Abstract:
An improved tilting rotor aircraft, a control system for an elongated shaft, and a method of controlling an elongated shaft are provided. Sensors are utilized to detect an amount of twist on a flexible and elongated shaft. A controller receives the signal and generates a command signal. A plurality of actuators receive the command signals and compensate for the twist in the flexible and elongated shaft.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/147,663; filed Aug. 6, 1999, entitled Method and Apparatus for Controlling Force Fights in a Rotating Shaft. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates in general to systems utilized for controlling rotating shafts, and in particular to systems which are utilized to minimize twisting loads on rotating shafts. 
     2. Description of the Prior Art 
     In many industrial applications it is not uncommon to have a relatively long and flexible drive shaft which is driven by two or more motors (such as electric or hydraulic motors typically). The goal is to synchronize the operation of the motors to avoid “force fights” between the motors. Force fights are conditions under which the motors are acting counter to one another. This typically results from a lack of synchronization in the motors, but also can be due to the mechanical properties of the shaft. For example, when the shaft is relatively long and flexible it is more likely to be subjected to “force fights” between a plurality of motors which are coupled thereto. Force fights are undesirable for a number of reasons. First, they reduce the overall power transfer from the shaft to other mechanical equipment. For example, in aircraft, such as tiltrotor aircraft, it is not uncommon for such a shaft to lose as much as twenty or twenty-five percent of its total power due to force fights. Second, force fights can generate twisting loads on the shaft which can damage or even break the shaft. In general, drive shafts that are subject to force fights are also subject to an increased occurrence of fatigue loads which in general result in part failure. 
     In the prior art, attempts have been made to better coordinate the operation of the motors which drive the shaft in an effort to minimize or eliminate force fights. These efforts have been only moderately successful. When hydraulic motors are coupled to the drive shaft, a controller has been utilized to individually control a number of electrical hydraulic valves (EHVs). A sensor assembly is utilized to detect excessive twists or the occurrence of force fights. The sensor data is provided as an input to a controller. The controller supplies command signals to the electrical hydraulic valves to moderate the operation of the hydraulic motors, all in a manner which attempts to reduce or minimize twisting loads on the shaft due to force fights from motors which are slightly out of synchronization. One central problem encountered in the prior art is that, in complex mechanical systems with a variety of individual parts, the operating tolerances on the individual parts combine to provide a substantial range of normal operation for the parts. It is difficult for the sensor assembly and controller to accurately measure and rapidly respond to conditions which indicate that undesirable twisting loads are present in the shaft due to force fights between motors. 
     SUMMARY OF THE INVENTION 
     It is one objective of the present invention to provide a control system which minimizes or eliminates the occurrence of force fights between motors which are cooperatively driving a single drive shaft. 
     It is another objective of the present invention to provide a control system which provides a simple numerical indicator of twisting forces acting on a drive shaft. 
     It is yet another objective of the present invention to provide a control system for use in minimizing force fights in a rotating drive shaft, which utilizes electrical resolver components in order to develop a relatively simple single numerical indicator of twisting loads on the drive shaft. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objectives and advantages thereof, will best be understood by reference to the following detailed description of the preferred embodiment when read in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a perspective view of a tiltrotor aircraft in an airplane mode of operation. 
     FIG. 2 is a perspective view of a tiltrotor aircraft in a helicopter mode of operation. 
     FIG. 3 is a block diagram representation of one prior art approach for controlling force fight in a drive shaft of a V-22 tiltrotor aircraft. 
     FIG. 4 is a block diagram representation of a first embodiment of a force fight control system in accordance with the present invention. 
     FIG. 5 is a block diagram representation of a second embodiment of a force fight control system in accordance with the present invention. 
     FIG. 6 is a schematic representation of the mechanical components of the second embodiment of the force fight control system of the present invention. 
     FIG. 7 is an electrical schematic representation of one resolver transmitter/transformer pair in accordance with the first embodiment of the force fight control system of the present invention. 
     FIG. 8 is a block diagram representation of the preferred force fight control system of the present invention. 
