Motor Module
Orientation
Figure 1.1 CAD concept and actual motor module
Coming into the P08201, the main objective is to improve base on the design of P07201, and one of the main goal is size and weight reduction of the overall robotic platform. With the objective in mind and also upon the request of the customer, several components have been changed. First of all, the turning plate is 6” x 6” with the height of 0.32”, which resulted in smaller space requirement in the stationary position. Also, in order to reduce the overall height, the wheel size has also been reduced. However, in order to preserve the original P07201 performance, the pitch ratio of the timing pulleys has been changed from 1:1 (top: bottom), to 2:1, thus hoping to achieve the maximum velocity of P07201 robotic platform.
Table 1.1: Estimated Performance Specification
Performance Specification:Estimated Max Speed / 2.69 m/s
Estimated Acceleration / 1.49 m/s^2
Estimated RPM on wheels / 676 RPM
Estimated Torque / 0.426 N-m
Along with the two major changes in the turning plate and wheel dimension, the acrylic plate thickness is also changed since the new fastening system, which is using aluminum bracket to re-enforce the connection between of acrylic plates. Comparing to the P07201 design, the use of Aluminum plate allows the assembling process to be more efficient due to the fewer screws. Therefore, even through P08201 team selected the same acrylic material as the team P07201, the plate thickness of P08201 is only 0.177” opposed to P07201’s 3/16”. The thinner acrylic plate can provide slim weight reduction of the motor modules.
Also, the fastening systems between the driving components have been changed. Upon the customer request, the team needs to look for ways to secure and connect gears and shafts without the possibility of shift. Also, the new design has to remain the easy access to change components. Therefore, the solution of using cotter pins to secure and connect the driving components. The uses of cotter pins can prevent the slide and slip of set screw on the connecting shaft but requires more precision machining to on the shaft to
The connecting system from the motor module to the platform uses the T-Nut which comes with the 8020 aluminum frame. Since the T-Nut fastening system comes with the 8020 Aluminum extrusion and also fits to serve as the connection between the turntable and the bottom of acrylic plate of encoder mount. Thus, the new connecting system requires even less machining and still grants the easy assembly that the team hopes to achieve.
Steering Angle:
The main design of the P08201 is a continuation of the P07201 robotic platform with the same ability of infinite rotation. Also, with the requirement to reuse components of P08201, the result is the old system and component being kept and reused. With the same ring gear from P07201, the new internal spur gear and pinion were chosen to produce a reasonable rate of rotation for accurate control. The spur gear was to be remains the same with 10 teeth for the 15.6:1 ratio. However, due to unable to find the same part, the ratio was changed to 13:1 which results in about 5.56 rpm or 10.58 s/rev.
Figure 1.2 Timing Belt and Miter Gear Display
Platform
The main design concept of current platform is also put into the consideration of a lightweight and strong platform to support the 10 kg minimum required payload. The P07201 teams selected acrylic tubes as their main structural material and screw for main connection between tubes. As the results of extra machining and drilling on the tubes to accommodate the fastening screws, the structural integrity was greatly damaged. This term, under the recommendation of customer, P08201 team selected 8020 Aluminum extrusion as the main structural material. The benefit of 8020 is its pre-designed gap for easy table attaching. Also, there are off the shelf fastening system available for purchase for quick assembly. The Aluminum extrusions are stronger and can stand a payload with less deflection with might cause a problem in the long term for the robotic platform. However, the weight of Aluminum becomes one main concern of our team. Due to the extra weight of Aluminum, the overall performance is lower, and there are more stress on the motor module and the battery.
As the basic structure, the team design starts with the basic unit of assembly, the Y-Bar. The Y-Bar contains two U shape ends for motor module attaching and a beam connects to those two sections.
Figure 2.1 Y-Bar concept and Structural analysis
With Y-Bar as the basic structural unit, the team can assemble the platform to meet the requirement in case there are multiple wheeled vehicles requested. The rest of the platform is to assemble the two Y-bars to fit the four wheel requirement with connector and the cargo platform to transport the payload. Figure 2.2 shows a top platform area for cargo transporting but also a lower part which is used to be spaced for electrical essentials of robotic platform
Figure 2.2: The Overall Concept and Actual Platform