Capacitive liquid level sensor for automatic chemical analyzer

A capacitive liquid level sensor for an automatic chemical analyzer, wherein a pipetting tube approaches a liquid level of the sample to be analyzed and is electrically connected to one terminal of an electrical bridge network. The electrical bridge network is activated by an A.C. current and produces an A.C. signal which has a phase change produced by a change in capacitance between the pipetting tube and the liquid level as the pipetting tube approaches the liquid level. The A.C. signal is converted into a D.C. signal proportional to the phase difference by means of a phase detector. A liquid level is detected by comparison of the D.C. signal with a preselected reference level.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a liquid level sensor for an automatic 
chemical analyzer, and more particularly to a capacitive liquid level 
sensor which is used not only as a liquid level sensor, but also as a 
pipetting tube in the automatic chemical analyzer. 
2. Discussion of the Background 
An automatic chemical analyzer automatically analyzes a plurality of 
samples such as patient serums according to a number of analysis items. 
The samples in a sample container are dispensed to an array of reaction 
vessels using a pipetting tube. Reagent solutions selected in accordance 
with the analysis items are fed to the reaction vessels using another 
pipetting tube. The reaction solutions in the reaction vessels are 
analyzed, for example, by ion selecting electrodes for measuring ion 
activities like sodium, potassium and chlorine or by a spectrometer for 
spectral analysis. The results of such analyses are displayed on a monitor 
or typed, analysis item by item, and, patient by patient. 
In the conventional automatic chemical analyzer, a liquid level sensor 
cooperates with such a pipetting tube for detecting the liquid level of 
the samples in the sample container or reagent solutions of the reagent 
solution tanks. A deep immersion of the pipetting tube results in excess 
liquid sticking to outside of the pipetting tube. This excess liquid, not 
only decreases accuracy of the pipetting, but also, causes contamination 
with another sample or reagent solution. The inaccuracy of the pipetting 
and contamination caused by the excess liquid sticking to the outside of 
the pipetting tube makes the analysis results inaccurate and less 
reliable. 
In other words, shallow immersion of the pipetting tube gives accurate and 
reliable analysis results. A liquid level sensor enables such a shallow 
immersion of the pipetting tube and avoids deep and blind immersion of it. 
A conventional liquid level sensor is described in for example, U.S. Pat. 
No. 4,451,433. The conventional liquid level sensor comprises a pair of 
electrically conductive members. One is a pipetting tube made of a 
chemical proof metal such as platinum or stainless steel and the other is 
an electrode made of a chemical proof metal wire. Such a pair of 
electrically conductive members provides a detection signal when both are 
immersed into a reagent solution or samples. 
However such a conventional liquid level sensor may cause inaccurate and 
contaminated pipetting because the electrode accompanying the pipetting 
tube is immersed as well as the pipetting tube. Liquid sticking to the 
electrode is liable to produce less accurate and more contaminated 
pipetting. 
Further, it is difficult in the conventional liquid level sensor to watch 
and evaluate whether or not the suction of the pipetting tube is normal. 
SUMMARY OF THE INVENTION 
It is, therefore, a primary object of the present invention to provide a 
capacitive liquid level sensor for an automatic chemical analyzer which is 
able to detect a liquid level only with a pipetting tube. 
It is another object of the present invention to provide a capacitive 
liquid level sensor which enables a visual determination as to whether or 
not the suction of the pipetting tube is normal. 
These and other objects are achieved according to the present invention by 
providing a new and improved liquid level sensor including an electrical 
bridge network, a pipetting tube connecting to an element of the 
electrical bridge network which when it approaches the liquid level 
changes a capacitance between the liquid level and the pipetting tube, a 
source of A.C. current connected to a terminal of the bridge network such 
that changes in the capacitance between the liquid level and the pipetting 
tube produce an A.C. output signal from the bridge network, a phase shift 
detector for comparing the phase of the bridge output A.C. signal with the 
phase of the A.C. current applied to the bridge network to produce a D.C. 
output signal dependent on the distance between the liquid level and the 
pipetting tube, and a comparator for comparing the D.C. output signal with 
a preselected reference level to determine whether or not the pipetting 
tube has reached the liquid level. 
It is another feature of the present invention that the liquid level sensor 
further detects the amplitude of the D.C. output signal to determine 
whether or not the suction by the pipetting tube is normal.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawings, wherein like reference numerals designate 
identical or corresponding parts throughout the several view, FIG. 1 
illustrates a block diagram of an embodiment of a capacitive liquid level 
sensor for an automatic chemical analyzer in accordance with the present 
invention. A pipetting tube 28 is made of a chemical proof metal such as 
platinum or stainless steel and is driven by a pipetting tube driver 38. 
