Patent Publication Number: US-10788586-B2

Title: Multichannel inertial measurement unit and integrated navigation systems on its basis

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to designing strapdown inertial measurement units and integrated navigation systems that are coupled to inertial measurements and navigation measurements of various physical nature. 
     Background of the Related Art 
     In a typical integrated navigation system, outputting navigation solutions as a result of coupling inertial measurements with navigation data of different physical nature (GNSS-measurements, measurements of odometers, altimeters, etc.), all sources of navigation information are connected to common computation unit via corresponding hardware interfaces like COM-port, RS232 interface, RS485/RS422 interface, CAN interface, MIL1553B interface, etc. The strapdown inertial measurement unit (IMU) in such a system has, as a rule, a single information and control hardware interface (RS232/484/422/CAN/MIL1553B, etc.) which is fully used for data exchange with single computation unit. The IMU generates inertial data with the aid of set of inertial sensors. This sensor set, as a rule, includes accelerometers with mutually perpendicular measurement axes and gyros with mutually perpendicular measurement axes. 
     For correct interpretation of measurements received from multiple sensors, the moment of time (called “epoch”) when each sensor&#39;s measurement was received must be known. Also, the time interval between two consecutive measurements from the same sensor must be known. To define these two parameters, an appropriate timescale must be introduced. the timescale is defined by the selection of some periodical synchronization signal and by selection of a time unit as a relation between a duration of this time unit and period of synchronization signal. The timescale can be generated by any device that can produce a periodic sync signal 
     All sensors are sampled using a common time scale, which may be generated by external computational unit or by IMU itself. In both cases, the IMU hardware interface also contains hardware synchronization line to transfer the time scale from source to destination (i.e., from a computation unit to IMU in one case and from IMU to the computation unit in another case). 
     If the IMU with a single hardware interface is used inside a navigation system that contains several separate computational or data processing modules the problem of hardware connection of one IMU with multiple computational modules arises. This problem has two well-known solutions. The first solution is to use additional hardware interface multiplexer/demultiplexer or switching device. One port of this device is connected to the IMU hardware interface. All other ports of this device are connected to appropriate hardware interfaces of separate computational units intended for IMU connection. This connection approach may be used for any type of hardware interfaces. Another approach to connect single IMU to multiple computation units is to organize some system bus. IMU and all computation modules (which require IMU data) are connected to common bus where IMU sends data through the bus and modules receive data from the bus. Only a limited number of widely used hardware interfaces support the bus connection. For example, the CAN and MIL1553B interfaces support common bus connections, but RS232 interface not support common bus connections. In the discussion below, only the first type of connection (without a common bus) is considered. 
     The first type of connection between IMU and computational modules requires the IMU and computational modules to operate synchronously, using a common time scale. In addition, it requires identical hardware interface settings (IMU data rate, data frame structure, etc.) for all modules connected to IMU. If computational modules are not specially designed for a particular navigation system, but are designed by different independent manufacturers, or purchased on a secondary market, it is often impossible to satisfy these two requirements. Trying to connect different computational modules with different time scales and hardware interface settings to a common IMU requires additional external matching devices (one external matching device per each computational module). These matching devices have two main purposes—to re-sample IMU data from an IMU timescale to a timescale of appropriate computational unit, and to rearrange resampled IMU data to new data frames matched with interface settings of an appropriate computational unit. 
     To build an integrated navigation system with a several independent computation modules whose algorithms use inertial measurements, it is reasonable to use an IMU with several independently-operating hardware interfaces (like COM port, RS232 interface, RS485/RS422 interface, CAN interface, etc.) for IMU data exchange. The IMU of this type with several independently-operating interfaces will be further referred to as a “multichannel IMU”, or MIMU. A MIMU is connected to each computation module via a separate hardware interface, which is adjusted in accordance with data communications protocol appropriate for this computation module. From the computation module&#39;s point of view, this connection looks like a dedicated point-to-point connection between an ordinary IMU and this module. The computation module can apply all required interface settings (time scale, sampling rate, data rate, data frame format, etc.) to the dedicated MIMU interface irrespective of connection status of other MIMU interfaces and computational modules. 
