Digital Controller Improves Power and Flexibility of Gas Turbine Driven M1A1 Tank

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1 CS THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS 345 E. 47 St., New York, N.Y The Society shall not be responsible for statements or opinions advanced in papers or in discussion at meetings of the Society or of its Divisions or Sections or printed in its publications- M Discussion is printed only if the paper is published in an ASME Journal. Papers are available ]^C from ASME for fifteen months after the meeting. Printed in USA. 91-GT-295 Copyright 1991 by ASME Digital Controller Improves Power and Flexibility of Gas Turbine Driven M1A1 Tank J. ERICKSEN E. GODERE A. WRIGHT TEXTRON Lycoming Stratford, CT ABSTRACT This paper describes the design and development of a Digital Electronic Control Unit (DECU) that replaces the existing Analog Electronic Control Unit (AECU) on the MlA1 battle tank's TEXTRON Lycoming AGT1500 gas turbine engine. This program marks the first application of a digital control on a vehicular gas turbine engine. The DECU preserves all functions of the AECU and is interchangeable, while allowing engine performance improvements, such as 20 percent fuel savings at idle. Diagnostic capabilities, using existing control sensors, were added to identify 90 percent of failed Line Replaceable Units. Controls strategies to achieve these results, such as adaptive routines, and some of the illustrative differences between Analog and Digital control implementations encountered in this application, are discussed. On 14 September 1990, two production Digital Electronic Control Units (DECU) were shipped to Lima Army Tank Plant in Lima, Ohio. These DECU's will be installed into M1A1 battle tanks to operate TEXTRON Lycoming's AGT1500 gas turbine engine (see Figure 1). Their advent marks the culmination of a two year development and testing program, which began when the Army Tank Command (TACOM) awarded funding to develop a digital tank engine control in May Since then, more than 20,000 miles and 4,000 hours of bench and vehicle testing has been done on the DECU. This engine was integrated into the M1A1 tank in It is the only gas turbine engine in an American tank and promises to hold its edge over the competing diesels into the next generation of tank. The Analog Electronic Control Unit (AECU) was introduced along with the turbine. At the time, analog controllers were the leading edge in technology, replacing the less expensive hydromechanical controller. Now, the DECU will gradually replace the AECU in the M1A1 system. Figure 1 M1A1 Battle Tank For logistical reasons, the DECU installation cannot require any system changes, and minimal system adjustments are allowed. The DECU must also preserve all of the AECU's functions to facilitate a vehicle retrofit. It was sold to the Army, however, to improve engine fuel economy and durability as well as to incorporate diagnostics to identify failed line replaceable units (LRU). Because of the advantages of a digital implementation, the DECU will result in performance and convenience benefits that are not available with an analog or hydraulic control device. The DECU development program reported herein illustrates the advantages of a digital system over its analog predecessor. THE DIGITAL ADVANTAGE Some goals of a control system are to increase performance envelope and to reduce operating costs. A Presented at the International Gas Turbine and Aeroengine Congress and Exposition Orlando, FL June 3-6, 1991

2 simple, inexpensive hydro-mechanical controller can guarantee that the minimum performance criteria are fulfilled; however, it sacrifices performance margin to protect against tolerance stack-ups, both for the engine manufacturing and control system manufacturing. An analog controller can introduce some feedback techniques, such as integrators or proportional gains via op-amps, resistors, capacitors, and some rudimentary decision-making concepts (select high or low or if-then structures) via transistors. It can also allow tracking of linear schedules and offer some flexibility in changing schedules. The fundamental advantage of a digital controller is that all control work is done through software. A digital controller allows the designer to do many