UAV Quadrotor

UAV Quadrotor

A quadrotor, also called a quadrotor helicopter or quadrocopter, is an aircraft that is lifted and propelled by four rotors. Quadrotors are classified as rotorcraft, as opposed to fixed-wing aircraft, because their lift is derived from four rotors. Unlike most helicopters, quadrotors use fixed-pitch blades, whose rotor pitch does not vary as the blades rotate; control of vehicle motion is achieved by varying the relative speed of each rotor to change the thrust and torque produced by each.

History

DeBothezat‘s design was created for a 1921 contract with the US Air Corps. After working on his design for over 2 years, he was able to develop a fairly capable helicopter, which was able to take on a payload of up to 3 people in addition to the pilot. His design was deemed underpowered, unresponsive and susceptible to reliability problems. In addition, instead of the calculated 100 meters cruising altitude, his craft was only capable of reaching a height of roughly 5m.

DeBothezats quad-rotor Design, 1922

The idea of a quad-rotor aircraft has existed since early in the 20th century. Throughout the 20th century very few distinct rotor-craft designs had been developed. The earliest workable designs for a quad-rotor were developed by George DeBothezat, Etienne Oemichen and D.H. Kaplan. Oemichen‘s quad-rotor design is the earliest mention of a complete four-rotor hovering vehicle in history. Oemichen‘s first design in 1920 failed in the initial attempt to become airborne, thereby requiring Oemichen to add additional lifting power and stability of a helium-filled balloon. After a number of recalculations and redesigns, Oemichen was able to come up with a design that actually was capable of lift off and even established world helicopter flight records of the time, remaining airborne for up to 14 minutes at a time by 1923.

Oeminchen No.2 Quad-rotor Design, 1922

The early designs were propelled by additional rotors located somewhere on the rear or the front of the craft, perpendicular to the main rotors. Thus, they are not true quad-rotor designs. It was not until the mid-1950s that a true quad-rotor helicopter flew, which was designed by Marc Adam Kaplan. Kaplan‘s quad-rotor design, Convertawings Model “A” quad-rotor, is arguably the most successful of the early designs of the rotor-craft. The prototype first flew in 1956, and did so with great success. The 2200 pounds craft was able to hover and maneuver using its two 90 horsepower motors, each capable of driving all four rotors in backup mode. Control in this case did not call for additional rotors on the sides of the craft, but was obtained by varying thrust between rotors. This also was the first quad-rotor design that was able to fly forward successfully.

Despite these early proofs-of-concepts, people saw little practical use for quad-rotors. They simply were not competitive with the performance specifications (speed, payload, range, etc.) of more conventional aircrafts. No production contracts were awarded and interest in quad-rotors waned.

Recently however, there has been renewed interest in quad-rotors from hobbyists. This old idea is returning with great potential after RC quad-rotors have shown that there is definite potential in the quad-rotor platform.

Convertawings Model A quad-rotor Design, 1956

Quad Rotor Dynamics

A quad-rotor is an aerial vehicle with four rotors arranged in a symmetric, square configuration around a central hub, which houses the battery and processing components. While flying, the quad-rotor is positioned with a propeller in front and back. Moving counter-clockwise from the front propeller, let Fi be the force of each rotor i for i = 1; 2; 3; 4 such that F1 and F3 rotate counter-clockwise and F2 and F4 rotate clockwise. To perform a stationary hover, all four rotors rotate at the same rate and the total thrust of the craft is equal to its mass, m.
Since both pairs of rotors spin in opposite directions, the net torque on the craft due to drag from the propellers is zero. 

To create yaw movement in a counter-clockwise direction, F1 and F3 are sped up inversely proportional to F2 and F4. As a result, the net torque on the craft is negative via the right hand rule and it will yaw while remaining at the same altitude. If F1 and F3 do not increase proportionally to F2 and F4 decreasing, the craft will move in the z-direction because the net thrust will no longer equal zero. To yaw clockwise F2 and F4 must increase proportionally to F1 and F3’s decrease. A roll to the left is accomplished by decreasing the speed of F2 while increasing F4. Again, F2 must decrease at the same rate that F4 increases to maintain zero net torque. 

Increasing F2 and decreasing F4 results in rolling to the right. Pitching forward and back is done in the same fashion as rolling, but with F1 and F3. The principle for maintaining an equal rate of change for the two opposing rotors is how the translation of the craft is determined. Due to either a pitch or a roll, the lift force is displaced in the x and y planes by angles and respectively, resulting in a horizontal force component that will translate the craft. The altitude of the quad-rotor is altered by changing the rate of all rotors collectively by the same amount. The thrust from each rotor exists only in the z-direction with respect to the body frame. The total thrust can be represented by u = F1 + F2 + F3 + F4 and Fi is the force of rotor i.

