Abstract:
An opto-electric levelmeter having a sensor that provides an output signal whose frequency represents the surface level of liquid in a container. The sensor&#39;s circuitry includes an oscillation circuit that uses “optical feedback” to modulate the sensor output signal. The signal is delivered to a monitor that provides an output signal for the user. The monitor also permits adjustment for both high and low levels of a particular container. The monitor sends electrical energy to the sensor, and receives the sensor output signal on the same electrical link.

Description:
TECHNICAL FIELD OF THE INVENTION 
     This invention relates to measurement devices, and more particularly to an electro-optic device for measuring the fullness level of liquid in a container. 
     BACKGROUND OF THE INVENTION 
     Continuous level sensing is used for a vast number of applications, perhaps the most familiar being for tanks containing liquids, such as a fuel tank. Unlike, a limit levelmeter (also known as a switch levelmeter), a continuous levelmeter must provide a continuous range of measurements from empty to full. 
     There are many different types of level meters, each type having a different principle of operation. Some of the more common types are float levelmeters, capacitive levelmeters, photoelectric levelmeters, and ultrasonic levelmeters. 
     Some levelmeters, notably ultrasonic levelmeters, have used signal frequency to provide level information. For example, one type of ultrasonic levelmeter uses an emitter to direct ultrasonic waves into a cavity above the liquid. The resulting waves resonate at the cavity&#39;s resonant frequency and at harmonics of that frequency. At a different liquid level, the resonant frequency is different. Thus, measurement of the oscillation frequency provides a measure of the liquid level. 
     SUMMARY OF THE INVENTION 
     One aspect of the invention is an opto-electric levelmeter for measuring the level of liquid in a container, comprising: a sensor that has at least a light emitting diode and a phototransistor. The sensor is operable to provide a sensor output signal whose frequency is related to the distance of the sensor from the surface of the liquid. More specifically, this distance is an “optical link” and is connected into the feedback loop of an oscillation circuit within the sensor. The sensor delivers this output signal to a remote monitor that has level detection circuitry for converting the sensor output signal into a signal representing the level of liquid in the container. The monitor has an adjustment circuit that permits adjustment of both the high and the low level of the level detection circuitry. 
     One advantage of the levelmeter is the low cost of its sensor. The sensor may be used with a remote monitor and is easy to install. It is not invasive as is a float type levelmeter, and it does not require a wide orifice as does an ultrasonic type levelmeter. 
     Because the sensor output is a frequency not an amplitude, the signal may be transmitted long distances. Resistance variations on a transmission line are less likely to affect the signal readings. The sinusoidal oscillations generate low electromagnetic interference. 
     The levelmeter has a built in signal transmitter, in that the same wire that delivers electrical energy to the sensor may also be used to transmit the sensor output signal to the remote monitor. Two independent settings, one for low level and one for a full level, permit the use of one sensor configuration for different container sizes, where the level of fullness is provided to the user as a percentage. The device may also be quickly and easily calibrated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a tank containing a liquid and having a levelmeter in accordance with the invention. 
     FIG. 2 illustrates the operation of the levelmeter of FIG.  1 . 
     FIGS. 3A and 3B illustrate the relationship between the sensor output signal and the output of the non linear conversion of FIG.  2 . 
     FIG. 4 illustrates one example of electronics circuitry for implementing the levelmeter. 
     FIG. 5 illustrates how the levelmeter may be automatically calibrated. 
     FIG. 6 illustrates how the level meter may be used for remote monitoring by a service provider. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates a tank containing a liquid, fitted with a level meter  10  in accordance with the invention. Although in the example of FIG. 1; levelmeter  0  is used with a closed container, it mat also be used with open containers and frowpaths. 
     Levelmeter  10  has two main components: a sensor  100  and a remote monitor  200 . Sensor  100  is connected to monitor  200  by means of a single cable and ground connection. Cable  300  carries signals in two directions. It carries an operating voltage from monitor  200  to sensor  100 , as well as a sensor output signal from sensor  100  to monitor  200 . A single port  100   c  in the sensor circuitry is used for this bi-directional electrical transmission. 
