2) Design Process [ATD]

Design Constraints

Upon determining what goal the device was meant to accomplish, our team explored numerous methodologies of achieving the desired functionality. The design possibilities were constrained in the following ways:

• The dish would be circular and flat, and large enough to require multiple "sprinkling" spots of the topping (i.e. one sprinkle of the topping container would not be enough to sufficiently coat the dish).
• The topping container would be styled after a typical cylindrical salt-shaker, with a diameter considerably less than that of the circular dish.
• Releasing the topping from the topping container would involve some means of manipulating the container externally rather than directly handling the topping. This manipulation would have the end result of moving the container in such a way that the topping would "shake" through the small holes of the container.

Topping Dispensing

After defining these constraints, our team acquired a container of onion powder that satisfied the requirements. Henceforth, the design process centered around distributing toppings from this particular spice container. It is, however, worth noting that the end product could be adjusted quite easily to distribute a greater variety of toppings from containers of different shapes and sizes.

Next, our team experimented with the topping container and found several primary ways of causing the topping to dispense:

• Shaking the topping container up and down, with a fast attack motion on the downward stroke and a slow release motion on the upward stroke.
• Tapping the bottom of the container, with the container fixed in space.
• Vibrating the entire container.

All of these methods involved placing the topping container in the same position: upside down with the holes facing the dish. After some testing, our team determined that a controlled shaking motion was the most reliable and controlled way of distributing the topping. The tapping motion proved unreliable between different container fill-levels, and the vibrating motion distributed consistently but continuously (rather than in individual bursts which was the desired outcome). Overall the shaking motion proved to be the best, not only because of its effectiveness at distributing the topping, but also because of its conduciveness to being implemented in a synchronized mechanism-centric device, rather than a mechatronic assembly centered around electronics and software.

Translation of Container

The shaking motion produced approximately-circular distributions of topping onto the dish. With these distributions being considerably smaller than the dish itself, the next step of the design process was determining how to move the topping container relative to the dish while performing the shaking motion, such that the topping was distributed across the dish. The team explored two primary modes of translating the container relative to the dish: a raster pattern based on rectangular coordinates, and a spiral pattern based on polar coordinates. It was determined that a polar coordinate system made more sense for topping distribution given the shape of the dish (circular). From here, the next step was determining how to accomplish the spiral movement of the container relative to the dish.

Mechanism Selection

It was decided, after some analysis, that the dish would be rotated, while another mechanism would translate the topping container radially over the dish. A third mechanism would then translate the container vertically with the desired velocity profile to dispense the topping. All of these mechanisms would need to be synchronized and driven from a single mechatronic element (such as a motor).

Some analysis (shown in detail in Section 3) was performed to determine the ideal ratio of these movements based on the sizes of the dish and the topping container. It was determined that the spiral would require approximately 3 "layers" across each radius of the circular dish to adequately distribute the topping. It was also determined that approximately 6 "shakes" of the topping container would be required with each rotation. 

Given these requirements, the following design decisions were made:

• The dish would be placed on a rotating disk sitting on a turntable.
  • The rotation of this disk would be driven by a stepper motor through a gear reduction, increasing the torque of the turntable.
• The rotation of the turntable would be linked to a crank-slider mechanism through another gear reduction, such that the crank slider completes a single cycle with every 6 rotations of the turntable.
  • The slider would translate the container across the radius of the topping distribution area.
  • A full cycle of the crank slider would move the topping container across the radius of the dish and then back again. Thus, with each cycle the topping container would move across the radius of the dish twice. Since the crank would be geared at a 1:6 ratio with the turntable, the slider would move across the radius of the dish once for every three rotations of the turntable, as desired.
• The rotation of the turntable would be synchronized with a cam that lifts and "drops" the topping container, such that the topping container is "dropped" six times for each rotation of the turntable.
  
  • The cam would be saw-wave shaped, causing the follower to raise slowly (linearly) and then drop suddenly.
  • Each "drop" of the topping container would cause the topping to shake out.
  • Rather than creating a smaller cam with a single saw-shaped profile, a larger cam would be created possessing 6 profiles in series. Thus, this cam could be driven at the same speed as the turntable, avoiding the need for additional gearing.
  • The cam would be placed on the edge of the disk holding the dish, creating a more compact and elegant design.

Figure 2.1: Overview of three primary subsystems