Abstract:
A method and system for controlling a propulsion system of a vehicle. The propulsion system includes at least one propulsion effector. The system also includes sensors for sensing vehicle position and motions, a control device for generating control signals, and a generator for generating desired vehicle forces/moments from the sensed vehicle position and motions and the generated control signals based on predefined vehicle compensation and control laws. Also included is a neural network controller for generating propulsion commands for the at least one propulsion effector based on the generated desire forces/moments, wherein said neural network controller was trained based on pregenerated vehicle control distribution data.

Description:
FIELD OF THE INVENTION 
     This invention relates to methods and system for performing propulsion control in a vehicle and, more particularly, to methods and system for accurately determining propulsion commands in real-time. 
     BACKGROUND OF THE INVENTION 
     While, as will be understood from the following description, the present invention was developed for generating accurate and time-critical propulsion commands for a vehicle&#39;s propulsion system. 
     Propulsion systems in vehicles, such as vertical takeoff and landing aircraft, satellites and ejection seats, provide forces for controlling the operation of the vehicle. These propulsion systems are controlled by controllers that provide control distribution commands to the propulsion systems. The control distribution commands must provide timely and accurate commands in order for the propulsion system to keep the vehicle operating within predefined limits. For example, the controller in a vertical takeoff and landing aircraft must accurately supply control distribution commands to the propulsion system according to analysis of pilot entered commands, aircraft position and motion, and predetermined limits of the propulsion system, aircraft, and pilot. If the control distribution commands are untimely or inaccurate, jet engine response can lag or misinterpret pilot inputs to the point of causing dangerous pilot induced oscillations. If the control distribution commands are inaccurate, jet engine total flow requirements will not be satisfied leading to engine flameout, degraded performance, or catastrophic failure. 
     Currently, controllers provide control distribution commands using iterative or multiple step algorithms implemented on digital microprocessors. This approach includes linear optimization or pseudo-inverse procedures that are inherently iterative in nature and have varying times of convergence. Iterative algorithms, when used for complex vehicle control, require extensive calculations and thus, have difficulty performing the required computations in real-time. When the iterative and multiple step algorithms cannot be executed in real-time, non-optimal approximate solutions are generated, thereby reducing the accuracy of the control distribution commands. 
     Another method of generating control distribution commands is a non-iterative linear technique, which uses mode switching when control limits are reached. This results in complex mode switching logic that is impractical to implement and verify. Another method implements multi-dimensional linearly interpolated data tables. This method requires excessive memory to store the data and is unable to execute the linear interpolation algorithms in real-time. 
     In summary, the present methods for providing control distribution commands require excessive microprocessor memory and complex mode switching, and are generally insufficient for providing real-time implementation. 
     The present invention is directed to overcoming the foregoing and other disadvantages. More specifically, the present invention is directed to providing a method and system for generating accurate control distribution commands in real-time. 
     SUMMARY OF THE INVENTION 
     In accordance with this invention, a method and system for generating propulsion commands for controlling a propulsion system of a vehicle is provided. The propulsion system includes at least one propulsion effector. The system includes at least one sensor for sensing vehicle position and motion, a control device for generating control signals, and a generator for generating desired vehicle forces/moments from the sensed vehicle position and motion and the generated control signals based on predefined vehicle control laws. Also included is a neural network controller for generating propulsion commands for the at least one propulsion effector based on the generated desired forces/moments, wherein said neural network controller was trained based on pregenerated vehicle control distribution data. 
     In accordance with other aspects of this invention, the vehicle control distribution data is generated off-line using a linear iterative search algorithm that determines propulsion effector commands from the desired vehicle forces/moments. The linear iterative search algorithm proportionally deviates desired forces/moments or deviates in a prioritized order from desired forces/moments to determine propulsion effector commands. 
     In accordance with still other aspects of this invention, the neural network controller is a unified or a decoupled structure with activation functions. The activation functions are sigmoidal, radial basis or linear functions. 
     In accordance with further aspects of this invention, the neural network controller is trained off-line using dynamic derivatives and a node decoupled extended Kalman filter technique or a gradient descent technique. 
     In accordance with yet other aspects of this invention, the neural network controller is implemented as an analog electronic circuit, a computer program, or an application specific integrated circuit. 