     Sensor  100  is an electro-optic probe installed in the upper portion of the container above the surface of the liquid contained therein. A light emitting diode (LED)  100   a  emits light, which is reflected from the surface of the liquid. A phototransistor  100   b  detects the reflected light. As explained below in connection with FIG. 4, phototransistor  100   b  provides an “optical feedback” to an oscillation circuit. A variable impedance oscillates as a function of the distance, L, between the sensor  100  and the surface of the liquid in the tank. 
     FIG. 2 illustrates the basic process of using levelmeter  10  to measure liquid level in a container. 
     As stated above, sensor  100  emits light to the surface of the liquid and detects the reflected light. This reflected light is used as optical feedback to an oscillation circuit. The output from sensor  100  is an analog signal whose frequency represents impedance variations as a function of L. 
     The relationship between L and F is generally logarithmic because the light propagates as a conical section of a sphere. However, reducing the angle of dispersion of LED  100   a  will make the relationship more linear. 
     Within monitor  200 , a variable impedance conversion circuit  201  converts the sinusoidal output of sensor  100  to a squarewave. A frequency to voltage conversion circuit  203  operates non-linearly to provide a linearized voltage response curve inverse to that of the frequency. 
     FIGS. 3A and 3B illustrate the relationship between the response of sensor  100  and the output of conversion circuit  203 . More specifically, FIG. 3A illustrates the frequency-distance response of sensor  100 . FIG. 3B illustrates the output voltage of conversion circuit  203 , which is proportional to L in a more linear manner. The linearization is accomplished with an inverse transfer function. 
     Referring again to FIG. 2, a level detector  205  translates the current signal to a signal representing the fullness level of the container. Level detector  205  is adjusted with an adjustment circuit  207  for both low level and high level. This may be accomplished with a pair of potentiometers, as described below in connection with FIG.  4 . The ability of monitor  200  to be adjusted for both the low and high level permits a single embodiment of levelmeter  10  to be used for different tank sizes. 
     The output of level detector  205  is delivered to an indicator driver  209 , which drives whatever elements are used to display a measurement for the user. For example, driver  209  may be used to drive a numerical LED display. An alternative display comprising a column of LEDs is explained below. 
     A alarm circuit  211  may be used to deliver a signal to a remote location or sound an alarm when the liquid is at an undesirably low or high level. 
     FIG. 4 illustrates one example of electronics circuitry for implementing levelmeter  10 . Sensor  100  is a two terminal device. As stated above, the same port  100   a  is used to both receive the electrical energy and to transmit a sensor output signal. 
     Sensor  100  receives DC voltage from monitor  200  via a regulator  401 . The regulated voltage is divided by diode  403  to provide a first voltage to a low current section of the sensor circuit, which is isolated from high current variations of the power section of the sensor circuit. Capacitor  405  stores energy during high current peaks through LED  100   a.    
     The light emitted by LED  100   a  and reflected by the liquid surface is sensed and converted to voltage variations by optical transistor  100   b.  Capacitor  411  provides low frequency isolation and phase shift. 
     Resistors  413  and  415  provide a reference voltage for the non inverting input of operational amplifier  417 . This reference voltage is one-half the supply voltage, Vr. Operational amplifier  417  provides voltage gain. Capacitor  419  and resistor  421  set a suitable bandwidth and amplification with the operational amplifier feedback, with resistor  423  limiting the gain. Using “optical feedback” from optical transistor  100   b,  a closed loop circuit within sensor  100  acts as a frequency modulated oscillator that oscillates at the resonant frequency of a loop that includes the optical link. The distance between sensor  100  and the surface of the liquid is the frequency modulator. Thus, the oscillation frequency is dependant on the distance, L. The sinusoidal waveform reduces interference and optimizes sensitivity. 
