Dubovikova, Nataliia

Karcher, Christian

Resagk, Christian

Digitale Bibliothek Thüringen

FORCE MEASUREMENTS BY STRAIN GAUGE SENSORSAS PART OF TIME-OF-FLIGHT FLOW RATE CONTROLNataliia Dubovikova, Christian Karcher and Christian ResagkInstitut of Thermo- und Fluiddynamics, Technische Universita¨t IlmenauP.O. Box 100505, D-98684 Ilmenau, GermanyABSTRACTContactless techniques are the most promising methods for liquid metal flow rate control and some ofthese methods are based on electromagnetic induction of breaking force acting on an electrically conduc-tive fluid which is moving through a static magnetic field. Providing flow analysis in case of aggressiveand hot liquids is a complicated task, especially when liquids composition and, hence, its physical proper-ties, are unknown. One of the techniques is time-of-flight Lorentz force velocimetry (LFV). By using themethod one can estimate volumetric flow rate without knowing of electrical conductivity, magnitude ofmagnetic field or characteristic dimension. The most important and crucial challenge within the techniqueis detection of small fluctuations of Lorentz force value. In this article we will focus on special aspectsof application of highly sensitive strain gauge force sensors within the framework of time-of-flight LFV.1. INTRODUCTIONThere are existing many kinds of multiply origin forces like compression, tension, bending, gravity andmany others. Measurement of these forces is in sphere of vivid interest of present-day science and industry.Different working principles are applied for the purpose [1]: comparison with well-known force (scales),force effect on elastic element (strain gauge), change of the pressure (pneumatic load cells), etc. Moreover,nowadays force measurements are used not only for direct force determination, but also for investigationvalues in different fields, such as bioengineering (bacterial adhesion study [2]) or material science (atomicforce microscope [3]). It is also common to use force measurement for flow rate control of conductive [4]and non-conductive materials [5].One of the flow control’s method which is based on force measurement is Lorentz force velocimetry(LFV). It is an up-to-date contactless technique for control of volumetric flow rates of hot, opaque, andaggressive fluids, where neither contact nor optical method can be applied. Since metal melts are excellentelectrical conductors, the employment of electromagnetic flow measurement devices are favorable. LFV isbased on the fundamental principles of magentohydro-dynamics (MHD) [6], [7]. The theory of this MHDeffect was well described by Shercliff in 1963 [8]. When an electrically conducting material passes themagnetic field lines stretched by an arrangement of permanent magnets, eddy currents are induced inside themoving material. These eddy currents interact with the applied magnetic field and, as a consequence, Lorentzforces are generated within the material. The generated force opposes liquid’s movement and, according tothe Newton’s third law, the same value of force is acting in the opposite direction on the source of magneticfield - permanent magnet. Because the force depends on velocity, it provides a velocity dependent forcesignal for flowmeter applications. According to Thess [4], the flow velocity has a linear influence on theresulting Lorentz force FL, hence it can be estimated by measuring the force:FL ∼ σvB20 . (1)c©2014 - TU Ilmenau 1URN (Paper): urn:nbn:de:gbv:ilm1-2014iwk-109:8 58th  ILMENAU SCIENTIFIC COLLOQUIUM Technische Universität Ilmenau, 08 – 12 September 2014 URN: urn:nbn:de:gbv:ilm1-2014iwk:3A drawback of the technique is that the measured force is not just the function of volumetric flow rate,but also depend on such factors as electrical conductivity σ of the melt, magnitude of the applied magneticfield B0, which difficult to control precisely during the experiment due to their temperature dependence.2. TIME-OF-FLIGHT LFVTo circumvent the deficit of accurate properties data, improved LFV technique - time-of-flight [9] method -is suggested. Feasibility of the method to determine flow parameters without probes and signal particles issignificant under condition of industrial manufacture. Time-of-flight LFV allows to control flow rate of liquidmetal and is unaffected by physical properties of fluid or by outer conditions. According to the