Detailed Design
STRUCTURE SELECTION
Since
the project was so open-ended, we decided that the feedback from our
sponsor should be our determining factor. We held a conference with
him, and discussed our observations with each model.
For
the Flemons’ model, we thought it looked very interesting and was
the best visual portrayal of tensegrity. It was also the original
recommendation of our stakeholder. However, after constructing the
prototype and experimenting with the cord tension for motion, due to
the abstract shape, it wanted to roll over on the side being
tightened. This was due to the decreased length at the spot and no
support at the front or rear. If more links were used, this may have
solved the issue, but that would become expensive and still wouldn’t
be a sure solution. An alternative solution would be to add on an
additional leg to the front and rear vertebra for stability; this
idea was dismissed fairly quick, too.
For
Danny’s model, it didn’t appear very tensegrity at first. Upon
explaining the actuation of it, however, the sponsor was able to see
how it used the principles of tensegrity. Our sponsor also pointed
out that this model would be very collapsible. Since the idea for
this project is a proof of concept, this would be very beneficial in
validating space exploration capabilities.
The
3 spindle model was discussed as well, but the use of springs
wouldn’t really fit with the tensegrity idea and this concept was
with no surprise quickly dismissed.
Another
structural issue we wanted to discuss was the placement of actuators.
The project description specifies no base plate and the use of
in-line actuation. We asked if that meant the actuators needed to be
suspended on the line itself, or if the actuators could be attached
to the vertebrae frame. He clarified attaching to the frame was
okay.
Ultimately,
he recommended we run with Danny’s model. He made up his mind
fairly quickly and we felt it would be the best to specialize in,
too. In addition to being the best liked approach, there was an
added bonus: by placing a fourth actuator at the tip of the
tetrahedron, it could tighten or loosen all three of the inner cords
evenly which would allow an accordion/inchworm movement as well as
help distribute the force as needed.
CURRENT STRUCTURE DESIGN
Upon
choosing the form of the structure, some requirements were made to
make sure it would work:
-
The frame would need to be
easy to fix of replace a portion.
If something breaks, we want to be able to fix it quickly without
having to re-machine the whole frame.
-
The actuators need to be
attached to the frame and be in-line with the cords.
As all of the components for the actuators are still being
developed and tested, the machining cannot be held up to long; thus,
there needs to be space for actuators and an easy way to attach
them.
-
All cords need to be the
same length when at the same tension.
Even using force actuation, the cords must be the same length
structurally and have the same level of pretension. This would also
allow easy future adjustability.
-
We cannot route the cords.
That was an issue from last year that the sponsor would like to
avoid this year. He wants to be able to see clearly what each wire
does.
Current Base Ideas
To
make attachments easy, it would be nice to have at least one flat
surface for each vertebra. The most suitable place for this was the
base, which is the connection of the legs opposite of the tip of the
tetrahedron. The concept is shown below.
Figure
4: The
base of a vertebra.
As
shown in Figure 4 above, there should be plenty of room for three
actuators. They will be placed on the back side of the base. This
is to help add balance to each vertebra as well as to prevent the
components from being in the way of actuation. Note, this will limit
the range it can collapse inside itself, but currently the range of
collapsing is limited by the size of the actuators in the nose cones,
as they cannot be allowed to collide. So this would be a way to
take advantage of that empty space. Still it must remain small for
slithering type motion as that will not be restricted by the nose
cone actuator size.
In
addition, there would be room to accommodate for controllers and mini
breadboards for circuitry if needed for future controlling.
Basically, it should prove fairly versatile. The spindle holes could
either be placed at the outer edge of the triangle tips or inner edge
of the triangle tips.
Leg and Tip
Attachments
There
were several ideas of how best to attach the spindles and tip of the
tetrahedron. We thought it best to make the tip its own separate
part and the spindles separate as well, as this would allow easy
replacement if needed.
For
the tip, there were two main ideas. One, make it a simple ball
shape; two, modify the ball shape to accommodate for an actuator to
be placed in the center. The later concepts are illustrated below to
better visualize.
Figure
5:
The
modified nose cone. This is the back view to see the actuator
placement.
And
as seen in the figure above, the modified shape would allow for an
actuator to be slid through the center. Basically, the tip is
shifted towards the base more to maintain the same angle for the
spindles. In addition, it should have a reduced weight compared to
the ball. However, the ball shape would be easy to manufacture. It
could be clamped down and simply move the angle and position of the
drill bit to drill the holes and tap as needed.
For
the legs, we thought it easiest to simply use cylindrical rods. As
far as attaching those to the base and tip there were again a few
ideas. One, thread the holes; two, use a pin to hold the position;
three, use a bolt. All three concepts are shown below.
(Include
all three ideas)
Figure
X: a.) Using
threads, b.) using pins, and c.) using bolts.