     FIGS. 9,  10 ,  11  and  12  are graphical representations of exemplary signals generated by the resolver transformer and resolver transmitter pairs of FIG.  7 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIGS. 1 and 2 in the drawings, a typical tiltrotor aircraft  11  is illustrated. Tiltrotor aircraft  11  has an airframe  13  and wings  15   a  and  15   b  coupled to airframe  13 . As is conventional, wings  15   a  and  15   b  terminate with tiltrotor assemblies  17   a  and  17   b , respectively. Fairings  18   a  and  18   b  for reducing drag are disposed between tiltrotor assemblies  17   a  and  17   b  and wings  15   a  and  15   b.  Tiltrotor assemblies  17   a  and  17   b  each generally include an engine, a transmission and gear box for driving prop-rotors  19   a  and  19   b , and a conversion actuator for actuating tiltrotor assemblies  17   a  and  17   b  between an airplane mode, as illustrated in FIG. 1A, and a helicopter mode, as illustrated in FIG.  2 . In the airplane mode, tiltrotor aircraft  11  can be flown and operated like a conventional fixed-wing propeller driven aircraft. In the helicopter mode, tiltrotor aircraft  11  can take-off, hover, land, and be operated like a conventional rotary wing aircraft or helicopter. In the airplane mode of flight the rotor disk is positioned substantially transverse to the air frame; in contrast, in the helicopter mode of flight the rotor disk is substantially parallel to the air frame. 
     FIG. 3 is a simplified block diagram representation of the prior art control system utilized in the V-22 tiltrotor aircraft in order to detect and minimize force fights between two hydraulic motors  103 ,  105  on one actuator gear box. The force fights are detected by utilizing a complex closed-loop algorithm that uses monitored actuator motor pressure to determine the load sharing between motors  103 ,  105 . Because of large changes in motor pressure and gear box efficiencies that occur with changes in temperatures, the V-22 force fight control system is relatively ineffective at low temperatures. As is shown in the view of FIG. 3, controller  111  is utilized to provide control signals to electrical hydraulic valves (EHVs)  107 ,  109 . Electrical hydraulic valve  107  is utilized to supply pressurized hydraulic fluid to motor  105 . The control signal sent over control line  115  increases or decreases the amount of high pressure hydraulic fluid directed through EHV  107  to motor  105 . The same is true for motor  103 . Controller  111  supplies control signals over control line  117  to EHV  109 . EHV  109  in turn increases or decreases the amount of high pressure hydraulic fluid which is supplied to hydraulic motor  103 . Motors  103 ,  105  cooperate to rotate drive shaft  101  at relatively high speeds. In accordance with this prior art approach, hydraulic pressure in one or more motors is monitored over monitor lines  119  and  120  by controller  111 . Controller  111  contains a mathematical model which maps pressure information from monitor lines  119 ,  120  to an amount of twist or force fight present in drive shaft  101  and drive motors  103 ,  105 . It sends control signals through control lines  115 ,  117  to electrical hydraulic valves  107 ,  109  to increase or decrease the high pressure hydraulic fluids applied to motors  103 ,  105 . 
     FIG. 4 is a simplified block diagram view of the first embodiment of the force fight control system of the present invention. As is shown, a relatively elongated and flexible drive shaft  201  is provided to transmit power in a mechanical system, such as a tiltrotor aircraft. A plurality of motors are physically coupled to drive shaft  201 , rotating it at a relatively high rate of rotation. In the view of FIG. 4, exemplary motors  203 ,  205  are shown coupled to drive shaft  201 . A greater number of motors may be coupled to drive shaft  201 . In the embodiment of FIG. 4, motors  203 ,  205  are hydraulic motors which rotate in response to receipt of high pressure hydraulic fluid. In alternative embodiments, motors  203 ,  205  may comprise electrical motors, or any other suitable alternative. As is shown in FIG. 4, electrical hydraulic valves  207 ,  205  are provided to supply high pressure actuating fluid to motors  203 ,  205 . Electrical hydraulic valves  207 ,  209  are under the direct control of controller  211 , which is preferably a microprocessor or industrial controller which is suitable for executing program instructions. The control signals are supplied to electrical hydraulic valves  207 ,  209  via control lines  223 ,  225 . Drive shaft  201  may also be coupled to one or more brakes, such as brake  213  which is utilized to slow or stop the rotation of drive shaft  201 . Brake  213  may be under the control of controller  211 , or it may be under the control of some other system or subsystem within the aircraft. A plurality of brakes may be provided which mechanically and selectively engage drive shaft  201 , but, in order to simplify this exposition, a single brake  213  is depicted which is representation of one or more brake or braking systems. 
