Advanced Power System Selection & Maximizing Efficiency in Multirotors

Transcription

1 Advanced Power System Selection & Maximizing Efficiency in Multirotors P R E S E NT ED B Y: LUCIEN B. MILLER PRESIDENT & CEO INNOV8TIVE DESIGNS, INC.

2 Welcome To The Workshop For a PDF file of todays presentation:

3 Welcome To The Workshop

4 Electric Power for R/C Models

5 Some Projects In The Works Scorpion SII

6 Scorpion SII Some Projects In The Works

7 Some Projects In The Works

8 How do you select a power system for a Multirotor Aircraft?

9 Power Systems For Multirotors For many people, Electric Power Systems are a mystery With some basic knowledge and understanding, the process used to select an Electric Power System is actually quite easy Power Systems need to be thought of as a Total System To work together properly, the components must be matched A properly matched power system can double flight times An improperly matched system can cause serious problems

10 Basic Power System Overview Battery Speed Controller Motor Propeller

11 Power System Component Details Each component of a power system has specifications These specifications give operating limits for each part Proper understanding of these parameters is a must Exceeding a components ratings will cause a failure Proper de-rating of component specs increases lifespan Mil-Spec: 50% de-rating, 80% is good for commercial use Pushing components to the Max greatly shortens lifespan

12 Batteries Provide DC Power to run the motors and electronics Parameters include: Voltage, Capacity & Discharge rate Lithium Polymer (Li-Po) cells are most common today High energy densities (150 Watt-Hours per Kilogram) Require special chargers to prevent damaging cells Can cause serious fires if over-charged or damaged

13 Battery Voltage Li-Po cells have important specific voltage parameters Fully charged, Li-Po batteries measure 4.20 Volts per cell Under load, they drop down to about 3.70 Volts per cell The voltage output is fairly flat over the entire discharge Li-Po batteries are essentially dead at 3.0 Volts per cell Li-Po s should be stored at 3.7 to 3.8 Volts per cell no-load Storing fully charged batteries will greatly shorten their life

14 Battery Voltage Discharge Curve 3.90 V Average voltage level is 3.7 Volts per Cell 3.60 V Dead at 3.00 V

15 Battery Capacity Typically measured in Milli-Amp Hours or mah Describes how much stored energy you have in the pack Larger packs will provide longer flight times 1000 mah = 1 Ah, So a 5000 mah pack is a 5.0 Ah pack Battery weight is directly proportional to capacity size R-O-T: Li-Po batteries weigh ~ 1 ounce per Ah per cell A 6-cell 5000 mah pack is ~ 6 cells x 5 Ah = 30 ounces

16 Battery Discharge Rate Also called C-Rate, it describes how fast you can pull energy from a battery pack without damaging it internally C is the Capacity of a cell, in a 5000 mah pack, 1C = 5 amps A 1C discharge rate will completely drain the battery s stored energy in 1 hour At 2C the battery will discharge in 30 minutes or ½ hour 6C = 10 minutes, 10C = 6 minutes, 15C = 4 minutes Most batteries have a C-rating of between 20C and 45C

17 Speed Controllers Converts DC energy into 3-phase AC to power motors Allows Flight Controller to vary motor speed Main ESC Parameters are Max Voltage and Max Current Should always be rated for the maximum motor current Require some airflow to keep the internal components cool Available with special Firmware specifically for Multirotors

18 Other Speed Controller Features Multirotor specific Firmware allow for faster refresh rates Response time: 400 Hz and 500 Hz vs common 50 Hz BEC or No BEC (Battery Eliminator Circuit) Linear versus Switching type BEC Circuits PWM (Pulse Width Modulation) Frequency, 8 KHz or 20 KHz Programmable or upgradable Firmware options

19 What a Speed Controller Does.125 ms = 8 KHz 100% Power 75% Power 50% Power 25% Power

20 Brushless Motors Converts electrical energy into thrust using propellers Simple design with nothing to wear out other than bearings 3-phase design is most commonly used for Multirotors Motors for Multirotors typically spin larger props at low speeds to maximize the efficiency of the propulsion system 3 main parameters: Motor size, Power rating and Kv Value Motor size can be confusing due to varying standards

21 Brushless Motors Most better quality motors use Stator Size to measure motors Motor power is determined by the Maximum Current Rating Motor power varies with input voltage, limited by Max Current A 30 Amp motor: 330 W on 3 cells, 440 W on 4 cells, etc. The Kv Value tells you how fast a motor will spin Kv Value has nothing to do with Power Output

