RU2810694C1 - Dual-axis micromechanical accelerometer with capacitive displacement transducer - Google Patents

Abstract

FIELD: measuring technology.

SUBSTANCE: invention is intended for measuring linear accelerations along two mutually orthogonal axes by measuring the electrical capacitance of capacitors formed by a movable electrode on an inertial mass and a stationary electrode on a stationary base. A two-axis accelerometer comprises single-axis accelerometers made in the form of a frame, to which an inertial mass is connected through elastic suspensions, the accelerometer is equipped with comb capacitors, and the capacitors with resistors form a resistive-capacitive bridge connected to a direct current source, and the bridge is switched by transistors controlled by a quartz oscillator. The accelerometer differs in that the suspension of the inertial mass to the frame is made of two S-shaped elastic elements and two single-axis accelerometers are placed in a stack one above the other, and the voltage is from two capacitors, the capacitance of which varies in different directions, and connected to the first inputs of two comparators, the second inputs of which are connected to the reference voltage, and the outputs of the comparators are connected to the two inputs of the “exclusive or” logical element, the output of which is connected to the current amplifier and, in the form of a duty cycle of pulses, carries information about acceleration along the line through one of the channels, and the converter board is placed on the inertial mass.

EFFECT: increase in the accuracy of converting the difference in capacitances without complicating the converter circuit.

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Description

The proposed invention relates to measuring technology and is intended for measuring linear accelerations along two mutually orthogonal axes by measuring the electrical capacitance of capacitors formed by a movable electrode on an inertial mass and a stationary electrode on a stationary base.

There are a large number of two-axis and three-axis accelerometers that use two or three inertial masses, which are suspended relative to the device body on elastic suspensions, equipped with comb capacitors, in which the distance between the electrodes changes due to the movement of the inertial mass relative to the accelerometer body. In single-axis accelerometers, inertial masses are suspended from the housing through elastic plates; two or three-axis accelerometers in this case are implemented by placing two or three similar accelerometers with individual masses in one housing. It is also known to mount the inertial mass on an elastic suspension in an additional frame, fixed relative to the body on an additional elastic suspension.

We adopted the accelerometer according to patent RU 2693030 C1 as a prototype. It uses a DC power converter. In two axes it is made on one substrate of two identical comb capacitive transducers equipped with an inertial mass, which is suspended on four Ω-shaped elastic elements to a frame that is rigidly fixed to the substrate through anchor elements. Also, the inertial mass, through four other Ω-shaped elastic elements, is connected to an intermediate frame, which is suspended on the elastic elements relative to the inertial mass and through the third four elastic elements is suspended to the outer frame, which moves along an axis orthogonal to the axis of motion of the inertial mass and which, through the anchor elements rigidly connected to the substrate. An intermediate frame was introduced to eliminate cross-connection between two mutually orthogonal measuring axes. The accelerometer is equipped with comb capacitors, one group of electrodes of which is rigidly connected to the inertial mass, and the second group is rigidly connected to the substrate. The comb electrodes form two capacitor pairs. The difference in capacitances in a capacitor pair is converted into two information signals carrying information about acceleration along two independent axes. In this case, both circuits with phase-sensitive rectifiers and power supply of resistive-capacitive bridges with alternating voltage at a constant frequency, and resonant circuits in which the power supply frequency of resistive-capacitive bridges is adjusted to the resonance can act as converters. The latest converters contain a frequency generator controlled by a voltage that is detected by a phase-sensitive rectifier. Converters in which resistive-capacitive bridges (RC circuit) are powered by alternating voltage require appropriate power supplies, which limits their use. Therefore, circuits powering RC bridges with DC voltage are more often used. At the same time, in such circuits, the charging of capacitors and their discharge are ensured by connecting in parallel with the capacitors a transistor controlled by a quartz clock generator. This uses the inertial property of the RC circuit, in which the charge time constant depends on the capacitance T=RC, C is the capacitance of the capacitor. In this case, the capacitance is converted into pulse duration, which is then converted into DC voltage using low-pass filters. Such converters are much simpler than converters operating on alternating voltage, and therefore have found wider application. However, their accuracy is much lower than AC-powered converters.

The purpose of the present invention is to eliminate the intermediate frame and a large number of elastic elements. The second goal is to increase the accuracy of converting the difference in capacitance without complicating the converter circuit.

These goals are achieved by placing two identical single-axis frame converters in a stack and introducing a capacitance difference converter into pulse duty cycle. In this case, in the known frame accelerometer with an RC circuit powered by DC voltage and switching a pair of capacitors, the voltage from the pair of capacitors is compared with the reference voltage on the comparator, the received pulses from the outputs of the two comparators are compared on the “exclusive or” logical element and thereby on its The output produces pulses with a variable duty cycle, which is determined by the ratio of the pulse duration and the period of the quartz clock generator. Moreover, the accelerometer is equipped with two S-shaped elastic elements that ensure the rectilinear movement of the inertial mass.

Three figures are provided to explain the operation of the accelerometer.

