CA1049289A - Electronic thermometer - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

An electronic thermometer system comprising a temperature sensing circuit for providing an analog signal representative of the temperature being sensed, a voltage to rate converter circuit responsive to the analog signal for providing uniform digital pulses at a repetition rate pro-portional to the temperature being sensed, and means for count-ing the digital pulses for a predetermined period of time for providing an indication of the temperature being sensed.

Description

1~49Z~39 F IELD OF INVENTION
This invention relates to an electronic thermometer system utilizing a voltage to rate converter circuit for trans-forming an analog si~nal representative of a temperature being sensed into a digital signal proportional to that temperature for measurement by digital circuits to indicate the temperature, and to such a system having an anticipation circuit for intro-ducing the final value of the temperature before it has been measured.
sAcKGRouND OF INVE~TION
Conventional electronic thermometer systems use a sensing element e.g. thermocouple, thermistor, diode whose impedance changes, or voltage or current output varies as a function of the temperature to which it is exposed. The signal is submitted to an ahalog measuring or indicating device such as a galvanic meter which displays the temperature. Such systems are satisfactory in sophisticated technical and scientific environments where personnel are accustomed to such equip~ent. However, recently, electronic thermomeh~rs have become increasingly more widely used in less technically sophisticated areas such as patient care by medical personnelO
Electronic thermometers quickly rivaled mercury thermometers in the area of cost and speed. The cost of using an electronic thermometer with disposable probe covers is compatible-with~the~
cost of purchase of, plus routine sterilization of mercury thermometers, and electronic thermometers can measure temperatures much more quickly conserving expensive and scarce personnel time. This area of'~application demands a more easily readable as well as smaller, lighter, more compact and less expensive electronic thermometer. In view of present electronic technology these aims are best served by making maximum use of digital as - 3 - ~

1~49Z89 opposed to analog implementation. One attempt to create an electronic thermometer using at least some digital circuitry resulted in a system in which the temperature sensing probe provides a signal to a bridge circuit. When the temperature sensed exceeds some predetermined reference, the bridge im-balance causes a signal to step a counter and charge a capaci-tor. Thê counter causes a resistor to be inserted in the bridge to balance it. Further increase in temperature causes the cycle to be repeated until, the capacitor has been charged sufficiently to trigger a switch to display the temperature:
when the changes in temperature are occurring at a slow enough rate relative to the thermal time constant of the thermometer it is assumed that the temperature sensed is at its final temperature within the desired accuracy. While such a system does operate in a digital fashion it requires many switches, extra resistors and circuitry in a large, heavy and expensive system. Increased accuracy in such a system is acquired at the cost of increased numbers of resistors and switches.
SUMMARY OF INVE~TIO~
It is therefiore an objectoof this invention to pro-vide an improved, smaller, more compact, lighter and less expensive electronic thermometer system making increased use of digital electronics.
It is a further object of this invention to provide such an electronic thermometer system which converts the analog temperature signal into a digital signal whose pulse rate is proportional to the temperature. -It is a further object of this invention to provide such an electronic thermometer system which indicates final temperature before it is actually measured.

1(~4~Z89 The invention features an electronic thermometer system having a temperature sensing circuit for providing an analog signal representative of the temperature being sensed.
A voltage to rate converter circuit responsive to the analog signal provides uniform digital pulses at a repetition rate proportional to the temperature being sensed represented by that analog signal. Means are provided for counting the digital pulses for a predetermined period of time for provid-ing an indication of the temperature being sensed.
