UAV PROPULSION

ESC

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An ESC (acronym for “Electronic Speed Controller“) is what allows the flight controller (covered in the next lesson) to control the speed and direction of a motor. The ESC must be able to handle the maximum current which the motor might consume, and be able to provide it at the right voltage. Most ESCs used in the hobby industry only allow the motor to rotate in one direction, though with the right firmware, they can operate in both directions.

Connectors

An ESC might initially be confusing because it has several wires exiting on two sides.

  • Power input: The two thick wires (normally black and red) are to obtain power from the power distribution board / harness which itself receives power directly from the main battery.
  • 3 bullet connectors: These pins are what connects to the three pins on the brushless motor. There are some standard sizes in the industry, but if you find the two are mismatched, you will need to replace one set.
  • 3-pin R/C servo connector: This connector accepts RC signals, but rather than requiring 5V on the red and black pins, most of the time an internal BEC provides 5V to power the electronics.

In certain instances, the manufacturer does not want to assume which connectors you are using, and leaves the wires for the motor connection and power input bare (they may provide bullet connectors in the packaging which you may or may not want/need and would have to solder onto the wires). The bullet connectors you received with the motors may also not match those of the ESC, so in this case, it’s simply best to replace one or the other. Your next question is obviously given three bullet connectors, which one plugs into which on the motor? As far as the connector for the power, this is entirely up to you – ideally you would use connectors to make the ESC easily removable in case of failure, or if you want to use it on a different project, but be sure that the positive on the ESC goes to the positive on the battery, and same for negative. In order to reverse the direction of rotation, swap any of two of the three connectors between the ESC and the brushless motor.

BEC

Most ESCs include what is called a “Battery Elimination Circuit” or BEC. This comes from the fact that historically, only one brushless motor was needed in a given RC vehicle, and rather than splitting hte battery, it would just need to be connected to the ESC, and the ESC would have an onboard voltage regulator to power the electronics.It is important to know the current which an ESC’s BEC can provide, though it is normally in the range of 1A or above and is almost always 5V.

In a multi-rotor, you need to connect four or more ESCs to the flight controller, but only one ESC is needed, and having power coming from multiple sources all being fed to the same lines can potentially cause issues. Since there is normally no way to deactivate a BEC on an ESC, it is best to remove the red wire and wrap it with electrical tape for all but one ESC. It is still important to leave the black (ground) wire in place for “common ground”.

Firmware

ESCs are not all equally good for use with multi-rotors. It is important to understand that before multi-rotors were around, that brushelss hobby motors were used primarily for RC car drives, airplane propellers and as primary motors in model helicopters. Most of these applications did not require very fast response time or rapid updating. An ESC equipped with SimonK or bheli firmware is able to react very fast (much higher frequency) to changes in input, which may mean the different between stable flight or a crash.

Power Distribution

Since each ESC is powered from the main battery, the main battery’s single connector must somehow be split amongst four ESCs. To do so, a power distribution board, or power distribution harness is used. This board (or cable) splits the main battery’s positive and negative terminals into four. It is important to note the type of connectors used on the battery, ESC and power distribution board may not all be the same, and it is best, whenever possible, to choose a “standard” connector (such as Deans) which is used throughout. Many inexpensive boards require soldering, as they do not want to assume you are using any specific connector. A very simply power distributor could involve a two input terminal block or soldering all positive connections together, and then all negative connections together..

SOURCE : ROBOTSHOP

UAV FRAMES & MATERIALS

UAV FRAME TYPES

Tricopter

Tricopter
  • Description: A UAV which has three arms, each connected to one motor. The front of the UAV tends to be between two of the arms (Y3). The angle between the arms can vary, but tends to be 120 degrees. In order to move, the rear motor normally needs to be able to rotate (using a normal RC servo motor) in order to counteract the gyroscopic effect of an uneven number of rotors, as well as to change the yaw angle. A Y4 is slightly different in that it uses two motors mounted on the rear arm, which takes care of any gyroscopic effects – no servo is therefore needed.
  • Advantages: Different “look” for a UAV. Flies more like an airplane in forward motion. Price is theoretically lowest among those described here since it uses the fewest number of brushless motor (and ESC).
  • Disadvantages: Since the copter is not symmetric, the design uses a normal RC servo to rotate the rear motor and as such, the design is less straightforward than many other multi-rotors. The rear arm is more complex since a servo needs to be mounted along the axis. Most, though not all flight controllers support this configuration.

