ABSTRACT:

The linear activation system for Engine Control of Aircraft is based on Brushless DC Motor. Engine control implies the throttle control of the engine by regulating the fuel flow.

The Brushless DC Motor (BLDC) is a 3- AC Motor with built in Hall sensors spaced 120o (electrical angle) apart to sense the shaft position and switch the current flowing in the 3- windings which is called Commutation. BLDC motors are a type of synchronous motor. This means the magnetic field generated by the stator and the magnetic field generated by the rotor rotates at the same frequency. BLDC motors do not experience the “slip” that is normally seen in induction motors.

The activation system consists of:
1. BLDC motor
2. Ball lead screw
3. LVDT
4. Electronics Module

Electronic module in turn consists of:
a. Servo Card
b. 3- PWM driver based on MOSFETS constructed in 3- H-bridge form
c. Power supply for electronics

The command voltage is analog DC voltage form (+10V corresponding to + 22mm of ball lead screw) and is algebraically summed up with the feedback voltage from the LVDT. The error thus generated is amplified and applied to a PWM generating circuit. The 3- PWM output drive is the BLDC motor whose rotor is connected to the ball lead screw. Thus the ball lead screw movement is dependent on the command input. The tip of ball lead screw controls the throttle of engine and therefore the fuel being injected.

Velocity and torque loops (forming inner loops) and employed to achieve required rate and torque specifications.

Compensation networks are employed to achieve required gain phase margin and thus the specific bandwidth. Command limiting is employed to avoid over travel of the ball lead screw in case of excessive command inputs.
. Introduction

Rustom 1 is a long Endurance UAV having an Altitude ceiling of 25,000 ft and endurance of 12 to 15 hours. In order to develop this in a short time, it is proposed to convert a manned aircraft to a UAV. After detailed comparison of aircraft, which are readily accessible to India, Rutan Long EZ aircraft for which fabrication plans are available has been selected. ADE has evaluated a Long EZ aircraft with Lycoming ‘O – 320’ engine available with a private owner in India and has ascertained that the aircraft, with suitable modifications, meets the needs of Rustom 1. In order to cut down project time, design of modifications of the LongEz aircraft to UAV and development of Actuators have been taken up under a lead-in Technology demonstrator project.

Actuators form the major sub systems of the UAV. While some Actuators, like the nose wheel retraction, air brake operation etc are proposed to be bought out, it is proposed to design and develop precision Actuators for Control surface and engine control operations. The requirements of the Actuator for the Engine Control has been worked out based on study of the aircraft and measurements made during Flight-testing of the aircraft. Design of the Actuator has been carried out and presented in this report.

2. General Description
Successful development of Lakshya and Nishant UAVs has encouraged ADE to take up development of Endurance UAVs. The first Long Endurance UAV proposed by ADE for development, is LER (Long Endurance & Range) UAV ‘Rustom-1’ covering altitudes up to 25,000 ft., providing endurance of 12 to 15 hrs and operating upto a line of sight up to a range of 250 Km. This will have a maximum payload capacity of 75 Kg. In order to develop Actuators and Airframe, which are long lead items, a lead – in project for Design & Development of Electro-Mechanical Actuators & Control Test Rig-Cum-Mockup of LER-UAV (ADE – 185) has been taken up.

2.1 Types of Missions

• Reconnaissance & Surveillance, Target acquisition and designation
• Battle Damage Assessment
• Monitoring enemy’s order of battle
• Other roles like monitoring of disaster sites and dangerous areas, search and rescue missions, patrolling of specified areas.

2.2 Basic Performance Requirements of LER-UAV Rustom

• Operating Altitude: ~22,000 ft
• Altitude Ceiling: 25,000’ ft
• All Up Weight 750 Kg.
• Maximum Speed 150 KTAS
• Endurance: 12 - 15 HOURS (on station) @ 150 km
• Operating Range: 250 Km (Line Of Sight Link)
• Take Off/Landing: CONVENTIONAL with EP. Take off up to 12,000 ft. Altitude should be possible.
• Pay Load Capacity 75 Kg (Max.)
• Pay Loads
1. EO GPA (CCD + FLIR) Nose mounted camera(Switchable)
2. SIGINT ELINT/COMINT/RWR
3. RADAR SAR
4. GENERAL IFF, Recorder

The air vehicle systems are to be capable of satisfying the environmental requirements encountered over the flight envelope specified. The Equipments should be either qualified to meet this environment or provided with suitable protection devices such as heating, cooling or thermal insulation, as may be required.

The UAV in its operational configuration shall withstand exposure to rain during operating conditions. The rainfall rate shall be upto 50 mm/hour for one hour.

2.3 Actuators for the UAV

For Rustom 1 UAV, a manned aircraft is being converted to UAV, to save time in development of Airframe. Rutan Long EZ aircraft has been selected after comparing various aircraft of the class. The aircraft is of canard type with pusher configuration and is having control surfaces namely, elevator (1), ailerons (2) and rudders (2). An air brake helps to reduce the speed during landing. It has two main wheel brakes. The operations of each Rudder (port and Starboard) and corresponding wheel brake are performed through corresponding (left & right) pedal. The nose landing gear is retractable.

