Abstract:
A method and apparatus for automated field calibration of temperature sensors uses a series of readings including a reading of a known source, such as an LED, for use in calculating a factor that is compared to a reference for adjusting the sensor output signal. Calibration readings are taken more frequently after start up to compensate for sensor drift during storage, as opposed to less frequent readings during operation to compensate for slower sensor drift while operational.

Description:
FIELD OF THE DISCLOSURE 
     The disclosure generally relates to methods of non-contact temperature measurement and, more particularly, relates to a method for automated self-calibration in optical temperature detectors. 
     BACKGROUND OF THE DISCLOSURE 
     Non-contact temperature instruments allow measuring the temperature of an object at a distance and are quick to respond. These operating features are particularly helpful when measuring the temperature of an object in a harsh or dangerous environment where physical contact is not an option. Such instruments generally operate by sensing the energy emitted from objects at a temperature above absolute zero in which the radiant infrared energy emitted by the object is proportional to the fourth power of its temperature. 
     To develop a measurement, some devices use a shield, often called a chopper to expose a sensor or detector, alternately exposing and blocking the target object, creating a modulated signal. 
     Many optical sensors or detectors, such as lead sulfide detectors, exhibit long term drift in their responsiveness, that is, the output as a function of incident radiation may change over time. Devices using such detectors must be periodically recalibrated using a source of known brightness, such as an incandescent bulb. One such method of calibration requires that the device is removed from service, exposed to the incandescent bulb, and then manually recalibrated. However, this requires that the measurement of the target object be interrupted during the calibration period. If the device is being used in a process control environment, either the process must be halted or the process must run out of control during the calibration period. Waiting for the incandescent bulb to warm up and stabilize may introduce further delays in the calibration process. 
     One attempt at automated self-calibration used an incandescent bulb arranged so that the chopper would expose the sensor to the target object, block the target object, expose the sensor to the bulb, block the target, and again expose the sensor to the target object. This approach requires the incandescent bulb to be run continuously, introducing its own brightness drift over time as a source of error. Moreover, when operated in this manner, the device is only measuring the target object one quarter of the time, reducing both the signal-to-noise ratio and its responsiveness to changes in target radiation. 
     SUMMARY OF THE DISCLOSURE 
     A non-contact optical temperature measuring device performs automated self-calibration using a reference calibration source, such as an LED, and a series of measurements with the chopper open, the chopper closed with the calibration source off, and the chopper closed with the calibration source on. The three readings may be combined and compared to a reference value to generate a calibration factor. The ratio of the combined readings to the reference value may be used to correct for drift of the sensor. An adjustment for the temperature of the calibration source may be made to further refine the accuracy of the correction factors. The calibration source may be activated on a periodic basis determined by the drift characteristics of the sensor being used, for example, every 5-10 minutes. 
     Because sensor drift may be exaggerated during periods of storage, especially storage at rated temperature extremes, calibration may be performed on a more frequent basis during the start up period of the device, for example, every three to five seconds. 
     These and other aspects and features of the disclosure will become more readily apparent upon reading the following detailed disclosure when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified and representative block diagram of an exemplary device used in non-contact temperature sensing; 
         FIG. 2  through  FIG. 4  are simplified and representative block diagrams showing exemplary device configurations for various readings taken by the device; 
         FIG. 5  is a flowchart illustrating an exemplary method of self calibration during operation over a period including initial device startup; and 
         FIG. 6  is a flowchart illustrating an exemplary method of collecting and applying readings used for self calibration. 
     
    
    
     While the disclosure is susceptible to various modifications and alternative constructions, certain illustrative embodiments thereof are shown in the drawings and will be described below in detail. It should be understood, however, that there is no intention to limit the disclosure to the specific embodiments disclosed, but on the contrary, the intention is to cover all modifications, alternative constructions, and equivalents falling within the spirit and scope of the disclosure as defined by the appended claims. 
     DETAILED DESCRIPTION 
     Referring now to the drawings, and with specific reference to  FIG. 1 , an apparatus for measuring temperature constructed in accordance with the teachings of the disclosure is generally referred to by reference numeral  100 . While the apparatus  100  can be used to measure the temperature of many objects, examples include, but are not limited to metal, glass, ceramics, and plastic. 
