Very Large Servos
FIGURE 11. Hitec HS-645MG
high torque servo.
test arm rod that I was using to
measure the torque with the lab
balance weights. The Hitec HS-
M7990TH monster torque servo shown
in Figure 15 weighs only 2.78 oz with
an aluminum heatsink case, but has a
massive 500 oz-in of torque at 6.0
volts. With titanium gears and dual ball
bearings, a magnetic rotary encoder,
and programming capability, this servo
is perfect for large robots — either for
leg or upper arm actuation. It exceeded
my torque measurement capability and
did not even slow down when grasped
by my fingers.
ServoCity makes what is called a
servo power gearbox shown in Figure
16. This particular setup used a Hitec
HS-785HB sailboat winch servo with 3-
1/2 revolution capability to drive up to
a 7:1 gear ratio to obtain 1,281 oz-in
of torque and a full 180º of rotation.
The model that I have tested has a 3:1
ratio and a 420º rotation for 549 oz-in
of torque. For $120, you cannot find a
servo anywhere with these capabilities,
nor machine the brackets with the
large ball bearings to take large loads.
ServoCity (through its parent company,
RobotZone) has a large variety of gear
and mounting accessories for many
Invenscience makes the Torxis
i00800 shown in Figure 17, and the
i01855 (industrial version) super high
torque R/C servos with an ultimate
torque of 3,200 oz-in. As with most
very large servos, this one requires a
separate 12 VDC supply at three amps
capacity, but uses a standard R/C PWM
input. It can travel 90º with an R/C
input or up to 270º with a
Though a bit slower than a small
servo, it has a 60º travel in . 5 seconds.
It is all metal gear construction with
dual roller bearings on the output shaft
and an aluminum 6061-T6 horn. These
are very massive and powerful servos
that can handle any human-sized
robot's requirements. In testing them, I
had to be very careful of any pinching
situation at the output horn.
FIGURE 13. Futaba 9352HV very high torque servo.
Servos from Robotis
The Dynamixel rotary actuator
series that I mentioned earlier has
become one of the most popular
servos for advanced robotics
experimentation. The basic AX- 12
actuator of the group is at the lower
right in the two rows of actuators
shown in Figure 18. It has a holding
torque of 229 oz-in and operates from
7. 2 to 12 VDC. Operating on a TTL half
duplex async serial protocol through
the daisy-chain interconnection, this
intelligent actuator is the servo for all
18 DOF in the Bioloid Premium kit.
Figure 19 shows the AX-12A and
10 of the different actuators in a speed
vs. torque graph. The Robotis website,
as well as the CrustCrawler and Trossen
Robotics websites, have extensive
information on these amazing servos.
The AX-12A and all of the AX and
more advanced MX series can be mixed
as all can operate on a single 12 VDC
supply. The servo’s built-in
microcontroller handles sensor
management and position control.
Each actuator can track its speed,
temperature, shaft position, voltage,
and load. The most advanced of the
series (shown on the bottom left) is the
MX-106T with 1,416 oz-in of torque
and an onboard 32-bit, 72 MHz Cortex
M3 controller with a PID control
algorithm (Proportional Integral
Derivative or ‘three-term control’), and
a contact-less magnetic encoder that
can operate at up to 3 MB/s with the
new TTL 2.0 bus structure.
I am very impressed with
this whole series which I have
been experimenting with
using the Bioloid’s controller
and various shop-made add-ons to test torque and speed.
I have found the AX-12A
actuators to be faultless in
operation with the Bioloid
Premium kit (which I will
review very soon). The
Dynamixel AX-12A and AX- 18
are plastic geared actuators
with a cored motor.
The Dynamixel MX- 28,
MX-64T, and MX-106T are
FIGURE 14. Futaba S9156
even higher torque servo.
78 SERVO 11.2012