Note: Descriptions are shown in the official language in which they were submitted.
~2~7~;
The present invention relates in general to
apparatus and a method for determining the level of material in
a container and in particular to a digital point levcl switch for
use with a gamma ray level detection device.
During many manufacturing
processes, it is important to know the level of the material in a
process vessel. In some cases, a nuclear device is utilized to
detect these levels, the device including a gamma ray source and
a geiger tube positioned wit-h respect to one another and the
vessel such that less gamma rays are received by the geiger tube
when the material in the container rises above the level of the
source detection line. The geiger tube responds to the ga~ma
rays by generating output pulses.
The conventional prior art point level switch is an analog
device which includes an amplifier fox amplifying the geiger tube
output pulses, a rate circuit for converting the pulses per unit
of time into a voltage having a magnitude proportional to the
pulse count rate, a trigger circuit for comparing the rate circuit
voltage with a reference voltage level to detect the level at
which material is to be added to the container and a relay driver
and relay for activating an alarm or an apparatus for adding
and/or removing the material. In general, these devices must be
simple, rugged and inexpensive. In order to operate at minimum
possible radiation levels, it is necessary to employ circuitry
~hich is accurate and reliable in an industrial environment In
an analog device, such circuitry tcnds to bc e~;pensive. ~or
example, cost considerations limit the use of precision dial
potentiometers for setting tl-e triggcr level or levels ~or the
circuit ~hen it is m~nuractured. Thereforc, tlle device oftcll mus~be
adjustcd in tlle field with tlle powèr on undcr operating condi~i~s.
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However, under hazardous operating conditions, it may not be
possible to make the necessary adjustments. In these cases, the
circuitry can be positioned in a location remote from the con-
tainer, but the cost of the device is increased.
Some attempts have been made to set the trigger levels at
the factory, but the settings can be changed accidently during
shipment or installation. Furthermore, since point level switches
are often required to operate in high vibration applications, the
potentiometers for setting trigger levels are vunerable to changed
settings and damage.
Another contributor to the vibration problem is the power
transformer for the geiger tube high voltage and signal circuit
power supply. The weight of this transfor~er makes effective
shockproof mounting of the electronics a problem.
The present disclosure concerns a digital point level switch
which eliminates the need for an amplifier, tlle analog pulse rate
to voltage converter, the potentiometers and the power trans-
former. Furthermore, the trigger level need not be adjusted and
the resistance to shock and vibration is improved over the prior
art devices.This switch utilizes the output pulses from the
geiger tube and the frequency of the power line in a race circuit.
The geiger tube output puls2s are accumulated ~y a first or level
colmter to a first predetermined total representing a level of
material in the container at ~llich a bclow level col-dition is ~o
be indicate~. The power line cycles are accumulated by a sccond
or interval counter to a second prcdctermined total rcpresentillg
a fixed lengtll counting intcrval.
The race circuit is rcspollsive ~o the coulltcr wllich rcaclles
its prcdeterl)lincd total irst to rc~set both of thc counteL-s. If
~Z~75
the first counter reaches the first predetermined total before
the end of the counting interval, a control circuit is set for
activatin a low level alarm for adc3ing material to the container.
Then the race circuit becomcs responsive to a third predeter-
mined total of the first counter, less than the first predeter-
mined total, representing a level of material in the container at
which a high level condition is to be indicated. The first and
third predetermined totals provide a hysteresis effect to prevent
the point level switch from "hunting" around the level at which
a high to low or a low to high transition is to take place.
After the container has been filled to the level represented
by the third predetermined total, the race circuit will be con-
trolled by the second counter. The second counter will reset the
control circuit and turn off the low level alarm (or activate a
high level alarm) and/or stop the filling of the container. ~ow
the level in the container must drop from the level represented
by the third predetermined total to the level represented by the
first predetermined level before the above alarm condition is
reversed.
Here described is a point level switch which is more economical
to manufacture and install than prior art switches. The point level
switch which has increased resistance to shock and vibration, and
does not require field adjustments. The point level switch whlch
can operate directly from a geiger tube and an a.c. power line.
