kept close to the body and ROS-compatible software converts the
linear length to angle, and vice versa
for position feedback. The upper arms
use a parallelogram design to allow the
lower arm to remain on a horizontal
plane (Figure 3).
Four inch wide lazy susans on top
and below the compartment for the
linear actuator make it easier for the
servos to pan the arms. The lower
arms are currently similar to AX- 12
CrustCrawler arms with dual grippers.
The base is 9” high — the same as
PR2’s base — and can hold up to five
large 12-volt sealed lead-acid batteries.
The wheel design and software was
derived directly from our original PRLite
prototype. The computer and
electronics are largely contained
throughout the torso.
The Autodesk design was then manually converted into
a URDF model for ROS to do the different transforms,
display, and planning for PR2Lite. The passive cyclic joints
are a problem for ROS’ URDF which requires an acyclic
FIGURE 1. The December 2011
SERVO Magazine with a
picture of the first PRLite
prototype beside Willow
FIGURE 2. The first prototype (left) beside the new
and improved PVC PR2Lite (right).
shoulder, the arm,
and the linear
actuator form a
triangle with two
actuator is along the
raises or lowers the
arm when the
extended/retracted. In our URDF, we artificially break the
cycle by fooling ROS into thinking that the complex
shoulder joints are just another Dynamixel joint.
FIGURE 3. The robot has a telescopic
body, lifting up by one foot. Also
notice the parallelogram shape of
the upper arms (green) which keeps
the elbows (red) horizontal — even
when the upper arms lift up.
New Linear Actuator
and Wheel Controllers
Before leaving for Berkeley, Robert
redesigned PR2Lite’s controllers for the linear
actuators and wheels. His firmware implemented
a protocol stack over RS-485 that broadcasted
datagrams or stream-based connections with
packet collision avoidance and retry capability.
The RS-485 provides much improved electrical
noise immunity compared to the I2C bus in the
The new design also uses MOSFETs to replace
the old relays. The wheel controllers utilize
photogates to act as encoders and provide PID
control. Unavailable at the time, Parallax has now
changed the design to their wheel controllers to
also support this functionality.
Although the microcontrollers on the RS-485
bus can try to avoid packet collisions, it is hard for
the PC with the USB serial port to do so. Frank
has changed the software to give priority to the
SERVO 08.2013 35
FIGURE 4. An output of
rosrun tf view_frames
gives an idea of the scale
of the modeling of
PR2Lite in ROS's URDF
System's Unified Robot