Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01P—MEASURING LINEAR OR ANGULAR SPEED, ACCELERATION, DECELERATION, OR SHOCK; INDICATING PRESENCE, ABSENCE, OR DIRECTION, OF MOVEMENT
  • G01P15/00—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration
  • G01P15/02—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration by making use of inertia forces using solid seismic masses
  • G01P15/08—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration by making use of inertia forces using solid seismic masses with conversion into electric or magnetic values
  • G01P15/13—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration by making use of inertia forces using solid seismic masses with conversion into electric or magnetic values by measuring the force required to restore a proofmass subjected to inertial forces to a null position
  • G01P15/131—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration by making use of inertia forces using solid seismic masses with conversion into electric or magnetic values by measuring the force required to restore a proofmass subjected to inertial forces to a null position with electrostatic counterbalancing means
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01P—MEASURING LINEAR OR ANGULAR SPEED, ACCELERATION, DECELERATION, OR SHOCK; INDICATING PRESENCE, ABSENCE, OR DIRECTION, OF MOVEMENT
  • G01P15/00—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration
  • G01P15/02—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration by making use of inertia forces using solid seismic masses
  • G01P15/08—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration by making use of inertia forces using solid seismic masses with conversion into electric or magnetic values
  • G01P15/125—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration by making use of inertia forces using solid seismic masses with conversion into electric or magnetic values by capacitive pick-up

