Patent Publication Number: US-9885232-B2

Title: Method for determining position with improved calibration with opposing sensors

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     See also Application Data Sheet. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
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     THE NAMES OF PARTIES TO A JOINT RESEARCH AGREEMENT 
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     INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC OR AS A TEXT FILE VIA THE OFFICE ELECTRONIC FILING SYSTEM (EFS-WEB) 
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     STATEMENT REGARDING PRIOR DISCLOSURES BY THE INVENTOR OR A JOINT INVENTOR 
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     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to determining position of a tool for operations at a location of the tool. In particular, the present invention relates to calibrating sensors for determining position of the sensors. Even more particularly, the present invention relates to calibrating sensors on a downhole tool to more accurately determine position of the downhole tool in an underground formation, such that setting location of activity or operations, such as drilling by the downhole tool, is more accurate. 
     2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98 
     Sensors to determine position are used in a tremendous number of important processes, such as deployment of vessels in space, movement of a video game controller, and injection of cells in a tumor. On large scales and small scales, determining position for operations at the determined position can be very important. Accurate sensors are crucial for the performance of those activities. Sensors are also known to direct oil and gas operations in a rock formation. The direction of the tool and the location of the wellbore are detected, so that the various downhole activities can be accurately placed in the formation. These downhole activities include drilling, injecting, and isolating zones in the formation. The accuracy of the sensor and placement of a wellbore can seriously affect the outcome of a drilling operation. 
     Sensors are calibrated to increase the amount of accuracy and precision so that the determination of position is also as accurate and precise as possible. Sensors in extreme environmental conditions are subject to error, due to those conditions, such as high temperatures. The environment of a sensor can include the depth, pressure and heat in a wellbore. Alternative environments for accelerometers also include high temperatures from electronic components in a circuit board of a video game controller, re-entry heat in orbit, and elevated temperatures from radiation treatment in cellular tissue 
     In the prior art, redundancy is used for increasing accuracy. U.S. Pat. No. 6,206,108, issued to MacDonald, et al. on Mar. 27, 2001, teaches a method for adjusting a drilling operation based on a system with multiple sensors to measure multiple parameters. The sensors correct each other, and each sensor measurement further refines an instrument reading downhole. In U.S. Patent Application No. 2010/0078216, published for Radford, et al. on Apr. 1, 2010, a system and method for downhole vibration monitoring for reaming tools includes a plurality of accelerometers, a plurality of magnetometers, and at least one temperature sensor. The plurality of accelerometers corrects or verifies other sensors to guide drilling. U.S. Pat. No. 6,648,082, issued to Schultz, et al. on Nov. 18, 2003, teaches a method for differential sensor measurement and a system to detect drill bit failure. The system incorporates a main sensor and individual sensors for other sensor values, which are compared to each other to create a self-correcting system. 
     The prior art further establishes mathematical models to increase accuracy. U.S. Pat. No. 8,818,779, issued to Sadlier, et al. on Aug. 26, 2014, teaches a system and method for real-time wellbore stability while drilling a borehole. The drilling operation is adjusted in real time according to sensor readings compared against a geomechanical model. U.S. Patent Application No. 2014/0231141, published for Hay, et al. on Aug. 21, 2014, discloses a system and method for automatic weight on bit sensor correction with a sensor arranged in a bottomhole assembly. The method comprises first taking a survey recording an initial depth within a borehole, calculating a prediction borehole curvature at a second depth, calculating a weight correction value based on the predicted hole curvature, and finally adjusting the borehole position with the weight correction value. 
     Adjusting for accuracy in the prior art focuses on external factors and conditions affecting readings, not the sensor itself. The mathematic models apply to a specific context for drilling operations, not the general accuracy and precision of the sensor. In a different context for a different activity (deep space, microsurgery), there is still a need to calibrate according to the components of the sensor itself. 
     The factory calibration of a sensor is addressed in the prior art. Upon assembly, the components of the sensor are calibrated before applied in a specific context with other distorting external conditions. U.S. Pat. No. 5,880,680, issued to Wisehart, et al. on Mar. 9, 1999, teaches calibration of a sensor according to a temperature model. One accelerometer is tested at the time of manufacture to determine a temperature model of how accuracy of the accelerometer is affected at different temperatures. In the drilling operation, a temperature sensor and the accelerometer are run in the wellbore, and the readings of the accelerometer and the readings from the temperature sensor are processed according to the temperature model. U.S. Pat. No. 7,234,540, issued to Estes, et al. on Jun. 26, 2007, teaches a system and method of a two-axis gyroscope and other sensors which, when incorporated into a bottomhole assembly, determines the direction of the wellbore and drilling tool in real-time. A number of corrective operations are applied to the sensors while downhole, including a scale factor correction for the temperature at the final position. 
