Reorientation, service operation of client spacecrafts etc. are very actual tasks nowadays. Most of such spacecrafts are non-cooperative. Although service operations of these satellites were held before, they are not fully automated. However, docking in manual mode needs huge amount of time and human aboard. That makes impossible to carry out the required number of service operations. In current work it is represented conceptual model of the robot -manipulator for capture and maintenance of non-cooperative client spacecraft. Mechanism of collet effector was proposed for this purpose. Payload Adapter interface PAS 1666 S, PAS 1194 C, PAS 1666 MVS, PAS 1184 VS was chosen as a docking point, because of this interface's convenience and prevalence. Proposed mechanism allows to work in conditions of client spacecraft's linear and angular dynamic position errors in a range of +/-5⁰ per minute and +/-0.1 meters per minute respectively. This is achieved by design of the robot-manipulator. Problem of jogless docking and methods of shock prevention are examined. In addition, the berthing and girth operations need axes coincidence of non-cooperative client spacecraft and service spacecraft. Having this in mind, phases of berthing and girth are described and main functions of service spacecraft's control system for solving this problem are considered.
Nowadays on the geostationary Earth orbit (GEO) there are about 1500 satellites. However reorientation,
motion or other service operations are needed for about 750 spacecrafts [
The analysis of reports Department of Mechanical Engineering Massachusetts Institute of Technology [
As follows, it is obvious that with further evolution of Astronautics execution of docking operations with NCSC will appear more often and involvement of human as an object, which controls docking process, will have no any economical, social or scientific significance.
Process of docking NCSC with SSC is performed using the script, which is presented in table 1. This script
includes nine stages, the first five of which are essential for orbital docking of SC [
Stages 1, 2 have complied technical solution [7, 8], and stages 3, 4 were conducted within Space Orbiter programs and described in publications [7-10]. Stages 6-8, which provide the process of “soft” docking, are performed only in manual mode and the process of their automation requires the development of new control tools and control systems. Table 1. Sequence of operations, which are held during docking with NCSC.
Suitable Docking point (DP) in modern SC is apogee motor nozzle (Fig. 1) and SpaceCraft adapter ring (S/C) Payload Adapter (Fig. 2) the technical characteristics of which are written in operating manuals [11-13]. These nodes are characterized by high rigidity and coherence of the position of center of mass of the devices what allows to move NCSC with mechanic tools, which are located in SSC [11-13].
Capture and maintenance of NCSC through the listed DP need compensation of error of the active positioning of the spacecrafts, which is caused by the features of radar sensor system, which is characterized by low resolution at the distance of up to 20 meters [15], has stationary character and is given by six degrees of freedom (DOF).
Fig. 1. Spacecraft “Cassini-Huygens” with available S/C adapter ring and apogee motor nozzle in process of connecting to PAS: 1 system block of SC; 2 nozzle of apogeum motor; 3 S/C adapter ring; 4 PAS (photo is used with permission of European Space Agency, Communication Department) 1
Typical structure of S/C Adapter, which remains on S/C after its departing from the NCSC's launch vehicle (Fig. 3), has mounting hardware (keys) to Payload Adapter, thank to pyrotechnic fasteners is provided “departing” of SC from launch vehicle through. After departing from the launch vehicle S/C Adapter remeins on the SC and is not used repeatably for the active existence of devise on the GEO. Mechanical characteristics of pyrotechnic fasteners' mounting hardware allow the NCSC to hold the S/C Adapter 's mounting hardware in case of uncoordinated docking with the following coordination of dynamic and mechanical parameters of leaked devices.
Accordingly, for providing automatic capture of SC and S/C Adapter's elements it is necessary to specify the technical capabilities of capture adapter with a diameter range of 800-1400 mm, while maintaining the coaxiality of console, finite effector, NCSC and SSC. It is also important to note the necessity of unstressed capture in conditions of dynamic positioning error of devices. In work [14], the possibility of such a capture is considered, however coaxiality of SC is not provided.