       FIG. 1  shows the apparatus  100  for measuring temperature in block diagram format. The apparatus  100  may include a sensor  102 , a chopper  104  having teeth or flags  106  operable to block or pass radiation from a target object  108 . The apparatus  100  may also include a calibration source  110 , shown in this exemplary embodiment as a light emitting diode (LED). A processor  112  may be used to control operation of the chopper  104  and LED  110 , as well as receive output signals from the sensor  102 . A memory  114  may be coupled to the processor and used to store in process readings, calibration data, executable code, lookup tables, etc. A display  114  may be used to read out temperatures measured by the sensor  102  as well as support calibration and device set up. An input/output (I/O) device  116  may be coupled to the processor and used to communicate measurement data, set up information, alarms, etc. with a process controller or other management station. 
     A clock or timer  118  may be used to measure intervals, both for operation of the chopper  104  as well as setting the time between measurements using the calibration source  110 . In some embodiments, the readings associated with the calibration source, for example, LED  110 , may require temperature compensation. Thermistor  120  may be used to measure the temperature of the calibration source  110  and appropriately adjust for changes in brightness vs. temperature. For example, a quadratic correction of the LED reading using measured temperature at the LED may be used. 
     The sensor  102  may be any of a variety of known sensors, for example lead sulfide, lead selenide, or mercury cadmium telluride. The chopper  104  may be a physical wheel with teeth  106  or may be a piezoelectric actuator attached to a flag used to pass or block radiation from the target object  108 . The chopper  104  may be responsive to control signals from the processor  112  for determining rotation speed or duty cycle or both. Alternatively, the processor  112  may simply sense chopper  104  activity and adjust calculations accordingly, if needed. The processor  112  may be a known single-chip computer or programmable logic controller and may include an analog to digital converter for conversion of the analog signal from the sensor  102  to a digital form for storage or further processing. The memory  114  may include both volatile and nonvolatile memory and may be used for both long-term storage of programs and settings as well as for storing process data generated during operation. The display  114  and I/O  116  may be known conventional devices suited to the task and operating environment. For example, in one embodiment the I/O may be a simple RS-232 serial interface, while a high-speed Ethernet (HSE) or other industrial standard may be used in a process control environment. 
       FIGS. 2 ,  3  and  4  show only those portions of the apparatus  100  required to illustrate various measurements taken in the calibration process. 
       FIG. 2  illustrates the apparatus  200  showing only the sensor  202 , chopper  204  with teeth  206 , target object  208 , and LED  210 . As illustrated, the chopper wheel is positioned so that radiation from the target object  208  passes directly to the sensor  202 . A signal proportional to the incident radiation may be transferred from the sensor  202  to the processor (not depicted). 
       FIG. 3  also illustrates an apparatus  300  with sensor  302 , chopper  304 , teeth  306 , target object  308 , and LED  310 . As illustrated, the chopper  304  has turned so that one of the teeth  306  has blocked the incident radiation from target object  308 . In one exemplary embodiment, the chopper  304  is turned to create alternating illuminating and blocking of the sensor  302  at a rate in the range of 260 to 320 Hz. 
       FIG. 4  illustrates an apparatus  400  with sensor  402 , chopper  404  with teeth  406 , target object  408 , and LED  410 . In this illustration, the chopper teeth  406  block incident radiation from the target object  408  while the LED  410  is illuminated under the control of the processor (not depicted). A signal corresponding to the radiation emitted by the LED is forwarded to the processor. 
     In operation, the apparatus  100  alternately takes readings at the sensor  102  of the target object  108  and a quiet value when the shutter  104  is closed to create a modulated signal output. In one embodiment, the ideal duty cycle between shutter open and closed is 50%. However, variations in manufacture and shutter operation may result in duty cycles that are not 50%. As discussed below, this variation in duty cycle may be measured and compensated for using a factor in the calibration equation. 
     To calibrate for long-term drift, a calibration reading may be taken periodically, for example, every several minutes or approximately every 50,000 shutter cycles. A calibration cycle may include a first reading of the chopper closed with the calibration source off and a second reading with the chopper open. These first two readings are identical to normal readings taken during operation. A third reading may be taken with the chopper closed and the calibration source on. The third reading may be temperature corrected as discussed above. It is easily seen that this singular third reading after a significant number of normal readings neither materially reduces the signal to noise of the measurement process nor jeopardizes an associated controlled process. 
