Abstract:
An admittance sensor includes a voltage divider network having a probe and an associated variable virtual admittance. The voltage divider network is operable to receive pulses and transmit attenuated pulses. Each attenuated pulse has a respective attenuated pulse magnitude based, at least in part, upon a magnitude of the virtual admittance at approximately the respective time at which the attenuated pulses are transmitted. The admittance sensor also includes a level detector operable to receive the attenuated pulses from the voltage divider network and operable to generate output pulses corresponding to each of the attenuated pulses received which exceed a predetermined attenuated pulse magnitude. The admittance sensor also includes a pulse detector operable to receive the output pulses and generate a signal if at least one of the output pulses is not received for a predetermined period of time.

Description:
TECHNICAL FIELD OF THE INVENTION  
         [0001]    This invention relates to detection devices, and more particularly to an admittance sensor for mass detection.  
         BACKGROUND OF THE INVENTION  
         [0002]    Admittance is the ease with which current flows in an alternating-current circuit. Its magnitude depends on the resistance, capacitance and inductance of the circuit.  
           [0003]    Charge transfer sensors are a variation of an admittance sensor and are generally related to capacitance measurement. The most common implementation is essentially an exercise in switching circuitry. A sensing electrode capacitor is charged with a reference voltage, and by simple switching that charge is transferred to a known capacitor.  
           [0004]    Mechanical switches can be used only at very low sampling speeds. Such switches are bulky and have very high power consumption. Integrated metal oxide semiconductor field effect transistor (MOSFET) switches are small and are capable of high speed operation but have an internal resistance that is temperature dependent.  
           [0005]    Some traditional charge transfer sensors have voltage regulators that do not have the transient response to charge large capacitance directly and at high sampling rates. The need for a good quality ground reference makes it difficult to use low sensitivity charge transfer sensors in applications like portable battery operated devices, fluid level measurement and gas mass evaluation, because poor quality ground greatly degrades range and sensitivity of conventional sensors.  
           [0006]    A given mass of liquid, gas or solid material can be electrically characterized by a complex arrangement of resistance, capacitors and inductance. Many conventional charge transfer sensors cannot evaluate all these parameters in a single network.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention provides an admittance sensor for mass detection that substantially eliminates or reduces at least some of the disadvantages and problems associated with previous detection devices.  
           [0008]    In accordance with a particular embodiment of the present invention, an admittance sensor includes a voltage divider network having a probe and an associated variable virtual admittance. The voltage divider network is operable to receive pulses and transmit attenuated pulses. Each attenuated pulse has a respective attenuated pulse magnitude based, at least in part, upon a magnitude of the virtual admittance at approximately the respective time at which the attenuated pulses are transmitted. The admittance sensor also includes a level detector operable to receive the attenuated pulses from the voltage divider network and operable to generate output pulses corresponding to each of the attenuated pulses received which exceed a predetermined attenuated pulse magnitude. The admittance sensor also includes a pulse detector operable to receive the output pulses and generate a signal if at least one of the output pulses is not received for a predetermined period of time.  
           [0009]    In accordance with another embodiment, a method for detecting an admittance variation includes receiving pulses and transmitting attenuated pulses through a voltage divider network. The voltage divider network has a probe and an associated variable virtual admittance. Each attenuated pulse has a respective attenuated pulse magnitude based, at least in part, upon a magnitude of the virtual admittance at approximately the respective time at which the attenuated pulses are transmitted. The method also includes receiving the attenuated pulses from the voltage divider network using a level detector and generating output pulses using the level detector. The output pulses correspond to each of the attenuated pulses received which exceed a predetermined attenuated pulse magnitude. The method also includes generating an analog output based upon changes in the attenuated pulse magnitudes using an amplification system. The method also includes receiving the output pulses and generating a signal using a pulse detector if at least one of the output pulses is not received for a predetermined period of time.  
           [0010]    Technical advantages of particular embodiments of the present invention include a sensor with the ability to detect an object approaching a probe based on variations in the admittance of the area surrounding the probe. Accordingly, changes in any of a number of characteristics affecting admittance, including capacitance, resistance or inductance, can lead to sensor notification of an approaching object.  
           [0011]    Another technical advantage of particular embodiments of the present invention is an admittance sensor with a controlled feedback loop for detection of objects approaching a probe at a particular speed. Accordingly, the sensor can be used in various applications, such as air bag deployment and automobile security systems.  
