Kate Gleason College of Engineering
Rochester Institute of Technology
Rochester, New York 14623
Project Number: P14347
Wildlife Tracker
Alexander PelkeyMechanical Engineering / Timothy Nash
Mechanical Engineering
Joseph Ciccarello
Electrical Engineering / Eric Peterson
Electrical Engineering / Frank Meola
Electrical Engineering
Abstract
Bow and arrow hunting is a popular outdoor activity. An arrow shot by a hunter may fatally injure the wildlife being hunted, though the animal may not drop in place. Animals that are wounded can travel a substantial distance before their death. To help locate the wildlife and avoid wasteful killing, a reusable wildlife tracking device was designed to be deployed with the shot arrow and detach upon impact. This device’s purpose is to track the movements of the injured animal and relay that information to a user pocket monitor, allowing a hunter to recover the wildlife as well as the tracking device for future use.
The result of this design was a proof-of-concept model articulating electrical and mechanical feasibility. The separate devices worked in tandem to facilitate communication of a specific location from the arrow attachment to the user monitor, capable of performance over long distances as necessitated by a hunter out in the expansive wilderness. This paper outlines the design process and end result of the feasibility system described above.
introduction
The design objective was to facilitate the retrieval of runaway game for a hunter that would otherwise escape after being fatally wounded. During hunting season (October to December), a hunter in New York State is permitted to hunt four deer, usually only one by bow. When a hunter locates and takes a shot at an animal, it is typical to wait up to 30 minutes before tracking a wounded animal. Therefore, time and distance become critical parameters during a hunting trip. Tracking game typically takes a hunter a few hours, and the time to reach the wounded animal can increase greatly if traditional methods of tracking aren’t successful. The wildlife tracker was designed to allow a hunter using a bow and arrow to quickly and efficiently track an injured animal that escapes after impact.
The end result was a complete proof-of-concept of two separate devices that function together to achieve a method of communication between an escaped animal and the hunter. The first device is a small arrow attachment that was designed to affix to a hunter’s arrow and secure itself to an animal’s hide after the arrow is shot. The mechanical structure was streamlined to be aerodynamic such that it did not interfere with the flight of the hunter’s arrow and therefore cause a deviation in accuracy or range. Inside the durable structure,a GPS receiver with a correspondingantenna was selected as a wayto receive satellite data. A transceiver was chosen as a method of sending this satellite data at a high frequency of around 915MHz to the receiving component. The transmitting device was designed to have a dipole antenna that stuck out the back of the arrow attachment to optimize antenna range and efficiency.The second component is an interactive user device complete with a receive antenna to receive the transmitted data, a Raspberry Pi inside to interpret the data using Python code, and an LCD screen whose purpose was to display the interpreted data as latitude and longitude coordinates. The outer workings of this component only required a solid enclosure large enough to fit the electronics inside. Both devices are powered by rechargeable polymer lithium batteries with sufficient electrical specifications.
When the entire circuit operates properly, the transmitter sends coordinates of its location to the receiver, which displays and updates them accordingly. The hunter is assumed to have access to a smartphone (iPhone, Android, etc.) such that they can use a GPS to map out directions and distance parameters to the target. In a heavily wooded area, it is still possible to utilize the powerful GPS components of a smartphone, so this was not seen as a problem for the hunter when considering the real-life application. The coordinates displayed on the LCD screen could be entered by the hunter into a GPS application on the phone such as Google Maps and updated in conjunction with the animal location to provide a clear route to the runaway game.
Methodology
Given the initial problem and situation, there were a number of customer requirements and demands to take into consideration. The most important requirement was to ensure the device was able to track the animal after being shot. Otherwise, the design would serve no purpose. Additional customer requirements that were critical included the need for the device to successfully attach to the arrow and detach from the animal, as well as the need for the device not to impede the accuracy of the hunter’s shot. Both of these requirements were established to reduce the hassle of using the wildlife tracker and to integrate its functionality into the hunting environment that the user is already used to. Accuracy of the tracking device was also a key customer requirement to reduce the frustration of a hunter searching a general area for fallen prey.
Due to the customer’s extensive technical background, many of the requirements presented correspond directly to quantitative engineering requirements. The distance of operation (2000 meters), location accuracy (1/100 meters/meter), minimum operation time (90 minutes), acceptable effect on accuracy(25mm from center at 25 meters), and operational temperature (-30 to 50 degrees Celsius) all were given as numeric requirements. Other engineering requirements required some testing to obtain reasonable values, such as the necessary holding force of the barbed hook into the animal hide and the acceptable drag coefficient of the arrow attachment. The importance of these requirements was based on the customer’s input as well as their relevance to the primary goal of obtaining an animal location.