technique(Fig. 1a), two identical measurement systems are mounted along a liquid metal channel one by one. Each offlow meters comprises of a permanent magnet pair and an attached strain gauge force sensor, so inducing ofLorentz force and measurement of the reaction force are made simultaneously. The measurement systems areseparated by a certain distance D in the direction of the flow. Electromagnetic pump, based on permanentmagnets, is used for pumping the flow in the channel. Within the flow, up-stream to the measurementsystems, artificial vortices are generated. A generator of vortices is located in such way, that created by itfluctuations can move with a liquid through magnetic field of permanent magnets. The vortices are passingboth pairs of magnets one-by-one, which gives us serial change of measured Lorentz force signals because offlow disturbances. Here, upon finding a cross-correlating function of the two force signals, we determine thetime-of-flight τ of the generated vortex structure passing the both flow meters. Then, according to the timea) b)Figure 1: Working principles of time-of-flight LFV (a) and signals of time-of-flight LFV (b). Velocity isdetermined by the measurement of time delay between peaks, caused by Lorentz force disturbance within theflow due to a moving vortex.between peaks τ and the distance D between measurement systems, observer can estimate mean velocityand, hence, flow rate Qv of the liquid (k - empirical coefficient):Qv = kD/τ. (2)The experimental facility consists of a closed channel with rectangular cross-section of height per width= 80 x 10 mm2. The entire duct is made of acrylic glass. The flow velocity is limited by the water pumpand can reach fluid flow velocities in the range of up to v = 1.0m/s. The channel is filled with eutecticalloy of Gallium, Indium and Tin – Galinstan – which is liquid under room temperature and has electricalconductivity in order 106. To increase the rate of turbulence within the channel, to create long-life vortexstructures and to increase the rate of usable signals, a vortex generator is mounted into the flow. Severalc©2014 - TU Ilmenau 2methods can be used for vortex generation: contact way – by immersed solid body, or contactless way –by magnetic obstacle, that are created by permanent magnets or electromagnetic influence. Application ofsolid body as the vortex generation provides good qualitative results in creation of Karman’s street in caseof laboratory conditions, but as a contact method it cannot be used in case of aggressive or hot medium, somagnetic obstacles were tested as well.3. FORCE MEASUREMENTS SYSTEMSimultaneous measurement of Lorentz force by two identical flowmeters is the main idea of time-of-flight LFV. Thereto in practice different force sensors can be used: load cells [10] and electromagneticforce compensators [11], optical systems [12,13]. Core requirements to the sensors for laboratory tests byLFV: high sensitivity and accuracy in low forces measurement range, because level of Lorentz force in ourlaboratory conditions doesn’t exceed 10−2...10−1N. In case of time-of-flight LFV applied sensors have to beeven more sensitive, because flow disturbances as a result of vortex movement are lower than 10−3N. In orderto satisfy the requirements commercial three-dimensional strain gauge sensors were chosen with declaredsensitivity value 40µN and measurement range 2N. The principle of the force sensor is the measurementof voltage alteration due resistance changes under the application of force or strain. Standard strain gaugescan sense the displacements as small as 5 m, what makes them capable to detect small changes of force.As sensors material aluminum was selected because of its non-magnetic nature and smooth stress-strainbehavior in measured force range.Every force sensor is connected with a pair of permanent magnets 10 x 10 x 90 mm3, as it shownat Fig. 2. The magnet system is equipped with two high-energy magnets made of NdFeB and reachesmaximum magnetic flux density of the order of 300 mT within the channel between magnets. Top sideof both sensors is rigidly fixed to loaded aluminium construction to avoid displacement damping due tobending stresses of coupling elements and mounting system itself. The whole measurement setup is fixedFigure 2: Force measurement system of time-of-flight LFV (one of two). Force FR, caused