1. Technical Field
The present disclosure relates to a reading circuit for a multi-axis MEMS gyroscope having detection directions inclined with respect to the reference axes, and to a corresponding multi-axis MEMS gyroscope.
2. Description of the Related Art
As is known, micromachining techniques enable manufacturing of microelectromechanical structures or systems (MEMS) within layers of semiconductor material, which have been deposited (for example, a polycrystalline-silicon layer) or grown (for example, an epitaxial layer) on sacrificial layers, which are removed via chemical etching.
Inertial sensors, such as accelerometers and gyroscopes, made using this technology are experiencing an increasing success, for example in the automotive field, in inertial navigation, or in the sector of portable devices.
In particular, integrated gyroscopes made of semiconductor material using MEMS technology are known. These gyroscopes operate on the basis of the theorem of relative accelerations, exploiting Coriolis acceleration. When an angular motion is applied to a mobile mass that is driven with a linear motion, the mobile mass “feels” an apparent force, called Coriolis force, which determines displacement thereof in a direction perpendicular to the direction of the linear motion and to the axis about which the angular motion is applied. The mobile mass is supported via springs that enable its displacement in the direction of the apparent force. On the basis of Hooke's law, the displacement is proportional to the apparent force so that, from the displacement of the mobile mass, it is possible to detect the Coriolis force and the value of the angular velocity that has generated it. The displacement of the mobile mass may for example be detected in a capacitive way, by determining, in conditions of resonance, the variations of capacitance (or, likewise, of the amount of charge) caused by the movement of mobile electrodes, fixed with respect to the mobile mass (or constituted by the mobile mass itself) and coupled to fixed electrodes.
MEMS gyroscopes have generally symmetrical sensing structures, comprising a pair of sensing masses for each reference axis about which a corresponding angular velocity is detected, the sensing masses being aligned to one another in a detection direction (generally coinciding with a corresponding reference axis). The reading circuit hence generally adopts a differential scheme based upon the differential capacitive variations associated to the sensing masses of each pair. In fact, whereas the Coriolis force tends to unbalance in opposite directions and substantially by the same amount the sensing masses of each pair (generating so-called “phase opposition” movements), external noise accelerations determine displacements thereof in the same direction and again by the same amount (generating so-called “in-phase” movements). By computing the difference of the electrical signals associated to the two sensing masses of each pair, it is hence ideally possible to isolate and measure just the contribution due to the Coriolis force and completely reject the contributions of noise.
For example, a biaxial MEMS capacitive gyroscope detects a first angular velocity directed along a pitch reference axis, designated in what follows by x, and a second angular velocity directed along a roll reference axis, designated hereinafter by y. By applying the Coriolis theorem, the following expressions are obtained:Mc=−2·J·Ωxωz;Mc=−2·J·Ωyωz where Mc is the moment of the Coriolis force acting upon the sensing masses (which, as will be described in greater detail hereinafter, perform a detection movement of rotation out of the plane of the sensor), J is the moment of inertia of the same sensing masses, Ωx and ωy are the unknown angular velocities of pitch and roll acting about the respective reference axes, and ωz is the driving angular velocity of the driving motion, directed along a vertical axis z (orthogonal to the plane of the sensor) and variable for controlling an electromechanical driving loop of which the MEMS structure of the sensor forms an integral part (the driving angular velocity also acting as a constant scale factor in the aforesaid expressions). In particular, this driving loop is made by an integrated circuit dedicated to generating and maintaining a movement of oscillation of a driving mass, to which the sensing masses are mechanically coupled, with driving angular velocity ωz and at the resonance frequency.
The reading circuit of the gyroscope, as described for example in US Patent Application Publication Nos. US2008190200 and US2007289382, assigned to STMicroelectronics Srl, is configured so as to read, through the differential capacitive variation associated to the pitch or roll sensing masses, a signal proportional to the vector product Ωx ^ ωz, in the case of pitch, or Ωy ^ ωz, in the case of roll, the signal being of a DSB-SC (Dual Side Band-Suppressed Carrier) type, i.e., amplitude-modulated with a carrier given by the driving angular velocity ωz, due to driving of the structure at the resonance frequency. Next, by means of a coherent demodulation, the signal is first brought into baseband and then appropriately filtered, obtaining at output an electrical signal (in particular, a voltage signal) proportional to the unknown quantity alone, namely the pitch angular velocity Ωx or the roll angular velocity Ωy.
In greater detail, and as illustrated in FIG. 1, the reading circuit, here designated by 1, has two distinct transduction chains, a first transduction chain, designated by 1a, for detection of the pitch angular velocity Ωx and a second transduction chain, designated by 1b, for detection of the roll angular velocity Ωy. Also indicated in FIG. 1 are the contact of the mobile (or rotor) electrodes Rot, in common to all the sensing masses, the contacts of the fixed (or stator) electrodes P1, P2 associated to the pair of pitch-sensing masses, and of the fixed electrodes R1, R2 associated to the pair of roll-sensing masses (as well as the corresponding pairs of capacitors, having a capacitance variable in a differential manner). In particular, FIG. 1 shows the case where a single fixed electrode is associated to each sensing mass. Furthermore, an excitation signal, for example a voltage step ΔVs, is applied to the mobile electrodes Rot during reading operations.
Each transduction chain 1a, 1b, of a fully differential type and made using the switched-capacitor (SC) technology, comprises:                a charge amplifier 2, i.e., a charge/voltage converter, designed to minimize noise contributions and designed to convert the differential capacitive variation received at input (due to the displacement of the mobile electrodes with respect to the fixed electrodes) into a voltage variation signal of a fully differential type, this signal being a signal of a DSB-SC type on account of driving at the resonance frequency of the driving mass; the charge amplifier 2 has for this purpose a positive input and a negative input connected to a respective fixed electrode P1, P2 of the sensing masses;        a demodulator (or mixer) 4, cascaded to the charge amplifier 2 and forming therewith a measurement chain for signals of a DSB-SC type of the transduction chain, which is designed to carry out coherent demodulation of the amplitude-modulated signals of a DSB-SC type; in particular, by means of a clock signal CK in phase with the carrier of the signal (the driving angular velocity ωz), previously processed so as to minimize its phase delay, a coherent demodulation is obtained such as to bring the output signal, once again of a fully differential type, into baseband and reject the spurious components superimposed on the same signal (which have the same carrier frequency but are out of phase with respect to the information component by an angle of 90°;        a sample&hold stage 6, cascaded to the demodulator 4 and designed using the switched-capacitor (SC) technique, which has the purpose of transforming the fully-differential demodulated signal into a single-ended signal; and        a filter stage 8, which is cascaded to the sample&hold stage 6 to form a measurement chain for baseband signals of the transduction chain and is designed to implement a transfer function of a second-order lowpass filter so as to reject all the undesirable components that fall outside the band of interest of the signal (typically up to 140 Hz), amongst which the known components, which, as a result of coherent demodulation, are at the resonance frequency (so-called residual offset) and at a frequency equal to twice the resonance frequency, and supply at output a useful analog signal OUT (of pitch or of roll) on a purposely provided output electrode 9a, 9b; the output signal contains the desired information of the pitch or roll angular velocity detected by the MEMS capacitive gyroscope.        