Abstract:
The invention discloses a structured set of electro-hydraulic pilot servo valves to operate an actuator arm to raise and lower an ammunition cradle. The servo valve structure and operational logic provides redundancy and reduces probability of failure. If one of the valves malfunctions, the remaining other two valves vote to carry more flow to compensate for the lost capacity thereby continuously maintaining system performance at pre-failure or near full capacity levels.

Description:
FIELD OF THE INVENTION 
     The present invention relates to Servo valve systems which comprise one of the significant logistics components of a gun system for safe and efficient transfer of ammunition. Specifically, the present invention relates to servo valve systems which cooperate, in a majority voting sequence, to enhance gun system ammunition transfer and efficiency by improving reliability, maintainability and performance. 
     BACKGROUND OF THE INVENTION 
     The reliability of an ammunition transfer system is dependent on the efficiency and performance of the valves and controls involved in operating the transfer mechanism. Specifically, servo valves form a critical link in an ammunition transfer system and a malfunction of these components is detrimental to the smooth operation of the transfer mechanism. More specifically, if a pilot valve sticks at any position, the main stage spool of the valve could stroke unpredictably to either endpoint. Valve sticking may occur due to silting, contaminant build-up in the valve seat and similar environmental problems. A malfunctioning pilot valve may result in the actuator accelerating out of control thereby making the ammunition delivery unreliable and susceptible to failure. 
     Generally, a servo system is a closed loop control system that produces an error signal used to cancel any differences between an output and input command. The error signal drives an actuator that corrects the difference so the output will agree continuously with the input. Prior art three-stage electro-hydraulic servo valves typically use a single pilot valve for main stage spool control. Thus, if the single pilot valve malfunctions, the servo system becomes inoperative because the actuator will not be functional. Failures of this sort are detrimental to guns such as MK 45 gun systems whose readiness and rate of fire depend, to a large part, on the ammunition loading and transfer mechanism. 
     In a servo system the output variable is measured, fed back and compared to the desired input function at the summing point. The difference between the two values is an indication of the error which must be corrected. This setup generally requires a closed loop system. Closed loop servo systems are classified according to the variable being controlled. The most common forms of control are velocity, position, torque and combinations of these. The present invention relates to a servo valve system which is based on position control. Preferably, a resolver is used in the feedback loop to obtain position control. For example, sensors are used to determine that the shaft of the actuator has arrived at the desired angular position by counting pulses and comparing them with the input and stopping the shaft when the counts are equal. Further, a velocity feedback sensor loop may remain in the system to help in stabilizing it. 
     A stable servo system will always return to a stable operating state unless there is a component failure. However, the reliability of a weapon handling system could be significantly improved by structuring the servo valve systems in a manner to provide high level of availability and reliability at all times. Current systems do not use a compound set of servo systems which are able to compensate or step-in to perform the designated operation without interruption when a servo system fails. Specifically, as it relates to ammunition handling systems, the inventors are not aware of any system which utilizes a redundant set of servo valve systems which enable automatic switching of operations from a non-functional valve to a functional one based on a comparative error signal. 
     Accordingly, there is a need to enhance the reliability and availability of the servo valve systems operating in weapon handling equipment. Specifically, the electro-hydraulic valves in use in the MK 45 gun including ammunition handling systems in many other weapon systems, require a reliable servo valve system tailored to provide high reliability within permissible weight and space-volume parameters. 
     SUMMARY OF THE INVENTION 
     U.S Pat. No. 5,440,966 issued on Aug. 15, 1995, discloses a material hand-off device and process which uses a high performance hydraulic actuator to transfer ammunition. The power levels and speeds at which the actuator operates result in a system that is very sensitive to component failure. Thus, in an attempt to remedy the situation, various concepts were considered. For the most part, the present invention is an innovative solution to the problem encountered in controlling the actuator which operates the ammunition delivery system. 
     It is one of the objects of the present invention to provide a valve system for ammunition handling systems which is reliable and enables redundancy in case of failure of one or more of the valves. Specifically, the invention provides a microprocessor controlled servo valve system including a plurality of servo valves and transformers having input and output communications with a main stage spool, hereinafter referred to as the main spool, operating an actuator. More specifically, the system of the present invention includes a resolver, device for converting resolver signals to digital signals, a plurality of digital to analog converters, a plurality of error summing devices, a plurality of amplifiers and a plurality of voltage converters. The main stage spool further includes operable connections to the actuator on one side and to the servo valves and linear variable differential transformers (LVDTs) on another side. Furthermore, the actuator is connected to the resolver and the device for converting the resolver signals is connected to the microprocessor on an output side and the resolver on the input side. The microprocessor also includes connections with the digital to analog converters. The error summing devices provide connections to the digital to analog converters on a first side, the plurality of voltage converters on a second side and the plurality of amplifiers on a third side. The amplifiers are also structured to provide input to the servo valves. 
