Design is the first phase in the digital manufacturing process. In this course, through a series of lectures and hands-on lessons, we’ll examine a designer’s approach to the design and manufacturing process—from concept to 3D model. We’ll start by applying design thinking to understand user needs, and then we’ll explore design criteria as we dive deeper into Autodesk® Fusion 360™ sketching, modeling, rendering, and documentation features.
Design Criteria: Component location
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From the course by Autodesk
3D Model Creation with Autodesk Fusion 360
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From the lesson
Conceptual Design
This week covers the fundamentals of laying out multiple sketches for a complex part. With a design like a quadcopter, it is important to understand the artistic freedom and the mechanical constraints. Through concept sketches, we explore the shape and form in various 2D views. This allows us to understand physical volume constraints for components like a controller, batteries, and camera while playing with the overall shape.
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In this design criteria lesson,
we're going to talk about component location.
After completing this lesson,
you'll be able to understand multirotor mass location and make informed design decisions.
First we need to talk about the mass of each component.
Now currently we have four motors,
each of which is 28 grams.
There will also be some mass associated with the propellers,
but this is negligible,
and since it's spinning,
there's a little bit more advanced dynamics that we need to talk about.
But for right now, let's just use 28 grams per motor,
and we have four of them.
The flight controller, which needs to be centered on
the quadcopter for the best performance, is seven grams.
Now I chose the Micro CC3D because it's
fairly small size and has all the functionality we currently need.
If we tend to upgrade the flight controller,
that mass could go up,
but it's always going to be centered.
So, it doesn't really affect the overall performance by shifting mass around.
The batteries are going to be a large portion of the mass.
We generally want to shoot from between 50 and 66 percent of the overall mass.
Now this includes the body,
all the components, and that's where we want the mass of the batteries to fall.
Because we talked about several different ones from
smaller capacity three-cells to a larger capacity four-cell battery,
we have a range here of about 100-265 grams each.
We're going to be looking at and planning our overall design for the larger batteries,
but we can also use the smaller ones as well.
So we're looking at two batteries at 265 grams each.
The ESCs or electronic speed controllers are currently 20 grams.
Now these aren't the lightest ones,
but we're using these because they're set for 40 amps of current,
and they have a 500 Hertz response.
So this means that the resolution on the response is very fine.
Now the reason that we're using these is because we want to be able to
switch back and forth between three-cell battery setup and a four-cell.
When we're on a three-cell setup,
we're only drawing about 27 amps max.
So, the 40 amp is a bit overkill for that scenario.
But if we decide to use a four-cell battery setup,
we're going to be pulling closer to 36 or 37 amps.
And in that case, we'll actually need the 40 amp.
So that's why we're using these,
and we're sacrificing a little bit of the weight
by having the ability to change our battery configuration.
The controller RX module that receives the commands from the ground is about 13 grams.
It's actually a little bit lighter than that,
but I'm using a higher rate just in case we decide to change that module.
I want to make sure that the location of that mass makes sense.
The PDB or power distribution board has
a fairly large range from 7-22 grams for some of the boards that I've played with.
Now the smaller end, the seven gram board
is the one we're going to use in the final build.
The 22 gram board has less functionality,
but it's much easier to use because it includes bullet connectors
that can be easily plugged into and out of when you're testing your setup.
So sometimes it makes sense to use
a heavier component while you're in the testing phase and
downgrade to a smaller component with completely soldered joints for the final build.
Our camera in servo are about 30 grams.
The overall package of the camera that was purchased was 44 grams,
but we removed one of the servos,
so that way we dropped some of the mass.
And lastly, the video transmit module that sends
a video signal back down to the ground is 10 grams.
Now one thing I did exclude from this list is the body.
Of course the body is going to be designed,
and we're shooting for about 300 grams.
Now let's talk a bit about the mass location.
When we're designing a quadcopter,
the plane at which all the propellers are spinning is considered the thrust plane.
Now the direction or the location of the center of gravity
in relation to that thrust plane changes how it behaves in the air.
If we have the mass in the center of gravity located in that thrust plane,
so it's a centerline to the thrust plane,
we have a very quick direction change.
This means that without the mass above or below,
it's very easy for it to change directions and roll about the center of its axis.
So, ideally, if you want a very responsive quadcopter,
you're going to work toward putting that mass directly in the center of the thrust plane.
If it's below the thrust plane,
you get increased stability,
but you reduce the agility.
Now if you notice a lot of the off-the-shelf quadcopters
that have camera gimbals on the bottom
of them or the larger ones that are used for filming and taking pictures,
these will have a setup where the propellers are all on the top,
and all the mass is below the thrust plane.
Now this means that it's very stable to fly,
but it is very sluggish and slow to change directions.
Now generally when you have
a camera-type quadcopter that you're taking aerial photography for,
the agility is not really that important.
But in our case, we're designing for something that is fast,
can accelerate, and get to locations quickly so that we can search.
Now if you put the mass above the thrust plane,
this is an interesting scenario,
and it has the tendency to flip.
Now, if you look at a lot of FPV or first-person-view race in quads,
a lot of times you can see that batteries might be placed on top of the chassis or below,
and it has differing effects on how the quadcopter maneuvers.