things that cannot be accomplished in analog circuitry alone. Algorithms, calculations of non-linear functions and intricate decision-making structures can be programmed in the software. Further, each control function in an analog controller requires a separate bit of circuitry, and adding functions requires board redesign or jumpering. In a digital controller, as long as there is adequate program memory and throughput, functions are added by inserting a module of code in the software. Consequently, a digital controller can perform more functions than an analog controller in the same space. Finally, software (or the microchips) can store information when power is removed, thus creating a means of retaining historical information. These traits allow more complicated scheduling which can decrease surge margin, coordinate sensor data for adapting control schedules to optimize a given engine's performance, incorporate more versatile protective modes for sensor failures, and reduce acceleration and deceleration times. In addition, the user can expand the controller's capability by adding more memory chips and modifying the existing architecture. Using a digital controller for development allows engineers to rapidly implement concepts from an abstract "brainstorm" to a testable box. Not only can schedules, gains, and constants be altered or tweaked on test location, but also programmed algorithms can be iteratively fine-tuned between tests without the component level cuts-and-jumpers required with an analog control. Further, internal variables can be accessed each program cycle, so it is possible to monitor the controller's decision making process. The engineer can identify the source of undesirable oscillations, unexpected fuel over-rides, and other control mode anomalies. Finally, the digital controller can adapt to vehicle system alterations by downloading new software and modifying the I/O circuitry. This creates fewer obstacles to retrofit than would be required by an analog controller. Also, once a "box" has been developed and put into production, the same control structure and electronics can be used on different (but similar) engines within the same system. SYSTEM DESCRIPTION In the M1A1 system the DECU receives commands from the Driver's Control Panel, feedback signals from the Hydro-Mechanical Unit (HMU), and speed and temperature inputs from the engine (see Figure 2). It processes this information to actuate the engine accessories, to meter fuel to the engine, and to schedule the engine's variable geometry through the HMU's hydraulic actuators. The DECU also transmits output information to the Driver's Control Panel to provide the operator with critical engine parameters. INPUTS 1COMPUTATIONS OUTPUTS FUEL FLOW ENGINE SENSORS SENSOR CONDITIONING FUEL CONTROLLING TI INLET TEMP COMPUTATIONS IT TURBINE INLET TEMP W, ELECTRO METERING I CUTOFF NET POWER TURBINE SPEED MECHANICAL IGV FLOW IGV ' BACKUP ^\ NR HIGH PRESS COMP SPEED COMMANDS PTS (ELECTRICAL) ' IGV FES LOGIC VALVE PTS FLOW ACCESSORIES FEEDBACKS DRIVER REQUESTS F/B TRANSDUCER r^ DETECTION (ELECTRICAL) START MODE STR OUTPUT CONDITIONING TWIST GRIP REQUEST (PLA) pla FEEDBACK (MECHANICAL) FAULT DIAGNOSTICS TACTICAL IDLE FAULT DISPLAY IGV PURGE PTS PROTECTIVE MODE RESET ENGINE ACCESSORIES STARKER VEHICLE IGNITION EXCITER PIVOT STEER/NEUTRAL ELECTRICAL POWER iiiii- DRIVER INFORMATION NET METER TIME METER Tr OVERTEMPERATURE LIGHT NPT OVERSPEED LIGHT PROTECTIVE MODE LIGHT THE AGT1500 MIL-STD DATA BUS ---^ INTERFACE ^--- DIAGNOSTIC CONNECTOR Figure 2 DECU System Operation Textron Lycoming's AGT1500 gas turbine engine consists of a counter-spun two-spool compressor system, an independent power turbine spool, and a recuperator (see Figure 3). A system of variable inlet guide vanes (IGV) controls air flow to the low pressure (LP) compressor. The interstage bleed valve extracts air from the high pressure (HP) compressor to relieve loading in the low power regime. The power turbine stators (PTS) modulate air flow to the power turbine (PT). M" DIFFUSER HOUSING RECUPERATOR VARIABLE SINGLE CAN POWER TURBINE COMBUSTOR STATORS VARIABLE AND SCROLL EXHAUST GAS INLET GUIDE ^ _... T6 t Tt -:.. 1'.s dh.^ 7 *0tj*V 1, II POWER LOWPRESSUREI P _, - t))... I \'I \ COMPRESSOR \f1( I -YI\u \\ CCESSORY HIGH HIGH LOW TWO STAGE REDUCTI GEARBOX PRESSURE PRESSURE PRESSURE POWER GEAR COMPRESSOR TURBINE TURBINE TURBINE Figure 3 AGT1500 Description THE HYDRO MECHANICAL UNIT (HMU) The HMU (see Figure 4) is a fuel metering device which contains an HP compressor-driven positive displacement pump, and solenoids to meter fuel flow to the actuator

