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Q. It is my understanding that
the HSR-8498HB servos that
are used in the RoboNova
humanoid robot have position feedback capabilities, so I bought a couple
of them from Tower Hobbies. I have
been trying for several days now to
figure out how to get position data
from these servos. From what I have
seen on the Internet, all I have to do is
send the servo a 50 μs pulse, and it will
return a position signal that is similar to
the regular pulse width to move the
servo. I am missing something here,
can you help me?
— Pete Senganni
A. Last November, I showed
how position feedback for the
HSR-8498HB servo (see Figure 1)
can be measured by sending a 50
μs pulse and then measuring the
pulse width of the signal that is
returned from the servo on the same
signal line. Since this was on the same
data line, a pullup resistor was needed
on the signal line.
At that time, I was planning
on talking about the HMI Serial
Communication Protocol and how
to interface it with a microcontroller.
I had all sorts of problems trying to
get this to work, which is probably
why there is almost no information
to be found on the Internet on how
to do this. So this month, I will talk
about the problems I had trying to
get this to work, and to get all of the
HSR servos from Hitec (www.hitec
rcd.com) to work with a computer and
Table 1 shows
a list of several
Figure 1. HSR-8498HB demonstrating
two reconfigurable housings.
different specifications for the robotic
servos that are currently available at
Hitec. These servos are programmable
and provide data feedback, such as
their current position, voltage draw,
and current draw. Each servo can be
assigned an ID number and they can
be daisy-changed together; up to 128
different servos can be controlled with
a single data line.
Programming features include
velocity, start/stop, min/max position
limits, center position, dead band,
proportional and derivative gains, and
forward/reverse direction control. All
of these features are available through
the Hitec Multi-protocol Interface
(HMI). The HMI protocol allows the
servos to be controlled with traditional
position control using the 1 to 2 ms
pulse width, position feedback using
the pulse width method, and via serial
data communications which will be
14 SERVO 01.2008
Figure 2. HMI Servo Programmer user interface.