Abstract:
A transducer assembly for measurement of downhole temperature and (if required) pressure includes a crystal resonator responsive to temperature and a pair of temperature sensors such as thermocouples or RTDs. One sensor is located with the crystal resonator and the other is exposed to external conditions such as those adjacent the pressure resonator. Thus, the sensors provide a correction signal for the resonator and allow improvement in the transient response of the transducer. A feedback circuit for correcting the resonator output is also disclosed.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a transducer assembly.  
         BACKGROUND ART  
         [0002]    Transducer assemblies are required for returning information as to (for example) temperature and pressure. A situation which present particular difficulty is in the reporting of conditions downhole in, for example, oil, gas, geothermal and other wells. The conditions of elevated temperature and pressure require a robust transducer which is able to offer an adequate service life and a reliable signal over that life.  
           [0003]    Downhole sensors are often based on crystal resonators, typically quartz. These can be cut (or otherwise selected and designed) so as to exhibit a resonant frequency which is responsive to ambient temperature, ambient pressure, neither, or a combination of both. Thus, a crystal resonator can be excited and will resonate at a frequency which, if measured, will give information as to the downhole conditions. Given the high frequencies at which the resonator operates, its signal is usually mixed with the signal of a reference resonator to produce a signal whose frequency range covers more readily measurable frequencies. The reference resonator would ideally have no temperature response, but in practice will usually have a small response. Nevertheless, so long as the reference resonator has a different temperature response as compared to the first resonator, a meaningful signal can be extracted.  
           [0004]    A third resonator is usually provided at the same ambient temperature but exposed to outside pressure via a suitable medium. The signal from this resonator can be mixed with that of the reference resonator in the same way. Transducers can thus comprise a temperature crystal, a pressure crystal, and a reference crystal.  
           [0005]    Resonant transducers of this type are described in U.S. Pat. No. 5,231,880 (Ward et al)., U.S. Pat. No. 3,335,949 (Elwood et al), and U.S. Pat. No. 4,802,370 (Eernisse et al).  
           [0006]    To protect the reference crystal and the temperature crystal from damage due to the external pressure, a housing is provided which is sufficiently robust to withstand the forces involved. The pressure crystal will typically be located in a compartment connected to the high external pressure. The reference crystal and temperature crystal are typically located in another compartment. This housing, or the bulkheads between the compartments, will have a significant thermal capacity and will therefore cause a delay in the response of the transducer to transients in temperature. The present invention seeks to overcome this difficulty.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention therefore provides a transducer assembly responsive to local temperature, comprising a first crystal resonator with a frequency response dependent on local temperature, a second crystal resonator with a frequency response with respect to local temperature which differs from the first crystal resonator, and a pair of temperature sensors, the first and second crystal resonators being located within a protective housing, wherein one sensor of the pair is sensitive to temperature within the housing and the remaining sensor is sensitive to temperature external to the housing.  
           [0008]    This allows the output of the temperature crystal to be corrected for temperature differentials between the two crystals and the real temperature of the pressure crystal to be determined. A transducer constructed in this way retains the wide temperature range of the resonator crystal-based sensors but can avoid the problems noted above. Sensors with a fine response to small differentials can be employed without those sensors having to have an adequate response to a wide variation in temperature.  
           [0009]    The second crystal resonator is (ideally) substantially insensitive to temperature. In practice there will be a slight response, but so long as this is characterised then it can be allowed for.  
           [0010]    It is preferred that the one sensor is within the housing and the remaining sensor is located outside the housing. This will then correspond to the locations of the crystals whose temperature is being sensed. Other arrangements are conceivable, however, in which the sensors are located elsewhere but are (for example) in a sufficiently intimate thermal contact with the crystal concerned, such as via a good thermal conductor.  
           [0011]    The housing will typically be one of a number of compartments within a larger assembly. Different compartments will operate at different pressures, for example.  
           [0012]    A third crystal resonator sensitive to ambient pressure can be provided. This is a common arrangement for transducers of this type. It is often the case that temperature is being measured (inter alia) to calibrate the pressure crystal which will also have a thermal response. As the pressure crystal must be exposed to the ambient pressure, it will usually be outside the housing (or compartment) containing the temperature crystal. Thus, the invention can provide more accurate calibration of such pressure crystals.  
