مستخدم:Midodedo77/ملعب

The application of micro electronics to gas turbine engine controls 1.1Introduction The justification for use of electronic control techniques to replace the pneumatic, mechanical and hydraulic computing mechanism which have been employed in the majority of engine control system manufactured over the last ten year. From figure (1) we can obtain the engine control elements and we can discuss them in the following:- 1.1.1 Fuel pumping:- The function of the engine fuel pump is to supply fuel at pressure high enough to overcome losses in the flow regulation components the pressure drop through the fuel distribution system and the back pressure from the engine combustion chamber. The pumps have many types a- Positive displacement pump (1) Gear pump (2) Piston pump (3) Vane pump B- Centrifugal pump (1) Full running pump

                             - Engine or bleed
                                 - Air turbine pump
                                 - Driven pump

(2) Vapor core pump

                                 - Engine driven pump

To drive these pumps with variable speed electric motor, permitting direct variation of flow to the engine without use of separate flow regulation device shown in figure (1) Adoption of this technique is unlike due to the very low power to weigh ratio.

• • The fuel pump is unlike to be directly affected by any change from hudromechanical to electronic control computation.

1.1.2 Flow regulation We can obtain it from figure (3) the lower valve is positioned by the computation system to vary the area of a metering orifice in the flow line to the engine the pressure drop across metering orifice is sensed across the ends of the second valve The basic functions which can be identified in the flow regulation system are

1) Measurement of flow (2) Comparison of measured flow with required flow

(3) A mean for variation of flow when an error exists between required and measured flow.
The can be measured by a flow meter giving an electrical output signals compared the signal of required flow in controller.

1.1.3Actuation Figure (4) show a typical method of valve actuation which amplified the computer output signal to adequate force level.

A piston head is located at one end of metric valve shown in figure (2),position of the valve is measured by a spring at the opposite end of the valve , which is compressed against a provide lever to provide force on the lever which varies with valve position  and hence actual flow to the engine . A second input signal spring on the lever is compressed by the control computer to provide a force signal representing required flow.

When an electronic computer is used, the input spring will be compressed by an electrical interface unite such as a motor or solenoid. 1.2Transducing and control computation In hydraulic system, transducing and control computing elements are normally physically integrated with the actuation and flow regulation components. This is necessary to avoid the problem associated with transmission of a multitude of force or displacement signals over long distance. The electronic system is easier to accept remote location of transducing. We can summaries the types of transducer as typical in figure (5) The function of transducers is involve only (inner loop) signals from the engine itself example of these are {engine speed limit &combustion chamber pressure limit }, others such as a steady stat control of the engine require an (outer loop)signal from the airframe, defining, a required engine operating condition. 1.3The benefit of electronic control 1- Automatic variation of engine thrust from minimum fuel consumption, as function of aircraft all up weight, day temperature, altitude, wind speed, as is desirable in civil transport engine operation. 2- Automatic adjustment of engine fuel flow variable geometry features during maneuver which reduce compressor surge margin due to high intake incidence level.

3- Provision of intercommunication between different controls systems on engine airframe 4- To facilitate provision of variety of engine resets in response of airframes signals 5- The system have high reliability an this can be obtain from two way, first of them is using high reliability component, and the other one can be achieve through provision of redundant component 6- The first coast of the electronic system would be lower. 7- The electronic system is lighter than the hydraulic one. 1.4The implementation of electronic engine control system There is a wide variety of approaches to the implementation of electronic engine control system, three of which are shown in figures 10, 11, 12 these figures are arranged to two engine aircraft but it can be drown to other configuration of aircraft. Why the engine mounting is preferred? ***

The engine mounting is preferred because 

1- It minimizes the number of critical signal lines to be taken into airframe and made on installation of engine. 2- The engine can therefore be tested with its system 3- It introduces the possibility of integrating certain engine signal transducers into electronic package.

Introduction to Microprocessor

2.1 Binary – coded decimal (B. C. D) codes: In a b.c.d code, every decimal number a unique representation in binary form. the most popular b.c.d code is the 8421 BCD code , in which the decimal values 0 to 9 are represented by the pure binary groups 0000 to 1001 , as shown in table 8.2 the ' 8421' part of the name of the code relates to the decimal of the columns in the code . thus the decimal number 38 is represented in 8421 B C D as two groups of four bits one groups representing the ' 3' in the tens columns and other groups representing the '8' in the units columns of the number as follow : 3810=0011 1000 in 8421BCD

2.2 Bits, Nibbles, Bytes, and Length: The word bit is a contraction of binary digit.

  The word nibble is used to describe a group of four bits, e.g. 0101 could be a nibble of data.
  A byte is a group of eight bits; the group 00111000cold could be a byte of data.
  The word length is an indication of the ability of the computer to deal not only with complex instructions but also large numbers. A computer with long word length can deal with more complex instructions than a computer having a short word length.

