Position feedback compensation accelerometers replace the mechanical spring with an "electric spring". The latter is understood as an electromechanical device that creates a moment that compensates for the inertial moment that occurs under the influence of the measured acceleration.
Rice. VI 1.23. Pendulum type compensation accelerometer
Rice. VI 1.24. Float variant of pendulum-type compensation accelerometer: 1 - inductive displacement transducer; 2 - liquid; 3 - float; 4 - torque sensor; 5 - amplifier
On fig. VI 1.23 shows one of the schemes of a pendulum-type compensation accelerometer. The deviation of the pendulum 1 under the action of acceleration is converted by the inductive sensor 4 into an electrical signal, which, after amplification, is fed to the winding of the torque sensor 2. The latter creates a compensating moment. The electric current at the output of the phase-sensitive amplifier 3 and the voltage drop Uiv (created by this current on the additional resistance) are proportional to the measured acceleration x. To dampen the oscillations of the pendulum, the amplifier contains a corrective element.
On fig. VI 1.24 shows a diagram of a variant of a float compensation accelerometer. The lifting force of the float is close to the weight of the entire moving part of the device. The center of gravity of the float is displaced relative to the axis of the float by an amount that provides the necessary pendulum. Float devices have high sensitivity due to small errors from friction forces.
The scheme of a compensation accelerometer with a mass on an elastic suspension, a capacitive signal sensor and an electromechanical sensor that creates a compensating force is shown in Fig. VI 1.25. This scheme makes it possible to weaken the influence of the hysteresis of the elastic suspension and the nonlinearity of its characteristics, provided that the rigidity of the "electric spring" is much higher than the rigidity of the elastic suspension.
Rice. VII.25. Compensatory accelerometer with elastic suspension: 1 - capacitive displacement transducer, 2 - inertial mass; 3 - elastic suspension, 4 - force sensor winding, 5 - amplifier
Rice. VII.26. Scheme of the integrating accelerometer
In a force sensor used in the "electric spring" circuit, the force developed must be proportional to the current flowing in its winding.
Integrating accelerometers. Ground speed can be determined by continuously integrating the horizontal accelerations from takeoff. To determine the distance traveled, the resulting value must be integrated again. Integration can be done in two ways with:
a separate electrical or electromechanical integrator, the input of which is an accelerometer signal proportional to acceleration;
mechanical or electromechanical integrating device combined with the sensitive element of the accelerometer.
Let's consider the last method in more detail.
On fig. VI 1.26 shows one of the possible schemes of an integrating accelerometer. Under the influence of acceleration directed perpendicular to the plane of the drawing, the pendulum 6 will deviate, and a signal occurs in the inductive sensor 5. This signal, amplified by the amplifier, will make the electric motor 3 rotate. A permanent magnet 2 is mounted on its axis, which, when rotated, causes eddy currents in the conductive cap 1. The interaction of eddy currents with the field of the magnet creates a torque applied to the axis of the pendulum. The torque is proportional to the rotation speed of the magnet a:
But the moment balances the inertial moment of the accelerometer, therefore, in the steady process
and the angle of rotation of the magnet will be proportional to the time integral of the measured acceleration:
where is the length of the pendulum; - mass of the pendulum; x is the measured acceleration.
Rice. VII.27. Scheme of an accelerometer with double integration of accelerations
Angle a (reduced by means of a gear reducer) is the output coordinate of the integrating accelerometer. A schematic diagram of an accelerometer with double integration of accelerations is shown in fig. VI 1.27. The sensitive element of the accelerometer is the pendulum 5, on the axis of which the stator 2 of the electric motor is mounted.
Inside the stator, the rotor 1 can rotate freely. The deviation of the pendulum from the zero position causes a signal in the inductive sensor 4, which is applied to the amplifier output amplifier - to the stator of the electric motor. The torque developed by the electric motor causes the rotor to rotate with acceleration
where is the moment of inertia of the rotor.
The reactive moment applied from the side of the rotor to the stator is also equal and directed towards the inertial moment developed by the pendulum 5 under the influence of acceleration X
In the equilibrium position, the moments and (applied to the axis of the pendulum) are mutually compensated. Equating Measure we find
The angle a (reduced by the gear reducer) is the output value of the double integration accelerometer. The error of the device is mainly due to the friction forces in the suspension of the pendulum and accumulates in proportion to the square of the time of its operation. This error can be reduced by reducing the friction forces and increasing the moment of inertia of the motor rotor.
