Robotic design

Mechanical Engineering Design

Robotic design requires a variety of design elements emanating from various engineering disciplines, including electrical, mechanical, and computer engineering, as well as insight from the fields of engineering psychology and child development on the usability of the robot by non-technological people and children. The mechanical design of mobile autonomous robots requires effective housing of the motors, sensors, and communication devices, as well as a concern for the robots agility and appearance.
Mobile robots must be agile and compact. Their movement mechanisms must be housed within their design in a manner which maximizes their sensing and communicating abilities, while not limiting their motion. A mobile robot’s operation environment must be a foremost consideration for effective design that will optimize travel and function needs within the defined environment. Not only does this mean that the mobile robot must be robust in order to withstand the hazards of its environment, but it must be also be economical and, therefore, not have added capacities that are beyond the scope and requirements of its operating environment. Mobile robot design is optimal when it is designed in a manner which is sparse, yet durable and can achieve maximum performance and durability within the environment in which it will operate.
Team Spot, two stationary and one mobile robot, was developed in a succession of phases. A Lego prototype began the design process was followed by three rounds of prototypes before the final team of robots was built.
A mobile and stationary robot was developed with Legos. Light sensors were employed to collect light intensity data and infrared communication was used to transmit data between the robots. From experimentation with this basic prototype, it was decided that robots would employ a team of two stationary and one mobile robot to find the light spot. The solution was defined with room for expansion.

First Prototype:
A team of two stationary and one mobile robot were to be built to find a spot of light. The elements of the first prototype determined the initial mechanical design.
Robot Components:
OOPic-R Microprocessor with Development Boards
Infrared transmitter and receiver with electronic bred-boards
Photo-resistors (light sensors) with electronic bred-boards
Motors for rotation of light sensors (Stationary Robots Only)
Motors for Mobility of Mobile Robot
OOPic-R microprocessors were used to control the robots and to give each robot autonomous control. The small size of the OOPic, its onboard data pins and LEDs had to be required to board to be integrated into the robots in a place that was easy to reach and easy to see. This was necessary for development and testing of the robots code as it was for downloading code and checking sensor values in the development stages.  The infrared transmitters had to be in line of sight or in close range to the infrared receivers to insure reliable communication. The photo-resistors had to be in the same, or close to the same, horizontal plane as the target spot of light, necessitating placement close to bottom of the robot.
Based on the requirements of the system, the stationary robots were designed to have three levels; a base, light sensing level, a middle, computer- processing level and a top communication level. The light sensing level was designed with a motor to rotate the light sensor 90 degrees. The initial stationary robot geometry was square shaped on each level, s this seemed like a good starting point from which to cut away and modified sections of the geometry as needed.
The mobile robot body was bought in a kit from Lynxmotion. Due to short development time, and the unknown needs of the mobile robot, a pre-made kit was a good way to start the design process. The kit insured that all the components of the mobile robot would work concurrently and the issues of integrating the mobile robot with the stationary robots could be more quickly addressed.   

Second Prototype:
The body design for the second prototype had a very different geometry from the first prototype. The mobile and stationary robots of the second design consisted of two levels that were round in shape. The lower level housed the light sensing components, while the upper level housed the OOPic-R chip board and the Bluetooth board and antenna. The lower level of the mobile robot also houses the motors for the wheels. Several issues that compromised the functionality of the first prototype were considered in the new design of the mobile and stationary robots. These modifications were tested and further modified in this stage of development.
The light sensors ability to determine a reliable light intensity value was strongly effected by the geometry of the robot. The square shape of the second level of the first prototype of stationary robot cast a shadow over the second level. The light sensing bred-board was housed on this level and the photo-resistor of this circuit was very sensitive to the environmental light. In order to get the best true reading of the direction of greatest light intensity, the second level of the stationary robot had to be changed. The modified design is circular in shape and contains a half-moon shaped cut-out above the mounting position of the light sensing circuit. The half moon cut-out will prevent a shadow being cast over the photo-resistors and allow them to read the true intensity of light at any spot. The bottom level of the stationary robots had a half moon shaped pop-out to place the light sensing circuit further out in front of the robot, and away from any disturbances in the intensity of incoming light. The combined effect of the first level pop-out and the second level cut-out was expected to solve the problem of shadow interference in the light intensity signal.
The motors on the stationary robot were geared down. The initial advantage of using a servo motor was the ready made, internal gearing system. However, for the purpose of scanning ninety degrees in search of the direction of greatest light intensity, the rotational speed of the motor was still to fast. Lego gears, because of their convenience availability, were used to slow down the rotational speed of the light sensors. The light sensing circuit was attached to the top of the smallest and outermost gear.
The mobile robot was designed with the same general geometry, but with slight modifications to the design of the stationary robot. The mobile robot had three light sensors, one pointed to the left, one to the right and one pointed straight ahead. This configuration of the light sensor gave the mobile robot the ability to intelligently decide if it was going in the correct direction and to alter its direction towards the direction of greatest light intensity. Due to the three light sensors on-board the mobile robot, its second level had a greater possibility of causing disturbance with the light intensity reading of the sensors, especially the side sensors, which were placed closer to the body of the robot. The second level of the mobile robot was shaped as a circle with the top third cut off flat. This design prevented disturbance with the incoming light intensity to the sensors.
The base level of the mobile robot was a round shape, similar to that of the stationary robot. To accommodate the three light sensors, three half-circle shaped pop outs were place in the front third of the base plate. The motors were also included in the design of the mobile robot base plate and their placement at equal radial positions, across from each other determined the amount of space available in the front of the base plate for the light sensors. The variations of design between the mobile and stationary robots accommodate their different light sensing schemes and are believed to allow for light sensing with little disturbance. 
This team of robots was prototyped in foam-core poster board. The advantage of this material was its ease of availability, ease of use and ease for modifications – the body pieces were cut with an exacto-knife.  The foam-core body pieces were fabricated together with hot glue, which also allowed for easy modifications to the initial design. Modifications to this design and the performance of the robot team were noted and used in the final robot designs.

Final Robot Team:
The final robot was design was based on the second prototype design. The modifications made to the second prototype, as it was outfitted with electronics and tested in its light finding ability, were included in the final design of the robots.  The mobile and stationary robots have a final design that is similar, but various features to meet their individual design requirements. They are both circular in shape, with cut-outs above the light sensors to prevent interference with the light sensors, however the mobile robot has pop-outs for each light sensor and a larger cut-out on its second level, while the stationary robot has one single smaller pop-out for its single rotating light sensor and smaller cut-out above the light sensor.
The robots were fabricated out of 3/8 inch thick Lexan. Lexan is a flexible plastic that comes in a variety of colors and thicknesses.  Clear Lexan was chosen for use in this project so that the inner components of the robots could be seen by observers of the robots, as well as school aged children watching the robots in classroom demonstrations. In the tradition of see-through phones, imac computers and other see-though and other see-though devices, it is interest and understanding of the components that compose the robots will increase if they can be seen while the robots are in motion.  Beyond its appearance, Lexan was chosen for its easy availability, cost, durability and ease in machining.
Each robot had a base plate, mid plate and top plate machined from ¼ inch thick Lexan.  To allow the microprocessor and communications equipment to be easily accessed in the final stages of testing and calibration of the robots only the base and mid-plates were used in the final building of Team Spot. The top plate of each robot was added after the initial demonstration of the robots to the Academy and Tufts Professors, with the purpose of protecting the microprocessor and Bluetooth boards, as well as to give the robots a cleaner appearance.