Project Overview
Worcester Polytechnic Institute’s Mechanical Engineering program has this lab-based class called Engineering Experimentation. It’s main focus is on microcontrollers and teaching students how to utilize devices like Raspberry Pi and Arduino. What the class also has is an open-ended final project where you work either by yourself or with a group partner to develop a product. Through this project, I decided to fully develop a 6-legged walking robot on my own.
A walker robot is a robot that utilizes 2-3 degrees of freedom per leg module to execute a walking motion for movement (think of how a crab or spider works)
A benefit of designing these robotic systems is that it greatly improves your sense for mechatronic based systems, your controls engineering skills, and your Design for Assembly skills due to how much electronics there are to factor
GIF Provided by roboticgizmos.com
Here is the finished CAD for the first version of my walker. All red and black parts are 3D printed out of PLA+ and Super PLA+ due to their strength and resistance to impact. Each MG996R Servo Motor has been dual supported through the use of threaded standoffs, heat set inserts, bearings, and button-head screws
For this project, I decided to utilize a Printed Circuit Board. A downside to making a walker is that there are a lot of electronics to factor in. So having a board that acts as more compact wiring alongside holding all electronics in place makes a PCB vital to this project
Recently I became a Lab Assistant for my WPI’s makerspace. The Prototyping Lab that WPI runs has a PCB Fabrication device, so we used this circuit board as a means of training me to know how to use the machine
I am able to control the walker’s movement through the use of an RC transmitter and receiver sold by Flysky. I am well experienced with RC devices, having used them in combat robotics, and felt like that was a great way to introduce myself to control systems. Here is a video showing a leg motion sequence
Version 1 Takeaways
Even though the class is over, there are some aspects of the walker that I would like a second version to resolve.
The first of these improvements is a more power-dense power supply. I have been calculating what would be the necessary battery size and it seems like this would make for the best option.
The second of these improvements includes a lighter frame. There were many instances where the walker would remain off balance due to how much weight was used for a billet 3D printed chassis.
Version 2 Scope
When developing upon version 1, I had 3 key factors in mind:
Reliably power all 18 servo motors - this system has an estimated 45 amps of stall current, requiring a more safe and reliable system to draw out power
Create better designed leg geometry - version 1 had a difficult time standing up due to its leg geometry. This created a pivot point, making movement equivalent to walking in a circus’ tight rope
Add modularity to the design - version 1’s billet chassis formed various difficulties with both manufacturing and assembly
V2 Design
Given the necessary design changes, I went with a stiffener plate approach to create that necessary modularity. This involved creating two carbon fiber sheets on the top and bottom of the chassis that acts as stiffener plates. Using a mix of M3 fasteners and M3 brass inserts that were added to each 3D print, each leg module is now separate from the chassis body
Each carbon fiber stiffener plate and 3D print were fabricated using WPI resources
V1 vs V2 CAD Designs
In addition to the added modularity of each leg subassembly, rubber-like feet were fabricated using 3D printing with Sainsmart thermoplastic polyurethane (TPU) filament. An improved leg geometry was developed to maximize the amount of balance that each leg is able to perform. To prevent slippage of each dual-support standoff, a c-clip was added
CAD V1
CAD V2
PCB Design Improvements
One of the major flaws in V1’s design was the implementation of all electronics into one PCB. Additionally, the amperage of 2 18650 Lithium Ion Batteries in series was not sufficient. To fix this, I made 3 critical design decisions:
Switched from lithium ion batteries to a 3S 5200 mAh LiPo battery - this decision greatly improved the run time of the 18 servos that need to be constantly powered to allow movement
Utilized 3 6V 10A UBECS - these electronics divided up the LiPo battery output into 3 different sources of 6 volts and 10 amps of DC electricity for each servo motor. The division of high amperage electronics from the PCB’s low amperage rating resulted in a more reliable wiring harness
Separation of the walker PCB from each PCA9685 Servo Driver - Doing this reduces cluttering of wiring, while also keeping the modularity that V2 aims to accomplish
V1 vs V2 Circuit Designs
PCB V1
PCB V2
Disassembly and Reassembly Process
V1 vs V2 Assembly Designs
Version 1