Abstract:
A reduced-order fail-safe inertial measurement unit system. A first inertial measurement unit device includes a plurality of accelerometers measuring linear accelerations and gyroscopes measuring angular velocities. A second inertial measurement unit device includes a reduced number of accelerometers and gyroscopes relative to the first inertial measurement unit device measuring at least two linear accelerations and at least one angular velocity. A processor receives acceleration data from the first and second inertial measurement units. The processor detects faulty data measurements from the first inertial measurement unit. The processor supplements the faulty data measurements of the first inertial measurement unit with transformed data generated as a function of the measurement data from the second inertial measurement unit. The processor applies predetermined transformation solutions to transform the measurement data from the second inertial measurement unit into the transformed data. The processing unit provides the transformed data to the safety applications of the vehicle.

Description:
BACKGROUND OF INVENTION 
       [0001]    The present invention relates generally to fail-safe Inertial Measurement Unit (IMU) systems. 
         [0002]    Critical safety systems such as those used in a vehicle typically utilize controllers to monitor vehicle operating conditions and employ controlling strategies for vehicle safety functionality and operations. In controllers that monitor safety-related operations, a redundant controller is required. For example, an IMU used to sense linear accelerations of a vehicle is typically a 5 degree of freedom (DOF) or 6 DOF IMU. For backup purposes should the IMU fail, a redundant IMU is utilized. The redundant IMU is identical to the main IMU having the same DOF. However, utilizing an identical IMU having the same DOF is expensive. As a result, it would be beneficial to reduce the cost and burden of utilizing redundant IMUs, but still maintain the safety monitoring and controls of the primary IMU. 
       SUMMARY OF INVENTION 
       [0003]    An advantage of an embodiment is the reduction in cost in eliminating a duplicate IMU used for safety related applications and the utilization of a combination full-order IMU and a reduced-order IMU. The system utilizes predetermined transformation solutions to determine unknown angular velocities of the first inertia measurement unit affected by a fault in the main IMU. The respective angular velocities are calculated as a function of a known angular velocity measurement value of the second inertial measurement unit, known linear accelerations measurement values of the first and the second inertial measurement unit, and predetermined transformation intermediate variables that are derived from data from the main IMU prior to the detection of the fault. The respective linear accelerations and angular velocities are provided to controllers such as safety controllers to maintain functionality of a safety application while the driver of the vehicle is notified of the fault, and is instructed to take manual control of the vehicle operation associated with the safety application. 
         [0004]    An embodiment contemplates a reduced-order fail-safe inertial measurement unit system. A first inertial measurement unit device is mounted on a vehicle. The first inertial measurement unit device includes a plurality of accelerometers measuring linear accelerations and gyroscopes measuring angular velocities. A second inertial measurement unit device is mounted on the vehicle. The second inertial measurement unit device includes a reduced number of accelerometers and gyroscopes relative to the first inertial measurement unit device. The second inertial measurement unit measures at least one linear acceleration and at least one angular velocity. A processor receives acceleration data from the first inertial measurement unit and the second inertial measurement unit. The processor detects faulty data measurements from the first inertial measurement unit. The processor supplements the faulty data measurements of the first inertial measurement unit with transformed data generated as a function of the measurement data from the second inertial measurement unit. The processor applies predetermined transformation solutions to transform the measurement data from the second inertial measurement unit into the transformed data. The processing unit provides the transformed data to the safety applications of the vehicle. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0005]      FIG. 1  is block diagram of an inertia monitoring system 
           [0006]      FIG. 2  is a vector model of the locations of the IMUs 
           [0007]      FIG. 3  a flowchart for a method for the overall process for recovering the faulted signals. 
       
    
    