     To calibrate for short-term drift after power up, the same three readings may be taken and processed, only on a much faster time period. For example, calibration readings may be taken every several seconds or approximately every 1,000 shutter cycles. 
     Operation and self-calibration of the apparatus  100  is discussed in more detail with respect to  FIG. 5  and  FIG. 6 , following. Sensors of the type commonly employed in optical thermal measurement devices may exhibit two kinds of drift. During long-term operation, a relatively slow drift is common. Capturing the data required to compensate for this slow drift may occur over a period of hours, or even days, and may be averaged with previous readings to smooth the calibration corrections. A second kind of drift may occur after storage, particularly after storage for long period of time at the extremes of its rated storage temperature. During operation after power up, this relatively fast drift may require compensation at a much quicker interval, for example, using measurements taken every several seconds. 
       FIG. 5  is a flowchart illustrating a method  500  of operating an optical temperature measurement device, such as apparatus  100  of  FIG. 1 . At block  502 , the apparatus may be started. As mentioned above, because the sensor  102  may drift when powered off, the calibration measurement period may be set to ‘fast’ at block  504 . At block  506 , the apparatus  100  waits for the sensor  102  to stabilize so that measurements may begin. At block  508 , readings may be taken and stored at the fast rate, for example, one calibration reading every 3-5 seconds. The current reading may be stored in a buffer area of memory  114  and averaged with previous readings already stored in the buffer. At block  510 , a determination may be made as to whether the buffer is full. If the buffer is not full, the no branch may be followed and an additional reading may be taken and used in the average. 
     If, at block  510 , the averaging buffer is full, the yes branch may be followed to block  512 . The averaging buffer size may be adjusted so that the expected short-term drift period is accommodated, or the cycles through the loop at block  510  may be adjusted accordingly. At block  512 , the time between calibration readings may be set to the longer measurement period, appropriate for correcting the slow drift associated with normal operation. At block  514 , calibration readings may be taken at the rate set by the longer period, for example, 10,000 cycles or greater. 
       FIG. 6 , a flowchart illustrating an exemplary method  600  of collecting and applying readings for self-calibration, is discussed and described. The method of  FIG. 6  may apply to the “get a reading” blocks  508  and  514  of  FIG. 5 . From a start at block  602  a determination is made at block  604  whether it is time to take a reading. If not, a delay is invoked as the no branch is followed from block  604  back to block  604 . When it is time to take a reading, the yes branch from block  604  may be taken to block  606 . If the chopper is not fully closed, the process waits until the chopper is fully closed, ensuring that radiation from the target object  108  is isolated from the sensor  102 . When the chopper is fully closed, processing continues at block  608 , where a first reading is taken and the analog output of the sensor  102  may be converted to a digital reading in an analog to digital converter (ADC), such as may be found in processor  112 . After the first reading at block  608 , the chopper is monitored at block  610  until it is fully open. When fully open, a reading is taken at block  612 . This reading corresponds to radiation emitted from the target object  108 . 
     At block  614 , the chopper is again monitored until it is fully closed. When closed, the calibration source  110 , for example, an LED, may be turned on at block  616 . At block  618 , another reading is taken corresponding to radiation emitted from the calibration source  110 . Using these three readings a factor may be calculated at block  620 . A calculation may be used to determine the factor: factor=reference number/((second reading−first reading)*n+third reading), where n is a measured number corresponding to the duty cycle of the chopper, approximately 0.5. The reference number is a number developed during calibration of the apparatus  100 , for example, as part of the manufacturing process. At block  622  the temperature of the calibration source  110  may be compensated and at block  624  the detector gain adjustment, or calibration, may be applied. 
     In an exemplary embodiment, the temperature corrected factor at block  622  may be stored in a buffer of recent readings and averaged, and the average value resulting is used to apply to the gain adjustment for readings until the next calibration cycle. 
     The aforementioned disclosure presents a method and apparatus offering significant benefits to users of optical thermal measurements. On-going calibration may be carried out during both early power-on use and over long periods, without interrupting operation for calibration and without introducing excessive noise or calibration source drift error. 
     The foregoing description of temperature measurement devices, methods of measuring temperature and determining calibration values for optical sensors have been set forth merely to illustrate the disclosure and are not intended to be limiting. Because modifications of the disclosed embodiments incorporating the spirit and substance of the disclosure may occur to persons skilled in the art, the disclosure should be construed to include everything within the scope of the claims to be presented and equivalents thereof.