22 Propellers Converts rotational energy from the Motors into thrust The most critical part of the system to maximize efficiency Typically measured in inches of Diameter and Pitch Available in Wood, Plastic, Composite and Carbon Fiber Used in matched pairs of normal and reverse rotation The most common source of injury in Multirotors

23 Prop Efficiency Increases with Size A propeller is essentially a rotating wing generating lift Induced drag increases as the square of the airspeed A larger prop spins slower to produce the same thrust levels Simply changing props can add 20 to 50% to flight times

24 Creating a Matched Power System Determine System Parameters Consider Payload Determine Mission Step 1 Step 2 Select Motors Select Propellers Determine Motor Performance Adjust System as needed Recalculate values for new Missions Step 3

25 Initial Starting Parameters 1000mm Hex Frame with Brushless Camera Gimbal Panasonic Lumix GH4 Camera for 4K Video 6-cell Li-Po batteries for powering the multirotor Desired flight time of minutes per charge Real-Time Telemetry downlink system for framing shots Retractable landing gear for clear shots Would like to use 16 or 18 propellers (19.6 Max)

26 System Weights Add up the weight of all the component parts for a rough starting point to do your power calculations Divide total weight by number of motors to determine the thrust needed to maintain a stable hover in flight = 1337 grams/motor

27 Select the Proper Motor Choose a motor that provides 3 to 4 times the required thrust at full throttle on the desired propeller size Look for one that can safely run different size props to allow you to adjust the power for varying load conditions For this specific example: 3 x 1337 = 4011, 4 x 1337 = 5348 It should develop the required thrust below 50% throttle Prop Data Charts are an invaluable asset for motor choice!

28 Some Possible Motor Options gm 194 grams 16x5.5-MR 3060 grams 18x5.5-MR Too Big 234 grams 16x5.5-MR 2634 grams 18x5.5-MR 3194 grams 273 grams 16x5.5-MR 4760 grams 18x5.5-MR 5799 grams 342 grams 16x5.5-MR 4692 grams 18x5.5-MR 5826 grams

29 We are looking for desired thrust of 4011 to 5348 gms On a 16 inch prop APC 16 x 5.5 gives 4760 grams of thrust This is 3.56 times the required 1337

30 We are looking for desired thrust of 4011 to 5348 gms On a 16 inch prop APC 16 x 5.5 gives 4760 grams of thrust This is 3.56 times the required 1337 Also notice the APC 18 x 5.5 prop can be used, 5799 grams of thrust, which is equal to 4.34 times the required hovering thrust

31 Other available data from this chart The Max motor current is 54 amps We will use a 60 Amp ESC The motor weight Is 273 grams This is very close to our 275 gm estimate

32 Now that a motor has been selected, we can look at the performance data for a specific prop on this motor

33 Propeller Performance Data

34 Thrust vs Throttle Shows the thrust level of a Motor & Prop as a function of throttle position This curve is a square function, but it is fairly linear between 40% and 80% throttle This graph is used to determine the throttle level required to produce the necessary thrust

35 Current vs Throttle Shows the current draw of a Motor & Prop as a function of throttle position This curve is a cubic function that rises rapidly above the 40% throttle level This graph is used to determine how much current each motor pulls to produce the required thrust

36 RPM vs Throttle Shows how fast the prop spins as a function of throttle position This curve is fairly linear from 10% to 90% throttle for most ESC s This graph can be used to determine how fast the prop will spin, to see if the Max RPM spec is exceeded

37 Efficiency vs Throttle Shows how many grams of thrust are produced for each watt of input power This curve usually peaks at 20% power, and drops in a linear manner to 80% power This graph can be used to compare different brands of props to determine the highest thrust efficiency

38 Step By Step Procedure for Calculating the Performance of a Multirotor Power System

39 Step 1: Determine the throttle level required to make enough thrust to hover the multirotor 1337 gm = 47.2 oz Draw a line from the thrust axis to the thrust curve Draw a line from the thrust curve to the throttle axis Read the throttle level 45%

40 Step 2: Determine the current draw of each motor in a hover Start from the bottom of the current chart at the 45% throttle level Draw a line from the Throttle Axis up to the current curve Draw a line from the current curve to the Current Axis Read the current level 6.5 amps 45%

41 Step 3: Determine the Propeller Speed if desired Start from the bottom of the current chart at the 45% throttle level 4200 Draw a line from the throttle axis up to the RPM curve Draw a line from the RPM curve to the Speed axis 45% Read the Prop Speed

