HOW MATRIX DESIGNED THE POWERWATCH
2014: THE STATE OF THE ART
When the Matrix team started the PowerWatch design, the start of the art in wearable devices featuring thermoelectric generators (TEGs) was large, bulky and inefficient. The reason? Other projects had started with off-the-shelf components. Given that the small amount of heat available on the back of the wrist, the available technology couldn’t harvest enough power from a small enough device to build a practical consumer product.
STEP 1: THERMAL ENERGY HARVESTING SYSTEM
TEGs require a heat source and a heat sink, also known as a “hot side” and a “cold side.” For a wearable device, the hot side needs to be material with high thermal conductivity in direct contact with the user’s skin. Matrix used an aluminum disk on the watch bottom. Copper, brass and precious metals also exhibit good thermal conductivity.
The biggest challenge the team faced was directing the heat flow through the TEG, and minimizing heat leakage around it. They iterated the mechanical design many times to accommodate various electronics and display options.
For the heat sink, they designed an aluminum watch body that maximized air flow. Wearables designed for the torso or head will have different form factors and require different heat sink designs.
STEP 2: ENERGY HARVESTING & POWER MANAGEMENT
Conventional thermoelectric technology cannot produce sufficient power in a wearable form factor. Matrix has developed high-output TEGs and efficient boost converter ASICs optimized for wearable system design. Matrix TEG and boost converters are up to twice as efficient as conventional parts. Coupled with our expertise in thermal systems design, we can produce wearables that produce dramatically higher power, enough to eliminate charging even for cellular wearables.
For more information, see our Technology page.
STEP 3: LOW POWER ELECTRONICS & DISPLAYS
To create wearables that never need charging, you need to choose high-quality, low-power components. Matrix uses Ambiq Micro’s Apollo 1 processor, one of the lowest power ARM processors on the market.
Next, Matrix optimized all of the PowerWatch software to minimize power consumption. Many instructions are not optimized for power-we’ve seen cases where instructions or subroutines consume more than ten times the necessary power. Imagine what happens when that instruction is called recursively in a module...
It was necessary to ensure that each component in the PowerWatch system dissipated the minimum power possible. These devices tend to be particularly power hungry:
- Radios (Bluetooth, WiFi, Cellular, GPS)
STEP 4: VERIFY ENERGY EFFICIENCY
Thermal design is so difficult because all systems tend toward thermal equilibrium. Once all the components are in place, it is time to verify that the system delivers as much power as was intended.
In the thermograph at the left, a PowerWatch is balanced on a finger. Notice that the hot finger (yellow) has warmed up the circular aluminum plate (orange) on the bottom of the watch bottom, while the adjacent lower watch body (purple) is cooler. This confirms that heat is flowing through the internal energy harvesting system as intended, rather than the whole watch simply equalizing in temperature.
STEP 5: TEST, TEST, TEST!
Testing is especially important for new technologies. Matrix has tested the PowerWatch across a wide range of environmental conditions including:
- RF emissions
- Environmental impact
- Water resistance (50–200m)
- Walking, running, swimming