     The prior art general calibration requires numerous measurements taken over many sensor orientations and temperatures, and the prior art methods only account for temperature affecting components of the sensors. However, temperature is not the only factor, especially for certain types of sensors. 
     The relative position of sensors has also been used to account for error. It is known to align solid state sensors in pairs and on different axes together. U.S. Pat. No. 6,767,758, issued to Geen on Jul. 27, 2014, teaches micro-machined multi-sensor system which provides one axis of acceleration sensing and two axes of angular rate sensing. The sensor includes a pair of accelerometers. Each accelerometer includes a pair of sense electrodes on the lateral axis and the longitudinal axis. U.S. Pat. No. 7,571,643, issued to Suguira on Aug. 11, 2009, teaches a system and method for downhole dynamics measurements which incorporates a sensor arrangement for measuring downhole dynamic conditions and may include a tri-axial arrangement of accelerometers within the housing. The MEMS solid state sensors are set in pairs on the x, y and z axes. The term “diametrically opposed” is used to describe a cancelling out effect of the arrangement. 
     Sensor positions are used for multiple reasons, including increasing data collection and triangulation from signals. Seismic signals and acoustic sensors can triangulate the readings. The data collection from an array uses the different relative locations of the sensors in calculations. U.S. Pat. No. 7,424,928, issued to Cox, et al. on Sep. 16, 2008, teaches a system and method for flexibly coupling sensors to a downhole tool for measuring seismic data, by isolating seismic receivers from vibrations in the drill string and enabling the differentiation of compression waves from shear waves. U.S. Patent Publication No. 2013/0070560, published for Zheng, et al. on Mar. 21, 2013, discloses a system and method for an acoustic tool with a sensor array. The sensors in the array are arranged back to back. The sensors are placed to triangulate the received signal. U.S. Patent Publication No. 2005/0150689, published for Jogi, et al. on Jul. 14, 2005, discloses a system and method for enhancing directional accuracy and control using bottomhole assembly measurements. There are two sensors taking different measurements of a bending signal on different axes. An estimate is calculated in order to predict borehole curvature. The sensors are placed in different orientations for more relevant data collection. 
     Collecting additional confirmation data for increased precision is prior art. U.S. Pat. No. 5,058,077, issued to Twist on Oct. 15, 1991, teaches a compression technique for generating a corrected well log which could include erroneous signals from a downhole sensor. The sensors are placed longitudinally and radially about the drill collar in order to obtain the advantage of the phase angle difference between pairs of sensors and the relative orientation and position of the tool within the borehole. U.S. Pat. No. 5,842,149, issued to Harrell, et al. on Nov. 24, 1998, teaches a system and method for a closed loop drilling system including multiple sensors which are used within the system to retrieve signals which are then compared to programmed instructions and models, and finally used to allow the system to automatically adjust to the new drilling parameters. Sensors are positioned for data collection, not calibration or error correction. 
     More modern accelerometers are small micro electro-mechanical systems (MEMS or micro-mechanical systems, MMS). One of the most simple and less expensive MEMS devices is an open loop MEMS device, which basically consists of a hinged micro machined silicon wafer. The silicon wafer is the sensing element that moves in the presence of a gravitational field or acceleration force. The open loop MMS sensor measures the departure from a neutral starting position of the wafer. Another MEMS device is a closed loop MEMS device basically consisting of a cantilever beam, such as a cantilever beam, and a proof mass on the beam. The cantilever beam can be maintained in a neutral zero force position by applying a current flow through a small magnetic element, which creates the exact force to neutralize the gravitational force acting on the cantilever beam. An additional amount of current is applied proportionally to the gravitational field vector being measured in a particular orientation as required to keep the cantilever beam in the neutral position. A magnetic force induced by the electric current returns the cantilever beam to the start or neutral position. Thus, by measuring the amount of current to return the cantilever beam to the neutral force position, the amount of acceleration force or gravitational force can be measured. The present invention is applicable to both open loop and closed loop sensors. There is particular utility for open loop sensors with the present invention. 