     The measurements are generated in MIMU by single set of sensors in the time scale which can be generated both inside MIMU and outside MIMU (by external clock source). If external modules generate their own synchronization signals for transferring their timescales to the MIMU, the MIMU resamples the measured data according these sync signals. This feature allows all separated computational modules to operate asynchronously, with their own timescales and to receive IMU data with dedicated synchronization. The MIMU can also generate synchronization signals to transmit its own time scale (within which its measurements have been done) for synchronization of external modules. 
     The use of a MIMU within the integrated navigation system can be considered as an exemplary embodiment of a system (for example) consisting of two units (separated logically and/or physically), requiring inertial measurements for their operation—a GNSS receiver and a navigation and control computation device. Software and firmware of each unit can be developed and manufactured independently of one another (even by different independent manufacturers). The MIMU is connected to each of these units via a separate hardware interface with individual settings. 
     Inertial measurements transmitted from the MIMU to the GNSS receiver through one of interfaces are coupled in the receiver with radio navigational measurements to improve the quality and availability of navigation solutions. Inertial measurements transmitted from the MIMU to the navigation and control computation device through another interface are used in algorithms of navigation and control of a moving vehicle on which the navigation and control system is installed (for example—for vehicle&#39;s yaw and roll stabilization algorithms) A GNSS receiver is connected to the same computation device as one of the navigation sensors. 
     Another exemplary embodiment of this invention includes various navigation devices (generally speaking, of different types), implementing algorithms of integrating inertial measurements with navigation data of different physical nature and combined in a single navigation and control system. According to this embodiment each device is connected to a separate MIMU interface and performs its coupling algorithm independently of other devices. The outputs of its operation are transmitted to a user for further processing and/or collecting the entire set of navigation parameters. 
     For example, a system of this kind may be assembled with separate roll stabilization computer, pitch stabilization computer, yaw stabilization computer, trajectory stabilization computer and logging device. All of these computing modules have to operate simultaneously, have to receive inertial data measured by a single IMU, but may operate using their own timescales and require their own IMU data reception rates. All these devices may be connected to separate MIMU interfaces and receive inertial data via dedicated point-to-point connections. Different versions of strapdown IMU and integrated navigation systems with such modules are known. 
     Reference [1] describes such an integrated navigation system consisting of two independent GNSS receivers and one IMU connected to a common processing module which further generates a navigation solution. The main drawback of this device is having only one interface of the IMU to connect to the data processing module. 
     Reference [2] describes a method of building a synchronization and reading unit to read out measurements from inertial sensors within IMU. The unit contains its own stable clock generator and input port to receive a synchronization signal with an external timescale. This version only has a single interface to transmit inertial measurements to the external user and lack of output synchronization signal to transmit the internal timescale to external users. 
     Reference [3] discloses an integrated navigation system with a couple of IMUs (both gimballed IMU and strapdown IMU), each of which is connected to some computation units. Each IMU in reference [3] has only one interface to exchange data, with one IMU being connected to several computation units (or several IMUs—to one computation unit) with the help of multiplexing/de-multiplexing circuits. Such a combination of some IMU and computation units into a common system is made to increase system redundancy, thereby enhancing its reliability and fault-tolerance. 
     Reference [4] discloses an integrated navigation system comprising some IMUs and some computation units. In reference [4] each IMU has only one interface, and combination of different IMUs, computation units and GNSS sensors is made based on a cascade connection. According to this connection, GNSS information and the information from one IMU is fed to the inputs of the first computation unit via different interfaces, some information from the output of the first computation unit and the second IMU is fed to the inputs of the second interfaces and so on. 
     Reference [5] describes an IMU with the configuration when there is no external synchronization signal. This device has only a data exchange interface. 
     Reference [6] discloses a method of designing IMU with reduced requirements to the output interface as raw data are stored and the stored information is further sent to the external sink according to its request. The information is outputted from IMU via the single interface which is the main difference from the proposed invention. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is related to a multichannel strapdown inertial measurement unit that substantially obviates one or more of the disadvantages of the related art. 
     The present invention is directed to a multichannel strapdown inertial measurement unit comprising a typical set of sensors (3D-gyro, 3D-accelerator, 3D-magnetometer, etc.) and an internal computation unit for reading and preliminary processing of sensor information, as well as for generating output data flow. The computation unit of the MIMU includes two or more bidirectional data exchange interfaces. Each of these interfaces can transmit and receive data irrespective of the other interfaces, the format of data exchange being individually adjusted for each interface. 
