Abstract:
A computational scheme for an INS which utilizes micro-electro mechanical systems (MEMS) sensors resolves issues of unnecessarily large computational burden and numerical instability which happen in using conventional methods using high-end inertial sensors. The first aspect is to omit computing Earth radii of curvatures that were used in a high-frequency application. The second aspect is to ignore the Earth rotation and the rotation of the local tangent plane with respect to Earth which are much smaller than noise and bias in the MEMS sensor output. The last aspect is to assume that the absolute amount of the platform pitch and roll angles is less than 90 degrees. According to the aspects noted above, the computational load will be one-fourth of the conventional method without degradation of accuracy.

Description:
FIELD OF THE INVENTION 
     This invention relates to a method involving a vehicle navigation system, and more particularly, to a computational scheme for the inertial navigation system (INS) which utilizes low-precision micro-electro mechanical systems (MEMS) sensors for simplifying the computational burden to compute the platform six degrees of freedom (DOF) motion. 
     BACKGROUND OF THE INVENTION 
     The INS is a widely used technology for guidance and navigation of a vehicle. The INS is composed of an inertial measurement unit (IMU) and a processor wherein an IMU houses accelerometers and gyroscopes which are inertial sensors for detecting platform motion with respect to an inertial coordinate system. An important advantage of the INS is independence from external support, i.e., it is self-contained. However, the INS cannot provide high accuracy at long ranges. This is because inertial sensors are subject to errors that tend to accumulate over time, i.e., the longer the drive time, the greater the inaccuracy. 
     More recent development in global positioning system (GPS) has made high accuracy vehicle navigation possible at low cost. However, since the GPS relies on GPS satellites, it is susceptible to jamming, RF (radio frequency) interference and multipath problems. Although the GPS provides accurate position and velocity over longer time periods, the GPS involves occasional large multipath errors and signal dropouts. Therefore, efforts are made to develop integrated INS/GPS navigation systems by combining the outputs of a GPS and an INS using the Kalman filter to remedy performance problems of both systems. 
     Inertial sensors used to be expensive and bulky, thus only used in precision application, e.g., aerospace and military navigation. For establishing an IMU package in a compact and inexpensive manner, efforts have been made to develop MEMS sensors resulting in commercialization of low-cost, small, but noisier MEMS inertial sensors. A computational scheme for an INS commonly known in the art offers an exact formula applicable to a system with high-end inertial sensors, for example, ring laser gyros, to track the platform six degrees of freedom without any conditions in the platform dynamics ([1] Titterton, D. H. and Weston, J. L., “ Strapdown Inertial Navigation Technology ”, Peter Peregrinus Ltd., Stevenage, Herts., England, U. K., 1997, [2] Savage, P. G., “ Strapdown Analytics ”, Strapdown Associates, Inc., Maple Plain, MN, 2000). 
     Because of the large amount of noise and bias, however, use of the conventional formula for a system with low-cost MEMS sensors is unnecessarily detailed and often results in numerical instability which often happens as soon as GPS signals are lost. Also, assumptions of free orientation, free speed, and free altitude made in the conventional method result in unnecessary computation for most of commercial vehicles whose pitch and roll angles are restricted within −90 to +90 degrees with low speed and altitude. 
     Therefore, there is a need of a new computational method for a MEMS based INS/GPS for larger stability and greater efficiency assuming larger sensor errors and restrictions in the platform dynamics. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the present invention to provide computational methods with light computational burden yet with guaranteed stability for a low cost INS/GPS navigation system which utilizes low cost MEMS sensors in its IMU to track the platform three-dimensional position, three-dimensional-velocity, and three-dimensional orientation assuming the representative dynamics of commercial vehicles. 
     One aspect of the present invention is to consider that Earth is flat when tracking the platform three-dimensional position, i.e., a local tangent plane is used. This assumption is applicable to low-speed and low-altitude vehicles, e.g., ground vehicles and vessels. This will reduce cumbersome computation of the Earth North-South radius of curvature (R N ) and East-West radius of curvature (R E ) from the INS high-frequency computation. 
     Conversions from latitude (φ) and longitude (λ) into Northerly displacement (N) and Easterly displacement (E) are still required when integrating the INS and GPS, which can be done by using the reference Earth North-South radius of curvature (R N0 ) and East-West radius of curvature (R E0 ) computed only once at the reference point. 
     Another aspect of the present invention is to ignore the Earth rotation and the rotation of the local tangent plane with respect to Earth when computing the angular rates of the platform. This can be justified in the most of MEMS gyro applications since they are much smaller than noise and bias in MEMS gyro output. 