A string accelerometer can be used to integrate the accelerations. It is a vibrating device consisting of a string whose natural frequency changes depending on its tension created by an inertial mass under the influence of a measured acceleration. The change in natural frequency is proportional to the square root of the string tension force, i.e.
where K is a coefficient depending on the size of the string and the magnitude of the inertial mass.
If the inertial mass is suspended between two strings having an initial tension, then in the presence of acceleration directed along the strings, the tension of one string will increase by a value and the other will decrease accordingly.
In this case, the natural vibration frequencies of the strings
The joint solution of these equations gives
If the sum of frequencies is kept constant in the measuring device, then the frequency difference is proportional to the measured acceleration x.
Rice. VII.28. Structural diagram of pendulum-type compensation accelerometer
In this case, the integral of the difference in the frequencies of natural oscillations of a two-string accelerometer for a certain period of time is proportional to the integral of acceleration, i.e., the increment in speed over the same period of time. To integrate the signals of a string accelerometer, digital-type integrators or pulse counters with a high degree of accuracy can be used. Methods for integrating accelerations using gyroscopic integrating accelerometers with gyro-pendulums are given in Chap. VIII, §6.
Determination of transfer functions of compensation accelerometers. The transfer function of a pendulum-type compensation accelerometer (Fig. VI 1.28) is determined using the block diagram shown in Fig. VII.28:
where and are the mass and arm of the pendulum;
Moment of inertia of the moving system;
Transfer coefficients of inductive sensor, torque sensor and amplifier;
R - output electrical resistance;
Transfer function of the corrective link.
The expression (VI 1.31) is converted to the form
The object of study is a microelectromechanical (MEMS) three-axis accelerometer LSM303DLH in combination with a three-axis magnetic field sensor.
The purpose of the work is to study the errors of this accelerometer, the creation of algorithmic and software for determining the statistical errors of the sensor.
The subject of the study is the methods and algorithms for determining the errors of the LSM303DLH MEMS accelerometer.
Figure 1 - Triaxial accelerometer LSM303DLH
The principle of operation of motion sensors (accelerometers and gyroscopes) is based on measuring the displacement of the inertial mass relative to the body and converting it into a proportional electrical signal. The capacitive method for converting the measured displacement is the most accurate and reliable, so capacitive accelerometers are widely used. The structure of a capacitive accelerometer consists of various plates, some of which are stationary, while others move freely inside the case. Capacitances are included in the circuit of the resonant generator. Under the action of the applied control electrical signals, the suspended mass oscillates. A capacitor is formed between the plates, the value of the capacitance of which depends on the distance between them. Under the influence of the acceleration force, the capacitance of the capacitor changes. Figure 2 shows the topology of a MEMS sensor.
Figure 2 - MEMS accelerometer topology
Figure 3 - Types of SE accelerometers
The main structural unit of microelectromechanical accelerometers is a sensing element, the schematic diagrams of which are shown in Figure 2. The sensing element (SE) includes an inertial mass (IM) - 1, elastic suspension elements - 2, a support frame - 3.
Rice. four - Schematic diagram of the MEMS accelerometer: 1 - IM, 2 - fixed electrodes, 3 - anchor, 4 - movable electrodes, 5 - frame, 6 - elastic suspension element, 7 - base (body)
The inertial mass (IM) is mounted at some distance from the base (case) using two pairs of elastic elements, suspension and anchors. The MI moves according to the measured acceleration b. The capacitive displacement meter is formed by comb structures of electrodes, of which the movable electrodes form a single structure with the MI, and the fixed ones, united by a frame, are fastened by a base (case).
The main causes of measurement error in a MEMS accelerometer are temperature, vibration, and cross-acceleration.
A change in the ambient temperature leads to a change in the value of the dielectric constant e, the gap between the pendulum plate and the covers.
Under the action of cross acceleration, there is an additional deformation of the elastic elements of the suspension and the corresponding movement of the pendulum. The movements of the pendulum along the y axis coincide with the direction of the sensitivity axis and are compensated by the torque sensor, i.e. errors are not introduced. Movements of the pendulum along the z-axis relative to the stationary electrodes of the displacement sensor change the effective area of electrode overlap and, without taking constructive measures, can lead to an accidental error. The possibility of this error is prevented by increasing the area of the electrodes on the caps.
