Definition and Motivation A Mars rover is an automated motor
vehicle which propels itself across the surface of the planet Mars
after landing, its a NASA project specified to do a number of
missions. The four goals of NASA's long-term Mars Exploration
Program are: 1. Determine whether life ever arose on Mars. 2.
Characterize the climate of Mars. 3. Characterize the geology of
Mars. 4. Prepare for human exploration.
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Key Facts Missions to Mars started in early 1960s. No human has
ever stepped on Mars yet. Such trip would be a one way trip. The
plan is to send a human in the next decade. When the Curiosity
rover touches the surface of Mars in August 2012, it will be the
fourth rover to make tracks across the Red Planet in fifteen years
after Sojourner 1997, and the rover twin Spirit and Opportunity
2003. 255 400 24 40 10 Average distance between Earth and Mars is
255 million km, it reached 56 million in 2003, and the farthest it
can reach is 400 million km. The Mars day is 24 hours and 40
minutes. The cruise from earth to mars takes about 10 months. MERs
(Mars Exploration Rovers) Spirit & Opportunity were launched in
2003, each was on a different part of the plant as to enable the
NASA team on Earth to work 24/7, or actually 24:40/7.
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Curiosity is a rover that was supposed to be launched in 2009.
It was launched on the 26 th of November, 2011 with a total cost of
2.5 billion. The name Curiosity was chosen through a naming
contest. The rover weighs almost 862kg. Adding to it the weight of
the spacecraft it rides in, and its landing system all together
weigh 3400kg. Curiosity is twice the weight of Spirit and
Opportunity together. The testing was designed to put the rover
through operational sequences in environmental conditions similar
to what it will experience on the surface of Mars. The testing
temperature approaches -130 C. Testing is done inside a lab and
outside.
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Specifications and Construction Dimensions: The rover Curiosity
is 3.0 m in length. Speed: Maximum speed is estimated to be 2.5
cm/s, however, average speeds will likely be 0.8 cm/s based on
variables including power levels, terrain difficulty, slippage, and
visibility. It is expected to traverse a minimum of 19 km in its
two-year mission. Processor: There are two identical on-board rover
processors.
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X-band UHF Communications: Curiosity has two means of
communication: an X-band transmitter and receiver that can
communicate directly with Earth, and a UHF band for communicating
with Mars orbiters. Communication with orbiters is expected to be
the main contributor to data return to Earth, since the orbiters
have both more power and larger antennas than the rover. Data
includes photos, system-status information, etc. Also, the
communication system enables scientists on earth to send data to
the rovers such as commands and software updates to the rover. At
landing time, 13 minutes, 46 seconds will be required for signals
to travel between Earth and Mars. Collected data need to be sent
back to Earth. There is a 20-minute round-trip delay because of the
far distance between Earth and Mars. The rover transmits at only 12
kilobits per second. The direct link to earth is only available for
about three hours per day because of the alignment of the planets
and the power requirements of the radio.
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Mobility systems: Curiosity is equipped with 6 wheels in a
rocker-bogie suspension system. Curiosity's wheels are
significantly larger than those used on previous rovers. Cameras:
There are different types of cameras embedded on the rover that has
different usages. Some are for taking photos of the landscape,
other are specified as a sensing instruments, another acquire
microscopic images of rock and soil. Robot Arm: The Robot Arm is
responsible of digging inside the rocks and soil of Mars, taking
samples and pictures, and putting the samples in the right place on
the rover so as to be able to send their pictures back to
earth.
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Energy System solar system Spirit and Opportunity (MERs) relies
completely on the solar system. This made night missions impossible
and day missions constricted by power. radioisotope thermoelectric
generator Curiosity is powered by a radioisotope thermoelectric
generator (RTG). Radioisotope power systems (RPSs) are generators
that produce electricity from the natural radioactive decay of
plutonium-238, which has the lowest shielding requirements and the
longest half-life. Heat given off by the natural decay of this
isotope is converted into electricity by the Seebeck effect,
providing constant power during all seasons and through the day and
night, and waste heat can be used via pipes to warm systems since
the rover will work in temperatures varying from -127 C to +30 C.
Curiosity contains 4.8 kg of plutonium-238 dioxide.
Slide 9
Robot Arm A robotic arm is a mechanical arm, with similar
functions to a human arm. The links of such a manipulator are
connected by joints allowing either rotational motion or
translational (linear) displacement. The robotic arm is designed to
resemble the human arm that is able to grip, pick and place various
objects. A nearly six-foot-long, five-jointed arm for the Mars
Science Laboratory is the most sophisticated robotic instrument
positioning system yet designed for a space science mission.
Slide 10
The arm is a 5 joint robotic manipulator consisting of: 1.
