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Autonomous on-comet operations aspects of the RoSETTA mission
CostrolEng. Practice, Vol. 2, No. 3, pp. 499-507, 1994 Copyright © 1994 Elsevier Science lad Printed in Great Britain. All rights reserved 0967-0661/94 $7.00 ÷ 0.00
Pergamon 0967-0661(94)E0013-6
AUTONOMOUS ON-COMET OPERATIONS ASPECTS OF THE ROSETTA MISSION K. Schilling, H. Roth and B. Theobold Institut fiir Angewandte Forschung, Fachhochschule Ravensburg-Weingarten, Postfach 1261, D-88241 Weingarten, Germany
Abstract. The aim of the ROSETrA mission is to return pristine material from a comet to Earth. This paper studies the operations constraints for the on-comet phase, and analyses an adaptive control concept for autonomous sampling of cometary materials from the scarcely known soil. The adaptive control algorithm identifies during sampling the crucial parameters, characterizing the cometary soil and the spacecraft's subsystem performances to readjust the drill's penetration rate and the Attitude Control System's cold gas thrusting activities.This approach exhibited advantages regarding the reliability of operations as well as in the efficient use of the spacecraft's limited resources. Key
words•
Spacecraft Operations, Autonomous Systems, Space Robotics, Adaptive Control, Mission Analysis.
a change of the target comet (due to scientifically even more favourable opportunities or due to potential launch delays). In order to avoid active - and thus very risky - periods near perihel the comet encouter will take place at a distance between 5 and 6 AU. This means the signal propagation delay will be about 40 minutes (one way) and ground control interaction is very limited during this critical period (cf. (Schilling, 1992)).
1. I N T R O D U C T I O N
One of the four cornerstones in the scientificprogramme of the European Space Agency (ESA) is the R O S E T r A mission for the exploration of comets. From the related experiments the scientists expect to gain further insight into the origin and the formation processes of our solar system. Two options have been analysed : - a comet rendezvous mission, staying for extended periods close to the nucleus and deploying a science station on to the comet's surface, - a comet sample return mission, to return pristine cometary material to Earth for detailed investigatious with extensive laboratory equipment. While the rendezvous mission can be performed within the technical and financial framework as a purely European mission, the sample return has to include the participation of other space agencies. The results discussed in this paper are based on the scenario of the sample return mission (Matra Espace et al., 1991).
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Figure 1:
According to this framework a launch in December 2002 will lead to an encounter with comet Hartley-2 in Februa~ 2008 and an Earth return in November 2010. But a requirement for the mission design is still to provide sufficient flexibility to accommodate
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The spacecraR configuration during oncomet operations with deployed anchors and drill equipment.
Thus challenging requirements regarding autonomous on-board reaction capabilities (cf. (Born-
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K. Schilling et al.
500
schlegel, 1992)) exist. In addition the knowledge on expected cometary environments is not very detailed. The broad variation range for the comet characteristics will only be narrowed during comet approach (cf. (Lafontaine, 1992)), when during the observation phase the orbiting spacecraft will transmit actual measurements from remote sensing instruments. Unfortunately the mechanical properties of the soil, determining the sampling scenario, cannot be completely derived from these measurements and will still vary in a broad band. Thus the on-comet operations require strategies to quickly adapt control actions to actual sensor data, gained during the sampling process. Descent and Landing Phase
Site Assessment
Anchoring
~rimary Sample Acquisition -core samples -volatile samples -surface samples
L, ,¢,
¢seeondary 0n-Comet Science
t
Preparation for lift-off
Lift-off
Figure 2 : Schematic of the on-comet operations flow 2. S C E N A R I O OF THE ON-COMET PHASE
After landing, in the nominal operations sequence the spacecraft is stabilized via cold gas thrusting, and it should autonomously reestablish as quickly as possible the communication link with the ground control centre, which will be lost during the final descent. Then first information about the landing site will be transmitted, to allow a quick assessment of the actual situation on the ground. Meanwhile the spacecraft must autonomously (due to the signal propagation delay) handle the interaction with the
uncertain environment and the maintenance of the radio link, despite disturbances. If the location is considered appropriate so far, a test anchor equipped with accelerometers and temperature sensors is used to obtain first data on the mechanical properties of the subsurface layers. If these data are satisfactory according to scientific and technical criteria, the anchors will be deployed, in order to provide a stable fixation for the on-comet operations, in particular for the sampling. The sample acquisition system (SAS) should then collect three types of samples : - core sample : to document the stratigraphic soil properties to a maximum depth of 3 m; mass: ca. 10 kg - volatile sample : volatile gases, expected to be bound in the ice of the comet's interior; mass: up to 100 g surface sample : samples from different surface locations within the reach of the SAS; mass : up to 5 kg. To perform these tasks the actual version of the SAS consists of the following three parts: - Drilling System, including the drilling table (to be lowered to the surface after landing) and sampling tubes, to be successively introduced into the borehole, - Robotic Arm, to feed the sampling tubes into the drilling system and to bring the acquired samples into the return capsule's storage container, - Surface Sampling Device, to be handled by the robotic arm in order to collect surface pieces. In particular when the core sample is acquired via drilling, the reaction forces and torques must be accommodated. There are layers of varying resistance and hard pieces embedded in fluffy material to be expected. Thus during peak load periods the anchors should be supported by cold gas thrusting to provide the required stability. There is a quick reaction capability necessary to prevent any sinking or side movement, as this would cause the drill to jam. Because the core sample is of the highest scientific importance, all measures must be undertaken to avoid such a jamming (cf. (Eiden and Caste, 1991)). To comply with the uncertainties of the cometary environment, back-up scenarios have been elaborated for extreme situations. Thus the landing site might be considered not suitable, either because there is no chance to access the pristine amorphous ice of the core, or if the carrying capacity of the surface is not sufficient. In this case there is a capability foreseen to move to a second site.
