UAV Composition PDF

MULTICOPTER CH2 – UAV Composition 1  MULTICOPTER What are the basic compositions of a multicopter system? 2  MULTICOPTER What are the basic compo...

MULTICOPTER CH2 – UAV Composition 1  MULTICOPTER What are the basic compositions of a multicopter system? 2  MULTICOPTER What are the basic compositions of a multicopter system? 3  MULTICOPTER What are the basic compositions of a multicopter system? 4  AIRFRAME A typical airframe only includes a fuselage and a landing gear. To cover overall types of multicopters, the duct is also taken as a part of the airframe 5  AIRFRAME: FUSELAGE Fuselage acts as the platform to carry all the equipment of a multicopter. The safety, durability, usability, and the performance of a multicopter are often highly dependent on the configuration of its fuselage. For a well-designed multicopter, all factors including the scale, shape, material, strength, and weight should be carefully taken into consideration. Configuration: 6  AIRFRAME: FUSELAGE Weight: determined by its size and material. Smaller fuselage weight means larger remaining payload capacity for the same thrust. Diagonal Size: is the diameter (usually in mm) of the circumcircle determined by the motor axes. It is used to indicate the size of an airframe. The diagonal size restricts the size of propeller. Material: 7  AIRFRAME: LANDING GEAR Landing Gear: Functions of which include the follows: Supporting whole multicopter when landing on the ground or taking off and keeping the level balance of the multicopter. Keeping propellers off ground at a safe distance. Weakening ground effect when multicopters take off or land. Consuming and absorbing impact energy when land on the ground. 8  AIRFRAME: DUCT Duct: The thrust of a multicopter with ducts is composed of two parts, i.e., the thrust of the propeller and the additional thrust induced by the duct. Functions of which include the follows: protecting the blade and ensuring personal safety. enhancing the efficiency of thrust and reduce noise. 9 PROPULSION SYSTEM CH2 – UAV Composition 10  PROPULSION SYSTEM A propulsion system includes propellers, motors, ESCs, and Battery. This system is the most important part of the multicopter, which determines the main performances such as the hovering time, the payload ability, the flying speed and distance. Moreover, components of the propulsion system have to be compatible with each other, otherwise they cannot work properly or even fail in some extreme cases. 11  PROPELLER Propeller: produces the thrust and torque The motor efficiency varies with the output torque (depends on the type, size, speed and other factors of a propeller). A good match ensures high efficiency condition: less power consumed for the required thrust eventually extendingthe time of endurance of the multicopter. Propeller Type: Described by a four-digit number, e.g., the APC1045 propeller implies that: Propeller Pitch: the distance a propeller would move in one revolution if it were moving through a soft solid, like a screw the propeller belongs to APC series, through wood” the first two digits: the diameter of the propeller is 10 in, the last two digits: the pitch of the propeller is 4.5 in, 12  PTICH Propeller Pitch: the distance a propeller would move in one revolution if it were moving through a soft solid, like a screw through wood” 13  PROPULSION SYSTEM: PROPELLER Chord length: varies along the radius. The nominal chord length is located at the 2/3 of the radius of the propeller. 2/3rd Moment of Inertia: Tendency of a body to resist angular acceleration. A smaller moment of inertia of the propeller can improve the response speed of the motor, resulting in better control and performance. 14  PROPULSION SYSTEM: PROPELLER Safe Rotation Rate: the materials of propellers used are flexible. So, when rotation rate exceeds a certain value, the propellers may deform, which will reduce its efficiency. Therefore, when calculating the safety rotation rate limit, all the possible conditions should be considered. Depending on the company, an empirical formula for the maximum speed of multicopter propellers, Max Propeller RPM (RPM) Max Speed of Multicopter Propeller = Prop Diameter (inches) 105,000 Revolution Per Minute RPM APC Max Speed of Multicopter Propeller = = 10,500𝑅𝑃𝑀/𝑖𝑛 10 (inches) 65,000 Revolution Per Minute RPM SF Max Speed of Multicopter Propeller = = 6,500 𝑅𝑃𝑀/𝑖𝑛 10 (inches) 15  PROPULSION SYSTEM: PROPELLER Number of Blades: typical propellers with different number of blades. The efficiency of two-blade propeller is better comparing with three-blade propeller. The maximum thrust of the propeller is increased with the number of the blades, BUT the efficiency is decreased with the number of the blades. To obtain the same thrust, a three-blade propeller has a less diameter compared with the corresponding two-blade propeller. Although the efficiency is reduced for a three-blade propeller, the endurance MAY be improved to a certain extent by the reduction of the size and weight of the airframe. 