Program, Project, System and Subsystem Requirements

By: Anne Stapleton and Mynor Perez

Program Requirements

  1. Design and build an object to fly around ECS-317 and safely land in the instructor’s hand.
  2. Project cost shall remain under $700.
  3. The object must be built by December 9th, 2013.

Project Requirements

  1. The requirement for flight time is dependent on the flight path. The program requirement states that the flying object will be flying around a classroom. The classroom, also specified in the program requirement, is ECS-317. The dimensions of the classroom are are 30 ft. w and 23ft. long. Assuming an oval flight path and a single orbit, the minimum distance to be travelled is C = 86.75 ft. This distance does not include launching or landing distances. This distance also assumes a perfect flight path. The flight duration is equal to t = C/v. The value for velocity is assumed to be 2.25 ft/sec. Using this value for velocity, the result for flight time is 39 seconds. Adding a margin of 100%, the flight time requirement is 78 seconds (1 minute and 18 seconds).
  2. In order to land in the instructor’s hand and meet the program requirement, the maximum diameter of the flying machine is 11”.
  3. The body of the flying machine shall be sleek and appealing to the eye of the sponsor (ie: resemble a “UFO”).
  4. The flying machine shall utilize six ducted fans instead of propellers for the entire flight time of 1 minute and 18 seconds.
  5. The UFO must be able to carry the payload during the entire flight time of 1 minute 18 seconds. The payload of the UFO is a spy camera, provided by the President.

System & Subsystem Requirements

Structural System Discussion

The project is required to pay tribute to the design of the UFO in the film The Day the Earth Stood Still.  Although the design will look similar, the slanted feature from the UFO depicted in the film will not be feasible due to size constraints. The top and the bottom must be the same length in order to maintain good airflow and prevent the ducted fans from slanting. The sizing of the enclosed components, (the ESC’s, EDF’s, and the battery), are also specific to the level one project requirement of being 11 inches and cannot be reduced to a lower area then what it currently available.

The material of the top and base of the UFO shall be structurally sound enough to be able to withstand impact from a crash. In order to estimate the impact force from a crash, the assumed weight of the flying object is 1.5 kg and the assumed altitude is 9 feet. Calculating the force (lb) using F = wh/2d = (3.3 lb)(9 ft)/(2)(9 ft) = 1.65lb. The material chosen for the structural body shall be able to withstand a minimum impact of 1.65lb in order to withstand a crash.

Power Storage

Power System Discussion

The power system shall provide sufficient battery power to feed the electrical components on board for the entire flight duration of 1.18 minutes.  Drawing from the hobby community, fuels include gas and electric. Gas has an energy density of approximately 46 MJ/kg, and batteries have an energy density of approximately 1.8 MJ/kg. Based only on this criteria, a gas engine would be the design choice; however, factoring in operation in a small enclosed space, and the cost associated with remediation of gas fumes (catalytic converters, higher stroke engines, etc.), an electric battery was selected.

Our UFO will consist of six ducted fans that can only operate with certain battery storage units that provide specific voltages and currents. From the EDF’s available, we found that the only battery storage unit that can provide enough power for our motors to satisfy our level one requirement of flight time is a Lithium Polymer battery.

The level one project requirement of having a maximum diameter of 11 inches prevents the battery storage unit from exceeding a certain length, width, and height. Since the EDF motors and ESC’s were approved, we were able to size the available space for the power storage unit. The EDF motors are about 50mm or 2 inches in diameter and if they were subtracted from the 11-inch base, it will result in 7 inches of available space in the center of the base. Since the EDF motors will need about an inch of clearance from the battery and the edge of the base, we will only be able to have a battery storage unit with 6-inch maximum length. There will also be six ESC’s that will be placed on the base as well. Each ESC is about 1.18 inches in length and one inch in width, which means if we put three on opposite sides of the battery storage unit we will have a total of about 4.2 inches on either side being used. This will leave us with about 2.65 inches of battery width that is available to use. As for the height, we have a shell that will be placed about 4 inches above the base, so our battery storage will able to be around 3 inches if we consider the needed clearance space for the ducted fans.