by system reactionon interaction of conductive liquid flow and space variable magnetic field, FY – multidirectional turbulenceand magnets disaligment, FZ – gravitational force.on a stone block and embedded in a box filled with sand in order to suppress the effects of vibrations ofthe surrounding building. The magnets are emplaced symmetrically to the channel both in vertical andc©2014 - TU Ilmenau 3horizontal (orthogonal to the flow) directions. The Lorentz force that is generated within intersection ofapplied magnetic field and conductive flow, have effect on both magnets at the same time as consequentreaction force. The main component of measuring force – resulting force FR – acts at the middle pointbetween centers of permanent magnets along the flow. Orthogonal component of the reacting force FY iscaused by multidirectional turbulent fluctuations within the flow and the imperfection of magnets positioningaccording to flow: disalignment or asymmetry. Value of FY could reach significant values, up to 10% ofFR, so accurate mounting of the sensors is necessary for qualitative force signal. Vertical component FZ iscaused mostly by gravitational force due to the weight of fastening system with permanent magnets whichis equal 2,56N . Asymmetry in vertical positioning of the magnets according to the height of the channel hasinsignificant effect on FZ (less than 0,1% of gravitational force).The voltage change due to the sensors’ resistance alterations are measured in complex with commercialstrain gauge amplifiers from the force sensor’s producer and with a high resolution digital data acquisitionsystem Agilent. The influence of primary strain, caused by outer conditions, temperature, mutual influ-ence of forces in supplementary directions and so on are neutralized by preparatory tarring of amplifiers sopreliminary state doesn’t have significant effect on the measurement result.4. RESULTSTo illustrate practical results of time-of-flight LFV we used cylindrical solid body to generate strong tur-bulence fluctuations within the flow. The cylinder has diameter 5mm and is mounted in the middle of thechannel, 10cm before the first measurement system. Every test endures up to three minutes, so impact of thechange of surrounding conditions with time is neglected. because of high density and, hence, inertia forcesof investigated liquid, it takes one minute for it to stabilize the flow after starting of the pump. Two forcesignals were measured simultaneously and the results are presented at Fig. 3.a) b)Figure 3: Typical force signals of time-of-flight LFV (a) and their power spectrum (b). Combination of twoforce signals, obtained during experiments with closed rectangular channel. The signals are very noisy dueto mechanical vibration of the channel and measurement system, power spectrum of signals shows intensityof different frequency spectrum componentsTime-of-flight LFV measurements results show high level of noise of different nature, primarily due tomechanical vibrations within system itself, electromagnetic pump and oscillations in the channel; as wellas electromagnetic noise due to electrical networks (50Hz peak on power spectrum diagram). Anothersignificant source of noise is natural frequency of strain gauge force sensors. Though declared value of thec©2014 - TU Ilmenau 4frequency is 180Hz, due to heavy magnets and their fastening system high level of dead load effects, thatdecreases value of sensors natural frequency to ∼30Hz.The distance between the sensors is estimated as 15cm with uncertainty ±0,1mm. To detect the timedelay in a noise-corrupted sequence, the normalized cross-correlation function of two force signals wasapplied (Fig. 4). Cross correlation is a standard method of estimating the degree to which two series arecorrelated. The maximal value of the function shows the time delay value between investigated curves.Under normalized cross-correlation the range of the data is -1 to 1 such that the closer the cross-correlationvalue is to 1, the more closely the information sets are.Figure 4: Normalized cross-correlation function of two force signals of time-of-flight LFV. The highest peakhas shown at time point 0,237s due to alternate effect of vorticies within the flow. The fluctuation werecreated by solid cylindrical obstacle.Within described experiment time delay value 0,237s is obtained which with the