The ROSETrA Mission If in a worst case due to hard soil no anchoring is possible, two core and one volatile sample can be acquired with a corer of smaller diameter and at a higher penetration rate. For this emergency sampling the limits for thermal disturbance of the samples (less than 5 K above surface temperature) must be relaxed. The vertical connterreaction force to drilling will then be provided by the thrusters. In order to acquire sufficient mass in a case of lowdensity materials, there is a capability foreseen to compress the samples and to repeat sampling operations.
3. C O M E T A R Y
SOIL MODEL
The current cometary models were mainly derived from astronomical observations and the spacecraft fly-bys of comets Halley in 1986 and GriggSkjellerup in 1992. In addition experiments to produce comet analogue material were performed at the vacuum chamber of DLR Cologne in order to derive characteristic material properties (cf. (Schwehm and Langevin, 1991)). This provided significant improvements in the understanding of comets. "However, despite all the efforts, the composition, the inner structure and the evolutionary processes of comet nuclei are still a matter of wide uncertainty and they will remain a field of scientific debate..." (St6ffier and Schwehm, 1989, p.17).
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-dust layer: cohesionless material with a density between 0.005 and 2 g/cm3; thickness up to lm, - crust layer: thin (few centimeters), hard (cohesion up to 4 MPa, Young's modulus up to 10 GPa) material formed by solar irradiation, - nucleus : hard (cohesion obtained in the laboratory between 1 and 4 MPa ; Young's modulus up to 800 MPa) and brittle material at low temperature.
4. EFFECTS OF PHYSICAL AND ENGINEERING
PARAMETERS
For ROSETFA on-comet operations typical parameters to be identified on-board in order to optimize the sampling / attitude control system (ACS) coordination will be cometary soil properties like size, hardness, cohesion and stiffness of the different layers, because they will influence the forces and torques caused by the drill. In the following table the different parameters affecting the on-comet operations are summarized. Table 2: Identification of characteristic parameters influencing the on-comet phase. Item
Cometary Properties
Parameter
- reaction forces of anchors - drilling forces and torques - landing pad creeping - crust fragmentation
Yo~g's medulus
- shearing stress - tilting of S/C
internal angle of friction
- foedpad sinking - shear stress - soil resistance
adhesion factor
- anchor uplift capacity
soil strati-
-
Table 1: Expected variation bands of typical cometary physical/mechanical parameters (redrawn from (St0ffier and Schwehn~ 1989)). Parameter
Ranse
Fraction of refractory dust
0- 100 % Porosity of refractory surface material 0-95 % Porosity below dust mantle 10-80 % 0.005 - 2.2 g/era3 Density of dust mantle Density below dust mantle 0.1 - 1.5 g/cm3 Grain size ofmonophase grains 0.001 - 100 Grain size of multiphase grains and composite l p m - l m aggregates 10"4- 102 MPa Co~ive st~n~h of bulk samples
Variations of the irradiation environment during the perihel passages in the comet's orbit should have caused an evolution of different layers close to the surface. A very porous and nonhomogenous surface structure must be expected, including loose dust material, boulders, pebbles and areas of exposed lower layers. Nevertheless potential safe landing locations can be modelled in a first engineering approach as follows :
Effe¢l
cohesion
graphy
comet dynamics
force and torque profile of drilling - anchor stability - thermal control (day/night cycle) - data transmission (contact periods)
5O2
K. Schilling et al. 5. C O M P U T E R Item
Parameter
Drilling Equipment
drill rotation rate
- sample heating - torques
pushing force
- causes vertical force to be accommodated by ACS, anchors - sample heating - sample acquisition time to reach given depth - power demand spacecraft stability
Anchors
Attitude Control System
(ACS)
Item
forces caused by gravity, comet dynamics, antenna movements, drilling, robot arm motion, motor vibrations, activation of gas pockets, etc. sensor - recognition of resolution tilting
reaction time
- jamming avoidance of drill
duration of thrusting
- counterreaction tuning
Characteristic
Parameter
m i s s i o n rate
influenced
b~,
antenna pointing accuracy - power allocation - comet dynamics / link geometry -
landing manoeuvre fuel consumption - anchor stability - maximum forces generated by drilling
cold gas propellant
-
available power
- coordination of operational activities
thermal control
-
Table 3 : Simulated adaptive control scenario Parameter
Type
Optimisation Criteria
Measuremems
- effectivity of attitude change counterreaction
data trans-
FRAMEWORK
In order to investigate technologies to deal with these challenging autonomy problems, exemplarily an adaptive control algorithm was analysed in this context. As the core sampling by drilling is the most critical period, this phase was selected for the computer simulation. The parameters to be considered are listed in Table 3.
Models
selection of thruster combination
Parameter
Spacecraft Resources
SIMULATION
Effect
comet dynamics (day/night) power consumption
Control Actions
Specific
Parameter
- attitude stability (primal) - duration of drilling ~seconda~) - drill / soil interaction - mechanical properties of soil layers - simplified spacecraft structure - ideal gyros - force sensors (between pad/soil, drill (vertical direction)) - torque sensors (between pad/soil) - c o l d g a s thruster (24 x 30 N) - hydrazin thruster (2 x 70 N) - drill feed velocit~
In order to perform meaningful simulations, models of the following spacecraft components have been implemented : - drill equipment, in order to evaluate the resulting forces and torques dependent on soil properties and pushing force/drill rotation rate, - finite element model of Lander structure, to couple the generated force at the drill with the resulting forces and torques at each of the three anchors, - attitude control system (ACS), to generate for the desirable counterreaction the optimum approximation within the range of available thrusters, - anchors, to determine the actual fixation capacity from a given stress/strain relationship, - spacecraft dynamics, to deal with the situation that the anchors are torn off. In the following sections, using the example of drill/soil interactions and of thruster selection, two important models for the control system will be discussed in more detail. The simulation tool was already designed in a modular way to support future extensions. Thus as
The ROSETrA Mission
503
soon as more detailed models regarding the interactions between anchors/drill and the soil are available the implemented models can be easily updated. Further improvements of the simulation environment should also address - thermal control aspects, - limitations of available power, data transmission requirements.
where kmo t, k, RA, LA are characteristic parameters of the motor and J is the relevant moment of inertia.