     As will be readily appreciated from the foregoing summary, the invention provides a new and improved method and system for providing control distribution commands to a vehicle&#39;s propulsion system. Because command processing time is greatly reduced, time delays in processing control commands and dangerous propulsion system responses are avoided. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a block diagram of a vehicle with a control computer for controlling a propulsion system according to the present invention; 
     FIG. 2 is a flow diagram illustrating the creation of a neural network controller used in the control computer of the present invention; 
     FIGS. 3A and 3B are diagrams illustrating neural model network structures used in the present invention; 
     FIG. 4 is a diagram of a single neuron model of the present invention; 
     FIG. 5 is a flow diagram illustrating the process of using a trained neural network in accordance with the present invention; 
     FIG. 6 is a side view of an ejection seat; 
     FIG. 7 is a rear view of the ejection seat shown in FIG. 6; 
     FIG. 8 is a data flow chart of an exemplary embodiment of the present invention suitable for use with an ejection seat of the type shown in FIGS. 6 and 7; and 
     FIG. 9 is a graph of three varying inputs into a control computer formed in accordance with the present invention and formed in accordance with a previously known method; and 
     FIGS. 10 and 11 are graphs comparing a control computer formed in accordance with the present invention and formed in accordance with a previously known method simulated in the ejection seat shown in FIGS.  6  and  7 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 illustrates the components of a neurocomputing control distribution system formed in accordance with the present invention. A vehicle  50  includes some type of propulsion system  56 , sensors and controls  58  for sensing propulsion system and vehicle operation and receiving control commands, and a control computer  60  for determining desired vehicle forces/moments and generating propulsion system control signals. Propulsion system  56  includes one or more control effector for providing propulsion. Examples of vehicle  50  are an airplane, a helicopter, an ejection seat, a satellite, or other vehicles with propulsion systems that require control. The control computer  60  includes a neural network controller  66  and memory  70 . Prior to implementation of the neural network controller  66 , the neural network controller  66  is trained off-line, using a simulation of the vehicle and its propulsion system to acquire knowledge of actual vehicle operation. The type of neural network and training process used is described in more detail below with respect to FIGS.  2 - 4 . 
     FIG. 2 illustrates the neural network controller  66  creation process. First, at block  80 , vehicle position and motions are sensed. An actual motion signal is generated based on the sensed vehicle position and motions. See block  82 . The types of positions and motions sensed are based on the nature of the vehicle  50 . Examples of positions are roll, pitch and yaw axes positions, and examples of motions are linear and angular velocity and acceleration. Simultaneously, at block  88 , command signals are generated from either pilot or operator input or from automatic preset commands. Then, at block  90 , desired linear and angular forces are calculated based on predefined vehicle compensation and control laws. These calculations are conducted off-line, i.e., they are not conducted in real-time. The off-line calculations are preferably performed on computer programmed to simulate the vehicle. As can be readily appreciated by those of ordinary skill in the art of propulsion control systems, the predefined compensation and control laws are determined by the nature of the vehicle and the type of vehicle sensors employed. 
     Next at block  96 , feasible propulsion system forces are calculated based on the generated desired forces/moments and the predefined limits of the propulsion system. Again these calculations are conducted off-line. Desired force/moment generation is performed for numerous and extreme possible combinations of vehicle positions and motions. As a result, feasible propulsion system forces are calculated for nearly all possible vehicle positions and motions. An example of one technique for generating feasible propulsion system forces is a linear iterative search algorithm that iteratively reduces the generated desired forces/moments by a proportionate amount until a reduced force/moment is within the predefined limits of the propulsion system. Other algorithms or techniques, such as deviating in a prioritized order from the desired forces, may be used to determine feasible propulsion system forces. All off-line computations of the control distribution data are performed on a vehicle simulation computer with comparable processing power to that of the vehicle&#39;s control computer. As a result, the off-line computations are performed as they would be in a traditional on-line control computer. Once the process for producing feasible propulsion system forces is complete, a neural network is established and trained according to the generated desired forces/moments and feasible propulsion system forces; the combination of which is the propulsion system&#39;s control distribution data. 
     Still referring to FIG. 2, at block  98 , the neural network structure is established. The neural network structure includes inputs for all the different desired forces/moments and control outputs for all the corresponding feasible propulsion system forces that are supplied to the control effectors of the vehicle&#39;s propulsion system  56 . As shown in FIG. 3A and 3B, the neural network structure used in the present invention is either a unified or decoupled multi-layer perceptron artificial neural network. Next at block  100 , the neural network structure teaching algorithm generates nodal weight values for the established neural network structure based on the generated control distribution data (desired forces/moments and the feasible propulsion system forces). Back-propagation algorithms, node decoupled extended Kalman filter methods, or other neural network training techniques can be used to perform this nodal weight value generation. Because these are recursive neural network teaching techniques, the weight values are adjusted for each set of control distribution data (desired forces/moments and corresponding feasible propulsion system forces). Proper setting of weight values increases the chance that all on-line generated data will be accurate. Once all the weight values have been generated for the neural network structure, the neural network is implemented as a controller in the control computer  60  of a vehicle  50 , as shown in FIG.  1 . See block  104 . Neural network operation in a vehicle is shown in FIG.  5  and described below. The neural network can be implemented as an analog electric circuit, a computer program, an application specific integrated circuit or any other structure that allows the neural network to function properly. 