     Another aspect of the present invention is to focus on the platform dynamics with −90 (deg)&lt;pitch&lt;90 (deg) and −90 (deg)&lt;roll&lt;90 (deg), i.e., a platform without making vertical ascent/descent nor rolling over more than 90 degrees, which is natural to most of commercial vehicles including ground vehicles, vessels, and even commercial aircraft. These conditions (limited degrees of rotation) reduce the number of necessary parameters to track three-dimensional orientation. 
     According to the present invention, the computational load will be dramatically reduced to one-fourth of the conventional method without degradation of accuracy in using the conventional precision method as long as: 
     (1) MEMS sensors are used; 
     (2) the platform speed and altitude are low, e.g., ground vehicles and vessels and commercial aircrafts; 
     (3) −90 (deg)&lt;pitch&lt;90 (deg) and −90 (deg)&lt;roll&lt;90 (deg). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  depicts a local tangent plane about a reference point,  FIG. 1B  depicts that N, E, and altitude (h) are obtained by numerically integrating v N , v E , and negative downward velocity (−v D ) along the local tangent plane about a reference point, and  FIG. 1C  depicts that when conversions of φ and λ into N and E are required, R N0  and R E0  will be used. 
         FIG. 2A  explains the Earth rotation and its amount,  FIG. 2B  explains rotation of the local tangent plane with respect to Earth, and  FIG. 2C  explains how the Earth rotation and the local tangent plane with respect to Earth are smaller than noise and bias in MEMS gyro output. 
         FIG. 3A  depicts a three-by-three direction cosine matrix denoted as “T nb ” to track platform three-dimensional orientation, i.e, roll, pitch, and yaw angles. “T nb ” represents “Transformation from the Sensor Body coordinate system to North-East-Down, i.e. local tangent plane, coordinate system”,  FIG. 3B  explains that only four parameters of T nb  are necessary to track the platform of limited orientation, and a platform, and  FIG. 3C  explains that all the nine parameters of T nb  are necessary to track the platform of free orientation. 
         FIG. 4  is a schematic block diagram showing an example of basic configuration of the integrated INS/GPS navigation system of the present invention which is typically mounted on a ground vehicle, a vessel or a commercial aircraft. 
         FIGS. 5A-5B  are schematic diagrams showing the IMU and the inertial sensors incorporated in the IMU where  FIG. 5A  is a schematic block diagram showing a basic structure of the IMU, and  FIG. 5B  is a schematic diagram depicting an IMU body coordinate system. 
         FIG. 6  is a flow chart showing an example of basic operational process for lightening the computational load for an INS in accordance with the present invention. 
         FIG. 7  summarizes the comparison of the numbers of necessary computations in one step of high-frequency INS computation between conventional method and this invention. 
         FIG. 8  shows the vehicle trajectory given by a navigation system in which a 6 DOF MEMS IMU and a GPS receiver are coupled via a Kalman filter. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention will be described in detail with reference to the accompanying drawings. The method and apparatus of the present invention takes advantages of the conditions featured in the platform dynamics and low-precision MEMS sensor performance. It should be noted that although a road vehicle is mainly used in the following description, the present invention can be advantageously implemented to other types of vehicles such as vessels, commercial aircrafts, etc. 
     To compute platform (vehicle) three-dimensional position, the conventional precision method tracks latitude φ, longitude λ, and altitude h by numerically integrate their time derivatives at a high frequency. The time derivatives of latitude φ are the functions of velocity v N , Earth North-South radius of curvature R N , longitude λ, and altitude h, while the time derivatives of longitude λ are the functions of velocity v E , Earth East-West radius of curvature R E , latitude φ, and altitude h. The time derivative of h is simply −v D . Note that computations of the Earth North-South radius of curvature R N  and the Earth East-West radius of curvature R E  take large burden. 
     Meanwhile,  FIG. 1A  depicts a local tangent plane with respect to a reference point that is a concept used in an INS tracking. This concept allows to numerically track the Northerly displacement N and the Easterly displacement E by directly integrating velocities v N  and v E , respectively, as depicted by  FIG. 1B , wherein the steps to derive the Earth radii of curvatures R N  and R E  requiring cumbersome computations are omitted. The computational method of the present invention gives enough accuracy in the navigation operation for low-speed and low-altitude vehicles, e.g., ground vehicles, vessels, and commercial aircrafts. In mathematical expressions, the above steps will be denoted in the following: 
     Position Equation in Conventional Method 
               φ   .     =       v   N         R   N     +   h                     λ   .     =       v   E         (       R   E     +   h     )     ⁢   cos   ⁢           ⁢   φ                     h   .     =     -     v   D             
Simplified Position Equation In New Method
 