     The errors from MMS sensors can originate from bias and hysteresis type distortion. Bias error happens because the cantilever beam is deformed by high temperatures and cannot return to the same neutral/null position with the same voltage. Hysteresis type distortion happens because the amount of deformation of the cantilever beam by high temperatures relates to the time spent at the high temperatures. The physical component, such as the silicon wafer and cantilever beam, further deforms from being exposed to the repeated higher and lower temperatures for different amounts of time. These components of the MEMS accelerometer are affected by additional errors, besides the temperature itself. The past effects of high temperature are not addressed in the current calibration methods, which have particularly high impact on MEMS accelerometers. 
     Bias errors and hysteresis errors are known for prior art accelerometers. The &#39;540 patent also includes a bias correction, which is obtained from a prior survey; and misalignment and gravity dependent corrections to the gyroscope axes. Use of a sensor will have bias errors, as the components drift when the sensor is in use. However, the MMS sensors, in particular open loop MMS sensors, are more susceptible to bias errors and hysteresis errors, especially with exposure to high temperatures over extended periods of time. MMS sensors can be calibrated according to the prior art, including attempts at bias correction; however, the prior art for regular accelerometers does not address MMS sensors, which are more prone to these errors than regular accelerometers. The sensitivity and durability for the components of MMS sensors are not the same as traditional accelerometers. The old solutions for error of the prior art can apply to MMS sensors, but those old solutions are not sufficient for reliability. Additional solutions are required for the MMS sensors to be used repeatedly in high temperature conditions. 
     The accuracy problem of MMS sensors is known, and the shortcomings of a factory temperature model calibration is also known. U.S. Pat. No. 7,168,507, issued to Downton on Jan. 30, 2007, teaches a system and method for recalibrating downhole sensors by comparing output values of two sets of sensors. The first set of sensors is inexpensive and comprised of less accurate MMS sensors, so they are placed close to the drill bit with a high risk of damage. The second set of sensors is expensive and placed in a more stable remote location. The second set of sensors are accelerometers measuring the same parameters, when the second set arrives at the same location where the first set took measurements. The first set of MMS sensors are calibrated by the readings from the second set. Instead of solving the reliability problem of the MMS sensors, the prior art solution is to retain a second set of the more expensive accelerometers to double check the MMS sensors. The &#39;507 patent acknowledges the known error rate of inexpensive MMS sensors, but the solution of adding a second expensive set of more accurate sensors remains expensive and redundant. 
     It is an object of the present invention to provide a method for determining position with improved calibration. 
     It is another object of the present invention to provide a method for determining position of a tool or a terminal device at a location for initiating activity. 
     It is still another object of the present invention to provide a method for initiating activity at a particular location determined by a calibrated sensor. 
     It is an object of the present invention to calibrate a sensor. 
     It is another object of the present invention to calibrate an MMS sensor. 
     It is still another object of the present invention to calibrate an MMS sensor for a bias correction. 
     It is yet another object of the present invention to calibrate an MMS sensor for a hysteresis correction. 
     It is another object of the present invention to provide a method for determining position with improved calibration of an MMS sensor. 
     It is still another object of the present invention to provide a method for initiating activity at a particular location determined by a calibrated MMS sensor. 
     It is an object of the present invention to provide a method for generating a plastic bias value for calibrating an MMS sensor. 
     It is another object of the present invention to provide a method for determining a plastic bias value based on temperature, time duration at a temperature, and gravity. 
     It is an object of the present invention to provide a method for generating an orientation bias value for correcting sensor readings of two MMS sensors in opposite orientations. 
     It is another object of the present invention to provide a method for determining an orientation bias value based on temperature, time duration at a temperature, and position of each MMS sensor. 
     It is an object of the present invention to calibrate a sensor after a factory calibration of the sensor. 
     It is an object of the present invention to calibrate a sensor with a plastic bias value concurrently with a factory calibration of the sensor. 
     It is an object of the present invention to continuously calibrate a sensor according to an adjusted plastic bias value continuously generated by each position data signal. 
     It is another object of the present invention to provide a method for determining position with a continuous calibration of an MMS sensor. 
     It is still another object of the present invention to provide a method for initiating and maintaining activity at particular locations determined by a continuously calibrated MMS sensor. 
     These and other objectives and advantages of the present invention will become apparent from a reading of the attached specification. 
     BRIEF SUMMARY OF THE INVENTION 
     Embodiments of the present invention include the method and system for determining position with improved calibration. Any device requiring accurate position data can determine location of the device with a sensor calibrated according to the present invention. The location and orientation of the device can be more accurately detected, so that the device can initiate or maintain activity in the desired location and orientation. Furthermore, the position data can be calibrated in real time so that a device can be actively guided, such as navigating a drill bit, while drilling through a rock formation. 