The most important accelerometer parameters are the range of measured accelerations, sensitivity, usually expressed as the ratio of the signal in volts to acceleration, non-linearity as a percentage of full scale, noise, temperature drifts of zero (offset) and sensitivity. Thanks to these qualities, they have found their application in many industries: military and civil aviation; automotive industry; aerospace instrumentation; robotics; military industry; oil and gas industry; sport; the medicine. In some cases, an essential characteristic is the natural frequency of the sensor oscillations or the resonant frequency, which determines the operating frequency band of the sensor. In most applications, the temperature range and maximum allowable overload characteristics are important, which are related to the operating conditions of the sensors. The defining parameters that affect the accuracy of determining acceleration are drifts of zero and sensitivity (mainly temperature), as well as sensor noise, which limits the resolution threshold of the device.
The sensitivity of the sensor depends on the resonant frequency of the mechanical subsystem, as well as the quality of the electronic transducer. The change in sensitivity with temperature is mainly due to the change in the elasticity coefficient.
The temperature drift of zero is due to a change in the coefficient of elasticity, thermal expansion, and technological errors in the manufacture of the sensor. The change in the parameters of the electronic part of the sensor under the influence of temperature, as a rule, is significantly less. Since the accelerometer measures the acceleration or the force causing the acceleration of an inertial mass, the physical model of the accelerometer is an inertial mass suspended from a spring fixed in a fixed case, a simple system with one degree of freedom x in the direction of the measuring axis. The inertial mass acquires acceleration under the action of an accelerating force (the resultant inertia force under the influence of acceleration) proportional to the mass m and acceleration a.
Power spectral density (noise density, µ g/vHz rms) in physics and signal processing - a function that describes the distribution of signal power depending on frequency, that is, the power per unit frequency interval. Often the term is used to describe the spectral power of electromagnetic radiation flows or other fluctuations in a continuous medium, for example, acoustic. In this case, it means power per unit frequency per unit area, for example: W/Hz/m 2 .
The main characteristics of the LSM303DLH accelerometer are shown in Table 1.
Table 1 - Main characteristics of the LSM303DLH accelerometer
Figure 5 - Block diagram of the LSM303DLH accelerometer
Figure 6 - Location of pins of the LSM303DLH accelerometer
Table 2 - Assignment of pins of the LSM303DLH accelerometer
Figure 7 - The structure of the motion processing system
Figure 8 - Block diagram of the LSM303DLH module
Microelectromechanical (MEMS) sensors have small weight and size characteristics, low power consumption and cost, and are highly resistant to overloads and shocks. Their main disadvantage is the relatively low accuracy. This fact is primarily due to the fundamental absence today of adequate and possible for use over long time intervals, the intended use of mathematical models of the errors of such sensors.
The most popular applications in the MEMS industry are micromechanical gyroscopes and accelerometers. Their main technical characteristics are dynamic range, sensitivity, frequency response, characteristics of noise components. When calibrating, the microcircuits are fixed with a sufficient degree of accuracy on a tilting turntable, which will make it possible to properly orient the axes of the accelerometers relative to the earth's axis and, therefore, determine their systematic errors. Also implemented is the ability to calculate the coefficients of influence of temperature and supply voltage on the main systematic error, especially characteristic of such sensors. The basis for the development of MEMS is microelectronic technology, which is used in almost all silicon-based products.
The use of MEMS technologies in modern electronic systems can significantly increase their functionality. Using technological processes that are almost the same as the production of silicon microcircuits, MEMS device developers create miniature mechanical structures that can interact with the environment and act as sensors that transmit the impact to the electronic circuit integrated with them. It is sensors that are the most common example of the use of MEMS technology: they are used in gyroscopes, accelerometers, pressure meters and other devices. Currently, almost all modern cars use the MEMS accelerometers discussed above to activate airbags. Microelectromechanical pressure sensors are widely used in the automotive and aviation industries. Gyroscopes are used in a variety of applications, from sophisticated navigational equipment for spacecraft to joysticks for computer games. MEMS devices with microscopic mirrors are used to manufacture displays and optical switches.
With the advent of microelectromechanical systems (MEMS), inertial sensors have received significant development. Advantages such as low cost, low power consumption, small size, and the ability to fabricate using batch technology have allowed inertial MEMS sensors to have a wide range of applications in the automotive, computer, and navigation markets.
Unlike traditional technology, microaccelerometers are etched using specialized techniques that combine mechanical micromachining of the polycrystalline silicon surface and electronic circuit technology.