     It is yet another object of the invention to provide a servo valve system for precisely operating a material handling system to enable exchange and transfer of ammunition between a plurality of cooperative mechanisms. Specifically, the servo valve system includes a control system, an actuator and a main spool. The control system further includes a central microprocessor unit with operable electronic and data connections to a resolver, a digital converter of signals from the resolver, a plurality of digital to analog converters, a plurality of summer devices, a plurality of voltage converters, a plurality of amplifiers, a plurality of servo valve pilots and a plurality of LVDTs. The actuator is preferably connected to the main stage spool at the input side and the resolver at the output side. Further, the main stage spool is connected to the LVDTs and the servo valves. 
     It is a further object of the invention to provide a servo valve system to drive and control the motions of an ammunition handling system for engagement with a moving target attached to a gun tube rotatable through an arc above and below a horizontal azimuth. The servo system and the ammunition handling system, in combination, preferably include a control system and a main stage spool operable via a set of three electro-hydraulic pilot servo valves. Each of the pilot servo valves, hereinafter referred to as servo valves, are connected to LVDTs and form a configuration in which the set of preferably three servo valves operate the actuator. Further, the ammunition handling system includes a cradle that is matingly engageable with a slide mechanism to transfer the ammunition thereto. Upon transfer to the slide mechanism, the ammunition is ultimately fed into the gun tube. The ammunition handling system therefore includes two dynamic systems. The first system being an actuator driven cradle and the second being a slide that is attached to the gun tube. Both the cradle and the slide are adjustable to raisable and tiltable positions relative to a horizontal azimuth. The actuator includes an extendable dynamic arm that is implemented to raise and lower the cradle for engagement with the slide. 
     It is yet another object of the present invention to provide a device-implemented method of maintaining the reliability of an ammunition supply system using a plurality of pilot servo valves to supply undiminished power to an actuator for driving an ammunition handling device in which ammunition is transferred to and from a gun chamber. In the preferred embodiment, the method includes the steps of providing at least three servo valves to operate a main stage spool. Further, the method includes operating an actuator while monitoring positions of the main stage spool. The method also includes correcting errors by summing signals from a plurality of digital to analog converters and a plurality of transformer position to voltage converters. 
     Generally, the majority voting three-stage servo valve system of the present invention incorporates three servo valves for the control of the main stage spool. Each of the servo valves is controlled by a separate electrical main stage spool position feedback control system. The main stage spool position is monitored by a triple redundant set of LVDTs. The result is three servo valves operating independently. This structure provides a feedback control system which is highly reliable and provides an efficient redundancy at a substantially reduced probability of failure. 
     The above features and advantages of the present invention will become apparent upon consideration of the following detailed description of several specific embodiments thereof, especially when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an elevation drawing of a MK 45 type gun system showing a typical ammunition transfer space-volume locus wherein an ammunition is transferred from a storage to a gun breech. 
     FIG. 2 shows a typical arrangement of a servo valve implemented to drive an actuator. 
     FIG. 3 shows the major components of the present invention and their cooperative structure comprising the invention. 
     FIG. 4 is a schematic showing a high level logic architecture including system operation and control of the servo valve system of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1, an ammunition handling system  10  is shown in which cradle  12  is raised to engage slide  14 . Ammunition supply  13  delivers ammunition  15  to be picked up by cradle  12 . Ammunition supply  13  and cradle  12  are designed to cooperatively operate to transfer ammunition from storage into slide  14  from where ammunition  15  is fed into gun tube  16 . Actuator  18  raises and lowers cradle  12  to shuttle between ammunition supply  13  and slide  14 . 
     The present invention is particularly focussed on actuator  18  where, preferably a plurality of servo valves operate two pistons. The precise and reliable operation of actuator  18  is critical for delivering ammunition  15  on time and further to prevent a “Run-Away” cradle which may collide with endpoints at excessive speed thereby damaging either ammunition  15  or slide  14 . Another concern with a malfunctioning actuator is the fact that the rate of fire and gun system performance are directly dependent on the reliability of actuator  18 . The speed of cradle  12  is dependent upon the hydraulic flow and performance efficiency of the servo valve system. Thus, the basis of the present invention is enhancement of the servo valve system to ensure a reliable operation of cradle  12 . 
     Referring now to FIG. 2, a servo valve structure  20  is shown including the major components. Actuator  22  is controlled by main stage spool  24 . Further, pilot servo valve  26  provides control for main stage spool  24 . The position of main stage spool  24  is monitored by a redundant set of LVDTs  28 . Generally the system operates to coordinate the position and speed of actuator  22  to thereby indirectly control systems that are operated/engaged by actuator  22 . 
     Referring now to FIG. 3, three pilot servo valves are structured as shown. The structure represents a general scheme in which actuators  32  are in communications with main stage spool  34 . The connection includes a circuit comprising hydraulic supply and return lines. Main stage spool  34  includes connections to servo valves  36  including a hydraulic supply and return circuit therein. Further, main stage spool  34  is connected to LVDTs  35 . Resolver  37  is implemented to monitor the position and velocity of actuator  32 . Generally resolver  37  is a rotary electromechanical transformer or equivalent that can sense position and/or velocity in servo control systems. Related to a synchro unit, resolver  37  preferably contains a rotor and a stator. There are three widely used techniques for converting resolver outputs into digital format: (i) tracking, (ii) successive approximation, and (iii) time phase shift. Although, the present invention is adaptable to any of the above conversion techniques, in the preferred embodiment, tracking is implemented. Processor  38  is used to process input and output. Specifically, input data is received from LVDTs  35  and resolver  37 . The input data is processed and communicated to servo valves  36 . Servo valves  36  comprise a closed-loop control system that produces an error signal which is used to cancel any differences between the output and the input command. The error signal drives actuator  32  and thus corrects the difference so that the output will agree continuously with the input. 