The structure of effector should provide the opportunity of capturing NCSC with consideration of such factors: the presence of errors in linear and angular desitioning; limited time of being at the distance of NCSC and SSC berthing; a large number of S/C Adapter's standards, that are diffrent at sizes and structure; the lack of standard markers on NCSC adapter; absence of friction force between NCSC and environment.
Then, taking into account the factors above, capture of NCSC by Robot Manipulator (RM) must meet the
requirements, specified in table 2 and described in [
Berthing requirements for SSC, which are listed in table 2 (points 1 and 2) are caused by specifications of NCSC, in particular, the generalized length of solar batteries of devices, arengment of antennas, the radius of cone № Description
to 10˚ per 1 min to 0.1 m per 1 min 0.6-3 m to 5000 kg of occurrence of SC, errors of linear and angular positioning of the SSC to NCSC. In such conditions, delivery of RM's effector to S/C Adapter is possible with the use of telescopic console, which is equipped with two angular hinges: double-axis hinge 1 and triple-axis hinge 2, which are located in places of console mounting to NCSC (p. А) and the final effector (p. B) in accordance/ kinematic structure of console is shown on Fig. 4.
Double-axis hinge 1 and telescopic link of robotic console 3 provide the work of finite effector in the polar coordinate system. Triple-axis hinge 2 provides cardan joint of finite effector, that allows to compensate for errors of its positioning. Such kinematic structure of robotic console eliminatece the occurrence of singularity of the links, however does not solve problem of final effector positioning in S/C Adapter area for committing the unstressed NCSC capture. The solution of this problem is possible only by equipping the effector by sensor devices, which are suitable for recognition and relative position determination of finite SSC effector and NCSC S/C Adapter.
Taking into account the linear and angular errors of the positioning of the devices, guaranteed effector’s positioning for its further girth and capture of S/C Adapter is possible in the segment of work zone (Fig. 5 p. 1). The ability to capture NCSC with deviation close to zero is also possible in zones, which are shown on Fig. 5 (p. 2 and 10). Another zones, which are shown on Fig. 5 are intended for the equipment SC or used as a part of device work in the technical operations.
Zone of insensitivity (table. 2, p. 5) and difference of the S/C Adapter's radii (table. 2, p. 6) impose restrictions on the choice of the effectore's construction and the set of sensor system tools. “Zone of insensitivity” means impossibility of the sensory system, which consisting of the SSC control system (CS), to give accurate distance coordinates and NCSC’s orientation. Fault 1 m with the speed 1˚ per minute can cause a collision of effector with S/C Adapter and give them relative acceleration.
Compensation of error may be achieved due to the physical impact of the effector on S/C Adapter after its girth. Adaptation to diameter PAS is achieved by applying effector with chain compression principle. Kinematic structure of such effector is shown in Fig. 6. The contact area of effector has a constant shape due to unification of the key construction of PAS S/C Adapter. Shape of the adapter's key is shown on Fig. 7.
Unstressed capture of PAS S/C Adapter is achieved by compensating force, which effector has applied, and the counteracting reactive force applied by the adapter in relation to the effector. Such phenomenon is achieved through the restrictions that are imposed by effector's links, after effector girth adapter and until the approach of capture. Monaural compression values of the S/C Adapter are determinate from the selected capture algorithm, force sensors, which are integrated in the composition of effector's actuators, and sensors of the optical leane interruption (Fig. 6, s2), which works when the ray crosses S/C Adapter elements. Prevention of incomplete capture (Fig 6, S/C1) is ensured by a coordinated action of SSC's vision and technical means.
The final fixation when capture arises due to the to reduction of the free zone of the adapter movement zone within the board lines of effector's zone (Fig. 6, CapZ), which occurs as a result of translation motion in the effector units in the corresponding hinges (Fig. 6, k7, k9).
