(Complete list of all blog entries in this series)

Leeds university asked for a short introduction and a short manual. Instead of just handing this over directly, I thought it a good idea to add this here as reference. It actually serves as a good starting point to the rest of the series... So here we go.

Skier impact monitor

A short introduction

Hendrik Lipka

Project introduction

The 'skier impact monitor' (or short 'SkiMon' as I will call it further on) is a small device designed to measure impact forces happening during downhill skiing. I came to this idea when my son starting with skiing school two years ago (being a cautious parent). But its not limited to skiing, it should work for all kind of sports where such impacts might happen. The only prerequisite I see is that the device needs to be mounted somewhere on the head, so a helmet is needed (so I will test it when doing bicycle tours with my son, but soccer is out of the question).

SkiMon collects data with a high frequency (1600 samples per second), but aggregates them into data with a seconds resolution. It collects average and maximum acceleration force (regardless of direction), and calculates the head injury criterion (HIC) values (ans stores the maximal calculated value).

SkiMon works in conjunction with a Android phone app, which shows the current values as well as the logged values for up to 1 hour.

Prerequisites

SkiMon comes with two devices: the monitor unit itself, and the wireless charging base. The charging base needs a USB power supply that is capabler of delivering up to 1A (it might work with less current, but I never tested that).

The provided Android application has been developed and tested solely on a Samsung Galaxy S4 mini. It should work on all Android phones with Android 4.4 or higher that come with Bluetooth Low Energy, though I did not test this.

Start up guide

SkiMon can run in one of three modes:

• sleep mode (not doing anything), conserving battery power
• measurement mode (collecting data, but no transmission)
• connection mode (collecting data, being connected to the phone)

Waking SkiMon up

The device has been put into sleep mode before sending it away. It can be put into measurement mode by pressing the button at the top side (use a ball pen or something like it - it is recessed to avoid touching it by accident). The LED will start blinking, indicating that it goes into connection mode and waits for a phone to connect. When not phone can be found after a minute, the LED will stop blinking and SkiMon goes into measurement mode. The same happens when the phone gets disconnected.

Pressing the button for about two seconds puts SkiMon back into sleep mode, regardless of being connected to a phone or not. A short button press starts the connection mode again, and SkiMon will wait for a phone to connect again (indicated by the blinking LED).

App usage

To connect the phone to SkiMon:

• start the SkiMon on the phone, after activating Bluetooth (though the app will ask for permission to do so by itself when needed)
• start SkiMon connection by pressing the button on the device
• press the „Connect“ button in the app
• the app will start scanning for devices, and connect to SkiMon automatically when its found

When connected, the app will show the average and maximum acceleration measured in the last second. Swipe the page to see all the graphs showing data for the last minute (in seconds resolution) and for the last hour (in minutes resolution). Note that the graphs for alerts are not functional yet, so they stay empty.

Charging SkiMon

When in connection or measurement mode, the battery should last for about 16 hours, maybe longer (depending on the actual capacity of the battery up to 24 hours). It can be charged wirelessly, so there is no physical connection needed (allowing for sealing the device in its final version)

• connect the charger base station to a USB power supply (should deliver 1A or so, though I was able to use my computers USB port too)
• the green PCB in the base station must be on the top
• put the SkiMon device right on the middle of the base station (with its button being on top)
• when charging starts, the small red LED at the side lights up
• then the LED goes off again, charging is finished

When the battery is completely empty, the charging might take up to 24 hours.

Current limitations & future development

The biggest limitation right now is the SkiMon enclosure. Since I consider the current SkiMon version as a prototype, no special care has been taken to make the enclosure robust. Firmware development requires access to the PSoC4 BLE on the main PCB, so a debugging connector needs to be accessible.

So the case was not chosen with withstanding high energy impact in mind, but mainly for size. So it probably will break during the impact testing.

An easy solution for that would be to seal / pot everything in a clear resin (apart from the button). But this would make everything unusable later on, so I stayed away from it (the resin can be dissolved, but this would also destroy the electronic parts)

Another limitation is that the software is not final. The logging itself is finished, and can be used fully to evaluate the severity of impacts. But there is no altering yet that would allow to react to hard impacts as soon as possible. So that would be the next addition.

I already have designed a PCB that can hold the complete circuitry in a 35x25mm size. That would allow to have a final version in an enclosure of 25×35x10mm, fully encapsulated in a clear resin. This version also features a CapSense button sensor instead of a physical button, so SkiMon can be sealed completely.