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Comparing Strain Gauges To Piezoelectric Sensors

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Preface

Various industries have a need for weighing or measuring forces as part of performing their principal tasks. Strain gauges and piezoelectric sensors are two common transducer technologies that serve these industries.

Both transducers are widely used for almost the same applications. However, differences exist between these sensors. For one, performance factors differ such as resolution, accuracy, and susceptibility to environmental errors. Secondly, utilization factors such as ease of measurement, signal transmission, equipment interfaces, and physical characteristics distinguish them.

This article compares and contrasts these two types of sensors. Its goals are to provide information to system designers in selecting components for various applications, and also to provide general information on how these systems relate to instrumentation and control systems.

Principle of Operation

Strain Gauges

A strain gauge sensor works by transforming an applied force to an electrical signal through elastic deformation of the strain gauge (see The Versatile Strain Gauge Load Cell for more information). These forces can be static or dynamic in nature. Examples include weight, acceleration, and pressure. Internal to the gauge, this deformation in turn changes the dimensions of the strain gauge material. The result is a change in the material’s electrical resistance proportional to the magnitude of the applied force.

The mathematical formula that relates the changes in dimension to change in resistance is shown below:

Where:

\large R\pm \Delta R=\rho \cdot \left ( \frac{L\pm \Delta L}{A\mp \Delta A} \right )

R=Resistance\, units=ohms\, (\Omega )

\rho =Resistivity\, constant\, in\, bar\, units=ohm\, meter\, (\Omega m)

L=Length\, in\, meters\, (m)

A=Cross\, sectional\, areas\, in\, meters^{2}\, (m^{2})

\Delta R=Change\, in\, resistance\, ohms\, (\Omega )

\Delta L=Change\, in\, length\, of\, meters\, (m)

The delta in resistance is then measured by transforming it to an equivalent change in voltage. A Wheatstone bridge setup is the most commonly used technique for a change in resistance to change in voltage transformation. The strain gauge(s) is/are included in one or more arms of the Wheatstone bridge so that changes in its resistance under physical loading causes a change in the voltage at the output terminals.

Quarter-Bridge Configuration

The mathematical formula for a quarter-bridge configuration, shown in Figure 1, is expressed below. Here dR is the change in resistance of the strain gauge:

\large V_{o}=\frac{V_{s}dR}{4R}

Figure 1. Quarter-Bridge Strain Gauge Configuration

This analog output voltage is very small (mV), hence it is always passed through a signal conditioning circuit that amplifies and filters it.

Other Configurations

Half-bridge and full-bridge Wheatstone Bridge configurations also exist. These are discussed in more detail in the articles, The Versatile Strain Gauge Load Cell and All About Electrical Connections Of Force Transducers.

Piezoelectric Sensor

A piezoelectric sensor makes use of a piezoelectric material that transforms the quantity to be sensed into an electric charge. The magnitude of the applied load is proportional to, and therefore determined by, the quantity of this electric charge.

The piezoelectric material acts like a mechatronic component: it deforms elastically and mechanically under the influence of the applied load, which causes a disorientation of the dipoles within its crystalline structure. An electrode that is appropriately connected across the surfaces of this piezoelectric material will sense the movement of electric charges. This is shown in Figure 2 below.

Figure 2. Piezoelectricity Generation

The mathematical formula for the amount of charge produced is expressed below:

q=a\cdot F\cdot K_{s}

where q is the electrical charge generated by force, F (in Newtons) applied across the faces of a piezoelectric device with a mechanical compliance of spring rate K_{s}, in dimensions of mN^{-1} and a more complex material constant, a, of dimensions C m^{-1}.

A charge amplifier then detects the electrical charge between the electrodes as either a charge source or a voltage source, depending on the amplifier design. The former is most commonly preferred as it involves the use of an operational amplifier. This design helps overcome the effects of stray cable, sensor, and amplifier capacitances.

There are several advantages to charge amplifiers. With modern microelectronic techniques, the charge amplifier can be embedded inside the transducer to shorten the gap between the electrode and the amplifier. Furthermore, the internal resistance of piezoelectric material can be very large, causing charge leakages that decay over periods of time; the charge amplifier compensates for this by shortening the decay period. The voltage output of the amplifier can then be processed by analog or digital techniques to indicate the sensed quantity. If you’re looking for a charge amplifier, check out our AnyLoad A2P-D2 Load Cell Amplifier.

Also, a piezoelectric sensor can act as a piezoelectric actuator in a process called the reverse piezoelectric effect. This effect occurs when a supply of alternating voltage to the sensor makes the piezoelectric material start to vibrate.

Other Resources:

Material of Construction

Strain Gauges

Strain gauge sensors use the strain gauge element as the underlying mechanism. The types of construction for these include thin-film, foil, and semiconductor strain gauges. A conductive material is bonded to a thin backing. This conductive material is usually copper-nickel, nickel chromium, platinum-tungsten alloys, and silicon for semiconductor gauges. The backing is non-conductive, and usually some form of polymer.

Piezoelectric sensors

Piezoelectric sensors use a piezoelectric material as its underlying mechanism. These materials can be naturally occurring crystals like Rochelle salt and quartz. They can also be synthetic. Synthetic materials generally fall into two types: (1) crystalline such as Lithium Sulphate and Ammonium Di-Hydrogen phosphate, and (2) polarized ferroelectric ceramics such as Barium Titanate.

Method of Fabrication

Strain Gauges

Bonded strain gauges are fabricated by attaching a length of a conductive strip to a thin backing. This is usually done through a process called micromachining. This process usually follows the following steps: (1) the conductive material is deposited onto a thin film, (2) a “patterning” process arranges the conductor in a grid pattern, and then (3) the fused materials are etched to remove the patterned parts, leaving the conductive wire.