Process of S/C Adapter's fixation is accompanied by mutually rotation of the effector, which prevents the occurrence of angular momentum between effector and the adapter. Such rotation is provided by hinges, which are shown on Fig. 6 (k2, k3, k4).
In program of the connection of SCs, that involves the joint movement of devices, a required condition is preliminary alignment of their axes. Moreover, design of satellites involves the axial position of the point mass center (MCP) that provides position of a common MCP on a vector of the main SC's engine. For convenience, process of berthing is divided into two phases: 1. berthing with capture; 2. orientation of SC on one common axis.
During the first phase RM CS initiate the execution of operations of effector preparation for capture. Coordination of axes of effector and S/C Adapter is achived.
Berthing SSC to NCSC and their relative position is determinated by service spacecraft Control System (CS), however CS sets positioning faults, that can cause invariant of SC position. In this case there is a need compensation of the fault by means of the console and the effector that are part of the robot-manipulator. Due to the pair of hinges, which are placed at the base and at the end of the RM's console and thanks to the telescopic pair of links, final effector can be positioned in the plane of S/C Adapter. Control of effector positioning is possible due vision means. The process of compensation of linear and angular faults within the plain is shown on Fig. 8.
It is important to note that at the stage of berthing robot's effector does not interact with NCSC, which leads to preservation of the Newton's first law.
Capture procedure provides aligment of SSC and NCSC mass center points (MCP) without their displacement by applying an equal number in magnitude and opposing forces in the direction of the effector. During the berthing RM can act on NCSC with essential force, which in the case of inaccurate capture, will give NCSC uncontrolled movement. For the capture of NCSC without increase acceleration to it, effector of RM has to cover structural components of S/C Adapter without contacting them. This procedure is shown on Fig. 9.
First stage is held on the “decision-making” distance (20 meters between NCSC and SSC). At this stage operations of initialization of sensory system, executive devices and RM CS are performed. At a distance ρ=20 meters, effector of RM is introduced into the work zone of manipulator. According to data from RM CS, means of technical vision and sensors and the position of links is carried out by coordinating the position of the effector in 3 coordinates. At the same time, angular orientation of effector stays random. (Fig. 9, e1).
On the second stage effector places in parallel to the plane of S/C Adapter. Second stage is carried at the “berthing” distance (about two meters between NCSC and SSC). At this stage the calibration of vision devices is performed according to sensors of orientation and position of RM's links. The classification of the S/C Adapter is carried out. Algorithm of berthing and capture is determinate (Fig. 9, е2).
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At the third, forth and fifth stages(Fig. 9, е3 - е5) operations of girth and capturing the S/C Adapter’s construction part are performed by the effector. Taking into consideration huge number of S/C Adapter versions, effector can change zone of girth and capture in depending on the chosen algorithm, which is selected in the second stage.
During the second phase CS of RM initiate coordination of SSC and NCSC axes. For this angle c (Fig. 8) reduces to zero. After aligment of axes hinge joint of SSC, RM, NCSC design is constant, and limiting relative displacement new position of the all MCP are considered. MCP position coincides with axis of the devices (which passing through their apogee engines). This orientation of the devices allows to use apogee engine of SSC as the motor of whole construction.
Researches, which were held in this work, showed that procedure of non-cooperative capture and docking operations are economical and scientific actual engineering problems and the development of the space industry of the human management requires the technical means for carrying out satellites, their recycling, execution of mechanical operations etc.
The proposed principle and the concept of the structure of the robot-manipulator of a service spacecraft will allow the procedure of capturing spacecraft with the mass up to 5000 kilograms in conditions of uncertain definition of their spatial position and proposed structure of the adapter design. Working on the principle of chain will allow to carry out girth, capture and holding of SC, which is equipped by PAS 1666 S, PAS 1194 C, PAS 1666 MVS, PAS 1184 VS with positional fault 10° per minute and 0.1 m3.
This publication would not be prepared without consultation of colleagues from SOE “Pivdenne”, PrJSC “NVK Kurs”.