           [0012]    Still another technical advantage of particular embodiments of the present invention includes an admittance sensor which consumes a low amount of power during operation. Accordingly, the sensor can be battery operated at maximum sensitivity and may thus be useful in battery operated security systems.  
           [0013]    Other technical advantages will be readily apparent to one skilled in the art from the following figures, descriptions and claims. Moreover, while specific advantages have been enumerated above, various embodiments may include all, some or none of the enumerated advantages.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 is a diagram illustrating an admittance sensor in accordance with a particular embodiment of the present invention;  
         [0015]    [0015]FIG. 2 is a schematic diagram illustrating electronic circuitry of an admittance sensor in accordance with a particular embodiment of the present invention; and  
         [0016]    [0016]FIG. 3 is a graph illustrating the relationship between a control voltage and a pulse width of an admittance sensor in accordance with an embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]    [0017]FIG. 1 illustrates an admittance sensor  10  in accordance with a particular embodiment of the present invention. Admittance sensor  10  may be used to measure the admittance near a probe  20  or to give a warning signal when a body of certain characteristics approaches probe  20 .  
         [0018]    Admittance sensor  10  includes a pulse generator  12 . Pulse generator  12  generates a pulse train  13  which is received by a voltage divider network  14 . Voltage divider network  14  includes probe  20  and an associated virtual admittance of the surrounding media around probe  20 . Pulse train  13  enters voltage divider network  14 . Voltage divider network  14  then transmits attenuated pulse train  15  which flows to a level detector  16 .  
         [0019]    Level detector  16  may produce output pulses  34  with pulse widths based upon magnitudes of pulse train  15 . Such magnitudes reflect the virtual admittance of the area around probe  20  at approximately the time at which pulse train  15  is transmitted by voltage divider network  14 . If the virtual admittance around probe  20  subsequently changes, then subsequent periods of output pulses  34  will change in proportion to the change in virtual admittance. If an object comes into proximity with probe  20 , the virtual admittance of the area surrounding probe  20  decreases. If such decrease is dramatic enough, then level detector  16  may not produce an output pulse for a certain period of time. In accordance with a particular embodiment, this will trigger pulse detector  18  to produce a signal output  19  to notify a user that an object is approaching probe  20 .  
         [0020]    Admittance sensor  10  also includes an amplification system  32 . Amplification system  32  may produce an analog output  33  displaying the magnitude of a voltage level  35  output by a feedback delay  30 . Voltage level  35  is based upon the pulse widths of output pulses  34 . As stated above, such pulse widths are based upon magnitudes of pulse train  15  which reflect the virtual admittance of the area around probe  20  at approximately the time at which pulse train  15  is transmitted by voltage divider network  14 . Thus, analog output  33  may reflect variation in admittance around probe  20 . Particular embodiments of the present invention may include one or both of signal output  19  and analog output  33  to detect a change in the virtual admittance and the relative magnitude of such change, respectively.  
         [0021]    Using the virtual admittance of the area surrounding probe  20  as a basis for signal output  19  of pulse detector  18  and analog output  33  of amplification system  32  is useful since admittance comprises capacitance, resistance and inductance in a single parameter. Thus, changes in any of these characteristics in the area surrounding probe  20  may produce an output based on the change in admittance resulting from an object coming into proximity with probe  20 .  
         [0022]    Admittance sensor  10  also includes rate control  28 . Rate control  28  controls the frequency of pulse generator  12 . Voltage divider network  14  also includes a resistor  22 , a conductor  24  and electrical impedance to ground of an area  26  surrounding probe  20 . Probe  20  of voltage divider network  14  may comprise any electrically conductive material, such as a rod, a plate, a pipe, a needle or a sphere. The size of probe  20  may vary without reducing sensitivity.  
         [0023]    As stated above, after attenuation of pulse train  13 , voltage divider network  14  transmits attenuated pulse train  15  into level detector  16 . If the amplitude of pulse train  15  is higher than a reference voltage of level detector  16 , then level detector  16  will produce output pulses  34  having a similar train of pulses but with a smaller period than the period of pulse train  15 . The period of output pulses  34  will be proportional to the variations in admittance of the space around probe  20 .  