The point level switch will more easily operate at lower radiation
levels than prior art apparatuses.
1~"2~.8P~5
Speclfic embodiments of the invention will now be described
having reference to the accompanying drawings, in which;
Fig. 1 is a block diagram of a point level switch embodying
the present invention;
Fig. 2 is a schematic diagram of the point level switch of
Fig. l; and
Fig. 3 is a schematic diagram of the power supply for the
geiger tube of ~igs. 1 and 2.
. .
Fig. 1 is a
block diagram of an embodiment of the present invention, which
can be utilized to control the level of a material in a container.
A ga~na ray radiation source 11 an~ a geiger tube 12 are posi-
tioned with respect to one another and the container (not sho~n)
such that as the level of the material in the container rises
above the indicator beam, less gamma rays are detected by the
tube 12. Each detected gamma ray generates an output pulse from
the geiger tube 12 to a first or level counter 13. The counter
13 is digital and generates an output signal when the predeter-
mined number of geiger ~ube output pulses have been received. A
second digital interval counter 14 receives input pulses from an
a.c. power line 15 to define a counting interval.
Each counter has an output connected to a reset circuit 16.
The level counter 13, upon reaching a first predetermincd number
of counts, and the interval counter 14, upon reaching a second
predetermined numbcr of counts, each genel-ate an output signal to
the resct circuit. If tlle interval coullter reaches the second
predetcrlllilled l~umber of counts first, thc resct circuit will
B~75
respond to the output pulse to rcset both counters and a new
counting interval is begun. If the level counter reaches the
fir~t predeterTnilled number of COUIltS ~irst, thc coun~crs will
also be reset and an output llip flop which is responsive to the
output signals from both counters, is set. The output relay 18
then provides a low level contact closure. As the material level
in the container increases, there will be a level, the desired
material level, where the interval counter will reach the second
predetermined number of counts first and will generate an output
pulse to reset both counters and the level control. Thus, the
present invention will automatically turn on and off a relay for
regulating the addition of material to a container in response to
the level of such material in the container or for providing
required high or low level alarms.
Fig. 2 is a schematic diagram of the present invention which
is disclosed in block diagram form in Fig. 1. In the descrip~ion
of Fig. 2, a true or one logic signal will be represented as a
"1" and a false or zero logic signal will be represented as a
"0" Furthermore, each logic element having three or more ter~ninals
will have those terminals identified with a number and referenced
by the element reference numeral, a dash and the terminal refer-
ence numb-erj such as 13-1 for the input terminal of the level
counter 13.
The geiger tube 12 has a cathode connected to a negative
polarity d.c. power supply (not shown), typically minus 480
volts, and an anode connected through a pair of resistors 21 and
22 to a positive polal-ity d.c. power supply (lot shown), typi-
cally fifteen volts. The junction of the resistors is connected
to the counter input 13-1. The resistor 22 functions to limit
current flow into the geiger tube 12 alld resistor 21 fullctions as
a load resistor to gcllerat:e a logic lcvcl sigll~l at the input 13-1.
--6--
1~l21~7S
I~len a gamma ray is detected, a negative polarit~ pulse is
generated on the ano(~e o~ the ge;ger tube w~lich in turn generates
a negative going transition in the magnitude of the signal at the
input 13-1.
The counter 13 may be a fourteen stage ripple-carry, binary
counter/divider having the input 13-1, outputs 13-2 through 13-13
representing the first and fourth through fourteen binary stages
and a reset input 13-14. A "1" to "O" change in signal at the
input 13-1 is added to the total count in the counter 13 ~ith the
total count available in binary form at the outputs. A "1" at
the reset input 13-14 will reset the counter and all its outputs
to zero.
The interval counter 14 is similar to the level counter 13
and has an input 14-1 connected to the clock signal source 15
through a current limitir.g resistor 23. The source 15 can be an
a.c. power source, typically the standard sixty hertz power line,
~herein the transitions from positive to negative half cycles
are counted by the counter 14. Thus, the total number OL counts
in the counter 14 represents elapsed time and the total number of
counts in the counter 13 represents detected gamma rays during
that time ~hich total is modulated by material level when the
material level is at or above the gamma ray beam.
