Ski Bums
Project undertaken in course year 2022-23 with the Stanford School of Medicine
Project Goal
This project aims to show how an additional safety system could augment existing ski binding technology by releasing the binding when the skier is in a position known to result in an ACL tear. The project seeks to show how a micocontroller, sensors and an electronically-activated release mechanism can combine to address ACL tears, a weak point in skiing safety.
Project Motivation
These serious ACL tears are largely preventable with a system that recognizes the two most common Mechanisms of Injury (MOIs) leading to an ACL tear
Prevention of 50% of ACL tears relieves strained mountain hospitals and Ski Patrol
Increased safety removes mental stress when skiing, resulting in a more fun experience
Background
The Anterior Cruciate Ligament (ACL) requires surgery when torn and results in a significant ($10,000+) and mental burden (reduced mobility, uncertainty about return activity). 55% of these injuries results from just two different Mechanisms of Injury
Two primary Mechanisms of Injury are Phantom Foot and Valgus External Rotation. Phantom foot results when the skis are crossed and the skier falls backward, and Valgus External Rotation results when the skis open up and the skier falls forward.
High Priority Requirements
System releases boot no more than 50 ms after first point of injury situation
Sensors detect angle and angular velocity thresholds to release binding
Electronics work down to -10F and meet IP66 water proofing
System works reliably so that binding will release even with lack of power
Bindings conform to ISO 5355 (specifies requirements tests for bindings)
Bindings release according to DIN standards
Ethical Considerations
Increasing cost of safety: total system cost is minor compared to lift tickets, ski gear
Making sure that the system is tested for skiers of all demographics and abilities. Modular aystem adapts to skier, eliminating potential for exclusion of any group
Solution
Flex sensors and IMU's on the skier's knees communicate with a micontroller. When the micocontroller senses the onset of either of the two critical MOIs, it actuates a solenoid, which catalyzes the process of mechanically releasing the boot from the binding
System hardware
Flex sensors are carried on sleeves over each knee, and IMUs are on the boot. These are wired to a control pouch on the hip. Wiring connects to bindings which would be used for release.
Side view of binding
View on the top shows binding in the locked position. Note the middle pin on the metal casing - that is holding the release mechanism.
In the lower image, the middle pin has been removed and the binding released. This is a concept model for how the binding might work, the pin to be removed by a motor or servo.
Approach
The two Mechanisms of Injury (MOIs) are Phantom Foot when the skis are crossed and the skier falls backward, and Valgus External Rotation, when the skis open up and the skier falls forward. The goal is to be able to measure angles between the skis and the bend angle of the knee to help determine if the skis are in a dangerous position while the skier falls forward or back.
Defining danger zones when to release
The red zones show where the binding needs to be released in all cases, the yellow zones show where to release depending on the angular velocity of the ski, and the green zones represent safe zones. While the specific threshold values are not known, this project looks to be able to measure angular positional and velocity differences between the skis
Test setup for Flex sensor angle measurement
The Flex Sensor is intended to measure the bend angle of the knee. The iPhone acted as a leveling measurement tool and was strapped on the outside of the knee. These level values were then compared the flex sensor's readings from the microcontroller.
Test setup for IMU angle measurement
Setup of the IMU sensor accuracy and placement experiment, with the IMUs strapped on the mid-foot in this particular trial. The experiment was run on flat ground and with a sloped incline; and then the IMU was also placed on the ski and at the toe for additional testing.
Ready to go
Student wearing the full system - ready to ski
Flex sensor angle measurement capability
Line graph showing correlation and trend between the leveling tool readout (iPhone) and the flex sensor readout. The slope value was 0.904 while the R-sq is 0.95, indicating a good fit.
IMU angle measuring ski twist
With a compass mounted to the ski while standing on Flat Ground, angular measurement of the compass vs the IMU sensor readout where the IMU is placed at the middle of the boot. The slope value of 0.992 with an R-sq of 1.0 indicates the IMU sensor is adequately measuring the angle as the ski twists.
Tests placing the IMU at the toe, and on the ski, gave similar results indicating that the IMU can be placed in any of those locations with acceptable results.
Other testing conducted
Latency test showing 18.4ms delay (STD of 0.95) between the IMU sensing the leg movement until the Arduino-based sends an output signal to the binding. That results in 29ms available for the mechanical release to function
Wire strength
EMF interference
External Release forces
Student team
Future Work
Binding design
Electrical interface between microcontroller and binding release
Packaging - reduce size and snowproof
Gather field data to determine thresholds for release