5.1 Model o f Drill/SoR Interaction
The quotient fn/fs is a characteristic material property dependent on the normal and the tangential components of the cutting force. It is approximated by an exponential function of the cohesion. A more detailed model would use friction coefficients depending on the penetration depth and the soil parameters instead of the constants employed here.
Similar to the torque model the resulting pushing force F r is composed of a friction part depending on the constant K F and of the cutting force:
-
" The drilling operations to acquire material samples from a depth of 3 m can generate significant attitude disturbances depending on the encountered soil properties. The task of this chapter is to relate in a simplified model the effects of soil parameters (like Young's modulus, cohesion) and drill characteristics (like pushing force, rotation rate) with the resulting forces and torques. In the low gravity environment on the comet these have to be compensated by anchors and by attitude control systems (ACS) thrusters. This coordination is the task of the adaptive control concept for core sampling to be further detailed in section 6. The drill's rotation rate n(t) causes reactive torques acting on the spacecraft. As presented in (Magnani et al., 1989) this torque T is composed of a shear torque (resulting from separation of the material) and a friction torque:
T = I [ D ~ - D 2 IE v(t) 8 t
, j
s n(t)
+ Krn(t),
where D a is the outer and D i the inner drill diameter, E S represents the cutting energy, v(t) is the drill penetration rate and K T is a friction coefficient. The cutting energy is approximated as a linear function of Young's modulus. This assumption has been derived from empirical data for different materials. In order to penetrate into the soil a pushing force F r has to be exerted by the drill. This pushing force Fr is generated by a penetration motor providing a penetration rate v(t) in reaction to an input voltage u(t), limited to some 40 V. The motor dynamics can be modelled by a linear transfer function of second order:
G(s)-- v(s) = k,.o, u(s) s 2 + R , s+ k LA 2 LAJ
Lf
J"(t)
Thus, in a first approximative model a functional relationship is established between the voltage u of the penetration motor, the control parameter, and the characteristic soil parameters cohesion and Young's modulus to derive the resulting pushing force F r and torque T. The challenge for the adaptive controller results from changes in orders of magnitudes for both soil parameters when the drill passes different soil layers.
5.2 Selection of Active Thrusters The compensation of forces and torques exceeding the anchor carrying capacity can only be achieved via thrusting, as the potential contribution of gyros to counter-actions is in the order of 0.4 N and thus below the expected torques by a factor of 500. The primary means are thus the 30-N cold gas thrusters, positioned on the Lander. In each edge of the cube shaped Lander body there are placed 3 thrusters pointing in the outbound directions of the cube axes. These 24 cold gas thrusters can be supported in emergency cases by the hydrazine thrusters positioned on the Mariner Mark II spacecraft, at the cost of chemical pollution of the landing site and of the samples to be acquired. There are 24 10-N thrusters placed similarily to the Lander configuration and two 70-N bipropellant-thrasters above the drill to press the spacecraft down in case the anchors fail. Given a required counter-action (force and torque vector) an appropriate combination of thrusters is to be activated to approximate the desirable resulting forces. Combinatorial optimisation methods are far from achieving real-time performance in such a huge search space of possible combinations. Therefore a look-up table of about 700 cases has
K. Schillinget al.
504
been generated, covering the most frequent situations as well as representative patterns from the range of potential force/torque scenarios, From this table a reasonable counter-action can be selected in real-time by a least-square method. To improve the approximation accuracy, the data base must be extended, increasing in parallel the duration for computation. Another interesting feature is the inclusion of alternative activation patterns leading to similar reactions. In case of a malfunctioning thruster only the patterns using this particular thruster are to be deleted in the table, then the next best approximation within the table will be selected, and thus a gracefully degrading redundancy concept results.
stability are active. In this case the reaction wheels are included for control in a nominal manner. In the current stage of simulations, limitations in the sensor resolutions and delays in the dynamics of actuators have not been taken into account. Figure 3 depicts the identification / adaption scheme and Fig. 4 shows the interconnections and interfaces of the control concept explained in more detail in the following. The mission management provides the supervising frame for the adaptive controller operations. This includes estimations regarding the most appropriate nominal penetration rates and drill rotation rates (according to soil properties derived from anchoring measurements).
6. A D A P T I V E C O N T R O L C O N C E P T F O R
CORE SAMPLING The task of the adaptive control concept is to coordinate reactions of the SAS, the anchors and the ACS according to the actual soil properties. As attitude stability is a prerequisite for drilling, it must prevent rotational as well as translational movements to avoid damage to the drill. This requires decision strategies working autonomously on-line to induce appropriate drill and ACS reactions to achieve an optimum between the spacecraft resources and the required operations time. The control action is modeled with respect to the characteristic soil parameter : cohesion c and Young's modulus E. The related values have to be identified during drilling in order to adapt the control strategies and controller parameters (AstrOm and Wittenmark, 1989). The essential sensor inputs are provided with respect to attitude by accelerometers and gyroscopes (to determine tilting or sliding of the spacecraft). For lift-off or critical limit detection force- and torque-sensors at the pads / anchors and at the drill are foreseen. In addition there are in the SAS sensors to measure - the penetration depth of the drill, - the voltage u of the penetration motor and - the temperature at the tip of the drill. The ACS means to react are control units like thrusters and reaction wheels. The effect of the reaction wheels is too small to deal with the torques and forces (generated by drilling) exceeding the capabilities of the anchors. As soon as the anchors are torn off and the spacecraft is in a free movement, the usual attitude control laws regarding
The optimal value for the drill rotation rate n will depend on the soil parameters. For example for hard material the rotation should be higher than for soft soil. However, at the beginning of the drilling process the soil parameters at the drill point are unknown. The determination of the initial value will be a typical task of the supervising mission management. After identification of the real soil parameters the mission management has to adapt and optimize the value of the rotational velocity. The rotational rate n(t) is supervised by a dedicated drill controller and can be assumed to be constant with respect to the adaptive controller design. Before the on-line identification of the soil parameters with successive adaptation of the drill controller parameters begins, the initial values of the controller parameter must be defined. At the beginning of the identification procedure a nominal desired value for the drill pushing force F r nominal has to be chosen. It should be high enough to provide a small penetration rate but small enough such that no anchor tearing off results at the beginning. The anchor capability limits are not known at the beginning of the drill process. But a first hint can be expected from anchoring. In order to be on the safe side, it will be a better procedure to start with a pushing force that is too small rather than too high. But it is a great advantage of the adaptive control concept that after the starting phase all further information about the soil characteristics, required for control, is directly acquired from the on-going drilling process. After convergence of the identification and adaptation algorithms, the desired nominal value of Fr can be increased. But care must be taken, that the resulting force F r pushing the drill down into the soil does not exceed the anchor capabilities. Anchor tearing off would be recognized by measuring the
The ROSETrA Mission
maintain a stable attitude in such situations, it is necessary to fire suitable thruster combinations for pressing down the spacecraft.
