Coriolis effect transducer

Details Coriolis effect transducer Coriolis effect transducer

1999-07-22

2003-02-11

Noland, Thomas P. (Department: 2856)

Measuring and testing

Instrument proving or calibrating

Volume of flow, speed of flow, volume rate of flow, or mass...

C073S001380, C073S504020, C073S861356

Reexamination Certificate

active

06516651

ABSTRACT:

BACKGROUND OF SUMMARY OF THE INVENTION
This invention relates generally to Coriolis effect transducers and more particularly to test, or calibration, structures for such transducers.
As is known in the art, Coriolis effect transducers are used to measure angular rate. More particularly, if a mass, m, with velocity, v, relative to the body of the transducer, experiences an angular rate &OHgr;
IN
about a rate sensing axis perpendicular to the velocity of the mass, the mass will experience a Coriolis acceleration, A
COR
=2v&OHgr;
IN
, along a sensitive axis perpendicular to both the velocity, v, and the rate sensing axis. Thus, a Coriolis force F
COR
is produced on the mass equal to mA
COR
along the sensitive axis. The motion of the mass is restrained in the direction of the Coriolis force by a mechanical or electrical restraint, such as a mechanical spring or electrical servomechanism. The mass will undergo a displacement y=F
COR
/k, along the sensitive axis, where k is a constant, such as the spring constant or reciprocal of the spring compliance. This displacement y may be measured by any displacement transducer, such as, for example, a device which measures difference in capacitance produced by a linear change in a gap (i.e., displacement) between plates of a capacitor, one of such plates being mechanically coupled to the mass and the other being fixed relative to the body of the transducer. The device then produces an electrical output signal V
OUT
proportional to this change in capacitance. Thus, such a Coriolis transducer produces an output electrical signal, V
OUT
=K&OHgr;
IN
, where K is a proportionality constant which is a function of the physical and electrical properties of the transducer.
In order to determine K, a calibration procedure is used. Such calibration procedure typically includes setting the transducer on a rate table and applying a known input angular rate, &OHgr;
IN TEST
, about the rate sensing axis while the output signal V
OUT TEST
is measured. The proportionality constant, K, is determined in accordance with K=V
OUT TEST
/&OHgr;
IN TEST
. While this calibration procedure provides a determination of K, it is time consuming and therefore adds cost to the transducer.
As is also known in the art, one technique used to provide relatively inexpensive Coriolis transducers is micromachining. One such micromachined Coriolis transducer is described in my U.S. Pat. No. 5,635,640 issued Jun. 6, 1995, the entire subject matter thereof being incorporated into this patent application.
It is also very useful to be able to verify the quantitative functionality of a packaged Coriolis sensor in its end use. Specifically, when such sensors are used for automotive rollover or anti-skid applications they become safety-critical items and it enhances their usefulness if their correct function can be established with high confidence every time they are used. As will be described, this invention provides a means of testing that function with high confidence.
In accordance with the present invention, a method is provided for testing a Coriolis transducer having a body with a mass, m, vibrate along a vibratory direction in a resonant structure and undergo a displacement along a sensitive axis in response to an angular rate about a rate sensing axis. The displacement is perpendicular to both the vibration and the rate sensing axis. The method includes: applying forces, F
TEST VIBRATORY
and F
TEST SENSITIVE
, on the mass along the direction of vibration and along the sensitive axis, respectively, in a predetermined ratio, N; and, measuring an incremental output V
OUT TEST
of the transducer in response to the forces, F
TEST VIBRATORY
and F
TEST SENSITIVE
. The F
TEST
vibratory is identical to the vibratory drive force in normal operation.
With such method, a known test input angular rate &OHgr;
IN TEST SIM
is simulated, such rate, &OHgr;
IN TEST SIM
, being a known function of a measured characteristic frequency of the resonant structure. Thus, the proportionality constant K=V
OUT
/&OHgr;
IN
=V
OUT TEST
/&OHgr;
IN TEST SIM
can be calculated without expensive rate table testing. Further, because the test can be performed with a transducer prior to packaging, electronics integrally formed on the same semiconductor wafer of the mechanical transduction structure may be easily trimmed to provide a desired proportionality constant K.
In accordance with another feature of the invention, the mass of the transducer is included in a resonant structure. During a test, or calibration, mode, with the forces, F
TEST VIBRATORY
and F
TEST SENSITIVE
having a frequency at, or near the resonant frequency of the mass, and in the absence of an input angular rate about the rate sensing axis, the velocity of the mass will be predominately F
TEST VIBRATORY
/D, where D is the damping factor of the body within the resonant structure. Thus, under such condition, a simulated input angular rate, &OHgr;
IN TEST SIM
={F
TEST SENSITIVE
/F
TEST VIBRATORY
}{BW/2}, where BW is the resonant bandwidth of the resonance structure is applied to the transducer. In response to such simulated input angular rate, &OHgr;
IN TEST SIM
={F
TEST SENSITIVE
/F
TEST VIBRATORY
}{BW/2}, the output of the transducer V
OUT TEST
will be K{BW}/2N; i.e., K=2N{V
OUT TEST)
/BW. Because the mechanism used to apply the vibratory force, F
TEST VIBRATORY
, on the mass along the vibratory direction and the mechanism used to apply the force to the mass along the sensitive axis, F
TEST SENSITIVE
, are fabricated in proximate regions of the transducer, they have matched physical and electrical characteristics. Thus, manufacturing variations incurred in the formation of one of the force mechanisms occur to the other one of the force mechanisms with their size ratio, N, being independent of such manufacturing variations. As a consequence, with the vibratory direction force to sensitive axis force ratio, N=F
TEST VIBRATORY
/F
TEST SENSITIVE
, being related to the ratio of the sizes of the mechanisms, rather than to the absolute size of each one of the mechanisms, such ratio, N, can be accurately fixed by the design of the structure. In short, the ratio N is independent of manufacturing tolerance. The remaining parameter, the resonance bandwidth, BW, of the resonant structure, is readily and quickly ascertainable from a frequency response measurement of the resonant structure. While the resonance bandwidth, BW, changes with manufacturing variations, it is measurable from a frequency response measurement which does not require application of a known input angular rate to the transducer.
In one embodiment, the vibratory direction force mechanism, F
TEST VIBRATORY
, during both the normal mode and the test mode, and the sensitive axis force mechanism, F
TEST SENSITIVE
, used during the test mode, are electro-statically driven fingers. Thus, N is merely the ratio of the number of fingers used to produce F
TEST SENSITIVE
to the number of fingers used to produce F
TEST VIBRATORY
. Consequently, an accurate determination of the proportionality constant, K is achieved.
In accordance with another embodiment of the invention, during the test mode, the forces F
TEST SENSITIVE
and F
TEST VIBRATORY
are applied with a frequency, &ohgr;
APPLIED
, less than the natural resonant frequency, &ohgr;
o
. In such case, the simulated &OHgr;
IN TEST SIM
={&ohgr;
o
2
}/{2N&ohgr;
APPLIED
}, in which case the natural frequency &ohgr;
o
is measured and used for the calibration (i.e., the characteristic frequency of the resonant structure measured and used for the calibration is the natural frequency &ohgr;
o
). In particular, the proportionality constant K=V
TEST OUT
/&OHgr;
IN TEST SIM
={2N&ohgr;
APPLIED
/&OHgr;
o
2
}{V
TEST OUT
} is determined when the vibratory direction and sensitive axis forces, F
TEST VIBRATORY
and F
TEST SENSITIVE
, respectively, are applied to the tran

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