         [0024]    If a mass approaches probe  20 , then the virtual admittance of the area surrounding probe  20  will decrease, resulting in a reduction in amplitude of pulse train  15  with respect to pulse train  13 . If the amplitude of pulse train  15  is not higher than a reference voltage, then level detector  16  will not produce an output pulse for a certain period of time. If there is no output pulse of level detector  16  for a predetermined period of time, then pulse detector  18  may produce signal output  19  to alert a user of an object approaching probe  20  or to trigger a device. The reference voltage may be set such that pulse detector  18  produces signal output  19  based on certain characteristics of an object approaching probe  20 . Such characteristics may include any characteristic which may affect the virtual admittance of the area surrounding probe  20 , such as the velocity or the mass of the object.  
         [0025]    Pulse detector  18  may be adjusted such that it may respond to any number of missing pulses. Such adjustment may be made based on whether a user desires to detect a slow or fast approaching object through pulse detector  18 .  
         [0026]    Admittance sensor  10  also includes feedback delay  30 . Output pulses  34  of level detector  16  provide a feedback signal. Feedback delay  30  delays the pulse train of output pulses  34  and transforms the pulse period of output pulses  34  to voltage level  35  which enters pulse generator  12 . The magnitude of the voltage level change is proportional to the variations in admittance in the area surrounding probe  20 . The delay of feedback delay  30  can be adjusted so that admittance sensor  30  can evaluate both slow and fast objects approaching probe  20 . The use of feedback delay  30  may allow for the detection of small admittance changes at probe  20  and automatic adjustment to slow-changing environmental conditions around the probe.  
         [0027]    As stated above, amplification system  32  may produce an analog output  33  which in effect displays the magnitude of the variation in admittance at probe  20 . This information can be used, for example, to measure gas flow in pipes and the level of fluids contained in tanks.  
         [0028]    As an example, if admittance sensor  10  is used to measure gas flow in a pipe, probe  20  would be located inside the pipe. A reference level may be set by a potentiometer (illustrated in FIG. 2) when there is no flow in the pipe. When the gas flow begins, the admittance of the area surrounding probe  20  decreases. Output  33  displays the proportional variation in admittance of the area surrounding probe  20 .  
         [0029]    In another example, if admittance sensor  10  is used to measure the level of a fluid contained in a tank, probe  20  may be a rod placed vertically into the tank. As the level of the fluid rises, the fluid covers a larger portion of the rod. Thus, the admittance of the area surrounding the rod decreases, and output  33  displays the magnitude of this decrease.  
         [0030]    Another application of an admittance sensor in accordance with an embodiment of the present invention may be in air bag deployment in automobiles. In this situation, the admittance sensor may be used to alert a pyrotechnic device that controls the air bag seconds before the impact of an object with the automobile. The device may be detonated at this time, reducing the risk of injuries to passengers in the automobile. This application may also allow for the use of less explosive pyrotechnic devices since device does not have to wait until impact to detonate. Several probes may be used with the admittance sensor to protect the automobile from different directions. The feedback delay of the admittance sensor may be set such that the admittance sensor may respond to a certain momentum (p=mv) at a preset distance from a probe.  
         [0031]    Other applications of an admittance sensor in accordance with particular embodiments of the present invention may include automobile and museum security devices. For example, an admittance sensor may be used to trigger an automobile alarm when a person approaches the automobile. An admittance sensor could also be mounted in a museum proximate to an art work to alert security personnel when a person approaches the art work.  
         [0032]    Admittance sensor  10  consumes a low amount of power during operation and thus may be used in battery operated devices, such as battery operated security systems.  
         [0033]    [0033]FIG. 2 is a schematic diagram illustrating electronic circuitry of an admittance sensor  110  in accordance with a particular embodiment of the present invention. Admittance sensor  110  includes a pulse generator  112 , a voltage divider network  114 , a level detector  116 , a pulse detector  118 , a rate control  128  and a feedback delay  130 .  
         [0034]    Pulse generator  112  includes a capacitor  150  electrically coupled to a transistor  152 . Pulse generator  112  also includes resistors  154 ,  156  and  162 , capacitor  158  and inverters  172  and  184 . In operation, capacitor  150  is discharged by transistor  152 . The discharge pulse at the base of transistor  152  is provided by the resistance-capacitance network of resistor  154 , resistor  156  and capacitor  158 . The width of this pulse may be very narrow (e.g., a few nanoseconds).  
         [0035]    After discharging, capacitor  150  begins to charge. A control voltage  160  and resistor  162  set the rate of charge. Capacitor  150  and resistor  162  have fixed values, while control voltage  160  at the emitter of a transistor  164  is variable. Control voltage  160  follows the voltage variations of the voltage across a capacitor  166 . Resistors  168  and  170  are used to set a start up voltage at the base of transistor  164  and a discharge path to capacitor  166 .  
