Abstract:
A torque measurement device associated with a power driven ground engaging device having a power supplying device and a load. The torque measurement device includes a rotatable shaft, a first detectable feature, a second detectable feature, a plurality of sensors and an electrical controller. The rotatable shaft has a longitudinal axis, a first end and a second end. The first end is connected to the power supplying device and the second end is connected to the load. The first and second detectable features are respectively associated with a first longitudinal position and a second longitudinal position on the shaft. The plurality of sensors include a first sensor and a second sensor. The first sensor is proximate to the first detectable feature and the second sensor is proximate to the second detectable feature. The first sensor produces a first signal and the second sensor produces a second signal as the rotatable shaft rotates about the longitudinal axis. The electrical controller samples the first signal and the second signal at a sampling rate. The first signal has a first frequency and the sampling rate is less than twice the first frequency. The electrical controller computes a torque measurement representative of the torque on the rotatable shaft utilizing the first signal and the second signal.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a torque measurement device, and, more particularly to a torque measurement device utilized with a controller having a low sampling rate. 
       BACKGROUND OF THE INVENTION 
       [0002]    In power supplying devices, such as agricultural machines the engine&#39;s power is directed to several aspects of the agriculture machine and the directing of the power is often done by way of rotating shafts. The torque supplied by the engine is often directed through a transmission system that may be utilized to multiply the torque applied to a load. The capacity of the engine in supplying power to the load may exceed the capacity of the shaft or that of a subsequently driven item if the load becomes too great. It is known to measure the torque by way of a drive plate that has retaining springs through which the torque is transmitted from the drive plate to a driven plate. The differential position between the drive plate and the driven plate, as torque is transmitted through springs connecting the two, is utilized to compute the torque being applied by way of the drive plate/driven plate arrangement. As torque is transmitted through the plate, the springs are compressed and fingers extending from the driven plate move relative to a reference on the driven plate. This difference then is detectable by a sensor and the information is conveyed to a display system for information to the operator of the agricultural machine. An example of this sort of measurement device is contained in U.S. Pat. No. 5,596,153, which includes a device for the measurement of engine rpm and torque transmitted through the drive plate to the driven plate. 
         [0003]    What is needed in the art is a cost effective torque measurement system that can be easily and inexpensively incorporated using the computational capabilities available on an agricultural machine. 
       SUMMARY OF THE INVENTION 
       [0004]    The present invention relates to a torque measurement device and method that utilizes a controller with a low sampling rate to measure torque on at least one drive component in an agricultural vehicle. 
         [0005]    The invention in one form consists of a torque measurement device associated with a power driven ground engaging device having a power supplying device and a load. The torque measurement device includes a rotatable shaft, a first detectable feature, a second detectable feature, a plurality of sensors and an electrical controller. The rotatable shaft has a longitudinal axis, a first end and a second end. The first end is connected to the power supplying device and the second end is connected to the load. The first and second detectable features are respectively associated with a first longitudinal position and a second longitudinal position on the shaft. The plurality of sensors include a first sensor and a second sensor. The first sensor is proximate to the first detectable feature and the second sensor is proximate to the second detectable feature. The first sensor produces a first signal and the second sensor produces a second signal as the rotatable shaft rotates about the longitudinal axis. The electrical controller samples the first signal and the second signal at a sampling rate. The first signal has a first frequency and the sampling rate is less than twice the first frequency. The electrical controller computes a torque measurement representative of the torque on the rotatable shaft utilizing the first signal and the second signal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is a side view of a tractor utilizing the apparatus and method of the present invention; 
           [0007]      FIG. 2  is a schematical representation of an embodiment of the torque measurement apparatus of the present invention utilized in the tractor of  FIG. 1 ; 
           [0008]      FIG. 3  is a schematical representation of another embodiment of the torque measurement device of the present invention utilized in the tractor of  FIG. 1 ; and 
           [0009]      FIG. 4  is a schematical representation of an embodiment of the method utilized in the devices of  FIG. 2  or  3  in the tractor of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0010]    Referring now to the drawings, and more particularly to  FIGS. 1-3 , there is illustrated a ground engaging vehicle  10  also known as a tractor  10 . Tractor  10  includes an engine/transmission system  12  that provides power to wheels  14 , which can be understood to be a load  16 . Engine/transmission system  12  can be understood to be a power supply device  12  with power therefrom being distributed to mechanical loads throughout tractor  10  and even by way of linking power shafts to other agricultural equipment, not shown. For illustration purposes and for the ease of understanding, wheels  14  can be considered a load  16 , although it is to be further understood that load  16  can be any mechanical load that is being driven by engine transmission system  12 . 
         [0011]    Schematical representations in  FIGS. 2 and 3  respectively illustrate two embodiments of torque measurement system  40  or  42  with a power supplier  12  conveying a mechanical power to a load  16  by way of a shaft  18 . Torque is supplied by power supplier  12  and is transmitted by shaft  18 . Toothed wheels  20 ,  22 ,  24  and  26  are positioned as shown in the two embodiments with the embodiment of  FIG. 2  utilizing only toothed wheels  20  and  22 . Toothed wheels  20 ,  22 ,  24  and  26  are to be understood to be a detectable feature and do not necessarily have to be toothed wheels, which are described herein for ease of explanation of the present invention. Detectable features  20 ,  22 ,  24  and  26  can also be, for example, optical or magnetic sources associated with separate longitudinal positions along shaft  18 . 
         [0012]    Sensors  28  and  30  are respectively located associated with toothed wheels  20  and  22  in  FIG. 2 . In  FIG. 3  sensors  32  and  34  are additionally respectively associated with toothed wheels  24  and  26 . Sensors  28 ,  30 ,  32  and  34  are communicatively coupled to controller  36  for receiving signals generated by the interaction of toothed wheels  20 ,  22 ,  24  and  26  with corresponding sensors  28 ,  30 ,  32  and  34 . 
