Abstract:
A method for testing inertial measurement devices on a multi-axis rate table without having to utilize slip rings to transfer signals between the inertial measurement devices and remote processors by incorporating a processor internal to the inertial measurement devices and transferring the signals directly to the processors for determining and storing the calibration coefficients of the inertial measurement devices internally so that they are self calibrating.

Description:
REFERENCE TO A RELATED APPLICATION 
   This application is a divisional application of and claims priority to U.S. patent application Ser. No. 10/011,459, filed Nov. 6, 2001, which is incorporated herein by reference and which has issued as U.S. Pat. No. 6,778,924. 

   BACKGROUND 
   1. Field of Invention 
   This invention relates to an improvement to eliminate the need for slip rings in calibration system utilizing a signal processor that receives the outputs of one or more motion sensing devices mounted on a rotary platform. The devices are subjected to known conditions of rotary motion and the outputs are compared with the known conditions to determine any errors there between to produce correction factors that are applied to the outputs to more accurately indicate the known conditions. In the preferred embodiment, the invention is used with calibration equipment for inertial measurement units (IMU&#39;s) mounted on a rate table and, more particularly, to a self contained system that avoids the use of slip ring connections for transferring signals from the IMU&#39;s to the signal processor. 
   2. Description of Related Art 
   Heretofore, the calibration of IMU&#39;s has required the mounting of the IMU under test on a multi-axis rate table, subjecting the IMU to predetermined known motions (e.g. rates and accelerations) in the various axes (e.g. three perpendicular axes such as pitch, roll and yaw), and at various known temperatures and other environmental conditions, determining the response of the IMU to these input environmental condition values and transmitting the output signals indicative of such values through a slip ring assembly to a remote computer or processor which includes test software that operates to compare the IMU outputs with the predetermined known values and to determine any errors so as to produce calibration coefficients which can be used to correct the errors in the outputs. The calibration coefficients are stored in a memory, internal to the IMU, and which are then used with calibration software in the IMU to add or subtract the correction values from future IMU outputs, at the various rates and temperatures, etc. 
   While it is not necessary for an understanding of the invention, it is believed helpful to consider the operation of the prior art in establishing the correction values. Three major causes of inaccuracy in an IMU are scale factor, bias and misalignment. These three factors may also vary with temperature so temperature drift may be considered a fourth factor. There are others, but these tend to be the most important to be corrected by the calibration system and will be used here for simplicity. 
   Again, for simplicity, assume that a single axis sensor, such as an accelerometer, is mounted on a base that will be used in actual practice and assume that this base is mounted on a rate table and in an oven. If the base is mounted so that the sensing axis of the accelerometer is parallel to the gravitational axis, the IMU should produce a signal indicating “+1 g” when upright and “−1 g” when inverted, without any motion of the rate table. (Outputs greater than 1 g are produced by rotation of a table such as would be experienced on a centrifuge). If there is a scale factor error of, say, +1.0%, then at +1 g, the output would read +1.01 g and at a −1 g, the output would read −1.01 g. 
   Accordingly, to correct this error, one would multiply the output by 1.00/1.01=0.99. If there was a bias error of say +0.2 g, then at +1 g, the output would be 1.2 g and at −1 g the output would be −0.8 g. To correct this error, the value (1.2+(−0.8))/2=0.4/2=0.2 g would be subtracted from the output. 
   Misalignment errors are normally determined when the sensing axis is perpendicular to the gravitational field where a perfect alignment would produce an output of 0.0 g. The correction factor for misalignment is determined by observing the acceleration error from the accelerometer as the accelerometer is rotated about its input mounting axis placed in the 0 g orientation. The misalignment correction factor is then calculated by using a small percentage of the other two acceleration axes outputs of an orthogonal system. 
   All of the tests will then be re-conducted at a plurality of various temperatures so that the memory will contain the proper correction coefficients for all temperatures to be encountered in actual use. It should be noted that in addition to temperature, there are other environmental conditions, such as humidity altitude, and vibration that may affect the outputs. 
   In all cases, all of the output signals must be sent through the slip ring assembly to the processor where the correction coefficients are determined and then all of the correction coefficients must be transmitted from the processor back through the slip ring assembly to the memory in the IMU. 
   Now working at an IMU level with three axis accelerometers, then the simplified tests above will have to be repeated along all three of the axis of interest but, although more complicated, the result will still be a lookup table in the IMU which will correct for the major errors in all three axes and at all temperatures to be encountered. It should be understood that the tests may use more advanced algorithms than the simplified equations above, to detect the errors along the desired axes. For example, by aligning the IMU along axes that are skew to the three major axes, a new vector output may be obtained. Then the new vectors may be mathematically resolved to provide the desired information along the three major axes without as many repetitions of the application of various g forces to the IMU. 
   In any event, and regardless of the processor algorithm used, the requirement for using a slip ring assembly to read the IMU output values presents a considerable problem since such assemblies are very expensive particularly, when it is desired to calibrate a large plurality of IMU&#39;s (e.g. 32 units) at the same time. 
   SUMMARY 
   The present invention eliminates the need for slip-ring assemblies to obtain the signals from the IMU&#39;s for presentation to the external processor by building the IMU&#39;s with internal processors capable of using the test software to determine the calibration coefficients and the calibration software to modify the IMU outputs for customer use. This also eliminates the need for external remote processors and simplifies the calibration process at a greatly reduced cost. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Exemplary embodiments of the present invention are described herein with reference to the drawings, in which: 
       FIG. 1  is a schematic representation of the prior art system for calibrating IMU&#39;s; 
       FIG. 2  is a schematic representation of the calibration system of the present invention; and, 
       FIG. 3  is a flow diagram for the calibration of IMU&#39;s using the present invention. 
   

