Monthly Archives: April 2009

Hitec Servo

These are some of Hitec servos that I looked at:

Size Speed

(Time/60° (s))

Torque ( Price


Bang per buck (Torque/Time/Price)
HS-475HB Standard 0.23 4.4 15 1.3
HS-645MG Standard 0.24 7.7 34 0.9
HS-985MG Standard 0.16 9.6 58 1.0
HS-755HB 1/4 0.28 11.0 25 1.6
HS-805BB Mega 1/4 0.19 19.8 34 3.1


Micro 0.17 3.6 41 0.5


Standard 0.23 10.3 48 0.9

For standard size analog servo, the HS-985MG performs best. It is based on a coreless motor. It provides the highest torque as well as the fastest speed.

The HS-475HB is the best value servo. It has the advantage of using Karbonite (just like kryptonite 🙂 ) gears. Karbonite is supposed to be stronger than nylon but has less wear and tear compared to metal.

The HS-645MG has good torque compromise without being overly expensive. It uses mostly metal gear.

For a mega 1/4 scale servo. The HS-805BB looks awesome.

Hobby Servo

RC or hobby servos provide a convenient actuator to be used in a small-scale robotics project.

Inside a servo there are a DC motor, a gear reduction system, a potentiometer as a position sensor, and some electronics that control the motor based on the desired position.

A hobby servo is great, as it provides all the above in a convenient small package. Due to its popularity, hobby servo is quite cheap.

However there are shortcomings in using a hobby servo in a robotics project:

  • No position feedback. Once commanded to move to a target position, we don’t know when or whether the servo has reached the target position
  • No torque sense. We cannot tell if the servo mechanism is unexpectedly obstructed.
  • Sub-optimal control system. The servo manufacturer cannot know what load the servo will be connected to, so as a result the control system will not be optimal. The problem shows up as: overshoot, slow response, and steady state error.

My plan is to use all the mechanical components of a servo, but replace the electronics with one that I design myself to solve the above issues.