Abstract:
The invention disclosed herein is a pressure transducer utilizing a Hall effect sensor to detect the displacement of the transducer&#39;s diaphragm. The pressure transducer includes a power supply to draw current from a 4 mA to 20 mA current loop and deliver current to the Hall effect sensor at an average rate of 4 mA or less.

Description:
RELATED APPLICATIONS  
       [0001]     This Application claims the benefit of U.S. Provisional Application No. 60/541,136, filed on Feb. 2, 2004. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The invention relates to differential pressure transducers and transmitters for measuring the pressure of a fluid.  
       BACKGROUND OF THE INVENTION  
       [0003]     The invention described herein is a pressure transducer utilizing a magnet and Hall effect sensor to convert mechanical movement into electrical signals. The pressure transducer without a Hall effect sensor is well understood in the art. A pressure transducer typically includes two plenums or chambers separated by a flexible diaphragm. The plenums are subject to a fluid, such as a gas or a liquid, under different pressures. This pressure differential causes displacement of the diaphragm in the direction of the plenum exposed to the lower pressure. The diaphragm is connected to a mechanical component, which moves as the diaphragm is displaced.  
         [0004]     For example, the mechanical component could be a piezoelectric strip having two ends, one end fixed to a plenum wall or other fixed point of reference and the other end fixed to the diaphragm, and thus moving with the diaphragm. The electrical properties of the piezoelectric strip change as it is bent by the movement of the diaphragm. The electrical change can then be processed by other electrical components to indicate the pressure value. Other devices to convert the mechanical movement into electric signals are also used to convert the movement of the diaphragm as recognized by one skilled in the art.  
         [0005]     In the present invention, a magnet and Hall effect sensor are used to convert the mechanical movement to electric signals. It should be understood that the power conserving features of the invention disclosed herein need not be limited to the use of a Hall effect device, but may be applied to implementations using other structures for sensing movement as well. The power circuit can also be used in other applications where a low average current draw is required.  
         [0006]     The use of a Hall effect sensor and a magnet allows for physical separation of the mechanical and electrical components of the pressure transducer, since the interaction between the magnet and the Hall effect sensor can take place with material between them. The preferred embodiment shows an arrangement where the fluid in the differential pressure sensor and the mechanical beam and magnet are separated from the electronics, including the Hall effect sensor, by a wall.  
         [0007]     Many pressure transmitter applications use a 4 to 20 mA current loop to operate. In such instances, the 4 to 20 mA current loop provides both device power and signal path for the control signal. Most current loops used in the applications are specified to run between 10 and 35V supply voltage. As a result of the supply voltage and current constraints, the pressure transmitter should be capable of operation at a power level equal to the lowest power available, or 4 mA×10V=40 mWatts. In other words, because the power available can vary with the current signal, and from application to application, the maximum amount of power that can be relied upon is 40 m Watts. Thus, the device needs to be able to run on that amount of power, even if a higher amount is available in some applications.  
         [0008]     A problem arises in that most Hall effect devices available today consume more power than 40 mW. For example, the HAL805 and HAL810 available from Micronas need at least 10 mA of current available at a minimum of 4.5V to work. This is a minimum power load of 10 mA×4.5V=45 mW to achieve operation. With such power requirements, not only is there no power left over to run the rest of the transmitter, there is not enough power to run the Hall effect sensor itself.  
         [0009]     The problem is solved by alternately applying Power to the Hall sensor, making a measurement, then removing power for an extended time interval. In such a fashion, the duty cycle of the Hall sensor is adjusted. The position of the diaphragm or beam of the transducer does not need to be continuously monitored. In most applications, the Hall effect sensor can be turned off for substantial portions of the time, thus saving substantial amounts of power. In typical applications, the duty cycle of the sensor can be set anywhere from 1:2 to 1:100, although other ranges beyond those figures are possible. While other ranges are possible, the limitations of the average current draw and surge current to the sensor still must be achieved. In some embodiments, the duty cycle or period applying power can be selectively varied by the user. Such a selective circuit may include a multi-position switch, or other structure to allow a user to select from a plurality of timing circuits.  
         [0010]     In the case of an integrated semiconductor sensor, such as the HAL810 from Micronas, the sensor can be turned on and a stabilized reading made in approximately 40 mSeconds. If a reading is taken once every 400 mSeconds, the average power requirement of the sensor can be reduced by a factor of ten. This means the average power draw of the Hall effect sensor is reduced to 4.5 mW for an average current draw from the sensor of only 1 mA at 4.5 volts. This is an amount of power that can be delivered to the sensor in a current loop situation. The remaining approximately 35 mW of available power can be used to run the rest of the transmitter.  
