Abstract:
A method of determining a speed of a wheel that involves providing a resolver having a rotor and a stator which resolver produces at least one output signal, operatively connecting the rotor to a wheel such that the resolver is rotated by the rotation of the wheel, producing digital signals indicative of a position of the rotor relative to the stator based on the at least one output signal, and calculating a velocity of the wheel from the digital signals. The digital signals may be used alone or in conjunction with analog wheel speed signals. Also a speed sensor for practicing this method.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present application claims the benefit of prior U.S. Provisional Patent Application No. 60/652,724, filed Feb. 15, 2005, the entire contents of which is hereby incorporated by reference. 
     
    
     STATEMENT OF GOVERNMENT INTEREST  
       [0002]     The invention described herein was made with government support under Contract No. Z20544 awarded by the Department of Advanced Research Projects Agency. The government has certain rights in this invention. 
     
    
     FIELD OF THE INVENTION  
       [0003]     The present invention is directed toward an improved wheel speed sensor, and, more specifically, toward a resolver-based wheel speed sensor capable of producing a usable speed signal over a range of speeds including zero.  
       BACKGROUND OF THE INVENTION  
       [0004]     Frequency-based wheel speed sensors such as tachometers are well known. However, at low speeds, the signal output by these sensors becomes unusable. These speed sensors, when used on aircraft, for example, often do not produce a usable signal at speeds below 5 to 15 knots.  
         [0005]     The outputs from these sensors may be used to control deceleration or differential wheel deceleration at high speeds or to control anti-skid brakes, for example. At low speeds, when deceleration is nearly complete and when skidding is unlikely to occur, it is generally not critical to obtain accurate speed information. However, in certain applications, those involving unmanned vehicles, for example, a wheel speed sensor that produces a usable signal down to and including zero speed would be highly desirable or necessary. The output of such a sensor would enable a controller to determine whether the vehicle is at rest or slowly rolling off a runway, for example, when there is no pilot in the vehicle to make such a determination.  
         [0006]     Resolvers are generally used to measure the position of a rotating member, such as a motor shaft attached to a rotor, with respect to a fixed element such as a stator. A resolver is essentially a rotary transformer designed so that the coefficient of coupling between the rotor and stator varies with the relative positions of the rotor and stator. Usually resolvers have two windings on a rotor and two windings on a stator positioned at right angles to each other. When a rotor winding is excited with an AC reference signal, the stator windings produce AC voltage outputs that vary in amplitude according to the sine and cosine of rotor position. The stator signals from the resolver are input to a specialized A/D converter known as a resolver-to-digital (R/D) converter. The R/D converter multiples input sine and cosine values by cosine and sine terms, forms the difference and drives its output angle until a null value is reached. The output of the R/D converter is a digital signal that indicates the position of the rotor with respect to the stator and may vary, for example, from 0 to 1 over the course of a single rotation.  
         [0007]     Some R/D converters, including models AD2S80A and AD2S83A made by Analog Devices of Norwood, Mass., can also produce an output voltage proportional to velocity. A resolver that outputs an analog position signal is discussed in U.S. Pat. No. 5,260,650, the entire contents of which are hereby incorporated by reference. However, these analog signals do not provide velocity information down to zero speed and are limited in their precision.  
         [0008]     It would therefore be useful to provide a vehicle wheel sensor that is usable over a wide range of vehicle speeds including zero speed and that does not suffer from the foregoing problems.  
       SUMMARY OF THE INVENTION  
       [0009]     These problems and others are addressed by the present invention, a first aspect of which comprises a wheel speed sensor that includes a resolver having a stator and a rotor operatively connectable to a rotating vehicle wheel and producing at least one output signal. A resolver-to-digital converter having an input receives the at least one output signal and produces digital output signals indicative of an angular position of the rotor with respect to the stator. The system also includes a digital controller having an input receiving the resolver output signals and calculating a velocity of the wheel based on the received resolver output signals.  
         [0010]     Another aspect of the invention comprises a method of determining a speed of a wheel that involves providing a resolver having a rotor and a stator which produces at least one output signal indicative of the relative positions of the rotor and the stator and operatively connecting the rotor to a wheel such that the rotor is rotated by the wheel. Digital signals indicative of the position of the wheel are produced from the resolver output, and a velocity of the wheel is calculated from these position signals. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     These features and aspects of the invention and others will be better understood after a reading of the below detailed description together with the following drawings wherein:  
         [0012]      FIG. 1  is a diagram schematically showing a speed sensing system including a resolver according to an embodiment of the present invention;  
         [0013]      FIG. 2  is a schematic view, partly in section, of the resolver of  FIG. 1 ; and  
         [0014]      FIG. 3  is a chart illustrating different operating states of the system of  FIG. 1 . 
     
