Quadruped Robot
Carly Beardall, Matt Johnson, Bryson Murray, Zach Walker, Eddie Yazzie
Advisor Dr. Sanford Meek
Control System
Introduction
Robots have many modern-day applications, ranging from industrial to
domestic. Quadruped robots, specifically, have many potential uses, such as
search and rescue, long distance travel, and exploration of inhospitable
environments. However, our team’s quadruped robot will be used for research
in animal dynamics.
Challenges
Controller
Simulink software and a dSPACE controller board provide the quadruped with velocity
control. Through the use of Control Desk software, variables from the Simulink block
diagram can actively be adjusted to change various aspects of the quadruped’s motion.
Transfer Function
An Inherited Project
Past teams designed and manufactured the structure and
mechanical components of the robot, although, at the time our
team took on the project, the quadruped’s movements had only
been simulated.
Model Transfer Function
The integration of a control system for the existing robot, which was designed and fabricated
by previous teams, proved to be more difficult than was originally anticipated. Redesigns of
mechanical components were necessary in order to accommodate electrical components
such as optical encoders, servo motors, and all of the required wiring. Modifications to the
following components were necessary for the success of the project:
Directionally compliant, passively
stable, under-actuated leg design
Transfer Function Curve Fit
What the Robot Will Be Used For
A skeleton of a tiger, an example of a quadruped animal
Determination of System Gains
• Root Locus Techniques
• Simulink and Sisotool
Root Locus Plot
(http://www.joelmongeon.com/Rigging.html)
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Motor mount plate – to accommodate encoders
Servo guide plates – to route Spectra cables to the feet
Interchangeable feet – requested by customer
Hip joint datums – to keep legs parallel
Mounting screws – for the servo cables
Upper thigh cantilever springs – to lower stiffness
Hip drive shafts – machined to fit D-collars
Drive motor keyways – made longer for better grip
Keyless bushings – for precise leg alignment
Hip Joints – redesigned to withstand torque
Hip Joint Redesign
There were flaws in the original design that had to be re-engineered in order to get the robot
to trot. For example, the original design of the hip joints used a friction joint to couple the
legs to the drive system. These joints slipped under the torque required for motion and had to
be redesigned. The new design uses a D-shaft and D-collar to couple the shaft to the leg using
a mounting screw that fastens the D-collar to the side of the leg.
Project Description
The main objective of this project is to design and
implement a control system to enable the existing
quadruped robot to trot. Trotting is defined as the
synchronized movement of alternating diagonal
pairs of legs in four-legged animals.
Conclusions
In order to achieve this objective, our team
focused on two designs:
• Control System – The control system
includes multi-faceted code, a Simulink
model, and a power system which work
together to control the two encoders, two
drive motors, and four servo motors. This
system not only synchronizes the legs to
produce a trotting motion, but also lifts the
feet so the robot will not trip over itself.
The control system for the quadruped uses a controller made by dSPACE. While this controller
is sufficient, it came with a steep learning curve. dSPACE uses its own dedicated program,
Control Desk, to interface a Simulink model with the controller. The Control Desk program was
not intuitive, therefore a lot of time was spent learning the program. In addition to this, our
team had to learn to use a Roboteq amplifier that can also be used as a controller on its own.
There were many options and settings available we did not need and had to work around. The
combination of learning dSPACE, Control Desk, Simulink, and the amplifier all at once resulted
in a significant challenge.
Unanticipated Redesigns and Modifications
The legs of the quadruped were designed to be directionally
compliant, so the force on each leg is directed to the center of
the robot’s body. The legs are also intended to be passively
stable and under-actuated in order to minimize the amount of
mechanical components on the robot.
Studies have shown that three separate
evolutionary paths led to a “knees-in”
leg configuration, which the vast
majority of quadruped animals now
exhibit. Our robot will be used for
research in quadruped animal dynamics.
Control System Complications
Simulink model of control system
Testing Platform
Original design from past teams
Testing Structure
The structure suspends the quadruped
from linear bearings, allowing the robot to
trot freely, while preventing it from falling
and incurring damage. The structure also
allows the quadruped to pitch up or down,
providing a more realistic trotting gait.
• Testing Platform – The testing platform
includes a structure to support and allow
the robot to trot freely, but safely, as well
as a treadmill with active velocity control
to allow the robot to stay in place.
The main goal was to engineer a control system that enabled the quadruped to perform its
intended function, which is to trot. The necessary sensors and component modifications
were designed, and the hardware was installed. The electrical components were configured
and connected such that the necessary inputs and feedback from the robot would be
possible. A complex Simulink model was designed to command all of the necessary motions
of the robot using feedback from the installed sensors. In addition, a testing platform that
protects the robot and uses an actively controlled treadmill to aid in testing was designed
and manufactured. There were many challenges along the way, but overall the project was
successful.
Acknowledgements
Treadmill
3-D model of current design
A treadmill with active velocity control,
via an Arduino microcontroller, maintains
the robot’s position. This allows continuous
testing and observation of the robot’s motion.
Model of testing structure and robot
Sponsored in part by the
National Robotics Initiative 1427422