   DETAILED DESCRIPTION 
   A prior art system similar to that described in the above referenced publication is shown in  FIG. 1 . In  FIG. 1 , the desired environment, such as various temperatures likely to be encountered in actual use, is shown by an arrow  6 . This environment may be provided by a heater, cooler or oven, (not shown) well known in the art. An IMU  10  such as a gyroscope or accelerometer, or a combination comprised of a plurality of gyroscopes and accelerometer, having an internal memory  11  is shown mounted on a platform  12  that is connected to a multi-axis rate table  14  whose purpose is to subject the IMU to various motions in various axes to determine how it responds. A shaft  16  is shown connected between the rate table  14  and the platform  12  and is shown carrying a plurality of slip rings  18  as part of slip ring assembly  19 . Slip rings  18  are connected by lines  20  to receive the IMU  10  outputs, shown by arrow  21 . Outputs  21  carry signals indicative of the motion sensed by the IMU  10  and slip ring assembly  20  presents these signals over a line shown as arrow  22  to a remote computer or processor  24 . Processor  24 , as described above, compares the IMU output signals with the known predetermined values of input motion provided by rate table, and operates to determine any errors and calculates the desired calibration coefficients. 
   At the conclusion of the test, the processor will transmit the calibration coefficients back through the slip rings  18  to the internal memory  11  or, if preferred, the rate table may be stopped and the processor  24  connected to download the calibration coefficients over a line shown as arrow  26 . Thereafter, IMU  10  operates with the stored calibration coefficient values to modify the outputs  20  at the various temperatures so that more accurate values for the motions detected by the IMU  10  are obtained for customer use. If it is desired to check the recalibrated outputs, the test may then be re-conducted and the new outputs again checked against the known environment inputs with any more accurate calibration coefficients stored in memory  11 . 
     FIG. 2  shows the apparatus of the present invention. In  FIG. 2 , the environment  6  the IMU  10  with the internal memory  11  and the outputs  21 , the platform  12  the rate table  14  and the shaft  16  are the same as in  FIG. 1  although shaft  16  no longer is required to carry slip rings as will be explained. The IMU  10  is shown mounted on platform  12  as in  FIG. 1 , but in  FIG. 2  the slip rings  18  the connectors  20  and the slip ring assembly  19  have been removed. 
   The processor  24  of  FIG. 1  has also been eliminated and a processor  34  having the same function is now shown internal to the IMU  10 . Processor  34  operates in the same manner as processor  24  of  FIG. 1 , but is now internally connected to the outputs  20  of the IMU and to the internal IMU memory  11  as shown by a double ended arrow  35 . Processor  34  compares the IMU output signals  21  with the known predetermined environmental values of input motion and temperature provided by rate table and operates to calculate the desired calibration coefficients. 
   With the present invention, it is no longer necessary to utilize slip rings to carry the signals to the processor or the calibration signals back to the internal memory, (or to wait for the conclusion of the test to store the calibration coefficients) since the calibration coefficients may be immediately stored in the IMU internal memory  11  during the test. As was the case in  FIG. 1 , the IMU  10  will then be able to operate with the stored calibration coefficient values to modify the outputs  20  of the IMU  10  at various temperatures so that more accurate values for the motions detected by the IMU  10  are obtained for customer use. 
   It should be noted that re-checking of the calibrated outputs against the known environmental values of the rate table input may now be accomplished without stopping the rate table, transferring the calibration values to the IMU memory and restarting the test, since the recalibration values are already stored in the IMU memory. This will also save the time that may be required to re-establish the desired temperature environment. It should also be noted that a multiplicity of IMU&#39;s may now be simultaneously tested without the difficulty of connecting each IMU to slip rings for external handling thus achieving a great saving in cost, time and complexity. 
     FIG. 3  shows a flow chart for the operation of the improved IMU calibration test. In  FIG. 3 , box  50  indicates that the first step is to load the IMU&#39;s on a prep-station. The prep-station is merely an initial place where the each IMU may be loaded with the test and calibration software to be used and where the predetermined rate table values and temperatures to which the IMU&#39;s are to be subjected are inputted. This is performed at the next step, shown by box  52 . Box  52  indicates that the calibration environment information from a box  54  and the test and calibration software from a box  56  are downloaded into the IMU. After downloading the necessary information and software into each IMU, the next step is to mount the IMU&#39;s on the rate table and to initiate the test as is indicated by a box  58 . As indicated in the next box  60  during the test, the internal IMU processor reads the IMU outputs, determines the errors, calculates the correction coefficients, and stores the correction coefficients in the internal memory. Thereafter, the IMU, using the calibration software, changes the outputs in accordance with the stored correction coefficients to provide the more accurate values for customer use. 
   It is thus seen that I have provided an improved calibration test system for IMU&#39;s and have greatly reduced the complexity, cost and time involved to test them. Of course, the invention may find application in calibrating devices other than IMU&#39;s which are tested on rotating tables and which employ slip rings to transfer the output signals to a remote processor that detects errors and stores them in the devices for future correction. 
   Furthermore, other changes to the structures and processes described herein may occur to those skilled in the art. For example, the software and environmental information may be loaded into the IMU after it is loaded on the rate table and environmental conditions other than temperature, (such as vibration or altitude conditions), may be applied to the test devices. 
   Accordingly, I do not wish to be limited to the specific showings used in connection with the description of the preferred embodiment. The scope of the invention may be determined by the claims appended hereto.