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
  • G01P15/125—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 the field of acceleration measurement.
• the subject of the invention is devices, such as accelerometers, and methods for measuring acceleration.
• Acceleration measuring devices comprising a support, a mass m movable relative to the support and elastic means opposing the displacement of the mass m with respect to the support. These devices comprise means for detecting at least one physical quantity representative of a displacement of the mass relative to the support. The detection means are associated with modulation means arranged to generate, from the detected value of the physical quantity, at least one modulated analog signal from which the acceleration undergone by the mass is determined.
• modulation means are for example known from GB 791827A.
• acceleration measuring devices having a digital output.
• a demodulator is connected to the modulation means and to an analog-to-digital converter so as to generate a demodulated digital signal representative of the displacement of the mass relative to the support.
• the acceleration is then determined by a numerical calculation.
• acceleration measuring devices have a precision which may be insufficient for certain applications.
• An object of the invention is to provide an acceleration measuring device and a measuring method acceleration method for reducing the difference between the actual acceleration experienced by the mass and the estimated acceleration value.
• the invention relates to an acceleration measuring device, comprising a mass movable relative to a support and resiliently biased into a neutral position, and means for detecting a physical quantity representative of a displacement of the ground relative to the support which are associated with a modulator to produce, according to the detected magnitude and a modulation signal, an analog modulated signal representative of the displacement of the mass.
• the device comprises an analog digital converter connected to the detection means and arranged to convert the analog modulated signal into a digital modulated signal, a demodulator connected to the converter and arranged to produce a digital signal demodulated from the digital modulated signal and characteristics of the signal of the digital signal. modulation, and means for calculating an estimated value of acceleration as a function of said demodulated digital signal.
• the invention it is sought to produce a demodulated digital signal that can be used by numerical calculation means to calculate an estimated value of acceleration experienced by the mass.
• the fact of performing the digital conversion of the analog signal before carrying out the demodulation operation makes it possible to reduce the volume of errors contained in the demodulated digital signal.
• a standard error related to the analog demodulation is a demodulation symmetry defect.
• the analog demodulation means comprise an operational amplifier introducing a error called offset which is defined by the voltage to be applied to the input of this operational amplifier so that the voltage at the output of this operational amplifier is zero.
• the analog signal is not demodulated directly but the digital signal.
• the demodulation operation from the digital data representative of the analog signal is performed by a numerical computation controlled so that fewer errors are generated during the demodulation by numerical calculation that is generated with an analog demodulation means. In particular, there are practically no more errors related to a demodulation dissymmetry.
• the invention also relates to a method for measuring an acceleration, comprising the steps of:
• the method of the invention makes it possible to reduce the difference between the estimated value of acceleration and the acceleration actually experienced by the moving mass.
• FIG. 1 shows schematically an accelerometer type device according to the invention
• FIG. 2 represents an electronic circuit of the device according to the invention, making it possible to implement the method according to the invention
• FIG. 3 shows the electronic circuit of Figure 2 according to an alternative embodiment.
• the invention relates to a device for measuring an acceleration experienced by a seismic mass m along an axis of acceleration A-A visible in FIG.
• a piece of mass m is suspended from a rigid support 1 via elastic means of stiffness k known.
• the support 1, which serves as a housing, is U-shaped in the middle of which is placed the seismic mass m.
• the elastic means 2 comprise an elastic beam (or elastic hinge) having one end connected to the mass m and another end connected to the base of the U so as to be flexed under the effect of the mass m along an axis perpendicular to the plane in which extends the U-shape and recall the mass m in a position of equilibrium or neutral position.
• the device comprises means for detecting the distance separating a first side of the mass of a first branch la from the U shape and / or the distance e2 separating a second side of this mass from a second branch lb of the U shape.
• the detection means here comprise a first pair of electrodes 5a, 5b so that one of these electrodes is on the first face of the mass m and the other of these electrodes is on the first branch la of the U-shape and vis-à-vis the electrode carried by the first face of the mass m. These electrodes vis-à-vis form a first capacitor whose capacity Cl is variable according to the distance el.
