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Strain gageli kuvvet transduseri dizaynı

Design of force transducer with strain gage

  1. Tez No: 21715
  2. Yazar: SİNAN FANK
  3. Danışmanlar: PROF. DR. AYBARS ÇAKIR
  4. Tez Türü: Yüksek Lisans
  5. Konular: Makine Mühendisliği, Mechanical Engineering
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 1992
  8. Dil: Türkçe
  9. Üniversite: İstanbul Teknik Üniversitesi
  10. Enstitü: Fen Bilimleri Enstitüsü
  11. Ana Bilim Dalı: Belirtilmemiş.
  12. Bilim Dalı: Belirtilmemiş.
  13. Sayfa Sayısı: 134

Özet

ÖZET Bu çalışmamda, kuvvet ölçümlerinde en yaygın olarak kullanılan kuvvet transduserleri ( load cell-yük hücresi ) hakkında genel, dizaynı ile ilgili temel bilgilere yer verilmiştir. Birinci bölümde, ilk olarak kuvvet birimleri, kuvvet standartları ve kuvvet ölçme metodları hakkında genel bilgiler verilmiştir. Daha sonra kuvvet transduseri çeşitleri, kuvvet transduseri veya load cell yapımında çok önemli bir yeri olan strain gage'ler ve çeşitleri, şekil değişimi ( strain ) ile gerilme arasındaki bağıntılar, dirençli tip strain gage'lerin yapıları, çeşitleri, malzemeleri, yapıştırıcıları, şekil değişimi ile elektriksel direnç arasındaki bağıntılar, strain gage'in elektriksel devresi ( wheatstone köprü devresi ), load cell'lerde oluşabilecek hatalar ve load cell kalibrasyonu ile ilgili bilgilere yer verilmiştir. İkinci bölümde ise, kuvvet transduseri veya load cell dizaynında ele alınacak konular ve dikkat edilmesi gereken hususlar anlatılmıştır. Bunlar araasında, şekil değişiminin meydana geldiği ve strain gage'in yapıştırıldığı yay elemanları ve çeşitleri, yay elemanı malzemeleri, transduserin strain gage sistemi, transduser tamamlama devresi, başlangıç ve çıkış ayarları ile sıcaklıktan dolayı oluşan kaymaların kompanzasyonu için kullanılan yardımcı dirençlerin bağlanması ve seçimi ile ilgili bilgilere yer verilmiş, son kısımda da temel transduser dizaynları ile ilgili basit teorik yaklaşımlar ve elektriksel çıkışlarının şekil değişimi ile olan ilişkileri ve sıcaklık ile kablo hatalarının hesaba katılması hakkında genel bilgiler verilmiştir.

Özet (Çeviri)