The word length of the majority of microprocessors is eight bits or one byte, while the word length of large computers is many bytes.

2.3 Introduction to the digital computer

The control unites: - Is a section of the computer which controls the logical operation of the machine and causes events to occur in correct sequence. Arithmetic and logic unit (ALU):- Is the section of computer which the mathematical and logical operation are performed. 2.4 Microprocessor and microcomputer A microprocessor is an integrated circuit chip containing contain several component of complete computer as shown in figure 2.5 Architecture of a simple microprocessor The accumulator is a register which is used a temporary storage location by many instructions. Uses the accumulator not only to hold data before a mathematical operation but also as a temporary storage location for the result prior to it being stored in memory.

2.6 Microcomputer three- bus system 

Microcomputer is organized around what is known as a three–bus structure, namely, the data bus, the address bus, and the control bus.

The data bus must be bidirectional. The address bus has 16 wires in it, allowing the CPU to have access to 216=65 536 locations (this is referred to as 64K locations, where 1K is equal to (210=1024 locations). The location may represent single cell within a large memory array, or it may represent a single device such as a light-emitting diode.

The control bus has a number of lines on it the, the actual number of lines depending on the function that the CPU needs to operate the microprocessor system. 2.7 A simple microprocessor –based system Input/ output (I/O) devices are needed through which we communicate with the CPU suitable interface units are required between three buses system and the output devices in order to convert between the voltage levels. A parallel mode, the function of the interface unite is to convert the serial signal into a parallel from in which all the bites are presented to the CPU in parallel. The interface unite would be described as a modem (modulator demodulator) which converts the logic signals into audio tons, so that programs and data can be stored on a cassette tap via a low–cost recorder.

2.8 Interrupt:- Each microprocessor has at least one pin on the CPU chip known as an interrupt pin. When a suitable logic signal is to applied to the pin (which may be a logic "1" or a "0", depending on the design of the CPU, it cause the operation of the program to be interrupt at this point.

The interrupt routine is written by the system programmer and causes the computer to take a special course of action in response to the interrupt signal. 2.9A digital to analogue convector When digital system has to transfer data to an analogue system it does so via a digital-to-analog converter, the conversation frequently being performed by means of a d.a.c. It comprises a number of resistors having a 2 to 1 resistance ratio, the circuit being such that logic"1"on the most significant data bus line. 2.10An analogue –to-digital convector Many transducers provide an analog output voltage and if this information is to be retained in a digital storage system it must be converted into its digital equivalent. 2.11 Time division multiplexing (T.D.M.) Multiplexing is the processor of carrying numerous signals along the same wire a digital systems multiplexing is performed by process known as time division multiplexing (t.d.m), which is illustrated in the figure. In the cause shown, four signals A, B, C and D are to be time division multiplexing and transmitted along a common transmission line.

Component of digital electronic control

3.1 Computer control strategy

A basic (d.d.c.) system is illustrated in the figure in this system a large number of transducers are sited around the plant, each transducer being connected to one input of multiplexer. A multiplexer can be regarded as the electronic equivalent of a switch with a contact or blade which rotates very rapidly so that it moves from one transducer to another the blade remaining in contact with the transducer long enough for an analogue to digital convector to sample and digitize or to quantize the analogue Signal. The quantized data are then transmitted along the data bus of the system the CPU. When CPU has analyzed the data from one or more transducer and has compared them with appropriate set point on the computer program it sends signals along the data bus to the valves controlling the system as follows. The digital signals produced by the CPU are converted into analogue signal by a d.d.c.

3.2 Microprocessor based control for gas turbine 3.2.1 Introduction An accepted view up to three years ago was that digital control was only viable economically for the more complex engines on supersonic aircraft. Work on digital controls for gas turbine started in the late 1960's since then a number of different engines have been run using both full authorities and supervisor components. In general this work has involved the more powerful mini computers but the theoretical research and development carried out on these units are applicable to the microprocessor system. Control of gas turbine engines by digital means present to fundamental new problem i.e. no loop problems occur tests have done the advantage flexibility during development and the ease of program adjustment. 3.2.2 Perspective The control box represents about 50% of the system cost. For an electronic control the constituent parts are shown in figure a) Means of conditioning the input signals either from the sensor or the pilot. b) Means for providing the output signals either for actuation display, warning or shutdown. C) The power supply and safety circuits. d) The control section (consisting of arithmetic and memory portions).

