Research on compression-rod lock–release mechanism with large load for space manipulator
Journal of the Brazilian Society of Mechanical Sciences and Engineering
Research on compression-rod lock-release mechanism with large load for space manipulator
Fei Yang 0 1
Honghao Yue 0 1
Yuliang Zhang 0 1
Jun Wu 0 1
Zongquan Deng 0 1
0 Beijing Satellite Manufacturing Factory , Beijing 100094 , People's Republic of China
1 State Key Laboratory of Robot Technology and System, Harbin Institute of Technology , Harbin 150080 , People's Republic of China
With the development of Chinese space station, the space manipulator with large load plays a more and more important role. At the same time, the lock-release mechanism for the space manipulator must be reliable. In this study, the locking point layout method was proposed according to the size and the structure of the space manipulator, and the number and the position of the lock-release mechanism were determined. The design of lock-release mechanism including compression rod for large load lock-release was presented. By the established finite element models of lock-release mechanism and space manipulator, the locking stiffness and reliability was verified. A test prototype of the lock-release mechanism was developed. Through the stiffness measured in each direction, the accuracy of the stiffness and the strength were tested. At last, a space arm vibration test under lock status was carried out. The results show that the lock-release mechanism can meet the design specifications. Technical Editor: Fernando Antonio Forcellini.
Space manipulator; Lock-release mechanism; Pyrotechnic device
With the rapid development of deep space exploration
], especially the construction and
application of space station, space shuttle and space robot, space
] which has been used widely in space has
become an indispensable part of on-orbit servicing systems
such as the construction for space station. According to
China’s space master engineering plan, astronauts will
work on the space station for a long time in the future, and
the space manipulator will be one very important tool to
help astronauts to act in extravehicular environment.
During the course of transporting and launching, the space manipulator will bear inertial force, vibration and impact load [8, 9]. For safety, space manipulator will be
folded and locked while launching by lock–release
mechanism (LRM) [
]; when the space manipulator reaches
at the pre-planned position in space, the LRM unlocks the
folded manipulator to make it carry out the space missions.
To ensure that the space manipulator can reach the space
station smoothly, space manipulator must be locked
reliably to resist the large impact load while launching.
Generally speaking, more than one LRM are adopted at
multiple locking points to lock the space manipulator for
higher system stiffness, however, multiple LRMs will bring
one enormous challenge for unlocking successfully at one
time, thus one LRM will be single failure point, each LRM
is related to the successful unlocking of the folded
At current, the LRMs, which are widely used in aero
nautic and aerospace field, are mostly based on the
principle of initiating action [
] with the advantages of
simple structure, large load, rapid separation and so on.
Although the initiating devices are mature products, these
devices have some disadvantages of severe impact, obvious
pollution, non-repeatable usage and so on. At present, the
research on LRM of large space manipulator in China is in
blank state, the LRM with the property of large load,
locking and unlocking reliably will promote space
manipulator’s engineering in the future application.
2 Design of the lock–release mechanism
The space manipulator consists of high strength arm and
concentrated mass. And the joints and the end effectors are
high-density mass points, and each joint is connected in
series into a manipulator with multiple degrees of freedom.
The space manipulator is in the folding state while
launching, the joints of the end effector on both sides are
bended, and the whole mechanism lies on the cylindrical
body surface. Considering the spatial configuration and
mass distribution of the manipulator, the location of the
locking mechanism according to the division is shown as
According to the property of space manipulator and the rocket, the design requirements of the LRM are as follows: 1. 2.
The fundamental frequency of the manipulator in
locked state is not \ 70 Hz;
The total mass of the lock–release mechanism is
\ 20.1 kg;
The preload force error of the lock–release mechanism
is \ 5%;
The structural rigidity of the lock–release mechanism
is not \ 5.0 9 106N/mm.
3 Design of LRM
The LRM is a reliable rigid locking device that is
connected by a compression rod and released by a pyrotechnic
cutter. This device can achieve the locking between the
side wall of the spacecraft and the payload by the screw
connection of the compression rod [
]. When the
manipulator reaches the designated working area, the
electric detonator is ignited, and the cutter cuts off the
compression rod at high speed causing by explosive gas, so
that the original fixed restraint is released and the release of
the payload is realized.
The principle of the LRM is to provide a pressing force
through the compression rod assembly to maintain
sufficient positive pressure on the two contact surfaces that
need to be locked. The compression rod carries the axial
load of the separation surface, and the friction of the
contact surface or the positioning surface carries the lateral
load. One end of the compression rod connects to the base
and the other end provides compressing payload to achieve
a rigid connection. The external structure design is shown
in Fig. 2.
