Abstract:
A remotely programmable integrated sensor transmitter device for measuring and reporting a physical quantity of a given medium comprises a sensor for measuring a physical quantity of a medium and providing an electrical output as a function of the property measured, a scaler module for receiving the electrical output and for producing a scaled analog signal as a function of the physical quantity and a scale selection input, and a data interface for receiving programming data from an external computer and for providing the scale selection output to the scaler module.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to temperature sensors, more particularly to a temperature sensor including an RTD element, a conditioning circuit, a scaler unit and a combined digital and analog transmitter, all in a single compact assembly. The remotely programmable integrated sensor transmitter can also be re-calibrated and re-scaled over its entire range (−200° C. to 800° C.).  
         BACKGROUND OF THE INVENTION  
         [0002]    The temperature is often a critical variable that needs to be measured accurately in various industrial processes. Today, the RTD (Resistive Temperature Detector) is the most popular device used in temperature control. With an RTD element, we can obtain a resolution of hundredth or even thousandths of a degree centigrade in an ambient or moderate temperature application.  
           [0003]    The temperature cannot be measured directly from the RTD element. It has to be calculated from the measurement of a dependant variable that has a known relation with the temperature at which it is exposed. Unfortunately this relation, which is well known by the people working in this field, is not linear and for most of the application it is not convenient to have a non-linear signal. To solve this problem, many solutions have been proposed. German Patent No. 2,459,623 to Bruyere discloses a design in which an extra resistor is connected between an amplifier output and input and a referenced resistor. This method of linearization is not that accurate, greater than one part in thousand over the entire range, and is also highly dependent on the component tolerance used in the circuit. That, obviously, poses manufacturing problems.  
           [0004]    Other techniques have been proposed in U.S. Pat. No. 4,000,454 and U.S. Pat. No. 4,556,330 where they used an external linearization apparatus in which the voltage impressed to the conditioning section (a Wheatstone Bridge) changes as the resistance of the element changes. However, this arrangement contained also some disadvantages. First, the external placement of the linearization module provides, often, inaccurate readings by the fact that the sensing element and the linearization module are not integrated. Secondly, those devices are known to be unstable, whereby the failure of one or more components result in the failure of the entire device. Finally, the use of separate components increases the manufacturing and installation cost and can also be a problem on installation where the clearance is limited.  
           [0005]    To overcome those problems, the U.S. Pat. No. 5,741,074 proposed a linear integrated sensing transmitter. This transmitter integrates, in a single housing, the temperature sensing device and a current transmitter. The linearization is performed via a feedback resistor, a return path resistor, and a constant current source, all connected to a voltage-to-current converter. This arrangement gives a good linearization and offers an accuracy of &lt;0.1% of the full-scale. However, this device has also many lacks. First, the device cannot be calibrated to take in account the tolerance of the RTD element or the components themselves. That implies problems in a manufacturing point of view by the fact that we have to take a special care to the components selection. If the device is not perfectly linear, or offset, there is no way to correct it. Secondly, to change the range of operation of the device, we have to re-calculate and change all the resistor values. That is also causing a problem because we need different configuration for different range of operation.  
           [0006]    The purpose of the present invention is to overcome those problems by suggesting an integrated transmitter, which includes the sensing element, a current transmitter and a scaler unit. The scaler unit gives more flexibility to the device and allows the calibration and re-scaling of the device via a digital communication between the device itself, and a hand-held programmer or a computer.  
         SUMMARY OF THE INVENTION  
         [0007]    Accordingly, an object of the present invention is to provide a remotely programmable sensor and device for measuring a physical quantity of a medium comprising a sensor for measuring the physical quantity of the medium and providing an electrical output; a scaler module for receiving the sensor output and for producing a scaled analog signal as a function of the measured physical quantity and a scale selection input; and a data interface for receiving programming data from an external computer and for providing the scale selection output to the scaler module. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    The other features and advantages of the invention will be apparent from the following detailed description of each drawing.  
         [0009]    [0009]FIG. 1 represents the mechanical construction of the integrated sensor transmitter.  
         [0010]    [0010]FIG. 3 is a block diagram of a digital embodiment of the present invention.  
         [0011]    [0011]FIG. 4 is a block diagram of an analog embodiment of the present invention.  
         [0012]    [0012]FIG. 5 a  is a block diagram of the remotely programmable integrated sensor transmitter configured for RS-232 communication.  
