FIGURE 6. Diagram of the
mechanism used to rotate
the wheels.
position. Unfortunately, the relays and the linear actuators
generate a lot of electrical noise. To help reduce this, the
control of the relays from the microcontroller is opto-isolated. A separate DC/DC power circuit is used to provide
power to the relays.
Many different power adapters are needed to power
different components. Originally, we tried to build DC/DC
power sources for each voltage type. Later, we decided to
just use AC power for many of the components via a 200W
DC-to-AC inverter. We can connect the battery to this
inverter and allow many components to run with their
original AC power adapters. The i7 computer runs on its
own 24V DC power supply, which is powered by two 12V
lead acid batteries.
drive is used for further turning
control. The wheel controller will
not simultaneously move the
wheels while pivoting them via the
linear actuator.
The torso is attached to the
base via two 12” drawer slides and
can be moved up and down using
a linear actuator. Attached to the
top of the torso is a Kinect that can
pan and tilt through a pair of
servos and three lazy susans. The
arms are made from a Bioloid
comprehensive kit and are
constructed to be similar to two
Because the AX- 12+ wires are daisychained, one wire
with poor connectivity can make the whole downstream
chain perform poorly or not function at all. The wires in our
Bioloid kit are modified to be customized lengths. Many
needed replacement until we produced reliable arm
performance.
The Base and Torso
We used wood for the base of our initial PR Lite
prototype. Wood is cheap and easy to cut and drill with
hand tools, allowing us to tweak the design continuously.
The base contains a bottom shelf with a tongue that
extends beyond the main cabinet and holds the Neato
LIDAR. A telescoping torso is attached to the front side of
the cabinet.
Inside the main cabinet are three shelves. The bottom
shelf contains the rotating wheels, a linear actuator, and
batteries. The top and middle shelfs hold the computer, all
the microcontrollers, the DC-DC and DC-AC converter, and
the Wi-Fi router. The Parallax wheels are encased in acrylic
structures modeled with CAD software which are each
attached to a 4” lazy susan. The four wheels are all
simultaneously pivoted by a single bar attached to a linear
actuator. The linear actuator has a 4” throw with feedback
and is controlled by a microcontroller. An H-bridge is
formed via relays and transistors. As illustrated in Figure 6,
the wheels are pivoted in place to one of three positions for
the wheels to move forward, pivot in place, or move
sideways. When moving forward or sideways, differential
66 SERVO 12.2011
Conclusion
Our completed PR Lite prototype was demoed at
RoboGames 2011 and featured PR Lite’s arms mimicking a
person’s arms. Many enhancements are planned to make
PR Lite even more like PR2 in both dimension and
capabilities. The main addition will be to add upper arms,
each with pan capabilities (using two lazy susans with
dynamo servos) and powerful lift capabilities (using a linear
actuator). Using our many lessons learned, our next
iteration will be fully sketched out using CAD software and
made from laser-cut ABS at the TechShop. We also want to
harden the prototype against loose wires and bolts by
fabricating some boards, and replacing many of the Bioloid
construction kit parts with simple bent aluminum bars.
Much of the remaining work is software. The robot
torso will be adjusted so it’s similar in shape and
dimensions to PR2, allowing us to more easily reuse code
intended for PR2. We will try to hook PR Lite arm planning
directly into the PR2 arm planning. We’ll revisit the
simulation for the next iteration of PR Lite (PR2 Lite). We
can envision PR Lite eventually being able to perform many
PR2 capabilities. We look forward to doing our own
hackathons! SV