     In accordance with the preferred embodiment of the present invention, a set of gears  215 ,  219  are coupled mechanically to drive shaft  201  in order to allow for monitoring of drive shaft  201  in order to determine the extent and amount of twisting loads on drive shaft  201  in order to determine whether or not force fights are occurring between motors  203 ,  205 . Transformer resolver  217  is coupled mechanically to gears  215  at one portion or region of drive shaft  201 . Transmitter resolver  221  is coupled to gears  219  at another, different portion or region of drive shaft  201 . Transformer resolver  217  and transmitter resolver  221  supply monitor signals via monitor channels  227 ,  229  to controller  211 . Controller  211  analyzes the monitor signals supplied by transformer resolver  217  and transmitter resolver  221  in order to determine what particular control signals should be sent to electrical hydraulic valves  207 ,  209  via control lines  223 ,  225 . Controller  211  may increase or decrease the amount of high pressure hydraulic fluid supplied to either or both of motors  203 ,  205  in order to compensate for a detected force fight between motors  203 ,  205  which manifests itself as a detected twisting load acting on shaft  201 . During normal operations, drive shaft  201  is rotated at very high speeds by motors  203 ,  205 , while transformer resolver  217  and transmitter resolver  221  continuously monitor for twisting loads and supply monitor signals to controller  211 . Controller  211  dynamically and continuously supplies control signals to electrical hydraulic valves  207 ,  209  in order to compensate for the detected twisting load on drive shaft  201 . 
     The embodiment of FIG. 4 combines information obtained form the transformer resolver  217  and the transmitter resolver  221  in order to provide a single numerical indicator of the amount of twist detected in drive shaft  201 . Since redundancy of control is a fundamental requirement of most flight systems, there should preferably be at least two separate data channels for monitoring twisting loads on drive shaft  201 . FIGS. 5 and 6 are a representation of a second embodiment of the force fight control system of the present invention which provides two independent separate data channels in order to provide two independent measurements of the twisting load on drive shaft  201 . This provides the necessary and desirable redundancy of monitoring signals which is essential for safe flight operations. 
     Since many of the components of the first embodiment of FIG.  4  and the second embodiment of FIGS. 5 and 6 are similar, similar reference numerals are utilized, where appropriate, and new reference numerals are utilized to indicate new components introduced in the second embodiment which were not present in the first embodiment. 
     Referring now to FIG. 5, the second embodiment is depicted in block diagram form. As is shown, relatively elongated and flexible shaft  201  is provided as part of a power transmission or the mechanical system. In the preferred embodiment, the shaft is utilized in a tiltrotor aircraft; although, alternative commercial uses are obvious and contemplated, including uses that are in plant or other commercial facilities which are not in any way related to vehicles or aircraft. As is shown, a plurality of motors including motors  203 ,  205  are mechanically coupled to shaft  201 . In this embodiment, the motors are hydraulic motors, but in alternative embodiments electrical or other motors can be utilized. Motors  203 ,  205  operate to rotate shaft  201  at a high rate of rotation. High pressure hydraulic fluid is supplied as an input to motors  203 ,  205  through electrical hydraulic valves  207 ,  209 . As is true for the first embodiment, electrical hydraulic valves  207 ,  209  are under the direct control of controller  111  which is preferably a microprocessor or industrial controller which supplies command signals to increase or decrease the amount of high pressure hydraulic fluid supplied to electric hydraulic valve  207 ,  209  during normal operations. Preferably one or more braking systems, such as brakes  213 ,  214  are also in selective mechanical communication with drive shaft  201 . They may be utilized to slow or stop the rotation of shaft  201  as is required by the particular application. In this embodiment, two gear assemblies  215 ,  217  are provided to allow for monitoring and detection of twisting loads which are applied to drive shaft  201 . 