         [0013]    [0013]FIG. 5 b  is a block diagram of the remotely programmable integrated sensor transmitter configured for RS-485 communication.  
         [0014]    [0014]FIG. 5 c  is a block diagram of the remotely programmable integrated sensor transmitter configured for FSK communication with HART protocol.  
         [0015]    [0015]FIG. 6 represents the flow chart of the program inside the scaler unit  
         [0016]    [0016]FIG. 7 is a tree diagram of the different calibration procedures available to the remotely programmable integrated sensor.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0017]    With reference now to FIG. 1 there is shown an exemplary embodiment of the integrated resistance temperature detector and transmitter of the present invention. The sensor comprises an elongated cylindrical housing  11 , 13  for receiving a miniaturized transmitter  15  coupled to a resistance temperature detector  17 ,  19 .  
         [0018]    The housing  11 , 13  is preferably fabricated from Inconel™ or a 316 stainless steel, although it can be fabricated from any suitable metal which is capable of protecting the sensing element  17 , 19  while quickly responding to changes in temperature. The housing  11 , 13  comprises a cylindrical tip portion  13  and a cylindrical transition portion  11 . The tip portion  13  and the transition portion  11  are connected together by crimping, soldering, bonding or welding the transition portion around the tip portion  21 , the assembled housing  11 , 13  defines a cavity therethrough  23 , 25 .  
         [0019]    In the exemplary embodiment shown in FIG. 1 the transition portion of the housing  11  has a length of 2½″ and an outer diameter of ⅝″ (0.625). The tip portion of the housing  13  has a length of 12″ and an outer diameter of ¼″. It should be understood that the above dimensions are merely illustrative and may be altered to adapt the sensor to different applications.  
         [0020]    Disposed within the cavity of the tip portion  25  of the housing is the resistance temperature detector  17 ,  19  which comprises a 100 ohm, 0.00385 alpha Class B type bulb  17 , although a Class A bulb can be substituted. Disposed within the bulb is a platinum resistive element  19 . The resistive element  17 , 19  includes a first platinum lead  27  and second platinum lead  29  which extend from within the bulb. Again it is to be understood that the present invention is not restricted to the above resistance temperature detector; other temperature sensing means, such as a thermistor or a thermocouple, fall within the scope of this invention.  
         [0021]    Disposed within the transition portion of the housing  11  is the miniaturized transmitter  15 . The exemplary embodiment shown in FIG. 1 incorporates a transmitter  15  with 4-20 mA output signal  31 , 33 . Again it is to be understood that the present invention is not restricted to the above output signal; other signal output means, such as voltage, frequency or digital, fall within the scope of this invention. The transmitter  15  is miniaturized using well know surface mount technology. At the input section of the transmitter  39 , there are four terminals, comprising of the first, second, third and a fourth input terminals. Minimum two input terminals are required from the sensing element  17 , 19 . At the output section of the transmitter  41 , there are four terminals comprising a first output terminal  31  and a second output terminal  33 , a first communication terminal  35  and a second communication terminal  37 . (Additional terminals may be used, depending on the communication protocol.) The transmitter is secured within the cavity  23  of the transition portion of the housing  11  with an amount of sealant or any suitable potting compound.  
         [0022]    [0022]FIG. 2 is a block diagram of a remotely programmable integrated sensor transmitter. A sensor  17 , 19  is in primary contact with a process medium, measuring particular processes of that medium, such as temperature, pressure, etc. by relating those properties to electrical signals, such as voltage, current, etc. In the preferred embodiment, the sensor  17 , 19  is used for measuring temperature and it is a resistance temperature detector, while it could also be a thermistor, a thermocouple, an IC sensor, etc. The sensor  17 , 19  creates an electrical signal as a result of a changing property of the process medium. The electrical signal is sent to a scaler module  10 . The scaler module  10  receives scale input information from a communication module  12 . The scale input information allows the scaler module  10  to convert the electrical signal received from the sensor  17  into a scaled analog signal according to a desired scale. The scaled analog signal produced is then output.  
         [0023]    FIG. is a block diagram representing one possible embodiment of the integrated sensor transmitter. At the extremity, the Resistive Temperature Detector (RTD)  17  is connected to the conditioning circuit of the integrated sensor transmitter  16 ,  18 . This conditioning module is composed of a Wheatstone Bridge  16  and an amplifier  18 . The Bridge  16  produces a small voltage across its extremities when the RTD resistance  17  changes with temperature. This small voltage is then amplified  18  and directed to the Analog to Digital converter  20 . The conditioning module  16 , 18  was designed in such a way as to be able to measure a change of resistance of the RTD  17  from 15 to 380 ohms. That covers the entire range of the integrated sensor transmitter which is −200° to 800° C.  