Accelerometer called a device that measures the projection of apparent acceleration*. As a rule, an accelerometer is a sensitive mass fixed in an elastic suspension. If there is an apparent acceleration, the deviation of the given mass from its initial position is used to judge the magnitude of this acceleration.
* Apparent acceleration is the difference between the true acceleration of an object and the gravitational acceleration.
Design
Accelerometers come in one, two, and three component types. From the name, they respectively measure the apparent acceleration along one, two and three axes (X, Y, Z).
Weightlessness
The true acceleration of an object in weightlessness is caused only by the gravitational force, and therefore the true and gravitational accelerations are equal. As a consequence, there is no apparent acceleration and the data of any accelerometer is 0 (zero). All systems that use an accelerometer as a tilt sensor cease to function. Example: The image position on a tablet or smartphone will not change when you rotate the body.
Scheme of the simplest accelerometer
So, the simplest accelerometer consists of a spring with a load attached to it and a damper, which suppresses the vibrations of this load. The greater the apparent acceleration, the more the spring is deformed, and the instrument readings change.
When there is a balance between the inertia force of the load and the force of the spring, the amount of displacement of this load from the neutral position is recorded, which indicates the amount of acceleration (deceleration). This value is recorded by some displacement sensor and converted into an electrical signal at the output of the device.
Technologies for building modern accelerometers
Depending on the construction technology, the following accelerometers are distinguished:
piezoelectric;
piezoresistive;
on variable capacitors.
Piezoelectric accelerometers are widely used in testing and measurement tasks. They have a very wide frequency range and sensitivity range. In addition, they can have various sizes and shapes. The output signal of such accelerometers can be charge or voltage. The sensors can measure both shock and vibration.
Piezoresistive accelerometers are usually characterized by a small sensitivity range, as a result of which they are more applicable to shock detection than to vibration detection. In addition, they are used in crash safety tests. These accelerometers mainly have a wide frequency range, and the frequency response can go down to 0 Hz (so-called DC sensors) or remain unchanged. This makes it possible to measure long signals.
Variable Capacitor Accelerometers, like piezoresistive ones, have a DC response. Such accelerometers have high sensitivity, narrow bandwidth, excellent temperature stability, and low error. These accelerometers measure low frequency vibration, motion and fixed acceleration.The principle of operation of any accelerometer is based on the property of bodies to maintain their position unchanged during the accelerated movement of the base on which they are somehow fixed.
Pendulum accelerometers with an electric spring (Figure 6) are used in systems for stabilizing the center of gravity of the launch vehicle in positional and integrating versions. A fairly large variety of design schemes of pendulum accelerometers is known. However, a common feature for them is the presence of a mechanical system associated with the pendulum, and an electrical or photo-optical (as well as electrostatic, capacitive) system for collecting useful information.
The compensation measurement method underlying most pendulum accelerometers, in principle, ensures high measurement accuracy. The implementation of this method in accelerometers is carried out using compensating power or torque devices based on various physical principles - mechanical, electromagnetic, electrostatic.
Magnetoelectric converters are currently the most widely used, in which a compensating moment or force is created due to the interaction of the magnetic field created by the feedback current that flows through the converter winding with the field of a permanent magnet. Such transducers provide the necessary moments (forces) with small dimensions and have an acceptable stability of parameters at this stage.
The principle of operation of a pendulum accelerometer with an open key (integrating option) is as follows. When an apparent acceleration W z , directed along the OZ axis, occurs, the movable frame with the pendulum, which tries to keep its position unchanged, will begin to unfold relative to the fixed frame. As a result of the relative rotation of the frames, the magnetic flux of the movable frame, crossing the turns of the winding of the fixed frame, will cause an electromotive force in it. The voltage taken from the winding of the fixed frame, after amplification in the amplifier, enters through the capacitor and flexible conductors to the winding of the movable frame and will cause a feedback current i os in it. This current, in turn, will cause a magnetic flux moving frame. The interaction of the magnetic flux of a permanent magnet with the average value of the magnetic flux from the feedback current will cause the mechanical feedback moment M os, directed against the moment of inertial forces M and.
If we assume that the apparent acceleration W z is constant, then in the steady state there will be equality between the indicated moments, i.e. M os =M and, and the measure of the measured acceleration can be the current i os in the feedback circuit of the pendulum accelerometer flowing through the winding of the movable frame.
