Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Patent
1984-11-14
1988-02-09
Salce, Patrick R.
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
310338, 310348, 310367, 310312, H01L 4108
Patent
active
047243511
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION
1. Technical Field
This invention relates to force transducers having double resonating beams extending between mounting pads, and more particularly, to a force transducer configured so that its beams vibrate inwardly and outwardly on opposite sides of a straight mean position in order to minimize the magnitude of longitudinal forces applied to the mounting pads by the beams.
2. Background Art
Double-beam resonators have been proposed for use as a force transducer to measure such physical properties as pressure, weight or acceleration. When used as a force transducer, a pair of substantially parallel beams extend from respective mounting pads. Since the mounting pads form stationary nodes, the mounting pads may theoretically be connected to a force-transmitting structure without the movement of the beams being coupled to such structure. Coupling of motion from the beams to the force-transmitting structure would absorb energy from the beams and thus degrade the quality factor or "Q" of the resonator.
One problem associated with double resonant beam force transducers that has not be adequately recognized is the coupling of energy from the beams to the force-transmitting structure because of longitudinal movement of the mounting pads toward and away from each other resulting from deflection of the beams. Longitudinal movement or "pumping" occurs because the distance between the mounting pads varies as the beams deflect from side to side. This longitudinal pumping is undesirable not only for its degrading of the Q of the transducer, but also because it degrades the linearity of the transducer. This nonlinearity arises when the resonant frequency of the beams at a certain force approaches the resonant frequency of the force-transmitting structure either alone or in combination with the mounting pads between which the beams extend. As a result, when the response of the force transducer is linearized using appropriate formulae, there is a residual error at certain values of applied force. The magnitude of the error depends upon the nature of the resonance in the surrounding structure, and it can range from relatively small values such as 2.5.times.10.sup.-5 to relatively large values such as 2.5.times.10.sup.-3 of full scale.
If the resonance of the surrounding structure could be accurately predicted, the longitudinal pumping phenomena would not present an insurmountable problem. This is because the surrounding support structure could be configured to have a resonance outside the resonant frequency of the beams in their normal range of operation. However, the resonances in the support structure tend to be very complicated, because the structures are physically large compared to the dimensions of the force transducer. For example, the double-beam force transducer typically vibrates at between 17 kHz and 40 kHz, depending on the specific design. The fundamental resonance of the support structure is typically in the 1 kHz range. Thus, resonant frequencies of the surrounding structure in the 17 kHz-40 kHz range are fairly high overtones of the fundamental, so that the mode spectrum of the support structure is very dense at the operating frequency of the resonator. These higher order resonant modes typically involve flexural, torsional and extensional distortions which cannot be readily identified, controlled or predicted. Thus, it is impractical to design a support structure having a well-controlled mode spectrum at the operating frequency of the force transducer. The ideal solution would be to have a support structure having no resonances over the entire operating range of the force transducer. However, since the operating frequency of a force transducer with zero force frequency of 40 kHz typically varies from 36 kHz to 44 kHz as the force varies from full scale compression to full scale tension, it is not possible to do so.
The degree of nonlinearity caused by longitudinal pumping is, to a large extent, a function of the magnitude of the longitudinal pumping. Thus, a reduction in the longitudinal pump
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EerNisse Errol P.
Kistler Walter P.
Quartex, Inc.
Salce Patrick R.
Todd Voeltz Emanuel
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