     DETAILED DESCRIPTION 
       [0008]      FIG. 1  describes a system that includes a first Inertial Measurement Unit (IMU)  12  and a second IMU  14 . The first IMU  12  is full-order IMU such that a six IMU or five-DOF IMU is utilized. 
         [0009]    The six-DOF IMU utilizes three components of linear accelerations (hereinafter referred to as linear attributes) and three components of angular velocities (hereinafter referred to as angular attributes). The three components of translation typically include directions an x-y-z coordinate system. The three components of rotation include rotational attributes such a pitch, yaw, and roll. 
         [0010]    A redundant or semi-redundant IMU is required for safety applications so that vehicle can continue functioning if a fault occurs in a primary IMU. 
         [0011]    Typically if a fault occurs with respect to the IMU, certain functions that are active, such as adaptive cruise control or other autonomous or semi-autonomous vehicle applications, require that such features be shut down and control of those features be relinquished to the driver of the vehicle. However, the system does not want to abruptly terminate automated features/controls until the driver is warned and control of the features for such applications is relinquished to the driver. Under such conditions, it is pertinent that the safety features remain operational for a respective amount of time (e.g. 5-10 sec) so that the driver of the vehicle can retain control of those features of the vehicle. The duration of time that is required depends on the feature or features that are currently active and the estimated time that would be necessary to work the driver and for the driver to take control of the system. 
         [0012]    Prior systems are known to use full-order redundant systems where the main IMU and backup IMU are identical. This is costly to utilize a full-order backup IMU. However, such failures of the main IMU are only partial failures, and as a result, not all of the dynamics of the six-DOF IMU are decoupled. That is, a failure in the first IMU  12  may only affect a single attribute of the IMU (e.g., one of the gyroscopes), and as a result, a majority of the attributes are still operational and providing valid data. Therefore, a reduced-order IMU may provide supplemental data for at least a duration of time that is necessary to regain manual control of the application by the driver. 
         [0013]    A reduced order IMU, such as a three-DOF IMU utilizes a reduced number of linear attributes and/or angular attributes relative to the six-DOF IMU. Preferably, the three-DOF utilizes a combination of the linear attributes and angular attributes. For example, the three-DOF may use x-direction accelerometer, y-direction accelerometer, and yaw rate. It should be understood that the invention is not limited to the exemplary three-DOF described above, and may utilize any combination that is designed for the vehicle. In addition, the exemplary system as described herein is vehicle based, but the architecture can apply to non-vehicular systems as well. 
         [0014]    These respective IMUs may be part of a safety external object detection system that senses external objects about the vehicle and monitors the vehicle operating conditions to determine safe vehicle operations for either alerting the driver of the vehicle or for controlling autonomous or semi-autonomous operations of the vehicle. For example, the data processed by the processor may be supplied to various subsystems that include, but are not limited to, autonomous braking subsystems  17 , autonomous steering subsystems  18 , and autonomous speed control subsystems  19 . These subsystems may function independently or in cooperation with one another. It should be also understood that the system as described herein is not limited to an automobile but may be used in other types of applications that utilize two IMUs including, but not limited to, planes, ships, and trains. 
         [0015]    The first IMU  12  is typically positioned close to the center of gravity of the vehicle. The location of the first IMU  12  is identified during the design and development stages of the vehicle and the location is selected based on the location that can provide the optimum inertia measurements for the vehicle. The location of the second IMU  14  is determined during the design and development stages of the vehicle and is required to be offset at least a predetermined distance from the first IMU  12 . Therefore, an absolute position of the second IMU  14  within the vehicle is not required; rather, it is a relative relationship between the first IMU  12  and the second IMU  14  that is pertinent. 
         [0016]    A selection for a location of the second IMU  14  should preferably meet the criteria that includes, but is not limited to, noise frequency not to exceed a predetermined number of times the natural frequency of the vehicle (e.g., 5×). This predetermined number is configurable and is based on experimental and/or simulation studies conducted on the location of the IMU location. A second requirement includes a noise amplitude not to exceed a predetermined percentage of the range. If the actual noise amplitude is above the predetermined percentage, then the transformation technique as describe herein cannot readily correlate sensor measurements between the locations of the two IMUs. A third requirement is the data should be raw sensor data and not filtered data. A fourth requirement is that the location of the IMUs must be at least a predetermined distance from one another. 
         [0017]    To supplement the redundant data (i.e., data found to be in error) from the first IMU  12 , solutions are identified for determining each respective attribute (e.g., accelerometer data). The solutions utilize known variable measurements sensed by the first IMU  12 , and transformation intermediate variables that can be derived from measurements of the second IMU  14 . However, due to the offset between the first IMU  12  and the second IMU  14 , the absolute measurements of the second IMU  14  are not directly replaceable by the absolute measurements of the first IMU  12 . Rather, transformation intermediate variables along with solutions are used to correlate data from the second IMU  14  with data from the first IMU  12  based on the relative displacement between the respective IMUs. Moreover, it should be understood that second IMU  14  must be offset from the first IMU  12  in order for the technique described herein to apply. 
         [0018]    The following describes the technique for supplementing data from the second IMU  14  with the failure data from the first IMU  12 . An assumption is made that both IMUs (e.g., accelerometers and gyroscopes) are fixed to the vehicle body such that their axes are co-directed. Vehicle body is assumed to be a rigid body, i.e. any structural compliance is ignored. A vector model is shown in  FIG. 2  for two reference frames represented by the first IMU  12  at point O and the second IMU  14  represented by point P. Absolute positions are not necessary since the technique described herein utilizes a relative relationship between the two bodies. A coordinate vector between the origins of two reference frames for linear attributes can be represented by the following: 
         [0000]        OP=x   p   i+y   p   j+z   p   k,   (1)
 
