BitTag

The BitTag is a novel data logger capable of collecting continuous activity data for 360 days – depending upon battery size. The operational concept is simple – BitTag contains an accelerometer that detects movement and, based upon the movement dynamics, determines if the subject animal is active. Each second BitTag generates a single bit of information – 1 if the animal is active, 0 if it is it is not. These bits are aggregated (counted) over a measurement period 1 second to 5 minutes; at the end of each aggregation period the counts are stored in non-volatile memory. The choice of aggregation period depends upon the length of the experiment and is constrained by the amount of available storage – 248 hours for raw data to 8660 hours for 5 minute aggregation. In section ?? we present the design of a new BitTag with external memory capable of storing 8738 hours of data aggregated over 15 seconds.

BitTag has been used in a variety of experiments with both captive and free animals including monitoring circadian rhythms, determining changes in activity around migration and egg laying, and when migrating animals fly.

The key component in the BitTag is a extremely low energy accelerometer – the adxl362. This accelerometer has special activity detection hardware that samples current acceleration along 3-axes at 6hz. When an animal is completely still there is 1g acceleration towards the earth due to gravity. In activity detection mode, the adxl362 tracks acceleration – when the change in acceleration is greater than a configured threshold, the device signals the BitTag processor that the animal is active. When the acceleration remains within a configured range for a configured time, the BitTag signals that the animal is inactive.

The detection algorithm is illustrated to the right. Consider a tag with two states – inactive and active. An inactive tag (a) remains inactive until it experiences acceleration greater than the “active threshold” (b). An active tag remains active until it remains within a configured acceleration range for a configured time (d). Notice that whenever an active tag exceeds the configured threshold, the center of the range moves (c-d). This tracking hardware is remarkably low energy (300nA), but very effective at tracking bird activity because flying naturally results in high acceleration changes.

The physical design of tags is just a small part of the system design problem. To be useful, the tags require a support “ecosystem” that enables tag configuration, battery charging, and data recovery. This ecosystem includes both software (tag firmware, configuration software, data processing), and support hardware. For example, we have developed a “standard” hardware base to which tags are attached for configuration and charging. Variations in battery chemistry necessitate (small) hardware differences in the bases – we use two distinct types of batteries NiMn and LiPo. Furtunately, these hardware differences are opaque to the system software.

Of course different tags will have variations in geometry, thus our base architecture includes a 3D printed adapter that is tag specific. The adjacent image illustrates one such adapter along with the hand tools used to finish the commercially printed component. The tags carry a small array of “test points” (covered with insulation during flight) through which all base/tag communication occurs. The mechanical issues of reliably connecting to the tags are considerable and are discussed in [link].

It is worth noting that we face a scale problem – supporting experiments with dozens of tags is considerably different than configuring and testing single tags. For example, consider that our partners need to prepare groups of 20-100 tags for flight simultaneously. Simply charging the tag batteries takes many hours (e.g. overnight), thus we have also developed chargers that can be “ganged” together and connected by a single USB connection.

The adjacent image illustrates three such chargers (only one USB connector is used per ‘gang’). These chargers can be built relatively inexpensively (under $50/each in small quantities).