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An effective braking system plays an important role in aircraft design and operation. Aircraft landing at high speed need to be slowed down as quickly as possible without losing control or running out of runway. A short landing is a requirement for all aircraft for economic and operational reasons. Controlling the aircraft while decelerating is just as important as having a short braking distance. Skidding or locking of the wheels has long been a problem on aircraft when landing on slippery or iced runways. When the wheels lock-up the aircraft can easily tip over and crash because of the inherent instability of aircraft landing gears.
Anti-lock Braking Systems or ABS were first developed and fitted to aircraft. The vehicle industry then saw the potential and a lot of development has happened since. Nowadays almost all cars are fitted with ABS. But smaller aircraft still don't have these systems as standard, possibly to keep costs down and save weight. Experienced pilots say they can "feel" when the aircraft is about to skid and compensate accordingly. In an Unmanned Aerial Vehicle, or UAV, there is no pilot to feel when this is about to happen thus another plan has to be devised.
The client, Denel Aviation, has asked that it's Seeker 400 UAV under development is to be fitted with an Anti-lock Braking System. Currently none of their other UAVs are fitted with ABS and this is a new challenge for their engineers. They have asked the student to design, build and test an ABS system to be fitted to their prototype UAV. An ADAMS simulation of the UAV landing dynamics indicated that the aircraft could crash if both main wheels lock during touchdown for a period exceeding 0.8 seconds.
The thesis includes
ALL RELEVANT ASPECTS
design and building of the actuating mechanism and speed pickup, design and programming of the controller software and testing and optimization of the system.
Table of Contents
List of symbols
The Seeker 400 UAV
The Unmanned Aerial Vehicle or UAV has become more widely used in military and reconnaissance applications in recent years. The UAV is piloted by onboard computers rather than a human pilot. This makes the UAV useful in hostile and dangerous situations. Denel's Seeker 400 UAV will be capable of flying for up to 16 hours without refuelling and has a service ceiling of 18 000 feet. It will be capable of carrying up to 100kg payload. This is largely due to its low drag, all composite design. (Basch, 2008) The Seeker 400 will have a 10m wingspan and all up weight of 500 kg. This makes it fall into the category of light aircraft.
The wheels of the UAV are non-retractable which makes the design of the braking system somewhat easier. Military aircraft like the UAV seldom have to take-off and land on small runways and uneven terrain. On landing the UAV's wheels could easily lock when braking and simulations have shown that the aircraft will be destroyed if not unlocked within 0.8 seconds.
The UAV is piloted remotely by computers, but still needs to be landed by an experienced radio control pilot. Unfortunately the pilot cannot act fast enough to prevent an accident if the wheels would lock-up. Thus the UAV needs to be fitted with an Anti-Lock Braking System.
The ABS system was first developed in the 1920's and remained on aircraft until the 1950's when motorcycles were fitted with ABS. In the 1960's when safety in cars became a priority Ford's engineers started playing with the idea of ABS on cars. (Hartman) Nowadays ABS systems are commonly found on almost all new cars.
In theory all ABS systems work on the same principles. A wheel that skids has less traction than one that is moving because the sliding coefficient is less than the static coefficient of friction. When a wheel skids it takes longer to stop the vehicle and impossible to control. What the ABS system does is prevent the wheel from skidding. When it is detected that the wheel has started to skid or lock up, the braking force is momentarily released enabling the wheel to rotate before braking force is reapplied.
Figure : An ABS diagram (Howstuffworks)The ABS system on a car works as follows:
Braking force is applied to the main cylinder by the driver, which in turn applies a braking force to the wheel. While braking the speed sensors read the speed of each wheel on the vehicle. If it is detected that one wheel is moving slower than the other wheels skidding/lock up has occurred on that wheel. The control system then lets a pump or solenoid valve fluctuate the pressure inside the hydraulic line to release/apply braking force to the specific wheel. This fluctuation happens in very quick succession. This allows the wheel to rotate while braking and not lock up.