     Returning to FIGS. 3A and 3B, the neural network includes an input layer, one or more hidden layers, and an output layer. The elements that make up each layer of a neural network are referred to as neurons or nodes  120 . Inputs are fed forward from the input layer to the hidden layers and then to the output layer. Recursive neural network structures may also have outputs from the layers fed back to preceding layers. The number of neurons  120  in each layer is determined before the network is trained. Typically, there is one input node for each input variable and one output node for each output. The input and output layers need not be directly connected (FIG.  3 B). In a unified neural network (FIG.  3 A), every input node is connected to every node in the following hidden layer and every node in a hidden layer is connected to every node in the following adjacent hidden layer or output layer, depending on the number of hidden layers. Each connection to a particular node is weighted. 
     The number of nodes in the input and output layers is determined before the network is trained and correspond to the number of calculated sets of desired forces and feasible propulsion system forces. The number of neurons in the one or more hidden layers is determined during training. FIG. 4 illustrates a single neuron model of a multi-layer perceptron neural network. Equation 1 describes the product of the summation  108  performed within the single neuron model  110  shown in FIG.  4 :                    y      _      in     j   i          (   t   )       =       ω     j   ,   0     i     +       ∑     k   =   1       N     i   -   1                ω     j   ,   k     i     ·       y   k     i   -   1            (   t   )                     (   1   )                                
     where: 
     j=node number 
     i=neural network layer number (0 to L) 
     k=weight value number 
     t=index to an input pattern 
     N i =the number of neurons in the i-th layer of the network 
     L=the number of layers in the network 
     ω j,k   i  is the k-th weight of the j-th neuron in the i-th layer. When k=0, weight value is the bias weight.            y   j   i          (   t   )       =     f        (         y      _      in     j   i          (   t   )       )                              
     is the product of neuron  110  or nodal activation function  112 . The nodal activation function  112  is a sigmoidal, radial basis, linear or other function. The nodal activation function sets a value produced by the neuron or activates or deactivates the neuron  110 . 
     Derivatives used in the training process of the invention are based on expressions for dynamic derivatives of recurrent multi-layer perceptrons. The equation used in the training process is written as:                ∂   +            y   j   i          (   t   )           ∂     ω     g   ,   h     1         =         f   ′          (         y      _      in     j   i          (   t   )       )       ·     {         (     1   -     δ     1   ,   i         )     ·       ∑     k   =   1       N     i   -   1                ω     j   ,   k     i     ·         ∂   +            y   k     i   -   1            (   t   )           ∂     ω     g   ,   h     1               +       δ     1   ,   i       ·     δ     g   ,   j       ·     [         ∑     k   =   1       N     i   -   1                δ     h   ,     k   +   1         ·       y   k     i   -   1            (   t   )           +     δ     h   ,   1         ]         }         ,     1   ≤   i                            
     where: 
     δ ij =the Kronecker delta 
     If i=j, δ ij =1 and otherwise it is equal to zero.          f   ′          (       y      _      in     j   i     )                            
     is the derivative of the neuron&#39;s activation function  112 . ∂ + /∂(•) is the ordered derivative. 
     If the neural network training method used is a traditional back-propagation algorithm, the weight values are determined such that neural network outputs closely match the feasible propulsion system forces that correspond to the training data. At the beginning of neural network training, the connection weights in the network are set to initial values that may be randomized. The training data is then presented to the network one at a time. In accordance with the present invention, the training data includes various sets of desired forces/moments and the predetermined corresponding feasible propulsion system forces for a wide range of vehicle operation. The training process continues through all sets of the training data adjusting the weight values as necessary. Each set provides its own distinct neural network input and output. Accuracy between the discrete values of training data is enhanced by applying random components to the training data as is appreciated by those of ordinary skill in the art of neural network design. 
     Once the neural network structure has been trained, i.e. all the weight values have been generated for the neural network structure, the neural network structure is implemented as a controller  66  in the control computer  60  of the vehicle  50 , as shown in FIG.  1 . The neural network structure can be implemented as an analog electric circuit, a computer program, an application specific integrated circuit or any other structure that allows the neural network structure to function properly. FIG. 5 illustrates the process of determining propulsion system forces with a fully trained neural network structure implemented as the controller  66 . At block  130 , the vehicle&#39;s position and motions are sensed and control signals are generated according to automatic or manual control inputs. Next at block  132 , desired vehicle forces generated from the sensed vehicle position and motions and the generated control signals based on predefined vehicle compensation and control laws. Finally at block  134 , the generated desired vehicle forces/moments are entered into the trained neural network structure. The entered information propagates through the neural network structure getting manipulated according to the trained nodal weight values, thereby generating outputs or feasible propulsion system forces. 
