Servo - January 2012

Part 2

by Matt Bunting

Pimp

My

Hexapod

www.servomagazine.com/index.php?/magazine/article/january2012_Bunting Discuss this article in the SERVO Magazine forums at http://forum.servomagazine.com

Introduction

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.

Motors

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 then implemented 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.