16  PROPULSION SYSTEM: PROPELLER Propeller Specific Thrust: also referred to as efficiency, is a very important parameter to measure the efficiency of energy transformation. The propeller specific thrust is defined as. Thrust Propeller Specific Thrust = Mechanical Power where Mechanical Power angular W = Torque N. m × Propeller Speed (rad/s) Material: includes carbon fiber, plastic, and wood. Though the propellers made of carbon fiber cost almost twice as much as those made of plastic, they are more popular due to following advantages: less vibration and noise because of its high rigidity; lighter and stronger; and more suitable for the motor with high KV. However, because of the high rigidity, the motor will absorb most of the impact when a crash occurs and the fiber blade can be treated as a high speed rotating razor which is too dangerous to human nearby. Propellers made of wood are much heavier and more expensive, which is suitable for multicopters with large payload capacity. 17.  PROPULSION SYSTEM MOTOR Mainly brushless DC motors for various advantages such as high efficiency, potential to downsize, and low manufacturing costs. Brushless DC motors are used to convert electrical energy (stored in battery) into mechanical energy for propeller. Based on the position of rotors, brushless DC motors can be classified into the inner rotor type and outer rotor type Considering that the motor of a multicopter is supposed to drive larger propellers to improve efficiency, the outer rotor type outperforms the inner rotor type as it can provide larger torques. Compared with the inner rotor type, speed of the outer rotor type is more stable. Therefore, the outer rotor type is more popular in multicopters and most other aircraft. 18 19  PROPULSION SYSTEM: MOTOR Motor Size: is represented by its stator size with four-digit number, such as motor 2212 (or written as 22 × 12). The first two indicates its stator diameter (22 mm) and the latter two indicates its stator height (12 mm). That means, the larger the former two are, the wider the motor is; the larger the latter two are, the higher the motor is. A wide and high motor has high power, which is more suitable for large multicopters. 20  PROPULSION SYSTEM: MOTOR KV value for motors: The KV value for brushless DC motors is the number of RPM that the motor will revolve when 1V (Volt) is applied with no load attached to the motor. For example, 1000KV just means that when 1V is applied, the no-load motor speed will be 1000RPM. A low KV motor has more windings of thinner wire, which means it will carry more power, produce a higher torque, and drive a bigger propeller. By contrast, a high KV motor can produce a low torque so that it can only drive a small propeller. http://madcomponents.co/index.php/mad5010-310kv/ No-load current and voltage: In the no-load test, the current passing through the three-phase winding of stator after applying nominal voltage (generally 10 or 24V) is defined as the nominal no- load current. 21  PROPULSION SYSTEM: MOTOR Maximum current/power: It is the maximum current or power the motor can undertake. For example, maximum continuous current “25A/30s” represents that the motor can work safely with continuous current up to 25A, beyond which for more than 30s the motor may be burnt out. The same definition can be applied for the maximum continuous power. Resistance: There is resistance in all motor armatures (. It is very small but cannot be ignored because the current flowing through the resistance is tremendously large and sometimes reaches tens of Amperes. The existence of the resistance generates heat during Mechanical Power angular W the running of the motor, which may overheat the = Torque N. m × Propeller Speed (rad/s) motor and reduce the efficiency. 22  PROPULSION SYSTEM: MOTOR Motor efficiency: an important parameter to measure the performance. It is defined as output/input: Mechanical Power (W) Motor efficiency = Electrical Power (W) where Electrical Power W = Input Voltage V × Effective Current A Mechanical Power angular W = Torque N. m × Propeller Speed (rad/s) The motor efficiency is not a constant. In general, it varies with input voltage (throttle) and load (propeller). For the same propeller, the efficiency of the motor may be reduced as the input voltage (current) is increased That is because the larger the current is, the more the heat (caused by the resistance) and other loss will be, which makes the ratio of the effective mechanical power reduced Mechanical Power angular W = Torque N. m × Propeller Speed (rad/s) 23  PROPULSION SYSTEM: MOTOR Overall specific thrust: The overall performance of the propulsion system depends largely on a well- matched combination of motor and propeller. To evaluate the efficiency of the motor and propeller together, the overall specific thrust is calculated as. g Thrust (g) Overall Specific Thrust = = Propeller Specific Thrust × Motor efficiency W Electrical Power (W) Electrical Power W = Input Voltage V × Effective Current A Mechanical Power angular W = Torque N. m × Propeller Speed (rad/s) Thrust Propeller Specific Thrust = Mechanical Power Since both the propeller specific thrust and motor efficiency are not constant, the overall specific thrust changes with the working condition. The overall specific thrust is often given by the motor producers. Taking a motor for example, the overall specific thrust under different states is displayed in the next slide, where “Efficiency (g/W)” is in fact the overall specific thrust. This will help designers to choose combinations of a motor and a propeller according to their requirements. 