Definitions

• the invention relates to micromachined accelerometers which in particular make it possible to measure the movement of the ground for geophysical applications (mapping of the subsoil by seismic method).
• the invention relates in particular to accelerometers which implement a mass-spring system, in particular when the mass forms a series of fingers which are inter-digested with corresponding fingers of a fixed part.
• each pair of fingers facing each other forms a measurement capacitor.
• the fingers constituting the capacitors can be used both for measuring the displacement by measuring the variation in capacity, and at the same time for recalling the mass in its original position, by application of a force electrostatic within each capacity thus formed.
• the return electrostatic force can be controlled on the previous capacitive displacement measurement.
• a second problem lies in the fact that the fingers prove to be fragile in bending. Whether they vibrate in resonance or are simply subjected to strong accelerations, these fingers are subject to damage by bending.
• the invention consists of an accelerometer with moving mass and fixed part using variations in capacitance to detect the movement of the mass, in which a first series of electrodes secured to the mass is prohibited with a series of electrodes integral with the fixed part, each mobile electrode forming, with an adjacent fixed electrode, a variable capacitance as a function of the position of the mobile mass, the accelerometer further comprising an electronic circuit provided for detecting the variation of at least one capacitance between moving mass and fixed part as an indicator of the displacement of the moving mass, and also to generate an electrostatic force to return the moving mass to its original position, the electronic circuit being designed to generate the electrostatic return force in a controlled manner on a previous displacement measurement, characterized é in that the repetitive restoring force thus generated is specifically chosen so that its mechanical power frequency spectrum has a zone with power that is substantially zero at the mechanical resonance frequency of the electrodes of the moving mass and / or of the fixed part.
• FIGS. 2a to 2c are plots representing the frequency spectra respectively of the noise linked to the mass return voltage, of a function of transformation of this tension into force with and without resonance of the fingers, and of the resulting force, with and without resonance of the fingers there again; ; '- Figure 3 shows the frequency spectra of a pulsed voltage and a servo signal in slots windowing this voltage; - Figure 4 shows interdigitated electrodes according to a variant of the invention; - Figure 5 shows interdigitated electrodes according to another variant of the invention.
• the mobile electrodes 4 are electrically isolated from the fixed electrodes 3 and 7.
• the electrodes 3 form with the electrodes 4 opposite a capacitor C1.
• the electrodes 7 form, with the electrodes 4 opposite, a capacitor C2.
• a voltage applied to the terminals of C1 produces an electrostatic force which tends to bring the electrodes 3 and 4 together, therefore to move the moving mass in one direction, while a voltage applied to the terminals of C2 tends to move the moving mass in the other direction.
• An electronic circuit is connected to each series of fixed electrodes 3 and 7 and to the series of mobile electrodes 4.
• this circuit is clocked to the rhythm of a clock, and cyclically applies, in successive phases, a measurement voltage across each capacitor, making it possible to measure their capacity (differential measurement of the two neighboring capacities). The displacement measured is indicative of the displacement of the movable plate 5 due to the acceleration punctually present.
• the duration of the phase of application of a measurement voltage, denoted Te and also called charging time, or also duration of the detector phase, is much less than the resonance period of the system (and therefore the vibration period of the ground).
• the control set up here consists in canceling the relative movement of the mass 5 by applying a force between the series of mobile electrodes and one or the other of the series of fixed electrodes (C1 or C2). It is an electrostatic force and it is the actuator phase when it is applied in a time separate from the detector phase. It is preferably the same electronic circuit which alternately measures the position of the moving mass and tends to bring it back to its initial position by applying the adequate voltages across the terminals of the capacitors C1 and or C2.
• the circuit defines a multiplexing between measurement and feedback, preferably with a discharge of the capacities between these two stages.
• the frequency range for multiplexing is for example 100 to 500 times the resonance frequency of the system.
• the recall of the mobile mass can be implemented simultaneously with the displacement measurement.
• the mechanical chip typically resonates at 500 Hz.
• the resonance frequency preferably chosen as close as possible to the ground vibration frequency, is, in the present example, adjusted by adjusting an electrostatic stiffness k e . This stiffness is superimposed on the mechanical stiffness and adjusted by the duration of the loading step of capacity measurement.
• the electrostatic stiffness is here chosen to lower the resonant frequency of the system, the mechanical stiffness being chosen deliberately beyond the high frequency of; the band of interest.
• This optional arrangement makes it possible to limit the slump, to reduce the inter-electrode distance, and therefore to use high electric fields (therefore a high electrostatic stiffness).
• This arrangement also makes it possible to optimize the performance in the useful bandwidth and to compensate for the dispersion of mechanical stiffness of the suspension springs of movable plates, dispersion typically observed with the usual manufacturing processes. Thanks to the electrostatic stiffness, the apparent frequency is reduced to 140 Hz to best reduce the noise in the useful band (0-200 Hz).
• the fixed and mobile electrodes have the shape of "fingers", usually parallelepipedic silicon beams connected to each other by a base like a comb.
• Each of these fingers has a resonant frequency corresponding to that of a beam embedded at one end.
• the resonant frequency of the fingers was typically 90 kHz and increased to 585 kHz after a first modification described in the text which follows.
• the inventors have identified that these fingers have a tendency to resonate considerably, and this with all the more amplitude as the ambient pressure is very low inside the chip.
• the resulting movement is responsible for the folding into baseband, by frequency transposition of the noise present in the servo force, and therefore for the degradation of the overall noise of the geophone, in particular when the maximum compensable acceleration is increased (A m a ⁇ ) with the actuator.
• the spectral components of the respective recall signal applied to the mass will be analyzed below.
• the curve 70 transformed into frequency of the pulse signal of duration Ta, is a cardinal sine of formula Sin (Pi.T.Fa) / (Pi.FaTa). (with a first zero at the frequency 1 / Ta).
• the curve 80 is therefore the result of the multiplication of the curve 60 by the curve 70.
• a first preferential arrangement is to choose a positioning of the cardinal sinus to place a return to zero of the energy on the resonance frequency of the fingers, the spectrum resulting from the product then also having a return to zero at resonance.
• This positioning is carried out for example by choosing an appropriate value of Ta, so that the value 1 / Ta is placed on the resonance frequency of the fingers.
• the frequency of the fingers is greater than Fe, Ta cannot be> Te (Te denotes the sampling period) .
• Te Te denotes the sampling period
• the choice of a spectrum thus placed allows a significant gain on the noise level of the accelerometer.
• the fixed and / or mobile fingers 3 and 4 are configured so that their resonance is reduced to such a natural energy well, a well due to the application of forces during the durations Ta in the actuator phase framed by returns to zero of the restoring force.
• the preferred frequency for the resonance of the fingers is that equal to 1 / Ta, corresponding to the first zero crossing of the cardinal sinus, transformed from the signal into slots.
• a finger variant is also provided with curved edges, for example with convex external curvature, but which can also be concave, forming a general shape of rounded trapezium. Such a shape turns out to be more compact and has an even higher resonant frequency.
• a finger shape with a width that decreases towards the free end is beneficial to the flexural strength, it can be advantageous to adopt a different shape, in particular to shift the resonant frequency to a higher frequency. It should be noted that this geometrical modification of the resonant frequency of the fingers, here exposed with reference to an internal source of vibration, also makes it possible to escape from other sources of vibration.
• the modification of the resonance frequency of the fingers makes it possible to escape from stress frequencies of external origin.
• the resonant frequencies of the fingers are placed outside the frequency ranges of vibrations of external origin, which would otherwise be stressful. It will be noted that the higher the resonance frequency, the lower the amplitude of the movement.

Landscapes

• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Pressure Sensors (AREA)
• Geophysics And Detection Of Objects (AREA)
• Fittings On The Vehicle Exterior For Carrying Loads, And Devices For Holding Or Mounting Articles (AREA)
• Air Bags (AREA)

Applications Claiming Priority (2)

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• 2003
  • 2003-08-13 FR FR0309901A patent/FR2858854B1/fr not_active Expired - Lifetime
• 2004
  • 2004-08-11 EP EP04786297A patent/EP1664803A1/fr not_active Withdrawn
  • 2004-08-11 WO PCT/FR2004/002125 patent/WO2005017538A1/fr active Application Filing
  • 2004-08-11 CN CNB2004800231918A patent/CN100570371C/zh not_active Expired - Fee Related
  • 2004-08-11 JP JP2006523033A patent/JP5269313B2/ja active Active
  • 2004-08-11 US US11/547,899 patent/US7552638B2/en active Active
• 2006
  • 2006-03-08 NO NO20061113A patent/NO339398B1/no unknown

Non-Patent Citations (1)

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