the use ofadditional viscous damping devices. Their deflection and thelevel and the positioning of the foundation block change if theload status changes—an effect which increases with 1
/f 2V for
a given foundation mass. Steel springs also show the feature ofinternal resonances at higher frequencies leading to a partialbreakdown of isolation.
Pneumatic springs avoid some of these disadvantages.Their support frequency is essentially independent of thesupported mass, their vertical damping ratio is adjustable, andtheir level can be kept constant when the load situation ismodified. If required, the foundation block can be temporarilypositioned on rigid feet, bypassing the isolation mechanism,e.g. for installation purposes. Pneumatic isolation elementsare built for vertical support frequencies down to 1 Hz oreven lower. The horizontal behavior is governed by the elasticproperties of the membrane; this type leads to an overall hybrid
system. Resonances at the support frequencies have to beconsidered, as in the case of steel springs.
Pneumatic elements are more expensive than steel springsand they require a compressed-air supply and an automaticlevel regulation with a permanent feed of compressed air.In most applications, these components are active duringworking periods (measurements), but in some very sensitiveenvironments they can be deactivated for periods of up toseveral days in order to avoid any disturbances from airflowand control. Modern vibration isolation systems for highlysensitive applications generally work with pneumatic springelements.
All types of passive isolation elements against externaldisturbances work on the basis of the ratio η, which leads toa high sensitivity against internal low frequency disturbances.An example of this effect is the above change of load status,footfall or other operator activities, occasionally with fatalconsequences. This effect can be counteracted through ahigher mass of the foundation block, which of course hasits limits. The foundation block, with overall dimensions of7.5 m × 7.5 m × 4.5 m for the gravimetric reference discussedhere, finally had a mass of approx. 325 metric tons, leadingto a ratio of 10:1 compared to the changing loads of up to30 tons. The base slab, on which the foundation is supported,has a thickness of 60 cm, reducing the incoming disturbancesin higher frequencies already to a considerable degree throughits bending stiffness.
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Meas. Sci. Technol. 23 (2012) 074020 R Engel et al
(a)
(b)
(c)
Figure 8. Dynamic response of the 3 ton weighing system due todiverter-operated water collection (curve #1: reading of the EFCload cell, curve #2: reading of the strain-gauge load cells, curve #3actuation of the flow diverter). (a) Response of the weighing overthe whole water collection time (meter readings: force unit!).(b) Zoomed view: diverter actuation directing the water flow into theweigh tank. (c) Zoomed view: diverter actuation redirecting thewater flow into the bypass direction.
The 13 pneumatic passive isolation modules were oftype IDE PD3001 (Integrated Dynamics Engineering GmbH[11]) with a load capacity of 25 tons each at 8.5 bars. Thenominal maximum working pressure was 9 bars in the state ofstatic equilibrium. A level control system was installed which
acted through the pressure control of three groups of elementsreacting to three rigid body motions. The horizontal degreesof freedom were controlled passively by the elements’ elasticmembranes. The positions and the grouping of the elementswere—within the limits of the possible geometry—optimizedin view of the effectiveness of pressure modifications resultingin required position changes.
First tests after the installation of the components showedthat the reaction time of the level control was insufficient. Twoadditional pneumatic elements were installed, together withsome additional modifications (e.g. wider hoses), reducing theeffective maximum pressure of 10.2 bars to 9.2 bars and to asatisfying level control also during the filling process.
6. Dynamic behavior of the liquid in the weigh tankas a source of disturbance
Whereas operator-caused impacts could be excluded duringthe measurement periods, some effects of the filling of the30 ton weighing tank on mechanical vibrations had to beconsidered.
• Startup of the inflow into the tank, acting as a load stepfunction (see figure 8) with decaying vibrations of allstatically deformed parts of the weighing system and ofthe foundation in their eigenfrequencies.
