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HOTDOCK: Design and Validation of a New Generation of Standard Robotic Interface for On-Orbit
Servicing
Pierre Letiera*, Torsten Siedela, Mathieu Deremetza, Edgars Pavlovskisa, Benoit Lietaera, Korbinian
Nottensteinerb, Maximo A. Roab, Juan Sánchez García - Casarrubiosc, Javier Luis Corella Romeroc, Jeremi
Ganceta
a Space Applications Services NV/SA, Leuvensesteenweg 325, 1932 Sint-Stevens-Woluwe (Brussels Area), Belgium,
firstname.lastname@spaceapplications.com
b Institute of Robotics and Mechatronics, German Aerospace Center (DRL), 82234 Wessling, Germany,
firstname.lastname@dlr.de
c MAG SOAR, Av. de Europa, 82, 28341 Valdemoro, Madrid, Spain. Mail: jsanchez@magsoar.com Mail:
jcorella@sanjorgetecnologicas.com
* Corresponding Author
Abstract
This paper presents the design and validation of HOTDOCK, a new generation of standard interface for on-orbit
and planetary applications providing mechanical, power, data and thermal coupling capabilities between payloads
and spacecraft, between spacecraft modules and as end-effector of robotic manipulators. The provision of standard
interfaces in the design of spacecraft buses and payload modules will become essential to enable the emerging
LEO/GEO on-orbit robotic servicing market. That includes payload management for spacecraft maintenance and
reconfiguration, large structures assembly in space and de-orbiting operations. Standard interfaces are also highly
interesting for supporting robotic operations in future deep space missions (LOP-G, Moon and Mars surface
operations). HOTDOCK features a compact and fully integrated androgynous and 90-degree symmetrical design.
The external form-fit geometry supports mating trajectories in a cone of up to 130-degrees, allowing for
simultaneous connection of three orthogonally mounted interfaces. The unique patented coupling mechanism, along
the circumference, allows stiff mechanical structural coupling with high load transfer. A central connection plate,
equipped with spring-loaded POGO pin connectors, offers re-configurable and switchable electrical power as well as
bi-directional high rate data transfer between connected subsystems. HOTDOCK can be optionally equipped with a
fluidic transfer capability for thermal cooling on top of the regular thermal conduction between two units. In its
nominal configuration called Active, HOTDOCK provides an actuation mechanism for the mating as well as
integrated control and interface electronics. A purely passive version, without active components, has also been
developed to offer a lower cost, volume and mass version. Both Active to Active and Active to Passive connections
are possible, allowing in each case power, data and thermal transfer capabilities. HOTDOCK has been adopted as the
reference Standard Interface in three projects of the European Commission’s H2020 Space Robotic Technologies
cluster (OG8 PULSAR, OG9 MOSAR and OG11 PRO-ACT). They respectively address large structure assembly in
space, modular satellite reconfiguration, and collaborative robotic planetary operations. More than 50 units are
currently being produced for integration in several ground demonstrators (TRL 4). Furthermore, HOTDOCK is used
as part of the Michigan Technical University “T-REX” project awarded by NASA (BIG Idea Challenge).
Keywords: Space robotics, standard interface, mating interface, on-orbit servicing, on-orbit assembly, planetary
exploration, robotic manipulation, thermal management.
Acronyms/Abbreviations
Low-Earth Orbit (LEO), Geostationary Earth orbit
(GEO), Technology Readiness Level (TRL), Thermal
Vacuum (TVAC), International Space Station (ISS),
Mission Extension Vehicle (MEV), Power and Data
Grapple Fixture device (PDGF), Wedge Mating
Interface (WMI), Space Station Remote Manipulator
System (SSRMS), Orbital Replacement Unit (ORU),
Brushless Direct Current (BLDC), Printed circuit Board
(PCB), Low-voltage differential signalling (LVDS),
Telemetry/Telecommand (TMTC), Pulse Width
Modulation (PWM), Finite State Machine (FSM),
Failure Detection, Isolation and Recovery (FDIR),
Electrical Ground Support Equipment (EGSE)
1. Introduction
Space Industry is currently evolving towards new
applications and paradigms, in parallel to the recent
evolutions in robotics technologies. Although already
applied in some extent on large structures like the ISS,
the concept of on-orbit servicing is now emerging for
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other scales and applications, through different projects
[1] and on-orbit demonstration initiatives, like the recent
re-fueling operation performed by Northrop Grumman’s
MEV-1 servicer [2]. It is opening new opportunities for
scientific but also commercial applications, including
satellite lifetime expansion, maintenance, inspection,
repair, or spacecraft / mission reconfiguration.
