Abstract:
An air bag calibrator that is used to calibrate a plurality of seat sensors within a seat simultaneously. The calibrator uses controlled pressure contained within an air bladder and an air press cylinder to apply a controlled force across an entire seat surface that actuates all of the seat sensors in a consistent and repeatable manner, a major improvement over existing seat calibration methods. This is accomplished by loading a seat onto the calibrator, recording a baseline seat sensor value for each of the seat sensors, extending the air press cylinder to an extended position and applying a controlled force across the entire seat surface. The air pressure in the air bladder is then increased or decreased to a target pressure for a predetermined time, at which time an actuated seat sensor values for each of the seat sensors is recorded. A PODS controller in contained within the seat that is used for controlling the deployment of air bags in a crash situation is then calibrated as a function of the recorded baseline seat sensor values and the actuated seat sensor values.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority from provisional application Ser. No. 60/203,768 filed May 12, 2000. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to automotive systems and more particularly to seat sensor calibration methods and systems. 
     BACKGROUND OF THE INVENTION 
     Recently, automotive manufacturers have developed Passenger Occupant Detection Systems (PODS) to aid in the decision making for deploying passenger side air bags. 
     Typically, these systems consist of a flexible laminate mat with printed resistors and cylindrical force concentrators. The force concentrators (FCDs) are bonded to the sensor mat over the printed resistors. The sensor mat is placed in the passenger seat between the seat foam and seat trim cover. When the surface of the seat is depressed, the sensor mat and printed resistors will wrap around the force concentrators, which creates a change in the circuit resistance. Based upon the pattern and the weight of the depression, the PODS system can determine whether or not to deploy the passenger side air bag in a crash situation. 
     PODS sensors must be calibrated in order to obtain optimal performance. There are several variables that can affect sensor performance, including material considerations associated with the seat and sensor components, product material or process variations, and crack preconditioning process. As a result of the multiple sources of variation, the PODS sensors must be calibrated after the seat bottom has been assembled. 
     The goal of the calibration is to quantify performance of the PODS when a load is applied to the seat surface. The basic calibration steps are to measure a sensor&#39;s value in an unloaded state, apply a known load to the seat surface, and record the sensor value in the loaded state. The sensor performance data and the seat model information is then used to calculate the sensitivity of the sensors. The challenge has been to develop a method for applying the load to a seat that produces more reliable, repeatable, and realistic calibration data. 
     There have been several methods and pieces of equipment used in an attempt to acquire good PODS calibrations. One method focuses on applying a known force to each seat sensor. The seat is contacted with a flat 1.25″ diameter swivel pad usually made of steel or delrin material. The system uses a robotic motorized Z-axis to apply force to the seat. A force sensor integrated into the Z-axis monitors the applied force. The Z-axis drives into the seat until the desired force level is achieved. While the system provides acceptable laboratory results, additional testing uncovered problems controlling the force that was applied to the seat surface. Noise tended to generate erroneous force readings. The serving nature of the robot&#39;s Z-axis also introduced instabilities to force sensor values. Finally, the system exhibited sensor registration problems associated with off-center contact on the seat sensor and non-uniform and non-planar seat foam compression. 
     New concepts were developed in response to the problems associated with the original calibrators. An air spring assembly was implemented to design the force sensor out of the system. The air spring produces constant force over its entire length of travel. Because of this fact, the robot is able to drive the Z-axis to a nominal depth and be certain of contacting the seat with known force regardless of the height variation of the seat surface. 
     In response to the registration issues, the steel/delrin swivel pad was replaced with a 6″ urethane-nylon laminate air bladder. Testing concluded that contacting the seat with a flaccid air bladder would produce acceptable sensitivity to seat sensor registration, allowing mis-registration up to +/−25 mm without adversely affecting the sensor values. Further, the air bladder is able to conform to non-planar surfaces and non-perpendicular surfaces, such as sensor locations in seat bolsters. 