Quadcopter

Quadcopter
  • Description: A “quadcopter” drone which has four arms, each connected to one motor. The front of the UAV tends to be between two arms (x configuration), but can also be along an arm (+ configuration).
  • Advantages: Most popular multi-rotor design, simplest construction and quite versatile. In the standard configuration, the arms / motors are symmetric about two axes. All flight controllers on the market can work with this multirotor design.
  • Disadvantages: There is no redundancy, so if there is a failure anywhere in the system, especially a motor or propeller, the craft is likely going to crash.

Hexacopter

Hexacopter
  • Description: A “hexacopter” has six arms, each connected to one motor. The front of the UAV tends to be between two arms, but can also be along one arm.
  • Advantages: It is easy to add two additional arms and motors to a quadcopter design; this increases the total thrust available, meaning the copter can lift more payload. Also, should a motor fail, there is still a chance the copter can land rather than crash. Hexacopters often use the same motor and support arm, making the system “modular”. Almost all flight controllers support this configuration.
  • Disadvantages: This design uses additional parts, so compared to a quadcopter which uses a minimum number of parts, the equivalent hexacopter using the same motors and propellers would be more expensive and larger. These additional motors and parts add weight to the copter, so in order to get the same flight time as a quadcopter, the batteryneeds to be larger (higher capacity) as well.

Y6

Y6 Hexacopter
  • Description: A Y6 design is a type of hexacopter but rather than six arms, it has three support arms, with a motorconnected to either side of the arm (for a total of six motors). Note that the propellers mounted to the underside still project the thrust downward.
  • Advantages: A Y6 design actually eliminates a support arm (as compared to a quadcopter), for a total of three. This means the copter can lift more payload as compared to a quadcopter, with fewer components than a normal hexacopter. A Y6 does not have the same issue as a Y3 as it eliminates the gyro effect using counter-rotating propellers. Also, should a motor fail, there is still a chance the copter can land rather than crash.
  • Disadvantages: This uses additional parts, so compared to a quadcopter which uses the same components, the equivalent hexacopter would be more expensive. Additional motors and parts add weigh to the copter, so in order to get the same flight time as a quadcopter, the battery needs to be larger (higher capacity) as well. The thrust obtained in a Y6 as opposed to normal hexacopter is slightly lower (based on experience), likely because the thrust from the top propeller is affected by the lower propeller. Not all flight controllers support this configuration.

Octocopter

Octocopter
  • Description: An octocopter has eight arms, each connected to one motor. The front of the UAV tends to be between two arms.
  • Advantages: More motors = more thrust, as well as increased redundancy.
  • Disadvantages: More motors = higher price and larger battery pack. When you reach this level. most users are looking at very heavy payloads such as DSLR cameras and heavy gimbal systems. Given the price of these systems, added redundancy is really important.

X8

X8 Octocopter
  • Description: An X8 design is still an octocopter, but has four support arms, each with a motor connected to either side of each arm, for a total of 8 motors.
  • Advantages: More motors = more thrust, as well as increased redundancy. Rather than using fewer yet more powerful motors, octocopters provide added redundancy in the event of a motor failure.
  • Disadvantages: More motors = higher price and larger battery pack. When you reach this level. most users are looking at very heavy payloads such as DSLR cameras and heavy gimbal systems.

UAV Size

UAVs come in a variety of different sizes, from “nano” which are smaller than the palm of your hand, to mega, which can only be transported in the bed of a truck. Although both very large and very small UAVs may get quite a bit of attention, they are not necessarily the most practical for hobbyists. For most users who are getting started in the field, a good size range which offers the most versatility and value is between 350mm to 700mm. This measurement represents the diameter of the largest circle which intersects all of the motors. Not only do parts for UAVs in this size range come in a variety of different prices, there is by far the greatest selection of products available.

Drone Size

Smaller UAVs are not necessarily less expensive than medium sized ones. This is largely due to the fact that the technology and time needed to produce small brushless motors or small brushless motor controllers is the same for small parts or for large ones. The prices for the additional electronics such as theflight controller, remote control, camera etc. tend not to change at all. The frame is normally one of the least expensive parts of a UAV, so although the frame for a small UAV may be half the price of a larger one, the overall price, with all parts needed, may still be very close.

UAV Materials / Construction

Below are the more common materials found in multi-rotor drones. This list does not include all possible materials which can be used and should be looked at as a guideline / opinion as to the use of each material to make the frame of a drone. Ideally the frame should be rigid with as minimal vibration transmission as possible.