The interest for space sustainability is also growing
very fast, with the main problematic of the space debris
and satellite end-of-life management, as illustrated by
the ESA Clean Space initiative [3] and the mission
CLearSpace-1, planned for 2025 [4], targeting the first
debris removal from orbit.
On a longer term, to get rid of the limitations of
launcher size and with the perspective to deploy bigger
structures in space with more features and capabilities
(e.g. large size telescopes), the concept of (autonomous)
large structures on-orbit assembly, based on robotic
technologies, will provide new opportunities of
missions [5][6]. Also, modular and reconfigurable
spacecraft design would create a complete shift in the
way to address space use and economy. These concepts
are currently evaluated at ground prototype level, as
illustrated later in this article.
Finally, besides the on-orbit applications, planetary
exploration and exploitation draw more and more
attention. Whether it be through fully autonomous
missions, or as preparatory activities for future human
presence on the Moon or Mars, there is a consensus that
robotic technologies are instrumental for surface
exploration and in-situ lunar resources utilization.
In order to tackle these new challenges, key robotic
technologies should be developed to ensure reliable,
sustainable and cost-effective solutions. Due to the
increasing need for components connection in the
applications identified above, a provision of Standard
Interfaces fixture sites in the design of spacecraft buses,
payloads modules or robotic manipulators will be of
paramount importance. Besides supporting mechanical
coupling, these interfaces should also allow for power,
data and thermal transfer. They should be optimized in
dimensions, mass, energy consumption, integration
complexity, reliability and costs in order to allow for a
wide and fast uptake by the space industry.
Most robotic interfaces with space heritage today
such as the Power and Data Grapple Fixture device
(PDGF) and the Wedge Mating Interface (WMI) have
been designed for robotic operations on the ISS using
the Space Station Remote Manipulator System
(SSRMS, a.k.a Canadarm2) and the Special Purpose
Dexterous Manipulator (SPDM, a.k.a Dextre). These
interfaces are generally too bulky and massive for the
emerging LEO/GEO robotic servicing market and future
exploration missions to the Moon or Mars.
In this context, Space Applications Services has
initiated the development of HOTDOCK, a novel
standard interface building block for commercial
exploitation in space. It is a mating device providing
androgynous coupling to transfer mechanical loads,
electrical power, data and (optionally) thermal loads
through a single interface. Integrated to spacecraft and
payload structures, it provides mounting points for
components assembly and reconfiguration. Mounted at
the tip of a robot arm, it works as an end-effector for
quick connect/release of spacecraft modules during their
manipulation or as a quick tool adaptor. HOTDOCK
eases the replacement of failed modules (ORU) as well
as swapping of payload elements, and provides
chainable data interfaces for building spacecraft
modules assemblies.
The following sections provide information about
the design of the HOTDOCK interface, describing its
different functionalities and characteristics. Following
this, on-going projects and demonstration activities
based on the implementation of HOTDOCK are
introduced.
Fig. 1. HOTDOCK Standard Interface
2. HOTDOCK Standard Interface Design
HOTDOCK is a compact mating device that
provides the following four interface functions:
- The mechanical interface that provides the
alignment, connection and mechanical load
transfer capabilities.
- The electrical power interface that offers the
possibility to control the transfer of power
through the component.
- The bi-directional data interface that ensures
high-rate communication between systems.
- The thermal interface that can optionally be
integrated to allow fluid and thermal energy
transfer.
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Fig. 1. Active and passive version of HOTDOCK with
highlight of the four main interface functions
Based on the same design and shape, different
declinations of HOTDOCK have been developed to
offer mass and cost optimization as function of the
application. While the Active interface has all the above
features, the Passive interface is a simplified version of
the Active, which provides the form-fit geometry and a
fixed power/data transfer interface, without any active
capture mechanism. As a third variation, the
Mechanical version offers only the form-fit geometry.
