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Our team was tasked to create a design that helps people with Cerebral Palsy, CP, eat. People with CP typically have a hard time using their arms due to uncontrollable shaking. This makes it nearly impossible for them to feed themselves with standard utensils, requiring outside assistance. The main goal of this design is to increase the user’s independence and use minimal outside input.

While people with CP have trouble using their arms to eat, they have enough motor control in their obliques and hip flexors to allow hinging. This could let them lean to a spoon or fork with food. The design would need to be small enough for an easy setup and portable to let them use it outside of the home.

The design must be able to relocate food from storage, a container or bowl, to an accessible location for the user. The design must be portable, so a light weight, battery powered design is ideal.

- Obi
- The Obi Robot is a 6 DoF robot arm with four food bowls to select from. It uses a spoon to gather the food, and the robot arm to deliver it to the user allowing hands free eating. Obi is a top-of-the-line assistive eating device but comes with a large price tag.

- Eli Spoon
- The Eli Spoon is a 2 DoF gyroscopic eating utensil that corrects rotation. This utensil can correct unwanted movement in two directions, allowing for easier eating. The design is very portable and much cheaper than the Obi robot.

- Self-Feeding Supports
- Self-Feeding Supports are a subclass or assistive eating utensils that support the user’s arm. This mitigates the effects of muscle weakness associated with CP and can allow some independence in self-feeding. The price of these devices can vary.

With these 3 designs in mind, we decided to select Concept Design 3. There were a few reasons that went into this decision. The first being ease of use seeing how the user would only need to push buttons to control the machine. Secondly, it is a pretty simple machine that would deliver the food to a reasonable position in 3-dimensional space for the user to take a bit.

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The First engineering analysis pertains to the loading the motor will see. We wanted to determine the torque, power, and current the motor would see when it was subjected to the applied loads to determine the required motor and battery size. The applied loads are the weight of the food, weight of the actuator, and force of the spring.

Assumptions:

The load directions and locations are constant.

The drawer slides apply no friction.

The motor rotates at a constant velocity.

With these assumptions in mind, we derived equations for the torque and power on the motor would need using static equilibrium shown in the figures below.

Arriving at the equations

Once the equations were derived, we wrote a MATLAB script to make calculations and plot the results shown in the figures to the right.

Our calculations show that the motor would see a maximum torque of 6.5 in-lbf or 7.5 kg-cm. The maximum power required to drive the 5v motor at 1 in/s would be 6 W with a current draw of 1200 mA. This tells us that a 10 kg servo should be able to handle the applied loads and a LiFePO4 7Ah Battery would be sufficient to supply it.

The Second Engineering analysis is with respect to the loads seen by the linear actuator. We wanted to determine the axial and perpendicular loads on the actuator to make sure they did not exceed the limits. The manufacturer rates the PA-07 for a static axial load of 6.5 lbf. With the same assumptions made as in the first engineering analysis, we derived equations for the axial and perpendicular loads on the actuator shown in the figure below.

Arriving at the equations

We calculated the results with the same MATLAB script shown to the right. Our calculations show the maximum axial force on the actuator as 0.622 lbf and the maximum perpendicular load as -0.514 lbf. These loads are well within the limits of the manufacturer’s rating. The actuator draws 100 mA with no load. The 0.622 lbf would increase this current to 124.4 mA.

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2023 Spring