Search

Modern nanosatellites are being utilised more widely for missions that require high pointing accuracy of their instrumentation. Therefore, attitude control methods that can ensure pointing requirements are always met are needed. This paper explores the feasibility, performance and practicality of applying existing Inverse Simulation (InvSim) techniques to the attitude control problem. A series of different attitude manoeuvres\ud are used to show the versatility of InvSim. Specific scenarios of an inertial pointing and spin-axis pointing rigid body 3U nanosatellite, with four reaction wheel actuators, are investigated. Performance of the InvSim controller is compared to Proportional-Integral-Derivative (PID) control through consideration of the pointing error, peak actuator power and total actuator energy of the attitude manoeuvres. InvSim is successfully implemented as an attitude controller, with comparable performance to traditional PID control in most cases, and performing better in the presence of higher nonlinear dynamics. In addition, two specific use cases are explored to show the superior practicality and versatility of InvSim, and its possible use as a design tool/aid.

No abstract available.

This paper describes the instrumented test firing of a rocket that seeks to combine the throttleability of a liquid-fueled engine with the simplicity of a solid motor. The concept is that a differentiated fuel and oxidizer rod is forced into a vaporization unit where its constituents transition into separate propellant gases, which are then mixed in a combustion chamber. The vaporization unit is heated by the combustion, and the throttle setting is adjusted by changing the force used to drive the solid propellant rod into the vaporizer, which naturally influences the propellant feed rate. In experiments using a solid propellant rod consisting of polypropylene fuel and a 1∶1.5 mixture of NH4ClO4 and NH4NO3 oxidizer, operations have been sustained for around 60 s. During testing, using propellant feed forces of between 250 and 900 N, propellant feed rates of between 100 and 300 mm/min have been achieved, which are in turn correlated to chamber pressures of between approximately 300 and 700 kPa. These correlated cycles of control input (the feed force), throttle response (the propellant feed rate), and implied thrust (the chamber pressure) demonstrate, for the first time, a simple solid rocket that can be throttled in real time.

In the effort to explore the subsurface of terrestrial bodies, we seek to obtain better samples from ever greater depths. Many organisations are working towards technologies that can achieve this goal whilst ensuring compatibility with the likely requirements of planetary landers in terms of mass, power, and dimensions.\ud \ud The Ultrasonic Planetary Core Drill (UPCD) was an FP7 funded project which aimed to develop such a planetary sub-surface sample acquisition system, developing the required drill hardware and testing it in a Mars analogue environment in Antarctica.\ud \ud The objective was to reach 30cm and containerise the samples using the least possible power, while operating at low weight-on-bit. This has been broadly achieved within a conceptually-deployable package.

With manned expeditions to planetary bodies beyond our own and the Moon currently intractable, the onus falls upon robotic systems to explore and analyse extraterrestrial environments such as Mars. These systems typically take the form of wheeled rovers, designed to navigate the difficult terrain of other worlds. Rovers have been used in this role since Lunokhod 1 landed on the Moon in 1970. While early rovers were remote controlled, communication latency with bodies beyond the Moon and the desire to improve mission effectiveness have resulted in increasing autonomy in planetary rovers. With an increase in autonomy, however, comes an increase in complexity. This can have a negative impact on the reliability of the rover system. With a fault-free system an unlikely prospect and human assistance millions of miles away, the rover must have a robust fault detection, isolation and recovery (FDIR) system. The need for comprehensive FDIR is demonstrated by the recent Chinese lunar rover, Yutu (or “Jade Rabbit”). Yutu was rendered immobile 42 days after landing and remained so for the duration of its operational life: 31 months. While its lifespan far exceeded its expected value, Yutu's inability to move severely impaired its ability to perform its mission. This clearly highlights the need for robust FDIR. A common approach to FDIR is through the generation and analysis of residuals. Output residuals may be obtained by comparing the outputs of the system with predictions of those outputs, obtained from a mathematical model of the system which is supplied with the system inputs. Output residuals allow simple detection and isolation of faults at the output of the system. Faults in earlier stages of the system, however, propagate through the system dynamics and can disperse amongst several of the outputs. This problem is exemplified by faults at the input, which can potentially excite every system state and thus manifest in every output residual. Methods exist for decoupling and analysing output residuals such that input faults may be isolated, however, these methods are complex and require comprehensive development and testing. A conceptually simpler approach is presented in this paper. Inverse simulation (InvSim) is a numerical method by which the inputs of a system are obtained for a desired output. It does so by using a Newton-Raphson algorithm to solve a non-linear model of the system for the input. When supplied with the outputs of a fault-afflicted system, InvSim produces the input required to drive a fault-free system to this output. The fault therefore manifests itself in this generated input signal. The InvSim-generated input may then be compared to the true system input to generate input residuals. Just as a fault at an output manifests itself in the residual for that output alone, a fault at an input similarly manifests itself only in the residual for that input. InvSim may also be used to generate residuals at other locations in the system, by considering distinct subsystems with their own inputs and outputs. This ability is tested comprehensively in this paper. Faults are applied to a simulated rover at a variety of locations within the system structure and residuals generated using both InvSim and conventional forward simulation. Residuals generated using InvSim are shown to facilitate detection and isolation of faults in several locations using simple analyses. By contrast, forward simulation requires the use of complex analytical methods such as structured residuals or adaptive thresholds.