An Active interface is able to connect independently to
another Active, Passive or Mechanical version. The two
simpler declinations of HOTDOCK present a reduced
volume and mass, do not include moving parts and can
conveniently and affordably be integrated on spacecraft
buses and payloads, in prevision of future potential
connection in space with an Active HOTDOCK.
The following table summarizes the main
characteristics of the HOTDOCK ground model. More
details on each of the key features are provided in the
next subsections.
Table 1. HOTDOCK characteristics and main features
Dimensions
(diameter x height)
Active: 148 x 70 mm
Passive: 120 x 35 mm
Mechanical: 120 x 25mm
Mass
Active (Thermal): 1.4 kg (1.55kg)
Passive (Thermal): 0.35 kg (0.5 kg)
Mechanical: 0.25kg
General features
Androgynous, 90° symmetry
Misalignment tolerances
+/- 15mm in translation
10° in rotation
Diagonal engagement
130 ° approach aperture
Load transfer (non-
destructive)
3000 N in traction
300 Nm in bending moment
Coupling sequence
20s
Power transfer
2.5 kW @ 120V (up to 4kW with
specific pin configuration), with
switch relay and power measurement
Data transfer
CAN, SpW, Ethernet, TTE
Thermal transfer
up to 2.5KW (conduction up to 50W)
Powering
24V, 0.15A (average consumption)
Control, TM/TC
CAN
2.1 Mechanical Interface
The mechanical interface of HOTDOCK has
interesting features for mating operations in space. It
presents a fully androgynous design, for coupling of
identical HOTDOCK. It also has a 90° mechanical
symmetry, allowing different orientation of payloads
and simpler robotic manipulation operations in
preparation to coupling. In addition, as illustrated in Fig.
2, the front face is equipped with a form-fit geometry
that enables precise mechanical guidance during the
alignment process, as well as allowing high load
transfer once mated.
Fig. 2. HOTDOCK mechanical interface elements
Fig.4 illustrates the tests and verifications that have
been performed to characterise the range of attraction
(misalignment tolerance) and the diagonal engagement
capabilities of the HOTDOCK form-fit geometry. The
form fit geometry allows guiding the final approach of
the interface before starting the mating process, with the
support of a compliant robotic manipulator. The
geometry allows 15 mm of initial misalignment in all
directions, as well as 10° in rotation. This enables
mating alignment without necessarily requiring external
visual servoing capability. This is furthermore
supported by Hall-effect sensors embedded in the
mechanical structure, that are used for proximity
measurement as well as verification of alignment and
relative orientation of the two mated interfaces (Fig. 3).
Fig. 3. Hall sensors for proximity, alignment and
orientation feedback
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Fig. 4. Left: Experimental result of HOTDOCK form-fit range of attraction; Centre: DLR Test setup to validate
HOTDOCK angular engagement; Right: Compatibility of HOTDOCK engagement with standard shapes
HOTDOCK offers a 130° approach aperture, enabling
the simultaneous mating between a number of
interfaces exposed in “rectangular” and “hexagonal”
configurations.
2.2 Locking Mechanism
The Active version of HOTDOCK is equipped with
a patented locking mechanism to enable the mating and
fixation to another HOTDOCK interface. The coupling
mechanism relies on four locking pins, embedded in the
locking ring structure (Fig. 2). During the mating
process, they are pressed with the mated partner, by the
motion of the locking ring. The locking mechanism is
compatible with a remaining misalignment of up to
2mm. The use of the full circumference of the device
allows high levels of load transfer. HOTDOCK has
been tested with loads of 3kN in traction and 300Nm in
bending moment.
The mechanism is driven by a BLDC sensor-less
motor connected to an internal gearbox and barrel cam
mechanism. An absolute position sensor measures the
motion, enabling the control of the connection states of
the interface. The same mechanism is also used to drive
the motion of the central power and data connector plate
and the thermal interface.
2.3 Power and Data Interface
The HOTDOCK power and data interfaces are
integrated in the inner section of the device, through a
PCB connector plate (Fig. 5). Both electrical power and
data are transferred via a set of spring-loaded connectors
also known as “POGO” connectors. They are
particularly tolerant to misalignment and prevent
accumulation of dirt or dust. This keeps the power and
data transfer interface simple and subsequently
improves its reliability.
