Implantable Inertial Sensor (A3035)

© 2020 Kevan Hashemi, Open Source Instruments Inc.

Contents

Description
Versions
Design
Set-Up
Battery Capacity
Modifications
Development

Description

[02-APR-20] The Implantable Intertial Sensor (A3035) is a wireless accelerometer and gyroscope that may be implanted beneath the skin of a small mammal or attached to the exterior of a fish. As we describe in our IIS Technical Proposal, the design of the IIS is motivated by an experiment to measure the movement of freely-swimming fish in a water tank. The IIS is equipped with a 13-mm helical antenna and two 5-mm helical leads we use to recharge the battery. Antenna and leads are insulated with silicone. The circuit itself is encapsulated in epoxy and silicone. To attach the device to a fish, we wrap it in a custom-made rubber sleave, which we can super-glue to the fish's scales or skin. We recharge the A3035's battery with a Batter Charger (A3033E).


Figure: A3035AV1 Circuit Board with ML920 Battery. The circuit is not yet equipped with antenna and charging leads. The long neck and programming extension we cut off during encapsulation.

Versions

The table below lists the existing versions of the Implantable Inertial Sensor (IIS).

Version Battery Volume
(ml)
Operating
Life (hrs)
Data Transmitted Comments
A3035A ML920 11 mA-hr 0.9 1.0 8 Acceleration xyz 16-bit 128 SPS
Gyroscope xyz 16-bit 128 SPS
Table: Versions of the Implantable Stimulator-Transponder (A3036).

The A3035B provides 128 sixteen-bit samples of each of x, y, and z components of acceleration, and x, y, and z components of rotation. The total number of samples per second is 6 × 128 = 768 SPS. Total quiescent current we expect to be around 1.2 mA, so we anticipate operating life of 8 hours for one charge of the 11 mA-hr battery.

Design

The IIS is managed by a field-programmable gate array (FPGA) in a 2.5-mm square package, LCMX02-1200ZE. This device provides both volatile and non-volatile memory as well as thousands of programmable logic gates. It is capable of implementing arbitrarily-complex stimuli in response to a single command.

S3035A_1: IIS Version A Schematic.
A3035AV1 BOM: Bill of materials for the A3035A.
A303501A.zip: PCB for A3035A, Gerber files and drawing.
A303501A_Top: Rendering of top side of A303501B circuit board.
A303501A_Bottom: Rendering of bottom side of A303501B circuit board.
Code: Logic Programs and Test Scripts.
LCMXO2-1200ZE: The programmable logic chip data sheet.
WLCSP-25: The 2.5-mm square BGA logic chip package.
BMA423: Micro-power accelerometer in 2-mm square package.
BMG250: Low-power gyroscope in 2.5 mm × 3.0 mm package.

Set-Up

Battery Capacity

[04-JUN-20] The A3035A is powered by an ML920 manganese-lithium rechargeable battery. This battery may be recharged through the A3035A's two recharge leads, which protrude a short distance from the device, within the loop of the antenna.


Figure: Example Discharge Curves for Manganese-Lithium Batteries. We charge the batteries through a 400-Ω resistor using various voltages 2.9-3.3 V. The ML621 is 6.8 mm dia, 2.1 mm thick, 5.0 mA-hr. The ML920 is 9.5 mm dia, 2.0 mm thick, 11 mA-hr.

We recharge the ML920 through two internal silicon diodes using an A3033B battery charger. Charging to 80% capacity will be complete in twelve hours, but 100% capacity requires a forty-eight hour charge.

Modifications

The A3035AV1 appears to need no modifications, but does require a booster power supply to start up.

Development

[03-APR-20] We receive 25,000 custom-made rubber bands that fit around our smaller-sized implantable devices, as shown below.


Figure: Rubber Sleave Around Various IIS-Sized Devices.

This rubber adheres well to super-glue. We can glue two bands together instantly with super-glue gel. With the rubber sleave around the device, we can glue it to the exterior of a fish, and pull it off later. We throw away the rubber sleave, recharge the battery, and use another sleave for the next experiment.

[20-AUG-20] We have 100 of A303501A in 10 panels of 10.

[13-OCT-20] We have 3 of A3035AV1 first article. Inactive current consumption 1.1 μA. Turn on and we are able to scan the logic chip.

[14-OCT-20] We prepare A3035A No1 with our initital P3035A01 firmware, which does not communicate with the accelerometer or gyroscope, but instead transmits zeros at 512 SPS. We get good reception, current consumption 173 μA. We load ML920 battery, but the battery cannot supply the start-up current reequired by the LCMXO2-1200 logic chip. We try ML1220, but it can't do it either, nor a BR1225, alghouth a BR2477 is able to get the board started, as well as a 10-mAhr LiPo battery. We can jump-start the board by attaching an external 2.6-V power supply at power up, and after that the board runs fine off its ML920.


Figure: A3035AV1 Circuit Board with ML920 Battery. The ML921 has capacity 11 mAhr.

We are able to read the BMA423 accelerometer chip identifier register (0x00) and the BMG250 gyroscope sensor time register. We see the values we read from the sensors in the SCT signal plotted in the Recorder Instrument. We conclude that all connections in the circuit work, and approve second article. We leave the A3035A with 11-mAhr battery transmitting.

[15-OCT-20-] After fourteen hours, the A3035A is still running.