According to the above features, from another broad aspect, the present invention provides an electronic thermo-meter system which comprises a temperature sensing circuit for providing an analog signal representative of the temperature being sensed. A voltage to rate converter circuit is also provided and responsive to the analog signal for providing constant width and amplitude pulses at a repetition rate which is proportional to the temperature being sensed. The converter circuit includes an integrating circuit having a first input for receiving the analog signal and a second input for receiv-ing a reference input, responsive to a difference between the first and second inputs for providing a voltage proportional to that difference. A pulse generating circuit is further provided for producing pulses of constant width and amplitude, each of the pulse provided in response to a predetermined out-put level of the integrator. Further provided is a feedback circuit for introducing each of the pulse to one of the first and second inputs to reduce the difference between the first and second inputs and rest the integrating circuit. Further-more, there is provided means for counting the pulses for a predetermined period of time for providing an indication of the temperature being sensed.
~ _ 5 _ 1049Z~9 DISCLOSURE OF PREFERRED EMBODIME~T

Other objects, features and embodiments will occur from the following description of a preferred embodiment and the accompanying drawings, in which:
Fig. 1 is a block diagram of an electronic thermo-meter system using a voltage to rate converter circuit accord-ing to this invention;
Fig. 2 is a more detailed block diagram illustrating one implementation of the system of Fig. l;
Fig. 3 is a schematic diagram of the probe, internal temperature reference circuit, bridge circuit, anticipation circuit, reference current switch, integrator circuit and constant width pulse generator of Fig. 2;
Fig. 4 is a graph illustrating the RC constant of the anticipation circuit and the thermal constant exhibited by the probe of Figs. 2 and 3;
Fig. 5 is a schematic diagram illustrating an alter-native construction for the probe of Figs. 2 and 3; and Fig. 6 is a schematic diagram of an alternative con-struction of the anticipation circuit of Fig. 1.
There is shown in Fig. 1 an electronic thermometersystem 10 according to this invention including a temperature - 5a -sensing circuit 12 which senses the temperature and provides an analog signal representative thereof to the voltage to rate converter 14. Voltage to rate converter 14 provides at its output digital pulses whose repetition rate is proportional to the analog input signal representative of the temperature being -~ensed. These pulses are accumulated by counting cir-cuit 16 to measure the temperature being sensed. ~his system is operated with power and control circuit 18.
Temper~ture sensing circuit 12 may include a probe 20, Fig. 2 for sensing a temperature to be measured and pro-ducing an analog signal representative thereof which is sub- ~
mitted through internal reference circuit 22 to bridge CI~CUit 24. Internal reference circuit 22, immediately upon unplugging of probe 20 automatically connects a matching circuit to bridge circuit 24 in place of the input from probe 20, so that the accuracy and operation of the system can be verified. Bridge circuit 24 provides a reference output on line 26 and on line 28 provides a varying output as a function of the bridge im-balance representing the analog signal which is a function of the temperature sensed by probe 20. In this specific embodi-ment, used primarily to take the temperatures of humans, the measurement range is typically from 90F to 110F. Thus reference output 26 of bridge circuit 24 represents the level of 90F, when output 28 of bridge circuit 24 is equal to refer-ence output 26 the thermometer probe 20 is measuring a temper-ature of 90F. When output 28 is at a predetermined deviation from the level of output 26 the probe 20 is measuring 110F.
Output 28 is fed to anticipation circuit 30 which senses the rate of change of the temperature being sensed by probe 20 and modifies the signal on output 28 from bridge circuit 24, thereby providing a signal at summing point 32 in voltage to rate con-verter 14 representative of the final value of the temperaturebeing sensed in advance of the actual sensing of that final value, In voltage to rate converter 14 the signal at summing point 32 is directed to the negative input of integrator circuit 34 whose positive input receives reference output 26 from bridge circuit 24, A difference between summing point 32 and reference output 26 at the input to integrator 34 cause it to provide a positive slope ramp at its output to constant width pulse generator 36, which provides a negative going output pulse of fixed width when the ramp reaches a predetermined voltage level.