     The MIMU computation unit is also responsible for generating a time scale according to which MIMU sensors are sampled and output data packets are generated. The MIMU time scale can be directly generated in a computation unit using an in-built stable clock generator, or can be set by external clock source. The MIMU can transmit its own time scale to its external users via data exchange interfaces. 
     To receive/transmit the timescale, in each MIMU interface there are input and output signals for receiving/transmitting clock signals, whose format is individually selected. If the timescale is received from an external clock source connected to the corresponding interface, the MIMU computation unit locks onto the input clock signal from this interface and synchronizes all internal measuring procedures and time diagrams in accordance with the received clock signal. Once a transition process of synchronization has been completed, all measurements and output data packets are generated according to the external timescale. Switching between different timescales transmitted to MIMU at the same time via MIMU interfaces is done in response to commands from external devices connected to these interfaces based on internal settings or based on MIMU external clock selection priority logic. 
     When the timescale is transmitted from the MIMU to external devices, all clock signals are generated from the single internal clock generator. The format of single clock signals which can be included into data exchange interface (frequency, duration, polarity of clock pulses etc.) is individually selected in settings of each interface. 
     An advantage of the proposed MIMU is availability of some independent exchange interfaces in the IMU, each of the interfaces is individually adjusted, interfaces can be connected to different computation units solving various navigation tasks, these interfaces not taking part in ensuring system redundancy. 
     Additional features and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description, or may be learned by practice of the invention. The advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE ATTACHED FIGURES 
       The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
       In the drawings: 
         FIG. 1  presents a general diagram of a multichannel inertial measurement unit. 
         FIG. 2  presents embodiments of formats for output MIMU measurements via different interfaces. 
         FIG. 3  presents an embodiment of MIMU comprising a GNSS receiver and a navigation computation unit. 
         FIG. 4  presents an embodiment of MIMU with some navigation devices of different types. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. 
     A generalized diagram of a strapdown MIMU is shown in  FIG. 1 . The strapdown MIMU  100  includes vector sensors such as a 3D-gyroscope  101 , a 3D-accelerometer  102 , a 3D-magnetometer  103 , and a computation unit  104 . The computation unit  104  is connected to several connectors  105  located on the MIMU housing and allows external users to be connected to the measurement information. 
     Signals from the vector sensors  101 - 103  are read out by a sensors readout unit  106 , which is intended for generating a set of instantaneous measurements of the vector sensors in digital form for the same time instant. This unit can include multichannel devices for analog to digital conversion (if output sensor signals are analog ones), pulse counters or any standard digital in-circuit interfaces (like UART, SPI, I2C, etc.), if the sensors  101 - 103  are digital. 
     The set of instantaneous measurements is sent from the readout unit  106  to a data pre-processing unit  107 . In the data pre-processing unit  107 , the obtained data is scaled, thermally corrected, and freed from sensitivity axes misalignment. Some other errors typical for inertial measurements are removed as well. The data pre-processing unit  107  can also average or numerically integrate raw data obtained from the readout unit  106 . This unit can provide data in one or some pre-set formats of measurements (scaled vector measurements, integrals of measurements over the pre-set time intervals, integrals of measurements etc.) to output such data to the user. 
     Data preparation for transmission via physical MIMU interfaces which are presented by the connectors  105  is done in interface controllers  108 . These controllers define the type of the physical interface working through corresponding the connector  105 , pack data in information packets to transmit via physical interface and so on. Interface controllers  108  are used for setting formats (type, pulse edge, polarity) for input and output synchronization signals. If a synchronization signal from an external source has one of the pre-defined formats, upon reception it is converted to the internal MIMU format and transmitted to a synchronization unit  109 . If the synchronization signal is transmitted from MIMU to the external user, the interface unit  108  receives the signal in an internal format from the synchronization unit  109  and converts it into one of the pre-defined formats assigned by the user. 