Introduction about UAV

UAV: FRIEND or FOE

An unmanned aerial vehicle (UAV ;also known as a remotely piloted vehicle or RPV) is an aircraft that flies without a human crew on board the aircraft.There is a wide variety of UAV shapes, sizes, configurations, and characteristics.To distinguish UAVs from missiles, a UAV is defined as a reusable, uncrewed vehicle capable of controlled, sustained motion powered by a jet or reciprocating engine.

The concept of unmanned aerial vehicles was first used in the American Civil War, when the North and the South tried to launch balloons with explosive devices that would fall into the other side’s ammunition depot and explode

The Curtiss/Sperry Aerial Torpedo made its first successful flight on 6 Mar 1918 at Long Island, NY.

In the 1960s, the US started to develop ‘drones’, which were unmanned vehicles built for spying and reconnaissance. This was after they lost a manned spy aircraft to the Russians.

There are many UAVs manufacturer & suppliers working around the world. Two of the well known manufacturers from these includes Northrop Grumman & General Atomics.

The UAVs can simply be controlled by sitting in the control room through GPS & other navigational systems.

 

Commercial Uses

• Agricultural Industry: UAVs equipped with fertilizer and pesticide dispersing equipment can be used to spray over large fields.
• Crop monitoring: Right now, only 10% of the crops in the world are being monitored by aircraft. Use of UAVs would greatly increase the region or area under surveillance.
• Coast watch: UAVs are being used by the coast guard for monitoring coastlines.
• Environmental control / weather research: Weather balloons are being used to monitor the weather on the ground.
• News broadcasting: UAVs are finding use in providing aerial video feeds for news events where reporters cannot get to in time.
• Air Traffic Control: UAVs can be used to monitor air traffic over busy airports.

Air Data Computer

Air Data Computer is an essential avionics component.It takes sensor’s input and compute different variables.MIL-STD1553 Data Bus is a military based high speed communication data bus. Consists of a Bus controller and  Remote Terminals. It is a dual redundant bi-directional  half duplex data bus.

Air Data Computer takes input from sensor  and give computed output to bus controller and data logger. Bus controllers polls each remote terminal and checks if any of them need any data. Master Slave Communication between Bus Controller and Remote Terminal. USB data Logger logs the data from Air Data Computer in LabView Compatible format.
 
Air Data Computer takes the input from the sensors and convert incoming  signals into digitized form.1553 interface module consists of  Bus Controller, which controls all the data traffic between remote terminals and Air Data Computer. The Remote Terminal module is responsible for requesting the 1553 Bus Controller to send the data and convert those signals into 1553 standard and vice versa.
 
 

More about RADARS

In the previous post, we come to know about what is Radar, its basic components and its applications. In this post, we will look into how Radar actually works and how we get the range. If we see the diagram, which was posted before in previous post as well, we see its main working. But it works on two main principles:

1. Pulse Repetition Frequency
2. Pulse Repetition Time

Pulse repetition frequency (PRF) or Pulse repetition rate (PRR) is the number of pulses per time unit (e.g. Seconds). It is a measure or specification ("pulses per second") mostly used within various technical disciplines (e.g. Radar technology) to avoid confusion with the unit of frequency hertz of the transmitted electromagnetic signal. That electronic frequency may be thought of as switched on and off to form the pulse train of an active sonar or radar system, and if the radar has a characteristic (or known fixed) PRR, can be used in Electronic Warfare as a measurable attribute that can be used to identify the type or class of a particular platform such as a ship or aircraft—in some cases, a particular unit. Electromagnetic (radio or sound) waves are conceptually pure single frequency phenomena while pulses may be mathematically thought of as composed of a number of pure frequencies which sum and nullify in interactions creating a pulse train of the specific amplitudes, PRRs, base frequencies, phase characteristics, etc.

The reciprocal of PRF (or PRR) is called the Pulse Repetition Time (PRT), Pulse Repetition Interval (PRI), or Inter-Pulse Period (IPP), which is the elapsed time from the beginning of one pulse to the beginning of the next pulse. Within radar technology PRF is important since it determines the maximum target range (R_max) and maximum Doppler velocity (V_max) that can be accurately determined by the radar. Conversely, a high PRR/PRF can enhance target discrimination of nearer objects such as a periscope or fast moving missile leading to practices of employing low PRRs for search radar, and very high PRFs for fire control radars, with many dual-purpose and navigation radars, especially naval designs having variable PRRs which might allow a skilled operator to also use a PRR adjustment to enhance and clarify a unclear radar picture, for example in bad sea states where wave action generates false returns, and in general for less clutter, or perhaps a better return signal off a prominent landscape feature (e.g. a cliff).

Measurement

PRF is crucial for systems and devices that measure distance.