{dot over (N)}=v N  
 
Ė=v E  
 
 {dot over (h)}=−v   D  
 
Note that a dot represents the time derivative in the above expressions.
 
     In integrated INS/GPS navigation application, it is often necessary to compare the latitude φ and the longitude λ possibly obtained by the GPS with the Northerly displacement N and the Easterly displacement E obtained by the INS. Although this process requires the Earth North-South radius of curvature R N  and the Earth East-West radius of curvature R E , this conversion can be performed by using the reference R N0  and R E0  which are computed only once per travel at a reference point as described in  FIG. 1C . 
     In mathematical expressions, the above step for comparing the latitude φ and the longitude λ with the Northerly displacement N and the Easterly displacement E will be denoted in the following. 
     Conversion Between (φ, λ) and (N, E)
 
 N=R   N0 (φ−φ 0 )
 
 E=R   E0 (λ−λ 0 )cos(φ)
 
     Regarding the platform rotational motion, in the conventional method, the following angular rates must be taken into consideration in navigation computation:
         (1) angular rate of Earth rotation ( i ω e ) ( FIG. 2A )   (2) rotation of the local tangent plane with respect to Earth ( e ω n ) ( FIG. 2B )   (3) angular rate of the platform with respect to the local tangent plane ( n ω b )   (4) gyro output equal to the angular rate of the platform with respect to an inertial coordinate system ( i ω b ) with noise and bias       

     In the MEMS application under the present invention, however, the angular rates  i ω e  and  e ω n  are much smaller than gyro noise and bias, thus are ignorable as depicted in  FIG. 2C . In the diagram of  FIG. 2C , the dots O 1 -O 8  indicate the outputs of the MEMS gyros (INS sensors shown in  FIG. 7A ). The Earth rotation  iω   e  is typically 0.004 deg/s as indicated in  FIG. 2A , and the rotation of the local tangent plane with respect to the Earth  e ω n  is typically 0.0004 deg/s as indicated in  FIG. 2B , the sum of which is within the range indicated by the lines A and B in  FIG. 2C . In contrast, the MEMS gyro noise and bias is about 0.1 deg/s, respectively as indicated in  FIGS. 2A-2B , thus, the MEMS sensor errors are much larger than the Earth rotation error and/or the local tangent plane rotation error as also shown in  FIG. 2C . Therefore, in the present invention, the angular rates of Earth rotation  i ω e  and the angular rates of local tangent plane rotation  e ω n  are ignored in the computation process, i.e., the Earth is considered stationary. This assumption is applicable to low speed and low altitude vehicles (ground vehicles, vessels as well as small commercial aircrafts). 
     This assumption (angular rates of Earth rotation  i ω e =zero) further contributes to simplify the calculation process of velocity of the vehicle as follows: 
     Velocity Equation under Conventional Method
 
 {dot over (v)} =−( i ω b + i ω e )× v+a+g  
 
Simplified Velocity Equation under New Method
 
 {dot over (v)}=−   i ω b   ×v+a+g  
 
where
     v=three-dimensional velocity vector with respect to the platform body coordinate system   a=three-dimensional accelerometer output with respect to the platform body coordinate system   g=three-dimensional gravity vector with respect to the platform body coordinate system   