         [0013]    As torque is supplied by way of power supplier  12  to load  16  by way of shaft  18 , shaft  18  is rotated about a longitudinal axis  38  causing toothed wheels  20  and  22  to rotate therewith. As the load is increased by load  16 , additional power is supplied by way of power supplier  12 , shaft  18  flexes about longitudinal axis  38  causing a variation in the signals from sensors  28  and  30 , which results in detectable flexing of shaft  18  that is then measured and the stiffness of shaft  18  is utilized in the computation of the torque being delivered through shaft  18 . The computation is undertaken by controller  36 , which for ease of understanding is a controller/computer that is associated with tractor  10  and has a relatively low sampling rate inherent with such controllers. Controller  36  is utilized for controlling various systems and displays in tractor  10  and is normally not dedicated to just the measurement of torque as illustrated in  FIGS. 2 and 3 . Although typically controller  36  is being utilized for other functions on tractor  10 , it is to be understood that a separate controller  36  is also contemplated by the inventors. 
         [0014]    A measurement of the amount of torque in a drive train component such as shaft  18  allows for the adjustment of the fuel delivery system to the engine and adjustments in transmission of tractor  10  so it can be as productive as possible while protecting the drive train illustrated herein as representative shaft  18  and equipment being driven thereby, which is represented herein as load  16 . By being able to cost effectively measure torque on various drive train components this allows the engine to run at a higher power level while the torque being supplied to the various components such as to the power take off (PTO) and the axles of tractor  10  when the torque is being monitored on each of the driven components. 
         [0015]    The shifting of the transmission is also improved by knowing how much power is being delivered to the ground or other driven component to thereby optimize the timing of a shift as well as the selection of an optimal gear ratio. Controller  36  can include this information so that if the PTO and/or hydraulics are being used, the shift algorithm can be modified based on power levels being consumed by the different driven components. Additionally, knowing and even recording delivered engine torque to particular driven components can assist in warranty determinations, for example, if a user installs an unauthorized power box that is utilized to boost engine power this can result in damaged driven components, which may then be evaluated as the user&#39;s fault. In the past shear bolts or other devices were utilized to fracture when the load became too great to prevent damage to other portions of the driven components. This protection, while effective, results in undesirable downtime as the shear bolts or fractured elements have to be replaced to resume operation. The present invention provides for the use of information from the torque being supplied to the various components so that damage to driven components is prevented by the monitoring and restriction of torque transmitted through various shafts  18 . 
         [0016]    While shaft torque can be measured by a variety of techniques, such as strain gauges and structural acoustic waves, these techniques require either slip rings or telemetry to obtain a signal from the shaft and are not systems that have an inherent robustness and are not appropriate for off-road vehicles. Magnetostriction and eddy current techniques are robust and less costly, but are too expensive for original equipment manufacturing applications. 
         [0017]    Now, additionally referring to  FIG. 4 , there is shown a method  100  that utilizes the elements of torque measurement system  40  or  42 . For ease of understanding torque measurement system  40  will be discussed with all of the attributes and elements associated therewith being equally applicable to torque measurement system  42 . Toothed wheels  20  and  22  have a multitude of teeth thereon that alter the magnetic field as they move, which is detected by sensors  28  and  30 . Sensors  28 ,  30 ,  32  and  34  have a magnetically flux responsive devices that may contain a magnetic source. Sensors  28  and  30  measure the alteration of the magnetic field caused by the passing of the ferrous material of teeth on toothed wheels  20  and  22 . As the teeth on toothed wheels  20  and  22  pass sensors  28  and  30  respectively two signals are generated and are transmitted to controller  36 . Controller  36  has a sampling rate that is less than twice the tooth pass frequency detected by sensors  28  and  30 , as such the signal is aliased. In a typical application the sampling rate of controller  36  is much less than the tooth pass frequency. Controller  36  receives signals from sensors  28  and  30  that can be understood to be two signals that are related substantially in frequency, but with a phase variation that is associated with the torque being applied to shaft  18 . Controller  36  calculates the frequency of at least one of the two toothed wheels as in step  102  of method  100 . Controller  36  determines the sample rate used to obtain the best signal to noise ratio at step  104 . As previously indicated, the two signals are signals which are respectively pulse trains representative of the passing of teeth on toothed wheels  20  and  22  thereby resulting in the acquisition of two pulse train signals at step  106 . At step  108 , the Fourier coefficients are computed for each of the two pulse trains and the coefficients are utilized at step  110  to compute the relative phase of the two signals. The relative phase of the two signals are compared then to a no load phase difference and the difference in relative phases is multiplied by the stiffness of shaft  18  to arrive at the torque as illustrated at step  112 . The no load phase difference is determined as part of an installation or calibration procedure and is determined when rotating shaft  18  without a load applied thereto or alternatively with a predetermined load. 
         [0018]    By computing the term of the Fourier series that aligns with the frequency determined at step  102  for each pulse train and multiplying one coefficient by the complex conjugate of the other, the relative phase of the two measured positions along the longitudinal axis  38  of shaft  18  is determined. The advantage of this method is that it employs the current tractor controller that has a limited data sample rate and only two magnetic sensors  28  and  30  thereby resulting a robust low cost torque measurement system  40 . 
         [0019]    It will now be undertaken to describe how the aliasing method of torque measurement works. First the method of torque computation knowing the phase shifts of the two pulse trains is undertaken then two methods for computing the phase shift are included herein. The first method utilizes the inner product and may be easier to understand. The second is a traditional discrete Fourier transform method and may be more in line with what is often seen performing a frequency analysis. And then a discussion of what may occur when the speed of shaft  18  is at a no phase speed where the algorithm is not capable of determining a phase difference and some of the inventive solutions of that situation, which is inherent when using a low sampling rate controller. 
         [0020]    Deflection of a torsionally loaded shaft  18  is Θ=TL/JG 
         [0021]    where: Θ=angular deflection [radians]
       T=Torque   L=shaft length   J=Polar moment of inertia   G=modulus of rigidity of shaft  18         
 