With an open key and complete idealization of all links in the feedback chain, we can assume that
(1.1)
Since M and \u003d mlW x, then when M os \u003d M and we get
or after integration under zero initial conditions
(1.3)
Obviously, the integral of the apparent acceleration is equal to the apparent speed, i.e.
(1.4)
where t k is the integration interval, therefore
With a closed key and the same initial data
Thus, the same pendulum accelerometer can be integrating with flexible feedback, and positional with rigid feedback. This circumstance is widely used in the initial exhibition of aircraft control systems and in controlling their movement in flight. So, with the key open, the accuracy of the initial alignment of the command instrument complex increases, since with flexible feedback, the statistical errors of the pendulum accelerometer with an electric spring, as the simplest circuit of an automatic control system, are excluded.
In accelerometers of the compensation type, an angle sensor (DU) is used to obtain information about the magnitude of acceleration. Photo sensors (PD) and capacitive-type sensors (ED) are the most widely used both in navigational and industrial models of accelerometers.
The use of PD allows the use of relatively simple electronic circuits to amplify the useful signal. In a typical compensating type accelerometer, such a control is used.
The main elements of this measuring device are:
SD LED;
Two photodiodes VD1 and VD2;
Shutter, rigidly fixed to the pendulum, and located between the light and photodiodes;
Analogue (linear) signal preamplifier DA surrounded by feedback resistance Roc;
Resistance that converts voltage into feedback current RI;
Torque sensor winding (DM) L.
The principle of operation of this pendulum accelerometer in the analog (standard) mode is as follows. When an apparent acceleration Ain occurs, directed along the axis of sensitivity, the pendulum and the shutter rigidly connected to it, seeking to keep the position unchanged, will begin to turn around relative to the body of the accelerometer. As a result of relative rotation, one of the LEDs will light up more than the other. As a result, there will be a potential difference at the output of the remote control. This voltage will be applied to the input of the preamplifier and, after amplification, in the form of a feedback current, will enter the DM winding. DM will form a compensating moment, which will return the pendulum to its original state. Thus, according to the value of the feedback current can be judged on the value of the apparent acceleration.
At the moment the accelerometer pendulum begins to move, it is affected by the static friction force, which introduces an error into the measurements (sensitivity threshold).
Golyaev Yu.D., Ph.D., Kolbas Yu.Yu., Konovalov S.F., Doctor of Technical Sciences, Professor,
Solovieva T.I., Ph.D., Tomilin A.V.
(JSC Research Institute Polyus named after M.F. Stelmakh; Moscow State Technical University named after N.E. Bauman;
MIEM NRU HSE)
The results of studies and comparative tests of silicon and quartz accelerometers in an inertial measuring unit are analyzed. The advantages and disadvantages of two types of accelerometers related to the material of the pendulum and their influence on the accuracy parameters that determine the accuracy class of inertial measuring units based on them are considered.
Investigations and comparative tests of the accelerometers in the inertial measurement unit. Golyaev Yu.D., Kolbas Yu.Yu., Konovalov S.F., Solovieva T.I., Tomilin A.V.
The results of investigations and comparative tests of Si-flex and Q-flex accelerometers in the inertial measurement unit are analyzed. The advantages and the problems of the above accelerometers connected with pendulum material are described and its influence on the accelerometers accuracy parameters as well as inertial measurement unit’s accuracy class are discussed.
Key words: silicon accelerometer, quartz accelerometer, inertial measuring unit.
Key words: Si-flex accelerometer, Q-flex accelerometer, inertial measurement unit.
Introduction
The most promising for use in systems that require high accuracy when operating in a wide range of accelerations and in harsh operating conditions are compensating accelerometers with pendulums made of silicon or quartz.
They find wide application in various industries, from navigation technology for space, rocket, aviation industries and ending with non-traditional applications in construction, in monitoring systems in inclinometers for measuring the profile of oil and gas wells during drilling.
The design schemes of silicon and quartz accelerometers are similar (see Fig. 1.2). The main structural elements are the pendulum assembly, which consists of an installation frame, an elastic suspension and a blade, a capacitive angle sensor and a magnetoelectric moment sensor, which provides compensation for the deviation of the pendulum blade under the influence of acceleration. The material of the pendulum plays a key role in the difference in the characteristics of the two types of accelerometers. In this case, one should keep in mind the main feature of the construction materials of the pendulum. It lies in the difference in the thermal coefficients of expansion (TEC) of these materials. The TEC of fused quartz is almost equal to the TEC of the material of the magnetic circuit of the accelerometer magnetic system made of superinvar 32NKD, while the TEC of silicon exceeds it by almost 5 times, which creates problems for basing silicon pendulums on superinvar parts. At the same time, silicon has a number of obvious technological advantages over quartz, both due to the use of MEMS technology, and due to the cheapness and availability of blanks, which are used as standard silicon “wafers” of the electronics industry.