           [0013]    The outputs of the first and second crystal resonators are preferably mixed within the transducer assembly as high frequency signals are best dealt with locally. It is also preferred that the output of the first crystal resonator (or the mixed output of the first and second crystal resonators) is combined with the output of the sensor pair.  
           [0014]    The frequency domain signal of the resonators can be combined by comparing it with the output of the combiner itself, and adjusting the latter if the difference is not that dictated by the voltage output of the pair of sensors. This is of course a form of feedback loop, which we have found to be stable and reliable. The combiner can convert the resonator signal and its own output into pulsed signals for comparison. Each pulse can represent a zero crossing of the frequency domain signal from which it is derived. If the pulse signals are of opposite polarity then they can be summed and integrated by way of comparison, which can also include the voltage output of the pair of sensors.  
           [0015]    It is also preferred that, after combining, the output of the combiner is converted to a frequency domain signal. This allows the transducer to be pin-for-pin compatible with an existing transducer. The output of the second and third crystal resonators (ie the pressure and reference crystals) are also preferably mixed within the transducer assembly.  
           [0016]    The temperature sensors are preferably thermocouples or temperature dependent resistors. The latter can be arranged in a bridge circuit together with a pair of fixed resistors.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    An example of the present invention will now be described by way of example, with reference to the accompanying figures, in which:  
         [0018]    [0018]FIG. 1 shows a transducer according to the present invention in section;  
         [0019]    [0019]FIG. 2 shows in schematic form the signal processing apparatus of the transducer shown in FIG. 1; and  
         [0020]    [0020]FIG. 3 shows an alternative circuit. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0021]    Referring to FIG. 1, the transducer assembly  10  comprises a housing  12  including a low pressure compartment  14  sealed from the process pressure and a high pressure compartment  16  connected to the process pressure to be measured. The two compartments are separated by an Internal wall  18  which is dimensioned to withstand the pressure differential involved. A port  20  leads to the high pressure compartment  16  and is threaded at  22  to accept standard connectors for pressure lines conveying liquid at the temperature and pressure to be measured.  
         [0022]    A pressure crystal  24  is located in the high pressure compartment  16 , and a circuit board  26  carrying a temperature crystal  28  and a reference crystal  30  is located in the low pressure compartment  14 . Pressure feedthroughs  32  allow wires  34  through the internal wall  18  to convey signals between the circuit board  26  and pressure crystal  24 .  
         [0023]    An electrical connector  36  provides communication with the circuit board  26 , allowing power to be delivered and signals to be extracted.  
         [0024]    The operation of the resonant pressure, temperature and reference crystals is described in U.S. Pat. No. 5,231,880 and U.S. Pat. No. 3,355,949, to which reference is made and the contents of which are hereby incorporated by reference. Although the present embodiment does not work in a directly analogous manner, the above patents do give detail as to the construction and operation of the crystals employed. In particular, it is noted that the crystals resonate at a frequency that is either substantially constant (or theoretically so) in the case of the reference crystal or is characteristic of the pressure and temperature conditions. Crystals can be made to respond with different sensitivities to either temperature or pressure, and thus whilst the three crystals cannot in practice be totally selective (or completely unresponsive), the three will react differently and simultaneous equations can be established to determine the two unknowns (temperature and pressure) from the three frequencies.  
         [0025]    Of course, in a well designed transducer the temperature and reference crystals will be isolated from the high pressure chamber by the internal wall  18  and will thus respond only to temperature. There will therefore be a difference in the frequencies which will be characteristic of a temperature. The temperature of the pressure crystal will then be known and thus the pressure can be deduced from knowledge of the resonant frequencies at that temperature.  
         [0026]    This depends on the pressure, temperature and reference crystals all being at the same temperature. Whilst this will be true at steady state, it will not be true during periods of changing temperature, due to the thermal inertia of the housing  12  and the internal wall  18 . According to the present invention, therefore, a pair of thermocouples  38 ,  40  are provided. A first thermocouple  38  is located adjacent the temperature crystal, whilst a second thermocouple  40  is located adjacent the pressure crystal. Feedthroughs  44  allow the wires of the thermocouple through the internal wall  18 . The thermocouples therefore measure the temperature difference between the two and permit the output of the temperature crystal to be corrected.  
