Inertial Reference Unit Research Articles

Mechanical Resonance Suppression Technique for Inertial Reference Unit Based on Band-pass Filter Disturbance Observer

Inertial reference unit (IRU) has been recognized as an effective method for photoelectric tracking and pointing system to suppress the disturbance. Its flexible support usually results in a low-frequency mechanical resonance within the bandwidth. Experimental identification indicates an obvious resonant peak at near 36.7Hz with an amplitude of 26.6dB. In this paper, disturbance observer based on band-pass filter (BPF-DOB) is combined with a notch filter to suppress the mechanical resonance. The asymmetry of the resonance peak and variation of the controlled object over the time are both addressed. Simulation and experimental results show that the stability accuracy of IRU system is improved to be 1.09 μrad, indicating an improvement of 9.9% relative to the traditional method and high robustness for time-varying resonance.

Avionics System Design using MBSE Methodology

Abstract: The West Air Sweden Flight 294 serves as a reminder of tragic crashes of aircraftdue to Avionics System failure. Due to malfunction of the Inertial Reference Unit (IRU), the aircraft lost its control resulting in tragic loss of life of the peopleon board. The Avionics System is a complex set of instruments, logical and physical links which must comply with the guidelines of Dob178c. The design of the system is complicated. This paper aims to understand the behavior of the Avionics System using the state-of-the-art Model Based System Engineering (MBSE) technology. This includes the two widely used tools – Capella and Simulink. We aim to understand the dynamic nature of avionics through these tools. Model Based System Engineering can be specifically beneficial because it will help the engineers to detect the flaws in the avionics system much earlier. This will potentially contribute to helping to prevent aircraft crashes due to avionics system failure. The idea of model-based system engineering includes the concept of taking a model as a reference to systematically solve the complex designs of any engineering stage. Before the concept of Model Based Systems Engineering came to be, Documents Based Systems Engineering was the trend in the system engineering department. However, it would take a lot of time, effort and accuracy to correctly pinpoint the information and accordingly design the system from a bunch of documents. MBSE, however, eases such kind of trouble by providing a model as a reference for the systems design

Review on inertial reference unit applied to photoelectric tracking and pointing system of moving platform

Review on inertial reference unit applied to photoelectric tracking and pointing system of moving platform

Bracket design and natural frequency analysis of civil aircraft inertial reference unit

The Inertial Reference Unit (IRU) is an important and indispensable airborne equipment for civil aircraft, which is used to provide information on aircraft attitude, heading, acceleration and so on. The design of its bracket will directly affect the signal accuracy of the equipment. In order to ensure the accuracy of the information of the measuring equipment, the stiffness design of the support has strict requirements to avoid the impact of vibration and elastic deformation on the accuracy of the output signal. Through theoretical formula derivation and finite element analysis, this paper summarizes the influence of different bracket structure forms on the natural frequency and obtains the scheme to improve the natural frequency of the bracket. By increasing the flange height, adjusting the lightening hole on the plane mounting plate of the bracket and making the plane mounting plate, frame and skin form a box-shaped structure, the torsional stiffness of the bracket is enhanced. The results show that the improved bracket meets the requirements of natural frequency which is greater than 110 Hz, which can provide a reference for the design and strength analysis of similar aircraft airborne equipment.

An adaptive frequency disturbance rejection control for an inertial reference unit under unknown disturbance

The inertial reference unit (IRU) can provide a reference beam for the line-of-sight stabilization system to achieve precise pointing and tracking. The reference beam leaves the IRU with minimal jitter and determines the accuracy of acquiring, tracking and pointing system. When the vehicle is moving, the disturbance usually gathers around one frequency. Meanwhile, the resonance introduced by the flexure support of the IRU seriously affects the stabilization accuracy. In this paper, an extended state observer with frequency adaptation is proposed to improve the performance of disturbance rejection. An auxiliary filter is used to optimize the disturbance frequency estimation and improve the control bandwidth. Simulation results and experimental data verify the effectiveness of the proposed method.

