Design Process - MMW

Mechanism Breakdown:

The Mercedes Mono-wiper can be separated into 3 linked 4-bar mechanisms. In order from actuator to end-effector, they are:

• Crank and Rocker
  • 
• Double Rocker
  • 
• Crank and Slider
  • 

The crank and rocker mechanism was driven by our actuator, and the double rocker was driven by the output of the crank and rocker. The crank and slider is driven in two rotations: The output of the double rocker is directly coupled to the frame of the local ground of the crank and slider, and the input of the crank is driven by a planet and ring gear. The use of the planet and ring gear is what allows the complex planar motion with only one degree of freedom. 

Design Considerations: 

• Rigid Frame - Reduce non-planar motion, ensure stability 
• Minimal Adhesives - Maintenance-friendly
• Low Friction - Lower wear of parts, higher efficiency, less risk of mechanical failure
• Lightweight - Higher efficiency, less vibrations

Because the mechanism had many moving parts, our joints needed to be low friction and rigid. For all our revolute joints, we used press-fit ball bearings. For our linear slider, we used lightweight monolithic plastic sleeve bearings. Every connection that was not press fit was made using fasteners. Whenever possible, we used slots instead of clearance holes so that minor adjustments could be made to different components, like the rail mounts or bearing block. One major development in our design that was not applied in the original mono-wiper was the use of herringbone gears for the planet and ring. The self-centering and anti-backlash properties of herringbone gears helped keep our slider plate in plane because it added one more point of contact aside from the central shaft.

The complexity in mechanical design allows for extreme simplicity in electrical design. No sophisticated controls algorithms or actuator drivers are necessary. The mono-wiper takes in a constant speed rotation from a DC motor. Some speed control could be implemented with either a variable power supply or pwm control of the motor, but with the correct motor and gearbox choice, these are also largely unnecessary.