         [0027]    [0027]FIG. 2 shows a schematic of the signal processing. The pressure, reference and temperature crystals  24 ,  30 ,  28  are shown with signals therefrom being mixed (as is known in the prior art) by mixers  46 ,  48 . Mixer  46  combines the signals of the pressure and reference crystals and produces a frequency domain signal f p  at 50 that is delivered elsewhere for interpretation as a pressure level. Mixer  48  combines the signals from the temperature and reference crystals to provide a frequency domain signal f T  which is representative of the temperature in the vicinity of the temperature crystal. This is fed to a correction circuit  52  which produces a frequency domain output f T+ΔT  which is representative of the temperature at the pressure crystal.  
         [0028]    External to the correction circuit  52  are the thermocouples  38 ,  40 . These are arranged in series but with polarities opposed. Thus, if both are at the same temperature then the emfs generated by each are equal but opposite and the output voltage will be zero. One end of the series is grounded and the other and is provided as a temperature difference signal V ΔT . The respective ends of the series are chosen such that a positive difference in temperature yields a negative voltage to the correction circuit  52 .  
         [0029]    Within the correction circuit  52 , the f T  and f T+ΔT  signals are fed to a Programmable Logic Unit (PLU)  54  which is programmed to act as a two channel zero crossing detector. A first channel acts on the f T  signal and produces a negative voltage pulse when the frequency domain f T  signal shows a zero crossing. Thus, the number of pulses and hence the average (negative) voltage output on this channel will be proportional to the frequency of the f T  signal. A second channel acts on the f T+ΔT  signal and produces a positive pulse when a zero crossing is detected. Thus, the number of pulses and hence the average (positive) voltage output on this channel will be proportional to the frequency of the f T+ΔT  signal.  
         [0030]    The voltage signals output on each of these channels, together with the voltage output of the thermocouple pair, are fed into a summing and integrating amplifier  56 . This adds the three signals and gives a voltage output which is a time integral of the sum. That voltage is passed to a V−f converter  58  which converts it to the corresponding frequency, output as f T+ΔT  at 60 and fed back to the PLU  54 .  
         [0031]    The operation of the circuit will now be described. The summing integrator will stabilise when the three currents flowing sum to zero. The average current (ie smoothing the pulsed nature of the signal) flowing from the first channel of the PLU  54 , ie that dealing with the f T  signal, will be:  
               i   T     =       -     F   T                         W                 V       R   1                 (   1   )                               
 
         [0032]    where V is the pulse height voltage and W is the pulse width. Note that i T  is negative since this channel outputs a negative pulse for each zero crossing. Likewise, the average current flowing from the second channel will be:  
               i     T   +     Δ                 T         =       +     F     T   +     Δ                 T                  W                 V       R   1                 (   2   )                               
 
         [0033]    The thermocouples are arranged so that a negative voltage V ΔT  is generated for a positive temperature difference ΔT between the pressure and temperature crystals, ie the pressure crystal being at a higher temperature than the temperature crystal. Thus: 
           V   ΔT   =−kΔT   (3) 
         [0034]    where k is the thermocouple constant in units of V/° C. The associated current is then given by:  
               i     Δ                 T       =     -       k                 Δ                 T       R   2                 (   4   )                               
 
         [0035]    If we sum the currents,  
                   F     T   +     Δ                 T                W                 V       R   1         -       F   T                       W                 V       R   1         -       k                 Δ                 T       R   2         =   0           (   5   )                 F     T   +     Δ                 T         =       F   T     +         k                 Δ                 T       R   2              R   1       W                 V                   (   6   )                       is                          F     T   +     Δ                 T         =       F   T     +     C                 Δ                 T                     (   7   )                     where           C   =       k                   R   1           R   2        W                 V                     (   8   )                               
 
         [0036]    It is then a matter of selecting k, R 1 , R 2 , W and V so that C has the same value as the temperature crystal response.  
         [0037]    It can be seen that the loop is stable since if a net positive (say) current flows into the integrator, then the output voltage will drop. This reduces the V−f frequency output, reducing the number of zero crossings, and reducing i T+ΔT . This will reduce the net current and stabilise the system. Thus, the loop will stabilise to a point where the difference (if any) between the f T  and f T+ΔT  signals is governed by the signal from the thermocouples.  