The fixed width pulse is delivered along feedback line 38 to reference current switch 40 which produces a positive going pulse having fixed width and fixed amplitude and delivers it to summing point 32, The presence of this pulse temporarily restores summing point 32 to the level of output 26 causing the integrator circuit output to drop resulting in a sawtooth output signal. Constant width pulse generator 36 then produces no further pulses to reference current switch 40. Therefore no further pulses are delivered to summing point 32 and the level at summing point 32 once again moves away from that at the reference output 26. This causes integrator circuit 34 to provide another positive ramp and the cycle to begin again.
Since the pulse fed back to summing point 32 has fixed width and fixed amplitude it is the rate of those pulses which must adjust to the relative imbalance between summing point 32 and reference output 26. Thus the greater the difference between these two inputs to integrator circuit 34, the higher will be the repetition rate of the pulses provided at the output of constant width pulse generator 36, this repetition rate is pro-portional to the temperature being sensed by probe 20. The 104~Z89 illustrated configuration of voltage to rate converter 14 in Fig, 2 which includes summing point 32, integrator circuit 34, constant width pulse generator 36, feedback line 38 and refer-ence current switch 40 is one example of a voltage to rate converter which may be used. A voltage controlled oscillator or other means for producing an output whose frequency varies in proportion to an analog signal may be used.
Counting circuit 16 includes digital counting and decoding circuit 42 which counts the digital pulses provided at the output of constant width pulse generator 46 for a pre-determined period of time and decodes that count to display the measured temperature on digital display 44.
Power and control circuit 18 includes a power supply 46 and an automatic on-off electronic switch 48 which controls all power to the system. Precision voltage regulator 50 pro-vides regulated voltage, PVR, to bridge circuit 24, reference current switch 40, integrator circuit 34, constant width pulse generator 36, and low battery voltage sensing circuit 52. The other input to low battery voltage sensing circuit 52 is the unregulated power supplied at the output of automatic on-off electronic switch 48. When the unregulated power supply vol-tage decreases to a predetermined level relative to the regu-lated voltage output provided by precision voltage regulator 50, low battery voltage sensing circuit 52 provides a signal to digital counting and decoding circuit 42 causing it to extin-guish the least significant digit of the temperature appearing in digital display 44.
Electronic thermome~er system 10 operates in two modes a time display mode and a temperature display mode.
Digital control logic 54 supervises system performance in each of these modes and controls the transition between them. In 1049Z8g the time display mode digital control logic 54 passes pulses from clock 56 to digital counting and decoding circuit 42, while in the temperature display mode digital control logic 54 directs pulses from constant width pulse generator 36 to digital counting and decoding circuit 42, The system is operated by actuation of start switch 58, In operation, when start switch 58 is actuated, automatic on-off electronic switch 58 is turned on to supply power from power sup~ly 46 to the rest of the system, and digital control logic 54 and digital counting and decoding circuit 42 are reset, Probe 20 in contact with the patient whose temperature is to be measured begins to sense the temper-ature, As temperature T, Fig, 2, sensed by probe 20 increases the resistance R of the thermistor used in probe 20 decreases the voltage E at output 28 of bridge circuit 24 decreases, increasing the negative current I at summing poind 32, The difference in levels of output 26 and summing point 32 causes pulses to be generated at the output of constant width pulse generator 36 at a repetition rate required to restore summing point 32 to the pro,per level, The repetitionrate of the pulses at the output of the constant width pulse generator 36 stabilizes in a short period of time to represent the final value of the temperature being sensed, This period may be reduced still further by the use of anticipation circuit 30 as exp~ained previously, Simultaneously with this action, upon the actuation of start switch 58, a latch signal on line 60 from digital counting and decoding circuit 42 enables automatic on-off electronic switch to stay on for a predetermined~l period of time after the start switch has been pressed and released, In this particular embodiment the period of time is equal to the duration of the time display mode plus the duration of the temperature display mode which in system 10 are twenty seconds and ten seconds, respectively. The signal on latch line 60 is controlled during the twenty second duration of the time dis-play mode by clock operated logic and for the additional ten second duration of the temperature display mode by an RC timing network. Simultaneously with the actuation of start switch 58 digital control logic 54 passes clock pulses from clock 56 to digital counting and decoding circuit 42. These clock pulses may have a duration of 100 milliseconds so that a count of ten such clock pulses by digital counting and decoding circuit 4Z
indicates one second. At the end of each second so accumulated digital display 44 is enabled to display the number 1 through 19 representing the time. At the end of the twentieth second digital control logic 54 transfers the system into the temper-ature display mode by permitting passage, for the period of one clock pulse, of the pulses at the output of constant width pulse generator 36 to digital counting and decoding circuit 42 which accumulates and decodes the count and causes the temper-ature to be displayed in digital display 44.