As
seen in Figure X a. above, threads would give stability and be easy
to machine. But due to the geometry, it would be unnecessarily
difficult to make threading both ends worth it. It would require
(whatever the two-way threading part is called) and even then it
would be difficult to get all three spindles to tighten at the same
lengths.
Pins
would work for either end, but may not give as much stability. To
make the pins work, a hole could be drilled through the base at the
correct angle to ensure consistency and side holes could then be
drilled through for the pin to be inserted. The pin configuration
would also be easy to machine. However, the use of pins may omit
certain placements of the actuator since they might physically be in
the way of each other, whereas threaded would not require any
external space.
For
the bolt shown in Figure X c. above, it would give great support and
be able to mount to any flat or rounded surface; the end of the leg
would just need to be flattened or rounded accordingly. However,
this spindle design might be difficult to machine. It may also run
into threading issues or space issues based on how the bolt is
secured down.
Ultimately
we thought it best to use the modified nose cone to enable the use of
an actuator there. Since threading is easy to machine and stable, we
also decided to thread the end of the leg that would insert into the
nose cone. For the other end of the leg, we will angle a hole
through the base frame as noted with the pin approach, but we are
hoping that no pin will be needed. By inserting the bars through
first, then threading them into the tip, the rigidity and angle of
the bar should prevent any further movement between the base and
legs.
Attaching Cords and
Length Adjustability
This
applies to the cords running from the base of one vertebra to base of
the next, as well as the cords running from the base to tip. There
were two main approaches to accomplish this. One, run the cord
through a peg/dowel and then wedge it in a hole in the base; two, use
the same idea as a guitar tuner. Both concepts are illustrated
below.
(Include
a picture of both cord attachment ideas)
Figure
X: a.) Using a
dowel setup, or b.) using a guitar tuner setup to attach the cord.
As
seen in the above Figure X, the dowel setup in a., it would be
efficient; however, it would be difficult to machine, and
press-fitting would also be problematic to adjust the cord length.
As for the guitar tuner concept, it would be easy to buy the part,
then simple enough to drill and tap a hole for it in the base. The
cord could be wrapped about the tuner, making a sure connection, and
it would allow easy cord length adjustment.
We
decided to run with the guitar tuner concept. It seemed straight
forward.
Actuator Attachment and Placement
To
actually attach the actuator to the base we had two main ideas. One,
a horseshoe clamp; or two, a clamp attachment similar to a scope
mount. Both can be seen in the figure below.
(Include
both actuator attachment ideas)
Figure
X: a.) Using
the “…”, and b.) using the clamp attachment to attach the
actuator.
The
horseshoe clamp seen above would probably be the simplest approach.
We already have a flat surface to attach it to and now that we have
our actuator dimensions, we could machine it really quickly.
However, the added bonus of the scope-type mount is that it could
accommodate any changes in actuator dimensions. Though it hopefully
shouldn’t be necessary to change actuators, but would just allow
adaptability.
As
for the actual placement of the actuators, the figure below
demonstrates where they will be located.

Figure
6: The
placement of actuators on the vertebrae structure. The smaller
actuators are shown in red, while the larger actuators in the nose
cones are shown in blue.
The
spindle of an actuator will be placed at one of the numbered
positions shown above. For example, an actuator would be mounted on
the back of the frame at position 4 and would have its cord attach to
the front of location 1. The actuator at the back of location 1
would then run to the front of the vertebra behind it, and so on.
This same concept is the same for all actuators 1-6. As for the
actuators in the nose cone positions 7 and 8, actuator 7 would
connect to locations 1, 2 and 3 with a three separate cords; while
actuator 8 would similarly connect to locations 4, 5 and 6, again
with three separate cords.
To
actuate, actuator 7 would unwind while actuators 1, 2 and 3 would all
tighten, thus moving nose cone 7 inside of the vertebra in front of
it. Then by using one way friction, when actuator 7 tightens back up
and actuators 1, 2 and 3 loosen, the vertebra it is in would be
pushed away and forward from it. This would simulate an inchworm or
accordion-like movement.
To
achieve slithering or snake-like motion, actuators 7 and 8 could be
held constant. Then by tightening actuators 2 and 5 and similarly
loosening actuators 3 and 6 on the other side, the structure would
want to turn to the side. By alternating this motion, and again with
the use of one-way friction, the structure would slither forward.
Its speed and magnitude will be driven by the original length of cord
and power of the motor.
FUTURE STRUCTURE WORK
There
is still room for additional equipment such as the use of load cells
or optical encoders. But we feel that it would be good to get a good
head start on machining and assembling the prototype so that we can
test our actuators and other equipment. This tends to be the best
way to test out our ideas and make sure we are on the right track, as
well as more readily notice any drawbacks we may encounter.