Upper and lower bases
The upper base is connected to the mechanical interface on
the side of the arm by 6 M6 screws. The lower base is
connected with the cabin bracket by 6 M8 hexagon screws.
An insulating pad is arranged on the connecting surface of
the LRM to prevent the influence of the heat source on the
robot arm. The upper and lower bases are positioned by a
trapezoidal groove. The advantage is that it has a
positioning effect on the lateral direction and can provide a
specific shear resistance in the compacted state to ensure
that the bending moment load would not affect the bearing
rod in the locking mechanism. The placement of the upper
and lower base can divide the impact load to the two
opposing locking mechanism. The design reduces the load
bearing requirements on the monomer-locking mechanism.
2. Compression rod and nuts
The pressing rod is the main bearing member of the LRM,
whose diameter is 4 mm at the minimum cross section. The
pressing rod is connected to the upper and lower bases by
loading nuts and the preload force is applied. Glue is
applied to the connection between the pressure rod and nuts
to prevent connection failure.
3. Pyrotechnic cutter
The cutting cutter uses gunpowder explosion to promote the cutter to break the titanium alloy compression rod and the cutting reliability is greater than or equal to 0.9993.
Copper ball gasket
4. Lock washer and spring
The separation spring can send the broken piece of the
compressing rod into the capture cap to prevent
interference on the separation surface, which may stuck the robot
arm. It is necessary to reserve enough space considering the
shape changing of the compression rod broken bar after the
5. Copper ball gasket
The copper ball gasket, which can reduce the additional
bending moment of the locking rod due to the deformation
during the loading process, cooperates with the upper
separation socket. The self-centering effect of the ball
gasket will improve the stress state of the compression rod.
4 Simulation analysis of dynamic characteristics
4.1 Static load analysis of the LRM
The LRM is mounted below the joint of the manipulator,
which is primarily responsible for the dynamic load
generated by the acceleration of the joint. The load acts on the
upper flange surface, and the confinement surface is the
upper surface of the base. The material of LRM and space
manipulator is 2A14 T6, and the material of compression
rod in the locking mechanism is TC4, as shown in Table 1.
The finite element model is shown in Fig. 3.
The load and constraint of the finite element model is
set. The calculation results show that the load in the X
direction is 1393 N and the load in the Z direction is 1500 N,
respectively, which are shown in Figs. 4 and 5.
From the results of the stress analysis, it can be seen that
the maximum stress of the LRM occurs on the main
bearing compression rod under the equivalent static load,
and it does not reach the ultimate stress of the material,
which means that the locking part can be locked. At the
same time, the stiffness of the LRM in the X and Z
directions can be calculated according to the stress–strain cloud
diagram of the locking mechanism, as shown in Table 2.
The values in the table uses the smaller one between the tension and compression state of X or Z direction.
4.2 Analysis on dynamic response
of the manipulator under locking condition
The space manipulator is fixed by 14 LRMs on the base of the deck, and the finite element mesh of the manipulator under the locked condition using PATRAN is shown in Fig. 6.
The random vibration can check the stability of the
connection between the components of locked manipulator.
Therefore, a random vibration load is applied to the model, which is shown in Table 3.
Analysis also is done by applying sinusoidal vibration
load to the finite element model of the manipulator in X,
Y and Z directions. The maximum resistance force values of the locking points of each locking mechanism are shown in Table 4.
5 System test
5.1 Pre-tightening force control test of lock–
To ensure the correct preload of the lock–release
mechanism, the loading capacity and the loading rigidity of a
single locking mechanism needs to be confirmed.
Therefore, the tightening force control requirement of the LRM
should be within ± 5% [
]. In the early research, the
torque wrench was used to apply constant torque; however,
the loading error was difficult to control in this way.
According to the structural characteristics of the lock–release mechanism, the displacement of the test point is used to determine the preload.
The loading device of LRM based on the above prin
ciple is consist of loading board, loading rack, laser
displacement sensor and others, which is shown in Fig. 7. The
LRM is fixed on the loading board. In the loading process,
the laser displacement sensor detects the displacement of
upper surface of compression rod all along.
To calibrate the magnitude relation between the
displacement amount of the clamping rod/load lever end face
and the actual preloaded force, the main bearing bar is
pasted on the strain gauges to record the data of laser
displacement sensor and the corresponding points of strain
gauge in the loading process. The actual loading
mechanism of LRM is shown in Fig. 8 below. The measuring
Fig. 4 Stress–strain cloud in
Fig. 5 Stress–strain cloud in
Loading surface Z
1.1 9 10-1
1.3 9 107
point of the LRM is the end of the load bar, and the
measurement data points are recorded in Fig. 9.