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Rice. 1. Structural scheme quartz accelerometer: 1 - pendulum blade; 2 - elastic beam suspension of the pendulum; 3, 8 - magnetic system; 4 - force sensor coil; 5 - pendulum assembly ring with mounting plates; 6 - pole tip; 7 - permanent magnet |
Rice. 2. Structural scheme silicon accelerometer: 1 – pendulum blade; 2 – elastic beam for suspension of the pendulum; 3, 7 - magnetic system; 4, 8 – quartz rings; 5 – force sensor coil; 6 - pendulum assembly frame with mounting plates |
Comparative analysis of features of silicon and quartz accelerometers due to structural materials
A more detailed comparison of the properties of quartz and silicon makes it possible to note the following features of devices due to the difference in materials:
The elastic modulus of quartz (107 GPa) is about two times less than that of silicon (160 GPa). This allows, with the same strength properties of the elastic suspension of the pendulum, to have half the rigidity of the quartz suspension compared to the silicon suspension and, for this reason, reduce the requirements in quartz devices to the value of the time and temperature zero drift of the compensation circuit amplifier by half;
– the thermal conductivity of silicon (157 W/(o C m)) is many times higher than the thermal conductivity of quartz (1.38 W/(o C m)). For this reason, less overheating of the blade and coils can be expected in silicon pendulums;
- quartz has TKR = 0.55 10 -6 1/ o С versus TKR = 2.6 10 -6 1/ o С for silicon. For this reason, quartz parts have significantly less dimensional change with temperature compared to silicon parts;
– TCR of quartz (0.55 10 -6 1/ o C) is ideally combined with TCR of magnetic cores from superinvar 32NKD, equal to 0.56 10 -6 1/ o C. Therefore, in accelerometers with a quartz pendulum, the problem of fixing the pendulum is much easier to solve and, therefore, substantially higher zero offset stability can be more easily achieved;
- silicon can also be well combined in TEC with a number of Invar-like alloys, however, commercially available materials, for example, 39N, have a passport TEC value close to silicon. But the spread of TCR with an allowable difference in the nickel content in the 39N alloy from 38% to 40% gives a spread of TCR from 2 10 -6 to 4 10 -6 1/ o C. This leads to significant problems when basing the pendulum and to the associated zero offset instability problem. Pyrex is an acceptable pair for silicon, but the use of intermediate layers when connecting the pendulum to the invar parts of the magnetic cores complicates the design of the accelerometer;
- quartz is an insulator, therefore, it cannot be used without sputtering electrodes used as movable electrodes of a capacitive angle sensor and current leads to them. Silicon has sufficient electrical conductivity to be used as a movable electrode of a capacitive angle sensor without additional deposition of electrodes and current leads;
– A monocrystalline silicon pendulum can be manufactured using well-established methods in the electronics industry and from standard blanks. Usually, for silicon doped with phosphorus, the method of photolithography and liquid anisotropic etching in a 33% aqueous solution of KOH at a temperature of 100 ° C to 107 ° C is used. Sometimes ion-plasma etching is used. It is important to note that the liquid etching process proceeds anisotropically, which makes it possible to ensure an unambiguous and exact correspondence between the templates used in photolithography and the shape of the manufactured pendulums. Anisotropic etching makes it possible to obtain complex shapes of the elastic suspension of the pendulum blade (flat beams, cruciform and X-shaped extensions). The protective film during etching is a layer of silicon oxide grown in an oxidizing environment (moist oxygen) at a temperature of ~ 1100 ... 1200 ° C. Silicon blanks - "wafers" used in the manufacture of pendulums, are mass-produced by electronic industry enterprises and are cheap. Group production of pendulums is easily realized. Until recently, quartz pendulums were made individually from special blanks and therefore were expensive. Fused quartz "wafers" that have now appeared allow for the transition to batch technology. But here the technological process is significantly complicated due to the need for multiple deposition of protective gold films with a chromium sublayer (up to 8 microns thick) and multiple photolithographs. Otherwise, it is not possible to obtain the required shape of the elastic bridge - the process of etching quartz in hydrofluoric acid is isotropic. The achieved shape of the elastic suspension is a flat elastic beam.
