by Matt Bunting
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Previously, I had discussed some of the various issues
with my current hexapod design. After going through the
current design and past upgrades, some of the breaking
points were explored. Every time I go through an upgrade
process, most parts show some signs of wear and tear.
Some parts break much faster than others. Here, I will
cover two parts from opposite ends of the spectrum. The
RX- 28 took two years of a lot of use before some of the
parts broke down. The head mechanism took a month
before the gear teeth broke.
I will review the new Robotis MX- 28 motor and
compare it to the RX- 28. The newer motor offers some of
the latest features that most servo motors wish they had.
Having heavily used the RX- 28 motors, I’m sure that I will
welcome all the new features.
The gears in the head mechanism were never properly
designed, and therefore did not last very long. Though there
are proper ways to design gears, some of the methods are a
little difficult to understand. The involute shape itself may be
easy to understand, but it’s not necessarily clear as to why
the shape works as well as it does.
The RX- 28 motors have been more than exceptional to
work with, providing great features such as being able to
read the motor load, current position, motor temperature,
input voltage, plus implement emergency shutdown and
configure the control system. These features can be
essential to both professional and hobby roboticists. Before
exploring the control features of the MX- 28, let’s first
compare it with the RX- 28.
This time, we’ll explore and
implement two upgrades for the
hexapod. The first revision involves
transitioning from the Dynamixel
RX- 28 motors to the MX- 28 motors.
Some of the new features of the
The involute curve is explained and
in 3D printed gears.
MX- 28 vs. RX- 28
Each MX- 28 comes in a nice package with plenty of
hardware. Figure 1 shows the contents of each kit. Each
motor kit includes a motor, servo horn with shaft bolt, bolts
for mounting to the servo horn, nuts and bolts to mount to
the body of the servo, and a cable for power and
communication. The kit is very similar to the RX- 28 motor
kits, but there are some notable differences. The white
washer shown in the photo is actually a thrust washer. This
helps to prevent off-axis torque from tweaking the servo
horn too much. The cable included is also only a three-pin,
not four. Lastly, this kit does not include a molex connector
to build your own cable, nor does it come with the mating
connector to solder onto your PCB. The thrust washer is a
nice addition, but I did like how the RX- 28 motors included
the additional cable components.
Figure 2 shows the RX- 28 and MX- 28 side by side. The
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