The outputs 13-6, -7 and -8, representing the seventh,
eighth and ninth stages o~ the counter 13, are connecte~ to the
inputs oE a ~AND gate 24. ~ NAND generates an "O" at its output
when all of its inputs are at "1" and generates a "1" for any
other conlbination o input sinals. With its three inputs con-
nected as shown, thc NAND 24 will produce a "O" when the counter
13 has co~lnted 448 pulses (256 + 128 + 64).
Outputs 14-8 and -9, represen~-ing tlle nintll and tclltll stagcs
o the coulltcr 14, arc conncctcd to ~oth inputs o a ~A~D 25
~h;ch will provide a "0" input when the timi,n~ counter 14 has
counted 768 line cycles (512 ~- 256), An output 24-4 of the NAND
24 is connected to an input 26-1 of a ~AND 26 and an output 25-3
of the NAND 25 is connected to an input 26-3 of the NAND 26. A
NAND 27 has an output 27-4 connected to an input 26-2 and the
NAND 26 has an output 26-4 connected to the reset inputs 13-14 and
14-14 of the first counter 13 and the second counter 14 respec-
tively. The N~ND's 26 and 27 are included in the reset circuit
16 of Fig.,l. If both counters have been reset, the NAND's 24,
25 and 27 will each generate a ~ such that the ~AND 26 generates
a "0" at the reset inputs to enable the counters to count.
The NAND 27 has a pair of inputs 27-1 and 27-2 connected to
an output 13-5 and -6, representing the sixth and seventh stages
of the counter 13, and an input 27-3 connected to an output 17-3
of a N~ND (RS) flip 10p representing the relay control 17 of
Fig. 1. The output 24-4 of the NAND 24 is connected to the set
input 17-1 of the flip flop and the output 25-3 is connected to the
reset input 17-2. Another output 17-4 of the flip flop is connecte~
to both inputs of a bt~fer. During each counting-time cycle
outputs 24-4 and 25-3 are "1" allowing RS flip flop 17 to remain
in whichever state it is already in. If the flip flop 17 is al-
ready in the "reset" state (17-4 = "1", 17-3 = "0") and the time
counter 14 reaches its predetel~ined time of 768 (512 + 256) line
cycles as shown in Fig, ~, the resulting "0" at 25-3 will cause both
counters 13 and 14 to be reset, the count to ~e resumed, and the
flip flop 17 will remain in the reset state. Should the flip flop
17 already be in the set state (17-4 = 0, 17-3 = 1), the "0" at
25-3 would Challge 17 to the reset state in addition to resetti~g
both countel-s for a resumption of the count,
If the flip flop 17 is in the reset s~a~e while'counter 13
rCnCIlCS il-S hi~,ll COt311~ preset of ~48(256 + 128 + 64) asdefined by
~Z~ 5
gate inputs 24-1, 24-2 and 2~-3 a~; shown in Fi~2, then the resulting
"0" output in 2~-4 switchcs ~he flip flop 17 to the set state
causing a "1" at the output of bufer 28 to the relay driver 24.
If the flip flop 17 is already set, input 27-3 will be "1". Thus,
when counter 13 reads its low count preset of 96 (64 + 32), as
defined by gate inputs 27-1 and 27-2, as shown in Fig. 2, both
counters 13 and 14 will be reset and the count resumed without
changing the state of flip flop 17.
A "1" at the sixth and seventh stage outputs 13-5 and -6
represents 96 (64 + 32) counts, a "1" at the eighth and ninth
stage outputs 13-7 and -8 represents 44~ (256 ~ 128+ 64) counts and
at the ninth and tenth stage outputs 14-8 and -9 represents 768
(512 + 256) counts. If the counter 14 totals 768 counts before
the counter 13 totals 448 counts, the NA~D 25 will generate a "0"
to the input 26-3. The N~ND 26 responds by generating a "1" to
reset both counters and start a ne~ counting interval. The "0"
generated by the NAND 25 is also applied to the reset input 17-2.