• The filling of the tank leading to a sloshing water surface.Small-scale turbulent motion quickly decreasing; a globalnon-turbulent amplitude of 0.1 to 0.2 m (zero-to-peakamplitude) was assumed to be possible (experimentallydetermined on a small-sized flow calibration rig).
The slosh motion [7, 8] is—without additionalprovisions—only very weakly damped and causes mainlyhorizontal long-time periodic forces, resulting—if notcompensated—in pitching motions of the foundation. Ananalysis based on the linear theory of potential flow in a staticvessel leads to a potential function for the velocity in theform of
� = A cos(ωt) cosh(β[z + h]) cos(kϕ)Jk(βr), (6.1)
Jk being the Bessel functions of first kind, kth order with thedynamic pressure
pdyn = ρ∂�/∂t. (6.2)
The eigenvalues β result from the boundary condition of zerovelocity normal to the static boundary
∂Jκ (ζ )
∂ζ |ζ= βR= 0, (6.3)
with the corresponding natural angular frequency
ω2 = gβ tanh(βh), (6.4)
h being the filling depth and R the radius of the vessel.Numerical simulations of the above potential equation
were carried out, including also design parameters of the weightank and varying filling depths.
Here, relevant impacts were detected only for the firsteigenmode, for a filling depth of 3.0 m at a frequency of 0.5 Hzand with a resulting horizontal force of dynamic pressure
7
Meas. Sci. Technol. 23 (2012) 074020 R Engel et al
1 2 5 10 20 50
Frequency [Hz]
10-2
5
2
10-3
5
2
10-4
5
2
10-5
Acc
eler
atio
n
[m/s
²]
1 2 5 10 20 50
Frequency [Hz]
10-2
5
2
10-3
5
2
10-4
5
2
10-5
Acc
eler
atio
n
[m/s
²]
6
54
3
1
2
Figure 9. Environmental impact on a directly supported foundation.(1) RMS soil vibrations by flow measurement. (2) Rigid bodyresonance of the foundation block. (3) Soil vibrations due to inflowat 0.04 m3 s−1 into the lower basin (storage tank, figures 1 and 2).(4) + (5) Internal resonances of the weighing system, excited byenvironmental vibrations. (6) Internal resonance of the weighingsystem, excited by inflow vibrations.
of 5.1 kN at a height of 2.6 m for a sloshing amplitude of0.1 m (figure 11). More of these impacts arose for higher-order eigenmodes with only one axis of zero displacement,but with rapidly decreasing amplitude.
These forces would cause rocking foundation motionswith accelerations of approximately up to 5 × 10−4 to10−3 m s−2, essentially higher than allowed according to designphilosophy, and of the same scale as the expected accelerationson a non-isolated foundation. These accelerations would—differently from those due to external sources—decreasewith the decaying slosh amplitudes, but only within periodsthat were not acceptable. The solution of this dilemma wasthe combination of the passive isolation system—effectivelyreducing external excitation at higher frequencies—with anactive one, correcting the weakness of the passive system atlow frequencies by eliminating the foundation and tank rigidbody motion.
Figure 12 summarizes all the impacts considered. Curves1 through 6 show the vibration acceleration due to the sameimpacts as in figure 9, but damped by the dynamic propertiesof the foundation with its vertical and rocking eigenmodes onthe pneumatic vibration isolators (see the legend in figure 12).
Figure 10. Model of the weighing system applied for finite elementmethod (FEM) simulation.
Figure 11. First eigenmode of sloshing in a static cylindrical tank.