Currently there are two types of anti-lock systems used on aircraft. Pulsed braking and skid monitor braking. With pulsed braking the brakes are put on and off in quick succession for a predetermined amount of time until the aircraft come to a stop. No concession is made for skidding while it is on. No speed sensors are needed for this design and it is rather crude, but rugged. With skid monitoring the speed of each wheel is monitored continuously and the braking force is released and reapplied in quick succession when skid is detected. (Yadav & Singh, 1995)
3.3 Client's requirements
Design, build and test an Anti-Lock Wheel Brake Servo for the main wheels of the Seeker 400 UAV. The servo must be used to activate the existing aircraft approved hydraulic brake slave cylinder.
Design, build and test a Speed Pickup for the main wheels as input to the controller.
Program the controller.
Test the integrated servo, speed pickup and controller with the existing hydraulic brake system on the UAV mockup.
The Faulhaber 50 mNm Brushless DC-Servomotor with integrated Motion Controller and RS232 interface is to be used for the servo actuator.
The design needs to be optimized for mass.
The system needs to be certified.
Objectives scope of work
Denel has supplied some of the hardware for this project. The system and components designed by the student has to fit together with these components:
Wheels- Including tire, rim, and axle.
Controller- Programmed using Labview.
All of the above components have already been certified. This means that none of the components can be modified (drilled/cut) in any way that would void the certification.
The complete actuating mechanism to apply force to the main cylinder has to be designed. It has to be set in motion by means of a Faulhaber 50 mN.m servo. The maximum amount of force that needs to be applied to the main cylinder is 800N. A reaction time of 0.2 s/mm also needs to be attained. This mechanism has to be as light as possible as weight is a huge problem in aircraft. The mechanism then has to be built, tested and optimized for performance.
The best and most adequate speed sensor has to be chosen to read the speed of the wheels. A literature study will be done to determine the most adequate sensor. The sensor pickup ring also needs to be designed and built. Because of space constraints it will be a huge challenge to find a place to mount the sensor.
With limited knowledge in the field of programming it will be a new learning experience for the student. The controller has to be programmed to control the system as effectively as possible. Choosing the correct ABS control methodology will play a massive role in the performance of the system. A complete literature study will be done to determine the most adequate system.
The completed mechanism will be installed in a mock-up of the UAV. All the systems will be tested on this full-scale mock-up UAV. The performance of the system will be tested by towing the UAV mock-up behind a vehicle over various surface conditions. The brakes will be applied to maximum force and braking distances measured. The speed, deceleration and internal systems dynamics will be monitored in real time by laptop. Adjustments will be made to the system where necessary.
All of the designed components have to be certified. This means that all the applicable codes need to be adhered to. A complete study of the relevant code's guidelines will be done in the following chapter.
5. Literature survey
5.1 Aircraft brakes
The purpose of aircraft brakes is to help stop the aircraft after landing, help steer the aircraft, hold the aircraft when parked and control the speed of the aircraft on the ground. The brakes turn kinetic energy into heat through friction with the brake discs. The heat is then dissipated into the surrounding air and components. Ultimately it is the friction between the tire and the ground which slows the aircraft. (Roskam, 2000, p. 57)
Figure 2 shows the aircraft's rim and braking calliper that will be used in this project. This is a hydraulic Cleveland brake kit with a split rim design. The brake disc is cast into the one half of the rim. Force is applied to a main cylinder filled with hydraulic fluid. The hydraulic fluid in turn applies pressure to the calliper's internal cylinder through a hydraulic line. This cylinder then applies a force to the brake pad and disc which slows down the wheel.
The clearance between the calliper, disc and axle is very tight. This will pose a challenge to the positioning of a speed sensor.
Figure : Cleveland brake kit (Google images)
Aircraft aren't usually stopped by only using the brakes. Because of the cost of tyres, wheel brakes are usually used in conjunction with a parachute, airbrakes or reverse engine thrust.