     However, while the air bladder created certain improvements, it also created additional problems. Assuming the contact force is always the same, the pressure on the seat surface would be fairly repeatable from seat sensor to seat sensor and machine to machine. With the air bladder design, however, the contact area was a function of the amount of air trapped in the bladder, the size of the bladder, air temperature, and the seat surface contour. 
     Methods were created to attempt to define and control these parameters. One method was to fill the bladder to a predefined pressure while pressing the bladder against a flat and rigid surface under a known force. Another method was to fill the bladder to a pre-determined height while pressing the bladder against a flat and rigid surface under a known force. These set-up methods attempted to control the amount of air in the air bladder, but did not solve production problems on non-planar seat surfaces and did not directly control the air bladder pressure. 
     Other calibration methods were attempted, and all of the methods possessed several common weaknesses. First, because all of the methods were single point calibrators, only one seat sensor could be calibrated at one time. As a result, it could take 2 to 4 minutes to calibrate a single-seat. 
     Second, testing indicated that a seat sensor&#39;s final position could move upwards of +/−35 mm from its nominal position due to uncontrollable seat assembly process and the nature of the materials used to construct the seat. Off-center actuation of the seat sensors produced unrepeatable and unreliable calibration date. 
     Third, since air was trapped in the 6″ diameter bladder, ambient temperature changes resulted in pressure changes within the air bladder, resulting in inaccurate sensor readings. 
     Fourth, integration of the robotic calibrators into different manufacturing lines for use on a wide variety of seat models, seat manufactures, and seat assembly processes would require custom calibration solutions in almost every instance. 
     Fifth, setting up, controlling, and monitoring process parameters was extremely difficult and time consuming. The interdependentness of air spring force and air bladder volume process parameters would require adjustments in an iterative manner to bring them within limits. In addition, it would be difficult to create a “transfer” standard that could be used to validate the operation of all machines. 
     As a result, a need exists for an improved method and system for calibrating seat sensors that produces reliable and repeatable results in a simple and efficient manner. 
     SUMMARY OF THE INVENTION 
     While the above methods for calibration focused on applying a controlled force to each seat sensor position with a different apparatus to contact the seat, the present invention focuses on two premises. First, it will be assumed that seat foam acts as a compressible fluid. Second, the amount of sensor mat wrap around an FCD and glue joint is directly related to seat surface pressure. A higher seat surface pressure would cause more sensor mat wrap thus generating a higher seat sensor value. In short, a consistent surface pressure acting on the seat surface will produce consistent reaction pressure in the seat foam, consistent seat deformation, and consistent and reliable sensor data. 
     The present invention incorporates these two premises into a calibration device generally referred to herein as a Big Bag Calibrator. On the Big Bag Calibrator, air bladder pressure is monitored and controlled directly. A precision pressure sensor is pneumatically tied to the air bladder and relays pressure information back to a computer. The computer determines whether to add or exhaust air to maintain the target pressure within the bladder. With the Big Bag Calibrator, a controllable pressure is applied to the entire seat surface that actuates all of the seat sensors in a consistent and repeatable manner. 
     This process eliminates the need for all of the indirect monitoring steps required for the prior art calibrators. Further, the process offers several advantages over the prior art. Since the Big Bag Calibrator tests all seat sensors at one time, the cycle time for testing seat sensors is substantially reduced. The Big Bag Calibrator also is simpler and less expensive to maintain than previous calibrators are. The expensive steel base, robot, servo amplifiers, motion control cards, and related control software are replaced with an inexpensive base and air cylinder. Further savings are realized in associated maintenance, downtime, and training. Further, the Big Bag Calibrator is relatively insensitive to sensor registration because the air bladder extends well beyond the edges of a typical car seat. 
