Patent Publication Number: US-2007107487-A1

Title: A calibration system

Description:
BACKGROUND  
      The invention pertains to sensors. Particularly, the invention pertains to calibration of sensors, and more particularly to self calibration of sensors.  
     SUMMARY  
      The invention is a system for static and dynamic calibration of sensors. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       FIG. 1  is a flow diagram of a calibration system;  
       FIG. 2  is a block diagram of a calibration system; and  
       FIG. 3  is a graph of an operating curve. 
    
    
     DESCRIPTION  
      A system may exist for the automation of static and dynamic calibration for a certain class of sensors. Static calibration may be performed via a slope seeking loop or algorithm. Dynamic calibration may be performed with both the slope seeking loop and a variation of the slope seeking set point.  
      The system may remove a need for manual adjustment of sensors to account for sensor drift due to ambient condition changes, and also the need for recalibration for operating condition changes. This may be significant if the sensors are wireless and there is a desire to minimize the need for manual adjustments.  
      The sensors may retain accuracy under changing ambient conditions and also recalibrate themselves to adjust to operating conditions and sensor aging. This may be done through using feedback control via sensing of the ambient conditions and the operating conditions.  
      A drift control feedback loop may be designed using identified empirical or semi-empirical constitutive models of sensor material. Similarly, a feedback loop may sense changes in operating conditions and use either a lookup table or a sensor model to change the gains at the sensor output.  
      Self-calibration of the sensors may be compelling for one or more of the following reasons. Accurate sensor calibration is time-consuming, expensive, and often manual. Aging of sensor parts compels periodic recalibration. Changes of operating conditions also necessitate recalibration. Sensor accuracy is compromised when operating conditions are not same as the calibration conditions. It is difficult to manufacture affordable sensors that do not need calibration or recalibration.  
      Self-tuning of the sensors may be compelling for one or more of the following reasons. The settling time of a sensor is different at various calibration points. The manufacture of sensors with uniform setting times is typically unaffordable. Thus, sensors generally require specific settings of calibration points. Aging and changes of operating conditions may change the settling time.  
      There may be a large class of or many sensors that transduce a difference between a control signal and an environmental signal to produce an output (typically electrical). Examples may include microphones, flow control valves, thermocouples, gimbaled mechanisms, and other relative-type measuring devices.  
      The present approach may be to adapt the operating point of a sensor for self-calibration. There is no need to do multiple sensors. A rough estimate of the operating curve may serve to eliminate individual calibration of manufactured sensors. The accuracy of a sensor may be well characterized through knowledge of the uncertainty in the transducer dynamics. Slope seeking is a key to an application of the present system.  
      The system may reduce sensor costs with the elimination or reduction of calibration tasks. The automatic calibration mechanism may be autonomous from the subject sensors.  
       FIG. 1  is a basic flow diagram of self-calibration and tuning system  10  for a sensor. An objective for a sensor to be calibrated may be represented by a “set sensor objective” block  11 . An objective setting from block  11  may go to a block  12  to run a static calibration to set sensitivity. An output of block  12  may go through an “insert a known signal” block  13 . The output with an inserted known signal from block  13  may go to a “calculate an objective” block  14 . An output of block  14  may be a calibration result of a settling time of the sensor. This output may go to a decision diamond  15  which determines whether the settling time of the sensor is approximately equal to the set or required setting time. If the answer is “yes”, then an output from diamond  15  may go to a block  16  to stop the calibration. If the answer is “no”, then an output from diamond  15  may go to a block  17  for an estimate of an optimal slope setting. An estimate of the optimal slope setting may go to the block  12  for a static calibration run to set the sensitivity of the sensor. That sensitivity setting may have a known signal inserted from block  13 . The resultant signal may go on to the “calculate objective” block  14 . The output from block  14  may be a new settling time compared to the set or predetermined settling time for the sensor at diamond  15 . If the settling times are not equal, then the system  10  may proceed again through blocks  17 ,  12 ,  13  and  14  for a settling time to approach or equal the settling time as prescribed. This cycle through blocks  17 ,  12 ,  13 , and  14 , and diamond  15  may repeat until the settling time is at least approximately equal to the set settling time. If the latter equality is attained, then the calibration may be stopped as indicated by block  16 .  
       FIG. 2  is a block diagram of a system  20 . Block  21  may represent a generator source of low frequency forcing relative to transducer dynamics of a sensor being calibrated and tuned. The generator source  21  may output a signal as represented by asinωt. The output signal of tracking compensator  47  may go to an adder or summer  22  where the signal may be combined with a perturbation from generator source  21 . The signal from adder  22  may provide a commanded input to set the slope through the exciting dynamics (F i (s))  23  to an adder or summer  24 . Also coming into a summer  24  may be a measured quantity  25 . The quantity  25  may be utilized for self-calibrating of a transducer  27  of a sensor when it is fairly static. The output  26  of adder  24 , including the output of exciting dynamics  23  and the measured quantity  22 , may go to the transducer  27 . Exciting dynamics  23 , measured quantity  25  and adder  24  may constitute a transducer signal source and interface module  48 . The characteristics of transducer  27  may be represented by an operating curve  28  with a slope shown by a tangent  29  on the curve  28 , which is at a calibration point  31 , as shown in  FIG. 3 .  
       FIG. 3  is a graph  32  revealing the operating curve  28  and slope  29  of the transducer  27 . The waveform represents the exciting dynamics  23 . The ordinate axis represents a transducer voltage and the abscissa axis represents ΔP (delta pressure in the transducer). The output of the characteristics block  32  of  FIG. 2 , which is represented by the graph  32  of  FIG. 3 , may go to a settling dynamics (F o (s)) block  33 .  
      In  FIG. 2 , the output of settling dynamics  33  may go through an amplifier  30 . An output of amplifier  30 , which may be the output of transducer module  27 , may go to an amplifier  34  and a washout filter  35 . The settling dynamics  33  may provide an output with an operating point on a required slope with a settling time. The operation may be achieved via a slope seeking algorithm and a settling time algorithm. The settling time algorithm may be a gradient descent or bisection algorithm. The output from dynamics  33  via amplifier  30  of transducer  27  may be to the variable gain K 1  amplifier  34 . An output  36  of the measurement by the sensor may be provided by amplifier  34 . Amplifier  34  may have a gain control input  37  with a signal for controlling the gain K 1  of amplifier  34 . The gain control signal  37  may come from a gain control and slope specification mechanism block  38 . Another output of block  38  may be a commanded or specified slope f′ ref  at calibration point  31 . Components  34 ,  38 , and  39  may constitute a dynamic calibration module  41  to optimize the settling time of the transducer  27 . The equations relating to mechanism  38  may indicate a relationship between slope, settling time, and amplifier gain over time steps. These equations may include 
 