Shoulder, elbow, wrist. 2. An axis at the base. 3. Fingers. The
whole robotic arm along with the rotating base is placed on a rover
which exhibits forward, backward, left and right movements. The
robot arm has four fingers: 1. Microscope. 2. Two fingers are
spectrometer to tell us in details what the rocks are made of, it
is responsible for elemental and iron- mineral identification. 3.
The RAT: the Rock Abrasion Tool which can grind into Mars rocks.
The RAT uses a diamond-tipped robotic grinding tool to scrape away
this weathered exterior, revealing a fresh surface.
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Slide 12
Base motor is mounted vertically on a horizontal plane.
Shoulder, elbow, tool pitch motors are mounted horizontally
relative to the base. The tool roll and grip motors are mounted at
the wrist joint.
Slide 13
Defining Parameters 1.Position and Orientation position vector
Once a coordinate system is established, we can locate any point in
the universe with a 3x1 position vector. The reference of the
position vector is the coordinate system of the robot arm base.
position position orientation Two axis are required to reach any
position in a plane; three axis are required to reach any position
in space. To fully control the orientation of the end of the arm
(i.e. the wrist) three more axis are required.
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orientation It is necessary not only to represent the position
of a point in space, but also to describe the orientation of the
arm in space. Assuming that the manipulator has a sufficient number
of joints, the arm can be oriented arbitrarily while keeping the
fingertips at the same position in space. In order to describe the
orientation of the arm, a coordinate system is attached to the
fingertips relative to the reference system.
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2.Carrying capacity or payload: 2.Carrying capacity or payload:
how much weight a robot can lift. 3. Speed 3. Speed: how fast the
robot can position the end of its arm. 4. Acceleration 4.
Acceleration: how quickly a link can accelerate. 5.Accuracy:
5.Accuracy: how closely a robot can reach a commanded position.
When the absolute position of the robot is measured and compared to
the commanded position the error is a measure of accuracy.
6.Degrees of freedom 7.Working Space
Slide 16
Robot Arms Design Degrees of Freedom (DOF) The degrees of
freedom, or DOF, is the number of joints on the arm, a place where
it can bend or rotate or translate. When building a robot arm you
want the less number of degrees of freedom allowed for your
application because each degree requires a motor, often an encoder,
and exponentially complicated algorithms and cost. If the degrees
of freedom are increased, the workspace becomes bigger.
Slide 17
The Workspace There are only two motions a joint could make:
translate and rotate. Each joint(DOF) has its limitations. Not all
joints can swivel 360 degrees. For example, no human joint can
rotate more than about 200 degrees. robot workspace The robot
workspace is all places that the end effecter(gripper, grinder,
etc) can reach. The workspace is highly dependent on the robot
configuration. The workspace is dependent on: 1. DOF
angle/translation limitations 2. Arm links lengths 3. The angle at
which something must be picked up at, etc.
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Fields of Study 1.Forward Kinematics Forward kinematics is the
method for determining the position of the end effecter, given the
joint angles and link lengths of the robot arm. In the following
example, the end effecter location with given joint angles and link
lengths will be calculated. For simplicity, a
three-degrees-of-freedom robot arm is considered. Assume that the
base is located at x=0 and y=0. The first step would be to locate x
and y of all the joints. Joint 1: x0=0; y0=L0.
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For x1 and y1: cos() = x1/L1 => x1 = L1*cos() sin() = y1/L1
=> y1 = L1*sin() Joint 2: sin() = x2/L2 => x2 = L2*sin()
cos() =- y2/L2 => y2 = -L2*cos() End effecter location: x0 + x1
+ x2, or 0 + L1*cos() + L2*sin() y0 + y1 + y2, or L0 + L1*sin() + -
L2*cos() The z component is dependent on alpha.
Slide 20
2.Inverse Kinematics Inverse kinematics is the opposite of
forward kinematics. This is when you have a desired end effecter
position, but need to know the joint angles required to achieve it.
If the robot arm wants to go to a specific point in space, what
angles should each joint go to? Derivation of the required
equations is quite complex, but are given by:
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What makes inverse kinematics so hard? Well, other than the
fact that it involves non-linear simultaneous equations, there are
other reasons too: 1. There is the very likely possibility of
multiple, sometimes infinite, number of solutions (as shown below).
2. The difficulty for the arm to choose which is optimal, based on
torques, previous arm position, gripping angle, etc. 3. There is
the possibility of zero solutions. Maybe the location is outside
the workspace.