Figure (3) Elements of microprocessor

3.2.3 Element of a microprocessor control 1) Structure All inputs have to be converted to digital form directly and transducer signal isolated. In order to reduce cost and weight d.c signal are fit to multiplexer and then to an analogue to digital converter. Shaft speeds pass directly to a frequency to digital converter while switch inputs are fed through input register. The reminder of the store consists of random access memory which is used to hold intermediate data used in computations. The output will be either to stepper motors or toque motors together with some output registries for condition signal for monitoring or display. 2) Self monitoring One of the advantages of a digital controller lies in its ability to monitor its own operation the processor is a powerful device when logical testes and decisions are

Required and is therefore ideally sited to testing the reminder of the circuitry in systematic manner, thus:- a) All input conditioning transducer and output devices can be checked by the computer. b) The analogue multiplexer and the ADC are checked by switching to two reference voltage source and verifying the output. c) The transducers and signal conditioning circuits can be checked for credibility by making sure that their values lies within a given range and that not only the instantaneous value but also the time relationship of any parameter is consistent.

Having checked during each cycle the behavior of the major portion of the controller it's now necessary to device means of checking the computer & store. This can be done by an independent monitor, which is checked automatically during ground check procedures. The output from the microprocessor is obtained by incorporating a routine in the main program which exercises relevant instructions. If the answer is correct & in the correct time interval then it can be assumed the unit is functioning correctly. Most computer faults cause the computer to abandon completely its logical program execution hence its device forms a simple means of safety checking the performance.

(3) Diagnostic facilities:

  To simplify the servicing & maintenance task a binary coded display can be available which isolates a fault to a line replacement board.

(4) Software:

  The computer software forms one of the most important parts of the system. The aim of the software is to provide an exact representation of the various control laws; to detect faults in both the input signals & the system outputs & to take appropriate action in the event of failure; to provide facilities for ground testing & for self monitoring. 

The software can be resolved into a no. of distinct modules. To a certain extent these modules can be designed, programmed & tested individually before they are incorporated into the final system. In order to control the execution of these modules a system executive program is required. The executive program is activated when it receives an interrupt from the system block.

3.2.4 Performance:

  The use of a digital system can obviously increase the accuracy of the control and thus allow engines to run at their best operating point but to date no new and practical 'optimized' set of control parameters have occurred. On the aircraft side complex engine thrust control is possible which has the effect of increasing engine life by reducing overall turbine entry temperature.

3.2.5 Engine testes with a microprocessor:

  The microprocessor digital engine control exercise was carried out in three basic stages:

a) Simulation testing. b) Open loop work with the engine. c) Closed loop engine control.

The first work at the engine consisted of open loop tests to check the various system elements while the engine was operating under manual control, in particular, the various transducer inputs could be calibrated, once this had been achieved, it was possible to set up the filter parameters to provide smooth signals, and to check that the failure detection subroutine was operating as desired, having checked and adjusted the control as necessary the control loops where closed. The purpose of the closed loop engine control tests was to demonstrate the main features of the microprocessor control program, these where:

1) Automatic starting. 2) Range speed governing. 3) Acceleration and deceleration: both partial and full scale, as well as hot re-slams. 4) Temperature limiting. 5) Failure detection and action.

Test results: Starting was achieved by first scheduling a constant light- up flow , until light-up was obtained and thereafter scheduling a starting flow as a function of speed until idle condition was reached, light up was detected by observing a rapid increase in jet pipe temperature caused by the burning fuel . Range speed governing of the H.P shaft speed was achieved to within 0.1% of the given demand, the control provided a steady throttle demand signal, which produces the desired flow to the engine. The loop gain of the system was determined in order to give the optimum value at every engine speed, the experimental values were found to be very close to the predicted values, the gain was introduced in the control program as a function of the shaft speed.

3.2.6 COSTS:

  The major benefit in cost will occur in the general applicability of the unit in that it will be suitable for variety of the engines and a variety of operating modes, development costs will be minimal once a given standard has been achieved.

A reduction in cost will also occur indirectly in that due to the simplified trouble shouting procedures through fault detection and self test, maintenance cost will be lower.