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.
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.
The Extension Bucket collects food from the intrinsic bowl, scoops it against the wall, and extends out to make getting a bite of food easier.
The Extension Bucket has two inputs. The first input is a linear actuator which drives into the food bowl and collects the bite. The second input is a servomotor which rotates the actuator to be parallel with the surface. Once parallel, the actuator extends to full length and stays extended until the user pushes the retract button. The retract button recedes the actuator and rotates it back to the starting position. The Food Bowl in the Extension bucket is mounted to drawer slides on the housing and has a low force spring that returns it to its starting position. The motors are controlled by buttons connected to an Arduino within the housing and their voltage is supplied by a 7Ah battery.
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.
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.
The Third engineering analysis is with respect to the 3D printed parts. We want to ensure that the parts can handle the weight of the servo, weight of the actuator with food, and force of the spring. The actuator weighs 0.3 lb, the spring gives an axial load of 0.7 lb, and that the food weighs 0.125 lb. We applied these forces to our 3D model in fusion and used the static simulation to determine minimum factors of safety for our 3D printed parts. The simulation results for factor of safety and Von Mises stress are shown to the right.
There are many loading situations on the actuator. Specifically, there is a different load case for each angle. The current angles are 40°, 5°, and 0°. Using these angles, the maximum stress occurred at the 5° angle where the spring force and weights of the food and actuator are all present. The minimum factor of safety is 15 and occurs where the servo will be mounted. We then loaded the actuator 0° with 5lbf in each direction to simulate a user taking a bite of food. The maximum stress occurred when the force was applied in a downward direction. The minimum factor of safety is 15 and occurs where the servo will be mounted.
The simulation for the spoon is to make sure it can handle the spring force and weight of the food. The minimum factor of safety is 10.36 and occurs where the spoon will be mounted onto the actuator.
The simulation for the spring catch is to make sure it can handle the max spring (3 lbf) and the spring force during use (0.7 lbf). The max spring force occurs when the drawer slides are fully extended. This is much further than the actuator will push the spring. The minimum factor of safety of the spring catch during use is 8.236 and occurs at the base of the spring catch. The minimum factor of safety at max spring force is 2.76 and occurs at the same location. Since the spring catch will not see the max spring force often, a factor of safety of 2.76 is acceptable.