     Furthermore, considering application to most of commercial vehicles, it is reasonable to assume that the platform orientation is restricted with the following conditions as depicted in  FIG. 3B . In the conventional method, a three-by-three direction cosine matrix denoted as “T nb ” to track platform three-dimensional orientation, i.e, roll, pitch, and yaw angles has been incorporated as depicted in  FIG. 3A . Here, “T nb ” is an orientation equation that represents “Transformation from the Sensor Body coordinate system to North-East-Down, i.e., local tangent plane, coordinate system”. As noted above, the present invention limits the range of rotation of the platform as shown in  FIG. 3B , which results in that only four parameters of orientation equation T nb  are necessary to track the platform (vehicle).  FIG. 3C  explains that all the nine parameters (three-by-three direction cosine matrix) of orientation equation T nb  shown in  FIG. 3A  are necessary to track the platform of free orientation (fighter, tactical aircraft, etc.) 
     Commercial Vehicle Orientation Conditions 
     As illustrated in  FIG. 3B , in the MEMS application under the present invention, the pitch and roll of the platform are limited to the ranges as follows:
 
−90 (deg)&lt;pitch&lt;90 (deg)
 
−90 (deg)&lt;roll&lt;90 (deg)
 
     When tracking an orientation with an INS, numerically tracking a direction cosine matrix (orientation equation) T nb , representing a transformation from platform body to the local tangent North-East-Down coordinates system is the common method. When assuming platform free orientation, it is necessary to track all the nine elements of orientation equation T nb  ( FIG. 3A ), however, under the orientation conditions described above, it is possible to reduce the number of required elements into four as indicated by the dot hatches in  FIG. 3A . 
     Since it can recover all the other elements of orientation equation T nb  from the sinusoidal functions of roll, pitch and yaw angles, the present invention need to track only the (1,1), (2,1), (3,1), and (3,2) elements of orientation equation T nb  whereas the free orientation assumption in the conventional technology requires to track all the nine elements. Namely, it is possible to recover sinusoidal functions of roll, pitch, and yaw angles in the following manner (r=roll, p=pitch, y=yaw): 
     Direction Cosine Matrix Recovering Procedure in New Method 
     Step 1.
 
sin( p )=− T   nb (3,1)
 
cos( p )=SQRT(1−(sin( p )) 2 )
 
     Note that cos(p)&gt;0 because −90 (deg)&lt;pitch&lt;90 (deg) 
     Step 2.
 
sin( r )= T   nb (3,2)/cos( p )
 
cos( r )=SQRT(1−(sin( r )) 2 ):
 
     Note that cos(r)&gt;0 because −90 (deg)&lt;roll&lt;90 (deg) 
     Step 3.
 
sin( y )= T   nb (2,1)/cos( p )
 
cos( y )= T   nb (1,1)/cos( p )
 
     In mathematical expressions, the above steps for estimating an orientation of vehicle will be denoted in the following: 
     Orientation Equation Under Conventional Method
 
 {dot over (T)}   nb   =T   nb   Rot ( i ω b − i ω e − e ω n )
 
where all the nine parameters are tracked.
 
Simplified Orientation Equation Under New Method
 
 {dot over (T)}   nb   =T   nb   Rot ( i ω b )
 
where only four parameters are tracked, and
 
where
 
               Rot   ⁡     (   ω   )       =       [         0         -     ω   ⁡     (   3   )               ω   ⁡     (   2   )                 ω   ⁡     (   3   )           0         -     ω   ⁡     (   1   )                   -     ω   ⁡     (   2   )               ω   ⁡     (   1   )           0         ]     ⁢     :             
mapping from a vector to a skew-symmetric matrix
 
     In a conventional INS computational method, errors accumulate on the elements of orientation equation T nb  resulting in that self consistency of the transformation matrix breaks. Here, self consistency means that
         each row is orthogonal to one another (orthogonality)   the sum of squares of the elements in each rows should equal unity [0] (normality)       