This does not change the output signals generated by the flip
flop 17 if the flip flop is already in the reset state. If it is
in the set state, the flip flop 17 is switched to the reset state.
If, while the flip flop 17 is in the set state, the counter
13 totals 96 counts before the counter 14 totals 768 counts, the
output 24-3 will change from "1" to "0" changing the NAND input
26-2 to be "0", and NAND output 26-4 to be "1", thus resetting
both counters 13 an~ 14 and resetting a new counting interval.
Ilowever, the flip flop 17 is in the reset mode, continuing to
apply a "0" at the input 27-3 to generatc the "1" to the NA~D 26
such that the counters 13 and 14 continue to count. If the
counter 13 tol:als 448 counts befol-e the counter 14 toLals 768
counts represcntin~ a lower than desircd level of ma~erial in Lhe
container, the NAND 2~ ill cilallge from "1" to "0" to the ill~Ut
~211~5
,-1. Thc NAND 2G responds by gelleratillg a "1" to reset both
counters and start a new counting interval The "O" generated by
the~NAND 24 is also applied to the set input 17-1. With the
input 17-1 at "O" and the input 17-2 at "1", the output 17-3 will
change to "l" and the output 17-4 will change to "O". The buffer
28 changes the signal at an output 28-2 from "O" to "1".
The output 28-2 is connecte~ to a gate of a triac 29 connected
in series with a relay coil 30 across an a.c. power source (not
shown). ~n en the signal at the output 28-2 switches from "O" to
"1", the triac is turned on to pass a.c. current and energize the
relay coil 30 providing a contact closure to indicate a low
material level. .Thus, as long as the flip flop 17 is in the set
condition, the buffer 2~ will generate a "1" and the material
will flow into the container.
The "1" at the output 17-3 is applied to the input 27-3 to
enable the NAND 27 to respond to "l"'s generated at the outputs
13-5 and -6. Such operation prevents the circuitry from "hunting"
around the 4~8 count total as the count rate fluctuates and to the
level of the material and radiation statistics. Thus, the con-
tainer will be filled to a level just above that required to re-
duce the count rate below 96 counts during the counting interval.
At that point, the counter 14 will total 768 counts before the
counter 13 totals 96 counts and the NAND 25 will reset the flip
flop 17 with a "O" at the input 17-2. The output 17-4 will
change to "1" to turn off the triac 29 and stop the filling of
the container. The output 17-3 will chan~e to "O" to disable the
NAND 27 and the "O" from the NAND 25 will gcnerate a "1" from the
NAND 26 to reset both of the counters. Now the counters will
continue to be resct by the 768 count total of the counter 14
until thc material ~alls to the lcvel wllcre the COUI~ter 13 totals
448 counts bcfore thc co~ tcr l~ tota]s 7G~ counts. Then the
-10-
375
illin~ operation will ~e startecl. 'I`hus, thc circuit of ~ig. 2
has becn disclosed as hav;ng a scvcnty-five percent hysteresis,
96 to 4~8 counts, ~ut any combination' of the outputs of the level
counter 13 could be uti],i,zed to obtai,n the desired hysteresis
and total counts. Also, any combinat:ion of outputs ~rom counter
14 could be utilized to different time intervals.
Although the relay coil 30 has been discussed in terms of
operating an apparatus for adding material to the container, it
could be utilized to actuate an alarm to indicate a lligh or low
level. Furthermore, the BUFFER 28 ~ould have its inputs connected
to the output 17 3 instead of the output 17-4 if it is desired to
have the relay energized for low material level instead of hi~h
material level.