Thus, today silicon is a more technologically advanced material and makes it possible to obtain cheaper products. At the same time, silicon is inferior to fused quartz in terms of its ability to provide higher accuracy characteristics of devices.
It can be seen from the foregoing that it is not possible to give priority to one or another type of accelerometers for use in a particular system without conducting comparative tests of devices based on both silicon and quartz.
Selection of accelerometers for comparative testing
The purpose of this research was to select an accelerometer that best meets the requirements for the accelerometric path for an inertial measuring unit (IMU).
Based on the specifics of the ISS application, which requires a short readiness time after power is applied (as a result, the absence of a thermostat) in a wide range of accelerations and temperatures, pendulum gas-filled devices were chosen for the ISS. These include quartz accelerometers of the QA-2000, QA-3000, A-18, BA-3, AK-6 types, as well as the newly developed AK-15, A-18T and AAK-02.
Since it is impossible to calibrate the accelerometer channels in the IMB product before use, the irreproducibility of the accelerometer parameters, namely the scale factor , zero offset and two angles that determine the position of the base plane, acquires the most important role. Errors increase all the more after exposure to extreme high and low temperatures, since in this case the temperature hysteresis of the parameters with short-term and long-term instabilities are added.
That is why for the initial assessment of the suitability of accelerometers for use in ISS, the non-reproducibility of the above parameters after exposure to both extreme high and low temperatures was chosen.
A detailed study of various types of accelerometers is given below.
Analysis of accelerometers for application in ISS
Currently, there are both mass-produced and newly mastered in production accelerometers that are close in parameters to the requirements for accelerometers in the IMU: scale factor irreproducibility 9 10 -5 rel.un., zero offset irreproducibility 8 10 -5 g, angle change reference plane orientation 40 "
.
Characteristics of accelerometers according to specifications or brochures are given in Table 1.
Table 1
|
Parameter name |
Unit meas. |
Requirements to accelerometers |
A-18 |
AK-15 |
VA-3 |
A-18T |
AK-6 |
E1 |
|
Scale factor non-reproducibility |
Rel. |
9 10 -5 |
15 10 -5 |
20 10 -5 |
24 10 -5 |
10 10 -5 |
8 10 -5 |
5 10 -5 |
|
Displacement non-reproducibility |
g |
8 10 -5 |
20 10 -5 |
3 10 -5 |
16 10 -5 |
10 10 -5 |
6 10 -5 |
8 10 -5 |
|
|
" |
40 |
30 |
4 |
20 |
20 |
10 |
20 |
|
Range of measured accelerations |
g |
40 |
40 |
20 |
50 |
40 |
20 |
50 |
|
Operating temperature range |
about C |
-50…+85 |
-60… |
-60… |
-55… |
-50… |
-60… |
-55… |
|
pendulum material |
silicon |
quartz |
quartz |
silicon |
quartz |
quartz |
||
|
Manufacturer |
ITT |
MIEA |
Electro-optics |
ITT |
sickle plant metalworker |
China |
||
|
Price |
thousand roubles. |
190 |
210 |
250 |
250 |
220 |
130 |
Preliminary checks of the accelerometers presented in the table showed that their parameters do not always correspond to those advertised. Therefore, it was necessary to develop a special technique for their thorough analysis in the temperature range. This technique provides for measuring the irreproducibility of parameters with high accuracy due to the fact that this characteristic is not subject to algorithmic correction and will have a decisive influence on the accuracy of the IMS accelerometer channel.
Accelerometer Test Method
When testing for the irreproducibility of the parameters, the following procedure was used, consisting of 5 stages.
The accelerometers were fixed on a dividing head in a heat and cold chamber. The temperature of +251 o C was set in the chamber, and the accelerometers were kept at this temperature for 2 hours. Then the accelerometers turned on. After 1.5 hours of operation, the scale factor, zero offset and deviation angles of the base plane of the accelerometers were measured. At the same time, the temperature of the accelerometers was controlled using the built-in thermal sensor. The measurement errors in this case were: according to the scale factor 0.5·10 -5 rel. units, by zero shift 1 10 -5 g, by angles of deviation of the base plane 10 " , at a temperature of 0.2 o C.