         [0026]    The shaft stiffness, K is T/Θ=JG/L and is a constant for any given shaft. While some alternation based upon temperature may be contemplated it is also understood that a temperature sensor may be utilized to modify the foregoing shaft stiffness calculation. 
         [0027]    Shaft  18  is outfitted with toothed wheels  20  and  22  separated by a length L. When shaft  18  is rotating at a sufficient speed, two sinusoidal waveforms are generated, one by each of the interaction between toothed wheel  20  and sensor  28  as well as toothed wheel  22  and sensor  30  resulting in the two signals represented by x 1 (t) and x 2 (t). The general equation is x i (t)=x i  sin(2πft−φ i ). 
         [0028]    where: x i =amplitude of sinusoid
       f=tooth pass frequency [Hz]   φ i =phase [radians], and   f=ZΩ       
 
         [0032]    where: Z=number of teeth on the wheels
       Ω=shaft speed [Hz]       
 
         [0034]    Toothed wheels  20  and  22  are designed so that magnetic pickups  28  and  30  generate a generally sinusoidal signal. Furthermore, toothed wheels  20  and  22  have a common number of teeth, and when no load is applied to shaft  18 , the relative phases of the pulse trains φ 0 =(φ 2 −φ 1  is constant. As shaft  18  winds up, due to torque, then φ=(φ 2 −φ 1  changes proportionally. If shaft  18  was to twist one full circular pitch of a tooth of the toothed wheels, then Θ=2π/Z, that is a  2 π phase shift of the magnetic pickup waveform occurs such that φ−φ 0 =2π. The relationship Θ=(φ−φ 0 )/Z and (φ−φ 0 )=ZΘ=TLZ/JG, in solving for torque we obtain T=(JG/LZ)(φ−φ 0 ) or more generally, for any elastic member of stiffness K, T=(K/Z)(φ−φ 0 ). 
         [0035]    Controller  36  reads magnetic pickups  28  and  30  calculates the toothed pass frequency of toothed wheels  20  and  22  at step  102 . Controller  36  cannot acquire the analog signals fast enough to evaluate the relative phases in the time domain. For example, suppose the tooth pass frequency is 1,500 Hz for wheels  20  and  22 , controller  36  would need to sample at more than 15,000 Hz to reasonably determine the phase in the time domain. By working in the frequency domain, the required sampling rate is twice the bandwidth. Since the analog signal frequency is centered at the tooth pass frequency and varies very little from there, one can successfully sample as low as 200 Hz. This signal is highly aliased (one sample for the passing of every 7.5 teeth), but the signal can be completely reconstructed because we know the frequency of the signal. 
         [0036]    Hereinafter two methods for determining the phase are presented. The first is referred to as the inner product method, the second computes a single line of the discrete Fourier transform (DFT). Each method assumes the tooth pass frequency ZΩ is known and that N samples of each pulse train x i (n) have been acquired at a sample rate of f s . 
         [0037]    The inner product method computes the inner product of the vectors x i (n) and sinusoids of frequency ZΩ. Trigonometric functions, such as the arctangent (herein ATAN 2), are used to obtain the phase. 
         [0000]    
       