One implementation of temperature sensing circuit 12 and voltage to rate converter 14 is illustrated in Fig. 3.
Probe 20 includes thermistor 62 connected in parallel with normalizing resistor 64. ~ormalizing resistor 64 is chosen to standardize the impedance of probe 20 at some reference level.
For example in this application where the temperature measuring range is 90F to 110F resistor 64 is chosen to standardize probe 20 in the center of the range at 100F to facilitate interchangeability of probes.
The output of probe 20 terminates in plug 66 illus-trated in Fig. 3 as part of internal reference circuit 22.

-~049~89 Plug 66 engages with socket 68 which includes switch 70.
Internal reference circuit 22 also includes a matching circuit 72 including one or more resistors 74, at least one of which is adjustable. Matching circuit 72 and probe 20 are selectively connected to bridge circuit 22 by switch 70. sridge circuit 22 includes four arms, resistors 76, 78, 80 and 82, respectively, Resistor 84 is an adjustable resistor used to balance the bridge. When plug 66 is engaged withssocket 68 probe 20 is connected in the bridge arm with resistor 78 and matching circuit 72 is disconnected therefrom. Conversely when plug 66 is unplugged from socket 6~, probe 20 is disconnected from the arm of the bridge including resistor 78 and matching circuit 72 is included in place of it.
Anticipation circuit 30 includes one or more capaci-tors 86, 88 and 90 which provide an RC constant which closely matches the thermal time constant of probe 20, Resistor 92 may also be used to adjust the RC time constant.
Adjustable resistor 96, shown for convenience as a part of anticipation circuit 30, is used in conjunction with resistor 94 to limit the current flowing from bridge circuit 24 to summing point 32 and thereby control the pulse repetition rate of the output of constant width pulse generator 36 for a given bridge imbalance. For example, resistor 96 is typically adjusted so that a temperature of 90F produces a zero pulse rate, a temperature of 100F causes a pulse rate of 1,000 pulses per second and a temperature ~f 110F produces a pulse repe-tition rate of 2,000 pulses per second. Resistor 95 is pro-vided at reference output 26.
The manner in which anticipation circuit 30 operates to provlde an advance indication of the final value of the temperature being measured before that final value is actually measured may be better understood with reference to Fig. 4 which illustrates thermal time constant 102 of probe 20 and the current characteristic 104 through the RC network of antici-pation network 30, curves 102 and 104 closely resemble a mirror image of each other: each has an initial steep portion 106, 108 and a terminal flat portion 110, 112. At the beginning of the temperature measuring cycle when the thermal time constant 102 indicates that the temperature of the probe 20 is increas-ing at a very high rate the current at output 28 o~ bridge cir-cuit 24, the current passed by capacitors 86, 88 and 90 is also high. The capacitors thus conduct heavily introducing addi-tional current to summing point 32 causing summing point 32 to experience a current flow representative of a temperature value which has not yet been sensed. As the rate of temperature increase slows so too does the rate of current increase and thus the current through the capacitors so that the final condition is correctly portrayed by summing point 32 well in advance of the time when the final value of temperature is actually sensed by probe 20.
Integrator circuit 34, Fig. 3, includes operational amplifier 98 with a feedback loop including capacitor 100.