Slide 22
3. Inverse Force Kinematics: After solving the inverse position
kinematics problem for the current end effecters position, the
inverse force kinematics then solves the following problem: "Given
the final position of the end effecter, what is the corresponding
joints forces and torques? The following relations derives the
torque required for maintaining the arm at a specific position. The
torque (T) is calculated using the following relation: The force
(F) acts at a length (L) from a pivot point. The force acting on an
object (causing it to fall) is the acceleration due to gravity g
=9.81 m/s 2 multiplied by its mass:
Slide 23
The force above is also considered the object's weight (W): The
torque required to hold a mass at a given distance from a pivot is
therefore: perpendicular This can be found similarly by doing a
torque balance about a point. Note that the length L is the
perpendicular length from the pivot to the force. Therefore,
replacing F with m*g, we find the same equation above.
Slide 24
In order to estimate the torque required at each joint, we must
choose the worst case scenario. In the figure, a link of length L
is rotated clockwise. Only the perpendicular component of length
between the pivot and the force is taken into account. We observe
that this distance decreases from L3 to L1 (L1 being zero). Since
the equation for torque is length (or distance) multiplied by the
force, the greatest value will be obtained using L3, since F does
not change.
Slide 25
It can be safe to assume that the actuators in the arm will be
subjected to the highest torque when the arm is stretched
horizontally. Although the robot arm may never be designed to
encounter this scenario, it should not fail under its own weight if
stretched horizontally without a load. The weight of the object
(the "load") being held (A1 in the figure next slide), multiplied
by the distance between its center of mass and the pivot gives the
torque required at the pivot. The equation takes into consideration
that the links may have a significant weight (W1, W2..) and assumes
its center of mass is located at roughly the center of its length.
The torques caused by these different masses must be added:
Slide 26
You may note that the actuator weight A2 as shown in the figure
below is not included when calculating the torque at that point.
This is because the length between its center of mass and the pivot
point is zero. Similarly, when calculating the torque required by
the actuator A3, its own mass is not considered. The torque
required at the second joint must be re- calculated with new
lengths, as shown below:
Slide 27
What is Rocker Bogie? The Rocker-Bogie system is the suspension
arrangement used in the Mars rovers. This mechanism enables a
six-wheeled vehicle to keep all six wheels in contact with a
surface even when driving on severely uneven terrains. road
holding/handling braking Suspension is the term given to the system
of springs, shock absorbers and linkages that connects a vehicle to
its wheels. Suspension systems serve a dual purpose: contributing
to the car's road holding/handling and braking. Generally,
exploration robots are driven on the rough surface which consists
of different sized stones and soft sand. For this reason, car
suspensions are not applicable for rovers.
Slide 28
Slide 29
Why Rocker Bogie in Rovers? The most important factor when
creating a suspension system for a rover is how to prevent it from
suddenly and dramatically changing positions while cruising over
rocky terrain. This might cause the flipping of the rover and it
will mean the end of the mission. The Rocker-Bogie design has no
axles or springs, and allows the rover to climb over obstacles,
such as rocks, that are up to twice the wheel's diameter in size
while keeping all six wheels on the ground. However, the rocker
bogie system in the rovers is very slow.
Slide 30
Number of Wheels Four-wheel drive could have been used, but
they do not climb obstacles very well, especially obstacles that
are larger than the diameter of the wheels. Six-wheel drive
provides enough flexibility to climb over obstacles. Thus six-wheel
system is used. Eight-wheel drive would provide better climbing and
vehicle control, however, more unnecessary wheels would mean more
weight. To transfer 1 pound to mars you would need 1 extra million
dollars, so weight is essential.
Slide 31
Each of the rover's six wheels has an independent motor. Each
wheel also has cleats, providing grip for climbing in soft sand and
scrambling over rocks. Rovers wheels have grown bigger starting
with Sojourner and ending with Curiosity.
Slide 32
Rocker Bogie Parts
Slide 33
How the Rocker Bogie Works Front Wheel: To go over an obstacle
the front wheels are forced against the obstacle by the rear
wheels. The rotation of the front wheel then lifts the front of the
vehicle up and over the obstacle. The middle wheel is the pressed
against the obstacle by the rear wheel and pulled against the
obstacle by the front, until it is lifted up and over. Finally, the
rear wheel is pulled over the obstacle by the front two wheels.
During each wheels traversal of the obstacle, forward progress of
the vehicle is slowed.
Slide 34
Rear Wheels: When a wheel climbs up a step(say the rear wheel),
the front and middle wheel are being pushed backwards and this
would result in in wheel slippage. This problem can be improved by
lowering the bogie pivot below the front wheel axis.
Slide 35
Loureen Qussouss Done By: Loureen Qussouss Esraa Sada Esraa
Sada Rami Abu Slayyeh Rami Abu Slayyeh Supervisor: Prof. Mohammad
Zaki-Khadar