     The method of the present invention includes assembling a first position sensor comprised of a first oscillation element having a first range of displacement in a first set direction according to gravity in a first orientation and a second position sensor comprised of a second oscillation element having a second range of displacement in a second set direction according to gravity in a second orientation. The first and second orientations are opposite each other. Each position sensor can be an accelerometer of MMS sensor. One embodiment includes each position sensor being an MMS sensor exposed to high temperature conditions. A corresponding position data signal is generated based on an amount of displacement in the set direction, and a temperature data signal is detected by a temperature sensor in proximity, in some cases close proximity, to the position sensors. The position data signals and temperature data signal are communicated to a processor module in communication with the position sensors and the temperature sensor. A position value is generated by the processor module based on the position data signals, the temperature data signal, and duration of a temperature of the temperature data signal. The position value is based on temperature and gravitational force, including how many times and how long the position sensors are exposed to the temperature conditions. The data signals and values are stored in the memory module. The position value is communicated to the terminal device so that activity of the terminal device corresponds to the position value. 
     The step of generating the position value further comprises generating an adjusted plastic bias value to set the position value, so that the position value accounts for bias and hysteresis errors and improves accuracy of the position value. The plastic bias value relates the temperature data signal, the duration of the temperature, and the position data signals, according to bias and hysteresis of the oscillation element of the position sensor. There is additional error correction based on the differences between the first position data signal and the second position data signal due to gravity. The method accounts for the bias error of the component of each position sensor, relative to temperature, the hysteresis error of the component of each position sensor, relative to duration of temperature, and the orientation of the gravitation field. The adjusted plastic bias value can at least partially cancel out hysteresis errors and other errors due to the electronics, physics and chemistry of the sensing element. Additionally, the adjusted plastic bias value correction to the position value is a continual temporal additional correction to the original sensor bias calibration factor. Note the plastic bias correction is applied to each one of the opposing sensors. In the present invention, the position data signals can be corrected so that measurements of gravity on the oscillation element and capacitance (open loop) or currents (closed loop) of the position sensors are corrected to the set position value. 
     The present invention can be continuous, unlike prior art sensors with only an initial or factory calibration. The step of generating the position data signals and generating the position value according to an adjusted plastic bias value can be repeated over and over. Alternatively, the step of generating the position data signals and setting the position value can comprise generating a plurality of the data signals and setting the corresponding plurality of position data signals according to the respective plurality of adjusted plastic bias values. 
     Embodiments of the system of the present invention include the first position sensor, the second position sensor, the temperature sensor, processor module, memory module and terminal device. The terminal device can be any device requiring position data for operation, such as a downhole tool in the oil and gas industry, a survey tool for mapping locations, or a mobile probe for tracking location of the sensor or a video game controller for translating movements into game action. The present invention can include any terminal device that uses location and orientation data. In some embodiments, the position sensors can be MMS sensors exposed to high temperatures. The system can also include another pair of position sensors generating another pair of position data signals and another position value adjusted according to another adjusted plastic bias value. The multiple sensor pair embodiment can further reduce errors and confirm accuracy of location and orientation. The system can also include at least one magnetometer in proximity to the temperature sensor. The field data signals from the magnetometer can be used to determine calculated parameters, such as compass azimuth and the earth&#39;s magnetic dip. These parameters use both the field data signals as magnetometer magnetic signals and position data signals as accelerometer gravitational signals, such that these parameters can also be improved with the plastic bias value of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a schematic view of the system of the present invention, showing the terminal device as a drilling and activity as drilling in a first orientation. 
         FIG. 2  is another schematic view of the system of the present invention, showing the terminal device as a drilling and activity as drilling in a second orientation. 
         FIG. 3  is a flow diagram of an embodiment of the method of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1-3 , the present invention includes the method and system  10  for determining position of a device  20  with improved calibration of a first position sensor  30  and a second position sensor  30 ′ of the device. The location and orientation of the device  20  are more accurate with the present invention. The position sensors  30 ,  30 ′ calibrated according to the present invention allow the device  20  to properly initiate or maintain activity in the intended location and orientation. When calibrated in real time, the device  20  can be actively guided, such as navigating a drill bit  22 , while drilling through a rock formation. Errors due to bias and hysteresis can be reduced by the method and system of the present invention. Previous sensors with large bias and hysteresis errors, such as MMS sensors exposed to high temperatures, in particular open loop devices, can now be incorporated into devices for reliable determination of position data. Errors due to gravity and a gravitational field can also be reduced by the method and system of the present invention. The error correction for bias and hysteresis are further modified by the adjustment according to consideration of gravity in opposing orientations. 
