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ax3-faq.md

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AX3/AX6 FAQ

Getting Started

Troubleshooting

Maximum recording duration

Approximate maximum recording duration from a single full battery charge. A typical configuration may be 100 Hz ±8 g. The accelerometer range (±g) and gyroscope range (°/s) do not affect the estimate. The auxiliary data channels (device temperature and light level indicator) are sampled at a lower rate in all modes.

Device Configuration Sample Capacity 6.25 Hz 12.5 Hz 25 Hz 50 Hz 100 Hz 200 Hz 400 Hz 800 Hz 1600 Hz 3200 Hz
AX3 packed^ 119009040 samples - ~65d (110d) ~48d (55d) 27d* 13.5d* 6.8d 3.4d 41.3h 20.6h 10.3h
AX3 unpacked^ 79339360 samples ~65d? (146d) ~65d? (73d) 36.5d 18d 9d* 4.5d 2.3d 27.5h 13.7h 6.8h
AX6 accel. only 158714880 samples ~244d? (293d) 146d 73d 36.5d 18.3d 9.1d 4.5d 55.1h 27.5h -
AX6 accel. + gyro. 79357440 samples - - 7-9d (36.7d) 7-9d (18d) 7-9d* 4.5d 2.3d 27.5h 13.7h -
  • d days (<= 400 Hz)
  • h hours (>= 800 Hz)
  • - an unsupported configuration.
  • ? an extrapolated best-guess
  • ~ more uncertainty in the value (charge-limited, and device-dependent)
  • () where a recording is likely to be battery-limited, times in brackets are the storage capacity (which could only be reached if recharged).
  • * where the storage- and battery-limits are similar, which is reached first may be device-dependent.
  • ^ AX3 packed data uses the sensor in a 10-bits per axis mode. AX3 unpacked uses the sensor at full resolution: 10-/11-/12-/13-bits per-axis depending on 2/4/8/16 g range (stored as 16-bit values). AX6 uses unpacked mode, stored as 16-bits per-axis, regardless of the range.

Sensor Data Characteristics

The AX6 and AX3 have the same physical dimensions, but the AX6 has more memory and battery capacity, and a different underlying sensor that allows it to (optionally) record gyroscope data.

Characteristic AX3 AX6
Sample Rate 6.25*/12.5/25/50/100/200/400/800/1600/3200 Hz 6.25*/12.5*/25/50/100/200/400/800/1600 Hz
Accelerometer Range ±2/4/8/16 g ±2/4/8/16 g
Gyroscope Range none 125/250/500/1000/2000 °/s
Underlying sensing device Analog Devices ADXL345 Bosch BMI160
Samples stored per 512 byte storage block Packed (10-bit mode): 120; Unpacked (full resolution): 80*. Accelerometer-only: 80*; Accelerometer+Gyroscope: 40.
Notes * 'Unpacked' recording mode only. * With gyroscope off.

Time and sampling

Time Zone and DST

The AX devices take the configuring device's local time at the time of configuration, and the time and date continue from there for any subsequently recorded data. They cannot adjust for time-zone changes or daylight savings, as they will not know the current country or locally-defined time zone or daylight savings rules, and also as that could lead to duplicate or skipped hours of the day. As long as you know the configuration time zone (the last configuration time is recorded in the .CWA data file), you could choose to later reinterpret these times in another time zone (including applying any DST change), but you would have to choose what to do with any duplicate or skipped hours of the day.

Sample timestamps

The AX devices record the raw output from an underlying movement sensor, configured to output at the rate you choose. This sensor's output rate will vary slightly compared to the onboard (and more accurate) real-time clock. Particular samples (on average, one every 40/80/120 samples) will be timestamped from this more accurate clock, to allow later reconstruction of timestamps for each sample. Epoch-based analysis methods can use the raw data directly, while algorithms requiring an exactly fixed rate can use a "resampling" loader to resample the data to a fixed interval. See next section for information on synchronizing between multiple devices.

Synchronizing data between devices, or with other devices

The AX devices have an internal real-time clock to keep track of time. When the device is configured, the internal clock is set to the system time of the configuring computer. This time is set to the nearest second, and the operation itself may be subject to some jitter (e.g. from the operating system performing other actions). Afterwards, as with any clock, it will be subject to clock drift, potentially of the order of around ±2 seconds per day.

There is no external communication while the AX devices are recording, so they cannot directly have their clocks set or read during normal use. However, depending on your application, you may have some options, including:

  • For a single sensor over, say, a week or so, this clock drift rate may be acceptable to directly combine with some external sensors, such as GPS or a mobile phone (assuming the configuring computer had a similar time when configuring).

  • If you are placing multiple accelerometers on a single moving body over a long period of time (e.g. a person), then there is some software that synchronizes the signal from devices that are likely to see a similar movement: timesync.

  • Where the setup/access allows (e.g. a lab-based recording, or one with frequent points of contact), you can introduce a "marker" -- a specific movement signal at one or more points (e.g. vigorous shaking before and after a session) that has its time externally recorded. For a lab-based session, it might be appropriate to video record the session in a way that captures the shaking times directly. It might be useful to introduce an external clock on a screen, e.g. this page: Time Sync Clock -- on some supported phones/browsers (e.g. Chrome browser on Android), you can hold the phone against the device and tap-and-hold the screen to introduce an optical and vibration marker/pattern for the time.

Event markers

The AX devices are specifically designed to avoid any user-facing interface, to minimise imapact on measurements. You can use external logging mechanisms to record events if you have a roughly-synchronized clock (see above).

For example, you can use the Time Sync Clock page to create event markers by tapping the screen, or clicking a mouse, or pressing keys on a keyboard, then download or clear the log of event markers with the buttons at the top of the page. The event log records the time and event type (e.g. which key was pressed), and can be used in conjunction with the recorded data.

Usage environments

The AX enclosure is designed for IP68 rated water resistance -- not technically "waterproof" (i.e. not ATM rated) as it is a static measure, and not dynamic. It was designed so that it can be used for daily activities, which includes washing and casual swimming. Importantly, not continuous immersion, substantial depth, extreme/saltwater/watersports activities, sauna, etc. Additional care should be taken to avoid impacts while immersed. The device must be removed and not used if there are any problems or indications of damage. The USB socket must be free of debris and completely dry before connection.

Technical Details