7. Realization of an adaptive vibration isolation ofthe weigh system foundation
The design and the construction of the gravimetric-referencesystem were an iterative sequence of
• theoretical analysis [5];• pre-investigations applied on down-sized test setups of
flow rigs and balance installations [4, 6];• functional layout of the balances’ location on a draft-
design body of the balance foundation;• draft design and dynamic simulation of the whole
weighing system [5];• activities of an iterative step-wise refinement and
fine-tuning both of the system’s steady-state behavior(measurement uncertainty of the weighing process[2]), the dynamic resistance to externally (figure 5)and internally (figure 10) excited disturbances and a
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Meas. Sci. Technol. 23 (2012) 074020 R Engel et al
0.1 0.2 0.5 1 2 5 10 20 50
Frequency [Hz]
10-2
5
2
10-3
5
2
10-4
5
2
10-5
5
2
10-6
Acc
eler
atio
n
[m/s
²]Sloshing: magnitude 100 mm
Transition time: 3 -12 minutes
3 -12 minutes
10 mm
1 mm
0.1 0.2 0.5 1 2 5 10 20 50
Frequency [Hz]
10-2
5
2
10-3
5
2
10-4
5
2
10-5
5
2
10-6
Acc
eler
atio
n
[m/s
²]Sloshing: magnitude 100 mm
Transition time: 3 -12 minutes
3 -12 minutes
10 mm
1 mm
12
3
4
5
6
Figure 12. External disturbances and internal expected resonanceson a 2 Hz isolated foundation. Above 1 Hz: (1) through(6)—environmental and internal impacts (see figure 8) modifiedthrough the 2 Hz support, damping ratio ζ = 0.2. At 0.5 Hz:foundation rigid body accelerations due to quasi-stationary sloshmotion of a filled tank. Sloshing magnitudes 100, 10, 1 mm, withdecaying intervals.
reasonable dynamic transition upon the combined stepand ramp-like mass force input signal (figure 4) [7];
whose outcome was a rigid body—a concrete ‘rock’ with amass of approx. 350 tons (see figure 13)—which was placedon a set of vibration isolators (figure 14) and which serves asthe installation platform for the three weighing systems [10].This foundation body is depicted in figure 13 in the stage of its‘raw conditions’, i.e. prior to the installation of the weighingsystems.
The spatial distribution of the vibration isolator elements,which can be seen in figure 13(b), had to be adapted to thelateral location of the center of gravity of the whole weighingsystem.
One essential dynamic disturbing impact is that thelateral position of the center of gravity varies depending onthe condition of which one of the three weighing systemsis selected for a flow calibration task. The time-dependentparameters of this dynamic disturbance input are determinedby the magnitude of the water flow rate running into the weightank and by the maximum water mass that is defined to stopthe inflow.
The functions that were implemented and dedicated to theindividual component parts of the vibration isolation systemare the level control of the balance foundation’s verticalposition and the control of the orientation so that the foundationblock’s normal vector is stabilized in a perpendicular direction.In order to stabilize both the foundation block’s height leveland its orientation, the set of 16 isolator elements wasfunctionally dedicated to three groups of isolators which serveas actuating devices in three autonomously operating controlloops, one for each of the three axes.
(a)
(b)
Figure 13. Balance foundation with the vibration isolation system.(a) ‘Raw conditions’ during construction. (b) Positions of thevibration isolators.
Figure 14. Vibration isolators supporting the weighing system’sfoundation [11].
The several actuating and control functions implementedin the vibration isolation system can be categorized as follows(depicted in principle in figure 15):
• passive vibration isolation: implemented through thepiston–cylinder compressed-air isolators;
• active three-axis level control via position sensors attachedto the foundation, and electronic controllers utilizing thepneumatic isolators as actuating devices;
• active vibration attenuation (superposed on the passivevibration damping effects) by three dynamic-signalcontrollers which receive their control signals from athree-axis velocity sensor and which are connected to 4three-axis linear electric drives;
• self-tuning adoption of the controller parameters in orderto provide stable and optimum dynamics operation;
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Meas. Sci. Technol. 23 (2012) 074020 R Engel et al
Compressed-air supply
S
Pneumatic vibration isolator
Linear drives(dynamic actuation)
sY
Valve 2
Valve 1
TC controller
Line
ar d
rive
s(3
axe
s)sX
3
Balance foundation
Vibration control loop
Leveling and positioncontroller: ± 0,05 mm
PID Controller
2
3
1
3
2
Velocity sensor (3 axes)
Position sensor 1 (3 axes)
x
S
xX
xZ
Yx
M
sZ
EngelC
M
M
Figure 15. Balance vibration isolation system: active level position control and dynamic feed-forward control.