     Therefore, it is necessary to perform orthogonalization and normalization at a certain cycle for a conventional INS. In the present invention, however, orthogonality is almost always achieved since only four elements of orientation equation are numerically integrated and the others are recovered using the exact formula as depicted in  FIG. 3A . Thus, custom normalization in the following manner is performed particular to this invention: 
                                                                         Direction Cosine Matrix Normalization Procedure for MEMS INS                                 I.   IF   T nb (2,1) 2  + T nb  (3,1) 2  &gt; 1           THEN   Δ ≡ T nb (1,1) 2  + T nb  (2,1) 2  + T nb  (3,1) 2  − 1                                   T   nb     ⁡     (     1   ,   1     )       =         T   nb     ⁡     (     1   ,   1     )       ⁢     (     1   -     Δ   2       )                       T   nb     ⁡     (     2   ,   1     )       =         T   nb     ⁡     (     2   ,   1     )       ⁢     (     1   -     Δ   2       )                       T   nb     ⁡     (     3   ,   1     )       =         T   nb     ⁡     (     3   ,   1     )       ⁢     (     1   -     Δ   2       )                                    conventional method found in [1],       II.   ELSE                T nb (1, 1) = sign(T nb (1,1)) {square root over (1 − T nb  (2, 1) 2  − T nb  (3, 1) 2  )}{square root over (1 − T nb  (2, 1) 2  − T nb  (3, 1) 2  )}                Direct normalization particular to this invention.                        
The mixture of the above methods I and II are preferable in the present invention.
 