Inertial Stable Platforms for Precision Pointing of Optical Systems in Aerospace Applications

Both airborne and space-based high-energy laser weapon systems present interesting challenges for precision line-of-sight control. Sub-microradian to microradian pointing accuracies are required against dynamic targets. In addition, “absolute” pointing and accurate inertial angular rate measurement are required to support mission requirements. This paper addresses these unique directed-energy weapon (DEW) precision-pointing requirements and their associated implementation alternatives in the context of strapdown and stable platform inertial reference technologies. The contributions of this paper consider the broader issues of DEW requirement drivers, integration of the stabilization system with the rest of the optical system, and design tradeoffs between stable platform and strapdown stabilization mechanizations. Both types of mechanisms are mounted to the gimbal of an imaging system using gryo sensors (gyros) and accelerometers to maintain a desired orientation in inertial space independent of the craft motion. Although both can include an optical reference that propagates through the optical path for optical alignment and jitter control, only in the stable platform is the optical reference stabilized. When combined on the same platform with a stabilized inertial measurement system and closed-loop stabilization with the optical reference, the mechanism becomes a stabilized inertial reference unit, which provides a stabilized optical reference to the electro-optical/laser pointing and jitter control systems.

Disturbance Attenuation and Pointing Control System Design for an Improved Disturbance-Free Payload Spacecraft

The low-frequency disturbances transmitted by flexible cables are difficult to be attenuated for a novel disturbance-free payload spacecraft, which decreases the payload’s pointing accuracy and stability. In this research, a new spacecraft configuration with a high-precision inertial reference unit composed of capacitive sensors and a spherical test mass is proposed. The disturbance attenuation and pointing control system is subdivided into four interconnected control loops. The payload can be isolated from disturbances in the all-frequency band by the active vibration isolation control loop and the drag-free control loops, and its high-precision pointing requirement can be satisfied with the attitude pointing control loop and the attitude tracking control loop. An integrated control strategy is proposed, and the control system is decoupled into 12 single-input single-output control loops by pre-compensating, which lays the foundation for feedback design. Through the amplitude-frequency response analysis, the control bandwidth is designed according to the Proportional-Integral-Differentive control algorithm. The numerical simulations show that the disturbance attenuation performance is better than −20 dB in the all-frequency band, and the pointing accuracy and the pointing stability are better than 10−6 deg and 10−7 deg/s, respectively. The new spacecraft configuration and the disturbance attenuation and pointing control system provide a general technical solution for payloads with high-precision and high-stability requirements.

Adaptive Sliding-Mode Control for Inertial Reference Units via Adaptive Tracking Differentiators

In this article, an adaptive sliding-mode control strategy is investigated for an inertial reference unit with an adaptive tracking differentiator (TD). A dynamic second-order system is established from a spring-mass-damper system by abstracting and analyzing the inertial reference unit. The adaptive TD is used to reduce overshoot and improve tracking speed for the inertial reference unit by designing adaptive parameters and hyperbolic tangent functions. An adaptive sliding-mode controller is designed to improve response rapidity via adaptive laws and adaptive gains based on an integral terminal sliding-mode function. Effectiveness analysis is shown on error convergence for the adaptive TD and the adaptive sliding-mode controller based on Lyapunov methods. A series of experiments are given to illustrate the effectiveness of the adaptive sliding-mode control strategy in the inertial reference unit.

Ground test results of the microvibration interference for the x-ray microcalorimeter onboard x-ray imaging and spectroscopy mission

Resolve is a payload hosting an X-ray microcalorimeter detector operated at 50 mK in the X-Ray Imaging and Spectroscopy Mission (XRISM). It is currently under development as part of an international collaboration and is planned to be launched in 2023. A primary technical concern is the micro-vibration interference in the sensitive microcalorimeter detector caused by the spacecraft bus components. We conducted a series of verification tests in 2021-2022 on the ground, the results of which are reported here. We defined the micro-vibration interface between the spacecraft and the Resolve instrument. In the instrument-level test, the flight-model hardware was tested against the interface level by injecting it with micro-vibrations and evaluating the instrument response using the 50 mK stage temperature stability, ADR magnet current consumption rate, and detector noise spectra. We found strong responses when injecting micro-vibration at about 200, 380, and 610 Hz. In the former two cases, the beat between the injected frequency and cryocooler frequency harmonics were observed in the detector noise spectra. In the spacecraft-level test, the acceleration and instrument responses were measured with and without suspension of the entire spacecraft. The reaction wheels (RWs) and inertial reference units (IRUs), two major sources of micro-vibration among the bus components, were operated. In conclusion, the observed responses of Resolve are within the acceptable levels in the nominal operational range of the RWs and IRUs. There is no evidence that the resultant energy resolution degradation is beyond the current allocation of noise budget.