         [0000]    and angular attributes can be represented by the following expression: 
         [0000]      ω=ω x   i+ω   y   j+ω   z   k.   (2)
 
         [0000]    Here ω is the angular velocity vector of the vehicle body. 
         [0019]    The relation between the velocities of two points P and O of a rigid body in terms of its angular velocity has the following form: 
         [0000]        V   P   −V   0 =ω×OP  (3)
 
         [0000]    where V P  and V 0  are velocity measurements of the second IMU and the first IMU, respectively. 
         [0020]    Taking the time derivative of eq. (3) generates the equation in the form of acceleration: 
         [0000]        a   P   −a   0 ={dot over (ω)}× OP +ω×(ω× OP )  (4)
 
         [0000]    where a P  and a 0  are absolute acceleration measurements from accelerometers and {dot over (ω)} is the angular acceleration vector. 
         [0021]    Using a standard identity of double cross vector products and taking a dot product of the both sides of eq. (4) and the vector OP generates the following expression: 
         [0000]      ( a   P   −a   0 ) OP =(ω· OP ) 2   −|OP|   2 |ω| 2   (5)
 
         [0000]    Here a known identity 
         [0000]      ({dot over (ω)}× OP )· OP= 0  (5a)
 
         [0000]    has been applied. 
         [0022]    Eq. (4) generates the following vector differential equation for angular velocity vector ω: 
         [0000]      {dot over (ω)}× OP=a   P   −a   0 −ω×(ω× OP )≡ b   (6)
 
         [0000]    or in scalar form 
         [0000]        z   P {dot over (ω)} y   −y   P {dot over (ω)} z   =b   x  
 
         [0000]      − z   P {dot over (ω)} x   +x   P {dot over (ω)} z   =b   y  
 
         [0000]        y   p {dot over (ω)} x   −x   P {dot over (ω)} y   =b   z   (7)
 
         [0000]    where vector b is defined as follows: 
         [0000]        b   x   =−a   0x ( t )+ a   px ( t )+ x   P ω y ( t ) 2   +x   P ω z ( t ) 2   −y   P ω x ( t )ω y ( t )− z   P ω x ( t )ω z ( t )  (8)
 
         [0000]        b   y   =−a   0y ( t )+ a   py ( t )+ y   P ω c ( t ) 2   +y   P ω z ( t ) 2   −x   P ω x ( t )ω y ( t )− z   P ω y ( t )ω z ( t )  (9)
 
         [0000]        b   z   =−a   0z ( t )+ a   pz ( t )+ z   P ω x ( t ) 2   +z   P ω y ( t ) 2   −x   P ω x ( t )ω z ( t )− y   P ω y ( t )ω z ( t )  (10)
 
         [0023]    System of differential equation (7) with respect to components of angular velocities is degenerate, since the determinant of the system represented by the left hand side of eq. (3) is zero. Consequently, the following condition holds true: 
         [0000]        x   p   b   x   +y   p   b   y   +z   p   b   z =0.  (11)
 