       FIGS. 1-2  show the system  10  including a first position sensor  30 , a second position sensor  30 ′, a temperature sensor  40 , a processor module  50 , a memory module  60  and a terminal device  20 . The terminal device  20  can be any device requiring position data for operation, such as a downhole tool with drill bit  22  in the oil and gas industry, as in  FIGS. 1 and 2 , a survey tool for mapping locations, or a mobile probe for tracking location of the sensor or a video game controller for translating movements into game action. In  FIGS. 1-2 , the device  20  is a downhole tool with a drill bit  22 . In this embodiment, the activity is drilling with the drill bit  22 . The present invention can include any terminal device that uses location and orientation data. 
     The first position sensor  30  is comprised of a first oscillation element having a first range of displacement in a first set direction according to gravity in a first orientation. The second position sensor  30 ′ is comprised of a second oscillation element having a second range of displacement in a second set direction according to gravity in a second orientation.  FIGS. 1-2  show the first oscillation element relative to gravity (G) and horizontal (H) and the second oscillation element relative to gravity (G′) and horizontal (H′). In  FIG. 1 , the first set direction (X) is shown horizontal and matching horizontal (H) in the first orientation of the first position sensor  30 . The second set direction (X′) is also shown horizontal and matching horizontal (X′) in the second orientation, opposite the first orientation. Gravity affects both sensors the same, but in opposite directions. 
     In  FIG. 2 , the first set direction (X) is shown at a pitched angle to horizontal (H) and the second set direction (X′) is also shown at a pitched angle to horizontal (H′). Gravity on the second oscillation element of the second position sensor  30 ′ is no longer the same as the first oscillation element of the first position sensor  30 . The force of gravity is no longer equal nor directly opposite the first position sensor  30 . 
     The accurate determinations of X and X′ determine the orientation of the device  20  or downhole tool, so that the drilling of the drill bit  22  is on the correct path. The first position sensor  30  and second position sensor  30 ′ generate respective first and second position data signals for what the respective oscillation elements detect as X and X′. The mechanical limitations of each oscillation element affect accuracy of the detected X and X′ data signal as the actual X of the device  20 .  FIGS. 1 and 2  also show the temperature sensor  40  for detecting a temperature data signal and being in proximity to the first position sensor  30  and second position sensor  30 ′. The environmental conditions experienced by the first position sensor  30  and second position sensor  30 ′ match the temperature sensor  40 . The effect of temperature on the first position sensor  30  and the second position sensor  30 ′ is detected by the temperature sensor  40 . 
     In some embodiments, the position sensors  30  and  30 ′ can be MMS sensors exposed to high temperatures.  FIGS. 1-2  show the device  20  as a downhole tool. The environmental conditions of using the downhole tool include extreme temperatures and pressures. The mechanical components must be durable and resilient because retrieval for repair is not a simple task at great depths in the rock formation. Prior art quartz accelerometers have been used for downhole tool. However, the costs and space requirements for these accelerometers as position sensors are demanding. The present invention allows the MMS sensor to replace a quartz accelerometer without losing accuracy and durability. 
       FIGS. 1-2  show the schematic illustrations of the processor module  50  being in communication with the position sensor  30  and the temperature sensor  40 . The processor module  50  receives the position data signals from both the first position sensor  30  and the second position sensor  30 ′ and the temperature data signal by transmission or by hard wiring. The processor module  50  generates a position value based on the position data signals for what the first position sensor  30  detected as X, what the second position sensor  30 ′ detected as X′, and the temperature data signal. The memory module  60  is also a schematic illustration in communication with the processor module for storing the position data signals, the temperature data signal, and the position value. 
     The terminal device  20  is shown in communication with the processor module to receive the position value. There can be a control means  24  for activity corresponding to the position value.  FIGS. 1-2  show a schematic control means  24  for the drilling action of the drill bit  22 . In the present invention, there can be other devices, control means and activities. For example, the terminal device can be survey tool with the activity being mapping or a mobile probe with the activity being tracking or a video game controller with the activity being game movement in a virtual reality. 
       FIGS. 1-2  also show various alternate embodiments. There can be another pair of position sensors  32 ,  32 ′, each being comprised of another oscillation element having another range of displacement in another set direction according to gravity in another opposite orientation arrangement.  FIG. 1  shows the pair  32 ,  32 ′ in the same opposing orientation at position sensors  30 ,  30 ′ for comparison data to position sensors  30 ,  30 ′.  FIG. 2  shows the pair  32 ″,  32 ′″ in a different opposing orientation relative to position sensors  30 ,  30 ′. The data from this pair  32 ″,  32 ′″ can contrast and confirm some of the gravity induced error of the position sensors  30 ,  30 ′. 