High-Precision Finite-Time Positioning Control for an Inertial Reference Unit With Asymmetric Loads and Mover Sideslip

In this article, a finite-time controller is designed on high-precision positioning for an inertial reference unit (IRU) with asymmetric loads and mover sideslip based on a finite-time extended state observer (ESO). The asymmetric loads caused by centroid deviation from central axis are a negative factor, which results in a negative load torque for IRUs. The mover sideslip is another negative factor that brings about output forces errors between actual output forces and theoretical output forces for voice coil motors. The finite-time ESO is designed to estimate and compensate the asymmetric loads and mover sideslip. The finite-time controller is proposed with high convergence rate for the IRU. Experimental results are presented to show effectiveness of the proposed finite-time control method in high-precision positioning under asymmetric loads and mover sideslip.

Research on inertial reference units prescribed performance integral sliding mode angle control

In this paper, a prescribed performance integral sliding mode control (ISMC) algorithm is proposed to realize high precision and rapid response for an inertial reference unit (IRU). To ensure rapid response, a tracking differentiator (TD) is introduced to arrange a transition process for a desired signal. Tracking error is limited within two prescribed performance function (PPF) bounds to improve transient performance of the IRU. By a prescribed performance error transformation, an error transformation system is established to transform the original constrained system into an equivalent unconstrained one for the IRU. A prescribed performance integral sliding mode controller is designed for the error transformation system to ensure stead-state accuracy. Stability of the prescribed performance ISMC algorithm is proved by a Lyapunov method. Comparative experiments are given to verify effectiveness and robustness of the prescribed performance ISMC algorithm for the IRU.

Robust Data-Driven Fault Detection: An Application to Aircraft Air Data Sensors

Fault detection (FD) is important for health monitoring and safe operation of dynamical systems. Previous studies use model-based approaches which are sensitive to system specifics, attenuating the robustness. Data-driven methods have claimed accurate performances which scale well to different cases, but the algorithmic structures and enclosed operations are “black,” jeopardizing its robustness. To address these issues, exemplifying the FD problem of aircraft air data sensors, we explore to develop a robust (accurate, scalable, explainable, and interpretable) FD scheme using a typical data-driven method, i.e., deep neural networks (DNN). To guarantee the scalability, aircraft inertial reference unit measurements are adopted as equivalent inputs to the DNN, and a database associated with 6 different aircraft/flight conditions is constructed. Convolutional neural networks (CNN) and long-short time memory (LSTM) blocks are used in the DNN scheme for accurate FD performances. To enhance robustness of the DNN, we also develop two new concepts: “large structure” which corresponds to the parameters that can be objectively optimized (e.g., CNN kernel size) via certain metrics (e.g., accuracy) and “small structure” that conveys subjective understanding of humans (e.g., class activation mapping in CNN) within a certain context (e.g., object detection). We illustrate the optimization process we adopted in devising the DNN large structure, which yields accurate (90%) and scalable (24 diverse cases) performances. We also interpret the DNN small structure via class activation mapping, which yields promising results and solidifies the robustness of DNN. Lessons and experiences we learned are also summarized in the paper, which we believe is instructive for addressing the FD problems in other similar fields.