Battery Size

Power storage includes the batteries to run the EDF motors and onboard computer. The requirements driving both of these systems require different storage solutions. The EDF’s require 27A (maximum) delivered over a period of only 72 seconds, resulting in a large storage requirement of 3250 mAH at a high discharge rate of 150C.

On the other hand, the Onboard Computers require only 61.3 mA of power delivered over approximately the same period of time (72 seconds). A study was conducted to determine the possibility of using a DC/DC converter allowing the sharing of power with the EDF battery.

The results of this study showed that the weight of a DC/DC converter can weigh anywhere from 12-20 grams while a coin cell battery only weighs about 2 grams. Consequently, a separate lightweight coin cell battery was baselined.

We also considered the use of an additional power source for the spy camera. We either had the option of using a 9v battery and using a DC/DC converter to power our onboard computer devices, or keep them separate and use a 9v battery for our spy camera and a coin cell battery for our onboard computer devices. The best option was to keep them separate because the weight of the DC/DC converter would be a total weight of about 60 grams while having a coin cell and a 9v battery separate will be a total of about 44 grams. This will save us about 15 grams from our total weight.

EDF Power Storage

Discussion: The EDF’s that were chosen are specific to the amount of voltage and current that is needed to power them. For that reason, we have to choose a battery that is capable of running the EDF’s at their rated voltage and current.

This battery also has to provide enough power to satisfy the level two requirement of powering six electric ducted fans. It also has to satisfy the level one project requirement of being capable of having flight duration of about 78 seconds.

Specifications of the Max Amps 3250 mAh LiPo Battery

Voltage (V) Amperage (A) Discharge (C) Weight (g)
4-cell, 14.8V 3250mAh 150C 327

The maximum amperage that our EDF’s are rated at is 27A each, which is 162A with all six-fan motors running at the same time. The battery specified above will have a discharge rate of 487.5A and will give us a flight time of 1.2 minutes: (3.25Ah*60min)/162A = 1.2 min.

Onboard Computer Power Storage

Discussion: The onboard computer power storage will be needed to power our controlling and communication units. Combined, these units need about 3V and 61.3mA to operate.

We have two options that can be used for our onboard computer power storage. The first option is to use a Universal Battery Elimination Circuit (UBEC) from our EDF power storage. And the second option is to use an additional power source via a lithium coin cell battery.

Our power storage options will need to satisfy the two to one weight ratio requirement, as well as our keeping the level one project requirement of flight time.

If we use the first option, which is a DC/DC converter connected to our EDF power storage unit, the weight total will be about 355 grams. Our second option is separating the EDF storage with the computer power storage, which will be about 340 grams.

The difference of about 15 grams will be important for keeping the two to one weight requirement. So the second option of using a coin cell battery will be the best choice.

Specifications of the Lithium Coin Cell Battery – CR1632

Lithium coin cell Voltage (V) Amperage (A) Weight (g)
CR1632 3V 120mAh 3

Onboard Spy Camera Power Storage

Discussion: The onboard spy camera will need a source of power in order to operate during flight. The power source can either be easily added on via a 9V power adapter that comes with the spy camera, or with a UBEC from our EDF power storage.

The problem with using a UBEC is that it will be an extra cost, which will exceed our level one requirement of cost management. Although the UBEC’s are relatively low cost, most of them are only offered through international warehouses, which will interfere with our level one requirement of the scheduled time for project completion. Not to mention the shipping and handling cost that will be added to the final purchase.

We looked into powering our on board computer system with this power storage unit, but there will be a need of a UBEC to lower the voltage so that the computer and communication systems can be powered. There will also be added weight when using a UBEC instead of the coin cell battery. This weight can tamper with the weight requirement.

Specifications of a 9V Battery

Voltage (V) Amperage (A) Weight (g)
9v 500mAh 43

Powertrain

Discussion: The powertrain encompasses the power conditioning and delivery system, plus the motors of the UFO.

Propulsion System Discussion

The propulsion system has to satisfy the level one project requirement of having a maximum diameter of a base that is 11 inches as well as the level one program requirement of flying safely around the classroom. Our motors will be ducted to achieve the safety requirement and will be a maximum of 2.5 inches in order to keep the UFO at 11 inches.

EDF Motors

Discussion: Taking into account the system and design requirement of having a propulsion unit that can only be a maximum of 2.5 inches in diameter and meeting the level one program requirement of flying safely in a classroom, we will have to use fans that have protection around the propeller units.