     The other position sensors  32 ,  32 ′,  32 ″,  32 ′″ are also in proximity to the position sensors  30 ,  30 ′ and the temperature sensor  40 , so that the other position data signals generated remain comparable to the position data signals of the position sensors  30 ,  30 ′. The other pair of position sensors  32 ,  32 ′ is in communication with the processor module  50 , so that the position value is determined by the position data signal from the position sensors  30 ,  30 ′, the pair of other position data signals from the other pair of position sensors  32 ,  32 ′, and the temperature data signal for further error reduction and confirmation of the detected position data of location and orientation. 
     A magnetometer  80  in proximity to the temperature sensor  40  and in communication with the processor module  50  is also shown in  FIGS. 1-2 . The magnetometer  80  generates a field data signal, which can also be used by the processor module  50  to determine an additional position value, such as compass azimuth or earth&#39;s magnetic dip. The plastic bias value for correcting the position data signal, the position data signals, and the field data signal, corresponding to magnetic field strength, determine the additional position value. Thus, the additional position value, such as azimuth, is now more accurate because of the plastic bias value. In some embodiments, there can be more than one magnetometer, such as magnetometer  82 , to further compare and confirm the additional position values from other field data signals. 
     Referring to  FIG. 1-3 , the present invention includes the method and system  10  for determining position of a device  20  with improved calibration of a first position sensor  30  and a second position sensor  30 ′ of the device. The location and orientation of the device  20  are more accurate with the present invention. The first position sensor  30  and the second position sensor  30 ′ calibrated according to the present invention allows the device  20  to properly initiate or maintain activity in the intended location and orientation. When calibrated in real time, the device  20  can be actively guided, such as navigating a drill bit  22 , while drilling through a rock formation. Errors due to bias and hysteresis can be reduced by the method and system of the present invention. Errors due to gravity measurement can be reduced when accounting for the bias and hysteresis errors of the present invention. Previous sensors with large bias and hysteresis errors, such as MMS sensors exposed to high temperatures, can now be incorporated into devices for reliable determination of position data. 
     The method of the present invention includes assembling position sensors  110  in opposing orientations. Each position sensor is comprised of an oscillation element having a range of displacement in a set direction according to gravity. The position sensors can be quartz accelerometers or MMS sensors. One embodiment includes each position sensor being an MMS sensor exposed to high temperature conditions with an oscillation element, such as the silicon micro machined wafer of an open loop sensor system or a cantilever beam of a closed loop sensor system. The wafer or cantilever beam is affected by gravity or acceleration forces, so there is an amount of displacement of the wafer or an amount of current in a particular orientation required to keep the cantilever beam in place. Thus, the amount of displacement or the amount of current and direction of current reveals the amount and direction of acceleration. The errors from this type of MMS sensor can originate from bias and hysteresis type distortion. Bias error happens because the oscillation element (wafer or cantilever beam) is deformed by high temperatures and cannot return to the same neutral/null position with the same current induced magnetic force. Hysteresis type distortion happens because of a latency and physical/chemical sticking in the oscillation element&#39;s ability to fully recover from temperature fluctuations and reversals. The oscillation element further deforms from being exposed to the repeated higher and lower temperatures for different amounts of time. The force of gravity also affects the distortion of the oscillation element (wafer or cantilever beam). In opposite orientations, the effect of gravity can be subtracted out or at least partially subtracted out, so that the distortion of position data signals does not remain in the position value generated by the processor module. 
     Position data signals are generated  120  based on each amount of displacement of the respective oscillation element in the respective set direction. In the embodiments with the wafer and cantilever beam, each position data signal corresponds to the measurement of gravity on the wafer or the cantilever beam, respectively. The capacitance measurement of the displacement of the silicon wafer can determine the position data signal for location and orientation for an open loop system. The current required to maintain the cantilever beam can determine the position data signal for location and orientation for a closed loop system. This position data signal is not a true position value, so the position data signal requires correction to a more accurate position value because the bias and hysteresis errors affect the accuracy of these sensor readings and because gravity distorts the respective oscillation elements in the first orientation and in the second orientation. 
     The prior art calibrations correct for temperature distortions, but not distortions due to changes in the components of the sensor themselves. When delicate components, such as oscillation elements in MMS sensors, are assembled for compact size, lower costs, and lighter weights of the position sensor, the prior art calibrations fail to adequately correct the position data signals. The smaller, lighter, and cheaper positions sensors cannot be used in devices that require high levels of accuracy. 
