7 LVDT Transducers III

Vinay Gupta

epgp books

    Learning Objectives

 

 

In this module we will study about transducers and how they are used in measuring various external forces. Our objective is

  • To develop understanding about the transducers Look into instrumental design
  • It’s functioning based on the application.
  • In the current module we will study about load cell and piezo-electric transducers.

    Introduction

 

Transducers were developed for converting non-electrical quantity into electrical signal. This electrical signal is further processed by some electrical or electronic circuit that supplies output to a device for indication or recording purpose. Sometimes, transducers are used as primary transducers where input signal is sensed directly. In some cases transducer is used as a secondary transducer. Here a detector first senses the input signal and it output is supplied to secondary transducer as an input signal.

 

In previous modules we studied about LVDT, Inductive and Capacitive transducers. In this modules we will study about other types of transducers like load cell, piezoelectric and photoelectric transducer.

 

Load Cell

 

A load cell is also known as pressure cell. It’s a device that can covert weight (pressure) or mechanical force into electrical signal. It is widely used for measuring dynamic and static forces. The heart of this device is load (weight, force or pressure) receiving element which is a elastic element having high tensile strength. This element is bonded to strain gauge bridge network.

 

The output of the load cell is derived from the deformation of the elastic element made of homogenous materials like steel alloys. One should carefully select the material and its structural configuration so that one can have linear relationship between the dimensional change and the quantity (force) under measurement.

 

A desirable material should have following properties: –

  1. A fairly large elastic strain limit with a linear stress-strain relationship, say upto 5000 micro-strains.
  2. Repeated overloading should produce low strain hysteresis (less than 2 micro-strain)
  3. Very low creep of less than 5 micro-strain over long period of loading
  4. Very low plastic flow due to strain

    Based on the design of the load cell element, one can have 3 different configurations

 

(i) Column (ii) cantilevered bending beam and (iii) shear element. Let us study about these configurations one by one –

 

1.  Column

 

Figure 1. illustrates the a typical design for columnar type load cell. Such columnar type cells are usually employed in load cell having capacity of 2250 kg or more. It had 2 strain gauges called active gauges that are bonded axially. The other two additional gauges are called the Poisson Gauges and are mounted 900 to axially positioned gauges. The load cell shown in the figure 1 is used only under compression. It has a fitting at the bottom of the base for attachment to the support structure with a load-receiving button at the top.

 

 

Figure 1. Compression Load Cell

 

During the construction of the load cell, the load-receiving element is shrunk-fit into the base in a compression type load cell. The lower most section of the load cell housing, also known as Can, is welded on to the base structure. On the top of the lower Can, a diaphragm is welded to the edge of the Can and the center column. The upper can is welded to the connection between the lower can and diaphragm. This completes outer shell of the load cell structure. Finally, the upper lip of the top diaphragm and the central column is welded to the top diaphragm. This seals the inner portion of the load cell which makes it impervious to gas and moisture. All the wires from the strain gauge are carried out through a glass-to-metal seal located in the lower can wall and are connected to the external cable.

 

2. Bending Beams

 

Bending beams type load cells are used for measuring forces and weights below 225 kg. There few variations in design but the most common one is dual-guided cantilever beam, as shown in figure-2.

 

Figure 2. Dual Guided Cantilever Beam Load Cell

 

It had four strain gauges that are mounted at the corners of a stabilized rectangle. When load is applied to the free end of this dual guided cantilever beam, the strain gauges bonded to the element undergo resistive change. This change is proportional to the forces or the load applied. In such type of cantilever beam configurations, the two gauges experience tension and two compression.

 

3. Shear Element Configuration

 

In this configuration, one can have 4 or 8 gauges bonded to specifically designed element and is wired to form Wheatstone bridge. A centrally loaded shear beam configuration is shown in figure 3.

 

Figure 3. Centrally Loaded Shear Beam

 

The strain gauges used in this design differ from those used in columnar and dual-guided cantilever design. The strain gauges are designed in such a way that they get activated when they are placed under a shear force. Such elements have an advantage of have higher capacities in smaller size and have low sensitivity to side load error. i.e. when load is not placed proper orientation.

 

The selection of a load for a particular application depends on the following factors –

(1)  Required Accuracy (2) Scale capacity (3) Type of Loading tensile (4) Number of cells required (5) loading conditions (6) Environment (7) Space Available (8) Desired output characteristics.

 

Piezoelectric Transducers

 

The word “Piezo”, is derived from a greek word, ‘piezein’ which means to squeeze or press. Certain crystalline and ceramic material can generate a potential difference across the opposing faces of the material when subjected to external mechanical force. Such types of materials are Piezoelectric materials and this phenomenon is called Piezoelectric effect. This effect is also reversible, i.e. on applying potential across the opposing faces of the material one can get changes in the physical dimensions of the material. This principle of electro-mechanical energy conversion is used for developing energy conversion transducers.

