Abstract:
A liquid level sensing apparatus, including a bridge circuit sensitive to an element the impedance between a liquid sample and a pipette for providing a signal corresponding to the impedance, a phase detector for phase-detecting the output signal of the bridge circuit, a high-pass filter for differentiating the phase detection signal and a circuit for inhibiting the passage of a variation component of a signal from a band-pass filter and passing a liquid level signal component.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a liquid level sensing apparatus for sensing the liquid level of liquid including serum in automatic chemical analysis. 
     2. Description of the Related Art 
     A well-known liquid level sensing apparatus used for an automatic chemical analyzer is disclosed in United States patent application Ser. No. 028,705 (filing data Mar. 20, 1987). This analyzer includes a bridge circuit, which generates a signal corresponding to a vertical displacement of a pipette for extracting a sample, a reagent, etc., when such movement is caused. The probe is connected as an element of the bridge circuit, and the bridge circuit generates an output signal corresponding to a change in the impedance between the probe and liquid level of sample accommodated in a sample container as the probe approaches the liquid level of the sample. When a CPU detects this signal, it provides a zero-setting signal corresponding to the input signal to an automatic phase controller. The automatic phase controller controls the phase of a reference signal from an oscillator according to the zero-setting signal to hold a phase difference of 90 degrees between the reference and input signals. 
     With this prior art liquid level sensing apparatus, the electrostatic capacitance between the pipette and liquid level is instable and varies due to vibrations of the pipette or the like stemming from the looseness of the mechanism of the analyzer for a predetermined period of time from the start of descent of the pipette. For this reason, the liquid level sensor detects the liquid level erroneously and produces an erroneous detection signal. 
     Further, the impedance between the pipette and sample surface is capacitive or inductive according to the kind of the sample. For example, if the sample is serum, the impedance is capacitive. With pure water, the impedance is inductive. With the variation of the detected impedance between capacitive and inductive impedances, negative and positive components corresponding to the capacitance and inductance, respectively, are generated as liquid level detection signals. When such opposite-phase detection signals are generated randomly, the liquid level is detected erroneously. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a liquid level sensing apparatus, which can prevent erroneous liquid level detection due to oscillation of the mechanism or other causes. 
     According to the invention, there is provided a liquid level sensing apparatus, in which the liquid level detection is inhibited for a predetermined period of time from the start of movement of a pipette for extracting the sample. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing an embodiment of the liquid level sensing apparatus according to the invention; 
     FIG. 2 is a graph showing the relation between the pipette position and electrostatic capacitance; 
     FIGS. 3A, 3B and 3C constitute a time chart showing signals input to and output from a phase detector; 
     FIG. 4 is a graph showing an output signal of an amplifier shown in FIG. 1; 
     FIG. 5 is a graph showing a mask signal; and 
     FIG. 6 is a graph showing a liquid level sensing apparatus. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, oscillator 11 which generates a signal at 10 kHz or above is connected to bridge circuit 12. Bridge circuit 12 comprises four resistors R1 to R4 in bridge connection. One of its opposed nodes is connected to a non-inverted input terminal of operational amplifier 13, while the other node is connected to an inverted input terminal of operational amplifier 13 as well as to pipette 10. The output terminal of operational amplifier 13 is connected to phase detector 15, which is in turn connected to oscillator 11 through phase shifter 14. 
     Phase detector 15 phase-detects the output of operational amplifier 13 according to a signal from phase shifter 14. The output terminal of phase detector 15 is connected through low-pass filter 16 and high-pass filter 17 to amplifier 18. The output terminal of amplifier 18 is connected to comparator 19. Comparator 19 receives positive and negative threshold voltages Vth1 and Vth2 and compares the output signal of amplifier 18 to threshold voltages Vth1 and Vth2. 
     The output terminal of comparator 19 is connected to one input terminal of AND gate 20, the other input terminal of which is connected to the output terminal of mask signal circuit 21. 
     Probe or pipette 10 is held by lift mechanism 22. Back electrode 23 is disposed below pipette 10. Container 24 accommodating sample 25 is disposed on back electrode 23 such that it faces pipette 10. 