3 (IGV and PTS) pistons and to the fuel nozzles. The HMU also contains transducers to measure actuator feedback and fuel valve position. programmable memory, 2K words of fault and calibration data, and 8K words of RAM. It has a 10-bit, buffered digital-to-analog (D/A) converter to accommodate analog signal outputs, and a 12-bit, buffered analog-to-digital (A/D) converter. A Universal Asynchronous Receiver/Transmitter (UART) chip allows communication with an RS 232 serial data link, through which the DECD is programmed and diagnosed. Inputs and outputs are shown in Figure 2. Figure 4 Hydro Mechanical Unit (HMU) 1. Fuel Pump 2. Quill Shaft 3. Bypass Valve 4. Metering Valve 5. Fuel Flow Solenoid 6. Linear Variable 7. Relief Valve Differential Transformer 8. Backup Solenoid 9. PTS Solenoid 10. IGV Solenoid 11. PTS Rotational Variable 13. Cutoff Solenoid Differential Transformer 14. Check Valve 12. IGV Rotational Variable Differential Transformer THE DRIVER'S PANEL The Driver's Panel contains switches for stop, start, starter only (purge), tactical idle and fault reset. The driver controls fuel with a twist grip throttle (Power Lever Angle (PLA)) and selects the transmission state (reverse, drive, pivot steer, and neutral) with a switch. The panel has lights to indicate engine overspeed and faulty fuel control, as well as an analog gauge to indicate power turbine speed measured in RPM. THE DECU HARDWARE The DECU is a 17" x 17" x 3" box weighing roughly 33 pounds (see Figure 5). It has four connectors on its front face and a streamlined backup battery on its rear face to fit into the limited space in the vehicle (3 cubic feet). On one side, there is an LED display window for viewing diagnostic fault messages. The DECU receives input signals and transmits output signals through harnesses attached to three of its four connectors. The remaining connector transmits diagnostic signals for test equipment and, during normal operation, is covered by a diagnostic cap. This diagnostic cap contains a signal pulse switch that activates the DECU's diagnostic built-in-tests. The core of the DECU is Bendix Engine Control's BX1750A 16-bit, complementary metal oxide semiconductor (CMOS) microprocessor, executing MIL-STD-1750A architecture with floating point standard. It has EEPROM chips, configured for 52K 16-bit words of Figure 5 The Digital Electronic Control Unit (DECU) THE BASIC CONTROL STARTING When the driver issues a start command the DECL engages the starter motor and the ignition exciters. Fuel request is ramped from 40 pph to 165 pph at a rate of 5 pph per second. When HP compressor speed (NH) passes 5 percent, the cutoff solenoid is disengaged, allowing fuel to flow to the nozzles. At 5C percent, NH speed, the starter is disengaged, and the start is completed. RUNNING Steady-state fuel flow is scheduled proportionally to PLA, ranging from 40 pph to 800 pph. This fuel flow request is modified by the minimum speed governors, which prevent NH and PT speed (NP) from dropping below their idle points, and the maximum speed governors, which prevent NP and NH from exceeding a maximum value. There are two NP idle governor settings: 30 percent when the driver's panel selects low idle and 45.6 percent when the driver's panel selects tactical idle. The minimum NH speed is 52 percent in the DECU, compared to 59 percent in the AECU. The maximum NP speed is 80 percent if the transmission is in pivot steer or neutral settings, and it is 103 percent if the transmission is in drive or reverse. The maximum NH setting is biased by Ti to generate 1500 shaft horsepower (shp) for all ambient temperatures (q.v. Manual NH Trimming, et al). An acceleration is signalled by a rate of change of PLA or NH, by switching from low idle to tactical idle, or by switching from pivot/neutral to drive/ reverse. Fuel request is ramped to the new PLA or idle setting. Fuel request rate is limited by a maximum T7 schedule and by a maximum rate of change of NH (NH-dot) limiter.