Traditional rotary drilling for planetary rock sampling, in situ analysis, and sample return are challenging because the axial force and holding torque requirements are not necessarily compatible with lightweight spacecraft architectures in low-gravity environments. This paper seeks to optimize an ultrasonic percussive drill tool to achieve rock penetration with lower reacted force requirements, with a strategic view toward building an ultrasonic planetary core drill (UPCD) device. The UPCD is a descendant of the ultrasonic/sonic driller/corer technique. In these concepts, a transducer and horn (typically resonant at around 20 kHz) are used to excite a toroidal free mass that oscillates chaotically between the horn tip and drill base at lower frequencies (generally between 10 Hz and 1 kHz). This creates a series of stress pulses that is transferred through the drill bit to the rock surface, and while the stress at the drill-bit tip/rock interface exceeds the compressive strength of the rock, it causes fractures that result in fragmentation of the rock. This facilitates augering and downward progress. In order to ensure that the drill-bit tip delivers the greatest effective impulse (the time integral of the drill-bit tip/rock pressure curve exceeding the strength of the rock), parameters such as the spring rates and the mass of the free mass, the drill bit and transducer have been varied and compared in both computer simulation and practical experiment. The most interesting findings and those of particular relevance to deep drilling indicate that increasing the mass of the drill bit has a limited (or even positive) influence on the rate of effective impulse delivered.