Then the accelerometers were turned off, and the temperature in the chamber was set to –501 o C, and the accelerometers were kept at this temperature for 2 hours. After that, the accelerometers were switched on for 1.5 h at this temperature, and the value of the scale factor, the zero offset, and the angles of deviation of the base plane were measured.
Then the described procedure was repeated at temperatures of +251 o C, +751 o C, +251 o C with the measurement of the scale factor, zero offset and deviation angles of the base plane of the accelerometers and control of the accelerometer temperature using the built-in temperature sensor.
Based on the five values obtained for each accelerometer, the temperature dependence of the scale factor, zero offset, and base plane deflection angles (a second-order polynomial) were calculated. For three values at +251 о С, the irreproducibility of these parameters was calculated, equal to the maximum deviation from the temperature dependence. This technique makes it possible to take into account all temperature errors up to the third order of smallness and provide the necessary measurement accuracy in the heat and cold chamber, which has a temperature setting error of 1 ° C.
The test results for specific accelerometers are shown in Table 2. For each parameter, the ranges of values obtained for several samples of accelerometers that simultaneously participated in the tests are indicated.
table 2
Characteristics of accelerometers according to test results
|
Parameter name |
Unit meas. |
A-18 |
AK-15 |
A-18T |
AK-6 |
E1 |
|
Scale factor non-reproducibility |
Rel.un. |
(10–15) · |
(16–18) |
(3–5) |
(3–7) |
(1–24) |
|
Zero offset non-reproducibility |
g |
(15–19) · |
(13) · |
(15–28) · |
(4–8) |
(4–6) |
|
Change the orientation angles of the reference plane |
" |
20–32) |
21–24) |
9–13) |
3–6) |
10–12) |
Conclusion
Of all the devices submitted for testing, none of the accelerometers fully complies with the requirements for the IIB accelerometer channel, however, to a different extent.
Accelerometer AK-6 complies with the requirements for IIB, except for the range of measured accelerations.
Accelerometer A-18 does not meet the requirements for ISS in terms of non-reproducibility of the scale factor, non-reproducibility of zero offset, change of orientation angles of the base plane.
The AK-15 accelerometer does not meet the requirements for ISS in terms of the scale factor non-reproducibility parameters and the range of measured accelerations.
The E1 accelerometer does not meet the requirements for the IIB device in terms of the scale factor non-reproducibility parameter (five devices out of six) . At the same time, a small part of the E1 devices shows exceptionally high accuracy characteristics, which indicates, on the one hand, a successful design, which is a copy of the American QA-3000 quartz accelerometer, and, on the other hand, the undeveloped technology for the production of these accelerometers.
The layout of the A-18T accelerometer does not meet the requirements for the IIB device in terms of the non-reproducibility zero offset parameter.
It should be noted that all tested accelerometers, except for AK-6, A-18 and AK-15, do not really correspond to the parameters specified in the brochures and specifications.
conclusions
All accelerometers with a silicon pendulum do not meet the requirements for zero offset non-reproducibility. This, apparently, is a disadvantage fundamentally inherent in accelerometers with a constructive scheme used in the A-18.
At the same time, all accelerometers with a quartz pendulum meet the requirements for the non-reproducibility of the zero offset and the change in the orientation angles of the base plane, and the other parameters are very close to the required ones.
Compliance with the requirements for the scale factor irreproducibility parameters and the range of measured accelerations for devices with a quartz pendulum is determined by the art of the designer and is quite achievable, especially when using modern magnets with a small temperature hysteresis.
The organization of group production of quartz pendulums from mass-produced quartz blanks (wafers) of large diameter with a minimum of manual operations using MEMS technologies will eliminate the disadvantage of quartz compared to silicon - the impossibility of using group technologies and will significantly reduce the cost of quartz accelerometers compared to prices prevailing on the Russian market . In this case, the absence of mechanical processing of pendulums will contribute to an increase in the accuracy of the instruments.
Since it is AK-6 that is closest in terms of accuracy to the requirements of the ISS, it is its design that should be taken as the basis for updating the accelerometer to the requirements of the ISS with a recommendation to introduce the latest group technologies in production that provide increased productivity and reduced cost. An increase in the measurement range of AK-6 is achieved without making structural changes. To reduce the warm-up time and increase the stability of the zero offset, the main fuel elements, primarily the feedback amplifier electronics, should be moved outside the body of the accelerometer itself. Carrying out these obvious improvements will make it possible to produce serial domestic accelerometers of the AK-6 type, which fully meet the requirements for the accelerometric tract of the IIB.
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