The pipetting tube driver 38 includes a drive mechanism (not shown), such 
as stepping motors and a rack and pinion mechanism, which causes the 
pipetting tube 28 to move horizontally and vertically from a home position 
H to a suction position A, from the suction position A to a discharge 
position B, from the discharge position B to a cleaning position C and 
from the cleaning position C to the home position H, as shown by the 
arrows 60. At the suction position A a sample container array 48 is 
disposed. The sample container array 48 includes sample cups 48a-48d 
loaded with samples, such as patient serums and is movable in direction 62 
to arrange each sample cup 48a-48d at the suction position A. As shown, 
the sample container array 48 includes four cups 48a-48d, but in practice 
the number of cups may be more or less than four. 
Also shown in FIG. 1 is a reaction vessel array 52 which circulates through 
the discharge position B. The reaction vessel array 52 includes a number 
of reaction vessels 52a-52d, the number of which likewise is not limited 
to four. At the cleaning position C, a cleaning section 50 is disposed to 
clean both the inside and outside of the pipetting tube 28. 
The pipetting tube 28 is linked to a microsyringe 40 through a flexible 
tube to pass a liquid or air. The microsyringe 40 sucks, holds, and 
discharges exact amounts of the sample. The microsyringe 40 is linked to a 
syringe 42 via valve 44. The syringe 42 sucks and discharges a sufficient 
amount of a cleaning liquid, such as deionized water, contained in a 
cleaning liquid bath 46 to clean the inside of the pipetting tube 28. The 
microsyringe 40 and syringe 42 are driven by a syringe driver 32. The 
valve 44 is common (COM) to the syringe 42, normally open (NO) to the 
liquid bath 46 and normally close (NC) to the microsyringe 40. In other 
words, when a control signal is supplied from a CPU 30 to the valve 44, 
the valve 44 links the syringe 42 to the microsyringe 40. Otherwise, it 
links the syringe 42 to the liquid bath 46. 
The pipetting tube is electrically connected to a terminal 13a of a bridge 
network 12 by a sensing cable 34. Since the pipetting tube 28 is made of a 
conductive material, the tip of the pipetting tube 28 is electrically 
connected to the terminal 13a of the bridge 12. From another terminal 13c 
of the bridge 12 diagonal to the terminal 13a where the sensing cable is 
connected, a compensating cable 36 extends near the pipetting tube 28, but 
is electrically insulated from both the pipetting tube 28 and the sending 
cable 34. In the preferred embodiment, the sending cable 34 and the 
compensating cable 36 are twisted together. 
The bridge 12 includes four impedance elements 12a-12d such as resistors 
and four terminals 13a-13d. An oscillator 10 is connected to the diagonal 
terminals 13b, 13d of the bridge 12 and also to a phase shifter 16. The 
ocsillator supplies an A.C. current to the terminals 13b, 13d of the 
bridge 12 and the phase shifter 16. In the preferred embodiment, the 
frequency of the A.C. current is more than 10 KHz. 
The stray capacitance Cx between the pipetting tube 28 and the liquid 
level, more precisely the ground of the sample container 48, is less than 
1 pF. The impedance of the stray capacitance Cx caused by the A.C. current 
applied from the ocillator 10 is represented as 1/jw Cx, and rewritten as 
Z(w)e.sup.j.phi.(w), where 
EQU Z (w)=1/2 Cx 
EQU .phi.(w)=tan.sup.-1 (-1/wCx), and 
w is a frequency of the A.C. circuit. 
In the case that Cx is 0.1 pF and w is 100 KHz, the impedance of the 
capacitance becomes about 10 M.OMEGA.. 
Accordingly, the resistors 12a-12d are preferrably set to about 10 
M.OMEGA.. 
The diagonal terminals 13a, 13c are connected to a differential amplifier 
14 which amplifies the output of the terminals 13a, 13d. The phase shifter 
18 varies the phase of the A.C. current and supplies the phase-varied A.C. 
current to a phase detector 16. The phase detector 16 converts the 
phase-varied A.C. current into a square wave and phase-detects the output, 
as shown in FIG. 3A, of the amplifier 14 with the square wave as shown in 
FIG. 3B. A low pass filter 20 converts the output of the phase detector 
into a D.C. signal and supplies the same to an amplifier 22. A reference 
level generator 24 generates a preselected reference level corresponding 
to the level when the pipetting tube 28 reaches the liquid level of the 
sample in the sample cup 48b. A comparator 26 compares the D.C. signal 
from the amplifier 22 with the preselected reference level and provides a 
detection signal to a CPU 30 when the D.C. signal exceeds the preselected 
reference level. The CPU 30 controls the pipetting tube driver 38 in 
accordance with the detection signal provided from the comparator 26. The 
CPU 30 further controls the sequences of the pipetting tube driver 38, the 
syringe driver 32, the valve 44 and the movements of the sample container 
array 48 and the reaction vessel array 52 and other units (not shown). 