       FIG. 4  is a schematic block diagram showing an example of basic configuration of the integrated INS/GPS navigation system for implementing the computational scheme of the present invention. An INS/GPS navigation system is typically mounted on a ground vehicle, a vessel or a commercial aircraft. The INS/GPS navigation system  20  includes an inertial navigation system (INS)  30 , a GPS (global positioning system) receiver  40 , and a Kalman filter  50  (KF 2 or integration Kalman filter). In the example of  FIG. 4 , unlike ordinary GPS navigation systems widely used in automobiles today, the integrated INS/GPS navigation system  20  of the present invention does not use any speed pulse sensor to detect the moving distance of the automobile. Therefore, it is no longer necessary to establish complicated wiring in the vehicle for connecting a speed pulse sensor to a navigation system processor. Further, it is unnecessary to adjust the outputs of the speed pulse sensor which was necessary in the conventional technology because the definition of speed pulse versus distance is different from manufacturer to manufacturer of speed pulse sensors. 
     In  FIG. 4 , the INS  30  is configured by an inertial measurement unit (MU)  32 , a low-pass filtering unit  34 , and a navigation equation unit (CPU)  36 . The GPS receiver  40  includes a Kalman filter  42  (KF 1 or GPS Kalman filter) in addition to GPS signal receiver circuits and a controller (not shown). Thus, this example shows a structure of a loosely coupled INS/GPS navigation system. As known in the art, in the case of a tightly coupled INS/GPS navigation system, GPS receiver  40  does not include a Kalman filter. The integration Kalman filter  50  (KF 2) includes a Kalman gain unit  52  which provides a gain (coefficient) to each difference between a GPS measurement and INS estimate in accordance with position tracking operations. 
     The IMU  32  has inertial sensors and a microprocessor. As noted above, the inertial sensors are established by MEMS (microelectro mechanical system) technologies to detect accelerations and angular rates of three coordinates of the platform (vehicle, etc). The microprocessor processes the detected signals from the inertial sensors. Because the integrated INS/GPS navigation system  20  includes inertial sensors, it can also estimate a vertical position of the vehicle with accuracy much higher than that of the ordinary GPS navigation system. The IMU  32  produces the output data, for example, 100 times per second (100 Hz). The output data from the IMU  32  is supplied to the low-pass filtering unit  34  in which high frequency components thereof are removed. The output data from the low-pass filtering unit  34  is supplied to the navigation equation unit  36  where the current position, velocity and orientation of the vehicle are estimated through the inertial navigation technology. 
     The GPS receiver  40  receives GPS signals from a plurality of GPS satellites and calculates the estimated location of the vehicle by comparing clock signals and position data included in the GPS signals. The GPS receiver  40  optimizes the obtained position data by the Kalman filter (KF 1)  42  to minimize the effects of noises on the satellite signals. Typically, the GPS receiver  40  produces the position and velocity data every one second (1 Hz). Each parameter of the position and velocity data produced by the GPS receiver is accompanied by accuracy information. 
     The estimated position data from the INS  30  and the estimated position data from the GPS receiver  40  are combined by the Kalman filter (KF 2)  50  which optimally estimates, in real time, the states of the navigation system based on such noisy measurement data. The Kalman gain unit  52  provides weight or gain to each difference between a GPS measurement and INS estimate in accordance with position tracking operations. The output of the Kalman filter  50  is provided to the navigation equation unit  36  which calculates the estimated position of the vehicle which will be displayed on a navigation monitor screen (not shown). 
     In this example, the navigation equation unit (CPU)  36  controls an overall operation of the INS  20  including the computational scheme of the present invention. Namely, the navigation equation unit (CPU)  36  stores the program to execute the simplified computation process of the present invention for position tracking of the vehicle. As noted above, the program is established based on the assumption that the Earth is flat when tracking the platform three-dimensional position, i.e., a local tangent plane is used, thereby simplifying the computation involving the Earth North-South radius of curvature (R N ) and East-West radius of curvature (R E ). Another condition for simplifying the computation is to ignore the Earth rotation and the rotation of the local tangent plane with respect to Earth when computing the angular rates of the platform. A further aspect of the present invention is to limit the rotation of the platform within −90 (deg)&lt;pitch&lt;90 (deg), and within −90 (deg)&lt;roll&lt;90 (deg), thereby reducing the number of necessary parameters for tracking the three-dimensional orientation of the vehicle. 
       FIGS. 5A-5B  are schematic diagrams showing the IMU  32  and the inertial sensors incorporated in the IMU  32 .  FIG. 5A  is a schematic block diagram showing a basic structure of the IMU  32  used in the INS  30  of  FIG. 4 . The IMU  32  includes a processor  33 , and the inertial sensors consisting of three (three-axis) accelerometers Acc  1 - 3  and three (three-axis) gyroscopes Gyro  1 - 3 . The accelerometers Acc  1 - 3  detect accelerations in the three (X, Y, Z) coordinates of the vehicle, and the gyroscopes Gyro  1 - 3  detect angular rates about the three (X, Y, Z) coordinate directions of the vehicle. The processor  33  calculates the accelerations and angular rates based on the signals from the inertial sensors Acc  1 - 3  and Gyro  1 - 3 .  FIG. 5B  is a schematic diagram depicting an IMU body coordinate system. In  FIG. 5B , characters P, Q, and R represent the vehicle angular rates about the IMU coordinate components x b , y b , and z b . 
       FIG. 6  is a flowchart showing an example of basic operational process of the INS/GPS navigation system using MEMS sensors for simplifying the computation for position tracking under the present invention. In the flowchart of  FIG. 6 , in the step  101 , the process assumes that the Earth is flat so that it is unnecessary to incorporate the curvature of the Earth, which will simplify the computation for tracking the three-dimensional position of the vehicle. In the step  102 , the process ignores the rotation of the Earth for computing the angular rates of the vehicle. Similarly in the step  103 , the process ignores the rotation of the local tangent plane relative to the Earth for computing the angular rates of the vehicle. By ignoring the rotation of the Earth and the local tangent plane, the computation for tracking the three-dimensional velocity of the vehicle will be simplified. 
     In the step  104 , the process limits the range of vehicle “pitch” within −90 degrees and +90 degrees. Similarly in the step  105 , the process limits the range of vehicle “roll” within −90 degrees and +90 degrees. By limiting the degree of rotation of the vehicle, the computation for tracking the three-dimensional orientation of the vehicle will be simplified because of the dramatic reduction of the parameters involved. In the step  106 , the INS finalizes the computation for tracking the vehicle incorporating all of the conditions established in the above steps  101 - 105 . Finally, in the step  107 , the results of tracking by the INS is integrated with the tracking by GPS to obtain the current position of the vehicle. 
     EXPERIMENTAL RESULTS 
     According to the methods proposed so far, the numbers of multipliers, dividers, and square roots reduce as described in the table of  FIG. 7  resulting in the approximate amount of one-fourth of the conventional computation. 
       FIG. 8  shows the vehicle trajectory given by the INS/GPS navigation system of  FIG. 4  in which the IMU  32  and the GPS receiver  40  are coupled via the Kalman filter  50 . The computation for the INS is carried out using the methods presented in this invention. It shows that the navigation system successfully tracks the vehicle motion entering and exiting a three dimensional parking building where GPS signals are dropped off for a long time inside the parking building. 
     Although the invention is described herein with reference to the preferred embodiment, one skilled in the art will readily appreciate that various modifications and variations may be made without departing from the spirit and scope of the present invention. Such modifications and variations are considered to be within the purview and scope of the appended claims and their equivalents.