Author Archives: stephen

Seeed Studio DSO nano oscilloscope review

I just received my Seeed Studio DSO nano oscilloscope.

I had difficulty justifying the purchase of this oscilloscope, as I had doubts on its usefulness and performance. However, the unit has such cool form factor that I just had to have it. The oscilloscope is modestly priced at USD $89. I justified buying it because, if for some reason the oscilloscope is not usable, then I still can use it as an ARM STM32 development board (Hey, this is the same CPU that powers Hexi’s locomotion engine).

DSO nano

The DSO nano is really small

Continue reading

Go Hexi!!

Meet Hexi. Hexi is my new 6-legged robot (hexapod) family member. He is demonstrating the three standard walking pattern for a hexapod: wave, tripod and ripple gait. For now he can only walk in a straight line.

Body: Lynxmotion Phoenix
Actuator: Hitec HS-645MG servos
Processor: ARM Cortex-M3
Battery: 5 Sub-C 5000 mAh (cheapbatterypacks.com)

Hitec Servo

These are some of Hitec servos that I looked at:

Size Speed

(Time/60° (s))

Torque (Kg.cm) Price

(US$)

Bang per buck (Torque/Time/Price)
Analog
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
Digital

HS-5085MG

Micro 0.17 3.6 41 0.5

HS-5645MG

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.