If you have it on top,
it makes it a little bit easier to flip over,
but harder to stop the flip.
If you have it below, it's harder to initiate a flip,
but it's much more over stable to help stop the flip.
So these are all things that we need to consider when we
talk about placing mass on our quadcopter.
Now if you're buying an off-the-shelf chassis, they're fairly configurable.
There's a lots of slots and holes where you can zip tie or fix different components.
So you can play around and move mass around and see how things behave.
So let's talk about the mass location with some screen shots of the final design.
So what we have here are the location of the battery,
which is roughly centered on the thrust plane.
Now you'll notice that the batteries are just a little bit below the thrust plane,
and that's because we have components above them.
We have the power distribution board,
the camera and the transmit module,
as well as the ESC servos.
We also have the motors.
In this orientation, we have the motors flipped and the propellers below them.
Now, in most cases, you'll have the motors pointing up and the propellers above.
Now, the reason that we're doing this is for airflow and protection,
but we'll talk about that in just a second.
So the batteries together are about 530 grams.
So this is by far the heaviest group of components.
So, having it just slightly below the thrust plane
allows us to offset the rest of the mass.
Now, we could move these up or down,
but in this instance we have them kicked up just
a little bit so that we have more protection during a crash or when landing.
The motors, in this case,
we only see two of them.
But if we look through it,
because we have two on each side,
we're talking about roughly 56 grams.
We have four ESCs or electronic speed controllers.
Now, in this case, we have 40 grams on each side.
Now if you'll notice, everything is working off of symmetry.
We're trying to keep everything as balanced as possible,
both left to right,
fore and aft, and up and down.
Now the ESC's locations are on the top side of the chassis right now,
but they actually can be affixed to the top or
the bottom depending on how we want to move the mass around.
Now if we look at this from the top,
let's take a look at the motor locations.
Again, everything is working off of symmetry,
the centerline of the vehicle in both directions.
We have the motors symmetric front to back and left to right.
Now this is important again for balance.
We want to make sure that everything is balanced,
and we keep the mass as close to the center point as possible to make it very agile.
Now if we take a look at the ESCs,
each of them are 20 grams and they're located in this area.
Even though we can't see them,
they are located underneath that section of the wing.
The camera and the tilt servo are located at the front.
Now the reason it's located here is for good visibility.
We want to make sure that we're not looking at the propellers,
we're not obstructing the view,
but we want to make sure that we're still protected.
So the camera is below the top and it's well above the bottom.
So we're not really worried about any collisions or obstructions.
And we also have the extensions around the propellers
to protect us if we were to bump into a wall or something.
And on the back side,
we have the video transmit module.
This is also fairly close to the location of the controller receiver.
Now the controller receiver is placed a little bit closer to center,
but again, this can move around.
Now right in the center, we have the CC3D,
which is the flight controller,
which is seven grams.
And again, it's best to place that at the center of rotation on the vehicle.
So if you draw across from each of the motors right in the center,
that's exactly where you want to place the CC3D.
It also is directional.
It needs to know which direction is forward,
and that way you can control the motors.
The power distribution board is located below this.
Essentially, the batteries, the power distribution board,
and the flight control are all sandwiched right in the center.
Now the power distribution board is sandwiched in the center
because it makes sense for symmetry in terms of
wiring for the ESCs in getting power out to the motors.
Lastly, let's talk about the batteries.
Now from this top view we can't see the batteries,
but they're located center, and again,
they're located just below the thrust plane.
Because they are the greatest mass,
we want to make sure that they're located at the center,
both one drawing across between all the motors,
and that we get them close to the thrust plane so we have a very agile craft.
Now let's talk about air flow restriction because this is also important.
We're able to place the mass wherever we want,
but we also have to be careful that we're not obstructing the airflow,
because having any obstructions in the air flow will create weird oscillations,
and also will just affect the performance of
the flight or reduce the flight time because the motors are having to work harder.
So you can see from this top view,
the propellers are mostly unobstructed by the chassis.
We made sure that no components were directly in the way,
everything is as close to center as possible.
Now we could also make this overall size a bit smaller,
but then we would start obstructing the flow of each of the propellers.
Now the circles that you see around the propellers are
just the path to make sure that we understand the full sweep,
and these are five inch by four and a half pitch props.
So this is the final size that we're going to go with and we have a good idea of how
everything was designed to make sure that we're not restricting that.
Another thing to consider just from a pure mechanical standpoint is the direction.
So we're talking about the direction.
You notice that the motors are all spinning toward the center of the chassis.
Now the direction again is important because we have to worry about obstructions.
But in addition to the direction they're rotating,
the proximity to other objects as well as the proximity to other propellers,
can greatly affect the performance as well as
increase the noise that a quadcopter makes during flight.
These are all things that we need to take into
consideration as we're designing our quadcopter.
We want to reduce the number of obstructions around the propeller,
we want to make sure that the chassis itself or the airframe is rigid,
and we also want to make sure that we're not
restricting the amount of thrust that we're applying.
Now this is one reason for flipping the motors over and
reversing them and trying to keep them away from the body as much as possible,
while allowing as much air flow to go through as possible as well.
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