     The synchronization unit  109  generates an internal timescale according to which all the operations of obtaining and processing the measurement information are implemented inside the MIMU and outputs measurement results to the external users. The synchronization unit  109  can receive the reference signal to generate timescale both from one of external sources via the controllers  108  and from an internal clock generator  110 . Internal synchronization signals generated by the unit  109  are used for clocking readout operations from the sensors  101 - 103  in the unit  106 , pre-processing operation in the unit  107  and operations of output packets generating and transmitting in the interface units  108 . The unit  109  also contains adjusted priority logic to select a single source of external synchronization, if two or more sources are available. 
     The clock generator  110  is used for clocking all computational operations and operations of inter-unit data exchange inside the computation unit  104 . The clock generator  110  can be also used as a source of reference signals to generate an internal timescale of the MIMU  100  when external synchronization sources are unavailable or turned off. 
       FIG. 2  shows an overview of a simultaneous transmission of measurements via different interfaces of the MIMU. Primary measurements of an analog quantity  201  are done by the readout unit  106  at a relatively high rate (of hundreds or thousands Hz) at discrete instants  202  counted by the clock generator  110 . External clock signal  203  can be fed to the input of one of the MIMU interfaces (exemplarily shown that counting instants of the external timescale coincide with rising edges of signal impulses  203 ). Each active signal edge  203  indicates to the beginning of an interval  204  of the external timescale with a current number N. If such a signal is present, then the MIMU internal timescale is synchronized such that exactly K equidistant discrete time instants  202  should fit between a pair of neighboring synchronization pulses  203 . An interval  205  of the internal timescale being synchronized with the external scale is designated as N:k, where: N is the number of the interval of the external timescale, k=0 . . . K−1 is the number of discrete time instant of the internal scale inside the interval of the external scale with number N. Synchronization of internal and external scales is made such that the instant  202  of the internal scale with number N: 0  corresponds to the active edge of signal  203  referring to the interval with number N. If the signal of the external synchronization  203  is absent, the instant  202  is generated by the clock generator  110  in accordance with the internal settings. 
     Each scalar sensing element of vector sensors  101 - 103  measures a parameter  201  varying in time. Depending on the sensor type, it can be a component of the angular speed vector, a component of specific acceleration vector, or a component of magnetic intensity vector. Raw data read out by unit  106  at time instants  202  is transmitted to the pre-processor  107  where this data are subject to different operations specific for preliminary processing of inertial measurements. 
     One of the exemplary pre-processing methods can calculate definite (short-term) integrals of neighboring sections of signal  201  of the same duration as Integral m calculated over time interval from instant  206  up to instant  207 , Integral m+1 calculated from instant  207  up to instant  208 , Integral m+2 calculated from instant  208  up to instant  209  and so on. For example, short-term integration results, calculated from accelerometers measurements, may be interpreted as vehicle&#39;s velocity increments. Also, short-term integration results, calculated from gyros measurements, may be interpreted as vehicle&#39;s attitude angular rotation increments. Calculated short-term integration results are transmitted to one of interface controllers  108  that generates packets  210  to transmit data (values of short-term integration results) to external users. The instant of ending the calculation of integration, which matches the moment of transmitting a packet with its numerical value and the moment of starting the calculation of the next integral, is indicated with synchronization impulses  211  generated according to the MIMU internal timescale to the external user. The type and format of packets  210 , as well as format of the synchronization impulses  211 , are selected in the interface controller  108  from the pre-defined set. 
     Another exemplary method of pre-processing raw data can be scaling and resampling (or decimation) of raw data obtained by readout unit  106 . Resampled and scaled measurements  212  are instantaneous measured values of signal  201  taken with a period increased in multiples of a period of time instants  202 . (Resampling refers to providing data at a different rate, e.g., when the original data was sampled at 100 Hz, resampled data might be at 74 Hz for example. Decimation refers to dropping some samples to achieve a lower rate, e.g., if the original data was sampled at 100 Hz, after decimation, 3 out of every 4 samples are dropped, resulting in a 20 Hz effective sampling rate.) The multiplicity of periods (resampling factor) and bandwidths of anti-aliasing filters are set in the settings of the corresponding interface controller  108  wherein the data packets  213  are generated from values after decimation. The instant of taking sample  212  of the signal  201  is indicated to the external user using synchronization impulses  214  of the corresponding interface. The type and format of the packets  213 , as well as format of the synchronization impulses  214  are selected in the interface controller  108  from a pre-defined set. 
