CONTROL SYSTEM ACTUATOR POSITION SYNTHESIS FOR FAILURE DETECTION

Abstract: The redundant sensed position signal samples of a position actuator are compared to the synthesized value from an actuator model and rejected for use in the sample period if the difference magnitude between the sample value and the synthesized value exceeds an accepted magnitude, the periodicity of sample rejection is recorded and if the periodicity is sufficiently high the sensed feedback signal itself is rejected for use and the signal's position sensor is recorded as failed if both samples are simultaneously rejected a diagnostic routine determines if the fault is in the sensors or the common actuator.

Description: This invention relates to electronic engine controls (EECs) for gas turbine engines, and more particularly to failure detection circuitry for use therein. As known, EECs electronically control operation of gas turbine engines based on selected control algorithms executed by the EEC. The EEC applies the control algorithms in response to sensed engine parameter signals to provide electrical control signals to engine mounted electro-mechanical devices. These include linear and rotary position actuators which position various engine valves, vanes, etc. to regulate engine performance. To ensure reliability, the EEC includes redundant control circuitry, i.e. primary and secondary control channels. The sensed engine parameter signals are also redundant, and are applied to each of the EEC channels. The sensed parameter signals include analog (amplitude and frequency) signal formats, as well as digital. EP-A-0093276 discloses a turbine control system using redundant speed sensors feeding primary and secondary speed computers in primary and secondary control channels for the actuator of a steam valve controlling the steam supply to a steam turbine. A valve control selector is connected between the primary and secondary control channels and the valve to connect one of the control channels to the valve and to switch to the other control channel when the one control channel is faulty. The use of a mathematical model of a control system is shown in EP-A-0093276. Digital electronic engine controls (DEEC) use digital signal processing to apply the control algorithms to the sensed parameter signals. For the analog sensed signal formats this requires analog-to-digital (A/D) conversion, all of which is well known. Since A/D conversion is sensitive to spurious noise associated with the analog signal it is also known to provide the DEEC with input selection logic. This logic selects the best signal from among the two actual sensed parameter signals. The executed control laws algorithms provide the output control signals, including actuator position command signals to the engine mounted actuators, which physically position the various valves and vanes in the engine. Since reliability of the actuator loops is critical to engine reliability, the redundant position feedback signals from the actuator are compared to detect gross malfunction. As a result, the DEEC can detect a faulty actuator control loop and use a backup actuator system (if available). The difficulty, however, is the ability to isolate the particular failed component in the actuator operating system, and then to identify the failed component to maintenance personnel. The object of the present invention is to provide fault detection and fault reporting routines for use in DEEC systems. This object is achieved in an apparatus according to the preamble of claim 1 by the features of the characterizing part thereof. Embodiments of the invention are described in the dependent claims. According to an aspect of the invention, the redundant sensed actuator position signals are sampled periodically and the samples compared to a synthesized position value obtained from an actuator model, if the difference magnitude between the sample value and the synthesized value exceeds an accepted magnitude the sample is rejected for use in the sample period and the periodicity of sample rejection is recorded, if the periodicity becomes sufficiently high the sensed feedback signal itself is rejected for use in the actuator control loop and the signal's position sensor is recorded as failed, if both samples are simultaneously rejected in the same sample period a diagnostic routine determines if the fault is in the sensors or the common actuator, and the actuator is deactivated. These and other objects, features, and advantages of the present invention will become more apparent in light of the following detailed description of a best mode embodiment thereof, as illustrated in the accompanying drawings, wherein: Fig. 1 is a conceptual illustration of an electronic engine control, which is used in the description of the present invention; Fig. 2 is a system block diagram of the failure detection circuitry of the present invention; Fig. 3 is a functional diagram of an actuator model response, for use in describing operation of the circuitry of Fig. 2; and Fig. 4 is a flowchart diagram illustrating the steps performed by the circuitry of Fig. 2. Best Mode for Carrying Out the Invention Fig. 1 is a simplified, conceptual block diagram of a digital electronic engine control (DEEC) 10 in which the present invention may be used. The DEEC is connected through sensor lines 12 and control lines 14 to a gas turbine engine 16. The engine shown is a twin spool turbofan engine with a low compressor 18 driven through shaft 20 by a low turbine 22, and a high compressor 24 driven through shaft 26 by a high turbine 28. The present invention, however, may be used with any configuration gas turbine engine. The sensed engine parameters 30 include redundant low turbine and