     As is depicted in FIG. 5, transformer resolver  217  and transmitter resolver  221  act as a pair to provide one independent data channel which provides a single numerical indication of the amount of twist load on drive shaft  201 . This is a differential measurement in that transformer resolver  217  provides a measure from one portion of drive shaft  201 , while transmitter resolver  221  provides a signal from another distally-located portion of drive shaft  201 . In the embodiment of FIG. 5, a second independent channel also provides a single numerical indication of the amount of twisting load on drive shaft  201 . More particularly, transmitter resolver  251  and transformer resolver  253  provide together a separate independent numerical indication of the twisting load on drive shaft  201 . Transmitter resolver  251  is mechanically coupled to drive shaft  201  through gears  215 , while transformer resolver  253  is mechanically coupled to drive shaft  201  through gears  217 . In accordance with this particular embodiment of the present invention, the combination of signals from transformer resolver  217  and transmitter resolver  221  provides one independent single numerical indication of the twisting load on drive shaft  201 , while transmitter resolver  251  and transformer resolver  253  provide another separate and independent numerical indication of the twisting load on drive shaft  201 . 
     Under most conditions, the single numerical value developed by each independent channel should be identical. Should some of the equipment fail, a single channel would be sufficient to continue operations until repairs can be made. Additional levels of redundancy can also be superimposed in addition to these two redundant channels. In accordance with the embodiment of FIG. 5, controller  111  receives monitoring signals from transformer resolver  217  and transmitter resolver  251  from one portion of drive shaft  201 , and also receives and monitors signals from transformer resolver  253  and transmitter resolver  251  from another portion of drive shaft  201 . Controller  111  combines the data in a manner which provides the two independent measurements of the twisting load on drive shaft  201 . 
     FIG. 6 is a pictorial and schematic representation of the mechanical components which are utilized in interconnect drive train  301  which mechanically links pylon conversion actuators  303 ,  305  and which is utilized to measure load sharing between the pylon conversion actuators  303 ,  305 . As is shown in the view of FIG. 6, the pylon conversion actuators  303 ,  305  terminate at an opposite end with a fixed lug  307 ,  309 . On each pylon conversion actuator  303 ,  305  a telescoping ball screw  311 ,  313  is provided. As is shown, the mechanical systems are symmetrical. They include a bevel gear set  315 , spur reduction gears  317 , and a hydraulic clutch  319 . Each rotor assembly has associated with it a planetary differential  321 . Each is mechanically driven by primary hydraulic power drive unit  323 , and is braked by primary hydraulic brake  325 . A remote brake piston  337  is also provided. A back up hydraulic power drive unit  327  is also provided to supplement the primary hydraulic drive unit  323 . Additionally, a back up hydraulic brake system  329  is provided to back up the primary hydraulic brake system  325 . The operation of these systems is monitored through angular displacement transducer  333  which is mechanically coupled through spur reduction gears  331  within gear box  337 . A chip detector  335  is provided to detect magnetic particles in the gear box which indicate damage or wear. The angular position of pylon conversion actuator  303  is monitored by angular displacement transducer  341 . The position of the tiltrotor is monitored by angular displacement transducers  351 ,  353 ,  355 ,  357 . In this manner, each rotor assembly has its own primary and back up drives and brake units. However, the operation of the tiltrotors is coordinated through drive shaft  301  and loads are measured and balanced through drive shaft  301 . As is shown, angular displacement transducers  371 ,  373  are mechanically coupled through gears  375 ,  377  to different portions of drive shaft  301  and are utilized to develop electrical signals which quantify the amount of twists on drive shaft  301 . As discussed above, the angular displacement transducers  371 ,  373  are two pairs of resolver transformer/resolver transmitter devices which generate electrical signals which can be combined to provide a single numerical indication of the amount of twists of drive shaft  301 . The resolver transformer and transmitter pairs are discussed above, but will be now be discussed in detail. 
