BIOSENSOR EXPLANATION

Back to Top

Mathematical Basis

A more thorough explanation of the Biosensor, including its mathematical basis, follows. The sensor consists of a piezoelectric transducer made of ceramics such as lead zirconate titanate (PZT) and a vibration pickup (made of PZT or ployvinylidene fluoride (PVF2) film). When an alternating voltage is applied across its electrode, as is well known, the PZT element is able to vibrate freely in the direction of its length.

The pickup detects a vibration that is generated in the rectangular PZT element. As the amplification is increased in this system, and since the pickup transducer detects this frequency and feeds a small alternating signal to a driving amplifier, the feedback circuit system oscillates at the resonance frequency. Hence, the driving amplifier always drives the PZT element at its resonance frequency. If the free end of the PZT element is pressed against a surface, as shown in this figure, the resonance frequency of the feedback system changes. This depends on the acoustic impedance of the object.

Such basic behavior of the Biosensor system can be explained and approximated in terms of the vibration mode of a finite rod as follows:

If the end of a finite rod is closed by the unknown impedance Z_x, at position l on its length, in general, the theoretical treatment of a vibrating rod is well known. After considering the resonance frequency for the loaded and unloaded conditions in the feedback system, the change in resonance frequency may be written as:

(1)

where V₀ is an equivalent velocity and Z₀ an equivalent impedance of the sensor system. b is the reactance of the impedance of the unknown object, Z_x, which may be expressed in the form:

(2)

where a is a resistance. The reactance, b, may be adequately written [5] as:

(3)

and

	(4)
	(5)

where r = density, n = Poisson's ratio, E = Young's modulus, S = pr² and r = radius of contact area. m_x is the inertia term and C_x the surface compliance, so the stiffness, k_x = 1/C_x. As is shown in eqns. (4) and (5), the inertia term, m_x, can be expressed as the third power of the radius r, and the stiffness, k_x, as the first power of r. If the contact area of the tactile sensor is r < 1.0, the stiffness term, k_x, will be larger than the inertia term, m_x. On the other hand, at r > 1.0, the stiffness term, k_x, may be neglected. Then the change in resonance frequency caused by the stiffness loading effect may be written as:

(6)

and for the mass loading,

(7)

where k₀ is an equivalent stiffness and m₀ an equivalent mass for an equivalent impedance of the sensor system.

In the above-mentioned principles, there will be losses occurring in the specimens that will contribute to the resistance, a. For example, wave attenuations take place within the medium and at the boundaries. However, such effects on the change in resonance frequency can be approximately included in the unknown impedance, Z_x.

Back to Top

Application and Data Display After starting a sample measurement on the computer, the user applies the Biosensor by gently pressing straight down (perpendicularly) on the sample for a few seconds. As shown in the figure, the electronic signals to and from the tactile sensor pass through an amplifier and filter before proceeding to a frequency counter where they are converted into digital signals. These signals are then sent to a central processing unit (CPU) where they are sequentially processed after which they are displayed on a computer monitor in "real time" and stored on the computer's hard disk. The recorded data are written in text format and are therefore easy to graphically display with analysis software.

Real time graphs run across the computer screen as the Biosensor is applied to the sample which appear similar to the ones in the diagram.

Back to Top

Data Interpretation

The tactile reading is expressed in Hz and is given as Df. As the tactile curve dips lower, this indicates a softer material. Hence, the softer the material, the more negative Df becomes. A less negative or a positive Df indicates a harder or stiffer substance.

The user can observe how the different amounts of pressure lead to corresponding changes in the tactile readings. Usually, a saturation point is quickly achieved, where increasing amounts of pressure do not significantly affect the tactile values. This is, of course, quite similar to the human sense of touch when used to gauge the stiffness of material. A particular value for stiffness, usually averaged from around the saturation point, can then be selected from the graph via the analysis software.

The Biosensor is generally used to determine stiffness values for comparative research; for example, the differences in softness among materials, or the location of hard substances within materials. As such, stiffness values in terms of Hz can be used for comparison and statistical analysis.

Due to its motor control, the Venustron System provides more data as it measures material while both pushing down on it and retracting thereby generating hysteresis curves.  These curves can be used to gain an understanding of the material's elastic properties.

Please see sample data and graphs from various research for a more in-depth understanding of the Biosensor's abilities.

Back to Top

1996 Axiom Co., Ltd.
22-2 Kashiwayama-machi, Koriyama-shi, Fukushima-ken, Japan
Tel: +81-24-962-0277 Fax: +81-24-962-0278 E-mail: info@axiom-j.co.jp