DESIGN OVERVIEW

​

The goal of this project was to design and build a robot capable of: (1) obstacle avoidance, (2) localization in a predetermined maze, and (3) the loading and delivery of a wooden block. The final design prioritizes the objectives of simplicity, low cost, and small footprint (cross sectional area). It uses five ultrasonic sensors for navigation and load detection, and three DC motors to achieve locomotion and load pick-up. 

​

​

​

Context

 

THE REQUIREMENTS

​

The rover had to fulfill the following requirements: 

• the  rover  should  navigate   around  the  walls  without  any  collisions 

• the starting  point  will  be  a  random  location  on  the  maze

• the drop off zone,  point  B, will be randomly selected

• the  rover  will show indication  (i.e.  blink  an   LED)  when  it:

  • arrives  at  the  loading  zone (denoted by "L/Z" in Figure A)

  • detects  the  load  at  the  loading  zone

  • returns  the  load  at  the  delivery  point  B   

• the  payload  will  be  less  than  2”  x  2”  x  2”  in  size

​

​

THE FINAL DESIGN

​

MAZE SOLVING STRATEGY

Upon entering the maze, the robot is programmed to randomly explore, enacting obstacle avoidance algorithms until it localizes itself in the maze. Obstacle avoidance is accomplished using sensor readings to generate a steering input, and modulate power given to the wheels. Localization uses these sensor readings to obtain location and orientation at a unique 4-way intersection (marked by the star shown in Figure A). The robot then follows a hard coded path to the loading zone (denoted by "L/Z" in Figure A), where it picks up a block, and back to a pre-specified drop-off zone (denoted by "B" in Figure A). 

​

OBSTACLE AVOIDANCE STRATEGY

For robot movement, a two-tiered PID system was employed. As motor speed proved difficult to control with simpler linear algorithms, the first PID controller used encoder values to adjust PWM duty cycles for the two motors to precisely control wheel position. Parameters were adjusted to help the robot achieve the desired position as quickly as possible, with minimal overshoot. Speed was controlled by incrementing the positional setpoint for the PID controller over time. A second PID controller was used to provide steering input, to help the robot drive in the center of the track. When a wall is present on either side of the robot, ultrasonic sensors provide feedback to keep the distance between the sensor and the wall equal to a target value. The output of this control system is used to modulate the desired setpoint of the first encoder PID (i.e if the right wall is too close, the right wheel should move faster than the left wheel). When the robot reaches the intersections without immediate walls, the desired motor powers of either wheel at the moment before the walls were ‘lost’ are maintained by the encoder PID until the robot again senses a wall. At this point the wall tracking PID is turned back on prior to the encoder PID. The robot prioritizes right turns, then left turns, then forward movement until localization is achieved.

​

KEY DESIGN FEATURES

​

Robust sensor selection: ultrasound sensors are used instead of infrared sensors because their effective range is longer, and their readings are not affected by surface type or color.

​

Minimized Power Requirements for Maximal Grip Strength and Range: the gripper is driven by one DC motor, has few moving parts, and requires no current to hold the block in place. Due to the need to grip and pick up the block, movement in orthogonal directions is required. For simplicity, the gripper uses a string and two sets of elastic bands, performing two orthogonal motions in sequence. 

​

Tiered Chassis: driven by the gripper dimensions and sensor clearance needs, the modified hexagonal tiers provide structure as well as organization while minimizing the rover's footprint.

​

Two-tiered PID Control: rover chassis position control layered on wheel peed control allows the rover to drive as straight as possible and dynamically adjust its heading.

​

Zero Turning Radius & Continuous Motion Navigation: minimizing the turning radius allows for minimal risk of wall collision while continuous movement (as opposed to incremental) allows for better PID control.

​

​

​

Process

​

DESIGN STAGES & DELIVERABLES

​

As outlined in the figure below, project responsibilities were divided into subsystems and organized using a Gantt chart.The subsystems tasks were done in pairs where multiple teammates had interest in the subject matter. As this project was a learning tool, I made sure that we were all derived the skills we wanted to learn within the 4 months we had with maximal support.