• the detection means comprise a second pair of electrodes 6a, 6b forming a second capacitor whose capacitance C2 is variable as a function of the distance e2.
• This type of accelerometer is known as an electrostatic pendulum accelerometer for measuring linear accelerations along a sensitive axis.
• the elastic beam exerts on the seismic mass m an elastic return force proportional to the stiffness k and the distance separating the center of inertia of the seismic mass m from its equilibrium position called "mechanical zero".
• This mechanical zero is defined as the equilibrium position (marked relative to the support case 1) obtained when the acceleration applied to the mass m along the sensitive axis A-A is zero.
• the estimated acceleration noted y is ideally deduced from the measurement of x denoted x (equilibrium position of the center of inertia of m) and the knowledge of k:
• Ax represents the value estimated by measuring the displacement of the center of inertia of the mass m according to A-A and is estimated by means of the detection means whose errors are added to the quantity to be measured:
• ⁇ could be deduced from the measurement of only one of the capacitors Ci or C 2 .
• Each electrode 5a, 5b, 6a, 6b has a surface S vis-à-vis the other electrode of the couple to which it belongs. To facilitate the calculations it is assumed that these surfaces S are here identical for the first capacitor and the second capacitor, but these surfaces could also differ from each other.
• the gap that is to say the real distance between the electrodes of a pair of electrodes, is denoted e ⁇ ⁇ :
• circuits of Figures 2 and 3 are examples of possible embodiments for the implementation of the invention. These exemplary embodiments make it possible to avoid the use of an analog demodulator which has the disadvantage of creating an offset biasing the demodulated digital signal Y and consequently the estimate of ⁇ .
• the detection means are associated with modulation means 4 adapted to generate a modulated analog signal S representative of a displacement ⁇ of the mass m with respect to the support 1.
• the modulated signal S is modulated according to a modulation signal M (t).
• These modulation means 4 comprise DC voltage sources V and U and first and second analog switches II, 12.
• the first switch II comprises:
• the second switch 12 comprises:
• Switches II, 12 selectively adopt a first configuration in which the first input terminal is connected to the output terminal and a second configuration in which the second input terminal is connected to the output terminal.
• the switches II, 12 are connected to a control means C comprising an excitation source Exc generating a modulation signal M (t).
• This modulation signal (t) alternately takes first and second values, and these control means C are such that when the signal M (t) takes its first value, the control means C control the passage of the switches II, 12 in their first configurations and when the modulation signal M (t) takes its second value, the control means C control the passage of the switches II, 12 in their second configurations.
• the modulation signal M (t) is preferably a periodic signal having a square-wave voltage.
• the detection means are connected to an operational amplifier having an input denoted "+” said non-inverting input and an input denoted "-" called inverting and an output connected to the inverting input -.
• the non-inverting input of the operational amplifier is connected to the electrodes of the first and second capacitors which are not connected to the switches II, 12.
• each of the capacitors has an electrode connected to an output of one of the switches and a electrode related to the non-inverting input of the operational amplifier.
• the output of the operational amplifier is also connected to its inverting input so that this input is subjected to the output signal.
• the signal generated at the output of this operational amplifier is a signal S which is modulated according to the modulation signal M (t) which controls the switches.
• M (t) which controls the switches.
• the non-inverting input of the amplifier receives a signal which is a function of the values of capacitors C1 and C2 which depend on the position of the mass relative to the support, it can be seen that this modulated output signal of the amplifier operational is also representative tative of a displacement ⁇ of the mass m with respect to the support 1.
• the electronic circuit of FIG. 2 also comprises a digital demodulator Dn.
• This digital demodulator Dn comprises an analog digital converter and digital demodulation means, in this case a digital computing unit OCN programmed to perform a digital demodulation operation.