The morphological analysis of the wildlife tracker consisted of six sub-functions: attaching the device to the arrow, keeping the device attached to the animal upon impact, indicating the location of the arrow attachment to the user device, transmitting and receiving the location to and from each device, and protecting the functional integrity of the device after being shot. The method of using a tether to attach the device to the arrow was chosen due to its simplicity and effectiveness. The tether allows for the device to open up enough to allow the arrow to travel through, while still remaining intact. Ideas such as utilizing a magnet, a spring-dowel pin, a clip, or an adhesive were considered for feasibility but left behind due to being less effective, more expensive, or more complex. To ensure that the device remained attached to the animal upon impact, the method of using a barbed hook was chosen over a pronged tip or a spring-activated clamp. Transceivers were used as a method to send and receive the location of the device after adhesion to the animal, which is obtained by GPS location. In combination with the GPS receiver to obtain a location and the transceivers to send and receive the location, a user device equipped with an LCD screen was used to display the current longitude and latitude of the device. In future iterations of this project, the method of sending the GPS data into a cellphone and using that information to display a location on an application such as Google Maps could be used, but this implementation did not have the time and resources to be accomplished in this iteration. It was decided that the most reasonable choice in protecting the device from being damaged after being shot from the arrow was to put all of the electronics into a waterproof enclosure. Analysis of each of the sub-functions allowed for the creation of carefully selected solutions.
Figure 1: Front of Center
Bow hunters typically experiment with the weight and length of arrows and broadheads in order to determine their preference. Some hunters prefer lighter faster arrows while others prefer heavier slowing ones with more impact. The majority of bow hunters, however, generally fall somewhere in between these two extremes. Arrows are either made of aluminum or carbon fiber between 27 inches and 34 inches long. Hunting arrows typically weigh 300 to 450 grains and broadheads weigh between 100 and 125 grains. Ballistic experts have developed a measurement called Front of Center (FOC) which is used in assisting to find the optimally balanced arrow. FOC is a ratio that compares the percentage from the middle of the arrow that the weight of the broadhead shifts the center of gravity (Figure 1). The websites Goldtip.com and ArcheryReport.com report that the optimal FOC ratio optimally lies between 7 to 15 percent, with the arrow being back-heavy at the lower limit and front-heavy at the upper limit. It is generally accepted that an arrow with a high FOC will be more accurate but tends to nosedive, while an arrow with a low FOC will not be as accurate but holds trajectory more easily. In order to ensure a tracking attachment does not alter the shot accuracy, the arrow, broadhead, and attachment must have an FOC within the preferred range.
The flight of hunting arrow cannot be easily modeled analytically due to the significant flex and elasticity of the shaft. During flight the center of the shaft can flex up to 3 inches from its resting position in either direction. This flexion changes the effective cross-sectional area and dissipates energy.
Figure 2: Sample arrow attachment geometries
The additional drag created by an attachment can be empirically calculated based on the additional weight of the attachment and the change in trajectory. Four dummy attachments were made, each in a different shape as shown in Figure 2 above. Tapered obround, obround, tapered, and streamlined (from left to right) were all tested. 125 grain practice tips were used with 450 grain carbon fiber arrows. The various geometries tested served as a comparison for deviation in arrow accuracy due to additional weight of the arrow and shape of the attachment. Equally important, an attempt was made to reduce the sound of the arrow with attachment during flight, as this could potentially startle a target animal and cause it to flee.
A hay target with a one inch by one inch bullseye was placed at a height of 20 meters in a near-level field. The experienced marksman used for this initial field testing wielded his own 65 pound Martin “MAG CAT” compound bow to shoot five baseline shots with a standard hunting arrow as well as five with each of the test specimens. The marksman was instructed to adjust his sights in order to hit bullseye with a standard arrow, and aim for the same spot each time without compensation for the addition of the attachments. The distance from the bullseye for each shot was measured with a scale using an X-Y coordinate grid, with the target at the origin.