In our application we will be using six ducted fans in order to create enough thrust to lift an encased UFO design (the level one requirement) and satisfy the requirement of having control authority.

Purchasing the brushless motors and propellers can easily create fans used in regular copter applications. Designing a shroud for the fans to create the safeing aspect will be difficult to achieve because of the weight requirements and the concern of not having the ducted fans being exactly the same in all dimensions. Thus building ducted fans was not realizable on a required scheduled time for completion.

Ducted fans can be found online for purchase, but most of them are only available from out of the country international warehouses. This would be an issue because it could also interfere with our level one program requirement of scheduling. For this reason research was done on the ducted fans available in the United States. The Dr. Mad Thrust 50mm 10 Blade Alloy EDF’s were in stock and available.

Although this ducted fan can create a good amount of thrust; they will not be able to counter rotate. Most of the ducted fans available are not able to counter rotate which will cause a problem when it comes to having control of the UFO during flight. When we researched available counter rotating fans that are available on the market today, the smallest ones that were found were 65mm in diameter, which is way too big for our application. If we were to purchase these counter-rotating ducted fans, the size of the base would be compromised.

counter rotating

 

The figure above shows a quad copter application that will need a set of clockwise rotating fans as well as one set of counter clockwise rotating fans. This is important to have because if the fans are rotating in the same direction, the copter will start to spin losing control over the yaw.

Since our application has six ducted fans rotating in the same direction, testing will be implemented to see if the UFO will start to spin while hovering. If this happens, a study will be done on possible ducts that will be placed under the fans that will redirect the airflow mimicking counter rotation.

Specifications of the Dr. Mad Thrust 50mm 10 Blade Alloy EDF

Input Voltage: 4S 14.8V lipoly
Rotor Diameter: 50mm 10 Blade
Outer Diameter: 53mm
Weight: 93g with 2mm bullet connectors
Motor: 20-40 Brushless Inrunner 3300kv
Max Amps: 27A
Max Thrust: 650g

EDF Power Conditioning and Delivery

Discussion: Once the brushless ducted fans are selected, the need of a power conditioning and delivery circuit will have to be selected. The power conditioning and delivery circuit that is most popular for hobby based helicopter applications is called an electronic speed controller (ESC). This circuit will need to deliver a constant voltage and regulate the current corresponding to the speed that is desired. By regulating the speed, we will be able to satisfy the level one project requirement of being able to control the flight time to 72 seconds.

An ESC is chosen by the maximum amount of current that is being drawn from the brushless motors. An example would be if our brushless motors were rated at maximum amperage of 18 amps, we want our electronic speed controller to be able to withstand that maximum with ease. If we have an ESC that is rated below 18 amps, and our brushless motor was reaching its maximum amperage, the ESC will overheat and be destroyed.

The ESC’s and the EDF’s communicate by detecting the back EMF from each brushless motor. This enables the ESC to then be able to control the speed of the brushless motor. Thus the need of having six ESC’s (corresponding to the six EDF’s) will be essential for controlling each fan separately. We would not be able to use one ESC to control two EDF’s because if there is a slight difference in the motors, the back EMF can destroy the ESC.

The EDF’s chosen are rated at maximum amperage of 27A. Thus the ESC’s chosen have to be rated above 27A to have breathing room in case we do reach maximum amperage. For this reason we chose a 30A ESC’s that can withstand spikes of up to 40A for 10 seconds.

Specifications of the New HobbyWing Flyfun ESC 30A for Airplane & Helicopter:

1.1 Output: Continuous 30A, Burst 40A up to 10 Secs.

1.2 Input Voltage: 2-4 cells lithium battery or 5-12 cells NiCd/NIMh battery.

1.3 BEC: 2A / 5V (Linear mode).

1.4 Max Speed: 210,000rpm for 2 Poles BLM, 70,000rpm for 6 poles BLM, 35,000rpm for 12 poles BLM.

(BLM: BrushLess Motor)

1.5 Size: 45mm (L) * 24mm (W) * 11mm (H).

1.6 Weight: 25g.

Cabling Subsystem

Discussion: The cable sizing and connectors have to withstand power coming from the LiPo battery and power coming from the coin cell battery. We will also be using connectors to mount our devises onto our onboard computer system units. This includes connections being made to the Tinyduino from the ESC’s, IMU, and Xbee. These connections will make sure the control systems and power systems are kept in check.

