How can robots use their motors and sensors to move around in an unstructured environment? You will understand how to design robot bodies and behaviors that recruit limbs and more general appendages to apply physical forces that confer reliable mobility in a complex and dynamic world. We develop an approach to composing simple dynamical abstractions that partially automate the generation of complicated sensorimotor programs. Specific topics that will be covered include: mobility in animals and robots, kinematics and dynamics of legged machines, and design of dynamical behavior via energy landscapes.
2.3.1 Actuator technologies
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來自 University of Pennsylvania 的課程
Robotics: Mobility
421 個評分
從本節課中
Behavioral (Templates) & Physical (Bodies)
We’ll start with behavioral components that take the form of what we call “templates:” very simple mechanisms whose motions are fundamental to the more complex limbed strategies employed by animal and robot locomotors. We’ll focus on the “compass gait” (the motion of a two spoked rimless wheel) and the spring loaded inverted pendulum – the abbreviated versions of legged walkers and legged runners, respectively.We’ll then shift over to look at the physical components of mobility. We’ll start with the notion of physical scaling laws and then review useful materials properties and their associated figures of merit. We’ll end with a brief but crucial look at the science and technology of actuators – the all important sources of the driving forces and torques in our robots.
Link to bibliography: https://www.coursera.org/learn/robotics-mobility/resources/pqYOc
與講師見面
Daniel E. KoditschekProfessor of Electrical and Systems Engineering
School of Engineering and Applied Science
In this section, we're going to consider actuators.
Actuators are very important in leg of robots because this is how the robot
exerts forces on the world.
These forces conserve to either manipulate the environment or
manipulate the robot itself.
We're going to consider different actuation technologies, and
their respective advantages and disadvantages.
The first actuator we'll think about are hydraulics and pneumatics.
These are typically considered as actuators, but really,
they're actually transmissions.
These transmissions rely on a source of compressed
fluid that's usually external to the actuators themselves.
For hydraulics, the working fuel is oil.
Where as for pneumatics, the working fluid is typically air.
So, for both of these systems, the force is proportional to the cross-section
area of the piston and the power consumed is proportional to the velocity.
They have very good specific force and
specific power because of the decoupled power source.
In this video, we have a very simple example of a hydraulic system.
There are two syringes filled with fluid, and
as one syringe moves, it causes the other syringe to move as well.
The force on the syringes and
displacement both relate to the surface area of the syringe.
In this video we see a simple example of a pneumatic system
where moving one cylinder causes the other to move.
The forces and displacement are also proportional to the area, however,
because the fluid is compressible, the relationship isn't as clear.
Friction in the walls of the cylinder is responsible for this discrepancy.
The innate compliance of the two working fluids dictates how the systems can work.
In this simple hydraulic system, if we tap the end of the syringe so that the system
is closed It's almost impossible to compress the hydraulic fluid.
In the pneumatic system, however, if we cap the syringe, the compressible fluid
means that as we push on the cylinder, we have effectively created an air spring.
These air springs were very useful in Mark Raibert's early hoppers,
which were all pneumatic.
These pneumatic hoppers used air as the working fluid.
And that's what's responsible for the hopping you can see in this video.
However, the spring properties of the leg
are also due to the compression of the air.
More recent Boston Dynamics robots use hydraulic transmissions.
In this video,
we see Spot, which is a hydraulic machine that has an electrical pump.
It's relatively quiet.
In this video, we see LS3, which also has a hydraulic transmission, but
has a gas engine, which makes it very loud.
[SOUND] Now we consider the implications
on actuation bandwidth in these transmissions.
Actuation bandwidth is how the amplitude of the signal decreases as
frequency increases.
Ideally, we would like very high actuation bandwidth so
that we get significant amplitude at high frequencies.
In this video, we see a simple pneumatic system where one cylinder is actuated
externally and we see the implications of movement in the other cylinder.
The larger cylinder is driven at increasing speeds and
we can see that the amplitude in the smaller cylinder decreases.
In this video we see a simple hydraulic system where the larger
driven cylinder causes displacement in the smaller cylinder.
As the frequency of the larger cylinder's drive is increased,
the smaller cylinder's amplitude decreases, but not as significantly.
This simple demonstration shows the bandwidth advantages
of hydraulic compared to pneumatic systems.
Now we consider electromagnetic actuators typically exemplified by electric motors.
These motors have considerably lower forces and
operate at very high speeds with high bandwidth.
The power costs operate these motors is typically proportional to
the torque squared.
The operation of these motors is governed by a speed-torque curve.
This means that the peak torque is achieved at no speed.
And as speed increases, the provided torque linearly decreases.
This can be modeled with a force damper system as seen in this image.
This force damper model is a useful abstraction of the speed torque curve.
This speed torque curve can be simply explained by a force damper model.
The force in this Model F is attenuated by a damper with constant beat.
Again, we see that at no speed, the full force of the actuator is available.
Whereas at larger speeds, the damper diminishes the output force.
Most electric motors are used in legged robots with the addition of a gear box.
The gearbox effectively increases the output torque and decreases the speed.
This allows the maximum power of the actuator to be achieved
at reasonable speeds.
While gears are very helpful to amplify torque they are also fragile and lossy.
In gear boxes there's considerable static, dry and viscous friction.
Furthermore the motor's inertia is reflected through the gear box so
that the inertia at the output is proportional to the gear ratio squared.
Gil Pratt made famous the notion of a series elastic actuator
in which a spring is placed between the load and the motor.
This is very helpful in that it protects the gears from shock loading
It also improves the compliance of the system and
it changes force control into a position control problem.
By changing the effective length of the spring
forces can be modulated when the actuator is in contact with the load.
For a series elastic system we must add a series spring to our four stamper model.
This spring of stiffness KS is added between the actuator and the load.
So proprioception in legging robotics is the information about one's own.
Information in this context is typically a force on the end effector.
It's possible to measure forces with an explicit sensor at the end effector.
But here we consider forces that are inferred by displacements at the actuator.
For this to work, actuator transparency is key,
so that information from the end effector
can travel to the actuator without being significantly attenuated or delayed.
Most delays will come in from any kind of compliance in the system,
such as a series elastic element.
Actuator transparency is much improved by now removing the gearbox.
By removing the gearbox,
we decrease the attenuation by all the sources of friction inside the gearbox.
These direct drive robots were first pioneered by Harry Asada in the 80s.
These machines were, however, not power-autonomous and needed to be
plugged into the ground as well as having the first link attached to the ground.
In our Minotaur machine, we have created a power-autonomous,
legged robot that is direct drive.
The actuators on the minuter machine are conventional of the shelf
rotary brushless electric motors.
When comparing the actuators in minuter to actuators in a Rex robot.
We see 100 fold decrease in reflected inertia
a 50 fold decrease in viscous friction.
And a four-fold decrease in both static and dry friction.
In this video, we see Minotaur taking a few short steps before making a jump.
What's important to understand here is that the machine must sense every time
the leg is in contact with the ground.
This is done using proprioception.
The actuators in the leg are commanded to be virtual springs, and
as the leg hits the ground, the deflection in the virtual springs is measured and
a force at the end effector is estimated.
As you can see, this works reliably and
ground detection is sensed near the beginning of the stride.
In this video,
we see a comparison between two actuators with very different transparency.
One is direct drive and one has a gearbox.
First we see the direct drive motor,
which is not controlled in any way, being driven into a fragile egg.
We see that the egg does not crack and that the actuator itself spins.
This spinning means that position information
can go back into the other system.
In the second part of this video, we see that a conventional
motor with a gear box is driven at a similar speed into the egg.
We notice that the egg breaks and that the actuator has not rotated at all.
This means that no information can get from the end effector
back into the system.
Now we consider what happens in biology.
Muscles in humans are typically very high in transparency.
If we consider our series elastic spring damper model from before,
the typical hill muscle model is very similar looking.
But, has the addition of a parallel spring of spring constant kp.
Muscles also have proprioceptive sensors built in as the muscle spindles can sense
the change in length and the Golgi tendon can sense tension in the muscle.
In this section we considered the advantages and
disadvantages of various actuators technologies.
Furthermore we motivated why it's important to think about
not only how the actuators are able to exert forces on their environment.
But what happens when the environment exerts forces onto the actuator?