In Fig. 3 there is shown a schematic diagram of the regulated
voltage multiplier power supply for the geiger tube 12. This
power supply eliminates the power transformer of the prior art
power supplies. A pair of input lines 41 and 42 are connected to
a 115 volt a.c. power supply (not shown) which is con~only avail-
able in manufacturing operations. The line 41 is connected
through a coupling capacitor 43 to an anode of a zener diode 44
which has the cathode connected to the anode of another zener
diode 45. The zener diode 45 has the cathode connected to the
input line ~2. The zeners are each rated at 120 volts to provide
a half wave rectified ~ave form clip,ped at 240 volts to approxi-
mate a square wave~
A diode 46 has tlle cathode connected to the anode of the
zener diode 44 alld the anode connected to the junction of a pair
of capacitors 47 and 4-~. The other side of thc capacitor 48 is
connected to the illpUt line 42 and thc other sidc of the capacitor
47 is conncctcd to the junction of thc anode of a diode 49 and a
filter rcsistor 51. Tllc cathodc of thc diodc 49 is conncctcd to
8~5
the anode o~ a diode 52 havillg ~he ca~hocle conllec~ed to tl~e anode
of the diode 46. A capacitor 53 is connected from the cathode of
diode 46 to the anode o~ diode 52. The other end of the resistor
51 is connected to the negative 480 volt output line 54 and a
capaeitor 55 is connected between the lines 42 and 54.
~ len tlle input line 41 is negative with respect to the input
line 42, the capacitor 48 will charge to 240 volts through the
diode 46 and the coupling capacitor 43 as the zener diodes 44 and
45 clip the applied a.c. wave form. Tllus, the junction of the
eapacitors 47 and 48 is at a negative 240 volts with respect to
the line 42. When the line 41 is positive with respect to the
line 42, the zener diodés will hold the junction of the capa-
eitors 43 and 53 near the potential on the line 42 and the capac-
itor 53 will charge to a negative 240 volts throug~ the diode 52.
~en the line 41 is again negative with respect to the line 42,
the capacitor 53 is clamped ~t a negative 240 volts b~ the zener
diodes 44 and 45 and, therefore, the junction of the diodes 49
and 52 will be 240 volts below the potential at the junction of
the capacitors 43 and 53 or a negative 4~0 volts. The capacitor
47 will now charge to 240 volts, a negative 240 volts at the
junction of the capacitors 47 and 48 and a negative 480 volts at
the junction of the diode 49 and the resistor 51. Thus, the
eapacitors 47 and 48 provide 480 volts to charge tlle output
eapacitor 55 and generate a negative 480 volt potential on the
output line 54 ~ith respect to the input line 42.
A diode 56 has an anode connected to the input line 41 and a
cathode connected to a positive fifteen volt output line 57
throu~h a pair of series connected current limiting resistoL-s 58
and 59. A filter capaci~or 61 is connected bet~eell the junction
of the resistors 5S ancl 59 and the input line 42. A fifteen volt
7.cner diodc has an anode conTlected to thc input line 42 and a
-12-
1~2~ 5
athodc conncctcd to thc output linc 57. ~ell thc line 41 is
positive with respect to the line 42, the capacitor charges
throu~ll the resistor 58 and the zener diode clips the wave form
at fifteen volts. Thus, this portion of the power supply is a
half wave rectifier wherein the resistors 58 and 59 function as a
voltage divider to permit the use of a lower voltage filter
capacitor 61.
If the geiger tube 12 of Fig. 2 is connected across the out-
put lines 54 and 57, 495 volts d.c. are applied to the tube to
render it operational. The capacitors 55 and 61 smooth the out-
put and provide current during the negative half cycle of the
a.c. input voltage. Thé output line 57 and the input line 42 can
also be connected to the logic elements of Fig. 2 to provide
operating power thereto.