high turbine speed signals (N 1A , N 1B and N2A, N2B) and redundant sensed position feedback voltage signals F A , F B from each of the various position controlled devices in the engine. This includes fuel valve positioning. The redundant (A, B) signals are presented through lines 12 to the inputs 32, 34 of both a primary control channel 36 and secondary control channel 38 of the DEEC. In the conceptual illustration of Fig. 1, each DEEC channel is shown to include signal selection logic 40, 42, and actuator command logic 44, 46. The selection logic selects the best available sensed parameter signal (e.g. best of redundant sensed position feedback) for presentation to the actuator command logic. The actuator command logic applies the selected sensed signal data to provide actuator position control signal outputs 48. The control signals are provided through lines 14 from one of the two channels as determined by channel command logic 50, all of which is known. Fig. 2 is a detailed block diagram illustration of the relevant circuitry of the DEEC 10 primary and secondary channels 36, 38, in which the present failure detection circuitry may be used. In the embodiment of Fig. 2 the failure detection is for use with one of the engine position actuators 52, and its redundant actuator position sensors 54, 56. In Fig. 2 the redundant sensors are labeled position sensor A and position sensor B. The use of redundant position sensors, and redundant drive coils described hereinafter, is required due to their location within the hostile engine environment engine and also to ensure single fault tolerance of the DEEC. In some configurations redundant actuator loops may be provided to protect against single actuator failures. The position sensors A, B (54, 56) each provide an actual sensed position feedback signal (F A , F B ) on lines 58, 60 back to the DEEC. Analog-to-digital (A/D) converters 62, 64 within DEEC 10 convert the amplitude intelligence of the sensed analog signals to equivalent digital signals having one or more bytes of digital signal bits, and provided on lines 66, 68 to the primary and secondary channels 36, 38 of the DEEC. The actuator 52 may be either a linear or rotary type actuator; it is assumed in the Fig. 2 embodiment that it is a linear, pneumatic actuator driven by torque motor 70. The torque motor alternately provides pneumatic fluid through lines 72, 74 to upper and lower chambers of the actuator, to extend or retract the actuator position, all of which is well known in the art. The torque motor includes redundant current drive windings 76, 77 each of which is driven by separate analog drive current signals from individual digital-to-analog (D/A) converters 78, 79. Each D/A converter receives the equivalent digital actuator drive signals through lines 80, 81 from the associated primary and secondary channels. The primary and secondary channels are redundant controllers. However, only one channel is in control at any one time. Therefore, only one of the torque motor coils 76, 77 is energized at any one time. Channel command logic 86 connects the output (82, 84) from the controlling one of the two channels to the DEEC output 48 (Fig. 1). It does this in response to command signals received through lines 87 from the primary channel. To facilitate understanding, the channel command function is illustrated with the pair of single pole - double throw switches 88, 90; one connected to each channel's actuator current signal output on lines 82, 84. As shown, only one switch is closed at a time. In Fig. 2, only those portions of the primary and secondary channels as are necessary to teach the operation of the present invention, are shown. Each channel includes its own signal processor; signal processor 92 for the primary channel and 94 for the secondary channel. Each processor includes a central processing unit (CPU) 96, 98, which are typically sixteen bit machines, of a type known in the art. Each CPU is connected through a signal processor bus 100, 102 to read only memory (ROM) 104, 106 and random access memory (RAM) 108, 110. The ROM and RAM memories are similarly of a known type, with memory capacity determined by the particular DEEC control application. The ROM stores the CPU program memory, including the DEEC control algorithms and the failure detection algorithm of the present invention. The RAM provides data storage. Each of the signal processors 92, 94 include input interfaces 112, 114 for receiving the digital equivalent signal of the sensed position feedback signals F A , F B on the lines 66, 68. Each processor further includes some number of input/output (I/O) interfaces 116-120 and 122-126. The two CPUs exchange the sensed feedback signal information through lines 127 connected to I/O interfaces 117, 123. Other peripherals connected to the processor busses, which are not necessary to an understanding of the present invention, are not shown. The operation of the DEEC in controlling the position of actuator 52, is well known. Briefly, with respect to Fig. 2, control information relating to the desired position of the actuator is received by the primary and secondary channels. The DEEC selection circuitry and preceding control law functions (not shown) process the parent system information to provide actuator position command signals on the input lines 130, 132 to the primary and secondary signal processors 92, 94. The position command information on these input lines is multiplexed together with other input information received by the processors. The received position command signal is received by the CPUs 96, 98 through I/O interfaces 116, 122 and the processor buses 100, 102. The CPUs provide the required actuator drive signal through I/O