Embedded Files
The Resistive Torque Measurement Apparatus can be assembled at a testing site and placed at various locations, where the intruder is then manually penetrated into and rotated in GM. The apparatus utilizes a mode switch to alternate between setup and data collection modes at each testing location. A high-level overview of the electronics package of the RTMA is described in the Programming Section, including the primary torque and angular measurement sensors, various environmental sensors, user interaction peripherals, and main computing and power distribution components.
Structural Components
Structural Components
The structural components can be broken down into four main elements: collapsible main legs, detachable sand feet, bushing hubs, and intruder bar (see component breakdown below). Material considerations were made with corrosion and sand resistance in mind, as well as durability. The apparatus frame is used to support and mount the electronics, sensors, and manual intruder bar. A tripod structure was selected to provide stability during setup and operation in the uneven beach terrain. The main legs and bottom bushing hub supports, are made from a lightweight aluminum square tubing to improve portability.
Collapsible Main Legs
Collapsible Main Legs
The bottom bushing hub supports are collapsible with the removal of a wing nut and thumb screw and the loosening of the wing nut in the center of the flat bracket, to allow folding against the main legs. This feature improves the transport of the apparatus since the outward leg splay characteristic of a tripod structure limits its compactness. The removed wing nut and thumb screw can be re-secured to the bottom hub support after it is folded into place or attached to the clear acrylic storage holster mounted on the electronics box.
Detachable Sand Feet
Detachable Sand Feet
The detachable sand foot design allows for installation of the boot-like component to each of the tripod legs upon apparatus setup. The Ring-Grip Quick-Release Pin enables the locking of the leg into the boot without additional tooling. The ball-bearing at the tip of the pin applies the appropriate amount of pressure to prevent accidental release of the pin from the sand foot during operation. The shoulder feature prevented over insertion of the pin during assembly. Forged steel stakes are used to secure the sand feet to the ground and maintain the desired position of the apparatus relative to the intrusion site. The stakes have an elbow eyelet that provides a tie point for a stainless steel wire which connects the stake to the main leg, preventing loss of the small component. The quick-release pin is also secured to the frame in this manner.
Bushing Hubs
Bushing Hubs
The final design utilizes two bushing hubs, upper and lower, supported by the three tripod legs. Each bushing hub is comprised of a press-fit bushing housing, two plates, and three hub supports. The bushing housing contains a low friction thermoplastic bushing, which supports and aligns the vertical movement of the intruder shaft. The lower bushing hub connects to the lower hub supports on the tripod legs. The upper bushing hub connects to the top of the main tripod legs and houses the depth locking and 3D printed angular measurement systems.
Intruder Bar
Intruder Bar
The intruder bar is a long, thin stainless steel component connected to the end effector, modeling the wingspan of the worm, and rotated manually by a handle bar situated at the top. Its primary function is to penetrate sand up to a certain depth ranging from 0 mm - 254 mm (10 in) and rotate under applied torque to characterize the resistive forces it experiences. The design of the intruder was intended to make it detachable, lightweight, and durable for repeated testing at the beach. The intruder is attached to the intruder bar with machine screws and locknuts. It is designed to be detachable for use with potential future geometries and sizes. The handle bar length, at 508 mm, was determined based on force distribution and biomechanics, aiding in the ergonomic design of the manual intruder bar.
Torque Measurement System
Torque Measurement System
The torque measurement system enables the transmission and measurement of torque from the operator to the intruder end-effector during manual rotation through granular media. It consists of the operator handle bar, upper and lower intruder bars, custom torque sensor couplers, a 50 Nm ATO Reaction Torque Sensor, and the intruder end-effector. The handle bar is welded to the upper intruder bar and connects to the upper torque sensor coupler via a depth locking cuff. Both the upper and lower couplers are secured to the torque sensor using, creating a rigid mechanical chain between the operator and the intruder tip. The handle bar length was selected based on ergonomic considerations to improve comfort and usability. Torque data is collected using a sensor and amplifier system, which allows the apparatus to record the torque experienced by the intruder during operation. This data is used to evaluate the resistance encountered as the intruder moves through granular media.