SUMMARY DESIGN OF FORCE TRANSDUCER WITH STRAIN GAGE Force is often measured for quality control and control of manufacturing machines and devices in production processes at manufacturing plants 'n addition to the measurement of strength of material and stress analysis of structures which are closely connected with safety in the field of engineering. Force is defined as the physical quantity acting on mass to accelerate it and the quantity that is called under the name of force. Load and weight has the same pysical unit also. Strain is a fundemental engineering phenomenon. It exist in matter at all times, due to external loads or to the weight of matter itself. Scientist and engineers have worked for centuries in attempt to measure strain accurately. The terms strain is used in engineering, refer to change in any linear dimension of a body, usually due to the application of axternal forces. Strain is given below equation; AL = total deformation ( the variatio of length ) L0 = original length Strain gage is one of the most important tools of the electrical measurement technique applied to the measurement of mechanical quantities. As their proper name indicates they are used for the measurement of strain. As a technical term strain compises of tensile and compressive strain, distinguised by a positve or negative sign. The strain gage based force transducer has been an important engineering tool since shortly after the invention of strain gage in the late 1930's. Broadly -VII-speaking, a transduser is a device which transforms on type of energy into another. At this mean, an ordinary glass thermometer is therefore a transducer ( heat energy converted into mechanical displacement of a liquids column ). We concern ourselves here with a specific class of transducers : devices which translate an input of mechanical energy into equivalent electrical signals for measuring and/or controling the input phenomena. This type of electromechanical transduser is usually located at the source of the pysical force or energy and respond the its magnitude. The readout or control instrumentation can then be positioned at any convenient distance from the transducer and connected to it by electrical wiring. Common examples of such transducers are used to measure fluid, pressure, acceleration, displacement, torque.etc. The word cell is often used for convevience to describe a compact transducers ( i.ejoad cell, pressure cell) Transducer can be manufactured on many different operating principles ( resistive, inductive, capacitive, piezoelectric etc.). The bonded metallic resistance strain gage, however, because of its unique set of operational characteristics, has easily dominated the transduser field. True orgin of strain gage transduser is lost in history. Lord Kelvin reported strain induced resistance change of electrical wires in the 1800's with below formula; R = p^ (2) A The resistance of any conductive path is a function of path length L, the path area A and specific resistance of material p. From this equation we can obtain gage factor which will be mentioned further section. In transducer design, strain and stress are very important quantities. Mechanical tension a 's significant for the stress materials. a = mechanical stress F = applied force -VIII-A = cross sectional area of material E = modulus elastisity of material a=4 A Mechanical stresses in materials cannot be practically measured directly on the spot. Yet strain in materials could be measured. Strain is present whereeverare in materials. Stra'n can be measured on surfaces. The basis for measurement of mechanical stresses via the strain measurement is formed by Hook's law which is valid for the unidirectional stress condition. e = strai E = modulus of elasticity eJE (4) All types of transducers are based on this principle. The principle of function can easily be explained by using a simple transducer for force measurement as an exaple. Therefore we can write below equation by using (3) and (4) formulas ; F / A = e. E or also F = e. E. A (5) The modulus of elasticity E and the cross-sectional area A are constant, The strain changes depends on the force F. In strain gage, the resistance R of on JO electrical conductor changes with ratio A- if it is stressed mechanicaly in Ro AL such manner that its length changes by a factor ® - -. The change in resistance depends partly on the change of the geometry of the conductor, and partlty on the change of specific conductivity p of the conductor material and poisson ratio \l due to changes of the material structure texture. This process is described by the following equation. -IX-AR,+ 0 dR0 i -- =e(1+2n +--.-) (6) i?0 tfe p If we know \ı and p values for best known materials, we can write down general simple relation, - = ke (7) The proportinate factor k is called gage factor, k value is given generaly on strain gage peckage by manufacturer. We know that strain gages-wiH- transform mechanical quantity“strain”in to electrical quantity“ variation of resistance ”. Both quantites are directly proportional: AR ro To detect variatin of resistance dueiQ strain, suitable electrical methods should be applied. The most important method which is almost always used, is the Wheatstone bridge circuit. The simplest form of the bridge circuit consist of four arms with resistors R1, R2, R3,fl4.Eşeh resistor can be strain gage. An output voltage Ua is proportional to bödge energising voltage Ue ( or exication voltage). We find that v^=- jRl+Ai?1 - *4+A*4 UE R^+AR^+Rz+ARz R^+AR^+R^ARa Usually all four resistors are equal : R1 = R2 = R3 = R4. Further conditions are that the source impedance of the bridge energizing generator is very large the impedance of the indicating unit is very large. Then the simplified equation is valid as follows:UE 4{ *, ^ Rz R4 AD Substituting =k.e we find: İ2 F=-^=4'(el-e2+G3+e4) 0) ^ Many types of load cell is exist in use. These are magnetoelastic, capacitive, inductive, piezoelectric type, strain gage type, hydraulic type, etc. But among these strain gage type load cell is widely used in serve. Although the strain gage type load cell was developed and commercialized about 40 years ago, its principal area of application was until recently, industrial - i.e in process control heavy machinery, test engineering and the like over the past decade, however, the load cell has invaded the weighing field. With the the need for scales having electrical output signals to operate the popular digital displays, and to interface with microprocessrs and computers, the strain gage load cell has come to represent the most practical weighing means. Electronic scales, on board weighing for trucks, based on strain gage load cells are now commonplace. Design of strain gage based force transducer many of criteria are considered, these are: a) Trancduser spring elements b) Spring elments materials c) The strain gage system of transducer d) Completing the basic transducer, adjustment and compansation of tempereture errors. The most critical mechanical component in any load cell, or strain gage transducer, is generaly spring elements. The function of spring element is to -XI-serve as the reaction for the applied load; and the focus the effect of the load into isolated, preferably uniform, strain field where strain gages can be placed for the load measurement. The strain level in the gaged area of the spring element respond in a linear-elastic manner to the applied load. Load cell spring elements will be divided into three classes according to the type of strain field used.These are: a) bending, b) direct stress, and c) shear, different forms of all three types are used contemporary transducer practice. Spring should exhibit the qualities of a high precision spring. These include, linearity in deflection versus force, low hysterisis, low creep and low stress relaxation. Since these are very important on transducer performance and cost, selection of the transducer spring element material deserves very careful attention. The selection process involves three main groups of considerations; namely the mechanical proporties of the material, its thermal proporties, and a number of characteristics which can be referred to collectively as manufacturing consideration. After design consideration and material selection for load cell spring elements, is done, the strain gage system for transducers should be selected and installed with the utmost care. The gage selection procedure for transducer application is sequence below : 1) Gage length 2) Pattern geometry 3) Gage series 4) Grid resistance 5) Optional features After strain gage have been selected, suitable adhesive for strain gage and transduser should be selected. Improper selection or application of adhesive willl surely degrade transduser performance, regardless of the quality of strain gage or spring materials. Finally, to arrive, at a copleted load cell which is capable of reliably accurate measurements. There are several other areas of transducer technology that require carefull consideration and meticilous implementation. These include gage-circuit wiring and soldering; bridge output adjustment and compensation, nad finally environmental protection of sensitive circuit elements. To achive. -XII-required accuracy, it is necessary to introduce auxilary or suplemantary resistors into circuit as means for adjusting the circuit output and compensating for the effects of temperature changes. There are four principal aspects of transducer behavior that commonly require such“tuning”: 1) Zero shift with temperature change, 2) Initial zero balance, 3) Span shift with temperature chance, 4) Initial span adjustment Expanded describtion and some calculation of these topics will be given in the following sections. -XIII-

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