Prof.ir. H.M.J.R. Soemers
Prof.dr.ir. J.B. Jonker
Prof.dr. M.C. Elwenspoek
Dr.ir. D.M. Brouwer
Dr.ir. J. Haneveld
Dr.ir. R.J. Wiegerink
Dr.ir. T.S.J. Lammerink
Dr.ir. A. Mehendale
July 2008- May 2010
mass flow sensor, coriolis sensor, MEMS, MST
A micromachined micro Coriolis flow sensor has been fabricated consisting of a silicon nitride resonant tube of 40 μm diameter and 1.2 μm wall thickness. First measurements with both gas and liquid flows have demonstrated an unprecedented mass flow resolution in the order of 10 mg/hr at a full scale range of 1 g/hr. The Coriolis type flow sensor consists of a vibrating tube being driven at a resonance frequency of about 2 kHz by Lorentz actuation. By itself, the oscillating tube has a periodic deflection corresponding to this eigenmode. A mass flow inside the tube results in Coriolis forces that cause a secondary deflection mode, different than the actuated eigenmode. The extent of this secondary deflection is a measure for instantaneous mass flow rate inside the tube. Figure 1 shows a schematic drawing. As the Coriolis force is proportional only to the angular velocity and the mass flow rate, the Coriolis sensor is insensitive to flow profile, pressure, temperature and properties of the fluid (density, viscosity, etc.).
Figure 1: Rectangular-shaped Coriolis flow sensor (ω is the torsion mode actuation vector, Fc indicates the Coriolis force due to fluid flow, which causes a secondary ‘flapping’ mode).
Figure 2: Photograph of the chip. The rectangular-shaped resonant tube size is 2.5 x 1.5 mm.
Figure 3: Chip mounted in the holder with fluidic and electrical connections and bar magnets for Lorentz force actuation.
A technique for realizing the resonant tube has been developed at the TST group; this involves creating a thin-walled buried channel in silicon nitride, and etching part of the formed channel (tube) free. Contrary to , , the deposited/grown silicon nitride tube has a small wall-thickness. Therefore the mass of the tube in relation to the mass of the moving fluid is small, increasing the resolution. Figures 2 and 3 show photographs of the realized sensor chip and package. The sensor structures were fabricated using the surface-channel technology described in  and outlined in Figure 4. Figure 5 shows a close-up of the silicon nitride sensor tube. The function of a thus fabricated sensor has already been demonstrated with several fluids. Figure 6 shows an image of a resulting vibration mode. Point P (see Figure 6) has been monitored by a laser vibrometer to measure the out-of-plane displacements of the tube due to the Coriolis force.
Figure 4: Outline of the fabrication process
Figure 5: Photograph of the silicon nitride resonant tube entering the silicon support.
Figure 6: Measured torsion mode using a PolyTec laser vibrometer setup.
In order to pick up the primary (excitation) and secondary (due to Coriolis force) motion, two promising sensor concepts have been identified – capacitive and optical-beam-deflection. In the present work, the optical beam deflection is investigated in detail. To this end, various tube shapes and integrated mirror-shapes (to reflect a beam) have been fabricated – these can be seen in Figure 7. A commercially feasible (qua cost and compactness) optical probe system (Figure 8) has been proposed; this will be tested alongside the various tubes to identify advantageous tube, mirror and light-beam forms. In addition to the motion sensors, a vacuum package and fluidic interconnection from the micro tube to an external system is investigated.
Figure 7: Some of the various fabricated Coriolis-tube and mirror shapes
Figure 8: Optical probe system concept
- P. Enoksson, G. Stemme and E. Stemme, J. MEMS, 6 (1997), pp. 119-125.
- D. Sparks, R. Smith, J. Cripe, R. Schneider, N. Najafi, Proc. IEEE Sensors Conference 2003, pp. 90-92.
- M.A. Dijkstra, M.J. de Boer, J.W. Berenschot, T.S.J. Lammerink, R.J. Wiegerink, M. Elwenspoek, Proc.Micromechanics Europe 2006, pp. 37-40.
- 1 Post-doc at Mechanical Automation group of CTW,
- 1 Post-doc at Transducer Science and Technology group of EWI
This research project is sponsored by Pieken in de Delta by order of the Dutch Ministry of Economic Affairs.