Mechanical design and determination of bandwidth for a two-axis inertial reference unit

The inertial reference unit (IRU) serves as a master reference for acquisition, tracking and pointing (ATP) system due to its ability to maintain stability with respect to inertial frame. However, the bandwidth of IRU system is usually limited by the dynamics of mounted inertial sensors and structural resonances. In this paper, magnetohydrodynamics (MHD) angular rate sensor (ARS) with extremely low noise at high frequencies (>3Hz) was combined with a conventional gyro serving as inertial sensing unit of the developed IRU. This sensing unit was verified experimentally to exhibit a nearly perfect transfer function with “unity” gain and “zero” phase error. A central hinge that allows only rotation about the actuating axes was used as the supporting structure. A structural parameter design approach based on analytical stiffness model was developed for the central hinge. The relative errors between theoretical stiffness values, finite element analysis (FEA) simulations and experimental data were found to be within ±13%. A double-T network notch filter with optimal parameters was implemented to suppress the low-frequency mechanical resonance. With the notch filter in the forward path, the open-loop dynamic model of the IRU was measured with the results indicating a completely damped resonant peak and an identification error less than ±0.5 dB in magnitude and ±5° in phase. The developed IRU was ultimately certified to achieve more than ±6 mrad as a drive range and more than 120 Hz as a closed-loop bandwidth under a digital controller designed with frequency shaping technique.

A resonance suppression method in platform style inertial reference unit via particle swarm optimization notch filter.

With the development of laser communication, remote sensing imaging, and other technologies, an inertial reference unit (IRU) plays an essential part in the line-of-sight (LOS) stabilization system used for acquiring, pointing, and tracking targets. The IRU provides a stable reference beam to realize accurate LOS pointing under external disturbances. Compared with the frame style IRU, the platform style IRU (PIRU) can achieve a higher bandwidth and better precision. However, mechanical resonance is introduced by a flexure hinge inevitably in the PIRU, which affects the performance of the LOS stabilization system. In this paper, an open-loop dynamic model of PIRU is established. Identification experiments are carried out with results indicating a 28.7 dB resonance peak at 27.07Hz in the x axis and a 30.3 dB resonance peak at 26.59Hz in the y axis. An asymmetric notch filter is used to suppress the resonance peak to achieve a higher control bandwidth. A fitness function is designed to represent the effect of resonance suppression. A particle swarm optimization algorithm is used to search for an optimal solution of the fitness function to obtain the parameters of the asymmetric notch filter. Experimental results show that the resonance peak is reduced by 97.88% and the system bandwidth reaches 159.31Hz.

An On-Orbit Dynamic Calibration Method for an MHD Micro-Angular Vibration Sensor Using a Laser Interferometer.

The magnetohydrodynamic (MHD) micro-angular vibration sensor is a significant component of the MHD Inertial Reference Unit (MIRU) and measures micro-amplitude and wide frequency angular vibration. The MHD micro-angular vibration sensor must be calibrated in orbit since the ground calibration parameters may change after lift-off. An on-orbit dynamic calibration method for the MHD micro-angular vibration sensor is proposed to calibrate the complex sensitivity of the sensor in high frequency. An absolute calibration method that combines a homodyne laser interferometer and an angular retroreflector was developed. The sinusoidal approximation method was applied, and the calibration system was established and tested using a manufactured MHD sensor. Furthermore, the measurement principle and installation errors were analyzed, including the eccentric installation error of the retroreflector, the tilt installation error of the retroreflector, and the optical path tilt error. This method can be realized within a rotation range of and effectively avoid the installation error caused by mechanical errors. The results indicate that the calibratable angular vibration frequency range is 25–800 Hz, and the angular velocity range is –7590 mrad/s. The expanded uncertainties of the sensitivity amplitude and phase shift of the calibration system for the MHD micro-angular sensor are and .

A novel method of calibrating a MEMS inertial reference unit on a turntable under limited working conditions

Micro-electro-mechanical systems (MEMS) inertial measurement devices tend to be widely used in inertial navigation systems and have quickly emerged on the market due to their characteristics of low cost, high reliability and small size. Calibration is the most effective way to remove the deterministic error of an inertial reference unit (IRU), which in this paper consists of three orthogonally mounted MEMS gyros. However, common testing methods in the lab cannot predict the corresponding errors precisely when the turntable’s working condition is restricted. In this paper, the turntable can only provide a relatively small rotation angle. Moreover, the errors must be compensated exactly because of the great effect caused by the high angular velocity of the craft. To deal with this question, a new method is proposed to evaluate the MEMS IRU’s performance. In the calibration procedure, a one-axis table that can rotate a limited angle in the form of a sine function is utilized to provide the MEMS IRU’s angular velocity. A new algorithm based on Fourier series is designed to calculate the misalignment and scale factor errors. The proposed method is tested in a set of experiments, and the calibration results are compared to a traditional calibration method performed under normal working conditions to verify their correctness. In addition, a verification test in the given rotation speed is implemented for further demonstration.