         [0024]    The condition shown in eq. (11) is equivalent to the condition in eq. (5a). Condition (5a) or (11) holds for ideal signals under rigid body assumption. In reality all measurements contain noise and identity (8) is no longer exact. However, eq. (7) can be used for estimating two “unknown” angular velocities components as long as a third angular velocity component is known. The following three scenarios may be used to determine unknown angular accelerations given known respective parameters. 
         [0025]    In a first scenario, the following components of the reduced order IMU are available: z-component of the angular velocity vector ω z (t), and x and y components of the linear acceleration a px (t), a py (t). Two unknown components of the angular velocity vector ω x  and ω y  satisfy the following system of two nonlinear differential equations: 
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         [0000]    where ω y  and ω x  are unknown angular velocities components determined after a detected failure, a ox , a oy  are known linear accelerations components of the first inertial measurement unit, a px , a py  are known linear accelerations components of the second inertial measurement unit, x p , y p , z p  are coordinates of the origin of the second IMU with respect to the first IMU, {dot over (ω)} z  and ω z  are known angular velocity rate and angular velocity of the second inertia measurement unit. 
         [0026]    The above equations (12) can be approximately solved in real time using, for example, Euler method. All measurements in equations (12) contain noise, therefore, equations (12) are, in fact, stochastic equations. The online numerical solution will satisfy the performance requirements only during some finite interval of time. 
         [0027]    In a second scenario, the following components of the reduced order IMU are available: y-component of the angular velocity vector ω y (t), and x and z components of the linear acceleration a px (t),a pz (t). Two unknown components of the angular velocity vector ω x  and ω z  satisfy the following system of two nonlinear differential equations: 
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         [0000]    where ω z  and ω x  are unknown angular velocity components determined after a detected failure, a ox , a oz  are known linear accelerations components of the first inertial measurement unit, a px , a pz  are known linear accelerations components of the second inertial measurement unit x p , y p , z p  are coordinates of the origin of the second IMU with respect to the first IMU, {dot over (ω)} y  is a known angular velocity rate and ω y  is a known angular velocity of the second inertia measurement unit.
 
Analogously to Scenario 1, the above equations (13) can be approximately solved in real time using, for example, Euler method.
 
         [0028]    In the third scenario, the following components of the reduced order IMU are available: x-component of the angular velocity vector ω x (t), and x and z components of the linear acceleration a py (t), a pz (t). Two unknown components of the angular velocity vector ω y  and ω z  satisfy the following system of two nonlinear differential equations: 
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         [0000]    where ω y  and ω z  are unknown angular velocity components determined after a detected failure, a oy , a oz  are known linear accelerations components of the first inertial measurement unit, a py , a pz  are known linear accelerations components of the second inertial measurement unit x p , y p , z p  are coordinates of the origin of the second IMU with respect to the first IMU, {dot over (ω)} x  is a known angular velocity rate and ω x  is a known angular velocity of the second inertia measurement unit.
 
Analogously to the first and second scenarios, the above equations (14) can be approximately solved in real time using, for example, Euler method.
 
         [0029]    To obtain data to solve for the respective parameters in eq. (12)-(14), sampling will be applied to obtain the three-DOF IMU signals (e.g., a x , a y , and yaw rate) and transformation expressions (12)-(14) that will be used to derive the redundant signals. Sampling may be based on a predetermined number of samples at a respective time interval (e.g., 10 samples at 10 ms). The samples utilized are obtained at a time just prior to the failure detection of the first IMU. It should be understood that the number of samples in addition to the sampling rate may vary depending on the different vehicle attributes that include, but are not limited to, type of vehicle, what system the information is being utilized for, processor capability, memory storage. 
         [0030]    The estimation will be applied for a predetermined amount of time while the driver is alerted of the error and directed to take control of the vehicle operation if an autonomous or semi-autonomous operation is enabled. The predetermined amount of time may vary based on various vehicle characteristics including, but not limited to, type of error, the system that is being disabled, or the number of systems that need to be disabled. 
         [0031]      FIG. 3  is a flowchart for a method for the overall process for recovering the faulted signals based on the techniques described herein. 
         [0032]    In step  20 , the technique is initiated. In step  21 , all data from the IMU is stored for the past predetermined number of samples. 
         [0033]    In step  22 , a determination is made as to whether the first IMU has failed. If the determination is made that the first IMU has not failed, then the routine returns to step  20  to continue monitoring the first IMU. If the determination that a fault has occurred with the first IMU, then the routine proceeds to step  22 . 
         [0034]    In step  23 , a sample average is determined based on the predetermined number of samples just prior to the failed signal. 
         [0035]    In step  24 , a determination is made as to whether the average noise amplitude is less than the predetermined percentage of the sensor range. If the determination is made that the amplitude is above the predetermined percentage, then a return is made to step  20 . If the determination is made that the amplitude less than the predetermined percentage, then the routine proceeds to step  24 . 
         [0036]    In step  25 , recovered signals not obtained by the first IMU are calculated based on the measurement data from the second IMU and the derived differential equations for co. 
         [0037]    In step  26 , the recovered signals are provided to a controller for maintaining operation of a respective safety feature for a predetermined time period to allow the driver to resume manual control the feature. 
         [0038]    While certain embodiments of the present invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention as defined by the following claims.