So
our next steps are:
- Machine
- Assemble
-
Attach
actuators and equipment to begin testing
ACTUATOR SELECTION
After discussing the possibilities with our sponsor, we immediately ruled out the linear
actuator since its performance was much worse than the other two. We continued to consider
the air muscles for a while since our sponsor liked them so much, but he did think that they
might be somewhat impractical. In the end,
we
decided to not use the air muscles because an air
compressor would be too big to make the robot practical, it would need to be transported to
NASA Ames, and the air muscles would not be feasible in space. That left the rotational motors,
which seemed to have some of the strongest advantages. Also, our sponsor mentioned that we
could mount them on the structure rather than hang them in line with the force. The only
condition was that they exert force in the direction of motion only. This made them even more
feasible.
We came up with two types of rotational motors that we could use: servo motors and DC
motors. The advantages of the servo motors were that they had readily available code and that
there were a lot of models available to choose from. The disadvantages were that while we
needed a force based control system for the robot, the force based controls for the servo that we
tested were very inaccurate and it would not be very feasible to use the position controls to
control output force. These disadvantages led us to use the DC motor instead.
The DC motors advantage was that the input current is roughly proportional to the torque
output. This means that we could more directly control the torque using a current controller. In
order to implement this we will need to build and test our own controllers. This is where the
actuation
of the tensegrity robot now stands.
Actuation
selection was modeled in appendix A.
CURRENT ACTUATOR DESIGN
Our
current design of the actuator system is to use DC motors that are
directly connected to the frame. There will be four motors
mounted on each section of the structure with three of the motors
holding the structure in tension with the other motor. The
motors will use spindles to wind and unwind the strings. Motion
will be achieved through changing the force that each of the two
groups exert on each other in a synchronized manner. Each of
the motors will be controlled by a single current controller and it
is hoped that a single microcontroller will be able to control
several of those. These will need to be mounted to the frame.
The power source will need to be connected to the robot by a
tether.
FUTURE ACTUATOR WORK
Now
that the electric motors have been purchased, future actuator work
will be in refining the method of forced based control. At the half
way point in the semester there are two different ideas for
controlling the force that each motor applies.
These
two types of force control do not alter the structure of the
tensegrity and can be investigated as parallel sub-projects without
inhibiting progress on the rest of the robot.
CONTROL SELECTION
With
the parameters of what we are supposed to have for our design, we are
thinking that both load cells and the current controller are going to
have to be incorporated. The parameters of the project says that the
robot has to be forced based and the force has to be displayed at all
times. This force based is going to come from the current controller
and the displayed force is going to come from the load cells that
would be attached to all the tendons in the design. With these two
control methods we are covering all parameters.
CURRENT CONTROL DESIGN
Figure
8:
Shows the current controller circuit board and how it is designed.
Figure
8 shows the way the current controller is set up. This design will be
able to control the torque based on the fact that current and torque
are proportional. We will control the current by calculating the
current over a known resistor and voltage. Then a map of the current
to the torque output of the robot will be made.
FUTURE CONTROL WORK
The
first few weeks of the second semester of this project will be used
to finish up the testing of both types of control. At the end of the
first semester the best option appears to be the combination of both
types of data acquisition. For the force control a current controller
is the most efficient method. However, if load cells are also
included then the tension can be accurately measured for every line
rather than the torque applied at each motor.
Both
circuits will be built by the team then ordered from a company that
will give more compact and reliable circuits. Extra circuits will be
purchased to insure that the robot can run for many years, as well as
clearly labeled circuit diagrams that convey how each one is built.
MOTION SELECTION
We
have decided that the most effective and easily incorporated one way
friction would be a cross country or telemarked climbing ski skin. If
it becomes necessary for us to have one way friction we will
incorporate these climbing skins.
CURRENT MOTION DESIGN
We
have not modeled anything yet, but we feel the ski skins will be the
best design for our project. Once we have our structure machined, we
will attach the ski skins to the bottom of the base frame. We can
adjust the foot print of the base frame to find the correct amount of
friction from the ski skins.
FUTURE MOTION WORK
If
it becomes apparent that we will need one way friction in our robot,
we will continue with one way friction design. The first step in our
one way friction design will be to set up an experiment to determine
both the static and kinetic friction coefficients in both directions
of the ski skin. This will be accomplished with a couple of very
simple experiments. The first experiment will be to find the static
friction coefficient this will be done with a tilt table and a sample
of our ski skin under a weight. The coefficient of static friction
will be found by the tangent of the angle at which the block starts
to slide. The coefficient of kinetic friction will be found by using
an Atwoods machine. An Atwoods machine consists of a block or a sled
with the surface on which you are testing the friction of; a
frictionless pulley, a level table, and a hanging weight. For the
Atwoods machine to work, photo gates will be setup a set distance
apart to calculate the velocity of the block on its path past the
photo gates. The full set up and experiment details can be found in
“Experiments in laboratory Physics” by Yanko Kranov Experiment 5.
Once the coefficients of friction are found, we can input them into
our math model so we can map the position of our robot to the force
that we apply in each of our strings.