     Other advantages include the fact that ambient temperature changes would not affect system performance, since the air bladder pressure is controlled directly. Also, the Big Bag Calibrator is easy to integrate into a wide variety of seat models, seat manufacturers, and seat assembly processes. Seats can be calibrated as individual seat bottoms, as full seats, or on a pallet containing a car&#39;s full complement of seats. These may be run off a single, machine without significant hardware or software changes. Finally, the Big Bag Calibrator directly and accurately controls the pressure applied to the seat surface, eliminating many unnecessary control problems. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
     FIG. 1 is a perspective view of a Big Bag Calibrator according to a preferred embodiment of the present invention; 
     FIG. 2 is a logic flow diagram for the calibrating the seat sensors using a Big Bag Calibrator according to a preferred embodiment of the present invention; 
     FIGS. 3A and 3B illustrate a front and side perspective view of a Big Bag Calibrator according to another preferred embodiment of the present invention; 
     FIG. 4 is a logic flow diagram for the calibrating the seat sensors using a Big Bag Calibrator according to another preferred embodiment of the present invention; 
     FIG. 5 is a perspective view of one preferred embodiment of a Big Bag Calibrator having a properly positioned air bladder; 
     FIG. 6 is a perspective view of FIG. 5 having an improperly positioned air bladder; 
     FIG. 7 is a perspective view of FIG. 5 having an improperly positioned air bladder; and 
     FIG. 8 is a perspective view of another preferred embodiment of the Big Bag Calibrator having a back edge bladder support that remedies the problems associated with an improperly positioned bladder. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIG. 1, a Big Bag Calibrator  10  is shown having as its major components an air bladder press cylinder  12 , an air bladder  14  and an air press plate  22 . A digital manometer  16  and a computer  20  are also coupled to the Calibrator  10 . 
     In operation, the air press cylinder  12  moves the air press plate  22  up and out of the way so that a seat (shown as  80  in FIGS. 5-8) can be manually loaded onto a fixture  18 . The seat  80  has a PODS controller (not shown) that is electrically coupled with the computer  20  and a plurality of seat sensors (shown as sensor mat  86  in FIGS.  5 - 8 ). The PODS controller is used to determine whether to deploy an air bag (not shown) in a crash situation. The use of the PODS controller is described below. 
     Once the seat  80  is loaded, the air press cylinder  12  then moves the plate  22  and air bladder  14  down on the seat  80 . The air press cylinder  12  maintains a force on an air press plate  22 . In a preferred embodiment, the air bladder press plate  22  maintains a force of approximately 400 pounds in the extended state to emulate a rigid body. Therefore, when the air bladder  14  is inflated or deflated to the target pressure, the seat  80  and the air bladder  14  are the only components that move. 
     The air bladder  14  conforms to the contour of the seat  80  and applies pressure to the seat surface.(shown as  81  on FIGS.  5 - 8 ). Preferably, the air bladder  14  is made of a urethane-nylon laminate material to allow the air bladder  14  to slide on the seat surface  81  and provide greater ability to conform to the seat contours. The bladder  14  can range in size from approximately 18″ by 24″ to 26″ by 26″. FIGS. 5 and 8 below show two preferred embodiments of configurations for the air bladder  14 . The digital manometer  16  preferably has a full scale sensing capability of 0 to 7.22 psi with a typical accuracy of +/−0.5% of Full Scale which translates to 0.0361 psi. Because the manometer  16  has a digital readout, pressure readings are captured and recorded manually. 
     Referring now to FIG. 2, a logic flow diagram with two ramp-up alternatives using a Big Bag Calibrator  10  according to one preferred embodiment of the present invention is shown. In Step  200 , a seat  80  is loaded into a fixture  18  underneath the air press assembly  12  and a cable (not shown) is connected with the seat harness (not shown). Next, in Step  210 , the PODS controller is powered up. Baseline sensor readings of the plurality of sensors  86  are recorded in the PODS controller in Step  220  prior to applying a load to the seat surface  81 . A computer  20 , preferably using Pure Imagination&#39;s software package Slug Recorder, is connected to the PODS controller. The software package is capable of capturing data from all the seat sensors  86  simultaneously. In the manual process, the data capture process is triggered manually. 
