Original subtitles

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.