forces and torques at the pads, together with the attitude measurement signals. In a case where the anchors are still fixed in the ground, the forces do not cause a recognisable attitude change.
If all anchors are torn out, the dynamics of the spacecraft has to be taken into account. In this case the nominal attitude controller comes into action. After stabilisation of the spacecraft by the attitude controller, drilling Without anchoring must continue. For the remainder the drill forces and torques have to be compensated by the ACS alone.
Critical situations will occur ff there are sudden changes in the soil characteristics. Expected changes derived from anchoring measurements can be anticipated by adaptation of F r through the supervising mission management. Otherwise it is possible, that the force F r will exceed the maximum force to be compensated by the anchors.
Thus, in all cases the drill controller is working as an underlying control loop optimizing the drilling process according to the measured soil parameters, for an anchored spacecraft as well as for a freemoving vehicle.
Possible reactions are - to activate thrusters to support the anchors for spacecraft stability purposes or - to lower the drill pushing force F r . The adaptive control algorithm should initiate an optimum combination of both activities.
By the adaptive control concept the anchors, sample acquisition system and cold gas thrusters are coordinated and support each other. This results in increased capabilities to guarantee spacecraft stability. Thus the reliability and efficiency of spacecraft operations during the on-comet phase Will be significantly increased.
As soon as one of the force sensors indicates liR-off, supported by the attitude sensors showing rotational or translational movements, the feed velocity has to be immediately reduced by decreasing the desired value of the pushing force F r nominal • In this case one or more anchors are torn out or loosened. To
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time in scc Figure 5 • Simulation of occuring forces, when the drill encounters hard material inclusions embedded in a softer environment • a) at the drill location, b) at the anchors (3-d force vectors) without counterreaction thrusting, c) at the anchors, when adaptive thrusting is provided. 7. S I M U L A T I O N R E S U L T S In simulations the adaptive control algorithm was tested on soil parameters varying within the ad-
missible range. The results were very encouraging, showing that this controller can even handle sudden, abrupt changes in material properties within acceptable limits. The worst case regarding
507
The ROSETrA Mission peak loads occurs when the drill encounters a hard inclusion in a soft material layer. The resulting force profiles are given in Fig. 5. Figure 5 a) displays the force resulting at the drill location. Figure 5 b) shows the forces acting on the anchors, when no counterreaction via thrusters is initiated. In the displayed situation a tearing off of the anchors results, as the maximum sustainable force of 150 N is exceeded. Figure 5 c) exhibits the remaining forces (within the capabilities of the anchors), when the algorithm activates the thrusters of the AOCS in parallel. The comparison of 5b) and 5c) shows that thruster compensation leads to significantly smaller acting peak forces and torques at the anchors. Thus only a short thrusting period can avoid tearing off of the anchors, preserving their counterreaction capabilities as the main contribution to future spacecraft stability. Further comparisons were performed with a conventional PI-controller. In the situation of Fig. 5 it led to unacceptable oscillations throughout the hard layer, while the adaptive controller quickly adapts to the new soil parameters, causing only quickly damped oscillation for the short initial period. For the other situations compared the adaptive scheme was also preferable.
8. S U M M A R Y
The aim of this paper was to analyse the constraints for autonomous ROSE'ITA on-comet operations. In particular for the very demanding period of core sampling it was shown that adaptive control methods can provide interesting contributions to support spacecraft autonomy capabilities. The results of this paper are - the development of an integrated anchor/drill/ ACS operations concept, to improve reliability and resource consumption efficiency, - first analyses on the drill control regarding transitions between the different soil layers. Regarding the application of adaptive control technology in the field of spacecraft autonomy, of interest are - capabilities to adapt to actual measurements, - a very attractive performance for poorly known environments, - applicability of engineering standards to the verification of algorithms (important in case of high quality assurance requirements). As assessed on the concrete example of ROSETTA on-comet operations, adaptive controls provide an
interesting approach to enable autonomous spacecraft reactions in an uncertain environment. Acknowledgements
Part of this work has been undertaken in an ESA contract. The authors thank J. de Lafontaine and E. Bornschlegl for their comments and support during that study.
9. R E F E R E N C E S
AstrOm,K.J./Wittenmark,B. (1989). Adaptive Control, Addison-Wesley. Bornschlegl, E. (1992) ASDMS : An Autonomous Spacecraft Data Management System. Preparing for the Future 2 , 4 - 6. Eiden,M.J./Coste,P. (1991). The Challenge of Sample Acquisition in Cometary Environment. ESA Journal 15,191 - 211. Lafontaine,J.de (1992). Autonomous Spacecra~ Navigation and Control for Spacecraft Landing. J. Guidance, Control and Dynamics 15, Vol.3. Magnani,PG. et al. (1989). CNSR - SAS System Definition Study. Tecnospazio Report, Doe.No. SA-O02, Annex 3. Matra Espace/JPL/Dornier GmbI-I/BAe/Aeritalia (1991). ROSE'ITA- System Definition StudyFinal Report. Schilling,K. (1992). Simulation of ROSE'I~A OnComet Operations. Annales Geophysicae 10, 141 -144. Schwehm,G./Langevin, Y. (eds.) (1991). ROSETTA/CNSR A Comet-Nucleus SampleReturn Mission. ESA SP-1125, Noordwijk. St6ffier, D., G. Schwehin (eds.) (1989). Physics and Mechanics of Cometary Materials. ESA SP-302, Noordwijk.