     In Step  230 , the air press cylinder  12  is extended via a manually operated valve (not shown) to its end of travel. The cylinder  12  exerts and maintains a force of approximately 400 lbs. in its extended state. Depending upon the volume of air left in the air bladder  14 , a force may be exerted on the seat  80  at this time. 
     In Step  240 , a determination is made as to how to drive the Big Bag to the bladder  14  target pressure. Two options are available. If the operator determines to approach the target pressure and maintain, proceed to Step  250 . If the operator determines to approach the target pressure on the fly and record, proceed to step  260 . 
     In Step  250 , the air bladder pressure is driven to its target pressure and maintained for a period of approximately 30 seconds within the specified tolerances. At that time, the PODS controller records the actuated sensor values. Typically, when the air press cylinder  12  is extended and the air bladder  14  is contacted to the seat  80  there is usually enough air trapped in the air bladder  14  to generate a pressure greater than the target pressure. As the seat  80  is compressed under the load of the bladder  14 , the air pressure in the bladder  14  decreases due to the increase in volume of the air bladder  14 . To expedite the process, air is exhausted from the air bladder  14  until the target pressure is achieved. Once the actuated sensor values are recorded, the computer  20  creates a calibration table for the PODS controller that takes into account the baseline sensor values from Step  220  and the actuated sensor values from Step  250  which determine the responsive curve for each sensor. The calibration table is used by the PODS controller to correct raw sensor values. This calibration table is downloaded into the PODS controller memory. The PODS controller then uses the corrected sensor values and deflection pattern information to determine whether to deploy the air bag in crash situations. The logic then proceeds to Step  270 . 
     In Step  260 , the process proceeds exactly as in Step  250 , with the exception that the actuated sensor values are recorded as soon as the bladder  14  target pressure is achieved, rather than waiting 30 seconds. Once the actuated sensor values are recorded and the calibration table is sent to the PODS controller, proceed to Step  270 . 
     In Step  270 , the big bag air press cylinder  12  is retracted, and in Step  280  the seat  80  is removed from the fixture  18  after powering down the PODS controller and disconnecting the cable. 
     Referring now to FIGS. 3A and 3B, an automated (production) version of the Big Bag Calibrator is depicted in two views. The automated Big Bag Calibrator  50  is shown having as its major components an air bladder press cylinder  52 , an air bladder  54 , a pressure sensor  56 , a controller  58  and a fixture  60 . The controller  58  consists of a power box  61 , a computer  63 , and an electronic application box  65 . Also electrically coupled to the computer  63  are a user interface monitor  67 , a user interface keyboard/mouse  69 , and a label printer  71 . The automated Big Bag Calibrator  50  may also contain a machine guarding  73 , a light stick  75  and a back edge bladder support  103 . The back edge bladder support  103  will be explained below with FIG.  8 . 
     In operation, the air bladder press cylinder  52  moves the bladder  54  up and out of the way so that a seat  80  can be loaded onto a fixture  60 . The fixture  60  has an electronic contact block (not shown) and a locating post (not shown) that are common to seat tooling designs. When the seat  80  is properly loaded, an electrical connection is established between the PODS controller (not shown) contained within the seat  80  and the controller  58 . Once the seat  80  is loaded, the air bladder press cylinder  52  then moves the air bladder  54  down on the seat  80 . The air bladder press cylinder  52  maintains a force on an air bladder plate  62  great enough to keep the air bladder plate  62  from moving when air bladder pressure is increased. In a preferred embodiment, the air bladder press cylinder  52  has a 4″ bore, a 7″ stroke, and maintains a force of approximately 675 pounds at 50 psi in the extended state to emulate a rigid body. Therefore, when the air bladder  54  is inflated, the seat  80  and the air bladder  54  are the only components that move. 
     The air bladder  54  conforms to the contour of the seat  80  and applies pressure to the seat surface  81 . Preferably, the air bladder  54  is made of a urethane-nylon laminate material to allow the air bladder  54  to slide on the seat surface and provide greater ability to conform to the seat contours. The size of the air bladder  54  is larger than the air bladder  14  in FIG. 1, and is approximately 26″ by 26″. The larger bladder  54  ensures complete coverage of the seat surface  81  with allowances for seat movement. 