         [0024]    The Analog to Digital converter  20  converts the analog signal to a digital value which is read by a digital scaler  22 . The high resolution of the Analog to Digital converter  20  allows a high precision measurement over the entire range of operation and eliminates the need for re-scaling the conditioning module. The Analog to Digital converter  20  has also, a built in auto-calibration feature. This feature allows a periodically auto re-calibration of the device to eliminate any drift due to a change of temperature.  
         [0025]    The Analog to Digital converter and the Digital to Analog converter are controlled by the digital scaler  10   a . The digital scaler  10   a  receives calibration parameters, range information and device information and identification (address) from a communication module  12 . The communication module  12  is connected to the external world for exchanging data and calibration of the device via a digital communication link  35 , 37 . The flexibility of the device allows for different modes of communication, as shown in FIGS. 5 a ,  5   b  and  5   c . The standard communication interface is RS-232, but a communication interface board can be mounted in piggy back with the integrated sensor transmitter to offer an RS-485 or FSK (Frequency Shift Key) with HART protocol. The digital communication will be explained in details later.  
         [0026]    [0026]FIG. 4 is a block diagram of another embodiment of the present invention. A sensor  17 , 19  is in communication with a conditioning module comprising a Wheatstone bridge  16  and an amplifier  18  of variable gain. The Bridge  16  produces a small voltage across its extremities when the RTD resistance  16  changes with temperature. This small voltage is then amplified by an amplifier  18  and output as a current signal of 4 mA to 20 mA. The conditioning module was designed in a such way as to be able to measure a change of resistance of the RTD from 15 to 380 ohms, covering the entire range of the integrated sensor transmitter. A variable resistor  34 , such as a potentiometer, is connected in parallel to the Wheatstone bridge  16  for controlling the resistances therein. The variable resistor  34  is controlled by the communication module  12 . The communication module  12  also controls the gain of the amplifier  18  in order to produce an appropriate analog output value.  
         [0027]    Digital Communication  
         [0028]    The digital communication allows configuration of the integrated sensor transmitter as well as the identification (address), calibration, re-scaling and reading of temperature. FIG. 5 a  shows the standard serial method of communication, over two wires with the RS-232 standard . In this configuration, the integrated sensor transmitter is connected to an interface communication module  39   a . This module  39   a  converts the low voltage signal (TTL-5V)  37  to an RS-232 standard  41   a  that can be read by a computer  43  or a hand held calibrator  49 . The module  39   a  can also provide the power for the integrated sensor transmitter.  
         [0029]    [0029]FIG. 5 b  represents the second method of digital communication available, the RS-485 standard. The integrated sensor transmitter is connected to an interface communication module  39   b . This module  39   b  converts the low voltage signal (TTL-5V)  37  to an RS-485 standard  41   b  that can be read by a computer  43  or a hand held calibrator  49 . The module  39   b  can also provide the power for the integrated sensor transmitter. The RS-485 standard allows longer distance between the integrated sensor transmitter itself and the computer  43  or other calibration device. It also allows for operation of multiple integrated sensor transmitters connected on the same link, each integrated sensor transmitter being identified by its unique address.  
         [0030]    [0030]FIG. 5 c  represents the third method of digital communication, via a Frequency Shift Key (FSK) superimposed on the current loop. In this configuration, the interface communication module  39   c  is a modulator/demodulator (MODEM) that converts the signal  37  from the integrated sensor transmitter to a FSK and superimposed this frequency signal on the current loop  41   c . This module  39   c  is mounted in piggyback with the integrated sensor transmitter and both are encapsulated in the hand of the integrated sensor transmitter. The digital data can then be achieved to a calibrator  49  having the FSK implemented or to a computer  43  via a modulator/demodulator (MODEM)  45  which convert the FSK to an RS-232 standard signal  51 . The main advantage of this method of communication is having only two wires coming out of the integrated sensor transmitter for powering, analog output and digital communication. In this configuration, the transmitter uses the HART protocol, which is a well known standard in the industry, to dialogue with other equipment. Independently the digital communication used, the 4 to 20 mA current loop  31 ,  33  is always available to read the temperature. The integrated sensor transmitter can work as a stand-alone unit without digital communication. The purpose of the digital communication is to allow for re-calibration or re-scaling of the device.  
