Kinetic Hybrid Smartwatch (2020)
This is a personal project that's done on the backburner around other projects and work going on, I just started this project as it explores ultra-low power design, WLCSP components and micro energy harvesters.
Utilizing a micro energy harvester, this design uses a kinetic mechanism that provides power to an ultra-low power ARM Cortex M0+ chip and a U-Blox GNSS module to receive GPS time updates. A simple motor mechanism as found in conventional quartz watch will be used to drive the hand mechanism.
This is an open-source design that is designed to be adapted and developed by others.
This board is still under development, so this page will be updates as it progresses.
Left: Rough layout of the initial components to have an idea of the amount of space needed.
Micro Generator History
The generator is built similarly to a stepper motor, with a stamped claw tooth design and a bifilar coil design. The stamped claw tooth stator is designed in such a way as to allow the magnets to be on the underside of the design, reducing the generator width. Kinetron has developed a couple of similar solutions over the last few decades, however their initial capacitor energy solutions had issues and had to be replaced, causing customer fatigue and caused the technology ultimately to fail in the public eye. In recent years there have been a few watches that integrate with the same technology and have used this to power smartwatches.
A lavet-type stepping motor would be used to power the hand movements—this is the same style of motor used in standard quartz movements, whereby an alternating electrical current switches the poles of a stator, such that a rotor flips between two positions. This is then geared to provide the sweeping motions. If cost appreciate, the gears will be machined through wire Electrical Discharge Machining (EDM), a process that can retain up to 1 micron accuracy whilst cutting. Failure to fabricate an accurate mechanism could cause a design transition to one of the SHARP memory displays, screens with the refresh rate of a standard TFT/OLED screen but with very minimal power consumption (15uW).
All of the components revolved around two main aspects; power usage and footprint. Keeping component count down is also important from a simplicity and BOM cost standpoint on this project.
The design incorporates a STM32L011E4 for the main microcontroller, a 32-bit ARM Cortex-M0+ running down to 76uA/MHz in a 2.1x2.1mm WLCSP form factor; this micro has 16KB of flash memory, and 512B of EEPROM, with 23 I/O pins, making it ideal for this application. Coming in at $1.80 for qty 1, this is a relatively low-cost microcontroller too.
Whilst daily time keeping will be retained by an internal Real Time Clock (RTC), monthly time keeping will be provided by the ZOE-M8B GNSS System-in-Package. This would be intermittently powered to update time every day or when changing time-zones. As it uses had a relatively high 6.7mA current draw it would not be running continuously. The 4.5x4.5mm footprint is also advantageous for this project. This is the singular most expensive component on the board, coming in at $29.00 for qty 1, or $14.70 for qty 100.
There's a few miscellaneous components, such as the TPS62840 high efficiency buck converter, with a 60nA quiescent current. The energy harvester IC that will be selected in response to the performance of the micro generator designs. A small lithium-ion battery and Battery Management Circuit will be designed, attention needs to be paid with vertical height to to keep these components away from the micro generator.
Aiming for a classic design on the watch, a homage watch case is what I'll be looking at here. Although CNC machining or DMLS printing a case could be considered, this would hugely increase cost and move focus of this project away from the electrical and energy harvester mechanism. There are also a multitude of hour indicator and hands available. A simple stainless-steel watch dial could be cut, bead blast and given a Physical Vapor Deposition (PVD) coating to provide color, providing a 1-2 micron thick pure metal or alloy coating. The case could also be PVD coated, as it provides excellent abrasion resistance and the finish is so durable topcoats are not necessary.