     One exemplary embodiment of the use for the MIMU is shown in  FIG. 3 . Such a system  300  includes a GNSS receiver  301 , the navigation computation unit  302 , and an MIMU  100  simultaneously connected to both of them. If the receiver  301  operates with one antenna  303 , it determines only its position and velocity of its phase center. If an additional GNSS antenna  304  is connected to the receiver, then in addition to position and velocity of antenna  303  the receiver can determine incomplete orientation (two Euler&#39;s angles of the three) of the base to which these two antennas are rigidly fixed. Another GNSS antenna  305  allows the receiver to define the complete orientation of the base (three Euler&#39;s angles). So the maximum set of navigation data S 306  that the receiver  301  is able to transmit to the navigation computation unit  302  includes position, velocity and 3D-orientation of the rigid base to which the GNSS antennas  303 ,  304 ,  305  are attached. 
     The MIMU  100  is connected to the receiver  301  and the navigation computation unit  302  via two different interfaces with different transmission and data presentation formats (in general case). For example, let the MIMU&#39;s measurements set S 307  transmit vector measurements of the 3D-gyroscope  101 , and the 3D-accelerometer  102  integrated over neighboring intervals with duration of 0.01 sec (orientation angle increments and velocity vector increments respectively) to the receiver  301  through the one MIMU interface. Let the MIMU&#39;s measurements set S 308  comprising corrected measurements of the 3D-gyroscope  101 , and the 3D-accelerometer  102  (components of angular vector and specific acceleration vector respectively) and the raw data of the 3D-magnetometer  103  be transmitted to the navigation computation unit  302  through another MIMU interface. Signal 1PPS S 309  generated by the GNSS receiver  301  is exemplarily considered as a signal of the MIMU external synchronization. The same signal is connected to the navigation computation unit  302  to synchronize its operation with the GNSS timescale. 
     Measurements S 307  are coupling in the receiver  301  with its radio navigation measurements to enhance the quality and availability of GNSS solutions. Measurements S 308  transmitted to navigation and control computation unit are used in the algorithms of navigation and control for a moving vehicle onto which the integrated system  300  is installed. The set of output parameters S 310  of the integrated system  300  is generated in the navigation computation unit  302  and can include not only the information S 306  but also some stabilization and control commands for the vehicle with the system installed. 
     Another exemplary embodiment of using MIMU with navigation devices of different types is shown in  FIG. 4 . The MIMU  100  is then simultaneously connected, for example, to one separate GNSS receiver  401  and to a vehicle navigation system  402 . The GNSS receiver  401  receives signals from navigation satellites via the antenna  303 . A steering angle sensor  403  and an odometer  404  are the raw data sensors for the vehicle navigation system  402 . Both the separate vehicle navigation systems  401  and  402  are connected to an external data sink  405  via the corresponding hardware interface. Vehicle data collector, board computer, or another similar device can be an example of an external data sink. 
     The MIMU data S 307  transmitted to the receiver  401  through one interface are used by receiver&#39;s firmware to improve the quality and availability of GNSS solutions. The MIMU data S 308  transmitted to the vehicle navigation system  402  are exemplarily used to detect possible slipping of the vehicle or to detect readout errors from the sensors  403  and  404 . The MIMU data S 307  and/or S 308  can be transmitted to an external user  405  along with measurements of the vehicle navigation systems  401  and/or  402 , respectively. 
     Measurements within the MIMU S 307  are synchronized with an external timescale by signal 1PPS S 309  generated inside the receiver  401 . This timescale is transmitted to the vehicle navigation system  402  with the aid of a clock signal S 406  generated within the MIMU S 307  and synchronized with the signal S 309 . 
     As will be appreciated by one of ordinary skill in the art, the various blocks shown in  FIGS. 1, 3 and 4  can be implemented as discrete hardware components, as an ASIC (or multiple ASICs), as an FPGA, as either discrete analog or digital components, and/or as software running on a processor. 
     Having thus described a preferred embodiment, it should be apparent to those skilled in the art that certain advantages of the described method and apparatus have been achieved. 
     It should also be appreciated that various modifications, adaptations and alternative embodiments thereof may be made within the scope and spirit of the present invention. The invention is further defined by the following claims. 
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