     In the method of the present invention, a temperature data signal is detected  130  by a temperature sensor in proximity to the position sensors. The device will have both the position sensors and temperature sensor exposed to the same conditions, such as the same temperatures, but the orientations relative to gravity will be opposed. The position data signals and temperature data signal are communicated to a processor module  140  in communication with the position sensors and the temperature sensor. The sensors and processor module can be connected so that data can be exchanged between these components. There is also a memory module in communication with the processor module. 
     The processor module generates a position value  150  based on the position data signals, the temperature data signal, and duration of a temperature of the temperature data signal. The position value can account for temperature distortions, gravity distortions, bias distortions and hysteresis distortions. The amount of time the position sensor is exposed to the temperature of the temperature data signal is used to determine the position value. The present invention includes more than the temperature alone to set the position value of the device. 
     Next, the position data signals, the temperature data signal, duration of the temperature and the position value are stored in the memory module  160 , and the position value is communicated to the device  170  so that activity of the device can correspond to the position value. When the position of the device is accurately determined, the activity can be initiated  180  in the correct place. In the example of the drill bit as the device or terminal device, drilling with the drill bit at the accurate location in the rock formation is very important for placing the wellbore in the right location for access to production zones in the rock formation. Missing the location has serious consequences for the success of the hydrocarbon production. In the example of the video game controller, the motion of the controller must correspond accurately to movement on the video display. In the example of a mobile probe, the location of the mobile device must be accurate for tracking movement of the mobile device. 
     Embodiments of the step of generating the position value of the present invention further comprise generating a plastic bias value  152  to correct the position data signals for the actual X of the device. The plastic bias value relates the temperature data signal, the duration of the temperature, and the position data signals, according to bias and hysteresis of the oscillation element of the position sensor. The method accounts for the bias error of these mechanical components of each position sensor  154 , affected by exposure to temperature, and for the hysteresis error of these mechanical components of each position sensor  156 , affected by duration and fluctuation of exposures to the temperature conditions. The position value is set by the plastic bias value  158  for a more accurate position value corresponding to the actual X of the device. 
     For each linear sensor, a prior art sensor calibration might be described by equation:
 
CAL=[RAW*SCALE( t )]−BIAS( t )  (Equation 1)
         wherein CAL is the prior art calibrated sensor reading or prior art position value,   wherein RAW is the original sensor reading or position data signal, SCALE(t) is a scale factor that is a function of temperature, and BIAS(t) is a bias factor that is a function of temperature. In the case of this prior art calibration, SCALE(t) and BIAS(t) can be determined by experimentation before the sensor is put into use. The prior art calibration is the factory calibration after the sensors are assembled.       

     For the present invention, the step of generating the position value is described by:
 
CAL=[RAW*SCALE( t )]−BIAS( t )  (Equation 1)
 
and
 
CAL′=CAL−PLASTIC_BIAS(temperature,CAL,time)  (Equation 2)
         wherein CAL′ is the improved, calibrated sensor reading for the position value of the present invention, and wherein PLASTIC_BIAS(temperature, field, time) is the Plastic Bias term or plastic bias value, which is a function of temperature, field and time. Field can refer to the position data signal or other measurement dependent on gravity. In Equation 2, CAL refers back to the calculation of the prior art position value determined by the position data signal (RAW). This new PLASTIC_BIAS term cannot be determined before the sensor is put to use. The plastic bias value can be active and continuous during use based on the actual sensor environment.       

     In the present invention, the position value requires further modification. As a function of temperature, gravitational field and time, wherein “field” can refer to the position data signal or other measurement dependent on gravity, the orientation in gravity matters. Bending, with the help of gravity, especially with high sensitivity at high temperatures, can affect the accuracy of the position data signal. To remove “the help of gravity”, the method further set the position value by adjustment according to opposing orientation  157 . 
     In one embodiment, there is a pair of opposed MMS sensors. One sensor, the PLUS sensor or first sensor  30  is aligned in the direction of interest. The other sensor, the MINUS sensor or second sensor  30 ′, is aligned opposite the direction of interest. The PLUS and MINUS sensors are the same type of hardware component, just oriented in opposite directions. 
     For each opposing pair, the output from the two (PLUS and MINUS) sensors are averaged, using equation:
 
COMBINED_RESULT=(PLUS−MINUS)/2
 
     The errors from MMS sensors from both bias and hysteresis type distortion are negated because the same deformation happens to both MMS sensors of the pair in opposite directions. The deformation error can be at least partially canceled out by the average of the combined result. Thus, an adjusted plastic bias value removes additional error of the plastic bias values, so that the position value of the processor module  50  is a more accurate determination of actual X of the device. 