The strain gauge is then carefully bonded to the surface of the sensor’s structural element with adhesives or other bonding agents. Proper bonding requires preparation of the surfaces. This involves the steps of cleaning, smoothing, roughening, and marking. The bonded strain gauge is then hermetically sealed to protect it against external mechanical and chemical damages.

It should be noted that there are also non-bonded strain gauges, but these are not as commonly used. The bonded strain gauge is preferable as it is more compact and easily embedded in the sensor’s structural element.

Piezoelectric Sensors

Synthetic piezoelectric materials like ferroelectric ceramics have randomly oriented internal electrical dipoles within their crystal structure. Therefore, fabricating sensors with these materials requires first polarizing them. This process is called poling. Poling happens by first heating the potentially piezoelectric powder material to a temperature level higher than the Curie point. The ferromagnetic properties of the crystal break down at this Curie point temperature. The next step to polling is then to apply a strong DC electric field of several kV/mm to the heated material and allow it to cool under this field. The result is the polarization of the material: the redistribution of the dipoles in the direction of the applied field. Hence a strong piezoelectric property is formed.

After cooling the electric field is removed and the material maintains the dipole orientation. Figure 3 below illustrates this process.

Figure 3. Poling Process

The most important thing to consider during poling is the geometry of the crystal, especially when applying the electric field; this greatly affects the sensitivity of the material.

Natural piezoelectric materials like quartz simply need to be cut by very fine precision tools to the required dimension that fits the application. However, these natural single crystal materials have very bad crystal stability and a limited degree of freedom.

The polarized material is later supported by a substrate which could also serve as the outer electrode of the piezoelectric sensor. The material is fixed such that the direction of the applied deforming stress is perpendicular to the direction in which charges are generated; that is perpendicular to the location of the electrodes’ placement. This is shown in Figure 4 below.

Figure 4. Direction of Stress and Direction of Movement Charges

Geometric Shapes and Designs

Manufacturers often categorize strain gauge sensors by the geometric shape of the structural housing unit to which they are fixed. These housing units can be in the form of a beam, S-shape, disc canister, or planar beam.

The design of piezoelectric sensors also includes housings in these shapes. The geometry of any sensor depends on the application requirements.

Both types of sensors can be used for multi-axial applications, and can support compression, shear, or bending stresses. Piezoelectric sensor designs are more compact and are more rugged in construction than those of strain gauges.

Sensor Characteristics

Original equipment manufacturers provide data sheets with the electrical specifications of their transducers. Some of these characteristics appearing on the data sheet include:

  • The Force Range: This is the rated capacity (minimum to maximum accurately detectable force) in Newtons. The force range of the piezoelectric sensor is typically 5KN to 1MN while the strain gauge sensor typically has a range of 5N to 40MN.
  • Loading Conditions: Piezoelectric load cells can only support dynamic loads such as vibrations, accelerations, and dynamic pressure measurements; strain gauge load cells can support both static and dynamic loads.
  • Creep: Strain gauge sensors have very low and insignificant drift in output when subjected to a load for a long time; piezoelectric sensors have very large drift in output value which results in errors for long time measurements.
  • Stiffness: Piezoelectric sensors have very high stiffness value.
  • Resonant Frequency: Unlike strain gauge transducers, piezoelectric sensors have a higher resonant frequency due to their stiffness. This value can be as high as 100,000Hz.
  • Sealing: Both sensors types are designed to have very excellent seals which offer a high degree of protection and operational safety in harsh environments. The most common sealing technique is the hermetic seal.
  • Temperature Effects: Both strain gauges and piezoelectric sensors are very sensitive to temperature changes in that they affect the zero-balance, sensitivity, and linearity. Because of this, manufacturers use various compensation techniques: strain gauge sensors use self-temperature compensating gauges; piezoelectric sensors adjust for temperature effects with a charge amplifier.
  • Repeatability: Both sensors can be designed to achieve excellent repeatability, or agreement between the results of successive measurements.
  • Linearity: Strain gauge sensors have a lower linearity error in comparison to piezoelectric sensors.
  • Sensitivity: This is the rate of change of the output as the desired input varies.  The sensitivity of the piezoelectric sensor depends on the material used and its geometry; it is rated in Pico Coulombs per Newton (pC/N). A strain gauge transducer’s sensitivity depends on the excitation voltage and the rated capacity value; it is rated in millivolts per volts (mV/V).

The values of these and other characteristics help designers choose the appropriate sensor for the application, and technicians calibrate the resulting measurement system.

Conclusion

Both strain gauge and piezoelectric force transducers are of undeniable importance to many force measurement applications. The choice between them depends on the application’s requirements. Manufacturers’ data sheets give the transducer’s input and output characteristics used in making the design choice.

Piezoelectric sensors offer excellent dynamic measurements, especially where compact size is required. Strain gauge transducers are more commonly used and offer excellent dynamic and static load measurements over a wide range of applications.

References

  • Piezoelectric Sensor, A Method for manufacturing a piezoelectric sensor and a medical implantable lead comprising such a sensor, US 8,626,313 B2
  • Comparative look at strain gauge and piezoelectric sensors by JAC Chapman, Elexsys
  • The Instrumentation Reference Book, Edited by Walt Boyes.
  • Jayant Sirohi, Inderjit Chopra , “Fundamental Understanding of Piezoelectric Strain Sensors,” Journal of Intelligent Material Systems and Structures.
  • Force Measurement Glossary by Tacuna Systems.
  • An Overview of Load Cells by Tacuna Systems.
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