     As indicated hereinabove, the objective of servo systems is preferably to maintain zero error with a response that is as rapid as possible. Closed-loop feedback control provides accurate positioning because it continually tries to correct any error that exists. However, if there is a delay in error correction due to poor system response, the error will increase until the system becomes unstable. As will be seen hereinbelow, the present invention implements unique structures and logic to provide a highly reliable servo system within a closed loop environment. 
     Referring to FIG. 4, main stage spool  34  output drives actuator  32 . On the input side, main stage spool  34  is connected to a plurality of servo valves  36  and LVDTs  35 . Resolver  37  is set to sense the position and velocity of actuator  32 . The signals sensed by resolver  37  are analog in nature and are converted to digital via converter  40 . The digital signal is routed as an input into CPU  38  for processing. After processing, the signal is converted back to analog by converters  43 . The analog signal is then directed into summer units  45  to correct any errors that may exist. Further, summer units  45  are connected to the output of converter units  46 . Converter units  46  accept analog signals from LVDTs  35  and convert them to voltage readings. Summers  45  correct any discrepancies and errors that exist between the input signals from converters  43  and converters  46 . The resultant, corrected signal is fed into driver amplifiers  47 . Amplifiers  47  direct the amplified signals into servo valves  36 . The signals from servo valves  36  are fed into the input side of main stage spool  34 . These signals are used to correct, adjust and modify the operations of actuator  32 , thereby forming a closed loop servo system. 
     Accordingly the structure and logic of the present invention enables servo valves  36  to work in any pair or three combination as shown. The majority voting concept is applied to a three-stage servo valve which improves the reliability of the valve package. For example, as shown in FIG. 2 , a conventional three-stage servo valve includes one pilot valve controlling the power stage. Thus, a majority voting three-stage valve system could be designed to provide three pilot valves controlling the power stage. If the probability of failure of a conventional three-stage valve is 0.01 the probability of a voting three-stage servo valve, as implemented in the present invention is 0.0003. 
     The present invention therefore provides a three-stage electro-hydraulic servo valve system. In accordance with FIG. 3 the overall configuration of the system includes a closed loop system comprising servo valves  36  each having independent LVDTs  35  and sharing a common main stage spool  34 . Servo valves  36  are integrated as shown to control main stage spool  34 . Thus, if any one of the valves fails, the other two valves will compensate and keep main stage spool  34  under control. Since any two servo valves among the three are a majority, the valve structure in accordance with the present invention provides at least two operational valves after the failure of any one of the three valves. 
     Accordingly, with reference to FIG. 1, the present invention is used to operate a high performance hydraulic actuator  18 . The power and speed at which actuator  18  operates require high level accuracy and sensitivity to component failure. The majority voting three-stage electro-hydraulic servo valve system of the present invention meets these stringent requirements. Specifically, actuator  18  rotates a 1000 pound weight through 25 degrees to 105 degrees. One of the end points is slide  14  which is dynamic. The time for motion varies from 0.35 seconds to 0.60 seconds depending upon the position of gun tube  16  and the angle through which cradle  12  must be raised to engage slide  14 . There are two motions involved. The first is encountered when cradle  12  is raised. During this operation the servo valve system may not use sleeve buffing to decelerate cradle  12  because the end point is dynamic, i.e., slide  14  may be moving. Thus, cradle  12  is under servo control during the raising cycle of cradle  12 . The second motion is encountered during a cycle in which cradle  12  is being lowered. In this case, the endpoint is non-moving and the actuator uses buffing on the sleeve to decelerate cradle  12 . In the prior art the servo valve was held wide open during the entire cradle lowering cycle. Specifically, the prior art solution to servo valve failure was the use of an abort circuit. The abort circuit predicted a response time of 40 msec. This was adequate except in situations in which when the pilot stage of the servo valve sticks causing the main stage spool to go hard in one direction. Solutions to speed up the abort cycle were considered but were found ineffective. The present invention overcomes the limitation of the prior art by implementing three servo valves which, even if one of the three fails, the other two compensate for the flow of hydraulic pressure to maintain the same power level to either raise or lower cradle  12 . Thus, the redundancy including the associated controls of the present invention provide a high level of reliability and maintainability to the ammunition supply system and thereby enhance gun performance. 
     While the preferred embodiments of the invention have been shown and described, it will be obvious to those skilled in the art that changes, variations and modifications may be made therein without departing from the invention in its broader aspects and, therefore, the aim in the appended claims is to cover such changes and modifications as fall within the scope and spirit of the invention.