 

Figure 4. Piezoelectric Transducer

 

The transducer with mechanical input and electrical output are used for measuring dynamic pressure, force, shock or vibratory motion. Piezoelectric effect can only be observed in crystals having asymmetrical distribution of charge. Due to relative displacement of positive and negative charges within the lattice one can observe contraction or expansion of the material. This displacement of internal charges produces potential different across the fitted electrode. This is illustrated in figure 4. Here electrodes are placed on the opposing faces of the crystals, where output voltages are collected for measuring an external force or pressure.

 

To better understand the piezoelectric properties of the transducing element in Piezoelectric Transducer let us consider a case of Quartz (SiO2) crystal. Figure 5 illustrates how opposing sides of the crystal get charged with application of external mechanical force. Under no load condition, positive and negative charges are arranged in center of the equisided triangular form.

Figure 5. Quartz Lattice Structure

 

The two opposing charges at the center point become neutral and no potential difference gets developed across the opposing face of the crystal. With the application of external force F, ions within the crystal get displaced within the structure and equi-legged triangles get deformed. This deformation of crystal lattice is shown in the figure 5b. Incase of application of tensile force, potential of opposite polarity is developed across the crystalline face adjacent to electrodes. Similarly, when piezoelectric material is under the influence of external electric field, crystal may contract or expand depending upon the polarity of the field applied.

 

The polarity & magnitude of induced surface charges is proportional to the direction &  magnitude of the applied external force. Therefore,

 

Q = d. F ——– (1)

 

where d is the crystal charge sensitivity in coulombs per newton (C/N) which is constant for a given crystal.

 

The force F brings a change in the thickness of the crystal by    t in meters

 

where A is the area of crystal in m2, E is the Young’s modulus of elasticity and t is the thickness of the crystal.

 

The charge at the electrodes gives rise to the output voltage Vout and is given by the following equation –

    where Cc is the capacitance in between the electrodes of the crystal.

 

As capacitance is given by

 

     Where A is the area of the crystal in m2, t is the thickness of the crystal and r is the relative permittivity.

 

Therefore, by solving and rearranging above equations we get

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Merits. Piezoelectric transducers are small in size and light. They are self-generating i.e. they do not need external power source. They operate over wide range of temperature, for quartz temperature range is of -200 to +300 0C, whereas for ceramic devices it is limited to +100 0C. Such systems have quite large outputs, for example, a quartz crystal of dimensions 2.5 mm (Thick) can have sensitivity upto 120 m V/kPa and with crystals having area of 1000 mm2, sensitivity is of 125 V/kN

 

Demerits. There output voltage generally gets affected by temperature variation of the crystals. When transducer is under constant deflection, it develops voltages across its terminals and slowly leaks of through its terminals. This decay of charge is very slow due to high leakage resistance of 1011 ohms. However this charge leakage is very rapid when a voltage measuring device is connected across its terminals. This prevents measurement of any static displacement of the measurand. In commercial systems, quartz element of high leakage resistance is used & input amplifiers of high impedance of 1014 ohms slows down the leakage allowing measurement of static displacements.

 

Applications of Piezoelectric Transducers

  1. These transducers are mainly employed for estimating forces and pressure. Due to their simple and robust construction they are used for measuring forces over a wide range i.e. from 1 N to 200 N having a linearity of ± 1%.
  2. Piezoelectric transducers due their high frequency response are mainly used in high frequency accelerometers. In operation their output voltage is typically of the order of 1-300 mV per g of acceleration.
  3. Quartz crystals can also be used as mass to frequency converter. A crystal controlled electronic oscillator has thin quartz plate. The frequency of electrical oscillations depends on the natural frequency of mechanical oscillation of the plate.
  4. Because of their rugged design and configuration they are employed for collecting data under challenging conditions. They are used in ballistics, blasts, internal combustion, fuel injection, flow instabilities, high intensity sound hydraulic or pneumatic pulsations in connection with problems associated with guns shock tubes, closed bombs, rocket motors, internal combustion engines, pumps, compressors, pipelines and oil exploration imploders.

    Summary

 

In this module we studied about transducers and how they are used in measuring various external forces. In the current module we studied about load cell, piezo-electric and photoelectric transducers. Our objective was to develop an understanding about the transducers and look into instrumental design and it’s functioning based on the application. One should note that almost all transducer design are based on the quantity to be measured. Depending on the application and objectives of measurement one should select a transducer most suitable for that investigation.

you can view video on LVDT Transducers III

References :-

  1. Electrical and Electronic Measurements and Instrumentation, Sawhney A. K., Dhanpat Rai & Sons, Reprint 1985
  2. Measurements and Instrumentation, Bakshi U.A., Bakshi A.V., Technical Publications, 2009
  3. Principles of instrumental analysis, Skoog, Douglas A., F. James Holler, and Stanley R. Crouc,. Cengage learning, Edition 2017
  4. Instrumentation, measurement and analysis. Nakra, B.C. and Chaudhry, K.K., Tata McGraw-Hill Education, 2003.
  5. Measurement and instrumentation: theory and application, Morris, A. S., & Langari, R. , Academic Press, 2012