Pergamon 0967-0661(94)E0013-6
AUTONOMOUS ON-COMET OPERATIONS ASPECTS OF THE ROSETTA MISSION K. Schilling, H. Roth and B. Theobold Institut fiir Angewandte Forschung, Fachhochschule Ravensburg-Weingarten, Postfach 1261, D-88241 Weingarten, Germany
Abstract. The aim of the ROSETrA mission is to return pristine material from a comet to Earth. This paper studies the operations constraints for the on-comet phase, and analyses an adaptive control concept for autonomous sampling of cometary materials from the scarcely known soil. The adaptive control algorithm identifies during sampling the crucial parameters, characterizing the cometary soil and the spacecraft's subsystem performances to readjust the drill's penetration rate and the Attitude Control System's cold gas thrusting activities.This approach exhibited advantages regarding the reliability of operations as well as in the efficient use of the spacecraft's limited resources. Key
words•
Spacecraft Operations, Autonomous Systems, Space Robotics, Adaptive Control, Mission Analysis.
a change of the target comet (due to scientifically even more favourable opportunities or due to potential launch delays). In order to avoid active - and thus very risky - periods near perihel the comet encouter will take place at a distance between 5 and 6 AU. This means the signal propagation delay will be about 40 minutes (one way) and ground control interaction is very limited during this critical period (cf. (Schilling, 1992)).
1. I N T R O D U C T I O N
One of the four cornerstones in the scientificprogramme of the European Space Agency (ESA) is the R O S E T r A mission for the exploration of comets. From the related experiments the scientists expect to gain further insight into the origin and the formation processes of our solar system. Two options have been analysed : - a comet rendezvous mission, staying for extended periods close to the nucleus and deploying a science station on to the comet's surface, - a comet sample return mission, to return pristine cometary material to Earth for detailed investigatious with extensive laboratory equipment. While the rendezvous mission can be performed within the technical and financial framework as a purely European mission, the sample return has to include the participation of other space agencies. The results discussed in this paper are based on the scenario of the sample return mission (Matra Espace et al., 1991).
•
o
•
-
•
•
•
• .-, ..
Figure 1:
According to this framework a launch in December 2002 will lead to an encounter with comet Hartley-2 in Februa~ 2008 and an Earth return in November 2010. But a requirement for the mission design is still to provide sufficient flexibility to accommodate
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The spacecraR configuration during oncomet operations with deployed anchors and drill equipment.
Thus challenging requirements regarding autonomous on-board reaction capabilities (cf. (Born-
499
K. Schilling et al.
500
schlegel, 1992)) exist. In addition the knowledge on expected cometary environments is not very detailed. The broad variation range for the comet characteristics will only be narrowed during comet approach (cf. (Lafontaine, 1992)), when during the observation phase the orbiting spacecraft will transmit actual measurements from remote sensing instruments. Unfortunately the mechanical properties of the soil, determining the sampling scenario, cannot be completely derived from these measurements and will still vary in a broad band. Thus the on-comet operations require strategies to quickly adapt control actions to actual sensor data, gained during the sampling process. Descent and Landing Phase
Site Assessment
Anchoring
~rimary Sample Acquisition -core samples -volatile samples -surface samples
L, ,¢,
¢seeondary 0n-Comet Science
t
Preparation for lift-off
Lift-off
Figure 2 : Schematic of the on-comet operations flow 2. S C E N A R I O OF THE ON-COMET PHASE
After landing, in the nominal operations sequence the spacecraft is stabilized via cold gas thrusting, and it should autonomously reestablish as quickly as possible the communication link with the ground control centre, which will be lost during the final descent. Then first information about the landing site will be transmitted, to allow a quick assessment of the actual situation on the ground. Meanwhile the spacecraft must autonomously (due to the signal propagation delay) handle the interaction with the
uncertain environment and the maintenance of the radio link, despite disturbances. If the location is considered appropriate so far, a test anchor equipped with accelerometers and temperature sensors is used to obtain first data on the mechanical properties of the subsurface layers. If these data are satisfactory according to scientific and technical criteria, the anchors will be deployed, in order to provide a stable fixation for the on-comet operations, in particular for the sampling. The sample acquisition system (SAS) should then collect three types of samples : - core sample : to document the stratigraphic soil properties to a maximum depth of 3 m; mass: ca. 10 kg - volatile sample : volatile gases, expected to be bound in the ice of the comet's interior; mass: up to 100 g surface sample : samples from different surface locations within the reach of the SAS; mass : up to 5 kg. To perform these tasks the actual version of the SAS consists of the following three parts: - Drilling System, including the drilling table (to be lowered to the surface after landing) and sampling tubes, to be successively introduced into the borehole, - Robotic Arm, to feed the sampling tubes into the drilling system and to bring the acquired samples into the return capsule's storage container, - Surface Sampling Device, to be handled by the robotic arm in order to collect surface pieces. In particular when the core sample is acquired via drilling, the reaction forces and torques must be accommodated. There are layers of varying resistance and hard pieces embedded in fluffy material to be expected. Thus during peak load periods the anchors should be supported by cold gas thrusting to provide the required stability. There is a quick reaction capability necessary to prevent any sinking or side movement, as this would cause the drill to jam. Because the core sample is of the highest scientific importance, all measures must be undertaken to avoid such a jamming (cf. (Eiden and Caste, 1991)). To comply with the uncertainties of the cometary environment, back-up scenarios have been elaborated for extreme situations. Thus the landing site might be considered not suitable, either because there is no chance to access the pristine amorphous ice of the core, or if the carrying capacity of the surface is not sufficient. In this case there is a capability foreseen to move to a second site.
The ROSETrA Mission If in a worst case due to hard soil no anchoring is possible, two core and one volatile sample can be acquired with a corer of smaller diameter and at a higher penetration rate. For this emergency sampling the limits for thermal disturbance of the samples (less than 5 K above surface temperature) must be relaxed. The vertical connterreaction force to drilling will then be provided by the thrusters. In order to acquire sufficient mass in a case of lowdensity materials, there is a capability foreseen to compress the samples and to repeat sampling operations.