• enhanced dynamic controller operation by additionallyapplying feed-forward signal paths in the control loops’signal structures.
When flowmeter calibrations are run, i.e. the gravimetricreference is in active use for determining the mass of thecollected water in the weigh tank, the above-mentionedelectronic level control is in the continuous-signal operationmode to provide the highest precision, i.e. the highest levelsignal resolution of ± 0.05 mm. In this case, a permanentconsumption of compressed air is caused by the pneumaticactuation of the isolator elements.
In the case when the water flow facility is not in activeuse, i.e. when it is kept in standby operation, the control loopstructure is altered by activating a mechanical on/off controllerwhose operation considerably reduces the consumption ofcompressed air.
8. Measurement characteristics of thevibration-isolated system under operating conditions
Besides disturbances generated by micro-seismic and otherambient sources, additional mechanical vibrations will beinduced by several components of the running system. Originsof these possible important disturbances are the oscillationscaused by the running pumps and by the circulating wateritself. They will be introduced into the foundation of thebuilding by the supporting systems, which are connected tothe bearing structures.
The impact of pump vibrations is reduced by careful staticand dynamic balancing and supporting on foundations withpassive vibration isolators.
The water current inside the pipes is in a state of turbulentflow, which will induce reaction forces and vibrations of the
structure, even if the supports are equipped with an elasticlayer between pipe and structure.
Further excitations are generated by the overflow water ofthe hydrostatic head tank and the bypass water of the diverterrunning down in a free flow jet to the storage tank locatedin the basement, where the foundation of the scales and thestorage tank are in close proximity. These jets generate pulsesand turbulent motions within the tank. Thus, the occurrence ofdisturbing impacts on the scales is unavoidable.
In order to verify the performance of the installed vibrationisolating system of the scale foundation and to prove thisadvantage, measurements were carried out. As a representativeexample, typical results achieved with the 30 ton scale areshown in figure 16. The weigh scale’s load cell is read outin time steps of 25 ms, for each of the depicted three runsat different kinds of operation, during an overall period of300 s. The magnitude of the scatter can be compared to thestate of ‘quietness’ when the foundation is settled down on thepassive vibration isolating system and the water circuit is notrun (background noise). If the water circuit is run at maximumspeed, an increase in vibrations can be observed even ifthe foundation is isolated passively. The scatter decreasessignificantly when the active isolating system, too, is underoperation.
In summary, it can be stated that any occurrence ofmechanical vibrations intruding into an EFC load cell eithervia its measurement input or via its base plane (disturbanceinput (4) in figure 4) causes—in addition to the ‘regular’ signalresulting from the force to be measured—an ‘irregular’ signaldue to the rectifying effect of nonlinear elements in the wholesignal path [3]. This is an analogous effect to that caused bya diode in an electronic rectifying circuitry where a directsignal and harmonics of the alternating-current input signalare generated as a response to the input signal.
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Meas. Sci. Technol. 23 (2012) 074020 R Engel et al
14.970
14.980
14.990
15.000
15.010
15.020
15.030
15.040
0 50 100 150 200 250 300
Time [s]
Bal
ance
rea
do
uts
[kg
] 3
3A 22A
1 1A
Figure 16. Error reducing effect of the balances’ vibration isolation system. Balance readings on 15 ton payload (calibration weights):(1) zero flow rate and vibration isolation system OFF; (1A) averaged readings; (2) flow rate of 2100 m3 h−1, vibration isolation system ON;(2A) ditto: averaged readings (difference from (1A): 0.4 kg); (3) flow rate of 2100 m3 h−1, vibration isolation system OFF; (3A) ditto:averaged readings (difference from (1A): 6.4 kg).