The power plant is Lycoming O–320 engine, which is a 4-cylinder air-cooled engine with horizontally opposed piston configuration to which the fuel is supplied by a “Throttle-body Injector”. The Engine has two pilot controls namely throttle and lean mixture controls.
The above controls will have to be operated through Actuators in the UAV. It is also proposed to have nose wheel steering for use during take-off and landing of the UAV.

Many actuators are available in market for operations of nose wheel retraction, airbrake operation etc. The main thrust is on development of control surface and Engine control Actuators, which require specific frequency response, accuracy, stiffness etc. Therefore, this review concentrates on design of these Actuators for Rustom 1 UAV.

The essential requirements for design of Actuators can be classified as:

1. Hinge loads
2. Response and accuracy requirements
3. Power requirement
4. Size, weight requirement
5. Natural and induced environmental requirements

The data on 1 was obtained through manned flights of LongEz. The requirements at 2 have been estimated from the existing control designs of Nishant, while comparing with Rustom UAV mission and examining the aero data available. The data on 3, 4 & 5 have been arrived at after analysis of Rustom.

This document discusses the details of requirement for Engine Control Actuator, options considered, and specifications. Also the details of specifications and engineering aspects are presented for detailed review.


3. System Requirements
Actuators are the final elements in the control chain of Flight Control System and Engine Control System. Rustom-1 being Long Endurance and Range UAV (LER-UAV) demands the actuators to be highly reliable and accurate to ensure continuous, failure free flight to accomplish its missions. Operation at high altitude for long hours requires proper selection of its constituent operation reliability design, fault tolerant aspects and thorough environmental qualification procedures.
Totally, ten actuators are required for operation of Rustom-1. Table-1 gives a functionally grouped list of the Actuators for the UAV.

The Actuators are also proposed to be scaled and used for Rustom – H, the Medium Altitude Long Endurance UAV which will be a 2000 Kg class of UAV operable up to 35,000 ft Altitude and will have 35 to 40 Hrs endurance.

Engine Control Actuator (ECA):

The data for the engine control actuators has been obtained from the throttle body injector manuals. For simplicity purpose both, the throttle control actuator and the lean mixture control actuator will be same. The basic data is given below.

Travel: 40 mm
Load: 100 N
Rate: 8 mm/sec

The Maximum load requirement is 100 N corresponding to the max spring force. The initial spring load is considered to be around 10 N (10%) and hence a gradient of 2.25 N/mm. The inertia load is approximated to be 0.350 kg.

The actuator + link combination is designed such that the load on the actuator will be minimum when the throttle is in “Fully-open” state and maximum when the throttle is at the “Closed” state. This arrangement will lead to a fail-safe condition where in the throttle is set to fully open condition in case of cable failure. The linkage will be suitably designed to achieve a 70% open position for the mixture control in fail-safe mode. The maximum load of 100N will be seen only when the actuator is in fully closed condition.

The specified rate of eight mm/sec will have to be met at the maximum load condition. Even though there is no requirement of the bandwidth specification for this application a Bandwidth of 1Hz @ 0.6 mm is specified for testing purposes. The rate of travel is to be obtained with the command shaper in the amplifier stage and the mechanical gear ratio will not be decided for this rate.

A low free-play value of 0.05mm and accuracy +0.2 mm is expected from the actuator. The stiffness requirement of the system is 500N/mm at the actuator.

The Values for supply & control voltages, MTBF, operating life and environmental requirements are same as that for the control surface actuators.

It is proposed have single unit housing servo electronics, power devices and the drive elements in a single package.
The sequence of tests will be as follows.

a. Functional tests
b. EMI / EMC tests
c. Endurance tests
d. Functional tests
e. ESS tests
f. SOF tests
g. Qualification tests

CONCLUSION:
The actuator design meets all the specified requirements in general. The specifications and design values are compared in the system parameters verification section of the actuator above.

The size for the actuator has been exceeded. The deviation varies 10% in the case of linear actuator. The weight is on the lighter side by about 10% in the actuator. The deviations will be addressed in the final production versions after evaluating the engineering development versions for all other specifications. The ECA design exceeds the size specification by around 20%.

The inertia ratio of 20% is exceeded in the actuator. The simulation does not include the effects of mechanical stiffness of individual components and backlash and hence no adverse effects are seen. As the mechanical stiffness is very high and the free play has been kept to the minimum, it is not expected to be a major issue.

The actuator is incorporating extended temperature industrial grade components for meeting the environmental requirements. The EMI/EMC requirements are addressed by using filter-pin connectors, EMI filter for DC-DC converter, differential op-amps for I/O and power bus filters.

Engine aircraft control - Free Legal Forms