         [0011]     Similarly acceptable duty cycles can be done with most other Hall effect sensors depending upon their power draw and start up and stabilization times. To achieve the power reduction, a suitable power supply is needed for the Hall effect sensor that can draw energy from the 4 to 20 mA loop on the average basis and deliver energy to the Hall effect sensor on the peak basis that the sensor requires. As described in the preferred embodiment, a low pass filter is utilized to draw power from the current loop and store the power until it is provided to power the Hall effect sensor. One skilled in the art will recognize that the power supply described herein is but one of many power supplies that can meet the goals described above. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is a sectional view of a three chamber pressure transducer incorporating a Hall effect sensor.  
         [0013]      FIG. 2  is an electrical schematic of the preferred embodiment for a Hall effect device power supply.  
         [0014]      FIG. 3  is a perspective view showing the positioning of a Hall effect sensor and magnet on a pointer display instrument. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0015]     With reference to  FIG. 1 , the pressure transducer  10  includes three isolated chambers or plenums, an upper pressure chamber  15  above the diaphragm  20 , a lower pressure chamber  16  below the diaphragm  20 , and a separate electronics chamber  17  above the upper pressure chamber  15 . A flexible diaphragm  20  separates the upper and lower chambers. The upper chamber  15  and the lower chamber  16  include ports  25 A and  25 B to allow fluid communication with the plenums defined by their respective walls. The upper chamber  15  includes a beam  30  affixed to the diaphragm  20 . The beam  30  is coupled to and moves in conjoined relationship with the diaphragm  20 . In the preferred embodiment, the beam  30  is coupled to the diaphragm  20  by a post  31  extending perpendicularly from the diaphragm  20 . A magnet  35  is attached to one end of the beam  30 . The beam  30  is attached to the post  31 , so that the beam  30  and magnet  35  are generally balanced about the post  31 , so as to minimize any twisting forces on the diaphragm  20 . One skilled in the art will recognize that the beam and post can take on many different shapes and configurations. In the preferred embodiment, the beam  30  is a spring upon which the diaphragm  20  applies a force proportional to the differential pressure on either side of the diaphragm  20 . Movement of the spring is in proportion to the force applied and hence displacement of the magnet  35  is in direct proportion to the differential pressure. Other structures to convey the movement of the diaphragm  20  to the magnet  35  are possible.  
         [0016]     In other embodiments, the post  31  and beam  30  may be eliminated, and the magnet  35  attached directly to the diaphragm  20 . Alternatively, a single structural member may attach the magnet  35  to the diaphragm  20 .  
         [0017]     The magnet  35  is positioned in working relationship with a Hall effect sensor  40  located in the electronics chamber  17 , thus permitting the magnetic field of the magnet  35  to generate a voltage response or signal in the Hall effect sensor  40 . Although in the preferred embodiment the Hall effect sensor  40  is located in a separate chamber from the magnet  35 , one skilled in the art will recognize that the Hall effect sensor  40  need not be located in the electronics chamber  17 , but may be located in the same chamber as the beam  30  or magnet  35 . The Hall effect sensor  40  and magnet  35 , power supply described herein may be used with two-chambered transducers, not needing a separate chamber for the electronics. Thereby, one skilled in the art will recognize the Hall effect sensor  40  could also be placed in the lower pressure chamber  16 .  
         [0018]     Because the Hall effect sensor  40  is not in direct contact with the beam  30  or magnet  35 , a chamber wall  50  may be placed between the Hall effect sensor  40  and the beam  30  or the magnet  35 . As shown in the preferred embodiment, the Hall effect sensor  40  and the magnet  35  are separated by the wall  50  defining the division between the electronics chamber  17  and the upper pressure chamber  15 . With this arrangement, the electronics of the transducer are separated from the fluid being monitored. As one skilled in the art will recognize from the teachings of this invention, the electronics chamber  17  may be eliminated, and the electronics, including the Hall effect sensor  40  can reside on the outside of the housing defining the outer boundaries of the upper pressure chamber  15  and the lower pressure chamber  16 . However, placing the electronics in a separate chamber provides protection from environmental elements, and is preferred.  
         [0019]     In the preferred embodiment, the electronics chamber  17  includes the Hall effect sensor  40  positioned so that it is magnetically coupled with the magnet  35 . The Hall effect sensor  40  includes wiring or other electronic pathways  47  to connect the Hall effect sensor to suitable electronic circuitry for displaying pressure information, or transmitting pressure information to control electronics. One skilled in the art will readily recognize such a transmitter or display circuitry would convert the voltage response of the Hall effect sensor  40  into a current signal and transmit the signal on the current loop.  