• the digital analog converter CAN comprises an input connected to the output of the operational amplifier for receiving the modulated analog signal S and an output connected to an input of a digital computing device OCN. Note that the CAN digital analog converter is also connected to the excitation source to achieve the conversion taking, for example, into account the modulation signal, however this link may be less direct.
• the digital demodulation means formed by the digital computing unit OCN comprise:
• OCN demodulation means demodulate the digital signal coming from the output of the analog converter CAN according to characteristics of the modulation signal M (t).
• the device of the invention further comprises calculating means CAL having an input connected to the output of the digital computing device OCN through which the signal Y passes. As a function of said demodulated digital signal Y, these calculation means CAL generate an estimated value Y representative of the acceleration ⁇ undergone by the mass m.
• the voltages U and V are so-called continuous voltages.
• the switches are wired so that when one of the capacitors has an electrode connected to one switch and subjected to the voltage U, then the electrode of the other capacitor which is connected to the other switch is necessarily subjected to the voltage V.
• This analog signal S is converted into a digital signal by the CAN converter and the demodulation means of the digital demodulator Dn (that is to say the digital computing device OCN) perform a demodulation.
• This demodulation consists in differentiating between successive successive values of the signal S taken in time by taking into account the characteristics of the modulation signal M (t) as the time slots for which the signal M (t) takes its first value and the time intervals where this signal M (t) takes its second value.
• the modulation signal M (t) is a periodic signal having a specific fixed frequency and the OCN demodulation means demodulate the modulated signal converted into digital as a function of the said own fixed frequency of the modulation signal M (t).
• the present invention limits errors by substituting an analog demodulator for a digital demodulator.
• the perfect symmetry of the digital demodulator Dn due to the digital computing unit OCN allows the use of a reference voltage U and a reference voltage V which are asymmetrical and also makes it possible to reject the offset of the analog part brought back to the input of the CAN digital analog converter.
• the numerical calculation means CAL calculate the estimated value of the displacement Ax of the mass m.
• FIG. 3 An alternative mode of the electronic circuit of FIG. 2 is presented in FIG. 3.
• This circuit is identical to that of FIG. 2 but also comprises a calculation function f making it possible to generate a demodulation signal M '' (t) in FIG. function of the modulation signal M (t).
• the calculation function can be summarized as the extraction of only one of the harmonics constituting the signal M (t) so as to make the device less sensitive to bandwidth limitations and linearity errors of the electronics.
• this last circuit of the figure 3 allows an estimation of ⁇ improved compared to the circuit of FIG.
• the acceleration measuring device of the invention can be used in a closed loop, in this case ⁇ is slaved to zero by applying to the seismic mass m a so-called counter-reaction force which is exactly opposed to the force of inertia. there. Divided by the seismic mass, this force constitutes the estimate of the acceleration.
• the detection means may be of the electromagnetic type.
• the excitation source Exc is arranged so that the modulation signal generated is independent, that is to say de-correlated, of the measured effective acceleration and therefore of independent / correlated dice of the physical quantity detected.
• this modulation signal has characteristics of fixed period and / or fixed frequency, that is to say constant.
• the characteristic (s) of the modulation signal M (t) used to generate the demodulated digital signal (Y), such as the own fixed frequency, are thus independent of the measured acceleration, which reduces the risk of occurrence of uncertainties in the modulation signal and, therefore, in the demodulation signal.
• the acceleration measurement made according to the invention thus has an even smaller uncertainty as the characteristics of the modulation signal used to perform the demodulation is / are independent of the variations of the acceleration.

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• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Measurement Of Mechanical Vibrations Or Ultrasonic Waves (AREA)
• Control Of Position Or Direction (AREA)
• Pressure Sensors (AREA)

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• 2011
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• 2012
  • 2012-06-13 EP EP12729466.8A patent/EP2721421A1/de not_active Withdrawn
  • 2012-06-13 WO PCT/EP2012/061201 patent/WO2012171957A1/fr active Application Filing

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