     FIGS.  6 - 8  illustrate the present invention implemented in the propulsion control system of a six degree of freedom ejection seat  150 . This example is for illustrative purposes only. FIG. 6 is a side view and FIG. 7 is a rear view of the ejection seat  150  that includes six controllable rocket nozzles  152 . Prior to operation of the ejection seat  150 , the neural network is selected and trained. As shown in FIG. 8, the input neural network training data is the determination of sets of desired ejection seat moments M DX , M DY , M DZ  and forces F DX , F DY , F DZ . The range of desired forces/moments is determined via off-line simulation from ejection seat sensor and reference data and ejection seat compensation and control laws. The sets of desired forces/moments are then calculated as evenly spaced values within the range. The neural network output training data are sets of feasible rocket nozzle control data  160  for each rocket nozzle  152 . The sets of best feasible rocket nozzle control data  160  is calculated off-line using engineering analysis software that simulates the ejection seat  150  and linear iterative numerical algorithms. Each set of feasible rocket nozzle control data corresponds to a set of desired moments and forces. Each set of feasible rocket nozzle control data includes an output that is supplied to a corresponding rocket nozzle. The best feasible rocket nozzle control data is produced based on seat kinematics, ejection seat capabilities and maximum propulsion system and individual nozzle thrust capabilities. Other ejection seat limitations may further define best feasible rocket nozzle forces. 
     Propulsion system control distribution input data is calculated for numerous ejection seat linear and angular attitudes, rates and accelerations that include maximum seat values to provide the greatest range of data for accurate neural network training. Prior to actual ejection seat operation, these angular attitudes and rates and accelerations of the ejection seat  150  are the same as those experienced by the aircraft, because the ejection seat  150  is fixed within the aircraft. The neural network is then trained off-line using the calculated control distribution input and output data that covers the entire range of seat operation. 
     The trained neural network structure is implemented in the ejection seat&#39;s control computer. The neural network structure can be implemented as software, application specific integrated circuits, or analog electronic hardware. Once the neural network structure has been implemented, the ejection seat is ready for on-line operation. 
     During on-line operation, desired forces/moments are continually being generated. The neural network determines feasible rocket nozzle forces based on the generated desired forces/moments. The desired force/moment data propagates through the trained neural network structure based on the type of network and the determined weight values to produce feasible rocket nozzle data for each rocket nozzle. The rocket nozzles respond to the produced feasible rocket nozzle data, when the ejection seat has been activated. 
     FIG. 9 illustrates time varying input values to a control computer of a vehicle. The time varying input values are three desired forces/moments (moments M DX , M DY  and M DZ ). FIGS. 10 and 11 illustrate a comparison of results of two control computers of the ejection seat shown in FIGS.  6 - 8  with the input values shown in FIG.  9 . One of the two control computers includes the previously known method of on-line searching and the other of the two control computers includes the present invention of a trained neural network to generate feasible force data. In this example, only desired moments were used for the comparison. In order to provide a proper comparison between the two control computers, each of the moments has been chosen to vary differently in time (0-1 sec) between preset desired moment values (−5,000 to +5,000) as shown in FIG.  9 . As a result, a wide range of ejection seat desired motion values are used in the comparison. 
     FIG. 10 illustrates the thrust of rocket nozzle  1  in pounds of thrust as determined by the two control computers over the same time period given the input moment values shown in FIG.  9 . The on-line searching control computer method is illustrated by the solid line and the neural network control computer is illustrated by the dotted line. It is noted that the neural network control computer maps nearly identical to that of the on-line search control computer. The difference in the output of the control computer for each of these techniques is virtually identical and thus, processing accuracy is maintained. 
     However, as shown in FIG. 11, the processing time of the on-line search control computer is significantly slower than the neural network control computer. An engineering work station accurately simulated the processing of each control computer with the input being the moment graphs shown in FIG.  9 . The engineering work station determined that the on-line search control computer took approximately 0.33 CPU seconds to determine rocket nozzle thrust data and the neural network control computer took approximately 0.01 CPU seconds to perform the same task. This decrease in processing speed of 30 times or greater significantly reduces the risk that can occur when processing lags the requirements of the dynamically moving vehicle. 
     In summary, the neural network control computer generates similarly accurate propulsion control data at a fraction of the speed of prior techniques. Because command processing time is greatly reduced, time delays in processing control commands and dangerous, lagging propulsion system responses are avoided. 
     While the preferred embodiment of the invention has been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.