Figure 1 shows a schematic design of a single-axis accelerometer.

Figure 2 shows the placement of two single-axis accelerometers in a stack.

Figure 3 shows a diagram of one channel of the converters.

The figure shows: frame 1, inertial mass 2 with the converter board, elastic suspension 3 in the form of S-shaped elastic beams, fixed electrodes of capacitors 4, movable electrodes of capacitors 5.

Figure 2 shows: the shelf housing 6, single-axis accelerometers 7, electrical connector 8.

Figure 3 shows: quartz clock generator 9, resistors 10, which together with capacitor 11 form RC circuits, transistor 12, which transfers RC circuits from the charge state to the voltage reset state, comparators 13, “exclusive or” logic element 14 , current amplifier 15, for transmitting information to the line.

A dual-axis accelerometer consisting of two single-axis accelerometers is proposed. However, in contrast to the accepted planar arrangement of single-axis accelerometers, their stacked arrangement is used. At the same time, as a capacitance converter, a converter is proposed, the output of which is the duty cycle of the pulses, proportional to the difference in the capacitances of a pair of capacitors, the capacitances of which change in different directions when the inertial mass moves.

RC circuits are connected to a stabilized constant voltage source through an individual resistor, and a capacitance discharge transistor is connected in parallel with the capacitor. This transistor is controlled by a quartz clock oscillator. Each capacitor is charged to a voltage that is compared to a reference voltage on the comparator. Since the duration of charging the capacitor to the reference voltage depends on the product of capacitance and resistance, a pulse is formed at the output of the comparator, the width of which is determined by the value of its capacitance. The ratio of the width of this pulse to the time between clock pulses of the clock generator determines the duty cycle of the pulses. Thus, the capacitance value is converted into the duty cycle of the pulses. The described converter allows you to convert capacitor capacitance ranging from several picofarads to pulse duty cycle if the clock generator frequency is selected in the range of several hundred KHz. The converter described above is a pulse-width converter (PWM). Pulses from the PWM converter of capacitors, the change of which is multidirectional when moving the inertial mass, are compared on the “exclusive or” logical elements, thereby determining the difference between the width of the pulses. In fact, the use of PWM conversion of the capacitance value into the duty cycle of the pulses and comparison of the pulse width on the “exclusive or” element implements a differential signal proportional to the difference between the capacitances of the two capacitors. This signal is proportional to the movement of the inertial mass, and therefore to the linear acceleration of the sensor body, since the movement of the mass corresponds to the movement of the elastic suspension, which creates a compensating force for the movement of the inertial mass. The output signals from the “exclusive or” logic elements are current amplified to ensure the required length of the communication line with the consumer. In this case, the converter can be made on a glass-ceramic substrate using hybrid integrated circuit technology and rigidly placed on an inertial mass.

Common features with the prototype are:

- making the accelerometer in the form of a frame with an inertial mass suspended through elastic elements,

- equipping the accelerometer with comb capacitors,

- power supply of the resistive-capacitive bridge with DC voltage,

- use of bridge switching with transistors controlled by a quartz oscillator

Distinctive features are:

- suspension of the inertial mass on two S-shaped elastic elements

- placement of single-axis accelerometers in a shelf,

- converting the difference in capacitance of capacitor pairs, the capacitance of which varies in different directions, into the duty cycle of pulses and thereby ensuring the transmission of information to the line in the form of duty cycle of pulses. The consumer can convert these pulses into digital code using known converters. For example, filling information pulses with pulses of a higher frequency and determining the number of these pulses.

Due to common and distinctive features, the dimensions of the two-axis accelerometer have been reduced. Since the number of elastic elements has been significantly reduced, the design of the accelerometer has been simplified. By converting the difference in capacitances into the duty cycle of the pulses, the number of electronic elements has decreased, which allows the proposed converter to be designed in a hybrid design with the converter board placed on an inertial mass, which also leads to a reduction in dimensions. In addition, since a differential circuit is ultimately used to convert the capacitance difference into the duty cycle of the pulses, the conversion accuracy is increased. Placing single-axis accelerometers in a single shelf, which simultaneously serves as the device’s body, which is made of metal, provides adequate shielding of the device from external interference.

Claims (1)

A two-axis accelerometer containing single-axis accelerometers made in the form of a frame to which an inertial mass is connected through elastic suspensions, the accelerometer is equipped with comb capacitors, and the capacitors with resistors form a resistive-capacitive bridge connected to a direct current source, and the bridge is switched by transistors controlled by a quartz oscillator , characterized in that the suspension of the inertial mass to the frame is made of two S-shaped elastic elements and two single-axis accelerometers are placed in a stack one above the other, and the voltage from two capacitors, the capacitance of which varies in different directions, is connected to the first inputs of two comparators, the second inputs of which are connected to the reference voltage, and the outputs of the comparators are connected to two inputs of the “exclusive or” logical element, the output of which is connected to the current amplifier and, in the form of a duty cycle of pulses, carries along the line information about acceleration along one of the channels, and the converter board is placed on the inertial mass.

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