Using the basic projectile motion and drag equations above, the drop in the shot due to the additional weight of the attachment can be easily calculated. This value is subtracted from the empirical data and the resulting drop distance can be attributed to the increase in drag from the attachment. Due to the similarity in weight of all four attachments being tested, the drop associated with weight can be cancelled out and the final shot locations can be evaluated with respect to one another in order to determine the effects of drag.
A solid prototype was printed with parts in both standard SLS and PolyJet plastic. A prototype 1/16 inch thick barbed hook was cut with a Waterjet machine and then sharpened using a belt sander. The barbed hook was placed into a slot in one side of the enclosure and secured with epoxy. An arrow with a standard four-blade broadhead was used in order to simulate a real hunting scenario. An added 45 degree bar was added to the bend in the hook to help reinforce it during impact. The setup of this test was nearly identical to that of the drag test in order to additionally evaluate the flight of the arrow with the prototype attachment. Due to the lack of strength associated with 3D printed plastics, this test was viewed as simply a full assessment of the attachment design, since the perceived chance of damage to the prototype was high. A piece of deer hide was placed over the hay target. Test shots were completed from 20 meters by our marksman both with and without the attachment. The measurements taken included longitudinal accuracy, drop, penetration depth, and barbed hook holding force.
The electrical components were all put onto one side of the arrow attachment, allowing the other side to exclusively hold the barbed hook as a way to separate the force of impact from the delicate electronics. The solid fixture that holds the barbed hook in place will be able to withstand higher impact forces compared to the earlier design where it was accompanied by half the electrical components with the idea of distributing the weight throughout the attachment. It was later deemed, however, that the difference in weight between both sides is negligible with this design.
Initially the major electrical options were as allows: transponders, GPS receivers, cellphone signals, and radio frequencies. Transponders were expelled due to the fact that Game Vector, a competition benchmark, already utilized this technology. Rather than piggy-backing off their concept, the decision was made to adventure into a more complex and feasible design. Cellphone signals were later expunged when possible forest coverage and wooded areas posed high possibilities of causing attenuation and possible loss to cellular signals. GPS in combination with RF signal transmission was determined to be the most viable option. GPS would allow for precise tracking and the most accurate final destination of the wounded animal. This facilitated the potential to rise above Game Vector and allow for a more precise method when locating the injured game.
To determine the location of the device after it had been shot from arrow, a GPS receiver was used. A GPS receiver is a passive device which, once powered on, begins searching for satellite signals. Once three satellites have obtained a connection with the receiver, its position can be determined. This method is known as trilateration. Each satellite has an approximate radius the device can be in,and when at least three of these approximate radii intersect, the location can be obtained. GPS receivers are now significantly smaller and cheaper than the used to be just a few years ago, making them ideal for this application. The GPS receiver used (GM series, GNSS model) was provided by Linx Technologies free of charge and provided the location in the form of NMEA UART (National Marine Electronics Association Universal Asynchronous Receiver/Transmitter) serial data. The size of the receiver was only .512 x .591 x .087 inches.
Transceivers were a staple in this project. They were the primary means to send and receive data amongst the system. The main constraint was not with power consumption, but rather the sizing the componentry as well as operational range. Linx Technologies offered RF module technological solutions to these needs in the form of the TRX-915-R250 module. The sizing of the components met the engineering requirements, and the range of operation (up to 4km) was more than sufficient for the goal specification. Furthermore, thanks to the developmental and evaluation modules the technology came test-ready and proved to be feasible for the project.
Antenna design was kept as simple as possible for proof-of-concept due to the complicated mathematics and sensitivities regarding antennas. For the arrow attachment, the GPS antenna used with the development board sufficed as a way to obtain satellite data. Since the electronic technologies used came from Linx, the receive and transmit antennas for each transceiver were also ordered from Linx to avoid the risk of being unable to interface efficiently with antennas from another manufacturer. These consisted of quarter-wave dipoles at the proper length to resonate well at the designed frequency of 915MHz (80mm). The receive antenna was ordered to have a 90 degree angle bend to ensure it would fit in the user device enclosure and stick out.
The batteries used served importance in ensuring operation of all electronics. For the GPS receiver and two transceivers, a set of polymer lithium ion batteries from SparkFun (3.7V, 100mAh) were utilized as a power source. In the ideal situation where weight and size are minimized, these batteries would work well. They were used because of their low weight, compact shape, satisfactory electrical requirements, the ability to be recharged, and ease of implementation. These contained a power wire and ground wire extending from the main battery part to facilitate testing and flexible usage, as well as placement with respect to other parts.