Abstract:
A fluid level detecting system provides an input signal at a specific frequency to a sample in a container. A probe contacts the energized sample and provides a signal to a level sensing circuit. The level sensing circuit amplifies the signal from the probe and then bandpass filters, tuned to the specific frequency, the amplified signal. This filters out extraneous signals received from, for example, a cover on the container, and specifically identifies when the probe has contacted the sample fluid by comparing the filtered signal to threshold levels.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Application Ser. No. 61/845,599, filed Jul. 12, 2013, which is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    In conventional test systems, for example, known immunoassay analyzers, a container  104 , as shown in  FIG. 1 , sometimes referred to as a cuvette, will include a liquid sample  108 . The liquid sample  108  usually only partially fills the container  104  and, therefore, presents a sample surface  112 . Generally, the container  104  is closed with a cover or septum  116  that may be made from either a flexible material, for example, rubber or a type of foil, for example, a metallic seal. 
         [0003]    In operation, referring now to  FIG. 2 , a pipette  204  is inserted through the cover/septum  116  in order to retrieve a portion of the sample  108 . In some known systems, the pipette  204  is driven down into the sample  108  a set distance that is expected to reach, for example, all the way to the bottom of the container  104 . This, however, has the drawback of coating the pipette  204  with sample material that then needs to be washed away in order not to contaminate successive pipettings from other containers  104 . In addition, by moving the pipette a set amount, the possibility of either aspirating an empty container  104  or driving the pipette through the container, is not reliably mitigated. 
         [0004]    In known conventional capacitive level sense techniques for identifying that the pipette is in contact with the fluid sample, there is a reliance upon a very small energy transfer from a capacitive probe to the object being detected, i.e., the sample surface  112 . This approach, however, has a high rate of false positives, i.e., a false determination that the pipette  204  has reached the sample surface  112 , as the technique responds to input from any surface on which the signal is present. Specifically, the conventional capacitive level sense systems do not work reliably with a container  104  having a septum or a foil seal closure. 
         [0005]    What is needed, therefore, is a level detection system that accurately determines the location of the sample surface  112  in a container  104  having a septum or a foil seal closure. 
       BRIEF SUMMARY OF THE INVENTION 
       [0006]    In one embodiment of the present invention, the level sense system more specifically identifies the fluid sample surface by energizing the sample and container with a signal at a specific frequency. A level sense circuit is provided that uses an amplifier and a bandpass filter tuned to the energizing frequency to distinguish the surface of the fluid sample from other portions of the container. As a result, only contact with energized sample fluid material will result in a response from the level sense circuit. 
         [0007]    In another embodiment, a surface detection method couples an input signal to either a probe or an outer surface of the container and detects an output signal from whichever of the two is not coupled to the input signal. A processed signal is generated as a function of the output signal and compared to a contact threshold level and when the processed signal is at least equal to the contact threshold is an indication that the probe has contacted the sample surface. 
         [0008]    In yet another embodiment, a probe positioning apparatus to place a probe within a sample in a covered container includes a signal source configured to couple a first signal at a first frequency f p  to either an outer surface of the container or the sample. A detector is coupled to whichever of the two is not coupled to the signal source in order to detect a second signal as a function of the first signal. A level detector is coupled to the detector and is configured to generate a third signal as a function of the second signal and a comparator, coupled to the level detector, is configured to compare the third signal to a contact threshold value. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0009]    Embodiments of the present invention may be better understood by referring to the following description in conjunction with the accompanying drawings in which: 
           [0010]      FIG. 1  is a conventional container holding a sample; 
           [0011]      FIG. 2  is a representation of the conventional container of  FIG. 1  with a pipette inserted therein; 
           [0012]      FIGS. 3A and 3B  are block diagrams of an embodiment of the present invention; 
           [0013]      FIG. 4  is a graph of an output signal in accordance with one embodiment of the present invention; 
           [0014]      FIGS. 5A and 5B  are embodiments of the present invention; 
           [0015]      FIG. 6  is a block diagram of another embodiment of the present invention; 
           [0016]      FIGS. 7A and 7B  are block diagrams of yet another embodiment of the present invention; and 
           [0017]      FIG. 8  is a top view of a combination assembly in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    The U.S. Provisional patent application Ser. No. 61/845,599 entitled “Fluid Level Detection System And Method,” filed Jul. 12, 2013, is herein incorporated by reference for all purposes. 