4 Deceleration fuel is controlled by a minimum schedule, which is determined to prevent flame-out in the combustor and roll-back of the HP compressor spool due to improper fuel-to-air ratio. The deceleration schedule dominates only when all other fuel requests (PLA, underspeed governors) disappear. Finally, the DECU controls temperature gradients across the engine recuperator by limiting fuel based on an estimate of recuperator gas temperature. Because this location is not instrumented, an algorithm simulates the average transient temperature of the recuperator, based on an existing measured temperature upstream of this position (T7). During-steady state operation, there are two PTS control methods, the first designed to minimize fuel consumption (called Fuel Economy schedules) in the idle and low power regimes, the second to approach 1500 shp at constant combustor exit temperature (T5). For low NH speeds, a constant PTS area (determined by feedback position) is held. As NH speed increases, PTS transitions to a closed-loop control, which schedules PTS to track a T7 schedule. During an acceleration, the PTS are stepped to fully open position to reduce temperature. During steady state, in the low power regime, the IGV are held closed and the bleed valve is held half open. As inlet-referred NH speed increases, the IGV are gradually opened and the bleed valve gradually closed. On an acceleration or deceleration, the IGV are stepped closed and the bleed pops open to prevent surge in the compressor. SOME SPECIAL DECU FEATURES MANUAL NH TRIMMING AUTO NH TRIM AND ENGINE HEALTH CHECK The turbine control governs HP compressor maximum speed. Because no two engines will produce exactly 1500 SHP at exactly the same compressor speed, each engine requires a unique maximum speed value. The proper 59 F referred speed for each engine is established at final engine test and is stamped on each engine. Programming the maximum HP compressor speed into the AECU is done by running up to maximum power and adjusting a potentiometer on the housing of the AECU until actual speed equals the speed calculated based on engine performance. This is a clumsy and time consuming task. With the DECD, manual trimming is done by selecting a sequence of operator inputs (Fault Reset followed by Tactical Idle) when the engine is not running. A routine is activated which displays the trim value in the diagnostic window. The operator can vary the trim in 4 hertz increments by pulsing the fault reset button. With use, the maximum speed (NH) to achieve 1500 shp changes. By observing engine inputs, the DECU continually recalculates the optimum operating speed to assure proper performance throughout the life of the engine. These same engine inputs are used to track engine performance and available power output using a performance-based algorithm, called the Engine Health Check. This information, which is displayed in the diagnostic window, can be used to determine the probability of successfully completing a given mission, and to improve system readiness by setting maintenance and replacement cycles for each engine. LOW OIL PRESSURE LOGIC When the HP compressor speed was lowered to conserve fuel (q.v. the Basic Control), the gear-coupled oil pump speed was reduced. This lowered flow to the lubrication system. When oil pressure drops below a critical level (11 psi), a pressure switch signals the DECU to shut down the engine. At the lower pump speed it was expected that almost fifty percent of the fielded engines would unnecessarily trip this switch at idle speeds. Rather than adjust the oil pump as part of DECU installation, the DECD was programmed to ignore the external shutdown signal and to step up the HP compressor idle point one percent per second. When oil pressure returns to an acceptable level, the DECU stops increasing the idle point and locks this point as the new minimum governor. If speed exceeds a predetermined level (60%) and the external shutdown signal persists, the DECU shuts down the engine and stores a diagnostic message in fault EEPROM to inform the user that oil pressure was low. Increasing the HP compressor idle speed, however, causes the engine to consume more fuel. The DECU, therefore, stores another diagnostic message in fault EEPROM for the user to adjust the oil pump whenever the Low Oil Pressure logic is triggered. PTS ADAPTIVE ROUTINE The PTS are mechanically powered by a piston actuator, which is driven by fuel delivered from the HMU. Originally, PTS were controlled closed loop to track a temperature schedule (see Figure 6). Electrical f Offset s RV DI Figure 6 PTS Loop Improved fuel scheduling (q.v. the Basic Control) required a known PTS position to be regulated. An open loop position request does not yield an exact position because of system tolerance stack-up. This stack-up results from variance in solenoid installed position, in fuel nozzle back pressure, in mechanical alignment of the stators on the engine, and in circuit components. An adaptive routine was developed to compensate for these errors. While the engine is shut down, the PTS 4

