Design and fabrication of an elephant anklet to encase a sensor stack collecting inertial data to monitor elephant health
Spring 2023 MAE 156B Senior Design Project
University of California, San Diego
Sponsored by San Diego Zoo Wildlife Alliance and Engineers for Exploration at UCSD
Background
In order for researchers to better understand elephant health and behavioral patterns, the San Diego Zoo Wildlife Alliance (SDZWA) is utilizing a sensor stack consisting of an accelerometer, gyroscope, and magnetometer to track elephant movement via their front leg.
To collect reliable and meaningful data from the sensors, there must be sufficient rigidity on the ankle; this device may endure high amounts of stress from water pressure and compressive forces. Therefore, these sensors will require a durable and waterproof enclosure to attach to the elephant's ankle.
Our Mission
Our goal is to design and fabricate a housing unit encasing inertia sensors that can attach to an elephant's ankle. During the design process, some physical parameters were emphasized, including:
In the event that the anklet falls off the elephant, the sensor enclosure must be strong enough to protect the sensors from any forces applied to it (For example, the elephant stepping on the device!)
The sensors can be damaged when coming into contact with moisture. We must create an enclosure that protects the sensors when the elephant goes for a swim or has some fun in the mud!
We want to create a system that allows easy access to the sensors for the scientists collecting data. Additionally, we must use materials that will not interrupt any signals from the sensors, causing loss of data.
Most importantly, we want to make sure that this system does not, in any way, disturb the elephants or cause any harm. We created an "elephant-proof," adjustable strap to ensure the system stays in place and secure.
Challenges
ELEPHANTS ARE EXTREMELY INTELLIGENT and can use their trunks to remove an anklet
SIGNAL DISTURBANCE may occur if the structure is made with strong materials like metal
The structure must be 3D PRINTED to be easily replicable, potentially reducing maximum strength
Our final design is comprised of four components:
1. Internal Sensor Stand
2. Waterproofing Component
3. External Strength Component
4. Adjustable Buckle on Firehose Straps
The INTERNAL SENSOR STAND (1) is chemically bonded to the lid of the WATERPROOF COMPONENT (2). The waterproof component is a Blue Robotics 2" diameter waterproof enclosure that is tested for waterproofing to ensure the sensors do not come into contact with moisture. Once the lid is screwed on, the waterproof component enters the EXTERNAL STRENGTH COMPONENT (3), 3D printed with PC-ABS filament. The external strength component is split into two parts to insert the waterproof component and is fastened with bolts and nuts. Finally, the external strength component was designed to feed the ADJUSTABLE BUCKLE ON FIREHOSE STRAPS (4) throughout the structure. These straps stay in place from the tight buckles and the nut and bolts required to strap the loose ends in place.
Prototype Performance
Because we used preexisting materials for the Waterproof Component (2), our main focus during experimentation was the durability of the External Strength Component (3). FEA simulation indicated the External Strength Component would be able to undergo a force of 30 kN (the average weight of an adult, female elephant) with slight a deformation of about 0.2 mm, without failure.
We used the Instron 3400 Series Ultimate Testing Machine (UTM) to conduct the strength test. The size of the load cells was a problem for Team 20, as the final design for the enclosure was significantly larger than the size of the load cells. To account for this problem, Team 20 decided to create a scale model that would fit within the area of the load cell. In order to make the scale model fit on the cell, it was scaled to 15% of the original size, shown below.
The UTM took data of the displacement, force, and time. These results were then converted into a stress-strain plot for the compression test. As seen in the figure on the right, the stress-strain curve follows a linear relationship up until it reaches a stress of 17.458 MPa, where the sample fails. This stress corresponds to a force of about 3 kN.
Since the model was scaled down to 15% of the original size, the force at which the full-scale model would fail was calculated. For simplification purposes in the calculations, the scale model uses an area of 0.15 m2 while the full-scale model uses an area of 1 m2 with x being the force at which the full-scale model would fail:
Using this equation, it was calculated that the full-scale model would fail under a force of 20 kN.
This result showed that the external strength enclosure ultimately failed to meet the requirement that to withstand the entire weight of an adult, female elephant (roughly 25 kN to 35 kN). There are a few reasons why these results came about, some of which include the accuracy of the scale model, the lack of fasteners in the scale model, and the infill that the model was 3D printed at. However, further experimentation with a full-scale model is recommended for a more accurate representation of the strength of the External Strength Component.