 f′   ref ( k+ 1)= f′   ref ( k )+ g   1 (τ s ( k )), and 
 
 K   1 ( k+ 1)= K   1 ( k )+ g   2 (τ s ( k )). 
 
      “k+1” and “k” indicate time steps. “τ s ” indicates settling time, “g 1 ” and “g 2 ” indicate a function of settling time relative to the set slope f′ ref  and amplifier gain K 1 , respectively.  
      A set slope f′ ref    39  may be sent as an input to a slope setting processor  42  of a slope seeker for static calibration module  43 . For the same sensitivity, the product of K 1  and f′ ref  may be a constant. The slope setting processor  42  may reflect the following equation, 
 
−(a/2)R{e −jφ jωF o (jω)C o (jω)F i (jω)}. 
 
      “a” may indicate a magnitude, “R” may indicate the real part, F i  may indicate the exciting dynamics, F o  may indicate the settling dynamics, and C o  may indicate the washout filter (if ω is small, F i (jω) and F o (jω) behave as constant gains). The slope setting processor  42  may output a slope setting to a summer or adder  44 . An output of washout filter sC o (s)  35  may multiplied with a phase shift signal  46  represented by sin(ωt−φ), at multiplier  45 . The output of multiplier  45  may go to adder  44  where it is summed with the slope setting from processor  42 . The output of adder  44  may include a signal representing a tracking error which is proportional to the difference between where one is and where one should be. This output of adder  44  may go to a tracking compensator  47  which may have a signal transformation aspect that is represented by C i (s)/s. The output of compensator  47  may be a setting signal that goes to adder  22  to be combined with the perturbation signal from the low frequency forcing generator  21 . The forcing may be an additive to the input of the exciting dynamics  23 . The sinusoidal signal may be added to perturb the current setting. The output path of adder  22 , along with the other processes, may be noted above.  
      In the present specification, some of the matter may be of a hypothetical or prophetic nature although stated in another manner or tense.  
      Although the invention has been described with respect to at least one illustrative example, many variations and modifications will become apparent to those skilled in the art upon reading the present specification. It is therefore the intention that the appended claims be interpreted as broadly as possible in view of the prior art to include all such variations and modifications.