Robot structure
The mechanical requirements needed to fulfill the challenge were to develop a mobile structure capable of moving around a one floor house and bringing objects from anywhere inside to the host. The robot design must be friendly to the operator and surraundings and bring the objects in time. So In order to accomplish this task we divide the design robot into 4 sections: base, lifting, arm and gripper.
In the first section we focus on the chassis design. This part must be capable of supporting the weight of the robot on the motors and remains stable with a low center of gravity. The dimensions width and tall must fit in a door. To achieve stability the arrangement of the heavy materials have to be in a position as low as possible.
In addition to the base we attached a structure that shall give rigidity and support to the elevator. This is composed of three aluminum profiles joined in a 'T' shape going in parallel with the elevator.
For the displacement of the robot we would use 4 motors of 45 kg* cm of torque and 83 rpm with a gearbox relation of 131: 1 with four omni wheels so as to achieve the ability to move in any direction without the necessity of turn around and also given a smooth movement.
For the lifting system we must elevate the arm from 15 cm to more than 1.5 meter and turn it a minimum of 180° degrees. By doing analysis we consider using a ball screw with four bars of aluminum giving it support. The ball screw will rotate thanks to a stepper motor and that way lifts the arm. Finally, we decided to add an angle of freedom to our elevator to give more freedom to the arm. For this, we use a radial bearing, two axial bearings, and finally, a ball bearing to achieve a stable 180-degree turn.
The arm specification is to be able to rotate and locate the gripper in the best way to catch the object. The object is no more than 5 kg, so it has to be strong enough to resist this weight in the extended position. The material should be light and strong, taking care not to give extra load to the upper section. The best and economical way to accomplish this is to use PLA as material and give 3 degrees of freedom using the scara model with bearing and steppers motors to achieve accuracy.
We made the design of the gripper using three servomotors, we decided to print the structure in PLA and use TPU for the "fingers". For the latter, we were inspired by grippers from the Festo company to design internal ribs so that the "fingers" can take the shape of the object they are holding to provide better support and adaptability. In a neutral position, the gripper provides an extra degree of freedom, rotating along the axis perpendicular to the right plane of the robot.
We designed a base capable of moving 30 kg of load, using 45 kg * cm of torque motors, omnidirectional tires, and a 12v 18 ah battery, we also designed a cover that will be manufactured with fiberglass to provide a better appearance. Finally, we designed two PCBs, the first one to regulate the necessary voltages for all the robot components, which includes protection for short circuits and high current demand. The second PCB was to control the motors and sensors of the base.
PCB of power: https://oshwlab.com/a00825779/home-base
PCB of base: https://oshwlab.com/joseacisnerosm1/homebase-atmega2560
We developed an elevator that can rotate thanks to a NEMA 17, it also has a 1610 ball screw also driven by another NEMA 17 with which we will be able to give it the necessary speed of .1m / s to have a final movement of 12 seconds. To control this elevator, we also developed a PCB where we have the stepper drivers and the logic for TOF sensor and limit switches to have feedback of the elevator movement.
PCB of lifter: https://oshwlab.com/AnelAlvarado/driverelevador
We were able to cover all the requirements of the arm using three NEMA 17 stepper motors. We also built a PCB to control these motors and the gripper servo motors, as well as some sensors to receive feedback from the movement of the arm and the gripper.
PCB of arm: https://oshwlab.com/a00831510/brazo_control_Home
In the gripper, we decided to use a 180 ° servo motor with 50 kg * cm of torque to move the gripper in the Y-axis, in addition to this we used two 180 ° servo motors with 15 kg * cm of torque for the "fingers". About the structure, we decided to print it in PLA and the fingers in TPU so that they take the shape of the object that we hold. On its last iteration, the designed gripper weighed an estimated 709.33 grams and was calculated to be able to pick up a maximum of 2.08kg.
