Quartz Crystal Microbalance
The Quartz Crystal Microbalance (Qcm/Qmb) is an very sensitive mass sensor, capable of measuring mass changes in the nanogram range [1].
Pressure Sensor Transducer
Qcms are piezoelectric devices fabricated from a thin plate of quartz with electrodes affixed to each side of the plate.
A Qcm-D (Quartz Crystal Microbalance with Dissipation monitoring) consists of a thin quartz disc sandwiched in the middle of a pair of electrodes.
Due to the piezoelectric properties of quartz, it is potential to excite the crystal into oscillation by applying an Ac voltage over the electrodes. Changes to this oscillation are directly proportional to mass changes on the crystal [1].
Various sorbent coatings can be used on the crystal covering in order to add element of selectivity to the sensor [2]. A number of distinct types of sensor control under similar basic principles, such as "Bulk Acoustic Wave (Baw)" and "Surface Acoustic Wave (Saw) sensors". Both sensors want an A.C. Voltage for configurations/operation. Baw sensors use the galvanic field in order to excite the quartz crystal to oscillate, and Saw sensors use wave propagation on the covering sensor [1].
a. Manufacturing Process
After being cut along inevitable crystallographic axis, the thin plates of the single piezoelectric crystal quartz are covered with thin gold electrodes on both sides [4].
The two sides of the crystal are then coated with polymer films. The coating technique could be any of the following [4]:
- Spray coating.
- Growth of Langmuir-Blodgett films.
- Self-Assembled Monolayers (Sams).
The coating will supply the conductivity and changing of mass.
b. Sensing Mechanism
The Qcm is basically a thin quartz wafer with electrode pads on each side [5].
The Qcm oscillates mechanically, when connected to an amplifier.
At the same time the amplifier oscillates electronically, with a inevitable frequency.
On the covering of the Qcm there is a coating of a sensitive chemical. Exposure of which to analyte vapour, cause the molecules of the analyte inter into the coating. The result will be an increase in mass, which causes a slowing in the frequency of oscillation.
Qcm are very sensitive to any wee changes in their mass, and for this presume the Qcm can measure changes in its frequency to 1 part in 108 [5]. normal operating frequencies are in the range from 10 to 30 Mhz. [4].
Surface Acoustic Wave Sensors (Saw)
As in the Qmb (i.e. Qmc) this sensor is based on the same principle i.e. When mass changes, frequency changes. The expedient utilises covering acoustic waves, with a frequency of about 600 Mhz [4].
a. Manufacturing Process
Two inter-digital transducers (Idt) are usually made up from thin metal electrodes and fitted on "a polished piezoelectric substrate", settled in the centre and enclosed by resonators [4].
The wavelength is carefully by the spacing of the Idt fingers.
One of the Idt surfaces will enlarge and ageement when an alternating current applied to it. The movement of the covering generates a wave (some scientists/researchers call it a "Rayleigh Wave"), which will pass through the substrate. A frequency counter settled in the Idt receiver will then article the frequency of the wave.
To minimise noise and temperature, as well as lower the frequency to be measured, a dual Saw set up may be constructed, and therefore, the reference signal from the Saw (uncoated) will be mixed with the sensor signal.
b. Sensing Mechanism
The physical properties of the covering can affect the wavelength/frequency of the covering wave itself. A thin layer of polymer coats the substrate, which is settled in the middle of the two Idts. The absorption of gas changes the mass of the polymer, and consequently the properties of the sensitive layer. The covering wave is not just affected by the convert of mass; it is affected by other factors, such as temperature, pressure, dielectric constant and viscosity.
Smart Sensors
Smart sensors are naturally sensors with microprocessors attached to them. When it comes to a theory design, a smart sensor can be:
Easier.
Cheaper.
More dependable and more scaleable.
Higher performance.
More rapid to design.
Obviously, these benefits are all obtained when microprocessors or computing resources are embedded on the sensor. Therefore, the processing of data is performed on the spot i.e. Within each individual sensor, instead of using a central theory controller. In addition to this, ordinary sensors production raw data; but only useful data is produced by a smart sensor. Many of the smart sensors can be really programmed and/or reprogrammed, thus rescue time and expense.
The feasibility of using such kind of sensors in any Mod depends on how small the expedient will be and on the final application(s), as well as the final cost of the expedient itself.
Najib Altawell
References
[1] Lee-Davey, J., (2004) "Application Of motor Olfaction theory For The Detection Of High
Voltage Transformer Oil Degradation"Cranfield University.
[2] Perera, A., Sundic T., Pardo A., Gutierrez-Osuna R., Marco S., (2002)"A conveyable Electronic Nose Based on Embedded Pc Technology and Gnu/Linux: Hardware, Software and Applications"
Ieee Sensors Journal, Vol. 2, No. 3, June 2002 235
[3] K. Persaud, G. Dodd, Nature 1982, 299, 352-355.
[4] Nose Office (2003) "Nose Ii - The Second Network on artificial Olfactory Sensing" University of Tuebingen Dec 2003 - Germany
[5] Finklea, H. O., lecture notes ( ) "Gas Phase Sensors"
Department of Chemistry West Virginia University
Morgantown, Wv 26506-6045.
© Altawell 2008
machine Olfaction gadget (Mod) Sensors (Part Three)