interfaces 120, 125 and lines 80, 81 to the D/A converters 78, 79. The torque motor responds to the drive current signals on the energized one of the two windings to position the actuator. The drive current produces a slew rate in the actuator (length per unit time, i.e. velocity) and the position sensors A, B (54, 56) both track the actual position together; each providing the respective feedback signals F A , F B on the lines 58, 60. The present invention performs: (a) a selection of the one of two redundant sensed position feedback signals (F A , F B ) which is then used by the actuator control algorithm executed by the CPUs 96, 98. The selection of which of the two sensed feedback signals to be used is provided by a selection routine which compares the actual sensed magnitude of each of the sensed position feedback signals with a synthesized feedback signal (F S ) which represents the response of an idealized "model" of the actuator loop. Fig. 3 illustrates the use of the actuator loop in synthesizing the position feedback signal. The model 140 includes the sum forward gain (G) 142 of the actuator loop which receives the error signal on line 144 which is equal to the difference signal magnitude between the actuator position command signal on an input line 146 and the selected one of the two actual sensed feedback signals (F A , F B ) on lines 148. The two signals are summed in summing junction 150. The product of the error signal and the forward gain produces an actuator current signal on line 152 to the actuator model, which is illustrated as the LaPlace integral transform K/S 154. The integral, which represents the actuator position, is provided on output line 156 as the output D (Displacement) and as the synthesized feedback signal (F S ). The model is used to synthesize the nominal feedback signal for a present sample interval in response to the use of the selected one of the two actual position feedback signals in a preceding sample interval. The simulated F S , however, is not used to close the loop. Fig. 4 is the flowchart of the source code listing for performing the fault isolation algorithm of the present invention in the DEEC control channel signal processors (92, 94). In operation, instructions 160 request the sampling of the two actual feedback sample values F A , F B , and the synthesized value F S . Decision 162 determines if the F A FAIL FLAG is set. This flag is set in response to the permanent rejection of the F A sensed signal, as described hereinafter. The rejection is precipitated by either a "hard" failure (i.e. mechanical or electrical malfunction) or a "soft" failure (i.e. out of tolerance), all of which is described hereinafter. Assuming that there is no present rejection of the F A sensed signal, decision 162 results in a NO answer. Decision 164 next determines if the absolute value of the difference signal magnitude between the sensed signal F A and the synthesized signal F S is greater than a selected tolerance (T). If NO, indicating the sensed signal is in tolerance, instructions 166 decrement a counter (C A ) by one count. As also described hereinafter, the invention counts each "out of tolerance" occurrence of the sensed signal, which is a rejection of the sensed signal, as a multiple count value. Typically the rejection count value is three. Each nonrejected (in tolerance) occurrence of the sensed signal sample results in the single down count. As a result it requires three nonrejected samples to cancel each rejected sample occurrence. The presence of a positive sum count indicates a failure mode condition; whether consistent or periodic rejected samples which occur within the "healing" period established. Following the single down count of instructions 166 decision 168 determines if the present sum count is less than, or equal to zero (indicating no immediately preceding rejected samples). If YES (no evidence of sample malfunctions), instructions 170 select the F A sensed signal sample as the actual feedback position of the actuator for the present sample interval. This is indicated by instructions 170 which set the output to F A , after which the routine branches back to instructions 160. If the present F A sample is out of tolerance, resulting in a YES to decision 164, instructions 172 increment the sum count value C A by the multiple count value M. As stated, the multiple count value is typically three. Decision 174 next determines if the sum count is greater than a maximum allowable count value. If YES, indicating the presence of a high frequency of failure occurrences, instructions 176 set the F A FAIL FLAG; permanently rejecting the F A sensed signal for the remainder of the flight. Instructions 178 record the message "sensor A failed". The recorded message is for maintenance purposes in aiding the maintenance personnel to locate the failed component. The message is recorded in the RAM of the controlling one of the two channels (RAM 108, 110). Alternatively, the preferred recording site is in nonvolatile memory. Preferably it includes an electrically alterable read only memory (EAROM) 180 (Fig. 2) which can be written to directly by each of the two DEEC channel I/O interfaces 118, 124, through lines 182, 184. The EAROM content can be read through lines 185 on the ground. This provides a failure history log which is accessible by maintenance personnel. Referring again to Fig. 4, following: instructions 178, a YES to decision 162, a NO to decision 174, and a NO to decision 168, decision 184 determines if the F B FAIL FLAG is set. The F B condition is not examined until the F A sensed signal sample is rejected. If NO, decision 186 determines if the difference signal magnitude between the F B sample value and the F