System Design

Performance specifications

Explicit requirements:

1. 1. Locomotion must be controlled/initiated through the use of legs.
  2. The device’s footprint should not exceed 5’ x 5’.
  3. The user should ride 2’ to 3’ off the ground.
  4. The device’s mass should not exceed 150 lbs.
  5. The device must be able to carry a person weighing up to 170 lbs.
  6. The seat position must allow for the user’s body to not be in contact with the ground.
  7. The user’s line of sight of the user to be clear when seated.
  8. The device must have a zero-degree turning radius.
  9. The device must be able to be shipped in a 3’x3’x3’ case.
  10. The device must be able to be disassembled into no more than 7 parts.
  11. The device must be able to fit through a 36” door when folded.
  12. The device must be robustly constructed: use nuts and bolts, machine screws, cable ties, proper soldering, etc.
  13. The robot shall maintain a sound level under 70dB during normal operation. 85dB peaks are acceptable for short periods of time.

Implicit requirements:

1. In order to minimize the noise made by the robot, we cannot use devices like linear actuators as they would make a lot of noise while functioning.

2. When the robot moves forward or backward, the person seated on the chair should not experience jerks, hence requiring a good suspension system in the legs.

Coolness Factor:

The robot is especially silent.

Functional Architecture

Basic Principle:

This robot will have 6 legs (hexapod) with each leg having 2 DOFs with which it would locomote forwards and backward. The robot will have a 170lb payload sitting on it the entire time, hence the goal would be to maintain balance during movement. The 6-legged system would help to accommodate this requirement. The robot’s motion will be controlled by a joystick controller. The goal is to move forwards and backward along a straight path while maintaining sufficient pace.

General Layout:

Cyberphysical Architecture

This diagram illustrates the major cyberphysical subsystems of our robot, the individual components which make up each system, and the electrical and data connections between each system and component. These systems are the User Control System, the Computation System, the Power Distribution System, and the Actuation System.

Final System Design Description & Depiction

Main Body

The main body consisted of two decks. The lower deck served two purposes: house all electronics including batteries, motor drivers, etc., and connect the legs to the main body via ball bearing mounts. The upper deck is meant to serve as the seat for the rider. The main frame is entirely made from 25x25mm 80/20 extruded aluminum t-slotted frames, connected together robustly using corner brackets, bolts, and nuts. The upper deck has a ¼” thick plywood plank on top of the 80/20 to serve as a comfortable seat for the rider. The Xbox adaptive controller and emergency stop buttons are also placed on top of the wooden plank for easy access to the rider.

Legs

Each leg of this robot is made from 2 pieces of 40x80mm 80/20 extruded aluminum t-slotted frames which are joined together by durable rotational joints. The leg is mounted onto the main frame via a long shaft and is rotated forward and backward using a DC stepper motor via a chain assembly. The two pieces of each leg have a rest orientation of 90º and have the ability to extend out using linear actuators as shown below.

Control System

The robot is controlled by the central Raspberry Pi, which reads user commands from the User Input Device and communicates motor commands to the Arduino microcontrollers via serial communication. The system runs on ROS Noetic in an Ubuntu 20.04 environment, and translates between robot motion commands from the user to motor commands to be executed by each of the microcontrollers. Each microcontroller is responsible for controlling the actuators of two legs. Each leg has a linear actuator, which is used to raise and lower the foot of the leg from the floor, and a stepper motor, which rotates the legs to achieve the robot’s motion.

Power System

The power system consists of two independent power supplies, tied together on a common ground. A 24V power supply powers the actuators of the system while a 5V power bank powers the computational devices. Both supplies are tied together on a common ground and the 24V supply has an E-stop which can be used to break the connection between the two 12V batteries.

Gait Control

As the name suggests, our tripod gait works by keeping at least three legs on the ground at a time. These are composed of two tripod groups, the odd legs and the even legs. With each “half-step” the system takes, they alternate being the active group, i.e. the group of legs which are rotated to achieve the desired motion. For a single half-step, the gait cycle is as follows:

Lift each leg of the active group, one-by-one.
Rotate all active group legs according to the direction of the robot's motion.
Lower each leg of the active group, one-by-one.
Lift each leg of the passive group, one-by-one.
Rotate all active group legs back to the neutral position.
Lower each leg of the passive group. One-by-one.
Swap the active and passive groups.

The direction of rotation each leg undergoes in step two for each robot motion direction can be found in the table below.

The Direction of Rotation of each Leg for each Robot Motion Direction

Leg Numbering and Grouping Diagram

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