24 PROPULSION SYSTEM: MOTOR g Thrust (g) Efficiency OR Overall Specific Thrust = W Electrical Power (W) 25  PROPULSION SYSTEM: ESC Electronic Speed Controller (ESC): The basic function of ESCs is to control the speed of motors based on the signal that autopilots send, which is too weak to drive brushless DC motors directly. ESCs also act as dynamic brake, or a power supply (battery) elimination circuit module for RC receiver or servo motors. The brushless ESC also acts as an inverter, transforming an onboard DC power input into a three-phase Alternating Current (AC) power that can be applied to brushless DC motors. There are some other auxiliary functions, such as battery protection and starting protection. 26  PROPULSION SYSTEM: ESC Maximum continuous/peak current: The most important parameter for brushless ESCs is current, which is usually represented by Ampere (A), such as 10 A, 20 A, and 30 A. Different motors need to be equipped with different ESCs. An inappropriate matching will burn ESCs or even cause motor failure. - The maximum continuous current: is the maximum continuous current in the normal working condition, while - The peak current is the maximum instantaneous current that the ESC can withstand. Each ESC will be labeled with a specified value, such as Hobbywing XRotor 15A which indicates the maximum continuous current allowed. When choosing the type of ESCs, attention should be paid to the maximum continuous current, which needs to be checked whether it leaves a safety margin (20% for example) so as to efficiently avoid burning the power tube. Taking 50A ESC for example, 10A is often left as a safety margin. Voltage range: The range of voltage allowing the ESC to work properly is also an important parameter. Usually, the index like “3-4S LiPo” can be found on the ESC specification, which means that the voltage range of this ESC is 3-4 cells of LiPo battery, i.e., 11.1–14.8V. 27  PROPULSION SYSTEM: ESC Resistance: Since all ESCs have resistance, the heating power cannot be ignored because the current flowing through them can sometimes reach tens of Amperes. Considering the heat dissipation, the resistance of ESCs with high current is always designed to be small. Refresh rate: Motor response has a great relationship with the refresh rate of ESCs. The higher the refresh rate is, the faster the response will be. e.g, multicopters require rapid thrust adjustment which is done by the rapid control of propeller angular speed. This demands the refresh rate of multicopter ESCs to be faster. Programmability: The performance of ESCs can be optimized by tuning internal parameters. There are three ways to set the parameters of ESCs, i.e., programmable cards, computer software via the USB, and RC transmitters. The parameters that can be set up include: throttle range calibration, low voltage protection, power outage value, current limitation, brakes mode, throttle control mode, switch timing setting, starting mode, and PWM mode setting. Compatibility: If the ESC and motor are incompatible, the motor is likely to be jammed, which may result in a fall and crash for a multicopter in the air. 28  PROPULSION SYSTEM: BATTERY Battery: Battery is used to provide energy. Time of endurance, which heavily depends on the capacity of batteries. Lithium Polymer (LiPo) battery and Nickel Metal Hydride (NiMH) superior performance and cheap price. The nominal voltage of a single cell of LiPo battery is 3.7V. Fully charged, the voltage can reach 4.2V. Several cells can be assembled together. Voltage is decreased gradually with the discharge of the battery. Ideally, the remaining voltage is in a linear relationship with the battery remaining capacity. However, in the late stage of discharge, the voltage may drop sharply, which may result in rapid thrust loss of multicopter. To ensure that a multicopter has enough power or capacity to return home before the carried battery runs out, it is necessary to set a safe voltage threshold for the battery. Besides, the output voltage will drop as the current of discharge is increased because of more voltage allocated to the internal resistance. It should be noted that the battery should not be completely discharged, otherwise it may have an irreversible 29 damage.  PROPULSION SYSTEM: BATTERY Connection: By combining battery cells in series (S), a higher voltage can be obtained, with capacity unchanged. On the other hand, by combining battery cells in parallel (P), a larger capacity can be obtained, with voltage unchanged. The letters S and P are used to represent for the series connection and parallel connection, respectively. For example, assuming that the voltage of one cell is 3.7V and its capacity is 100mAh, then 3S1P represents three cells in series connection (total voltage is 11.1V, capacity is 100 mAh). For the 2S2P battery, its total voltage is 7.4V and total capacity is 200mAh. Capacity: The milli Ampere-hour (mAh) or Ampere-hour (Ah) is a how much electrical charge a particular battery has. The capacity of 5000mAh for a LiPo battery means that the discharge of the battery will last for an hour with