Quadcopter Design and Dynamics.

Transcription

1 Lecture 2: Quadcopter Design and Dynamics Lecture 2 Page: 1 Quadcopter Design and Dynamics colintan@nus.edu.sg

2 Lecture 2: Quadcopter Design and Dynamics Page: 2 Objectives of this Lecture The objectives of today s lecture are: To help you understand the various frame designs, and their advantages/disadvantages. To help you understand the various components in a multicopter. This will help you design and build your own multicopters.

3 Lecture 2: Quadcopter Design and Dynamics Page: 3 What We Will be Covering Multicopter Frame Design X-frame. Y-frame, hexacopter, octocopter, etc. Multicopter components Frame components, motors, speed controllers, propellers, flight controllers, batteries, battery eliminator. Multicopter dynamics Mathematical models.

4 Lecture 2: Quadcopter Design and Dynamics Page: 4 MULTICOPTER FRAME DESIGN

5 Lecture 2: Quadcopter Design and Dynamics Page: 5 Multicopter Frame Designs Common Frame Designs X-Frame Y-Frame H-Frame Hexa-Frame Octo-Frame

6 Lecture 2: Quadcopter Design and Dynamics Page: 6 Multicopter Frame Designs Frame design affects stability Y-frame has less roll and yaw stability than X, hexa and octo frames. H-frame has less pitch stability.

7 Lecture 2: Quadcopter Design and Dynamics Page: 7 Multicopter Frame Designs Frame design affects reliability. Quadcopter flip of death Most often caused by a failure in one motor (In this video the failure was caused by rain)

8 Lecture 2: Quadcopter Design and Dynamics Page: 8 Multicopter Frame Designs In Hexa and Octo frame designs, other more power can be added to other propellers to balance out a failed motor. Note: APM 2.6 does not support this feature.

9 Lecture 2: Quadcopter Design and Dynamics Page: 9 Multicopter Frame Designs Hexacopter and Octocopter frames provide much heavier lifting capacity. Smaller high-speed props also produce less vibration. Very popular with video production companies.

10 Lecture 2: Quadcopter Design and Dynamics Page: 10 Frame Size Multicopter frame size is measured in millimetres. Measurement is taken from centre of a propeller spinner, to the centre of the spinner of the propeller directly opposite. A 450-sized quadcopter measures 45 cm across.

11 Lecture 2: Quadcopter Design and Dynamics Page: 11 MULTICOPTER COMPONENTS

12 Lecture 2: Quadcopter Design and Dynamics Page: 12 Components of a Multicopter Frame Centre Plates Usually made of plastic or carbon fibre. Used in pairs to hold the arms. Holds the battery, flight controller, flight radio and other electronics.

13 Lecture 2: Quadcopter Design and Dynamics Page: 13 Components of a Multicopter Frame Arms Usually made of nylon, plastic or carbon fibre. Holds the motors and electronic speed controllers.

14 Lecture 2: Quadcopter Design and Dynamics Page: 14 Components of a Multicopter Frame Power Distribution Board Distributes power and control signals to the motors. Many multicopter designs use a printed circuit board integrated with the centre plate.

15 Lecture 2: Quadcopter Design and Dynamics Page: 15 Components of a Multicopter Frame Skids Long skids useful for mounting cameras under the multicopter. Long skids tend to get caught on the ground causing the multicopter to flip. Short skids. Easier to take off and land. Less chance of flipping.

16 Lecture 2: Quadcopter Design and Dynamics Page: 16 Active Components - Motors Motors Types of Motors Brush motors A commutator is used to reverse the current in the rotor to make the rotor spin.brushed motors overheat more easily and are less efficient.

17 Lecture 2: Quadcopter Design and Dynamics Page: 17 Active Components - Motors Brushless motors No brushes. Uses a generated AC voltage to reverse rotor polarity causing it to spin.

18 Lecture 2: Quadcopter Design and Dynamics Page: 18 Active Components - Motors Inrunner Motors These are brushless motors, and the spindle is connected to an internal rotor that spins. Inefficient at high speeds, requires gearbox. Outrunner Motors Also brushless. The casing itself spins.