3. C O M E T A R Y
SOIL MODEL
The current cometary models were mainly derived from astronomical observations and the spacecraft fly-bys of comets Halley in 1986 and GriggSkjellerup in 1992. In addition experiments to produce comet analogue material were performed at the vacuum chamber of DLR Cologne in order to derive characteristic material properties (cf. (Schwehm and Langevin, 1991)). This provided significant improvements in the understanding of comets. "However, despite all the efforts, the composition, the inner structure and the evolutionary processes of comet nuclei are still a matter of wide uncertainty and they will remain a field of scientific debate..." (St6ffier and Schwehm, 1989, p.17).
501
-dust layer: cohesionless material with a density between 0.005 and 2 g/cm3; thickness up to lm, - crust layer: thin (few centimeters), hard (cohesion up to 4 MPa, Young's modulus up to 10 GPa) material formed by solar irradiation, - nucleus : hard (cohesion obtained in the laboratory between 1 and 4 MPa ; Young's modulus up to 800 MPa) and brittle material at low temperature.
4. EFFECTS OF PHYSICAL AND ENGINEERING
PARAMETERS
For ROSETFA on-comet operations typical parameters to be identified on-board in order to optimize the sampling / attitude control system (ACS) coordination will be cometary soil properties like size, hardness, cohesion and stiffness of the different layers, because they will influence the forces and torques caused by the drill. In the following table the different parameters affecting the on-comet operations are summarized. Table 2: Identification of characteristic parameters influencing the on-comet phase. Item
Cometary Properties
Parameter
- reaction forces of anchors - drilling forces and torques - landing pad creeping - crust fragmentation
Yo~g's medulus
- shearing stress - tilting of S/C
internal angle of friction
- foedpad sinking - shear stress - soil resistance
adhesion factor
- anchor uplift capacity
soil strati-
-
Table 1: Expected variation bands of typical cometary physical/mechanical parameters (redrawn from (St0ffier and Schwehn~ 1989)). Parameter
Ranse
Fraction of refractory dust
0- 100 % Porosity of refractory surface material 0-95 % Porosity below dust mantle 10-80 % 0.005 - 2.2 g/era3 Density of dust mantle Density below dust mantle 0.1 - 1.5 g/cm3 Grain size ofmonophase grains 0.001 - 100 Grain size of multiphase grains and composite l p m - l m aggregates 10"4- 102 MPa Co~ive st~n~h of bulk samples
Variations of the irradiation environment during the perihel passages in the comet's orbit should have caused an evolution of different layers close to the surface. A very porous and nonhomogenous surface structure must be expected, including loose dust material, boulders, pebbles and areas of exposed lower layers. Nevertheless potential safe landing locations can be modelled in a first engineering approach as follows :
Effe¢l
cohesion
graphy
comet dynamics
force and torque profile of drilling - anchor stability - thermal control (day/night cycle) - data transmission (contact periods)
5O2
K. Schilling et al. 5. C O M P U T E R Item
Parameter
Drilling Equipment
drill rotation rate
- sample heating - torques
pushing force
- causes vertical force to be accommodated by ACS, anchors - sample heating - sample acquisition time to reach given depth - power demand spacecraft stability
Anchors
Attitude Control System
(ACS)
Item
forces caused by gravity, comet dynamics, antenna movements, drilling, robot arm motion, motor vibrations, activation of gas pockets, etc. sensor - recognition of resolution tilting
reaction time
- jamming avoidance of drill
duration of thrusting
- counterreaction tuning
Characteristic
Parameter
m i s s i o n rate
influenced
b~,
antenna pointing accuracy - power allocation - comet dynamics / link geometry -
landing manoeuvre fuel consumption - anchor stability - maximum forces generated by drilling
cold gas propellant
-
available power
- coordination of operational activities
thermal control
-
Table 3 : Simulated adaptive control scenario Parameter
Type
Optimisation Criteria
Measuremems
- effectivity of attitude change counterreaction
data trans-
FRAMEWORK
In order to investigate technologies to deal with these challenging autonomy problems, exemplarily an adaptive control algorithm was analysed in this context. As the core sampling by drilling is the most critical period, this phase was selected for the computer simulation. The parameters to be considered are listed in Table 3.
Models
selection of thruster combination
Parameter
Spacecraft Resources
SIMULATION
Effect
comet dynamics (day/night) power consumption
Control Actions
Specific
Parameter
- attitude stability (primal) - duration of drilling ~seconda~) - drill / soil interaction - mechanical properties of soil layers - simplified spacecraft structure - ideal gyros - force sensors (between pad/soil, drill (vertical direction)) - torque sensors (between pad/soil) - c o l d g a s thruster (24 x 30 N) - hydrazin thruster (2 x 70 N) - drill feed velocit~
In order to perform meaningful simulations, models of the following spacecraft components have been implemented : - drill equipment, in order to evaluate the resulting forces and torques dependent on soil properties and pushing force/drill rotation rate, - finite element model of Lander structure, to couple the generated force at the drill with the resulting forces and torques at each of the three anchors, - attitude control system (ACS), to generate for the desirable counterreaction the optimum approximation within the range of available thrusters, - anchors, to determine the actual fixation capacity from a given stress/strain relationship, - spacecraft dynamics, to deal with the situation that the anchors are torn off. In the following sections, using the example of drill/soil interactions and of thruster selection, two important models for the control system will be discussed in more detail. The simulation tool was already designed in a modular way to support future extensions. Thus as
The ROSETrA Mission
503
soon as more detailed models regarding the interactions between anchors/drill and the soil are available the implemented models can be easily updated. Further improvements of the simulation environment should also address - thermal control aspects, - limitations of available power, data transmission requirements.
where kmo t, k, RA, LA are characteristic parameters of the motor and J is the relevant moment of inertia.
5.1 Model o f Drill/SoR Interaction
The quotient fn/fs is a characteristic material property dependent on the normal and the tangential components of the cutting force. It is approximated by an exponential function of the cohesion. A more detailed model would use friction coefficients depending on the penetration depth and the soil parameters instead of the constants employed here.