the current of 5000mA when the voltage of a single cell is decreased from 4.2 to 3.0V. However, the discharge ability will be decreased along with the process of discharge, and its output voltage will also be decreased slowly. As a result, the remaining capacity is not a linear function of the discharge time. 30  PROPULSION SYSTEM: BATTERY Current of Discharge (mA) Discharge rate: Discharge Rate C = Capacity (mAh) For example, the discharge rate of a battery will be 0.2C when its nominal capacity is 100mAh and the discharge current is 20mA. When the maximum discharge rate of a battery with the nominal capacity of 5000mAh is 20C, the maximum current of discharge is calculated as 5000mAh × 20C = 100 A. The total current of a multicopter cannot exceed its maximum current limit of the battery; otherwise, the battery may be burnt out. The battery having higher discharge rate can generate more current, which can be applied to multicopters demanding higher current because of heavier bodies and more motors. Resistance: Resistance of a battery is not a constant value, and it varies with the power status and service life. The resistance of a rechargeable battery is relatively small in the initial state. However, after a long period of use, because of the exhaustion of electrolyte and decrease in chemical substance activity of the battery, the internal resistance will be increased gradually until to a certain degree where the power in the battery cannot be released, hence, the battery can be regarded as being run out of. 31  PROPULSION SYSTEM: BATTERY Energy density:is the amount of energy stored in a given system or region of space per unit volume or mass, and the latter is more accurately termed specific energy. In general, the units for energy density and specific energy are (Watt × hour)/kg and (Watt × hour)/L, i.e., Wh/kg and Wh/L, respectively. Batteries with higher energy density are more popular due to the contradiction between volume (weight) and endurance for a product. Lithium-ion battery as a kind of clean energy is getting more and more attentions and is widely used in many applications. The energy density of Lithium-ion batteries varies from chemistry to chemistry and the energy density can range from 240 to 300 Wh/L (double of the NiCd, 1.5 times of NiMH). 32 COMMAND AND CONTROL SYSTEM CH2 – UAV Composition 33  COMMAND AND CONTROL SYSTEM Radio Controlled (RC) transmitter, RC receiver, autopilot (also known as the flight controller), Global Position System (GPS) receiver and Ground Control Station (GCS) belong to the command and control system. RC Transmitter and Receiver: RC transmitter transmits commands from remote pilots to the corresponding receiver. Then, RC receiver passes the commands to the autopilot after decoding them. Finally, the multicopter flies according to the commands. Some flight parameters can be set on the transmitter, such as the throttle direction, stick sensitivity, neutral position of RC servo motors, function definitions of channels, record and remind setting of flight time, and lever function setting. Advanced functions include battery voltage and current flight data of multicopters. At present, there are several open source transmitters. 34  COMMAND AND CONTROL SYSTEM RC Transmitter and Receiver: (Cont.) Frequency: The RC transmitter and the receiver communicate by radio waves, and the commonly used radio frequency is 72MHz (old) and 2.4GHz (new). The 2.4GHz radio communication technology has the following advantages. 1) High frequency. 2) Less chance of co-channel interference. When several transmitters work together, this technology allows frequency-hopping automatically to avoid mutual interference. 3) Low power consumption. 4) Smaller volume. Since the control wavelength is very short, transmitting and receiving antennas can be shortened greatly. 5) Rapid response and high control accuracy. Although the 2.4GHz RC transmitter can deal with the co-channel interference, some problems still exist. For example, the 2.4GHz microwave is of good straightness. i.e. the control signal has a bad performance when there exists an obstacle between the RC transmitter and the multicopter. As a result, the transmitting antenna and receiving antenna should be maintained line of sight; and the obstacles between them, such as houses and warehouse, should be avoided. 35  COMMAND AND CONTROL SYSTEM RC Transmitter and Receiver: (Cont.) Modulation: Pulse Code Modulation (PCM) implies the encoding of signal pulses, and Pulse Position Modulation (PPM) refers to the modulation of high-frequency signal. By operating sticks on the transmitter, the value of potentiometer varies accordingly. By the encoding circuit, it can be read and converted into a pulse coded signal, namely PPM or PCM, which will be further modulated through a high-frequency modulation circuit and sent by high-level circuit. The advantages of PCM are not only the strong anti-interference capacity, but also the convenience to be programmed by a computer. Compared with PCM, PPM is easier to realize and cheaper, but is more susceptible to interference. 