Introduction to space Robotics :

Robot is a system with a
mechanical body, using computer as its brain. Integrating the sensors
and actuators built into the mechanical body, the motions are
realised with the computer software to execute the desired task.
Robots are more flexible in terms of ability to perform new tasks or
to carry out complex sequence of motion than other categories of
automated manufacturing equipment. Today there is lot of interest in
this field and a separate branch of technology ‘robotics’ has
emerged. It is concerned with all problems of robot design,
development and applications. The technology to substitute or
subsidise the manned activities in space is called space robotics.
Various applications of space robots are the inspection of a
defective satellite, its repair, or the construction of a space
station and supply goods to this station and its retrieval etc.
With the over lap of knowledge of kinematics, dynamics and control
and progress in fundamental technologies it is about to become
possible to design and develop the advanced robotics systems. And
this will throw open the doors to explore and experience the universe
and bring countless changes for the better in the ways we live.
Free Flying Space Robots
Fig. Free Flying Space Robots


The proposed robot is of
articulated type with 6 degrees of freedom (DOF). The reason for 6
DOF system rather than one with lesser number of DOF is that it is
not possible to freeze all the information about possible operations
of the payload/racks in 3D space to exclude some DOF of the robot.
Hence, a versatile robot is preferred, as this will not impose any
constraints on the design of the laboratory payload/racks and provide
flexibility in the operation of the robot. A system with more than
six DOF can be provided redundancies and can be used to overcome
obstacles. However, the complexities in analysis and control for this
configuration become multifold.
The robot consists of two arms
i.e. an upper arm and a lower arm. The upper arm is fixed to the base
and has rotational DOF about pitch and yaw axis. The lower arm is
connected to the upper arm by a rotary joint about the pitch axis.
These 3 DOF enable positioning of the end effector at any required
point in the work space. A three-roll wrist mechanism at the end of
the lower arm is used to orient the end effector about any axis. An
end effector connected to the wrist performs the required functions
of the hand. Motors through a drive circuit drive the joint of the
arm and wrist. Angular encoders at each joint control the motion
about each axis. The end effector is driven by a motor and a pressure
sensor/strain gauges on the fingers are used to control the grasping
force on the job.
main subsystems in the development of the manipulator arm are
  • Joints

  • Arm
  • Wrist
  • Gripper


A joint
permits relative motion between two links of a robot. Two types of
joints are

  1. Roll
    joint –

    rotational axis is identical with the axis of the fully extended
  2. Pitch

    – rotational axis is perpendicular to the axis of the extended arm
    and hence rotation angle is limited.

main requirements for the joints are to have near zero backlash, high
stiffness and low friction. In view of the limitations on the volume
to be occupied by the arm within the workspace, the joints are to be
highly compact and hence they are integrated to the arm structure. To
ensure a high stiffness of the joint the actuator, reduction gear
unit and angular encoders are integrated into the joint.

Each joint consists of

  • Pancake type DC torque motors
    (rare earth magnet type) which have advantage over other types of
    motors with respect to size, weight, response time and high torque
    to inertia ratio.
  • Harmonic gear drive used for
    torque amplification/speed reduction. These gear drives have near
    zero backlash, can obtain high gear ratios in one stage only and
    have high efficiency.
  • Electromagnetically actuated
    friction brakes, which prevent unintentional movements to the arms.
    This is specifically required when the gear drive is not
    self-locking. In space environment, where the gravity loads are
    absent (zero ‘g’ environment) brakes will help to improve the
    stability of the joint actuator control system. i.e. the brake can
    be applied as soon as the joint velocity is less than the threshold
  • Electro optical angular
    encoders at each axis to sense the position of the end of the arm.
    Space qualified lubricants like molybdenum disulphide (bonded
    film/sputtered), lead, gold etc. will be used for the gear drives
    and for the ball bearings.


The simplest arm is the pick
and place type. These may be used to assemble parts or fit them into
clamp or fixture. This is possible due to high accuracy attainable in
robot arm. It is possible to hold the part securely after picking up
and in such a way that the position and the orientation remains
accurately known with respect to the arm. Robot arms can manipulate
objects having complicated shapes and fragile in nature.


Robot arm comprises of grippers
and wrist. Wrist is attached to the robot arm and has three DOF
(pitch, yaw, and roll). Wrist possesses the ability to deform in
response to the forces and the torques and return to equilibrium
position after the deflecting forces are removed.
Gripper is attached to the
wrist of the manipulator to accomplish the desired task. Its design
depends on the shape and size of the part to be held.
In the future, robotics will
make it possible for billions of people to have lives of leisure
instead of the current preoccupation with material needs. There are
hundreds of millions who are now fascinated by space but do not have
the means to explore it. For them space robotics will throw open the
door to explore and experience the universe.