5 are manually aligned in the desired idle position. The DECU is instructed to store the measured feedback value of this position by a sequence of operator inputs. When the engine has started and achieves idle, the DECU modifies PTS request until the feedback position lines up with the stored value. This enables the DECU to hold a desired PTS position despite the actuation system inaccuracies. FUEL ADAPTIVE ROUTINE Although the PTS adaptive routine was developed first, the fuel adaptive routine, which is similar in concept, had a much more significant performance impact. The fuel metering system operates similarly to the PTS system (see Figure 7). Tolerances, like the PTS system tolerances, cause large variations between requested fuel flow and actual fuel flow from one system to another. The controller normally operates closed loop on speed, where these errors have no impact. There are, however, three critical points where fuel request is open loop: on a start, at minimum fuel request, and at maximum fuel request. These errors are most apparent at the minimum request and during a start where the magnitude of the error is comparable to the request magnitude. Electrical Offset flow to fuel DRIVER fuel nozzles SOLENOID, AND fuel adaptive routine has a powerful impact on performance on starting and at idle. RESOURCEFUL EXPANSION Having only the existing M1AI system inputs and outputs limited the expansion of the DECU's flexibility. Therefore, the DECD assigns different functions to some of the discrete inputs depending on the mode of operation. For example, there was a need for a new input from the driver's panel to activate pre-shutdown logic to open the PTS. The starter only switch, which activates only the starter if the engine is not running, was reassigned to initiate this logic if the engine is running at operational speeds and temperature. This approach of assigning multiple functions to an input or output was also used to communicate with the driver. By flashing a light at different frequencies, the system denoted either that the vehicle was running on the backup battery, that the engine was in pre-shutdown mode, or that the fuel control was faulty. ntagnosttcs The DECU was designed with three areas of fault identification: static detection of LRU hard faults, intermittent faults detected during operation, and system diagnostic faults. These faults are captured using existing sensors normally used for control purposes. request METERING VALVE fuel feedback LV D T Figure 7 Fuel Loop Two field incidents illustrate these problems. The first occurred at Fort Knox during the idle fuel consumption demonstration. On one of the vehicles, there was a 30 pph error between fuel request and metered fuel. At idle, the minimum fuel request of 40 pph resulted in an actual flow of 70 pph. Both NH and NP ran above their governors, even though the throttle had not been advanced. Consequently, no fuel savings resulted from the fuel economy schedules (q.v. the Basic Control) until the fuel adaptive routine was developed and implemented. The second incident occurred at Yuma Proving Ground during the summer. A 20 pph fuel error caused consistent start aborts due to over-temperature. Testing was conducted which determined that 20 pph fuel error resulted in 100 F hotter starts. Therefore, when the ambient temperature passed 100 F, it was common to see hot re-starts whose temperature (T7) peaked around 1400 F to 1500 F and caused start abortion as they exceeded the 1360 F limit. The fuel adaptive routine computes the average error between fuel request and measured feedback. This compensation is added to fuel request to generate a modified fuel request that ultimately eliminates system error. Although the concept is simple, the j Hard faults are determined with the DECU installed and powered. Upon activation of the diagnostic button, a voltage is applied to all of the input and output circuits for less than a second, during which time the signal levels are captured. These voltage and current readings are then compared to predetermined limits in the software to evaluate whether any short or open circuits exist. In order to establish confidence in the fault criteria, a twenty vehicle survey of sensor impedance was conducted to determine fault limits. If any faults are reported during this test, they are translated to a message and displayed on the diagnostic window. Twenty-one different faults addressing seventeen potential devices external to the DECD can be identified. Intermittent faults are detected during normal operating conditions. Whenever any of the control inputs exceeds a rate limit more than once or violates range limits, an intermittent fault is stored in fault EEPROM. In some cases the existence of one fault may introduce other associated faults. For example, the shorting of a primary winding of a transducer can generate a fault in the secondary feedback winding. Therefore, routines were developed to identify the initial fault and to mask unique electrical couplings. System diagnostic faults are detected by algorithms based on system behavior and vehicle interaction. For example, a failed fuel solenoid is diagnosed when the fuel feedback signal differs by a certain amount from the fuel request. Also, without any pressure sensors installed, more obscure problems such as a fuel line restriction can be reported.

6 CONCLUSION The DECU development program has provided a functional equivalent to the existing AECU without requiring system modifications. The DECU provides the user with conveniences, such as push-button NH trimming, automatic adjustments to operating schedules, engine health check, and a flashing light to tell the user the engine is cool enough to shut down. Cost savings, such as fuel economy schedules, fuel scheduling to protect the recuperator, and diagnostics to reduce engine and accessory trouble-shooting are available with the introduction of the DECU. These features have been proven with 4000 hours of bench and vehicle testing. As the demand for more sophisticated and precise system integration increases so will the requirement for a digital controller. The DECU's inherent ability to be easily reprogrammed with additional functions, has demonstrated its worth in an ever changing system. The DECU will prove to meet the challenge of the future economic constraints with it's flexibility to adapt to more than one application. 6