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The Technology

Vectron capitalizes on a solid foundation of bulk and surface acoustic wave technology as well as the associated electronics expertise integral to Vectron's Frequency Control Products. Furthermore, Vectron's broad base of experience in RF, signal conditioning, embedded system electronics design and advanced packaging technologies allows Vectron to succeed in demanding industrial application environments.

Image of a Sensor Element which is the core of the products used in some of SenGenuity's impressive product solutions
BAW Technology
BAW Technology
Cross-sectional view of the sensing element and the displacement profile Packaged sensing element to allow one surface of the sensor to interact with the target fluid
The sensing element (Figure 1a) consists of a thin piezoelectric bulk acoustic wave (BAW) plate that operates in the thickness shear mode of vibration (TSM). As shown in the figure, the displacement profile is throughout the thickness of the plate and maximum at the surfaces. Because the displacement is parallel to the surface of the plate, and the fluid interface, it makes the TSM BAW sensing element an ideal solution for measuring the viscosity of a fluid. Just like many other lab based methods of viscosity measurement, motion applied to the fluid is shear; however, unlike many other methods of viscosity measurement, the displacements are atomic scale and at a frequency of 5.25 million cycles per second. The sensing element is packaged to allow one surface to interact with the fluid under test (Figure 1b).

The SenGenuity ViSmart® viscosity sensor is a commercially available, robust, reliable and cost–effective threaded–bolt viscometer for integration into in–line, real–time monitoring and process control systems for scalable applications (Figure 2).

Cabled viscosity sensor Connectorized viscosity sensor
The sensor has no moving parts (other than the atomic scale vibration of the surface) and, due to the high frequency of the vibration, several millions of vibrations per second, is independent of flow conditions of the liquid and immune to vibration effects of the environment. High temperature electronics are utilized that allow a very wide operating temperature range for the sensor.

The importance of these acoustic sensors lies in the distinctly different measurement principle. Whereas one class of mechanical devices measures kinematic (flow) viscosity and the other class measures intrinsic (friction) viscosity, the AW sensors measure acoustic impedance, (ωρη)1/2, where ω is the radian frequency (2πf), ρ is the density and η is the intrinsic viscosity.

The viscosity measurement is made by placing the BAW TSM resonator in contact with liquid. The liquid's viscosity determines the thickness of the fluid hydro–dynamically coupled to the surface of the sensor. The sensor surface is in uniform motion at frequency, ωρ=2πf, with amplitude, U. The frequency is known by design and amplitude is determined by the power level of the electrical signal applied to the sensor. As the shear wave penetrates into the adjacent fluid to a depth, d, determined by the frequency, viscosity and density of the liquid as d=(2η/ωρ)1/2, as depicted in Figure 3.
Hydro-dynamically coupled fluid layer on the surface of the BAW viscometer with a shear wave penetration defined by the penetration depth d.
Figure 3: Hydro–dynamically coupled fluid layer on the surface of the BAW viscometer with a shear wave penetration defined by the penetration depth d.
Acoustic viscosity is calculated using power loss from the piezoelectric resonator into the fluid. The unit of measure is acoustic viscosity (AV) and is equal to ρη, (g/cm³ o cP) or ρ²ν, ((g/cm³ )²o cSt). Acoustic viscosity is thus equal to density times the dynamic viscosity or density–squared times the kinematic viscosity.

The acoustic wave resonator supports a standing wave through its thickness. The wave pattern interacts with electrodes on the lower surface (hermetically sealed from the liquid) and interacts with the fluid on the upper surface. The bulk of the liquid is unaffected by the acoustic signal and a thin layer (on the order of microns) is moved by the vibrating surface. As shown in Figure 4, for the SenGenuity bulk acoustic wave viscometer operating at 5.25MHz, the penetration depth into the fluid is ideal for measuring the viscosity of homogeneous fluids like lubricants and the viscosity measurement will not be susceptible to large particles or debris because the small penetration depth makes them virtually unnoticeable.
AW penetration depth (um) vs. kinematic viscosity.  This graph assumes an operating frequency of 5.25MHz and a constant density of 0.850g/cm3.
Figure 4: AW penetration depth (um) vs. kinematic viscosity. This graph assumes an operating frequency of 5.25MHz and a constant density of 0.850g/cm³.
Image of a SAW based waver which is the core of the technology used in some of SenGenuity's impressive product solutions
SAW Technology
SAW Technology: Rayleigh Surface Acoustic Wave Delay Line

Starting with the Rayleigh surface acoustic wave (SAW) delay line, we can see that propagating wave is confined to the top surface of the substrate. For a particle on the surface of the substrate, the propagation of the Rayleigh wave will cause the particle to experience a vertically aligned elliptical motion. Because of this, the SAW is a very sensitive probe for measuring mechanical and electrical properties on its surface. We also note that since there is a vertically polarized displacement, the Rayleigh SAW can only be used for gas sensing or physical sensing applications. Putting the SAW in an aqueous environment will result in the SAW being completely damped out due to energy loss into the liquid.
Zoomed in outline drawing of the surface displacement on a SAW surface substrate Outline drawing of the sideview of the delay path on the SAW surface substrate
The Rayleigh SAW is sensitive to mechanical and electrical properties occurring on its surface. For mechanical properties, they are sensitive to mass loading and visco–elastic changes like stiffening and softening. For electrical properties, the devices can be sensitive to any property that interacts with the electrical field that is coupled to the propagating acoustic wave. This effect has been given the term electro–acoustic interactions. The Rayleigh SAW is also sensitive to stress or strain coupled into the SAW substrate through the packaging. Because of this, Rayleigh SAW devices make great platforms for torque and pressure sensing applications. Rayleigh SAW devices can also be tailored with special cuts of piezoelectric substrate to create a very linear SAW frequency versus temperature dependence. The result is a very high resolution temperature sensor.