     The method may further include corrections based on the additional components, such as a magnetometer  80 , as in  FIGS. 1-2 . The field data signals can determine magnetic field strength, which may also affect the oscillation elements of the position sensors  30 ,  30 ′. 
     The present invention provides a method for determining position with improved calibration. Under extreme conditions, such as high temperatures, sensors can be calibrated to provide more accurate location information. The method allows for a terminal device to initiate activity at the properly determined location. For example, a more accurate path of the drill bit on a bottom hole assembly can be mapped for a drill bit. Also, the sensitive movement of a handheld controller can be capture for more accurate movement simulated in a video game. The position value based on orientation and location data is more accurate with the calibration of the present invention. The present invention can account for high temperature conditions of a wellbore or heat sinks from a computer system. 
     The method and system of the present invention calibrate a sensor, particularly an MMS sensor. Although relatively simple and inexpensive, the durability and reliability of MMS sensors has prevented the adoption of this micro-machine technology in many technical fields. Bias errors and hysteresis errors could not be addressed with current systems. The particularities of such small and sensitive parts required extra accommodations to insure reliability of the components. Certain conditions, such as temperature, had to be avoided in order to rely on the data from these MMS sensors. With the present invention, an MMS sensor can now be calibrated to accurately determine position. Activity can be initiated at a particular location determined by a calibrated MMS sensor. 
     The present invention provides a method for generating a plastic bias value for calibrating an MMS sensor. Beyond the prior art factor calibration based on the effects of temperature, the present invention gathers additional data and generates a different value to refine the readings from the sensor. The plastic bias value accounts for temperatures experienced by the sensor, time duration at each temperature, and the orientation or gravity value being detected during the exposure to those temperatures. The history of sensor can be considered so that bias and hysteresis errors no longer reduce the reliability of the sensor. For an MMS sensor, the plastic bias values can be used to form a plastic bias model, which can be used for a batch of MMS sensors assembled under the same conditions. Although the single history of all plastic bias values of one MMS sensor will provide the most accurate data for that one MMS sensor, there is still utility for creating a plastic bias model for all MMS sensors in the batch from which the one MMS sensor was taken. 
     The present invention provides a method for generating an adjusted plastic bias value further correcting position values based on position data signals of two MMS sensors in opposite orientations. In addition to correcting for bias and hysterersis errors, the adjusted plastic bias value accounts for errors due to different gravity on the pair of sensors. With opposing orientations, the effect of gravity on the position sensors can be at least partially canceled out. The temperatures, time duration at each temperature for each sensor, and position of each MMS sensor can be used to further cancel out errors due to gravity on the MMS sensors. The opposing orientations allow the subtraction of errors due to the different effect of gravity, when the sensors are in opposite orientations. 
     Prior art factory calibrations based on temperature remain compatible with the present invention. When the plastic bias value is determined to calibrate the MMS sensor after a factory calibration, the sensor readings still correct the sensor for bias and hysteresis and temperature. When the plastic bias value is determined concurrent with the factory calibration, the heating and cooling of the factory calibration are incorporated into the plastic bias values of the sensor. When the plastic bias value continues to be used to calibrate the MMS sensor, the sensor readings still correct the sensor for bias and hysteresis. 
     In an embodiment of the present invention, the sensor is continuously calibrated with a plastic bias value. The real time history of the exposure of the sensor is used to calibrate so that the sensor readings are the most accurate to exactly what conditions were experienced by the sensor. For an MMS sensor, the reliability problems due to high temperature exposure, bias and hysteresis are now addressed so that MMS sensors can be used in more diverse applications. The activities initiated at the determined position have increased accuracy, specific to the conditions exposed to the sensor. The prior art models and extrapolations are no longer the sole basis for error correction and calibration. Furthermore, sustained activities, such as drilling, can be tracked with the increased accuracy, such that the wellbore formed can be mapped with better reliability. Real time navigation through the formation is guided by accurate position information, instead of being guided by projections and theoretical models. The continuously calibrated MMS sensor actively adjusts sensor readings for advantages and benefits beyond the prior art factory calibrations and prior art multiple sensor arrangements. 
     The foregoing disclosure and description of the invention is illustrative and explanatory thereof. Various changes in the details of the illustrated structures, construction and method can be made without departing from the true spirit of the invention.