The PCB is split in four sections, with 24
independent POGO pins/pads, arranged in mirror to
ensure the androgynous and 90° symmetry
characteristics of the interface, offering also the option
of redundant data transmission. The flexibility of the
connector layout makes it easy to integrate the POGOs
in various patterns, as function of the application and
the required power/data transfers needs.
Fig. 5. HOTDOCK central power and data connector
plate, equipped with POGO connectors
In the current configuration of the power interface
(used in the demonstration projects described
afterwards), it can transfer up to 20A @ 120V. It can
also be equipped with an internal power switch relay or
control an external one, while also measuring the
current and voltage of the power line.
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The data interface has been currently validated for
Ethernet, CAN and SpaceWire bi-directional
transmission. It is equipped with an LVDS switch to
enable signal data routing through the different
quadrant, as function of the interface orientation, while
keeping required track impedance for high-speed
signals.
2.4 Thermal Interface
Thermal management is an important topic in space
environment. HOTDOCK can be equipped with an
internal thermal interface to enable fluid transfer
between two connected HOTDOCK units. Developed in
collaboration with MAG SOAR, the HOTDOCK
thermal interface consists of 8 hydraulic Stäubli
connectors (four male and four female) integrated in a
3D printed titanium structure (Fig. 6). The thermal
interface deployment relies on the regular HOTDOCK
actuation mechanism. The additional weight for the
thermal interface is 150g. With a maximum fluid
pressure of 16 bar, the interface can transfer up to
2500W of heat power. Fig. 6 illustrates the test setup to
characterise and validate its performances.
Fig. 6. HOTDOCK thermal interface and test setup for
thermal transfer characterisation
2.5 HOTDOCK Controller and Operations
The HOTDOCK controller is implemented as a
round-shaped custom PCB to seamlessly fit at the back
of the HOTDOCK mechanism (Fig. 8). As illustrated in
the avionics architecture diagram, the controller is
interfaced on the outside with the HOTDOCK CAN
control TM/TC line, the HOTDOCK power line (24V)
and the power/data transfer lines. On the other side, it
interfaces with the central POGO connector plate (front
connection, section 3.3). The PCB controller is also
equipped with an external interface to support update of
the firmware and for debugging purpose.
The HOTDOCK controller manages the following
tasks:
- Motor control: field-oriented control of the
brushless motor (section 3.2), by interfacing the
front-end chain (H-bridge, gate drive and buffer)
through PMW signal.
- Sensors interface: processing of the analogue and
digital internal signals of HOTDOCK including
the Hall effects proximity/orientation sensors, the
absolute encoder locking sensor, the
voltage/current of the interface power transfer
and the 4 internal temperature sensors, used to
monitor the state of the device.
- Local management: with the implementation of a
Finite State Machine (to control the different
states of the interface, Fig. 7) and FDIR
management.
- Local electrical power conversion: from the 24V
powering bus, for the PCB components.
- TM/TC: to enable telemetry and telecommands
with an external OBC/EGSE through CAN bus.
- LVDS switch: control of the connector plate data
routing.
- Power relay: latching relay control to enable the
bi-directional control of the power flow through
the interface. It can also be used to control
external relays for higher current limits.
Fig. 7. HOTDOCK Finite State Machine
implementation
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Fig. 8. HOTDOCK avionics architecture and integrated PCB controller board
3. HOTDOCK’s Applications
This section introduces the different on-going
projects, ranging from on-orbit servicing to planetary
applications that supported the development of
HOTDOCK. In those projects, HOTDOCK is involved
as a key element for different ground demonstrators
with completion foreseen in the first quarter of 2021.
3.1 H2020 MOSAR
MOSAR is a H2020 European Commission (EC)
funded project, led by Space Application Services and
part of the European Space Robotic Technologies
Strategic Research Cluster [7][8]. It aims at
demonstrating and developing key technologies for
modular and reconfigurable on-orbit space assemblies.
Fig. 10 illustrates the MOSAR concept through an artist
impression. The baseline scenario involves a servicer
spacecraft carrying spacecraft modules, each of them
being related to a specific function for the client satellite.