     The pressure sensor  56  is coupled to a controller  64  that is capable of outputting pressure information to the controller  58  via a communication protocol. The pressure sensor  56  preferably has a full scale sensing capability of 0 to 1.5 psi with a typical accuracy of +/−0.5% of Full Scale which translates to 0.0075 psi. The pressure sensor  56  data is used to decide how much air to move in and out of the bladder  54 . 
     Referring now to FIG. 4, a logic flow diagram with two ramp-up alternatives using an automatic Big Bag Calibrator  50  according to one preferred embodiment of the present invention is shown. In Step  400 , a seat is loaded into a fixture  60  underneath the air press assembly, a cable is connected, with the seat harness, and the seat fixture is slid into position beneath the Big Bag Press assembly  52  such that the seat extends into the electrical contact block. Next, in Step  410 , the Controller  58  is powered up by turning on the application box  65 . Baseline sensor readings are recorded in the PODS controller in Step  420  prior to applying a load to the seat surface  81 . A computer  63  is connected to the seat  80  and the controller  58  and is capable of capturing data from all the seat sensors  81  simultaneously. 
     In Step  430 , the air press cylinder  52  is extended to its end of travel. The cylinder  52  exerts and maintains a force of approximately 675 lbs. at 50 psi in its extended state. Depending upon the volume of air left in the air bladder  54 , a force may be exerted on the seat  80  at this time. 
     In Step  440 , a determination is made as to how to drive the Big Bag Calibrator  50  to the target pressure. Two options are available. If the operator determines to approach the target pressure from the high side, proceed to step  450 . If the operator determines to approach the target pressure from the low side, proceed to step  460 . 
     In Step  450 , the air bladder  54  pressure is directed by the computer  63  to be driven to its target pressure and maintained within the specified tolerances. At that time, the actuated sensor values from the pressure sensor  56  are recorded. Typically, when the air press cylinder  52  is extended and the air bladder  54  is contacted to the seat  80  there is usually enough air trapped in the air bladder  54  to generate a pressure greater than the target pressure. As the seat  80  is compressed under the load of the bladder  54 , the air pressure in the bladder  54  decreases due to the increase in volume of the air bladder  54 . To expedite the process, air is exhausted from the air bladder  54  until the target pressure is achieved. Once the actuated sensor values are recorded, the computer  63  creates a calibration table for the PODS controller that takes into account the baseline sensor values from Step  420  and the actuated sensor values from Step  450  which determine the responsive curve for each sensor. This calibration table is downloaded into the PODS controller memory. The calibration table is used by the PODS controller to correct raw sensor values. The PODS controller then uses the corrected sensor values and deflection pattern information to determine whether to deploy the air bag in crash situations. The logic then proceeds to Step  470 . 
     In Step  460 , the process proceeds similarly as in Step  450 . However, in this scenario, when the air press cylinder  52  is extended and the bladder  54  contacts the seat  80 , the volume of air is small enough such that the air trapped in the bladder  54  generates a pressure less than the target pressure. In this case, air is pumped into the bladder  54  to increase the air pressure to the target pressure. Once the target pressure is achieved and maintained within specific tolerances, actuated sensor values are recorded. The computer  63  then creates a calibration table that is sent to the PODS controller as described above. The logic then proceeds to Step  470 . 
     In Step  470 , the big bag air press cylinder  52  is retracted, and in Step  480  the seat is removed from the fixture  60  after powering down the controller  58  and disconnecting the cable. 