The functional relationship between the preload stress
and the displacement of the loading rod is given as
r ¼ 4774:4d2 þ 1408d þ 4:2019:
According to the Eq. (1), when the preload stress reached 447 MPa, the displacement of compression rod end is 0.191 mm. The displacement result is close to the finite element calculation, which is 0.185 mm, indicating
that the test did not appear theoretical bias. The error is
mainly due to the workpiece size deviation and test device
error. Repeating the loading test twice with the same
loading process, the dynamic strain gauges show that the
stress values are 458 MPa and 462 MPa, respectively, and
the calculation errors are 2.46 and 3.35%, respectively;
when the loading amount of the compression rod is
0.191 mm, the error is less than the technical indicators
required preloaded error of 5%.
5.2 Static load test of lock–release mechanism
Static load test is that locking the fixed mechanism in the test device and applying loading on a specific surface by drawing press. The tensile/compressive load and load displacement are recorded during the test, and the
pull/pressure curves are obtained. The experiment is shown
in Fig. 10.
The X direction load is given on the flange of the upper base, and the Z direction load is given on the top on the upper base (Fig. 11).
Through the test software, the relationship between the load in X/Z direction and the displacement of the locking mechanism is obtained, and rigidity curve is drawn in Fig. 12.
By fitting the data, the relationship between the load in
the Y and Z directions of the LRM are obtained as follows:
Y : fy ¼ 17; 707d
Z : fz ¼ 72; 046d3
19; 365d2 þ 4621:6d
Fig. 11 Stiffness test of LRM.
a Stiffness test in X/Y b stiffness
test in Z
Fig. 12 Stiffness test curve of
The vibration test is carried out on a 40-ton vibration
table of the mechanical environment test station in
Changchun institute of optics and fine mechanics and physics, Chinese academy of science. The whole test system is shown in Fig. 13.
During the test, the test device is mounted on the
vibration table by bolts. The control sensor is mounted on
the vibration table and the direction is the same as the
vibration direction. The rest of the acceleration sensor is
placed on the shell surface of each joint of the manipulator.
The sensors are attached as shown in Fig. 14 below, and the test is shown in Fig. 15.
Through the given input conditions, the sweep tests in
the three directions are completed. The input of the X, Y,
Z three directions are shown in Table 5. The vibration test
results of the X, Y and Z directions of the robot arm in the
locked state are shown in Figs. 16, 17, and 18. The results
of the vibration test of the acceleration sensor are listed.
The frequency range is within 200 Hz.
From the curve results, it can be seen that the funda
mental frequency in X, Y and Z directions are 84,105 and
87 Hz, respectively, and the fundamental frequency in X,
Y directions are lower than the mechanical analysis of
89.2 Hz. The error is mainly due to the machining and
assembly error of the prototype and the stiffness of the
tooling during the test will affect the overall test results,
resulting in a slightly lower resonant frequency. The data
obtained from the experiment show that the fundamental
frequency of the three directions of the manipulator is close
to that of the finite element analysis, and the correctness of
the analysis result is verified, the LRM meets the design
The sinusoidal vibration experiment is completed by the
given inputs. The sinusoidal vibration stress test results of
the two opposite LRMs are shown in Fig. 19.
The test data show that the stress of each LRMs does not exceed the ultimate stress of the material under given sinusoidal vibration loads.
Fig. 19 Stress test data of two
LRMs of end joint
85 105 125
Base frequency f(Hz)
85 105 125
Base frequency f(Hz)
Base frequency f(Hz)
According to the reliable locking requirement for space
manipulator during the launching course, one design of
lock–release mechanism is presented. To ensure the
locking stiffness for manipulator, upper base is connected to the
lower base reliably by compression rod with enough
preload. The finite element model of lock–release mechanism
is established; the locking stiffness between upper base and
lower base is verified. Multi-point distributed locking
simulation model for space manipulator including 14 lock–
release mechanisms is built; the reaction force for 14 lock–
release mechanisms is obtained under sinusoidal vibration
load. The principle prototype of lock–release mechanism is
developed, and the locking stiffness is tested using tension
machine. The space manipulator is locked by 14 lock–
release mechanisms and the test is carried out, the test
results show that the lock–release mechanism can
effectively ensure the structural safety of the space manipulator
Acknowledgements This work was financially supported by
SelfPlanned Task (NO. SKLRS201614B) of State Key Laboratory of
Robotics and System (HIT), the China Postdoctoral Science
Foundation Funded Project (2015M580268), the Heilongjiang Postdoctoral
Science Foundation Funded Project (LBH-Z15077) and the
Fundamental Research Funds for the Central Universities (Grant No.
Open Access This article is distributed under the terms of the
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