• Radar
• Laser range finder
• Sonar

Different PRF allow systems to perform very different functions. But at this stage, we will only about RADARS only.

Radar

PRF is required for radar operation. This is the rate at which transmitter pulses are sent into air or space.

Range ambiguity

A radar system determines range through the time delay between pulse transmission and reception by the relation:

For accurate range determination a pulse must be transmitted and reflected before the next pulse is transmitted. This gives rise to the maximum unambiguous range limit:

The maximum range also defines a range ambiguity for all detected targets. Because of the periodic nature of pulsed radar systems, it is impossible for some radar system to determine the difference between targets separated by integer multiples of the maximum range using a single PRF. More sophisticated radar systems avoid this problem through the use of multiple PRFs either simultaneously on different frequencies or on a single frequency with a changing PRT.

The range ambiguity resolution process is used to identify true range when PRF is above this limit.

Low PRF

Systems using PRF below 3 kHz are considered low PRF because direct range can be measured to a distance of at least 50 km. Radar systems using low PRF typically produce unambiguous range.

Unambiguous Doppler processing becomes an increasing challenge due to coherency limitations as PRF falls below 3 kHz.

For example, an L-Band radar with 500 Hz pulse rate produces ambiguous velocity above 75 m/s (170 mile/hour), while detecting true range up to 300 km. This combination is appropriate for civilian aircraft radar and weather radar.

Low PRF radar have reduced sensitivity in the presence of low-velocity clutter that interfere with aircraft detection near terrain. Moving target indicator is generally required for acceptable performance near terrain, but this introduces radar scalloping issues that complicate the receiver. Low PRF radar intended for aircraft and spacecraft detection are heavily degraded by weather phenomenon, which cannot be compensated using moving target indicator.

Medium PRF

Range and velocity can both be identified using medium PRF, but neither one can be identified directly. Medium PRF is from 3kHz to 30kHz, which corresponds with radar range from 5 km to 50 km. This is the ambiguous range, which is much smaller than the maximum range. Range ambiguity resolution is used to determine true range in medium PRF radar.

Medium PRF is used with Pulse-Doppler radar, which is required for look-down/shoot-down capability in military systems. Doppler radar return is generally not ambiguous until velocity exceeds the speed of sound.

A technique called ambiguity resolution is required to identify true range and speed. Doppler signals fall between 1.5 kHz, and 15 kHz, which is audible, so audio signals from medium-PRF radar systems can be used for passive target classification.

For example, an L band radar system using a PRF of 10 kHz with a duty cycle of 3.3% can identify true range to a distance of 450 km (30 * C / 10,000 km/s). This is the instrumented range. Unambiguous velocity is 1,500 m/s (3,300 mile/hour).

The unambiguous velocity of an L-Band radar using a PRF of 10 kHz would be 1,500 m/s (3,300 mile/hour) (10,000 x C / (2 x 10^9)). True velocity can be found for objects moving under 45,000 m/s if the band pass filter will admit the signal (1,500/0.033).

Medium PRF has unique radar scalloping issues that require redundant detection schemes.

High PRF

Systems using PRF above 30 kHz function better known as interrupted continuous-wave (ICW) radar because direct velocity can be measured up to 4.5 km/s at L band, but range resolution becomes problematic.

High PRF is limited to systems that require close-in performance, like proximity fuses and law enforcement radar.

For example, if 30 samples are taken during the quiescent phase between transmit pulses using a 30 kHz PRF, then true range can be determined to a maximum of 150 km using 1 microsecond samples (30 x C / 30,000 km/s). Reflectors beyond this range might be detectable, but the true range cannot be identified.

It becomes increasingly difficult to take multiple samples between transmit pulses at these pulse frequencies, so range measurements are limited to short distances.

What is a RADAR?

A radar in it’s simplest form is a parabolic dish that uses pulses of radio waves to locate objects. These include but are not limited to ships, terrain, aircraft, weather formations and missiles.

To give a better understanding of what a radar is, some of my friends decided to make an educational radar to help teach the future batches of Avionic students and practically show them it’s functionality.

As they put it, the Educational Radar is an ideal learning tool to understand the concept and working of Radar technology. It detects the range, altitude, direction, or speed of both moving & fixed objects.

A few of it’s applications are as follows.

• Weather forecasting
• Air traffic control

BASIC COMPONENTS

• A transmitter that generates the radio signal with an oscillator such as a klystron or magnetron  and controls its duration by a modulator.
• A waveguide that links the transmitter and the antenna.
• A duplexer that serves as a switch between the antenna and the transmitter or the receiver for the signal.
• A receiver. Knowing the shape of the desired received signal (a pulse), an optimal receiver can be designed using a matched filter.
• An electronic section that controls all those devices and the antenna to perform the radar scan ordered by a software.