An algorithm and observability research of autonomous navigation and joint attitude determination in asteroid exploration descent stage

In this paper, an autonomous relative navigation and joint attitude determination algorithm in asteroid exploration descent stage is researched based on feature point information of perpendicular asteroid surface image observed by optical navigation camera, distance vectors from spacecraft to asteroid measured by three angled installed lidars and relative velocity increment measured by accelerometer when the relative distance vector to the centroid of asteroid can not be obtained. The inertial attitude of spacecraft is determined by sun vector, star vectors and inertial angular velocity respectively measured by sun sensor, star trackers and inertial reference unit. Also, in order to obtain measurement error model transferred from sensor noise, a covariance matrix solver considering error correlation is presented via the error model of normalized vector to first order. Numerical simulation and improved observability evaluation of filtering are undertaken to discuss the results of complete sensor observation and weak observation of lidars, and verify the effectiveness of the presented relative navigation and attitude determination algorithm.

Sensor fault detection and isolation for aircraft control systems by kinematic relations

This paper presents a new approach to Fault Detection and Isolation (FDI) for sensors of aircraft. In the most general case, fault detection of these sensors on modern aircraft is performed by a logic that selects one of, or combines, the three redundant measurements. Such a method is compliant with current airworthiness regulations. However, in the framework of the global aircraft optimization for future and upcoming aircraft, it could be required, e.g., to extend the availability of sensor measurements. Introducing a form of analytical redundancy of these measurements can increase the fault detection performance and result in a weight saving of the aircraft. This can be achieved by exploiting the knowledge of the kinematic relations between the measured variables. These relations are exactly known giving the advantage that no model-mismatches need to be accounted for. Furthermore these relations are valid over the whole flight envelope and general for any type of aircraft. Two example applications will be presented, showing the applicability of the method for the FDI of air data sensors and measurements of the inertial reference unit.

Calibration of a Novel MEMS Inertial Reference Unit

This paper describes a novel low-cost inertial reference unit (IRU) developed for spin-stabilized sounding rockets, as well as a method to calibrate the IRU using a one-axis rate table. The IRU contains three orthogonally mounted microelectromechanical system (MEMS) gyros and is intended for use in a low-cost system for attitude determination of sounding rockets. The high spin velocity of the rockets (4-6 r/s) causes sensor misalignments and scale factor errors to have a large effect on the gyro outputs. The performance of the IRU is considerably improved by detailed calibration, including temperature compensation. For sounding rocket applications, the total rate error is typically reduced by more than one order of magnitude. The calibration procedure uses a ramp profile calibration maneuver input at all three sensor axes and a Kalman filter to estimate the 12 error parameters.

The THEMIS Constellation

The Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission is the fifth NASA Medium-class Explorer (MIDEX), launched on February 17, 2007 to determine the trigger and large-scale evolution of substorms. The mission employs five identical micro-probes (termed “probes”), which have orbit periods of one, two and four days. Each of the Probes carries five instruments to measure electric and magnetic fields as well as ions and electrons. Each probe weighs 134 kg including 49 kg of hydrazine fuel and measures approximately 0.8×0.8×1.0 meters (L×W×H) and operates on an average power budget of 40 watts. For launch, the Probes were integrated to a Probe Carrier and separated via a launch vehicle provided pyrotechnic signal. Attitude data are obtained from a sun sensor, inertial reference unit and the instrument Fluxgate Magnetometer. Orbit and attitude control use a RCS system having two radial and two axial thrusters for roll and thrust maneuvers. Its two fuel tanks and pressurant system yield 960 meters/sec of delta-V, sufficient to allow Probe replacement strategies. Command and telemetry communications use an S-band 5 watt transponder through a cylindrical omni antenna with a toroidal gain pattern. This paper provides the key requirements of the probe, an overview of the probe design and how they were integrated and tested. It includes considerations and lessons learned from the experience of building NASA’s largest constellation.