     FIGS. 5 and 8 illustrate two preferred embodiments that may be used by either of the Big Bag Calibrators as depicted in FIGS. 1 and 3 to calibrate passenger side seats  80  to aid in the deploying of passenger side airbags. In FIG. 5, the width of the bladder  14 ,  54  is set approximately equal with the depth of the seat  80  (from front to back), while in FIG. 8 a back edge bladder support  103  is added to remedy the problem if the bladder is positioned beyond the back edge of the seat  80 , as depicted in FIG.  7 . Also, FIG. 6 illustrates where the position of the bladder  14 ,  54  is too small. 
     FIG. 5 depicts a seat  80  placed in the Big Bag Calibrator  10 ,  50 . The seat  80  is composed of, starting from the bottom, a sheet metal seat pan  82 , seat foam  84 , a sensor mat  86 , an FCD  88 , and a seat cover  90  (sometimes referred to as trim). The sensor mat  86  contains a bottom mylar sheet (not shown), a plurality of silver ink conductors  92 , a resistor pad (not shown), and a top mylar sheet (not shown). 
     The position of the bladder  14 ,  54  is set wherein the rounded edge of the bladder  14 ,  54  extends beyond the back edge  101  of the seat  80  in all extremes of seat movement. This position is crucial for optimizing calibration. 
     To optimize sensor performance, the bladder  14 ,  54  position must be properly controlled. If the bladder  14 ,  54  position does not extend beyond the seat  80 , as depicted in FIG. 6, a step  99  in the seat foam  84  is created. This step  99  in turn causes the sensor mat  86  to further wrap around the FCD  88 . This causes more cracks (gaps) in the silver ink conductors  92 , leading to a higher resistance value, which in turn causes the sensor value to increase, resulting in inaccurate sensor values which can affect air bag deployment. 
     Further, if the position of the bladder  14 ,  54  extends too far beyond the back edge  101  of the seat  80 , as depicted in FIG. 7, the back edge  101  of the seat foam  84  is pulled down and the sensor mat  86  is flattened. Thus, the sensor mat is not properly wrapped around the FCD  88 , and the silver ink conductors  92  will read lower than anticipated sensor values. 
     As shown in FIG. 5, in one preferred embodiment of the present invention, the position of the bladder  14 ,  54  is set wherein only the rounded edge of the bladder  14 ,  54  extends beyond the back edge  101  of the seat  80  in all extremes of seat movement. This ensures that the sensor mat  86  properly wraps around the FCD  88  for more consistent and repeatable sensor values. 
     In another alternative preferred embodiment, as shown in FIG. 8, a back edge bladder support  103  is added. The back edge bladder support  103  supports the bladder  14 ,  54  to prevent the sensor mat  86  from flattening as in FIG.  7 . This is useful because it allows the same Big Bag Calibrator to be used for calibrating different sizes of seats without having to adjust the position of the bladder  14 ,  54 . 
     This process eliminates the need for all of the indirect monitoring steps required for the prior art calibrators. Further, the process offers several advantages over the prior art. First, since the Big Bag Calibrator tests all seat sensors at one time, the cycle time for testing seat sensors is substantially reduced. Second, the Big Bag Calibrator is much simpler and less expensive to maintain than previous calibrators. The expensive steel base, robot, servo amplifiers, motion control cards, and related control software are replaced with an inexpensive base and air cylinder. Further savings are realized in associated maintenance, downtime, and training. Third, the Big Bag Calibrator is relatively insensitive to sensor registration because the air bladder extends well beyond the edges of a typical car seat. Fourth, since the air bladder is controlled directly, ambient temperature changes would not affect system performance. Fifth, the Big Bag Calibrator is easy to integrate into a wide variety of seat models, seat manufacturers, and seat assembly processes. Seats could be calibrated as individual seat bottoms, as full seats, or while on a pallet that contains a car&#39;s full complement of seats. These may be run off a single machine without significant hardware or software changes. Sixth, the Big Bag Calibrator directly and accurately controls the pressure applied to the seat surface, eliminating many unnecessary control problems. 
     While the invention has been described in terms of preferred embodiments, it will be understood, of course, that the invention is not limited thereto since modifications may be made by those skilled in the art, particularly in light of the foregoing teachings.