All language subtitles for 04_design-considerations-continued.en

So now we've studied the effect
of thrust to weight ratio,

let's now look at the power
consumption of each robot.

So in this picture I show you
six different robots that

we've built in a laboratory.

Each one uses a different motor,
has a different frame and

has a different payload.

So because of that the thrust to
weight ratio is different, and

the power consumed is also different.

If you plot the power drawn as
a function of thrust for a given robot,

you'll find that the slope of this
curve is roughly 200 watts per kilo.

If you look at the power consumed and

the power delivered by different
types of batteries, you'll find

the blue dots show the power consumption,
which is around 200 watts a kilo.

And, thankfully, the batteries
produce more than 200 watts per kilo.

So this gives you some idea
of how to pick batteries so

that you can actually support
the power consumption for

the motors and provide extended life for
the quadrotor.

So when you think about system design, you
have to think about battery selection, and

when you think about battery selection,

you have to think about
the power consumption.

In addition to power consumption,

you also have to think about the total
energy carried by the battery.

In this plot, we show the specific power

plotted against the specific energy for
a variety of batteries.

On the y axis you see watts per kilo, on
the x axis, you see watt hours per kilo.

You'll see that most lithium polymer

batteries produce around
200 watt hours per kilo.

There's really nothing on
the right side of this band.

To contrast that with how humans perform,

if you look at a piece of
adipose tissue or fat.

That carries about 10,000
watt hours per kilo.

This is several hours of magnitude more
energy then is carried by batteries.

If you look at the power consumption,

robots consume about 200
watts per kilo per hour.

If you look at humans,

we consume a lot less than that
to walk around, or even to run.

In fact,
if you look at the fastest man on Earth,

Usain Bolt, he's estimated to
consume about 20 watts per kilo.

So our robots are ten times more
inefficient, than possibly the most

inefficient man on Earth, as he runs
the hundred meters race in ten seconds.

Even if you look at bicyclists
like Lance Armstrong,

he consumes about six watts per kilo.

So the moral of the story is our robots
are inefficient, actually hovering

is an inefficient mechanism, so we needs
lots of power to power our robots.

And if you look at lithium polymer
batteries which represent the best choice

of batteries today,
they don't carry a lot of energy.

So what do we do when we
need a lot of power, and

we don't have batteries
that carry a lot of energy?

Well, we can try to reduce our weight and
go on a diet.

And that's what we try to do in the lab,
we try to build smaller and

lighter quad-rotors.

If you at the mass distribution
in a quad-rotor and

look at different components,
how they contribute to the total mass,

you will see a lot of variability.

You'll see that the batteries contribute
about 33% of the total mass and

the motors plus propellers contribute
about 25% of the total mass.

Of course, if you add sensors
like laser scanners and cameras,

the increases the total mass also.

If you take a laser scanner with
a range of about 30 meters,

it consumes about 10 watts for
operation, but

because it weighs 270 grams, it consumes
another 50-60 watts for mobility.

Here's a camera system that
weighs about 80 grams.

it costs us 1.5 watts to operate this
camera plus an additional 15 watts for

mobility.

So when thinking about the payload
we want to also think about

the power consumed in addition
to the thrust to weight ratio.