Cable Specifications

  • 2mm gold bullet connectors will directly connect the ESC’s to power from the LiPo battery.
  • 2mm headers are used to connect signal wires from the IMU, TinyDuino, Xbee, and ribbon cable from the ESC.
  • Power wires connect the ESC, LiPo battery, and motors

Onboard Computer System

Discussion: The onboard computer encompasses power conditioning (voltage regulators), the microcontroller, peripheral subsystems (including those responsible for monitoring and safety of the battery), and all onboard software (excluding Xbee). Due to its’ complexity, level 3 requirements for software are defined in a separate section.

Microcontroller Modules

Discussion: The microcontroller will be needed to control the ECS, which will in turn control the EDF. Since there are several microcontrollers that are available to us, we decided that the best option would be to use a microcontroller that we had sufficient knowledge on with the software and hardware aspects. For more on the reasons why we chose our microcontroller please see the trade off study on microcontrollers.

The Arduino microcontroller was the best choice for the UFO because as a team, we have the most experience with the Atmel hardware and software. We will be using the TinyDuino, which has the same capabilities as an Arduino Uno but is only the size of a quarter. This microcontroller will provide us with enough I/O pins to make the necessary connections needed to control the EDF’s speed during flight. This will fulfill the level one requirement for the velocity needed to complete the flight path of the UFO.

Specifications for the TinyDuino

Voltage (V) Amperage (A) I/O Ports Atmel Atmega 328P
2.7-5.5V 1.2mA @ 3V 14 digital, 6 analog/digital 32kb Flash, 2kb RAM, 1kb EEPROM

 Safety Circuit

Discussion: Power safety is extremely important for our UFO to satisfy our level one requirement of having safe flight around a classroom. The power safety unit will be protecting the Lithium Polymer battery because of the danger factors if not used properly. There are several power safety circuits that can be used to protect the battery from being destroyed. One option would be to use a protection circuit module that prevents the Li-Po battery from over-charging and over-discharging. This option comes at a cost due to the extra weight that would be added on. Our second option would be to use the on board protection from our ESC’s. The low voltage protection mode and low voltage cutoff protection threshold shuts the power off as soon as the battery drops below a certain voltage. If the voltage from the Li-Po drops below its minimum rated voltage, then the integrity of the battery will be lost.

Using the on board capability of battery protection from the ESC’s will keep our Li-Po safe and maintain the integrity of the battery. It will also cut down on weight and cost deficiencies.

Safety Features for the New HobbyWing Flyfun ESC 30A for Airplane & Helicopter:

  • Safety Arming Feature: Regardless the throttle stick position, the motor will not spin after battery connected.
  • Throttle Calibration: Throttle range can be configured to provide best throttle linearity, fully compatible with all market available transmitters.
  • Programmable Items:

Brake Setting: brake enabled / brake disabled.

Battery Type: Li-xx(Li-ion or LiPo) / Ni-xx(NiMh or NiCd).

Low Voltage Protection Mode(Cutoff Mode): Gradually reduce the output power / Cutoff the output power.

Low Voltage Cutoff Protection Threshold (Cutoff Threshold): low / medium / high.

Start Mode: normal / soft / very soft.

Timing: low / medium / high.

Reset: reset all the programmable items to their default settings.

  • Full Protection Features: Low voltage cutoff protection / Over-heat protection / Throttle signal lost protection.

Software

Discussion: Arduino code embedded on the Tinyduino microprocessor will interpret the serial data coming in from the wireless controller via the XBEEs and turn it into fan thrust values for the UFO to move. Also, the onboard software will read the thrust, position, and velocity data and use control algorithms to stabilize the system. The software has to comply with the level one requirements of having safe flight around a classroom. This will be done by altering the fan speed in order to steer the aircraft as well as achieving stable flight.

For preliminary software design for testing on our UFO, we will create software to test each EDF for thrust. This software will be incrementing PWM by 10% until full PWM is achieved. For results please refer to the blog post Pre Burn-in Test Results vs. Post Burn-in.

