Mind / Iron
by Bryan Bergeron, Editor ;
Robotic Chia Pets
As a young experimenter, I remember trying to use
the ground return current from the city power station to
power something — anything. By driving copper stakes in
the ground about two meters apart, I was able to create
a 500 mV AC source. At the time, it wasn’t enough to
power any electronic device I had on hand. Today —
thanks to developments in energy harvesting, including
devices designed to work at very low voltages — 500 mV
is huge. The only problem is, it’s AC and not DC.
There are natural sources of DC voltage that can and
have been harvested, including plants. It’s been
documented since at least the 1970s that plants generate
a potential between the plant tissue and the soil. More
recently, with the advent of ultra-low-power devices,
experimenters have used plant ‘batteries’ together with
charge pumps to power digital counters and other
electronic circuitry.
For example, third place in the 2010 International
Design MSP430 Ultra-Low Power Challenge (sponsored by
Texas Instruments) was a device run by a plant battery.
The experimenter, Carlos Cossio, connected five potted
plants in series to achieve a voltage of 3.2V open circuit
— enough to power the MSP430 and some associated
hardware.
The voltage generated by a plant is due to the pH
gradient in its tissues, as opposed to some sort of
chemical reaction. In other words, this isn’t one of those
6 SERVO 07.2010
lemon juice as an electrolyte battery, but a different kind
of battery based on a different mechanism. Because the
potential is between the soil and plant tissue, you have to
use separate pots if you want to connect the plants in
series to create a higher voltage.
As you might expect, plant batteries have a high
impedance and relatively low voltage, and the voltage
varies from one plant species to the next. The common
jade plant is an excellent voltage source, in case you’d
care to experiment on your own. For contacts, you need
a nail or other conductor stuck in the potted soil, and a
straight pin/alligator clip to get at the tissue of the plant.
I imagine that a long-term battery connection needs to
be sealed from microbes that could destroy the plant
tissue.
From the title of this editorial, you can probably
guess where I’m going with this ... plant-powered robots.
I’m talking about small, plant-powered motors, sensors,
and control electronics. There’s a body of literature out
there on BEAM robotics that suggests this is doable. I
don’t envision jade-covered mechatronics circling
underfoot, but perhaps a small carpet roamer powered
by hundreds of smaller plants. You’ll have to design the
system so that daily watering doesn’t destroy the
onboard electronics and motors.
Before you start building your motorized chia pet
robot, I suggest you read the excellent article, Ultra-Low
Voltage Nanoelectronics Powered Directly, and Solely,
from a Tree, by Carlton Himes, Eric Carlson, and others,
published in the IEEE Transactions on Nanotechnology,
2009. There are full reprints on the Web.
By the way, in case you’re thinking about ways of
cashing in on this area, there’s at least one patent that
has been issued by the US patent office on plant
batteries. Check the patent office ( www.uspto.gov) for
details. You can also check our sister publication, Nuts &
Volts, for an upcoming article by Carlos Cossio on his
entry in the Texas Instruments contest.
If you’ve seen The Matrix, you know that animals
also generate electrical potentials. There may come a day
when we each have communicators or at least mp3
players sewn into our skin, with electrodes taking power
from our muscle tissue. Perhaps small rodents will provide
electrical power as well as cortically-directed control of
robots. However, plants are a better bet in the near-term
— they’re more politically correct. SV