         [0019]    In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the various embodiments of the present invention. It will be understood by those of ordinary skill in the art that these embodiments of the present invention may be practiced without some of these specific details. In other instances, well-known methods, procedures, components and structures may not have been described in detail so as not to obscure the embodiments of the present invention. 
         [0020]    Referring now to  FIGS. 3A and 3B , a level detection system  300  includes an AC signal source  304  coupled to the container  104  via an AC output coupler  308 . The AC signal source  304  outputs a signal with a predetermined amplitude and predetermined frequency (f p ) that is coupled through the container  104  to the fluid sample  108  and the cover/septum  116 . In one embodiment, the output signal is an RF signal with an amplitude of 100 mV-1 V (peak-to-peak) with the predetermined frequency (f p ) in the range of 100-450 KHz, and, in one embodiment, 250 KHz, generally in order to avoid FCC interference. A pipette/sensor  316  is inserted into the container  104  through the cover/septum  116  and is configured to provide a raw output signal  318  representing the signal detected from the AC signal source  304 . A level sense detector (LSD)  312  is coupled to the pipette/sensor  316  to receive the raw output signal  318 . 
         [0021]    Typically, the fluid sample  108  in the container  104  will exhibit a requisite level of conductivity. In one embodiment, the fluid sample  108  may be an ionic fluid and the container  104  may be made of a material that can carry an RF signal such as, for example, glass, styrene, polypropylene and polyethylene. In this configuration, the cover/septum  116  does not have the same signal level induced upon it as the signal level that is induced upon the fluid sample  108  and, therefore, available at the surface  112  of the fluid sample  108 . As a result, and as will be explained below, the system will distinguish when the pipette/sensor  316  touches the cover  116  from when the pipette/sensor  316  contacts the fluid sample  108 . 
         [0022]    Referring now to  FIG. 3B , the LSD  312  receives the raw output signal  318  and processes this signal, as described below, to provide an output signal  320 . As shown in  FIG. 4 , when the pipette/sensor  316  is urged into the container  104 , or the container is urged onto the pipette, the output signal  320  will indicate an increase in signal amplitude to a first threshold level T 1  upon its touching, or penetrating through, the cover/septum  116 . This first threshold level, however, will not be as high as a second, or “contact,” threshold level T 2  that will be detected upon the arrival of the pipette/sensor  316  at the surface  112  of the fluid sample  108 , as shown. Thus, the system can differentiate between the pipette/sensor  316  arriving at, or penetrating, the cover/septum  116  and the pipette/sensor  316  arriving at, i.e., contacting, the surface level  112  of the fluid sample  108  in the container  104 . 
         [0023]    Advantageously, once the surface level  112  of the fluid sample  108  is detected, the system can minimize the insertion depth of the pipette/sensor  316  into the fluid sample  108  and, therefore, minimize the amount of cleanup of the pipette that will be necessary. In addition, a system may be preprogrammed to insert the pipette a predetermined distance beyond the detection of the surface  112  of the sample  108  in the container  104  in the event that the lumen of the pipette is either set back from the sensing portion of the pipette/sensor  316  and/or to assure that there is complete insertion of an opening to the lumen of the pipette to assure that air is not aspirated into the system. Further, the system may halt further movement of the pipette/sensor  316  into the container  104  if a distance the pipette has moved since the detection of the cover is greater than some predetermined safety distance. This would prevent aspiration of either an empty container or one with too little sample volume and also prevent the pipette from being driven into, and possibly through, the bottom of the container. 
         [0024]    Referring now to  FIG. 5A , in one embodiment of the present invention, the level sense detector  312  includes an analog amplifier  504  to receive the signal  318  detected by the pipette/sensor  316 . An output of the amplified signal is provided to a bandpass filter  508  that is centered, ±5%, about the predetermined frequency f p  provided by the AC signal source  304  to provide the output signal  320 . The amplifier  504 , in one embodiment, is a high gain amplifier and therefore the implementation of the bandpass filter  508  allows for improved sensitivity while, at the same time, reducing a false detection of the surface  112  of the fluid sample  108  due to unwanted and extraneous signals. 