         [0031]    Program  
         [0032]    In the digital scaler  22 , a program manages the functioning of the integrated sensor transmitter. FIG. 6 shows the flow chart of this program. During the boot up sequence, the processor first initializes the memory and all the peripherals  53 . After that, it retrieves the calibration and range information from the memory  55  and performs the initialization and calibration of the Analog to Digital converter  57 . The program then enters in the main loop and check for an external command on the serial port  59 . If data is present on serial port, then the scaler unit accomplishes the task associated to the code in accordance with the communication protocol  61 . The next step is to check for auto-calibration of the Analog to Digital converter  63 . If this is the case the scaler unit commands the auto-calibration  65 . The following action is the reading of the input signal from the sensing element via the Analog to Digital converter  67 . The result, under a digital format, is then filtered digitally by an algorithm implemented in the processor  69 . If the result is higher or lower than a certain threshold value, the sensing element is considered failed  71 . If this is the case the current output is set to a maximum  87  or minimum  85  value depending the configuration  73 . If the signal read from the sensing element is in the limit of operation, then a factory offset factor is applied  75  to it. The factory offset and others calibration features are explain in the next section. After the factory offset, the reading is linearized by the processor  77  and the input  79  and output  81  calibration are performed. Finally, the current loop is set  83  to the corresponding reading.  
         [0033]    Calibration and Scaling of the Remotely Programmable Integrated Sensor Transmitter  
         [0034]    One of the most important features of this invention is the fact that the device can be re-calibrated and re-scaled at any time without having to change the device physically.  
         [0035]    This is a big advantage for a manufacturing point of view because all the devices are assembled with the same components. No need to change any resistors or amplifiers to match a different range. The fact that the device can be calibrated allows also a certain tolerance for all the components, including the sensing element. That tolerance is compensated by the calibration. For the end-user this is also an advantage. The sensor can be re-scaled for any particular application and this mean that the same sensor can work for different section of the same process, which are not in the same temperature range, and keep a high resolution on the output. The calibration and/or re-scaling are performed via the digital communication link. FIG. 7 shows all the different calibrations that can be performed on the integrated sensor transmitter.  
         [0036]    A total of 5 different calibrations are available  89 . The first one is the output calibration  91 . This action allows the calibration of the output current generated by the Digital to Analog converter. It is performed on two points located at the extremity of current range, at 4 mA  101  and 20 mA  103 . Two output calibration parameters are then calculated by the computer  43  or hand held calibrator  49  and then stored for output calculation.  
         [0037]    The second calibration procedure is factory offset  93 . The factory offset is performed by recording the reading form the sensing element at a pre-determined temperature  105 . This reading represents the offset from this particular device to the theoretical values used for linearization.  
         [0038]    The third calibration procedure is the operating range  95 . The operating range can be set anywhere inside the total span of the integrated sensor transmitter, which is −200° to 800° C. When the range is changed, default values of calibration (Zero and Span) are set  107  to have the output current swings from 4 to 20 mA between the low and high values of the range. The range value is stored in the digital scaler module  22 .  
         [0039]    The fourth calibration procedure is the 1 Point Calibration  97 . This calibration is perform by adjusting the offset parameter  109  in order to have the output current from the integrated sensor transmitter matched with the known temperature at which the sensing element is exposed. For this calibration, one external reference is needed.  
         [0040]    Finally, the fifth calibration procedure is the 2 Points Calibration  99 . This calibration is performed when we want to get the most accurate precision form the integrated sensor transmitter on a given range. Two external reference points are needed for this procedure. When the sensing element is at the first reference point the value is then recorded  111  in memory. After, the sensing element is brought to the second reference point and the value is recorded  113  in memory. The last operation is the calculation of the new calibration parameters (Zero and Span)  115 . This is performed by the processor in the digital scaler  22  and can be done at any time.  
         [0041]    It will be understood that numerous modifications thereto will appear to those skilled in the art. Accordingly, the above description and accompanying drawings should be taken as illustrative of the invention and not in a limiting sense. It will further be understood that it is intended to cover any variations, uses, or adaptations of the invention following, in general, the principles of the invention and including such departures from the present disclosure as come within known or customary practice within the art to which the invention pertains and as may be applied to the essential features herein before set forth, and as follows in the scope of the appended claims.