distance 15cm betweensensors gives us mean cross-sectional velocity 0,633m/s without applying of any calibration coefficient, atthe time as potential probe measured 0,809m/s in the middle of the channel.Our experiment procedure results in evaluating the travelling speed of any vortex structure within theflow. We observe that the raw signals are very noisy and the cross-correlation is weak, so proper filteringof raw signals is necessary. The improvements of filtering technique is a part of the future investigation,directed on decreasing noise-to-signal ratio at the frame of time-of-flight LFV.5. CONCLUSIONSThe feasibility of time-of-flight LFV for flow rate measurements of liquid metals in closed channel hasbeen successfully demonstrated under laboratory conditions as fully contactless method. There is no need inmechanical contact of probes and liquid or in additional information about physical properties of the mediaand material. The present technique measures the transit time of tagging vortieces that are transported bythe flow and registered by two Lorentz force flow meters that are arranged in a certain distance betweeneach other. The flow rate determination is based on cross-correlation function between two simultaneouslymeasured force signals and is independent of liquid properties and magnetic field strength. Strain gaugeforce measurement system, that was applied for the tests, is sufficiently sensetive to detect small changesof Lorentz force due to moving vorticies within the flow. Experimental results are well compared with theresults of local velocimetry technique - potential probe; the difference between measured results is primarilycaused by the difference of mean and bulk velocity values in channel flow.c©2014 - TU Ilmenau 56. ACKNOWLEDGEMENTSWe would like to express appreciation to DFG for funding of our Research Training Group Lorentzforce velocimetry and Lorentz force eddy current testing and this research particularly (GRK 1567). Ourthanks are extended to Prof. Kolesnikov for his useful and constructive scientific discussions and to AnnaKholodova for her support in measurements and signal processing.References[1] R. Pallas-Areny and J. Webster, Sensors and signal conditioning 2nd ed., New-York: John Wiley andSons, 2001, 632 p.[2] E. Fllman, S. Schedin, J. Jass and others, Optical tweezers based force measurement system forquantitating binding interactions: system design and application for the study of bacterial adhesion,Biosensors and Bioelectronics v.19 (2004), 1429 p.[3] H. Butt, B. Cappella and M. Kappl, Force measurements with the atomic force microscope: Technique,interpretation and applications, Surface Science Reports 59 (2005), 1152.[4] A. Thess, E. Votyakov, and O. Zikanov, Theory of the Lorentz force flowmeter, N. J. Phys., 9, (2007),p. 299.[5] A. P. de Camargo, T. A. Botrel, R. G. Vieira and others, Load cell adoption in an electronic drag forceflowmeter, Sci. Agric. (Piracicaba, Braz.), v.68, n.3 (2011), p.275[6] P. A. Davidson, An Introduction to Magnetohydrodynamics, (Cambridge: Cambridge University Press,2001).[7] R. Moreau, Magnetohydrodynamics (Dordrecht: Kluwer 1990).[8] J. A. Shercliff, The theory of electromagnetic flow-measurement (Cambridge: Cambridge UniversityPress, 1962).[9] D. Jian, Ch. Karcher, Electromagnetic flow measurements in liquid metals using time-of-flight Lorentzforce velocimetry, Meas. Sci. Technol. 23 (2012) 074021 (14pp).[10] I. Rahneberg, et al. Novel concept of a high precision 6-DOF force/torque transducer, NCSLI AnnualConference, National Harbor, MD, USA August 22-24, (2011).[11] C. Diethold, F. Hilbrunner, Force measurement of low forces in combination with high dead loads bythe use of electromagnetic force compensation, Meas. Sci. Technol. 23 074017 (2012).[12] A. Wegfrass, et.al., Flow rate measurement of weakly conducting fluids using Lorentz force velocime-try, Meas. Sci. Technol. 23 074017 (2012).[13] C. Diethold, et al. High precision optical position sensor for electromagnetic force compensated bal-ances. IMEKO TC3 Conference (2010).c©2014 - TU Ilmenau 6

Force measurements by strain gauge sensors as part of time-of-flight flow rate control

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Force measurements by strain gauge sensors as part of time-of-flight flow rate control

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