    
     DETAILED DESCRIPTION  
       [0015]     Referring now to the drawings, wherein the showings are for purposes of illustrating preferred embodiments of the invention only and not for the purpose of limiting same,  FIG. 1  illustrates a velocity sensing system  10  including a resolver  12  having an input shaft  14  connected to a wheel  16 , which may comprise, for example, the wheel of an aircraft. The resolver may be connected in a manner similar to the manner that tachometers are currently connected to wheels as is known to those of ordinary skill in the art. Other arrangements wherein the resolver shaft is angled with respect to or offset from the wheel axis and operatively connected thereto with suitable gearing could also be used depending on the geometry of the aircraft or other vehicle in which the velocity sensing system is used.  
         [0016]     As illustrated in  FIG. 2 , resolver input shaft  14  is connected to a rotor  18  in the resolver  12 , and as the input shaft  14  rotates, it causes rotor  18  to rotate with respect to resolver coils  20 . One of the stator coils is excited with, for example, a 20 kHz reference signal from resolver excitation circuit  21 , and as the rotor rotates, two of the stator windings produce AC voltage outputs that vary in amplitude according to the sine and cosine of shaft position. These signals are output on resolver first and second output lines  22  and  24 . A second, independently excited redundant resolver (not shown) may be also be used to increase the reliably of the system.  
         [0017]     An R/D converter  26  has a first input  28  connected to resolver first output line  22  and a second input  30  connected to resolver second output line  24 . R/D converter  26  may comprise, for example, an R/D converter sold by Analog Devices as model number AD2S80A or AD2S83A. Among the various outputs of R/D converter  26  are a digital position signal output  32  and an analog velocity signal output  34 . Position signal output  32  of R/D converter  26  is connected to a first input  36  of a controller  38  and analog velocity signal output  34  of R/D converter  26  is connected to an input  39  of an analog/digital converter  40 , the output of which is provided to controller  38 . The R/D converter samples the output signals on first and second output lines  22  and  24  at a rate based on the highest expected wheel speed and produces a digital readout indicative of the angular relationship between the shaft and the stator. Preferably, the signal is sampled at a rate that is at least twice the anticipated maximum wheel speed, expressed in revolutions per second, to avoid the possibility of aliasing. A higher sampling rate, such as four samples per revolution, or higher, could be used to provide unambiguous direction information and a higher tolerance for a missed sample. For purposes of this discussion, a sampling rate of approximately six times per revolution will be considered.  
         [0018]     The position signal on position signal output  32  is a digital signal that varies from 0 to 1 as the rotor rotates with respect to the stator and rolls over to 0 from the upper end of the range. The number of signals output per second may be determined based on a given application; in embodiment described below, 500 signals per second are output. These signals are received by controller  38 , which may comprise the main computer of an aircraft, which receives the signal on first input  36  and, using a time reference signal from time reference circuit  37 , takes the derivative of this signal to determine a velocity based on the changing position indicated by the R/D converter output signal.  
         [0019]     An aircraft operates over a wide range of speeds. At landing, an aircraft may be moving at 200 knots, well over 200 miles per hour, and the aircraft wheels may therefore rotate as fast as about 6000 rpm. Resolvers that can operate at speeds up to around 20,000 rpm are known; however such resolvers sample at a bit rate that does not provide sufficient precision at low speeds, speeds measured in feet per minute, for example, to be useful in many situation. According to one embodiment of the present invention, this issue is addressed by operating the R/D converter in different modes which modes are selected by a selector  41  controlled by controller  38  by sending a mode selection signal on line  44  to a mode input  46  of R/D converter  32 . In the present embodiment, R/D converter  38  is shiftable between a first mode operating with 12 bit precision and a second mode operating with 16 bit precision; however, other operating modes having different precisions and the use of more than two operating modes is also within the scope of this invention and will depend on, for example, tire diameter and the maximum anticipated rotation rate of the wheel on which the sensor will be mounted.  
         [0020]     With reference to  FIG. 3 , the system  10  operates in a first, high speed, low resolution state  50  as long as the detected analog speed remains above a first value, 50 radians per second, for example. When the detected analog speed drops below this rate, a transition illustrated at  52  occurs to a second, low speed, high resolution state  54 . System  10  continues to operate in this second state  54  until the detected analog speed increases to above a second level, 80 radians per second, for example, and then transitions at  56  back to first state  50 . This active switching allows for the use of highly accurate position information at low speeds while using the same resolver and R/D converter to provide good speed information at higher speeds.  