Wood

If you want your frame to be as inexpensive as possible, wood is a great option, and will greatly reduce build time and additional parts required. Wood is fairly rigid and has been a proven material time and time again. Although the aesthetics may suffer, replacing a broken arm after a crash is relatively easy and “dirt cheap”. Painting the arm helps hide the fact that it’s wood. Ensure you use wood which is straight (no twisting or warping).

wood

Foam

Foam is rarely used as the sole material for the frame and there tends to be some form of inner skeleton or reinforcement structure. Foam can also be used strategically; as propeller guards, landing gear or even as dampening. There are also many different types of foam, and some variations are considerably stronger than others. Experimentation would be needed.

foam

Plastic

Most users can only access and work with plastic sheets (rather than 3D plastic shapes or objects). Plastic tends to flex and as such is not ideal. Used strategically (such as a cover or landing gear), plastic can be a great option. If you are considering 3D printing the frame, consider the time needed to print the part (versus buying a plastic frame kit), and how rigid the part will be in the air. 3D printing parts (or the entire frame) has so far been more successful on smaller quadcopters. Using plastic extrusions may also be an option for small and medium sized drones.

plastic

Aluminum

Aluminum comes in a variety of shapes and sizes; you can use sheet aluminum for body plates, or extruded aluminum for the support arms. Aluminum may not be as lightweight as carbon fiber or G10, but the price and durability can be quite attractive. Rather than cracking, aluminum tends to flex. Working with aluminum really only requires a saw and a drill; take the time to find the right cross section (lightweight and strong), and try to cut out any non-essential material.

Aluminum

G10

G10 (variation of fiberglass) is used as a less expensive option than carbon fiber, though the look and basic properties are almost identical. G10 is mostly available in sheet format and is used largely for top and bottom plates, while tubing in carbon fiber (as compared to G10) is usually not much more expensive and is often used for the arms. Unlike Carbon Fiber, G10 does not block RF signals.

g10

PCB

Printed Circuit Boards are essentially the same as fiberglass, but unlike Fiberglass, PCBs are always flat. Frames <600mm sometimes use PCB material for top and bottom plates, since the electrical connections integrated into the PCB can reduce parts (for example the power distribution board is often integrated into the bottom plate). Small quadcopter frames can be made entirely out of a single PCB and integrate all of the electronics.

pcb

Carbon Fiber

Carbon fiber is still the #1 sought-after building material due to its light weight and high strength. The process to manufacturer carbon fiber is still quite manual, meaning normally only straightforward shapes such as flat sheets and tubes are mass produced, while more complex 3D shapes are normally “one off”. Carbon fiber impedes RF signals, so be sure to take this into consideration when mounting electronics (especially antennas).

Carbon fiber

SOURCE : ROBOTSHOP

How Is Carbon Fiber Made?

The Manufacturing Process Of This Lightweight Material

Also called graphite fiber or carbon graphite, carbon fiberconsists of very thin strands of the element carbon. Carbon fibers have high tensile strength and are very strong for their size. In fact, carbon fiber might be the strongest material there is.

Each fiber is 5-10 microns in diameter. To give a sense of how small that is, one micron (um) is 0.000039 inches. One strand of spider web silk is usually between 3-8 microns.

Carbon fibers are twice as stiff as steel and five times as strong as steel, (per unit of weight). They also are highly chemically resistant and have high temperature tolerance with low thermal expansion.

Carbon fibers are important in engineering materials, aerospace, high performance vehicles, sporting equipment, and musical instruments–to name just a few of their uses.

Raw Materials

Carbon fiber is made from organic polymers, which consist of long strings of molecules held together by carbon atoms.  Most carbon fibers (about 90 percent) are made from the polyacrylonitrile (PAN) process. A small amount (about 10 percent) are manufactured from rayon or the petroleum pitch process.  Gases, liquids, and other materials used in the manufacturing process create specific effects, qualities, and grades of carbon fiber. The highest grade carbon fiber with the best modulus properties are used in demanding applications such as aerospace.

Carbon fiber manufacturers differ from one another in the combinations of raw materials they use. They usually treat their specific formulations as trade secrets.

Manufacturing Process

In the manufacturing process, the raw materials, which are called precursors, are drawn into long strands or fibers. The fibers are woven into fabric or combined with other materials that are filament wound or molded into desired shapes and sizes.

There are typically five segments in the manufacturing of carbon fibers from the PAN process.

These are:

  1. Spinning. PAN mixed with other ingredients and spun into fibers, which are washed and stretched.
  2. Stabilizing. Chemical alteration to stabilize bonding.
  3. Carbonizing. Stabilized fibers heated to very high temperature forming tightly bonded carbon crystals.
  4. Treating the Surface. Surface of fibers oxidized to improve bonding properties.
  5. Sizing. Fibers are coated and wound onto bobbins, which are loaded onto spinning machines that twist the fibers into different size yarns. Instead of being woven into fabrics, fibers may be formed into composites. To form composite materials, heat, pressure, or a vacuum binds fibers together with a plastic polymer.