         [0025]    The output signal  320  is provided as an input to a comparator circuit  512  including two comparators  516 - 1 ,  516 - 2  that are set, respectively, to determine if the amplitude of the output signal  320  has passed either of the first T 1  or second T 2  threshold values described above. The outputs of the comparators  516 - 1 ,  516 - 2  are provided to a controller  520  for determining the location of the pipette. The controller  520  processes the information and controls a pipette actuator  524 , for example, a stepper motor or the like, for moving the pipette. 
         [0026]    In another embodiment, as shown in  FIG. 5B , the output of the bandpass filter  508  is provided to an analog-to-digital converter (ADC)  528  and the output of the ADC  528  is provided to the controller  520  in order to identify the surface  112  of the fluid sample  108  by comparison to the first T 1  or second T 2  threshold values. One of ordinary skill in the art will understand that the controller could be any type of processor capable of receiving the first T 1  or second T 2  threshold values as either analog levels or digital signals. Further, a hybrid approach of analog and digital components may also be implemented and embodiments of the present invention are not limited to one or the other and are presented here merely for explanatory purposes. Of course, the function of the comparator device  512  may be incorporated into a device that includes the controller  520  as would be understood by one of ordinary skill in the art. 
         [0027]    In another embodiment of a level detection system, referring now to  FIG. 6 , the AC signal source  304  may be coupled to a pipette/transmitter  604  and the level sense detector  312  is coupled via an AC input coupler  608  to the container  104 . As the pipette/transmitter  604  is lowered into the container  104 , a lower amplitude signal will be detected to the level sense detector  312 . Upon arrival of the pipette/transmitter  604  at the surface  112  of the fluid sample  108   108 , however, the level sense detector  312  will detect a higher amplitude signal and determine, therefore, that the pipette/transmitter  604  has arrived at the surface  112  and the system will respond accordingly. 
         [0028]    In yet another embodiment of the present invention, referring now to  FIGS. 7A and 7B , a combination assembly  704  includes a pipette/transceiver  708  inserted through an AC coupler  712  where the pipette/transceiver  708  and the AC coupler  712  are isolated from one another by a seal  716 . In one version, the pipette/transceiver  708  is connected to the level sense detector  312  and the AC coupler  712  is connected to the AC signal source  304 . Alternatively, and as shown in  FIG. 7B , the level sense detector  312  is coupled to the AC coupler  712  and the AC signal source  304  is coupled to the pipette/transceiver  708 . It should be noted that the combination assembly  704  is not shown to scale and one of ordinary skill in the art would understand the relative sizes and geometries necessary to provide a proper configuration for any container  104  on which the assembly  704  would be applied. Operation of either of these embodiments is along the lines as described for the embodiments above. 
         [0029]    Referring now to  FIG. 8 , a top view of the combination assembly, from the direction A as shown in  FIG. 7A , the combination assembly  704  is, generally, circular as is the pipette/transceiver  708  and the seal  716 . Of course, the geometry of the combination assembly would be adapted to correspond to the geometry of the container. 
         [0030]    In the foregoing embodiments, the pipette/sensor  316  or pipette/transceiver  708  was described as being lowered into the container  104 . Of course, one of ordinary skill in the art would understand that the pipette/sensor  316  or pipette/transceiver  708  may be held stationary while the carrier/cuvette  104  is pushed up onto the pipette/sensor  316  or pipette/transceiver  708  until the level  112  of the fluid sample  108  has been detected. Further, the sensor may be separate from the pipette and offset some predetermined distance such that the sensor touches the septum and fluid prior to the pipette. 
         [0031]    In addition, embodiments of the present invention may operate in a system where the container does not include a cover. Thus, the determination of the pipette reaching the fluid would be based on the signal meeting or exceeding the second threshold value T 2  without there having been a crossing of the septum and the corresponding signal response. 
         [0032]    Having thus described several features of at least one embodiment of the present invention, it is to be appreciated that various alterations, modifications and improvements will readily occur to those skilled in the art. Such alterations, modifications and improvements are intended to be part of this disclosure and are intended to be within the scope of the invention. Accordingly, the foregoing description and drawings are by way of example only, and the scope of the invention should be determined from proper construction of the appended claims, and their equivalents.