36 37 38  COMMAND AND CONTROL SYSTEM RC Transmitter and Receiver: (Cont.) Channels: One channel corresponds to one separate operation, and generally there are six-channel transmitters, eight- channel transmitters, and ten-channel (or more) transmitters to control multicopters. The operations needed include: throttle control, yaw control, pitch control, and roll control. In this way, an RC transmitter requires four channels at least. Considering the mode transition and control of camera gimbal, transmitters with at least eight channels are recommended. 39  COMMAND AND CONTROL SYSTEM RC Transmitter and Receiver: (Cont.) Mode: RC transmitter modes refer to the way how an RC transmitter is configured to control a multicopter, i.e., the relationship that sticks correspond to movements. For example, “Mode 1”: pitch/yaw on the left stick, throttle/roll on the right (also called right-hand mode, popular in Japan, more suitable for fixed- wing aircraft); “Mode 2”: throttle/yaw on the left, pitch/roll on the right (also called left-hand mode, popular in the U.S. and other parts of the world including China, more suitable for multicopters). 40  COMMAND AND CONTROL SYSTEM RC Transmitter and Receiver: (Cont.) Throttle: Relates the voltage/current signal to be sent to the motors The motor speed will be increased when the stick is higher than the midpoint and decreased when lower than the midpoint. Total thrust correlated with the deflection of the throttle control stick. motor will stop with the throttle control stick at the bottom and work at a full speed with the throttle control stick at the top. The transmitter can also be set to recover back to the midpoint automatically once it is released. Remote control distance: The control distance of an RC transmitter is restricted by its power. For example, the effective control distance of the “Md-200” is claimed to be 1000m. In order to extend the control distance, power amplifiers and antennas can be used. 41  COMMAND AND CONTROL SYSTEM Autopilot: A multicopter autopilot is a flight control system used to control the attitude, position, and trajectory of a multicopter. It can be semi-automatically (needs commands from remote pilot) or fully automatically. Autopilots have a control framework which is often based on Proportional-Integral-Derivative (PID) controllers. A multicopter autopilot can be divided into the software part and hardware part. The software part is the brain of a multicopter and it is used to process and send information, while the hardware part generally includes the following components: (1) GPS receiver. To obtain the location information. (2) Inertial Measurement Unit (IMU). It includes: the 3-axis accelerometer, 3-axis gyroscope, and electronic compass (or 3-axis magnetometer). It is used to obtain attitude information of a multicopter. (3) Height sensor. The barometer and ultrasonic range finder are used to obtain the absolute height (altitude) and relative height (distance to the ground), respectively. (4) Microcomputers. It acts as a platform to receive information and run algorithms to produce control command. (5) Interface. It acts as a bridge between the microcomputer and the other devices, such as the sensors, ESC, and RC receiver. The three main functions of the autopilot are: (1) Perception. It is used to solve the problem of “where the multicopter is.” (2) Control. Control is to solve the problem of “how the multicopter flies to a desired position.” (3) Decision. Decision is to solve the problem of “where the multicopter will go.” 42  COMMAND AND CONTROL SYSTEM Ground Control Station (GCS) Software: An important part of a GCS is the software. Remote pilots can interact with the software using the mouse, keyboard, button, and joystick. So, way points can be planned by remote pilots for multicopters in advance. Furthermore, remote pilots can monitor the flight status in real time and set new missions to intervene flight. Besides, the software can record and playback flight for analysis. 43  COMMAND AND CONTROL SYSTEM Radio telemetry: Radio telemetry refers to using Digital Signal Processing (DSP) technology, digital modulation and demodulation, radio technology to transmit data with high accuracy, and it is equipped with functions of forward error correction and balanced soft decision. It is able to send and receive data in less than 10 ms and shows some parameters such as field intensity, temperature, voltage, state error statistics, alarm, and network management. One end of radio telemetry is connected to the GCS software, and the other end is connected to the multicopter. Communication is performed using certain protocols to maintain the two-way communication of a multicopter and the corresponding GCS. 44 RADIO TELEMETRY 45  COMMUNIC ATION PROTOCOL Communication protocol is also called communication regulations, referring to as the convention of the data transmission on both sides. The convention includes uniform rules of data format, synchronization method, transmission rate, procedure, error checking, and correction, and definition of control characters, which should be recorded by both sides of communication. It is also called link control regulations. The formulation of communication protocol is advantageous to the separation of GCS and autopilot. As long as communication protocols are obeyed, the GCS software can be compatible with different autopilots. 46