The accuracy of the level detection depends upon the strength
of the radiation, ~he number of gamma rays emitted over a time
interval, and the length of the counting time interval. Since it
is important from a cost standpoint to operate at the minimum
possible radiation levels, it is necessary to utilize relatively
long counting time intervals such as one or two minutes. In an
analog device, the circuitry required to obtain the required time
constants becomes too costly and unreliable. The present inven-
tion can utilize relatively short counting time intervals, the
output 14-8 represents 4.27 seconds at sixty hertz, or relatively
long counting time intervals, the output 14-13 represents 137
seconds at sixty hertz. Thus, in operations whcre tl-e rate of
change in level the material does not requirc rapid rcsponse, the
present invention can function at as much as a factor o~ ten
lower ratiation souL-ce stren~th than the convelltional analog
point level switch. ~urthcrmore, l~he coul~ters 13 and 14 can cach
l~ave any two or more o~ their outl~uts connectcd to a NAND lavin~
-13-
~ llZ~ 5
le rcquirecl null~ber o~ inputs to (~!)t-a;l~ y d(si.rccl COUllt ~C)~:
between one and the maximum 16,38~
In summary the present invention is preferably utilized in a
system of detecting the level of material in a container and
activating an alarm or adding materia1. when the material has
-eached a first predetermined level at which material is to be
added. A radiation source such as a gamma ray source, and a
detector means, such as a geiger tube generate a pulsed output
signal at an average pulse rate that is inversely proportional to
the level of the material in the container. A first counter
means is responsive to the pulsed output signal for accumulating
a pulse count and generating an output signal for a first pre-
determined total number of pulses counted representing the first
predetermined level. A second counter means is responsive to
clock pulses such as the cycles of an a.c. signal, for accumulatin~
a pu~se count and generating an output signal for a second pre-
determined total number of pulses counted representing a counting
time interval. A resetting means is responsive to the first and
seeond predetermined total output signals for resetting the first
and second counter means and means are responsive to the first
predetermined total output signal for generating a signal indi-
eating that the material has reaehed the first predetermined
level.
The first counter means also generates an output signal for
a third predetermined total number of pulses counted representing
a second predetermined level of the material. The resetting
Means is responsive to the third predetermined total output
sigl1al during the time the inc3icati.ng signal is being genera~ed.
The indicating mcans is responsive to the second predeterm;l1ed
total out~>ut signal for terMinatinc~ the generatiol~ of the indi-
cating signal fo~ clicatincr~ t11at the mal-crial has rec~chccl th~
secon(3 p1~ec3e~el~mil1ecl level. Tl1e indicatil1g means c.ln be a col1~rcl
~ 7S
means for genera~in~ a control s;gnal for activatin~ an alarm or
means for adding material to the container. The indicating or
control means can include a flip flop means which is set by the
first predetermined total output signal to generate the control
signal and an enable signal and is reset by the second predeter-
mined total output signal to terminate the generation of the
control signal and the enable signal. The resetting means can be
responsive to the enable signal and the third predetermined total
output signal for resetting the first and second counter means.
Also described here is a method for determining
the ,evel of material in a container comprising the steps of
generating detection puises at an average rate inversely propor-
tional to the level of the material in the container, generating
clock pulses at a predetermined rate, simultaneously accumulating
said detection pulses in a first counter means and said clock
pulses in a second counter means, generating an output signal
rom the first counter means for a first predetermined total num-
ber of pulses counted representing a first predetermined level of
the material, generating an output signal from the second counter
means for a second predetermined number of pulses counted repre-
senting a counting time interval, resetting the first and second
counting means in response to the one of the first and second pre-
determined total output signals which is generated first, and
generating a signal in responsè to the first predetermined total
output signal to indicate that the material has reached the first
predetermined level. The method can also il~clude the steps of
generating an output si.gnal from thc first counter means for a
third prcdeterlllined total num~er of pul5es counted repl-esellting
a second prccletcrmincd level of the material and rcsctting thc
first and second COUntitlg mcallS il- rcspollse to the gencra~ion of
the tllird prcclctcrn-inecl total oul:put sigllal aftcl tlle first
]5-
t3 75
predetermined total output signal. has been generated. The metllod
further can include the step of ~.:erminating the gcneration of the
signal representing the first predetermined level in response to
the generation of the second predeteL^mined total output signal to
indicate that the material has reached the second predetermined
level.
In accordance with the provisions of the patent statutes, the
principle and mode of operation of the invention have been ex-
plained and illustrated in its preferred embodiment. However, it
must be understood that the invention may be practiced otherwise
than as specifi.cally illustrated and described without departing
from its spirit or scope.
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