Angular Measurement System
Angular Measurement System
The angular measurement system employs a 30 tooth XL pulley mounted concentrically with the intruder shaft onto a steel turntable bearing, which spins freely from the intruder shaft. A non-concentric Same Sky AMT13 incremental capacitive encoder connects to this pulley via a second 30 tooth XL pulley and timing belt. A custom axial dog clutch system using semicircular ‘shelves’ to engage rotationally when pressed together is used to actuate the pulley system. The top half of the dog clutch is rigidly connected to a depth setting collar on the intruder shaft, allowing for operators to engage with the bottom half of the dog clutch mounted onto the concentric pulley once the desired penetration depth is reached.
Control and Data Logging System
Control and Data Logging System
The control and data collection system was designed for the easy collection, storage, and transmission of testing data of the apparatus at multiple locations during an extended testing period. The system is driven by an Arduino Mega 2560 REV3 microcontroller with an additional Adafruit SD Card Shield for onboard storage and Adafruit Bluefruit LE UART Friend for wireless communication. Toggle switches are used for powering the system as well as swapping between setup and collection modes. A 10kΩ potentiometer is used to record an analog input for the intruder depth, which is displayed on a 16x2 LCD display with I2C communication. The LCD screen is additionally used to indicate different modes of operation of the apparatus and display live measurements of measured torque while in collection mode. The system is powered by an Anker 325 USB power bank, providing more than 60 hours of use for the system. All electronics are contained within a sealed PLA 3D printed enclosure to deter water and dust ingress.
Summary of Performance Results
Summary of Performance Results
Integrated testing of the completed apparatus was first performed with one measurement taken in each of the following sand conditions at La Jolla Shores beach:
Dry sand: no interaction with the ocean in multiple hours
Damp (compacted) sand: little interaction with the ocean in multiple hours, denser and more compacted than dry sand
Wet sand: recently washed over, but allowed to desaturate before each test
Submerged sand: fully saturated, with legs and intruder of apparatus fully submerged throughout the duration of the test
In each sand type, the depth locking cuff was unlocked and the intruder penetrated as deep as possible by the operator. The maximum depth reached in each sand type was recorded and the depth locking cuff was locked at that depth. Two rotations were performed for each test location.
Soil moisture, maximum penetration depth, and average and peak torque were measured in each sand type. These data supported the prediction that measured torque would increase in wet and compact sand conditions.
During these tests, the originally produced aluminum intruder was bent during the second test, and thus following tests reflect a lower average and peak torques. This also suggested that this intruder material/geometry were not operable for the final design of the apparatus.
A second stainless steel intruder with the same geometry as the first was manufactured and mounted on the RTMA and further tests were performed. Dry, damp, and wet sand were tested at 1, 2, and 3 inches of penetration. Penetration to these depths was easily feasible for the operator. Again, at least two rotations at each depth were performed.
This data was then processed via MATLAB and plots were generated. Each figure shows torque vs. both time and angle, along with showing the angle vs. time, which shows how quickly the apparatus is rotated. This allows for correction in analysis for torque dips that occur due to apparatus stoppage. The legend indicates the time of test (for identification), and the depth and soil moisture for each test for comparison.
Dry Sand
Wet Sand
Submerged Sand
The completed RTMA was able to produce useful measurements at various depths across various beach sand conditions. Torque, angular position, and various environmental data were measured and stored as CSV data files, allowing for analysis and comparison of the properties of granular media. The RTMA fulfilled all functional requirements of the project, with a full list of capabilities listed below:
Stability and rigidity in various beach sand conditions, including submersion in up to 150 mm (6 in) of water
Convenient and quick collapsibility and portability: <10 minutes to setup and break down, and <5 minutes to perform each test
Precise torque measurement up to 50 Nm, <0.1 Nm sensitivity
Precise angle measurement, <0.1° sensitivity
Environmental sensing capability, measuring GPS coordinates, ambient temperature and humidity, and soil moisture
"Push-of-a-button" testing system, allowing for convenient use by only two operators
Rapid data transfer and analysis, with each test file storing all environmental data along with torque and angle measurements taken at 50 Hz
Purpose built data analyzer, allowing for convenient data comparison and visualization
The results found by the RTMA suggest interesting trends regarding how torque varies with sand conditions, depth, angle, and time. Research using the RTMA will continue under the Gravish Lab to determine resistance trends in various GM conditions, and what they suggest about the future of bio-inspired robotics.
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