After the pipetting tube finishes its suction and leaves the liquid level, 
the CPU 30 checks the amplitude of the D.C. signal provided from the 
amplifier 22 to determine whether or not the suction is normal. 
Now the operation of the above-described embodiment will be explained. 
First, the pipetting tube 28 is lowered from its home position H to the 
suction position A. During lowering the stray capacitance between the tip 
of the pipetting tube 28 and the liquid level of the sample cup 48b 
increases gradually. The disturbance capacitance Ce between the bridge 12 
and the pipetting tube 28 also varies and is detected by the cable 34 
because the distance between them changes. But this disturbance 
capacitance Ce is cancelled by the compensating cable 36. 
Referring to FIG. 2, which illustrates the equivalent circuit of the bridge 
12, the terminals 13a and 13b contain the stray capacitance Cx and 
disturbance capacitance Ce therebetween. However, the capacitance C'e 
nearly equal to the capacitance Ce is detected by the compensating cable 
36. Both the capacitances C'e and Ce are the same polarity to the input 
terminals of the differential amplifier 14. Accordingly, the disturbance 
capacitance is cancelled by the compensating cable 36. 
The output of the bridge 12 depends on only the stray capacitance Cx. Its 
phase .theta. varies in accordance with the stray capacitance. 
The phase detector 16 receives the output, as shown in FIG. 3A, of the 
bridge 12 through the amplifier 14 and phase-detects the same in relation 
to the square wave as shown in FIG. 3B to produce the phase-detected 
signal as shown in FIG. 3C. The phase-detected signal is supplied to the 
low pass filter 20 to produce the D.C. signal. If there is 90 degree phase 
difference between the output signal as shown in FIG. 3A and the square 
wave FIG. 3B, the D.C. signal becomes zero. This is explained more 
precisely as follow. 
The Fourier series el(t) of the square wave is described as 
##EQU1## 
where A is the amplitude of the square wave. Then the output signal e2(t) 
is written as 
EQU e2(t)=S(t) sin(wt-.theta.) 
where S(t) is the amplitude of the output signal and .theta. represents the 
phase difference between the output signal and the A.C. current. The 
phase-detected signal e3(t) is represented as 
##EQU2## 
After filtering by the lowpass filter 20, e3(t) is approximately as 
follow: 
##EQU3## 
This means that the D.C. signal is proportional to the amplitude S(t) and 
cos .theta. which depends on the stray capacitance Cx. 
In the preferred embodiment, the phase of the square wave as shown in FIG. 
3B is adjusted or calibrated by the phase shifter 18 so that the D.C. 
signal becomes zero when the pipetting tube 28 is located at the home 
position H. This D.C. signal increases as the pipetting tube 28 approaches 
the liquid level of the sample as shown in FIG. 4. 
FIG. 4 illustrates the respective D.C. signals wherein the sample quantity 
in the sample cup of the volume 1000.mu. varies from 50 .mu.l to 1000 
.mu.l. These experimental results indicate that the D.C. signals change 
significantly when the pipetting tube is near the liquid level. If the 
preselected reference level generated by the generator 24 is set to 
approximately 1 volt, the liquid level will be completely detected with 
respect to a volume of liquid from 50 .mu.l to 1000 .mu.l. 
Since the capacitance Cx is proportional to the dielectric constant of the 
material, the D.C. signal provided from the amplifier 22 increases or 
decreases according to the dielectric constant between the pipetting tube 
22 and the ground 49. FIG. 5 indicates the D.C. signals regarding toluene 
(C.sub.6 H.sub.5 CH.sub.3), butanol (C.sub.4 H.sub.9 OH), ethanol (C.sub.2 
H.sub.5 OH), methanol (CH.sub.3 OH), and water (H.sub.2 O) of the same 
volume. Thus, the D.C. signals depend on the dielectric constant of the 
material. FIG. 6 shows curves of the D.C. signals of water and serum, 
obtained by varying the volumes thereof. The dielectric constant of the 
serum is higher than that of water. Furthermore the larger the volume of 
the sample is, the higher the D.C. signal is, as shown in FIG. 6. 
Therefore the preselected reference level is determined according to the 
material and its range of volume. 
When the CPU 30 receives the detection signal from the comparator 26, the 
CPU 30 controls the pipetting tube driver 38 so that the pipetting tube 28 
is immersed as shallow as possible, but deep enough not so as to suck air. 
Then, the microsyringe 40 is activated by the CPU 30 such that the 
pipetting tube 28 sucks a certain amount of the sample. In the preferred 
embodiment, the microsyringe 40 and the tube between the pipetting tube 28 
and the microsyringe are filled with the cleaning liquid. There is an air 
bubble to isolate the sucked sample and the cleaning liquid in the 
pipetting tube. 