Smaller aircraft seldom have any of these additional braking devices. They depend on friction and wheel braking to stop within the limits of the runway. Light aircraft usually don't need to stop as quickly as larger aircraft because most runways are designed to accommodate larger aircraft and thus runway length is not a problem.
The Seeker 400 UAV will be used for military applications. This means that the aircraft would have to be able to land on shorter and rougher runways. To get the shortest possible landing distance without locking the wheels an ABS system is essential.
The slip ratio is an important parameter to any braking system. The slip ratio is defined as:
Slip ratio= [1- (Wheel RPM) / (Wheel RPM)] (Roskam, 2000, p. 57)
brakes on brakes off
The main objective of a braking system is to apply the maximum braking force without skidding. This is dependent on the friction coefficient between the tire and the runway which is a function of slip ratio and the condition of the runway. Figure 3 shows the effect of slip ratio on the ground friction coefficient. A slip ratio of zero is a free rolling wheel, while a ratio of one is a fully locked wheel.
Figure Coefficient of friction versus tire slip (Somakumar & Chandrasekhar, 1999, p. 613)
The objective of the ABS system will be to keep the tyre slip at an optimum value to allow maximum braking force. A maximum practical deceleration value of 0.5g can be attained with anti-skid carbon brakes. (Roskam, 2000, p. 61)
5.2 Aircraft Anti-lock Braking Systems
ABS is essential from a safety and maintenance standpoint. A tire can blow out in seconds or form a flat spot in an even shorter period of time. Sometimes skidding can cause the aircraft to become unstable and crash. These problems can be eliminated by fitting a skid control system. (Currey, 1982, p. 7.55)
Figure : ABS components (Currey, 1982, p. 7.59)Standard skid control systems are available from companies like Bendix and Goodyear (Figure 4). These systems are very expensive and don't cater for smaller aircraft like the Seeker 400. Because these systems are actuated by a human pilot they aren't designed to suit UAVs. Although they cannot be used in UAVs the underlying concepts remain the same.
A convenient way to compare ABS control systems is to consider three systems: Mark 1,2 and 3. These different versions of the anti-skid control system and any anti-skid system today, all have the same components. A wheel-speed sensor, control valve and control box. What sets the different systems apart is the way they control the valve to apply and release braking force.
Early control systems such as the Mark 1 were considered "bang bang" systems. Brake pressure was released once a tire entered a deep skid and was reapplied when the tire started rotating.
The Mark 2 systems later developed used wheel speed sensors to get a signal. The signal was then differentiated to obtain wheel deceleration. When this deceleration exceeded a fixed value the brake pressure was released and slowly reapplied until it reached the fixed value again. This process was repeated until the aircraft came to a stop.
The Mark 3 or adaptive brake control systems later arrived. These systems used the concept of tire slip explained earlier. The speed was determined from a speed pickup on the wheel, which was compared to the real wheel speed. This slip error was then computed. The brake pressure was then modulated around the optimum slip point. Small adjustments are continuously made to keep the tire slip in this optimum range. (Currey, 1982, p. 7.58) Figure 5 shows the optimum slip range.
Figure Coefficient of friction versus tire slip (Somakumar & Chandrasekhar, 1999, p. 613)
The three different methods followed to prevent skidding can now be compared. (Figure 6) From this graph one can see that the method of controlling slip is by far the best. The optimum curve is the shortest possible distance that the aircraft can stop in if no skidding occurs and the friction coefficient is kept at its optimum value. A similar graph will be drawn to determine the optimum stopping distance of the UAV during testing. By comparing test data to this graph we would be able to judge how good the completed system is.
Figure Stopping distance vs friction coefficient (Currey, 1982, p. 7.57)
5.3 Speed sensors
5.4 Programming in Labview
5.6 Experimental investigation
Base case investigation/ measures of success