     In the above liquid level sensing apparatus, when oscillator 11 is operated to supply an oscillation signal to bridge circuit 12 and lift mechanism 22 is driven according to a control command from, for instance, a CPU for controlling the analyzer to cause descent of pipette 10, the electrostatic capacitance between pipette 10 and back electrode 23 is increased as shown in FIG. 2 with decrease of the distance between pipette 10 and back electrode 23. In the graph of FIG. 2, electrostatic capacitance value Cs corresponds to the position of pipette 10 before the start of descent of pipette 10, and the electrostatic capacitance increases gradually from Cs to C1 with the descent of pipette 10. Electrostatic capacitance value C1 is obtained immediately before the end of pipette 10 reaches the surface of a sample, e.g., serum. When the end of pipette 10 reaches the surface of the serum, the electrostatic capacitance is suddenly increased from C1 to C2. 
     In the graph, variations Cw of the electrostatic capacitance that occur in an initial stage of the descent occur due to a swing or vibrations of pipette 10 caused due to defective dimensional accuracy of lift mechanism 22. The capacitance variation is of the order of several pF, and the period tM of variation is several 10 to several 100 milliseconds. If the variations are detected, an error is generated in the liquid level detection. For this reason, there is provided a circuit for masking the variation component, as will be described later. 
     The change in the electrostatic capacitance between pipette 10 and back electrode 23 is detected by bridge circuit 12. More specifically, the electrostatic capacitance provided between pipette 10 and back electrode 23 is connected in parallel to resistor R4 of bridge circuit 12, so that with a change in the electrostatic capacitance bridge circuit 12 provides a signal corresponding to the capacitance change to operational amplifier 13. Operational amplifier 13 amplifies this signal and provides a signal as shown in FIG. 3A to phase detector 15. 
     Phase detector 15 is receiving the output signal from phase shifter 14. Phase shifter 14 phase-shifts a signal provided from oscillator 11, and supplies the shifted signal to phase detector 15. Phase detector 15 converts the output signal of phase shifter 14 into a pulse signal as shown in FIG. 3. According to this pulse signal the output signal of operational amplifier 13 is phase-detected to produce a signal as shown in FIG. 3C. 
     The phase detection signal is supplied to low-pass filter 16. Low-pass filter 16 supplies the low-frequency component of the phase detection signal to high-pass filter 17. High-pass filter 17 differentiates the output signal of low-pass filter 16. Thus, a gently changing component of the output signal of low-pass filter 16 is converted into a flat component, while a sharply changing component of the output signal is passed through filter 17 directly. With the provision of this high-pass filter a signal corresponding to a change in the capacitance generated at the time of the start of fall of pipette 10 is suppressed and not detected as any detection signal. 
     The output signal of high-pass filter 17 is amplified by amplifier 18 to be supplied as signal Vs having a waveform as shown in FIG. 4 to comparator 19. Comparator 19 compares signal Vs to threshold values +Vth1 or -Vth2 If signal Vs is beyond the range between threshold values +Vth1 and -Vth2, comparator 19 detects detection signal. More specifically, signal Vs which is below the threshold level is ignored as error signal. 
     Now, threshold values +Vth1 and -Vth2 will be described. 
     In case where sample 25 is a serum or a reagent for reaction thereof or like liquid having high dielectric constant and containing ions, the entire liquid is held at the same potential, so that the impedance change substantially coincides with a capacitive component change. For this reason, the phase detection output signal greatly contains the positive component. In this case, signal Vs is compared to threshold value +Vth1. In case where sample 25 is pure water or like liquid having low dielectric constant and substantially showing no electric conductivity, the entire liquid is not held at the same potential, so that the impedance change substantially coincides with an inductive component change. Thus, signal Vs greatly contains a negative component. In this case, signal Vs is compared to threshold value -Vth2. The sample thus can be reliably detected irrespective of whether it is inductive or capacitive. 
     The output signal of comparator 19 is supplied to one input terminal of AND gate 20, to the other input terminal of which is supplied a mask signal as shown in FIG. 5 from mask signal circuit 21. Mask signal, as noted before, has a constant-time pulse duration tM which can sufficiently cover the period tM (several 10 to several 100 milliseconds), of initial stage of descent of pipette 10, during which the electrostatic capacitance is stable and varies. With this mask signal supplied to AND gate 20, the instable signal component is never detected as liquid level detection signal as shown in FIG. 6, and a true liquid level detection signal is provided through AND gate 20. The mask signal circuit comprises a mono-stable multivibrator which provides a mask signal having a predetermined pulse duration in response to a detection signal detecting the upper dead center of the pipette. 
     In the above embodiment the phase detection signal is filtered through the low-pass and high-pass filters, but it is possible to filter the signal through a band-pass filter.