42 Step 4: Determine the Propeller Efficiency if desired Start from the bottom of the current chart at the 45% throttle level 9.4 Draw a line from the Throttle Axis to the Efficiency curve Draw a line from the Efficiency curve to the Efficiency axis 45% Read the Prop Efficiency

43 Summarizing the Information We chose the Cobra CM-4515/ Kv Motor With the APC 16x5.5-MR prop, this motor will make 1337 gm of thrust at 45% power, pulling 6.5 amps of current on 6 cells In a stable hover, the props will be spinning at 4,200 RPM The overall system efficiency is 9.4 grams of thrust per watt We can now calculate flight times and check battery size

44 Calculating Flight Time 6 motors at 6.5 amps each equals 39 amps total in a hover A pair of 6-cell 5000 mah batteries = 10 Ah of total capacity Discharge rate = Current Battery Size, 39 A 10 Ah = 3.9C Flight time = 60 minutes C-rate, = 15.4 minutes This would drain the battery completely, which is not good! Appling 80% de-rating factor: 15.4 x 0.8 = 12.3 minutes This is for stable hovering only, moving around lowers time

45 How maneuvering affects flight time In a hover at 45% power each motor pulls 6.5 A At 50% power each motor will pull 8.2 amps At 55% power each motor will pull 10.5 amps 10.5 amps 8.2 amps 6.5 amps These higher power levels will cut into your flight time 45%

46 Re-Calculating Power Requirements Taking gentle maneuvering into account, the average current draw of the motors will be closer to 8 amps on average 6 motors at 8 amps is 48 amps which is a 4.8C discharge C = 12.5 minutes, de-rated by 80% = 10 minutes Accounting for light maneuvering we can fly for 10 minutes What do you do if you need to fly longer for a job?

47 Adding Batteries for Extra Flight Time Adding batteries will increase flight time, but You do not get as much extra time as you might think Twice as many batteries will not give double the flight time Extra battery weight make the motors work harder You eventually reach a point of diminishing returns You can also gain some efficiency by moving to larger props

48 Re-Calculating With Extra Batteries Original weight was 8024 gms, with 1600 gms of batteries Adding 2 more batteries gives 20 Ah capacity and 9624 gms Each motor needs to make or 1604 gms of thrust The higher thrust levels needed will require more current Re-running the calculations will give the new flight times

49 Step 1: Determine the throttle level required to make enough thrust to hover the multirotor 1604 gm = 56.6 oz 56.6 oz Using the same method as before the new throttle level goes up 45% to 50% to maintain a stable hover

50 Step 2: Determine the current draw of each motor in a hover Start from the bottom of the current chart at the 50% throttle level Again, using the method as before the new hover current is 8.2 amps per motor 8.2 amps

51 Step 3: Check the Propeller Efficiency to see Change Start from the bottom of the current chart at the 50% throttle level 8.7 Draw a line up and over to see the new efficiency drop 8% from 9.4 to 8.7 gm/w The motors have to work harder AND the propellers are less efficient

52 Calculating New Flight Time 6 motors at 8.2 amps each equals 49.2 amps total in a hover Four 6-cell 5000 mah batteries = 20 Ah of total capacity C-rate = Current Battery Size, 49.2 A 20 Ah = 2.46 C Flight time = 60 minutes C-rate, = 24.4 minutes Applying 80% de-rating factor: 24.4 x 0.8 = 19.5 minutes 2 batteries gave 12.3 minutes, 4 batteries give 19.5 minutes A 100% increase in battery size only gives a 58.5% increase in flight time due to the motors having to work harder

53 Other Flight Time Considerations Other factors shorten flight times that should be considered Wind can play a huge part in reducing flight times A 15 MPH cross wind requires flying sideways at 15 MPH to maintain a steady position relative to the ground The hovering throttle level goes up from 45% to 60% Hovering current doubles from 6.5 to 13 Amps per motor! This crosswind effectively cuts your flight time in half

54 Monitoring Flight Times Changing weight and wind can greatly reduce flight times Current draw can vary dramatically from flight to flight This makes flying Multirotors with a flight timer unreliable In-Flight battery monitoring is the only accurate way Real time monitoring also alerts you of bad batteries What monitoring options are currently available?

55 In-Flight Battery Monitors Radio Built-in LED Smart-Phone Module Beeper

56 You Don t Want This to Happen!

57 Picking a Power System Does Not Have To Be Hard Use a methodical approach Take advantage of published data Choose reliable components Take care of your equipment Monitor your flight times Keep logbooks to find trends

58 Additional Information Company Website: ATTF Podcast: Lucien s Blog Site: Drone Info: Address: lmiller@innov8tivedesigns.com For a PDF file of todays presentation: