Table of Contents
To satisfy the customer need of reusing as many parts from previous designs as possible, the team decided to base its overall structure of the motor module around the previous team (P07201). The motor module was divided into several sections which include the yoke, encoder box, turning plate, and the driving components connecting the first two sections. The yoke is located below the turn table and is used to provide housing for the horizontal driving shafts and the wheels. The encoder box is used to house steering and drive encoders, as well as a support to mount the motors. The vertical shaft transmits power from the motor to the horizontal shaft located in the yoke using a set of miter gears.
Along with the changes in the turning plate and wheel dimension, the acrylic plate thickness was also reduced. The fastening system between acrylic plates used aluminum brackets to re-enforce the connection between them. This made the assembly process faster and more efficient.
The fastening system for the driving components has also been changed. At the customer's request, the team looked for ways to secure and connect gears and shafts while maintaining easy access to change components. The solution was to use cotter pins to secure and connect the driving components.
The main design of the P08201 required the robotic platform to have infinite turning angles. Also, because of the requirement to reuse components of P07201, the old system and components were reused when possible. The same ring gears and spur gears were initially chosen to provide a reasonable rate of rotation for accurate control. The spur gear was however changed due to an inability to find the original part. As such, the ratio was changed from 15.6:1 to 13:1 which results in about 5.56 rpm or 10.58 s/rev.
- 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
A major design requirement is a lightweight platform which is strong enough to support a payload of 10 kg. The P07201 team selected acrylic tubes as their main structural material. However, machining and drilling required for the fastening screws compromised the structural integrity. This team, under the recommendation of the customer, selected 8020 Aluminum extrusion as the main structural material. The benefit of 8020 comes from a slot and gap along the length of each of the four sides for easy construction and mounting of other, non 8020, components. Also, off the shelf fastening system are available for quick assembly. The Aluminum material used in 8020 is stronger and can withstand a greater payload with less deflection then the previously used acrylic tubing. The basic unit of assembly for the frame design is the Y-Bar. The Y-Bar contains two U shaped ends for attaching the motor modules, and a beam connecting to each of the two ends.
With the Y-Bar as the basic structural unit, the platform can be easily reconfigured to meet the requirements verifying design needs. The rest of the platform was used to connect two y-bars to fit the 4 wheel requirement with connector and the cargo platform to transport the payload.
- 8020 Frame
- Very strong and versitile, allows for many mounting configurations
- Quick connection of components to frame
- Able to hold much more than the required 10Kg
Areas for Improvement
- Miter gears skip under moderate loads
- Place more support near miter gears
- L-Bracket was used as a temporary fix, which worked except for under very heavy loads
- Belt Skip under extreme load
- Use a belt tensioner mechanism to adjust for belt stretch and wear
- Timing Belt Ratio
- Hard to accelerate, need to change gear ratio from 2:1 to about 1:1
- Will help eleviate some of the miter gear and belt skip problems
- Steering Gear
- Use a gear with a key way or setscrew. Press fit gear tends to slip under heavy load, rendering the connection useless. Temporary fix was to weld the gear to the shaft, however, the success rate was only 50% as the gear is easily burned through when welding.
- Slop in Motor Module Assembly
- Use a new, tighter turn table which has less play
- Use a more rigid material to minimize flex
- Minimal clearance between ground and yoke
- Decrease length of sides under wheel axle to allow more clearance
- Encoder Mounting
- Need to increase size of encoder to box to fit encoders, currently it is too small to fit the encoders
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