1. 20 devices were purchased but it seems that even after extensive charging, the two sensors in blue cases may not work, possibly as a result of malfunctioning batteries.
Description
AX3 is a data logger. It features a state of the art MEMS 3-axis accelerometer and Flash based on-board memory. The device incorporates a real time quartz clock, temperature sensor, and light sensor. The device is ideal for collecting longitudinal movement data. They are worn on the wrist, and provide a compact and ecological method for motion capture, especially to look at acceleration data. The accelerometer has configurable sample rates, adjustable sensitivity and a low power mode. The sample rate affects both the maximum sampling period and the battery life. Higher sample rates reduce the amount of time the device can record for and use extra battery power during recording. Under normal recording conditions, a fully charged battery can last for 14 days, although the longest recent recording time on the lab devices went 92 hours (2021/10). The accelerometer has a low power mode to reduce the current consumption and increase battery life, but this setting increases the amount of noise present on the accelerometer output and may not be suitable for high precision applications.
A. How to set up and use
Information on how to use the Axivity sensors is available in several locations:
- AX3 Product Description
- Contains purchasing information and technical specifications
- AX3 Downloads Page
- Contains datasheet, firmware, drivers, software, sample data, and user guide
- AX3 GUI Wiki on Github
- Contains download to OmGui and descriptions
- AX3 User Guide
- Contains extensive information on usage including what the LED lights mean
During the coronavirus pandemic, these devices were not charged for about two years. When beginning charging for usage with MusicLab Copenhagen, many displayed the error message "lost track of timing" and a warning that these devices should be charged once every 3 months to maintain battery life. After testing the 20 sensors it seems that only the 18 in the black cases, but not the two in the blue cases, have functional batteries.
The AX3s come with a charging dock that can connect up to 10 at a time; however, it seems that this is not the best way to connect them to the computer for arming recording and data extraction because some devices are not identified by the computer when they are attached to the dock. Additionally, the devices that are registered in the computer do not display their true battery charge (with values that are much lower than those displayed when devices are connected individually). Therefore, it is recommended to only use the connector dock for charging, but to connect the AX3s individually when arming for recording, checking battery life, or extracting data. The user manual states, "It is always recommended to only deploy devices with a battery charged over 85%".
Devices can be armed for recording using the OmGui, clicking record, and configuring them based on your desired settings (e.g. sampling rate, recording start). The devices can either begin recording immediately upon disconnect from the computer or at a specific interval.
Devices that are not recording have a very faint blue flash of light. Devices that are recording will either not flash at all or when recording is configured with the "Flash during recording" option, they will flash with a bright green light.
While devices seem to acquire the same computer time when recording is started, it is still important to calibrate or time align the devices, as is recommended on the website here (search for section "Data Markers"). For the MusicLab Copenhagen experiment in which motion was recorded from most participants using the MusicLab App, audience with AX3s were instructed to tap the sensor in the same way audience members were tapping their smartphones.
B. Software and Associated computers, licenses
The Axivity sensors work well with OmGui (Open Movement Graphical User Interface) which is open source and available on Github here.
C. Exporting Data
Downloading from the devices produces a .cwa file (Continuous Wave Accelerometer Data). This file can be exported to raw CSV or resampled CSV or WAV. To learn more about analysis, stay tuned to updates from the MusicLab Copenhagen project.
D. Use cases
These devices have previously been used for group studies about emotional engagement pertaining to perceived musical intensity peaks in electronic dance music (Solberg & Jensenius, 2016). Some sensors (n = 7) were also used at MusicLab Copenhagen to measure a subset of the audience who were unable to use their mobile phones to measure motion. Specifically they were inserted in the pouch of the phone holder and worn on participants' chests.
E. Testing time alignment
To test the timing alignment across devices, 3 devices were placed in the same RITMO phone holder pouch, worn around the neck and placed on the chest. Thus the three devices were tapped at the same time. Using ginput in Python to select the start of the last tap in each of the signals provided timepoints that were at most 38 ms apart. These differences could be caused by human error or it could be that there are slight misalignments of the signal. If greater precision in time alignment is necessary, further testing should be conducted.
F. Drift
From Alexander: "I recall Kristian Nymoen checking the drift in hour-long recordings. He found that the clocks were running slightly differently, which made them
drift up to one second per hour."
From Finn: Synchronisation taps were recorded on six AX3 sensors during the MusicLab Copenhagen experiment. All sensors were initialised against the same computer clock a few hours before the experiment. On the first synch cue, they differed by as much as 1.1 s (std 0.50). On the second synch cue, 96 minutes later, they lagged by a further 100 ms on average, but variance had not increased substantially between them. For experiments where sychronisation over long periods is particularly important, using a synchronisation protocol before and after measurement would be helpful.
G. Using Multiple AX3s
From the NM Standstill 2024, we found that it is possible to simultaneously plug in at least 12 AX3s (the number we had available). This was done with the help of two powered USB hubs and several micro-USB cables. Some micro-USB cables do not transfer data and can only charge the device, so if an AX3 doesn't show up in the OmGui, try with another cable.
Select multiple, hit Record, set Parameters/Settings. Now all of them will say "interval...". You MUST unplug the AX3s for them to start recording at your defined time interval. Before the start of each participant, tap thrice for a sync cue.
The OmGui lets you download multiple files at once. You can do this by selecting all and clicking on the download button.