Project: Development of a Non-contact Angular Rate Sensor for Vehicle Navigation
Reference: CSU 28/4/67
Last update: 26/01/2010 17:39:14
The objective is to develop a low cost solid state angular rate sensor which can be used in vehicle guidance systems. The sensor will be developed to meet a number of transport applications, but in order to focus the work, one specific application, namely GPS, will be concentrated on.
To develop a low cost solid state angular rate sensor that could be used in vehicle guidance systems. The sensor would be a vibratory planar device, utilising non-contact detection methods. Research will be conducted into the ZnO piezo-electric material to be used, the choice of wafer substrate and micro-fabrication techniques. Work will also be undertaken on the theoretical design to maximise the detection signal and the surface acoustic wave device required.
Transense Technologies Plc
Building 77,, Cherwell Innovation Centre, Bicester, Oxfordshire, OX6 3HD
Cost to the Department: £111,379.00
Actual start date: 25 January 2001
Actual completion date: 13 November 2003
Summary of results
- Following detailed FE analysis and practical experimentation, the original cylindrical design for the structure was modified to maximise the induced strain levels at the point of instrumentation for the practical levels of excitation that could be expected using an acoustic coupled drive source.
The resultant structure consists of a slotted cylindrical section with eight prongs. To evaluate this new structure a number of prototype units were constructed from aluminium and Zerodur (a low temperature coefficient glass material).
Initial trials confirmed the accuracy of the FE mode land provided increased levels of strain at lower resonant frequencies.
With consideration to the instrumentation stage alternative fabrication techniques were adopted for the resonant structure including:
- Machining the slotted structure for a single piece of material.
- Machining an open ended slotted cylinder and bonding on different thickness top-plates were asked to consider designs with both 0.5 mm and 1mm thick tops plates.
The early experiments employing a piezoelectric sounder proved disappointing. The research team then adopted an electromechanical sounder with a resonant frequency (in the range 1.7kHz to 2.2kHz) more closely matched to that of the structure that was calculated to be 1.8kHz and actually measured at 1.839 kHz providing a very close correlation with the theoretical model.
- To increase the induced strain in the structure two sounders were employed to work in a push-pull configuration. The research team developed project specific electronics to enable careful phase and amplitude matching between the two sounders.
To evaluate the response of the structure under acoustic excitation, measurements of strain on the top surface of the structure were taken using semiconductor strain gauges mounted in a 14 bridge circuit. Both dual and single source excitation were tried. Strains in excess of 20 micro-strains in the radial direction and in excess of 15 micro-strains in the tangential direction were observed with single source excitation. Higher strain levels were achieved with dual sources. However, the results for the gauged structure clearly indicated an undesirable frequency split that was not present on the non-gauged structure.
To investigate the resonant response of the structure further with the absence of the frequency split induced by gauging, a capacitive sensor system was implemented. This involves a structure being metalised and employing a proprietary capacitive sensor system. The initial trials, although hampered by component failures, have shown positive results to the extent that the capacitive sensor can now be considered a viable 'fall back' option should the SAW device approach be deemed un-workable in the future.
Initial trials indicated that, if refined, the lift-off technique for the production of SAW devices would offer a number of advantages. Samples were produced and evaluated for acoustic transmission and found to exhibit the appropriate response between opposing IDTs. However, producing SAW devices onto Si unfortunately led to a number of problems associated with electrical leakage. Additional experiments using a process of producing ZnO depositions onto glass wafers with gold IDTs showed more promise.
Although a series of material and resonator parameters were obtained for the initial resonators, production of subsequent devices was compromised by a change in the photo resist material used. This prevented any useful under-baking causing severe adhesion problems. To progress a separate mask was developed for the base electrode and SAW devices with a base electrode beneath a thick ZnO layer we reproduced.
To further investigate the manufacturing process ZnOof various thicknesses was deposited onto silicon and Zerodur. Results indicated that SAW velocity decreased with increasing ZnO thickness, however, it was reported that a ZnO thickness of approximately 0.9 Áproduced optimum response.
Experiments with Zerodur were not as successful (possibly due to problems with mask alignment on the much smaller wafer) as those on silicon. Given the limited time left with the programme it was agreed that the team would concentrate on the use of silicon wafers for the remaining period of their research.
An extensive series of trials to establish the optimum process conditions for sputtering ZnO were conducted. The trials varied the substrate material, pressure and temperature under controlled experimental conditions and the uniformity of the resultant ZnO deposits analysed.
The study concluded that the optimum process conditions were:
Fused Quartz substrate
18 ÁBar = P = 23 ÁBar
Employing the optimum conditions a batch of sample SAW devices were produced. The static strain evaluations comparing the output of the SAW device with that of conventional strain gauges (devices bonded to a diaphragm structure that could in turn be stressed) indicated higher levels of sensitivity than would have been expected.
New mask designs have now been agreed which allow for a high level of redundancy in SAW selection. These masks have been submitted for manufacture to RAL.
Departmental Assessment Status: Project completed prior to implementation of Departmental Publication Scheme.