Similar to the torque model the resulting pushing force F r is composed of a friction part depending on the constant K F and of the cutting force:
-
" The drilling operations to acquire material samples from a depth of 3 m can generate significant attitude disturbances depending on the encountered soil properties. The task of this chapter is to relate in a simplified model the effects of soil parameters (like Young's modulus, cohesion) and drill characteristics (like pushing force, rotation rate) with the resulting forces and torques. In the low gravity environment on the comet these have to be compensated by anchors and by attitude control systems (ACS) thrusters. This coordination is the task of the adaptive control concept for core sampling to be further detailed in section 6. The drill's rotation rate n(t) causes reactive torques acting on the spacecraft. As presented in (Magnani et al., 1989) this torque T is composed of a shear torque (resulting from separation of the material) and a friction torque:
T = I [ D ~ - D 2 IE v(t) 8 t
, j
s n(t)
+ Krn(t),
where D a is the outer and D i the inner drill diameter, E S represents the cutting energy, v(t) is the drill penetration rate and K T is a friction coefficient. The cutting energy is approximated as a linear function of Young's modulus. This assumption has been derived from empirical data for different materials. In order to penetrate into the soil a pushing force F r has to be exerted by the drill. This pushing force Fr is generated by a penetration motor providing a penetration rate v(t) in reaction to an input voltage u(t), limited to some 40 V. The motor dynamics can be modelled by a linear transfer function of second order:
G(s)-- v(s) = k,.o, u(s) s 2 + R , s+ k LA 2 LAJ
Lf
J"(t)
Thus, in a first approximative model a functional relationship is established between the voltage u of the penetration motor, the control parameter, and the characteristic soil parameters cohesion and Young's modulus to derive the resulting pushing force F r and torque T. The challenge for the adaptive controller results from changes in orders of magnitudes for both soil parameters when the drill passes different soil layers.
5.2 Selection of Active Thrusters The compensation of forces and torques exceeding the anchor carrying capacity can only be achieved via thrusting, as the potential contribution of gyros to counter-actions is in the order of 0.4 N and thus below the expected torques by a factor of 500. The primary means are thus the 30-N cold gas thrusters, positioned on the Lander. In each edge of the cube shaped Lander body there are placed 3 thrusters pointing in the outbound directions of the cube axes. These 24 cold gas thrusters can be supported in emergency cases by the hydrazine thrusters positioned on the Mariner Mark II spacecraft, at the cost of chemical pollution of the landing site and of the samples to be acquired. There are 24 10-N thrusters placed similarily to the Lander configuration and two 70-N bipropellant-thrasters above the drill to press the spacecraft down in case the anchors fail. Given a required counter-action (force and torque vector) an appropriate combination of thrusters is to be activated to approximate the desirable resulting forces. Combinatorial optimisation methods are far from achieving real-time performance in such a huge search space of possible combinations. Therefore a look-up table of about 700 cases has
K. Schillinget al.
504
been generated, covering the most frequent situations as well as representative patterns from the range of potential force/torque scenarios, From this table a reasonable counter-action can be selected in real-time by a least-square method. To improve the approximation accuracy, the data base must be extended, increasing in parallel the duration for computation. Another interesting feature is the inclusion of alternative activation patterns leading to similar reactions. In case of a malfunctioning thruster only the patterns using this particular thruster are to be deleted in the table, then the next best approximation within the table will be selected, and thus a gracefully degrading redundancy concept results.
stability are active. In this case the reaction wheels are included for control in a nominal manner. In the current stage of simulations, limitations in the sensor resolutions and delays in the dynamics of actuators have not been taken into account. Figure 3 depicts the identification / adaption scheme and Fig. 4 shows the interconnections and interfaces of the control concept explained in more detail in the following. The mission management provides the supervising frame for the adaptive controller operations. This includes estimations regarding the most appropriate nominal penetration rates and drill rotation rates (according to soil properties derived from anchoring measurements).
6. A D A P T I V E C O N T R O L C O N C E P T F O R
CORE SAMPLING The task of the adaptive control concept is to coordinate reactions of the SAS, the anchors and the ACS according to the actual soil properties. As attitude stability is a prerequisite for drilling, it must prevent rotational as well as translational movements to avoid damage to the drill. This requires decision strategies working autonomously on-line to induce appropriate drill and ACS reactions to achieve an optimum between the spacecraft resources and the required operations time. The control action is modeled with respect to the characteristic soil parameter : cohesion c and Young's modulus E. The related values have to be identified during drilling in order to adapt the control strategies and controller parameters (AstrOm and Wittenmark, 1989). The essential sensor inputs are provided with respect to attitude by accelerometers and gyroscopes (to determine tilting or sliding of the spacecraft). For lift-off or critical limit detection force- and torque-sensors at the pads / anchors and at the drill are foreseen. In addition there are in the SAS sensors to measure - the penetration depth of the drill, - the voltage u of the penetration motor and - the temperature at the tip of the drill. The ACS means to react are control units like thrusters and reaction wheels. The effect of the reaction wheels is too small to deal with the torques and forces (generated by drilling) exceeding the capabilities of the anchors. As soon as the anchors are torn off and the spacecraft is in a free movement, the usual attitude control laws regarding
The optimal value for the drill rotation rate n will depend on the soil parameters. For example for hard material the rotation should be higher than for soft soil. However, at the beginning of the drilling process the soil parameters at the drill point are unknown. The determination of the initial value will be a typical task of the supervising mission management. After identification of the real soil parameters the mission management has to adapt and optimize the value of the rotational velocity. The rotational rate n(t) is supervised by a dedicated drill controller and can be assumed to be constant with respect to the adaptive controller design. Before the on-line identification of the soil parameters with successive adaptation of the drill controller parameters begins, the initial values of the controller parameter must be defined. At the beginning of the identification procedure a nominal desired value for the drill pushing force F r nominal has to be chosen. It should be high enough to provide a small penetration rate but small enough such that no anchor tearing off results at the beginning. The anchor capability limits are not known at the beginning of the drill process. But a first hint can be expected from anchoring. In order to be on the safe side, it will be a better procedure to start with a pushing force that is too small rather than too high. But it is a great advantage of the adaptive control concept that after the starting phase all further information about the soil characteristics, required for control, is directly acquired from the on-going drilling process. After convergence of the identification and adaptation algorithms, the desired nominal value of Fr can be increased. But care must be taken, that the resulting force F r pushing the drill down into the soil does not exceed the anchor capabilities. Anchor tearing off would be recognized by measuring the
The ROSETrA Mission
maintain a stable attitude in such situations, it is necessary to fire suitable thruster combinations for pressing down the spacecraft.
forces and torques at the pads, together with the attitude measurement signals. In a case where the anchors are still fixed in the ground, the forces do not cause a recognisable attitude change.