S value is greater than the selected tolerance (T). If NO, instructions 188 decrement the sum count C B , for the F B sensed signal, by one. Decision 190 next determines if the C B sum count value is less than or equal to zero. If YES, instructions 192 select the F B sensed value as the output for the present sample cycle. If the F B sample is out of tolerance, a YES to decision 186 is followed by instructions 194 which increment the C B sum count by the multiple count value M (typically three). Decision 196 next determines if the count C B is greater than the maximum. If YES, indicating that the F B sample is also rejected, decision 198 determines if the F A FAIL FLAG is set. This decision is a diagnostic determination. If the F A FAIL FLAG is already set, and now the F B sensed sample value is rejected, there may be a common cause to both failures; namely, the actuator itself may be at fault. Therefore, in response to a YES to decision 198, decision 200 determines if the difference signal magnitude between the F A and F B samples is greater than a selected error (E) amount. If NO, this indicates that the sensor outputs are tracking within a relative tolerance band; each, however, are out of tolerance with respect to the synthesized F S value. It is assumed in response to a NO from decision 200, that the fault does not lie in the sensors but instead in the actuator itself. As a result, instructions 202 record an "actuator failed" message in memory, i.e. in EAROM 180. If the answer to decision 198 is NO, meaning that there is no apparent trend to sensor failures, instructions 204 set the F B FAIL FLAG, indicating permanent rejection of the F B sensed signal. Following instructions 204, or a YES to decision 200 (the two sensors are not tracking within an acceptable E error band) instructions 206 record a "sensor B failed" message in memory 180 (Fig. 2). Following instructions 202, 206, or a YES to decision 184, instructions 208 request that the actuator be depowered to a predetermined, safe condition. This is necessary since neither of the two sensed position values were found to be acceptable, or the actuator itself was determined to be at fault.

Claims: Apparatus for diagnosing failures in an actuator loop of a digital electronic engine control (DEEC) which controls the actuator position in each of a plurality of successive sample intervals in dependence on the difference signal magnitude between an actuator position command signal sample and an actual actuator position signal sample selected from one of two sensed actuator position signals provided from first and second position sensors associated with the actuator, comprising: model means (140), for simulating the signal response of the actuator loop to the difference signal magnitude between the actuator position command signal sample (POSITION COMMAND) and the selected actual position signal (F A ; F B ) sample from a preceding sample interval, to provide a synthesized actuator position signal sample (F S ) representative of the expected actuator position in a present sample interval; characterized by: signal selection means (40, 96, 42, 98), responsive to the actual position signal samples (F A ; F B ) of each sensed actuator position signal and responsive to said synthesized signal sample (F S ), for determining the difference signal magnitude between each actual position sample (F A ; F B ) and said synthesized sample (F S ) to select for use by the DEEC, a first one of the actual samples (F A or F B ) whose difference value is within a maximum position difference limit (T), for rejecting for use in a sample interval an actual sample (F A ; F B ) having a difference signal value greater than said maximun position limit, for providing a fail signal (F A FAIL; F B FAIL) identifying as failed any of said position sensors providing an actual signal sample having a difference magnitude greater than said maximum limit in a successive number of sample intervals; and recording means (RAM 108, 110), for recording each of said fail signals from said selection means to provide a failure diagnosis record for the actuator loop. The apparatus of claim 1, wherein said signal selection means, in response to both of said actual samples (F A ; F B ) having a difference magnitude greater than said maximum position limit, includ means for providing an actuator loop failure signal (208) to the DEEC to command actuator shutdown, for determining the difference signal magnitude (164; 186) between said actual samples, and in response to an actual sample difference signal magnitude less than an error difference limit, for providing a failure signal (176; 204) identifying the actuator as failed. The apparatus of claim 1, further comprising: counter means, responsive to each of said actual sample rejections by said selection means, for counting (1'; 194) each sample interval rejection as a plural count value and each sample interval nonrejection of a particular actual sample as a single count value, said counter subtracting (166; 188) said single count nonrejections from said plural count rejections to provide a sum count signal indicative of the accrued rejections and non-rejections in each present sample interval, and detection means, responsive to said sum count signal, for providing (174; 196) a maximum sum count signal in response to said sum count value exceeding a maximum selected count value, and wherein said selection means is responsive to said maximum sum count signal for providing (176; 204) said fail signal in the presence thereof.

Inventors: SPOCK, WAYNE RICHARD, URBANIK, PETER JAMES
Applicants: UNITED TECHNOLOGIES CORPORATION
Application year: 1985
Main classification: G06F15
IPC codes: G05B9, G05B23, F02D45