Below is a sample of the Arduino code that was implemented for the thrust testing.

// ESC WIRING:

// ESC black wire > pin 14 (GND)

// ESC red wire > no connection (it is +5V from the ESC UBEC)

// ESC white wire > pin 9 (AD1)

// ESC PROGRAMMING PROCEDURE:

// 1.MIN_SIGNAL, then switch on transmitter

// 2.Connect LiPo battery pack, listen for ‘SELF TEST’ = ‘123’ indicating normal voltage range, then 4 beeps for 4s battery, then one long beep for ‘SELF TEST PASS’

// 3.Ready for normal flight 

  Serial.begin(9600);

  Serial.println(“Program begin…”);

  Serial.println(“Normal Start Up.”);

  motor.attach(MOTOR_PIN);

  Serial.println(“Now writing minimum output.”);

  Serial.println(“Press any key and press enter once the PS is connected for 30%”);

   motor.writeMicroseconds(MIN_SIGNAL);

while (!Serial.available());

Serial.read();

 motor.writeMicroseconds(THI_SIGNAL);

 Serial.println(“30%.”);

 Serial.println(“Press any key and press enter for 40%”);

The while loop will start the testing at 30% and will be continued until 100% is achieved.

Once the thrust is known and the prototype is built, software will be designed to have the six EDF motors operate simultaneously with one another. Once the EDF motors activate simultaneously when power is applied, the following code will be used to test the hovering of the aircraft.

#define MIN_SIGNAL 700 //0%
#define MAX_SIGNAL 2000 //100% throttle
#define RANGE 1300
#define MOTOR_PIN_L 2 //I02 LEFT FANS E, A
#define MOTOR_PIN_R 3 //I03 RIGHT FANS B, F
#define MOTOR_PIN_F 4 //I04 FORWARD FAN C
#define MOTOR_PIN_B 5 //I05 BACK FAN D
Servo motorL;
Servo motorR;
Servo motorF;
Servo motorB;

Serial.println(“HOVER DIALOG”);

In this sample of the software, the EDF motors are split into four sections (left, right, front, back). Two fans will be placed in the left fans pin, two will be in the right fans pin, one will be in the forward pin, and the last will be placed under the back pin. By doing this we will be able to control the speed of the fans and the movement of the UFO in order to create a stable flight.

Wireless Communications

Discussion: A communication device has to be on board the UFO in order to have flight capabilities and maneuver around the classroom. Our communication device should be able to communicate to our microcontroller and the on ground flight control device seamlessly.

There are several communication devices that can be used like WiFi devices or Bluetooth devices (page 25, Trade Off Study three). Based on cost and ease of integration between the TinyDuino microcontroller we chose the XBee wireless communicator.

The XBee has an indoor range of up to 100 ft, which is plenty considering the classroom will only be about 40 ft from corner to corner. The XBee can also be easily interfaced with the TinyDuino microcontroller. The XBee is also low voltage and low current so the Lithium coin cell battery can power it.

Specifications for XBee 802.15.4

Voltage (V) Amperage (A) Range (Indoor) Weight (g)
2.8-3.4 V 50mA 100ft 3g

Ground Flight Control

Discussion: To communicate to the onboard communication device we need a ground flight control device that will be able to move the UFO up, down, forward, backward, and side-to-side. This will satisfy the level one requirement of flying safely in a classroom.

The design we found on the Internet includes a joystick that will move the object forward, backward, and side-to-side in the X and Y direction. (http://nootropicdesign.com/projectlab/2012/02/18/xbee-robotics-platform/). It will also have two buttons that will move the object up and down in the Z direction. This device will be portable so a 9v power supply will be added to the design, but since the XBee can only withstand a rated voltage of about 3.3 volts, a voltage regulator will be implemented. The Paralax joystick has analog pins that only operate with a voltage of 1.2, so the implementation of voltage dividers will be of importance.

All of the commands will be transmitted to the UFO via the onboard XBee communications device. Rather than using a breadboard, the parts will be soldered onto a perforated board and will be placed in a project box to keep the components safe.

Software will be designed once the joystick is built. This will include interfacing between the both XBee’s, Arduino microcontroller, and making sure the controller’s commands are interpreted properly so that the UFO will react accordingly.

Wireless Controller