If all anchors are torn out, the dynamics of the spacecraft has to be taken into account. In this case the nominal attitude controller comes into action. After stabilisation of the spacecraft by the attitude controller, drilling Without anchoring must continue. For the remainder the drill forces and torques have to be compensated by the ACS alone.
Critical situations will occur ff there are sudden changes in the soil characteristics. Expected changes derived from anchoring measurements can be anticipated by adaptation of F r through the supervising mission management. Otherwise it is possible, that the force F r will exceed the maximum force to be compensated by the anchors.
Thus, in all cases the drill controller is working as an underlying control loop optimizing the drilling process according to the measured soil parameters, for an anchored spacecraft as well as for a freemoving vehicle.
Possible reactions are - to activate thrusters to support the anchors for spacecraft stability purposes or - to lower the drill pushing force F r . The adaptive control algorithm should initiate an optimum combination of both activities.
By the adaptive control concept the anchors, sample acquisition system and cold gas thrusters are coordinated and support each other. This results in increased capabilities to guarantee spacecraft stability. Thus the reliability and efficiency of spacecraft operations during the on-comet phase Will be significantly increased.
As soon as one of the force sensors indicates liR-off, supported by the attitude sensors showing rotational or translational movements, the feed velocity has to be immediately reduced by decreasing the desired value of the pushing force F r nominal • In this case one or more anchors are torn out or loosened. To
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time in scc Figure 5 • Simulation of occuring forces, when the drill encounters hard material inclusions embedded in a softer environment • a) at the drill location, b) at the anchors (3-d force vectors) without counterreaction thrusting, c) at the anchors, when adaptive thrusting is provided. 7. S I M U L A T I O N R E S U L T S In simulations the adaptive control algorithm was tested on soil parameters varying within the ad-
missible range. The results were very encouraging, showing that this controller can even handle sudden, abrupt changes in material properties within acceptable limits. The worst case regarding
507
The ROSETrA Mission peak loads occurs when the drill encounters a hard inclusion in a soft material layer. The resulting force profiles are given in Fig. 5. Figure 5 a) displays the force resulting at the drill location. Figure 5 b) shows the forces acting on the anchors, when no counterreaction via thrusters is initiated. In the displayed situation a tearing off of the anchors results, as the maximum sustainable force of 150 N is exceeded. Figure 5 c) exhibits the remaining forces (within the capabilities of the anchors), when the algorithm activates the thrusters of the AOCS in parallel. The comparison of 5b) and 5c) shows that thruster compensation leads to significantly smaller acting peak forces and torques at the anchors. Thus only a short thrusting period can avoid tearing off of the anchors, preserving their counterreaction capabilities as the main contribution to future spacecraft stability. Further comparisons were performed with a conventional PI-controller. In the situation of Fig. 5 it led to unacceptable oscillations throughout the hard layer, while the adaptive controller quickly adapts to the new soil parameters, causing only quickly damped oscillation for the short initial period. For the other situations compared the adaptive scheme was also preferable.
8. S U M M A R Y
The aim of this paper was to analyse the constraints for autonomous ROSE'ITA on-comet operations. In particular for the very demanding period of core sampling it was shown that adaptive control methods can provide interesting contributions to support spacecraft autonomy capabilities. The results of this paper are - the development of an integrated anchor/drill/ ACS operations concept, to improve reliability and resource consumption efficiency, - first analyses on the drill control regarding transitions between the different soil layers. Regarding the application of adaptive control technology in the field of spacecraft autonomy, of interest are - capabilities to adapt to actual measurements, - a very attractive performance for poorly known environments, - applicability of engineering standards to the verification of algorithms (important in case of high quality assurance requirements). As assessed on the concrete example of ROSETTA on-comet operations, adaptive controls provide an
interesting approach to enable autonomous spacecraft reactions in an uncertain environment. Acknowledgements
Part of this work has been undertaken in an ESA contract. The authors thank J. de Lafontaine and E. Bornschlegl for their comments and support during that study.
9. R E F E R E N C E S
AstrOm,K.J./Wittenmark,B. (1989). Adaptive Control, Addison-Wesley. Bornschlegl, E. (1992) ASDMS : An Autonomous Spacecraft Data Management System. Preparing for the Future 2 , 4 - 6. Eiden,M.J./Coste,P. (1991). The Challenge of Sample Acquisition in Cometary Environment. ESA Journal 15,191 - 211. Lafontaine,J.de (1992). Autonomous Spacecra~ Navigation and Control for Spacecraft Landing. J. Guidance, Control and Dynamics 15, Vol.3. Magnani,PG. et al. (1989). CNSR - SAS System Definition Study. Tecnospazio Report, Doe.No. SA-O02, Annex 3. Matra Espace/JPL/Dornier GmbI-I/BAe/Aeritalia (1991). ROSE'ITA- System Definition StudyFinal Report. Schilling,K. (1992). Simulation of ROSE'I~A OnComet Operations. Annales Geophysicae 10, 141 -144. Schwehm,G./Langevin, Y. (eds.) (1991). ROSETTA/CNSR A Comet-Nucleus SampleReturn Mission. ESA SP-1125, Noordwijk. St6ffier, D., G. Schwehin (eds.) (1989). Physics and Mechanics of Cometary Materials. ESA SP-302, Noordwijk.