19 Lecture 2: Quadcopter Design and Dynamics Page: 19 Active Components - Motors Motors are rated according to continuous draw current, and kv. Continuous draw current, in amps. Tells you how much current the motor uses in normal operation. Kv Measures the number of revolutions per minute per volt. E.g. a 1000Kv motor produces 1,000 RPM per volt.

20 Lecture 2: Quadcopter Design and Dynamics Page: 20 Active Components - Motors We can find the maximum power of a motor: P = I x V Here I is the continuous draw power of the motor, and V is the maximum voltage of your battery. This in turn affects what you can do with your multicopter. E.g. higher power is required for acrobatics.

21 Lecture 2: Quadcopter Design and Dynamics Page: 21 The Importance of kv Due to the way it is built, a lower Kv motor has higher torque. This allows the motor to spin larger propellers and provide larger lifting capacity. A higher Kv motor has lower torque but is more reactive. You would use a smaller propeller with a high Kv motor. Smaller propellers react much faster to changes in current to the motor, providing better control.

22 Lecture 2: Quadcopter Design and Dynamics Page: 22 Active Components - Motors To summarize: Motors with larger continuous current draw produce more power and let you lift heavier objects. Motors with smaller Kv let you lift heavier objects. Motors with larger Kv react faster to changes in control input.

23 Lecture 2: Quadcopter Design and Dynamics Page: 23 Active Components - ESC An Electronic Speed Controller (ESC) lets you control the RPM of your motor. Selecting a speed controller is simple: Must match the voltage of your battery. Usually stated as an S value (see later) Must exceed the current draw of your motor (rated in Amps)

24 Lecture 2: Quadcopter Design and Dynamics Page: 24 Active Components - ESC The ESC provided with your F450 frames have the following specifications: Battery voltage : 2S to 3S (7.4v to 11.1v) Sustained current: 30A Burst current for 10s: 40A

25 Lecture 2: Quadcopter Design and Dynamics Page: 25 Batteries Almost all multicopters operate using LiPo batteries. Batteries are rated with 3 values: S value Determines maximum voltage output. mah value Determines capacity of the battery. C value Determines maximum current draw.

26 Lecture 2: Quadcopter Design and Dynamics Page: 26 Batteries S value: This tells you the number of cells in the battery. 2S -> 2 cells 3S -> 3 cells Each cell produces approximately 3.7 volts. 2S battery produces 7.4v, 3S battery produces 11.1v, etc.

27 Lecture 2: Quadcopter Design and Dynamics Page: 27 Batteries Milli-amp Hours (mah) This measures the battery s charge storing capacity. In a 2700 mah battery, you can draw 2700 milliamps for one hour, or 1 milliamp for 2700 hours.

28 Lecture 2: Quadcopter Design and Dynamics Page: 28 Batteries C Rating This is the maximum safe continuous discharge rate of your battery. To find discharge rate, multiply the C rating by the capacity. E.g. maximum current discharge for a 25C, 2700 mah battery is 25 x 2700 = 67,500 ma or 67.5 A. Discharging higher than this will cause your battery to heat up, bloat, and possibly explode. Thus if you are using an 80A ESC, a 25C 2700 mah battery would be insufficient.

29 Lecture 2: Quadcopter Design and Dynamics Page: 29 Propellers Propellers are rated using two values: Diameter in inches. Pitch in inches A 10x4.5 propeller for example has a 10 diameter, and travels forward 4.5 inches with each revolution.

30 Lecture 2: Quadcopter Design and Dynamics Page: 30 Propellers Propellers can be clockwise or counter clockwise. Every quadcopter would use 2 clockwise and 2 counterclockwise propellers.

31 Lecture 2: Quadcopter Design and Dynamics Page: 31 Propellers Propellers can be made of: Plastic Cheap. Suffers from aeroelasticity, can lose efficiency and cause vibrations. Carbon fibre More expensive. Rigid, less vibrations and less aero-elasticity.