         [0038]    The operation of the circuit can also be described by considering specific examples of situations. A first situation is steady state—ie a settled system with no temperature difference. Thus, f T  and f T+ΔT  are the same, as ΔT=0. Both signals will therefore be converted into equal but opposite pulses and the net will be zero. The sum (over time) of the signals fed to the integrator  56  will be zero and its output will not change The output of the V−f converter (f T+ΔT ) will thus be steady and the system will remain in the same state.  
         [0039]    A second situation arises when the temperature in the vicinity of the pressure crystal then rises slightly. This will not be reflected at the temperature crystal immediately due to the thermal inertia of the system. There will therefore be no change in f T  initially. As there will be a temperature differential, ΔT will be positive and V ΔT  will be therefore negative giving rise to a negative current flowing into the integrator from the thermocouples. As f T  has not changed, there will be no change in the net (zero) current from the two channels of the PLU  54 . There will thus be an imbalance, in that a negative net current will flow into the integrator, and as this is based on a standard inverting amplifier the output will rise. This will cause the V−f converter to raise f T+ΔT . Eventually, f T+ΔT  will generate more positive pulses than the negative pulses generated by f T , and the PLU  54  will be causing a net positive current to flow into the integrator. This will eventually balance the net negative current i ΔT , and a new steady state will be reached. In this steady state, f T+ΔT  exceeds by an amount governed by V ΔT . Thus, the circuit is correcting f T  for the temperature differential between the temperature and pressure crystals.  
         [0040]    If we than assume that the system reaches a new thermal equilibrium, is the ambient temperature around the temperature crystal rises to match that around the pressure crystal, then V ΔT  will fall to zero as f T  rises correspondingly. Thus, there is a reduction in (the negative value of) i ΔT  but an increase in the number of (negative) pulses generated by the f T  signal. These balance, leaving no net current change into the integrator, no net change in its output and no change to f T+ΔT . Thus, the output of the transducer does not change, reflecting the fact that there has been no change to the actual temperature around the pressure crystal.  
         [0041]    It is not essential to use this circuit. Other circuits or integrated circuits could perform a similar function. Indeed, no correction need be provided at the transducer itself and the V ΔT  signal from the thermocouples could be brought out together with the frequency domain signals from the crystals for processing elsewhere, The ΔT signal could be digitised separately and the three signals combined in firmware or software. In a simple arrangement, the V ΔT  signal could be used to trigger an indicator or alarm when above a certain level to signify that the measured values are at a lower accuracy due to thermal errors. However, processing the signal in the transducer as described above gives a accurate signal in a transducer that can be made pin-for-pin compatible with an existing transducer.  
         [0042]    Another advantage of extracting the V ΔT  signal from the transducer is that it can be used (for example in software) to detect long term drift of the thermocouple output. As the long term average of ΔT will be zero, a very low pass filter output of V ΔT  should be zero and the use of such a filter (with a time period of at least an hour and preferably greater than 24 hours) can provide a correction signal.  
         [0043]    [0043]FIG. 3 shows an alternative arrangement not relying on thermocouples. Instead, a pair of temperature dependent resistors (RTDs)  62 ,  64  are provided, one located adjacent the temperature crystal and one adjacent the pressure crystal. These are arranged in a bridge circuit with two other calibration resistors  66 ,  68  to give a voltage difference which varies with the temperature difference. This is fed to the summing and integrating amplifier  56 ′. The remainder of the circuit is as shown in FIG. 2.  
         [0044]    It will be appreciated by those skilled in the art that many variations may be made to the above-described embodiments without departing from the scope of the present invention. For example, although three separate crystal resonators are shown, these could be combined such as is shown in U.S. Pat. No. 4,872,765. In this document, the temperature and reference signals are supplied by different harmonics of the same physical crystal.  
         [0045]    In another alternative arrangement, the PLU  54  could be re-configured to issue a train of pulses whose sign and magnitude is dependent on the sign and magnitude of an f ΔT  signal that was the difference between f T  and f T+ΔT . This would avoid any need to match the resistances R 1  and to match the pulse widths of the i T  and i T+ΔT  signals. The magnitude of the pulse train is of course dependent on the pulse height, width and frequency.