         [0021]     When the detected velocity is 0, controller  38  sets a zero flag or otherwise produces an output to indicate that all motion has stopped. A no motion state or stopped state  62  is entered when no or substantially no change in position is detected. For example, if the output of resolver  12  does not change by more than plus or minus one count over five consecutive samples, stop state  62  is entered, and a signal may be produced indicating that the wheel has stopped rotating. If the position of rotor shaft  14  changes a given number of times in a single direction over a period of time, three changes in the same direction over the course of five samples, for example, motion state  60  is entered and a velocity output will be produced. Controller  38  remains in the low speed, high resolution mode  54  while the vehicle is in the stopped state. The outputs of controller  38  can be used to provide velocity information on a display screen, not shown, or be used by other control systems that require velocity information for taking appropriate control actions.  
         [0022]     The analog speed signal can be used over a wide range of speeds, down to and including low speeds such as 0.1 radians per second, for example. For an 18 inch diameter aircraft wheel, for example, this rate of rotation represents a vehicle speed of 0.9 inches per second or 4.5 feet per minute. An unmanned aircraft moving at such a speed could easily cause damage to itself or surrounding objects if allowed to roll at that rate for even a short period of time. With the digital speed sensor measuring these low speeds and the stop signal or flag confirming that motion of the vehicle is truly stopped, better control over unmanned vehicles in particular can be provided.  
         [0023]     Controller  38  can provide a velocity signal that is based on either the velocity calculated from the position information received at input  36  or on the analog velocity signal that is input to analog/digital converter  40 . The application that is using the velocity signal will play a role in determining which of these signals is selected. For example, the analog signal has little or no latency and reflects the velocity of the wheel in real time. However, the precision of the analog signal, in digital terms, is limited to approximately 11 or 12 bits. Speeds calculated from the position information output by R/D converter  32 , on the other hand, can have 16 bit precision or higher. Because these speeds are based on a comparison of two wheel positions, however, they will have a small latency, on the order of 10 msec. When using a velocity signal to predict the onset of a wheel skid, it may be preferable to sacrifice precision and obtain the velocity in real time. Such applications would thus rely on the analog velocity signal produced by R/D converter  32 . When controlling wheel deceleration, on the other hand, the extra precision provided by the digital position information may more than make up for the small latency inherent in such signals. The digital signal would then be used for such applications. The availability of two signals to the controller also provides a degree of redundancy. These signals, may, for example, be compared periodically, and in the event that a difference between the speeds indicated by the analog speed signal and the digital position signal differ by more than a predetermined amount, a possible error condition can be noted. One or the other signal, such as the analog speed signal, could then be selected for use alone until the source of the difference in speeds indicated by the analog and digital signals is determined.  
       EXAMPLE 1  
       [0024]     The operation of the system may be further understood with reference to the following example which is based on an 18 inch diameter tire moving at a maximum speed of 200 knots wherein the maximum desired sample interval is 1 radian. This provides over six measurements per wheel revolution.  
         [0025]     The 18 inch wheel, having a circumference of about 54.5 inches, moves at a linear speed of 200 knots, or 337.6 feet per second. The wheel will therefore rotate through an angle of 1.333 radians for each linear foot that the vehicle travels on a support surface such as a runway. At 200 knots, this wheel will have an angular velocity of 450 radians per second. Therefore, the desired sample rate is at least 450 Hz, and a sampling rate of 500 Hz will be assumed for this example. A wheel moving at one foot per second will rotate 9.17 arc minutes between samples taken at 500 Hz: 
 
1 foot/sec*1.333 radians/foot*0.002 sec/sample*57.2958 degrees/radian=0.1528 degrees=9.17 minutes. 
 
         [0026]     Each bit must therefore represent no more than 9.17 minutes, and this level of resolution requires 12 bit precision. Greater precision can be obtained for lower maximum rotation rates.  
         [0027]     Various advantages are provided by the speed sensors according to embodiments of the present invention. Resolvers do not use permanent magnets or optical devices, the performance of which tends to be temperature limited. Therefore, higher temperature operation should be possible with a resolver-based speed sensor. In addition, resolvers operate even at low wheel speeds as opposed to tachometers which often require gearing to speed up the wheel speed so that one wheel revolution produces several turns of the tachometer. Thus, resolvers require fewer components and may be generally more mechanically reliable that tachometer based systems.  
         [0028]     The present invention has been described herein in terms of a specific embodiment; however, it will be understood that obvious modifications and additions to this embodiment will become apparent to those skilled in the relevant art upon a reading and understanding of the foregoing description. It is intended that all such modifications and additions form a part of the present invention to the extent they fall within the scope of the several claims appended hereto.