A walking manipulator is used to support the assembly,
replacement or upgrade of the client satellite’s
components. In MOSAR, HOTDOCK plays a central
role to support the mechanical, data, power and thermal
interconnections between the modules and with the
spacecraft bus. HOTDOCK also equips the two end-
effectors of the walking manipulator to provide an
interface point to the modules and the spacecraft
structure [9]. The principle of modules manipulation
using HOTDOCK has already been demonstrated at the
occasion of IAC’19 [10]. MOSAR will extend this
demonstration to a set of meaningful scenarios,
including data, power and thermal transfer, by the first
quarter of 2021.
3.2 H2020 PULSAR
The H2020 EC funded project PULSAR, part of the
same research cluster, is developing and demonstrating
key technologies for large in-space assembly, and more
specifically, primary mirror of a large telescope (Fig. 9)
[11]. The ground demonstrator involves HOTDOCK as
an end-effector for the robotic manipulator, and used to
manipulate and assemble the mirror tiles. Once the
assembly process is completed, HOTDOCK is ensuring
the mechanical integrity of the telescope structure, and
allows data/power transmissions to monitor and control
the alignment of active optic mechanisms that are
embedded within each tile.
Fig. 9. Simulator representation of the PULSAR
scenario (Credit: Magellium/ONERA)
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Fig. 10. Left: Artist representation of the MOSAR concept; Centre: Detailed design of the MOSAR ground
demonstrator; Right: Preliminary MOSAR test setup with module manipulation through HOTDOCK
3.3 H2020 PRO-ACT
In complement to on-orbit applications, the
research cluster also investigates technologies for
planetary applications [12]. The H2020 EC funded
project PRO-ACT aims to develop and demonstrate
manipulation and collaboration of robots for the
assembly of a lunar in-situ resource utilization
facility. HOTDOCK is used for manipulation and as a
tool changer interface for the VELES robotic
platform, as depicted in Fig. 11. It allows the
connection of an active robotic hand or
electromechanical tool (e.g. driller), and supports the
direct manipulation of elements.
Fig. 11. VELES platform (credit: PIAP Space),
equipped with HOTDOCK standard interface as tool
changer
3.4 ESA MIRROR
MIRROR is an ESA project, led by Space
Applications Services. This project aims to develop
robotic systems for future large structure assembly in
space. Large structures are divided into subassemblies
that are launched, disassembled and later assembled
in orbit by the robot system. The robotic concept,
depicted in Fig. 12, is a multi-arm robot equipped
with HOTDOCKs. HOTDOCK allows for a high
level of modularity of the robot and enables a new
paradigm for assembly procedures through the active
inter-connection between the robotic arms, the torso
and the assembly structure.
Fig. 12. Artist representation of the MIRROR concept.
3.5 NASA BIG Idea Challenge T-REX
T-REX, awarded by NASA to the Huskyworks
Lab of the Michigan Technological University and
depicted in Fig. 13, is a concept of rover that lays
down lightweight, superconducting cable connected
to a lander. Once the rover reaches its final spot after
traversing rocky crater terrain into the permanently
shadowed regions, the rover can serve as a recharging
hub and communication relay for other robots. In this
context HOTDOCK is being used as the standard
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Fig. 13. Artist representation of T-REX mission
(Credit: Michigan Technological University, Paul van
Susante).
interface for powering, charging and communicating
with other rovers.
4. Conclusions
This paper has presented the design and
applications of HOTDOCK, an innovative standard
mating interface solution for on-orbit servicing and
space robotics. The HOTDOCK product line offers
attractive features for short and long-term space
applications. It is currently deployed in four
demonstration projects both in Europe and in the US,
in order to demonstrate its capabilities and
performances. Up to 50 laboratory quality models are
under production for the needs of these projects, and
30 more are already confirmed for 2021. From a
technology readiness level of 4+, Space Applications
Services is actively working on the maturation of
HOTDOCK towards TRL6+ and the validation of a
space qualified model by end of 2022.
Besides the space market, we are also
investigating potential HOTDOCK exploitation in
terrestrial applications, including e.g. automotive and
energy (including off-shore) industry. HOTDOCK
technology can be scaled down or up as well as
adapted to specific conditions, including e.g.
underwater environment, to address a wide range of
applications.
Acknowledgements
This work was partially funded by European
Commission under grants number 821966 (H2020
MOSAR), 821858 (H2020 PULSAR) and 821903
(H2020 PROACT).
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