Manufacturing Challenges

The manufacture of carbon fibers carries a number of challenges, including:

  • The need for more cost effective recovery and repair.
  • The surface treatment process must be carefully regulated to avoid creating pits that could result in defective fibers.
  • Close control required to ensure consistent quality.
  • Health and safety issues
  • Skin irritation
  • Breathing irritation
  • Arcing and shorts in electrical equipment because of the strong electro-conductivity of carbon fibers.

Future of Carbon Fiber

Because of its high tensile strength and lightweight, many consider carbon fiber to be the most significant manufacturing material of our generation. Carbon fiber may play an increasingly important role in areas such as:

  • Energy. Windmill blades, natural gas storage and transportation, fuel cells.
  • Automobiles. Currently used just for high performance vehicles, carbon fiber technology is moving into wider use.  In December 2011 General Motors announced that it is working on carbon fiber composites for mass production of automobiles.
  • Construction. Lightweight pre-cast concrete, earthquake protection.
  • Aircraft: Defense and commercial aircraft.  Unmanned aerial vehicles.
  • Oil exploration. Deep water drilling platforms, drill pipes.
  • Carbon nanotubes. Semiconductor materials, spacecraft, chemical sensors, and other uses.

In 2005, carbon fiber had a $90 million market size. Projections have the market expanding to $2 billion by 2015. To accomplish this, costs must be reduced and new applications targeted.

SOURCE : ABOUT.COM

Electronic Speed Controller & Propellor

An Electronic Speed Controller, or “ESC” controls the speed of the motor.  ESCs will have a power limit.  The more power an ESC can handle, the larger, heavier and more expensive the ESC will be.  When choosing an ESC, it needs to match or exceed the motor’s peak amperage.  If the peak amperage of the motor is 13 amps, then an ESC rated at 15 amps will be sufficient.  An ESC with a lower rated amperage will overheat and possibly fail.

Some common features of an ESC are a low voltage cutoff.  The low voltage cutoff will cut the power to the motors when the voltage drops to a specific level.  This is a protection feature for LiPo batteries.  If a LiPo battery’s voltage drops below its minimal voltage, it can permanently damage the battery.  The low voltage cutoff protects the battery from dropping below its minimal voltage.

Some ESCs can be programmed to have different throttle responses, adjust the low voltage cutoff limit, reverse the motor’s direction and change the switch rate.

PROPELLOR

Propeller choice is one of the most important decisions of your quadcopter.  These are the footwear of your quadcopter.  Propellers affect the agility, stability and efficiency of your quadcopter.

Propellers commonly come in 2, 3 and 4 blades.  The more blades on the propeller, the less efficient they become.  However, more blades produce less noise and are able to handle higher power requirements.

Propellers are specified by their diameter and pitch.  The diameter is measured length of the propeller.  The pitch is how far the propeller will advanced in one revolution.  For example, a 10×4 propeller has 10 inch diameter and will travel 4 inches in one revolution.

Propeller Size

The diameter of a propeller dictates how much thrust can be generated.  The larger the propeller the more thrust can be generated and also the more energy is needed to spin the propeller.

Propellers come in two spinning directions: clockwise and counterclockwise.  The spinning direction is also referred to as “tractor” (counterclockwise) and “pusher” (clockwise) propellers.  Tractor propellers are more common than pusher propellers.  A quadcopter needs a matched set of tractor and pusher propellers.   Because pusher propellers are less common than tractor propellers, propeller choice will be dictated by which propellers are available in pusher configuration.

Importance of Propeller Pitch

I discovered that my initial propeller choice of a 3-blade 8×6 propeller was the root of all my frustration in trying to stabilize Scout’s flight.  After weeks of tuning Scout’s stability, I began to hit a wall.   Even with the best tuning, Scout would still drift and sway during flight.  I could not get Scout to hover in one place.   I began to track down why Scout was so unstable.  I initially thought it was too much vibration that was overloading the sensors.  I added more foam padding to the sensor board and balanced the propellers.  The stability marginally improved, but not as much as I would like.

I then thought it was the ArduPirates code that was the problem so I switched to the ArduCopter code.   Scout was still unstable.  I then remembered I had bought a set of 2-blade 8×4 propellers.  I decided to give them a try.  Eureka!  Scout’s performance was remarkable.  Scout transformed into a different animal.  Without changing the tuning settings from the previous propellers, Scout’s stability is as smooth as glass.  I surmised that the issue was not the 3-blade to 2-blade choice but the pitch of 6 inches was creating choppy turbulent air and the quadcopter could not stabilize.

I recommend using APC propellers.  They are both rugged and perfectly balanced from the factory.

SOURCE : SCOUTUAV