         
           
             
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         [0000]    X i  is a complex number and the phase of each pulse train is: 
         [0000]      φ i =ATAN 2 [lm ( X   i ), Re ( X   i )]. 
         [0000]    The relative phase between the two signals is: 
         [0000]      φ=ATAN 2 [lm ( X   2     X     1 ), Re ( X   2     X     1 )]. 
         [0038]    Using a Discrete Fourier Transform Method is now described. This method computes the single line of the DFT that bests represents the pulse train. To simplify the equations, three new variables are presented. The first is a frequency ratio,  Y =ZΩ/f s . The second, k, is an integer has to do with how many times the tooth pass frequency is greater than the sampling frequency and is used to identify whether the frequency of interest is a positive or negative frequency in the range of 0 to (½)f s . The third, m, is an integer between 0 and N/2, identifies which line of the DFT we are interested in computing. The next few lines are pseudo-code for computing the relative phase between the signals. 
         [0000]    
       
         
               
               
             
               
               
             
               
               
             
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 if FLOOR( Y ) = CEIL ( Y  − ½) 
               
               
                   
                 comment: kf s  &lt; ZΩ &lt; (k + ½)f s   
               
             
          
           
               
                   
                 k = FLOOR( Y ) 
               
               
                   
                 m = ( Y -k)N 
               
             
          
           
               
                   
                 else 
               
               
                   
                 comment: (k-1/2)f s  &lt; ZΩ &lt; kf s   
               
             
          
           
               
                   
                 k = CEIL( Y ) 
               
               
                   
                 m =(k-  Y )N 
               
               
                   
                 end if 
               
               
                   
                   
               
               
                   
                 
                   
                     
                       
                         
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                 φ = ATAN2[lm(X 2   X   1 ), Re(X 2   X   1 )] 
               
               
                   
                   
               
             
          
         
       
     
         [0039]    Dealing with the No Phase Speeds previously discussed, there is inherent to the low sampling rate technique for computing torque by using aliased signals, a finite number of predictable speeds wherein the phase is not known. Examining the first summation equation we see that when 2ZΩ/f s  is an integer then e −j2πZΩ(n/fs)  is real for all n and no phase information is obtained. This condition is exactly the same as when m=0 or N/2 in the second summation equation. Since the number of teeth on the toothed wheels and the sample rates may be fixed, the spacing of the phase dropouts is ΔΩ=f s /2Z[Hz]=30f s /Z [rpm]. 
         [0040]    To get around this problem, in specified speed ranges one can choose another sampling rate or choose to observe the pulse train from a second pair of toothed wheels  24  and  26  with different tooth numbers from toothed wheels  20  and  22 . The selection of the number of teeth on the two sets of toothed wheels is carefully chosen so that the problematic zones are not overlapping.  FIG. 3  shows a schematic of a shaft with two pairs of toothed wheels  20  and  22 , and  24  and  26 , along with sensors  28 ,  30 ,  32  and  34  for resolving the latent phase. 
         [0041]    Another way to solve this problem is to use at least two separate sampling rates with the two rates being carefully chosen so that the problematic zones are not overlapping. This is exemplified in step  104 , where a sampling rate is selected for a better signal to noise ratio. For example, if the sampling rate is at 200 Hz and the rotation of shaft  18  is at a rotation speed such that the a no phase speed is experienced then the controller shifts to another sampling frequency such as 175 Hz to thereby allow the system to then make a torque measurement. 
         [0042]    An additional consideration of the signal processing contemplated by the inventor is the use of a window function applied to the time acquired data by controller  36 , the window being zero-valued outside of a predetermined interval. For example, a function that is constant inside the predetermined interval and zero elsewhere is referred to as a rectangular window, which describes the shape of the graphical representation thereof. When the time acquired data is multiplied by the window function, the product is then zero outside of the predetermined interval. In addition to the use of a rectangular window technique the use of a shaped window, such as a Hamming function or a Hann function may be used as the window. These functions are useful to help resolve the signals and to improve accuracy of the measurements. 
         [0043]    Having described the preferred embodiment, it will become apparent that various modifications can be made without departing from the scope of the invention as defined in the accompanying claims.