         [0020]     With careful selections of a sensor, the arrangement in  FIG. 1  senses the motion of the beam  30  linearly. Thus, the arrangement is useful for pressure transmitters as well as pressure switches.  
         [0021]      FIG. 2  shows an implementation of a power supply  90  for the HAL810 Hall effect sensor of the preferred embodiment. It can be easily adapted to other Hall effect sensors. The 5 volt supply shown in the power supply schematic of  FIG. 2  is derived from a linear regulator (not shown) running from the 10 to 35 Volt input of the current loop. This 5V supply also supplies all the other transmitter power requirements. The linear regulator is preferred because it is usually the lowest cost method of providing power even though it limits the available power to the entire transmitter to 4 mA at 5 volts or 5V×4 mA=20 mW. If more power is needed the linear regulator can be replace by a switching regulator and nearly 100% of the 40 mW available can be achieved depending upon the efficiency of the switching regulator.  
         [0022]     The basic concept of the supply is to low pass filter the supply current drawn by the Hall effect sensor such that only the average current of the sensor is drawn from the 5V supply. The supply is electrically connected to and forms part of the current loop by wires W 1  and W 2 . Capacitor C 7  along with resistors R 5  and R 7  and Q 2  form the low pass filter  100  when Q 2  is on. An average current is drawn through the resistors R 5  and R 7  and transistor Q 2  based on the average voltage difference between the 5V supply and the voltage across C 7 . The ripple in the average current is determined by how much the voltage across C 7  changes when Q 3  is turned on and current is supplied to the Hall effect sensor (not shown) through its connection to connector PJ 6 . The voltage across C 7  must remain high enough to meet the minimum voltage requirements of the Hall effect sensor. Ideally C 7  would go to an infinite capacitance as the resistance goes to zero. As long as the minimum supply voltage for the Hall effect sensor is less than the minimum voltage of the 5V supply a practical compromise for the values can always be found. In the preferred embodiment, the resistor and capacitor values are selected to provide a 1:9 duty cycle. Such a cycle provides power to the Hall effect sensor for 40 mSec, and charges for 360 mSec. Other duty cycles may work so long as the average current draw is below 4 mA, and the surge current to power the sensor is at least 4 mA for a time duration necessary to obtain a stable reading.  
         [0023]     The purpose of transistor Q 2  and resistor R 11  is to form a start up current  110  for start up conditions. Because C 7  starts at complete discharge, the current draw on the 5V supply can be higher than what is available. By separating the start up charging resistor value from the running value, the start up surge can be limited.  
         [0024]     Q 4  and R 10  form a discharge circuit  120  to provide a quick discharge path for C 7  when the 5V supply shuts down. This prevents C 7  from back feeding the 5V supply and causing a poor shutdown for the transmitter. Other supply arrangements that smooth the current draw from the 4 to 20 mA loop are possible.  
         [0025]     When powered, the Hall effect sensor  40  provides a signal to wire W 3 . The signal may be a voltage response, or, more preferred, the signal is a pulse width modulated voltage signal. The type of signal will depend on the output of the sensor  40 . In other embodiments, the signal may be a current signal, or other stream of data. In the case of a signal based on voltage, the signal is thereafter conveyed to a transmitter to convert the signal to a current signal for transmission in the current loop.  
         [0026]      FIG. 3  shows the invention as part of a pointer display instrument. The instrument  60  includes a pointer  70  which moves in response to a force induced on the helix  71  by the c-shaped magnet  72 . The c-shaped magnet  72  is mounted to one end of a leaf spring  75 , while the other end of the leaf spring  75  is anchored by clamp  77 . The leaf spring  75  is coupled to the diaphragm (not shown) by post  79 , allowing the leaf spring  75  and the c-shaped magnet  72  to move as the diaphragm is displaced by the differential pressure exerted on it. As the c-shaped magnet  72  moves, it magnetically interacts with the helix  71 , causing the pointer  70  to move.  
         [0027]     A Hall effect sensor is placed so it is magnetically coupled to the c-shaped magnet  72 , to thereby produce an electric signal corresponding to the movement of the c-shaped magnet. In the alternative, a separate magnet  80  can be placed on or coupled to the leaf spring  75 . A Hall effect sensor  85  is placed in proximity to the magnet  80 , to produce a signal when the magnet moves.  
         [0028]     Using this arrangement, the same beam deflection controls both the pointer and transmitter and is done without adding inaccuracy to either output. This allows the addition of transmitter capability to existing pointer display designs with a minimum of additional parts. Thus, the invention described herein can be used with a wide variety of indicators, consistent with the general principles described herein.