         [0036]    When the voltage of capacitor  150  reaches a high threshold level of Schmitt-trigger inverter  172 , an output pulse  174  goes low. The elapsed time between this event and the end of the discharge pulse at the base of transistor  152  sets the width of output pulse  174 .  
         [0037]    Control voltage  160  is always higher than the threshold voltage of inverter  172 ; but when the two values approach, the width of output pulse  174  becomes very large. In a noise-free environment this width can be almost infinite.  
         [0038]    Rate control  128  includes an inverter  176 , a resistor  178  and capacitor  180 . Rate control  128  is electrically coupled to a voltage source. When output pulse  174  of inverter  172  goes low, the output of inverter  176  goes high during a separation time  182  set by resistor  178  and capacitor  180 . Separation time  182  is a time between the pulses of output pulse  174 . When capacitor  180  charges to the high level threshold of inverter  176 , the output of inverter  176  goes low. Inverter  184  generates a reset pulse discharging capacitor  150 , and a new cycle begins.  
         [0039]    Voltage divider network  114  includes resistor  188 , probe  190 , an area  192  surrounding probe  190  and virtual ground  194 . Voltage divider network  114  attenuates output pulse  174  based upon the admittance of area  192  surrounding probe  190 . Output pulse  174  then flows to level detector  116 .  
         [0040]    Level detector  116  includes an inverter  186 . After a pulse has been attenuated by voltage divider network  114 , if it is high enough to change the output of inverter  186  from high to low then pulses  210  are generated having a width proportional to the difference between the height of the pulses at the input of inverter  186  and the threshold level of the inverter.  
         [0041]    Feedback delay  130  includes resistors  196 ,  168  and  170 , a capacitor  166  and transistors  164  and  198 . Feedback delay  130  is electrically coupled to a voltage source. Resistor  196  converts pulses  210  into current pulses amplified by transistor  198 , and the voltage across capacitor  166  increases. Transistor  164  completes the feedback loop increasing control voltage  160  and thus making the width of output pulse  174  smaller.  
         [0042]    The procedure automatically reverses if output pulse  174  does not have the required height after being attenuated by voltage divider network  114 . In this case, there is no charging pulse at a point  200 , and capacitor  166  discharges slowly, draining current by the network of resistors  168  and  170  and the base of transistor  164 .  
         [0043]    Amplification system  132  includes potentiometer  202 , resistors  224  and  226  and amplifier  228 . Amplification system  132  is electrically coupled to a voltage source. Control voltage  160  is amplified by amplification system  132  to give an output  204  proportional to the admittance variation resulting from a mass near probe  190 . The mass may be ionized gas, a fluid or a solid. Output  204  may be logarithmic to allow for a wide range of input capacitance without changing the components of admittance sensor  110 . Potentiometer  202  is used to set a reference level in amplification system  132  making output  204  adaptable to various applications.  
         [0044]    Pulse detector  118  includes resistors  206 ,  216  and  218 , a capacitor  208 , a transistor  220  and inverters  212  and  222 . Resistor  196  can be used to adjust the response time of feedback delay  130 . If the response time of feedback delay  130  is very fast, the movement of slow objects will not be detected by pulse detector  118 .  
         [0045]    Additional gain control is given by resistor  206 . Resistor  206  controls the discharging time of capacitor  208  if pulses  210  are interrupted over a preset time. Voltage capacitor  208  can reach the threshold of the input of inverter  212  switching its output from high to low. If resistor  196  has a very low resistance, feedback voltage  200  quickly compensates any increment in the admittance around probe  190 , and output  214  can be set to detect only fast approaching objects.  
         [0046]    [0046]FIG. 3 illustrates the relationship between the control voltage  160  and the width of output pulse  174 , both of FIG. 2. When the control voltage approaches the threshold level of inverter  172  of FIG. 2, the curve becomes almost horizontal, increasing the pulse width to several decades of magnitude with respect to the minimum pulse width. As illustrated, working near the threshold voltage of inverter  172  of FIG. 2 can provide a very wide range of pulse widths with which to operate.  
         [0047]    Although the present invention has been described in detail, various changes and modifications may be suggested to one skilled in the art. It is intended that the present invention encompass such changes and modifications as falling within the scope of the appended claims.