     The frequency of oscillation is a function of a number of internal and external factors. These factors include the following: internal electronics gain and phase shift, optic dispersion geometry, molecular structure of the reflecting surface, light intensity of the illumination, ambient light, and distance to the reflecting surface. By making each of these factors substantially constant except the distance to the reflecting surface, and by providing an appropriate phase shift, the oscillation can then be related to the level of fluid inside the container. 
     The power supply to monitor  200  is regulated by transistor  431 . Transistor  431  can be switched with positive and negative voltage, which permits automatic monitoring or manual activation. Transistor  433  provides voltage to sensor  100  using a small-valued resistor  435  for short circuit protection against base-emitter damage. 
     Resistor  437  provides current to voltage conversion. When the voltage across resistor  437  is over 0.7 volts, transistor  433  is saturated. The Schmitt trigger inverter  439  switches to a low state. When sensor  100  oscillates, the output of inverter  439  oscillates at the same frequency but as a square waveform. 
     Capacitors  441  and  443  and diodes  445  and  447  operate as a non-linear frequency-to-voltage converter. The voltage across capacitor  441  varies according to a response curve inverse to that of the frequency and is proportional to L. This response curve has the characteristics of the curve explained above in connection with FIG.  3 B. 
     The voltage across capacitor  441  is connected to a level detector circuit  449 , whose output represents the liquid level being measured. An example of a suitable level detector circuit  449  is the LM31914, manufactured by National Semiconductor Co. 
     Potentiometers  451  and  453  and their associated control circuitry  452  provide digital potentiometers with self-contained non volatile memory. Potentiometer  451  sets the high level reference, and potentiometer  453  sets the low level reference. Typically, these devices operate with a slider, whose settings are stored in the integrated memory. If the output of levelmeter  10  is expressed in terms of “percent full”, then the same configuration of levelmeter  10  can be used for different container sizes. 
     Each output of level detector  449  is connected to a different one of a set of LEDs  455 . Each LED  455  is associated with a different level of the container. For example, if the output of level detector  449  is at the highest level, the LED represent “full” would be lit. An over/under level indicator circuit  457  may also be used to provide an alarm when the container is over or under a predetermined limit. 
     As described above, the output information from sensor  100  is frequency information, which permits a simple communications link between sensor  100  and monitor  200 . However, if desired, a current transmitter could be used. 
     FIG. 5 illustrates how levelmeter  10  may be automatically calibrated. In FIG. 5, it is assumed that levelmeter  10  provides readings in the form of a series of LEDs  200   a.  For example, a reading in which the bottom LED is lit would indicate a low liquid level. 
     A light-isolated sensor housing  53  contains a segmented disk  53   a,  which is rotated by a motor  53   b.  Each segment of disk  53   a  has reflective characteristics similar to those within the container in which sensor  100  will actually be used. Sensor  100  is placed in housing  53 . A first segment is placed in view of sensor  100 , representing low level conditions. Then, a second segment is placed in view of sensor  100 , representing high level conditions. For each position of disk  53   a,  the appropriate LED  200   a  turns on. 
     Calibration unit  51  has two phototransistors  51   a  and  51   b,  one of which detects the “full” LED  200   a  and the other of which detects the “low” LED  200   a.  Calibration unit  51  then delivers a calibration signal to monitor  200 , which sets potentiometer  451  or  453 . This automatic calibration could be similarly accomplished with any type of optical display other than LEDs. 
     FIG. 6 illustrates how monitor  200  may be activated and used to signal low level conditions. A switch  61  may be manually or automatically activated. As an example of manual activation, where levelmeter  10  is used for a propane gas tank, monitor  200  could be placed in a residence. The gas level could be checked by pressing a button (switch  61 ) connected to monitor  200 . A low level signal could result in an alarm or other signal to the user. The same signal could be delivered to remote location, such as the office of a liquid fuel provider. The fuel provider might also remotely activate switch  61 , thereby providing remote monitoring without the need for intervention by the homeowner. 
     Other Embodiments 
     Although the present invention has been described in detail, it should be understood that various changes, substitutions, and alterations can be made hereto without departing from the spirit and scope of the invention as defined by the appended claims.