Thus, any reduction of disturbing vibrations that mightinteract with the EFC load cell coincides with the reduction ofthe systematic erroneous shift of the weigh signal due to theabove described effect.
In the diagram of figure 16, both the erroneous effect ina weighing system without vibration isolation provisions andthe significant improvements of the measurement accuracyby utilizing appropriate vibration isolation measures can beobserved.
For testing purposes, the pumps were run to provide a flowrate of 2100 m3 h−1, the maximum flow rate of the calibrationfacility.
• Curve #1: acquisition of the EFC data under undisturbedoperating condition with 15 000 kg (calibration weight)being placed on or suspended below the weigh scale,respectively.
• Curve #2: measurement data acquired from the EFC loadcell with the balance foundation in working position, i.e.lifted, and the active vibration isolation system switchedon. The payload again amounts to 15 000 kg.
• Curve #3: represents operation under the same flow andpayload conditions as curves #1 and #2, but with thevibration isolation system deactivated and the balancefoundation lowered and placed directly on the soil insidethe building.
More or less scatter of the ‘raw’ (sampling period is25 ms) signals (curve #1 through #3) can be observed infigure 16. The magnitude of scatter depends on the testingmode which is run during testing. Moving average signalfiltering had been applied during each of the three operatingmodes of testing; these signal averaging results are representedby the curves #1A, #2A and #3A.
It could be seen that in the case where the weighingsystem is operated without any appropriate vibration isolationprovision, this causes a systematic error shift of the EFC signalwhich amounts to approx. 6.4 kg: an error level that exceedsthe signal resolution of the load cell (10 g) 640 times.
As an additional preventive measure, prior to the flowcalibrations which are aimed at low measurement uncertainty,the weigh scale’s calibration is principally carried out with theflow system running.
9. Conclusions
Experiences and conclusions, which were acquired or derivedfrom the design, construction and operation of a high-precisionvibration-isolated weighing in a liquid-flow calibration facilityunder the conditions of both high-amplitude mechanicalexcitation caused by the measurement process itself and thespectral noises caused by the fluid in flow, can be summarizedas follows.
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Meas. Sci. Technol. 23 (2012) 074020 R Engel et al
• Although classified as a static-weighing measurementprocess (flying start-and-finish flow calibration applyingstatic weighing [1]), an essential part of the system’sdesign had to be dedicated to system dynamicsaspects.
• With weighing applications in high-accuracy liquid flowcalibration facilities, the magnitude of the disturbancesin the measurement process may exceed the resolutionthreshold of the weigh sensors considerably.
• The dynamic behavior of several processes that have animpact upon the accuracy of the weighing process has tobe taken into account.
• Primarily, harmonic excitations that cover a widefrequency spectrum have to be dampened down to a levelwhere they are lower than the input threshold sensitivityof the weighing system’s electronics.
• Coincidently, in designing the vibration isolation system,its dynamics must be designed in a manner that provides asufficient frequency bandwidth, i.e. mass reading responseof the weighing system as short as possible.
• An additional dynamic disturbing impact on the weighingsystem results from sloshing or liquid oscillations withinthe weigh tank in which the liquid is collected.
• In the case where more than one weighing system (PTB’swater flow facility comprises three balances: 300, 3000and 30 000 kg) is arranged on a common foundation(equipped with vibration isolators), the problem solutionresults in a multivariate and multi-objective optimizationtask.
• In order to meet all the above boundary conditions, amultivariate multi-loop control scheme is unavoidable.
• With PTB’s water flow facility, adaptive variable-structuremulti-loop control schemes were applied in order tonot only meet maximum noise damping, but also anoptimum balance response of the three different-sizedweighing systems and minimum energy consumptionwhile the weighing systems are in standby operationmode.
Acknowledgment
The authors would like to thank Mr A Eggestein, LiquidFlow Laboratory of PTB Braunschweig, for having carried outmeasurements which delivered the data used for the analysisof the weighing systems’ responses, and, thus, contributed tothe realization of this paper.
References
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