32 Lecture 2: Quadcopter Design and Dynamics Page: 32 Propeller Power Requirements The amount of power a motor must put out to turn a propeller is given by the following equation: W = k R 3 D 4 P R = propeller RPM, D = propeller diameter inches, P = propeller pitch in inches, and W is power required. k is a factor that depends on: Design of the propeller Blade thickness, width, airfoil profile, etc. A typical value is 5.3 X 10-15

33 Lecture 2: Quadcopter Design and Dynamics Page: 33 Propeller Power Requirements This equation tells us: Increasing propeller diameter results in a large increase in motor power requirement. This in turn means drawing much more power from your ESC and battery. Increasing propeller pitch results in a small increase in power requirement. And of course, more RPM = more power needed.

34 Lecture 2: Quadcopter Design and Dynamics Page: 34 Propeller Power Requirements E.g. specification: 1000 Kv motor at 11.1v = 11,100 rpm 10 propellers with 4.5 pitch Power requirement: 5.3 x x 11,100 3 x 10 4 x 4.5 = W Current draw = / 11.1 = 30A. Your ESC must be able to supply at least 30A of power, and if you are using a 3700 mah battery, you need a rating of at least 9C.

35 Lecture 2: Quadcopter Design and Dynamics Page: 35 Propeller Power Requirements Lets take the exact same specifications, but this time we use a 12 propeller with the same 4.5 pitch. W = 5.3 x x 11,100 3 x 12 4 x 4.5 = W I = / 11.1 = 60A A 2 change in propeller diameter has increased current draw by double! Your ESC must be able to supply 60A or it will overheat. If your battery has a capacity of 3700 mah, it must now have a discharge capacity of at least 17C.

36 Lecture 2: Quadcopter Design and Dynamics Page: 36 Propellers and Thrust You can also estimate thrust from a propeller using the following equation: F = R. D3.5 P. ( R. P V 0 ) D is propeller diameter in inches, R is propeller pitch in inches, R is RPM. This gives us thrust F in newtons. This equation is only an approximation. It assumes an air density of kg/m 3, the standard air pressure at sea level. It also ignores the propeller profile.

37 Lecture 2: Quadcopter Design and Dynamics Page: 37 Example You are designing a quadcopter which will carry 4kg (40N) in total. Your total thrust needed is generally taken to be twice the weight of your quadcopter, i.e. 80N. Too little thrust and your quadcopter is unresponsive. Too much thrust and your quadcopter is too twitchy. Since there are four propellers, each prop must generate 80/4 = 20N of thrust.

38 Lecture 2: Quadcopter Design and Dynamics Page: 38 Example We make the following assumptions: We continue to use 1000 Kv motors. We continue to use 3S batteries, producing 11.1v Maximum RPM is therefore 11.1 x 1000 = 11,100 rpm Our propeller pitch is fixed at 4.5

39 Lecture 2: Quadcopter Design and Dynamics Page: 39 Example It is too difficult to derive D from the thrust equation given earlier, so what we will do instead is to use the equation to generate thrust for propeller sizes from 7 to 12 Prop Dia (inches) Thrust (N)

40 Lecture 2: Quadcopter Design and Dynamics Page: 40 Example From here we see that an 11 x 4.5 propeller is ideal. The power required at full RPM is: W=5.3 x x R 3 x D 4 x P = 478 watts Given we are using a 3S battery producing 11.1v, current draw is 478/11.1 = 43A. So we know we need an ESC that can supply 43A sustained. If our battery capacity is 6000 mah, we need a battery rated at least 8C (8 * 6000 = 48A)

41 Lecture 2: Quadcopter Design and Dynamics Page: 41 Flight Controllers Unlike airplanes, multicopters are unstable. If you apply equal thrust to all propellers, your quadcopter is very likely to flip over at the slightest disturbance. A flight controller is required to keep the multicopter stable.

42 Lecture 2: Quadcopter Design and Dynamics Page: 42 Flight Controllers In addition to stabilization, modern flight controllers include: Return to Launch : Fly back autonomously to starting point and land. Loiter : Stay in one place at a fixed altitude. Mission Planning: Fly autonomously through various GPS waypoints, before coming back to land. Telemetry: Transmit important data like flight speed, direction, altitude, battery life, etc.

43 Lecture 2: Quadcopter Design and Dynamics Page: 43 QUADCOPTER DYNAMICS

44 Lecture 2: Quadcopter Design and Dynamics Page: 44 Quadcopter Control Pitch, Roll and Yaw are accomplished by varying the speed of the motors.