Robotic rovers which are designed to work in extra-terrestrial environments present a unique challenge in terms of the reliability and availability of systems throughout the mission. Should some fault occur, with the nearest human potentially millions of kilometres away, detection and identification of the fault must be performed solely by the robot and its subsystems. Faults in the system sensors are relatively straightforward to detect, through the residuals produced by comparison of the system output with that of a simple model. However, faults in the input, that is, the actuators of the system, are harder to detect. A step change in the input signal, caused potentially by the loss of an actuator, can propagate through the system, resulting in complex residuals in multiple outputs. These residuals can be difficult to isolate or distinguish from residuals caused by environmental disturbances. While a more complex fault detection method or additional sensors could be used to solve these issues, an alternative is presented here. Using inverse simulation (InvSim), the inputs and outputs of the mathematical model of the rover system are reversed. Thus, for a desired trajectory, the corresponding actuator inputs are obtained. A step fault near the input then manifests itself as a step change in the residual between the system inputs and the input trajectory obtained through inverse simulation. This approach avoids the need for additional hardware on a mass- and power-critical system such as the rover. The InvSim fault detection method is applied to a simple four-wheeled rover in simulation. Additive system faults and an external disturbance force and are applied to the vehicle in turn, such that the dynamic response and sensor output of the rover are impacted. Basic model-based fault detection is then employed to provide output residuals which may be analysed to provide information on the fault/disturbance. InvSim-based fault detection is then employed, similarly providing input residuals which provide further information on the fault/disturbance. The input residuals are shown to provide clearer information on the location and magnitude of an input fault than the output residuals. Additionally, they can allow faults to be more clearly discriminated from environmental disturbances., {"references":["S. Kassel, \"Lunokhod-1 Soviet lunar surface vehicle,\" DARPA, Tech.\nRep. September, 1971.","M. B. Quadrelli, L. J. Wood, J. E. Riedel, M. C. McHenry, M. Aung,\nL. A. Cangahuala, R. A. Volpe, P. M. Beauchamp, and J. A.\nCutts, \"Guidance, navigation and control technology assessment for\nfuture planetary science missions,\" Journal of Guidance, Control, and\nDynamics, vol. 38, no. 7, pp. 1165–1186, 2015.","R. Isermann, Fault-Diagnosis Systems: An Introduction from Fault\nDetection to Fault Tolerance. Berlin: Springer-Verlag, 2006.","R. Dearden, T. Willeke, R. Simmons, V. Verma, F. Hutter, and S. Thrun,\n\"Real-time fault detection and situational awareness for rovers: Report\non the Mars technology program task,\" in Proceedings of the IEEE\nAerospace Conference, vol. 2, Big Sky, MT, 2004, pp. 826–840.","V. Verma, G. Gordon, R. Simmons, and S. Thrun, \"Tractable particle\nfilters for rover fault diagnosis,\" IEEE Robotics & Automation Magazine,\nvol. 11, pp. 56–66, 2004.","K. Ferguson, \"Towards a better understanding of the flight mechanics\nof compound helicopter configurations,\" PhD thesis, University of\nGlasgow, November 2015.","R. Hess, C. Gao, and S. Wang, \"A generalized technique for inverse\nsimulation applied to aircraft manoeuvres,\" Journal of Guidance, Control\nand Dynamics, vol. 14, pp. 920–926, 1991.","D. Thomson and R. Bradley, \"Inverse simulation as a tool for flight\ndynamics research – Principles and applications,\" Progress in Aerospace\nSciences, vol. 42, no. 3, pp. 174–210, May 2006.","D. Murray-Smith, \"The inverse simulation approach: A focused\nreview of methods and applications,\" Mathematics and Computers in\nSimulation, vol. 53, no. 4-6, pp. 239–247, October 2000.\n[10] D. J. Murray-Smith, \"Inverse simulation and analysis of underwater\nvehicle dynamics using feedback principles,\" Mathematical and\nComputer Modelling of Dynamical Systems, vol. 20, no. 1, pp. 45–65,\n2014.\n[11] D. Murray-Smith and E. McGookin, \"A case study involving continuous\nsystem methods of inverse simulation for an unmanned aerial vehicle\napplication,\" Proceedings of the Institution of Mechanical Engineers,\nPart G: Journal of Aerospace Engineering, vol. 229, no. 14, pp.\n2700–2717, 2015.\n[12] K. Worrall, D. Thomson, and E. McGookin, \"Application of inverse\nsimulation to a wheeled mobile robot,\" in Proceedings of the 6th\nInternational Conference on Automation, Robotics and Applications\n(ICARA 2015), Queenstown, February 2015.\n[13] K. Worrall, D. Thomson, E. McGookin, and T. Flessa, \"Autonomous\nplanetary rover control using inverse simulation,\" in 13th Symposium\non Advanced Space Technologies in Robotics and Automation (ASTRA\n2015). Noordwijk: ESA/ESTEC, May 2015. [14] S. Rutherford and D. G. Thomson, \"Improved methodology for inverse\nsimulation,\" Aeronautical Journal, vol. 100, no. 993, pp. 79–85, 1996.\n[15] K. J. Worrall, \"Guidance and search algorithms for mobile robots:\nApplication and analysis within the context of urban search and rescue,\"\nPhD thesis, University of Glasgow, 2008.\n[16] K. J. Worrall and E. W. McGookin, \"A mathematical model of a Lego\ndifferential drive robot,\" in Proceedings of the 6th UKACC Control\nConference, Glasgow, 2006."]}

This paper presents the findings of a recent field trip by the Ultrasonic Planetary Core Drill team to the Mars analog site at the Haughton Mars Project. Results of this trip are compared to results obtained drilling permafrost simulant in the lab.

Control Moment Gyroscopes (CMGs) are not often considered for use on small satellites and, as a result few small satellite missions have implemented CMGs as on-board actuators. There are many reasons for this, but mainly this is due the complexity of the mechanical and control system needed to implement an effective CMG, and also because off-the-shelf CMG systems are generally made for the larger satellite market. .CMGs offer many advantages over reaction wheel systems. When used on a small satellite, a CMG based control system can provide the ability to perform very fast slew maneuvers, making CMGs very attractive to high resolution small satellite imaging missions. The CMG described in this paper incorporates two motors; a Brushless DC Motor (BLDC) and a Stepper Motor. The BLDC provides an efficient means of driving the inertia disk to store the angular momentum, whilst the stepper motor provides precision gimbal control. In order to keep mass and power consumption low, both motors are controlled from a single FPGA. The FPGA runs all associated commutation, speed and position control for both motors and also provides the command and telemetry interface to the rest of the spacecraft. The resulting system is a compact, power efficient design that is ideal for small satellites.