     FIG. 7 is an electrical schematic of a resolver transmitter/transformer pair. As is shown, resolver transmitter  401  is electrically connected to resolver transformer  403 . Resolver transmitter  401  includes a primary winding  405  which is a cylindrical, two-phase perpendicular rotor winding. The primary winding  405  is inductively coupled to a symmetrical two-phase, perpendicular stator winding  409  (represented in the view as indicator  51 - 53  and indicator  52 - 54 ). The resolver transmitter  401  generates a sine output and a cosine output in response to rotation. The output data is representative of the angular position of the rotor. Resolver transformer  403  employs a symmetrical two-phase perpendicular stator winding as the primary windings  411  (represented in the view as inductor S 1 -S 3  and inductor S 2 -S 4 ) which is inductively coupled to a cylindrical single-phase rotor winding  413  (inductor R 2 -R 4 ). Resolver control transformer stators  411  are excited by either a resolver transmitter stator  409  or a resolver differential rotor winding  413 . To reduce loading effects resolver control transmitter  403  has medium to high impedance stator windings. When the resolver control transformer  401  is mechanically positioned, single-phase electrical data is generated on the rotor winding  413 . This output data is proportional to the sine of the difference between the electrical input angle of the resolver transmitter  401  or the resolver differential and the resolver control transformer rotor angle. With no twisting load applied to the drive shaft, the resolver transformer generates a sine output. In contrast, with no twisting load on the drive shaft, the resolver transmitter generates a cosine output. The combination of these waveforms is essentially zero output. However, when maximum twisting loads (in this particular application the maximum of 10% twisting load is all that can be monitored) the outputs of the resolver transformer and/or resolver transmitter are shifted to add the signals together and generate a non-zero output. Experimentation has revealed that the amount of twisting between zero degrees and ten degrees of twist is essentially linear. Accordingly, varying amounts of phase shifting of the sine and cosine signals will generate all additive output (preferably RMS) which provides a linear identification of the amount of twisting. In accordance with the present invention, the controller is utilized to selectively actuate one or more hydraulic motors in order to abate the detected twisting. 
     With reference again to FIG. 7, an excitation current is supplied to primary winding  405  of resolver transmitter  401 . The excitation current is seven volts RMS. In the view of FIG. 7, this is applied between pins  1  and  2  of resolver transmitter  401 . The frequency of the excitation signal is very high (thousands of Hertz). The resolver transmitter  401  generates two outputs. One output is between output pins  3  and  4  and is representative of the sine output. The output is a function of the stimulating current, a constant, and a sine of the angle. The second output is a cosine signal which is developed between pins  5  and  6  and which is a function of a constant, the excitation current, and the cosine of the angle. The sine and cosine signals are supplied to terminals  3 ,  4 ,  5 , and  6  of the resolver transformer  403 . The output developed by resolver transformer  403  is a function of the sine and cosine of the angle between the stators and the rotor. The formula for the output is provided in the view of FIG.  7 . Essentially, the voltage output from pins  1  and  2  of resolver transformer  403  is a function of the sine and cosine of the angle of rotation from one end of the shaft and the sine and cosine of the angle of rotation of the other end of the shaft. In this manner, the data is combined to determine a single value which is indicative of the amount of twist between the two ends of the shaft. More particularly, the system is utilized to measure the amount of rotation of one end of the shaft relative to the other, all as measured with respect to a stable frame of reference, such as the position of the airframe. Polarity is utilized in order to indicate the direction of twist. 