Next, the pipetting tube 28 holding the samples moves from the suction 
position A to the discharge position B. In the case of plurality of 
analysis items to be performed on the sample, the pipetting tube 28 at 
once sucks and holds sufficient sample to discharge into the reaction 
vessels in amounts equal to the analysis items. The pipetting tube 28 at 
the discharge position B discharges the held sample into the reaction 
vessels in amounts equal to the analysis items, synchronized with the step 
by step movement of the circulating reaction vessel array 52 and 
activation of the microsyringe 40. 
After finishing discharging the sample, the pipetting tube 28 moves from 
the discharge B to the cleaning position C. The cleaning section 50 at the 
cleaning position C washes the outside of the pipetting tube with a 
cleaning liquid such as deionized water, which is also provided via the 
activation of the syringe 42 to the inside of the pipetting tube 28. 
Cleaning liquid is discharged from the pipetting tube 28 to clean the 
inside of it. 
Then, the cleaned pipetting tube 28 is returned from the cleaning position 
C to the suction position A. The sample container array 48 moves one step 
to arrange the next sample cup at the suction position A. In this way the 
above operation is repeated. 
The reaction vessels loaded with the samples moves to the reagent pipetting 
section (not shown). In this section, the reagents corresponding to the 
analysis items are discharged into the respective reaction vessels by a 
reagent pipetting tube which sucks and discharge such reagents. In the 
preferred embodiment, the reagent pipetting tube also includes a liquid 
level sensor the same as that provided for the pipetting tube 28, as shown 
in FIG. 1. This liquid level sensor contributes not only to accurate 
discharge of reagent and less contamination with another reagent, but also 
detection of the rest of the reagent. 
After discharge of the reagent, the reaction vessels loaded with the 
reagent solution are transferred through a reaction section (not shown) 
which includes a temperature-maintained bath kept over the range between 
25.degree. C. and 37.degree. C. Then the reagent solutions are analyzed by 
analyzers, such as ion selective electrodes, a spectrometer and a 
photometer. The CPU 30 acquires such data analyzed by the analyzers and 
outputs the analyzed data to the monitor or printer, sample by sample and 
item by item. 
In the preferred embodiment, the pipetting tube 28 sucks much sample little 
by little, synchronized with the gradual immersion of the pipetting tube 
28. After detection of the liquid level, the pipetting tube 28 is immersed 
deep enough to suck a portion of the sample, but not so deep as it sucks 
the entire amount of the sample. After suction of the portion of the 
sample, the pipetting tube 28 is further lowered to repeat the suction 
until it finishes suction of the entire amount of the sample. Suction of 
portions of the sample is facilitated by the microsyringe 40 driven by the 
syringe driver 32 under control of the CPU 30 responsive to the detection 
signal provided from the comparator 26. 
Next, the monitoring operation of this embodiment according to the present 
invention will be explained. After finishing sucking the sample, the 
pipetting tube 28 leaves the liquid level holding the sample, and moves 
from the suction position A to the discharge position B via the home 
position H. When the pipetting tube 28 is located sufficiently far away 
from the liquid level, like at the home position H, the D.C. signal 
becomes independent of the liquid level. Furthermore, since the phase 
detector 16 is calibrated so as to output zero when the pipetting tube 28 
is located at the home position H and filled with deionized water, the 
D.C. signal is not zero, if the pipetting tube 28 holds a sample having a 
dielectric constant which is different from that of water. 
FIG. 7 shows a curve of the D.C. signal when the pipetting tube 28 holds 
different volumes of serums as a sample at the home position H. As shown 
in FIG. 7, if the pipetting tube 28 holds more than approximately 50 .mu.l 
of the serum, the D.C. signal is between zero and one volt. It is verified 
that the D.C. signal is zero when the pipetting tube 28 sucks air 
accidentaly, because the dielectric constant of air is less than that of 
water. This D.C. signal is reliable in this embodiment, because the 
disturbance capacitance Ce between the pipetting tube 28 and the bridge 12 
is cancelled by the compensating cable 36. 
The CPU 30 checks the D.C. signal while the pipetting tube 28 is located at 
home H on the way from the suction position A to the discharge position B 
after the suction of an amount of serum over 50 .mu.l. At that time, if 
the D.C. signal is not detected, the CPU 30 will interrupt the next 
procedure and cause the pipetting tube 28 to suck the same serum again. 
Occasionally the liquid level sensor takes the liquid level from a bubble 
in the sample and the pipetting tube 28 sucks air. But in this embodiment 
such an erroneous suction is automatically detected before discharge. A 
conventional liquid level sensor only detects the liquid level of the 
sample, but does not detect whether the suction was normal or not. 
Obviously, numerous modifications and variations of the present invention 
are possible in light of the above teachings. It is therefore to be 
understood that within the scope of the appended claims, the invention may 
be practiced otherwise than as specifically described herein.