     FIG. 8 is a block diagram view of the preferred control system utilizing the present invention. As is shown, the left nacelle  490  and the right nacelle position  492  are applied to the interconnecting drive train IDT  513 . The position of the interconnecting drive train  513  is sensed directly by IDT sensor  515 . The output of IDT sensor  515  is supplied as an input to tracking gain module  517 , which represents a frequency sensitivity gain which is used to eliminate the twist in the IDT. The resulting output signal is limited by tracking limit module  519  which controls how much authority can be used to equalize the two nacelle positions. Making the nacelles&#39; position equal is what eliminates the twist in the IDT. To put it simply, the twist in the IDT is measured, and the resulting signal is used to command the right nacelle position to track the left nacelle position, which eliminates the twist in the IDT. The output of the tracking limit module  519  is supplied as an input to right nacelle position command input  521 . Right nacelle position command input  521  receives negative feedback from tracking limit module  519 , but it also receives negative feedback from the actuator position  523  which provides an indication of the actuator position, as well as from EHV position module  525  which provides an indication of the EHV position. The three negative feedback loops are summed and supplied as an input to amplifier module  522  which amplifies the signal and supplies it as an input to the EHV (electrical hydraulic valve) with electrical hydraulic valve position changing five percent per milliamp input. The electrical hydraulic valve is supplied as an input to motor  529  which is supplied as an input to the gears  531 . 
     The left nacelle position is monitored by actuator position module  503 . The output of actuator position module  503  is supplied to left nacelle position command input  501 . Similarly, two negative feedback loops are provided. Negative feedback is provided from actuator position module  503 , but negative feedback is also provided from EHV position sensor  505 . The left nacelle position signal is combined with a negative feedback and supplied as an input to amplifier module  502 . The output of the amplified signal is supplied as an input to EHV  507 . The output of EHV  507  is applied as an input to motor module  509 . The output of motor module  509  is applied as input to gears module  511 . 
     FIGS. 9,  10 ,  11  and  12  depict in simplified form four resulting output signals for four different twist conditions. FIGS. 9A through 9D depict a situation in which the left end is rotated plus twenty-five degrees and the right end is twisted a negative twenty-five degrees from an arbitrary reference point, resulting in a total IDT twist of fifty degrees. The representation is simplified in that the excitation signal depicted in FIG. 9A is depicted at a much lower frequency than is actually utilized. There are several orders of magnitudes in difference between the frequency of the excitation signal and the rate of rotation of the shaft. The excitation signal is a constant amplitude, constant frequency, excitation signal of a relatively high frequency in comparison to the rate of rotation of the shaft. 
     FIG. 9B is a graphical representation of the amount of rotation on the left end of the shaft with respect to time. In this view, the X-axis is representative of the time and seconds, while the Y-axis is representative of the degrees of rotation of the left end of the shaft. FIG. 9C is a graphical representation of the twist on the right end of the shaft with respect to time. The X-axis is representative of time in units of seconds, while the Y-axis is representative of the degrees of rotation measured at the right end of the shaft. FIG. 9D is representative of the output signal generated by the control system of the present invention. As is shown, the X-axis is representative of time in seconds, while the Y-axis is representative of the output signal in volts The amplitude of the output signal is a measure of the cumulative amount of twist on the shaft. The polarity of the twist is indicated by the phase angle of the signal. As is shown in FIG. 9D, the direction in which the signal is going (positive or negative) indicates the polarity of the information. As is shown in FIG. 9D, the positive going portion  601  indicates a particular polarity. 
     FIGS. 10A,  10 B,  10 C, and  10 D are identical to the situation depicted in FIGS. 9A through 9D, with the exception of the polarity of the twist. As is shown, the phase angle is 180° out of phase in FIG. 10D from that depicted in FIG.  9 D. Accordingly, the polarity is different. In FIG. 9D, the negative-going portion  605  is identified to illustrate this point. 
     FIGS. 11 and 12 depict different amounts of twist on a shaft with different polarities. In both FIGS. 11 and 12, a total of 20° of IDT twist is depicted. As is shown, the amplitude of the output signal of FIGS. 11D and 12D are representative of 20° of IDT twist. However, the signals are 180° out of phase, which indicates differing polarity for the twist. 
     Although the invention has been described with reference to a particular embodiment, this description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments as well as alternative embodiments of the invention will become apparent to persons skilled in the art upon reference to the description of the invention. It is therefore contemplated that the appended clams will cover any such modifications or embodiments that fall within the scope of the invention.