Constant width pulse generator 36 includes limiting resistor 114 connected to the emitter of the unijunction transistor 116 whose emitter is connected to ground through capacitor 118.
Base 120 of transistor 116 is connected to ground through resistor 122 and base 124 is connected to the regulated power supply through resistor 126. Transistor 116 is biased to be in the off condition until the output of integrator 34 reaches a predetermined threshold level at which capacitor 118 has charged sufficiently so that the emitter of transistor 116 has reached the intrinsic ratio value of approximately 50% of the voltage :1049Z89 across bases 120 and 124. Transistor 116 then conducts provid-ing a discharge path from the emitter through base 120 and resistor 122. The RC circuit consisting of capacitor 118 and resistor 122 fixes the width of the pulses generated by the circuit. When capacitor 118 is sufficiently discharged so that the emitter of transistor 116 is below the intrinsic value transistor 116 turns off. The output from base 120 is fed to base 128 of transistor 130 whose collector 132 is connected to the regulated power supply through resistor 134, Emitter 136 is tied dire~tly to ground. Transistor 132 inverts and ~mpli-fies the output of transistor 116 to feed back a negative going pulse of fixed width to reference current switch 40.
Reference current switch 40 includes transistor 140 which receives the feedback from constant width pulse generator 36 over feedback line 38 through resistor 132 connected to its - base 144. When in response to a negative going pulse on feed-back line 38 base 144 goes low, transistor 140 conducts causing its collector 146 to rise toward the supply voltage to which its emitter 148 is connected, thereby increasing the current flow through the resistor 150 to summing point 32.
Voltage to rate converter 14 is not restricted to use with any particular configuration of temperature sensing circu~t 12. For example, one or more temperature sens~tirve diodes 62', Fig. 5, may be used as the temperature sensing element in place of thermistor 62. The junction voltages of diodes 62' vary in response to changes in temperature to which they are exposed.
Thus the quiescent voltage e.g. 2 volts at junction 152 of resistor 151 and diodes 62' varies as a function of the temper-ature sensed and that voltage variation may be delivered on line 28' to voltage to rate converter 14, either directly or through anticipation circuit 30, completely eliminating the need for bridge circuit 24, Anticipation circuit 30 may advantageously be con-figured differently. For example, since positive pulses from reference current switch 40 are fed directly to summing point 32 in which anticipation circuit capacitors 86, 88 and 90 are connected, these capacitors tend to decrease the magnitude and shape of these positive pulses thereby lowering the loop gain through integrator circuit 34, constant width pulse generator 36, and reference current switch 40. This problem may be overcome by using the approach shown in Fig, 6 where the anticipation circuit is no longer connected to summing point 32 at the negative input to integrator circuit 34 but rather is supplied to the reference output 26 at the positive input of integrator circuit 34. Capacitors 86, 88 and 90 are connected to the negative input terminal of amplifier 154. The other ends of these capacitors are connected to variable output 28 as before which also, as before, is connected through resistor 94 to summing point 32, and the negative input of integrator circuit 34.
The positive input of amplifier 154 is connected to reference output 26 which is also connected as before through resistor 95 to the positive input of integrator circuit 34, an adjustable feedback resistor 156 is connected between the input and output of amplifier 154. Thus the configuration shown in Fig. 6 removes the connection of the anticipation circuit to the summing point 32, so that the pulses from reference current switch 40 are no longer effected by capacitors 86, 88 and 90.
The configuration of Fig. 6 also illustrates that the antici-pation function can be accomplished by adnusting the rreference output 26 instead of varying output 28, Other embodiments will occur to those skilled in the art and are within the following claims:

Claims (2)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. An electronic thermometer system comprising:
a temperature sensing circuit for providing an analog signal representative of the temperature being sensed, a voltage to rate converter circuit responsive to said analog signal for providing constant width and amplitude pulses at a repetition rate which is proportional to the tem-perature being sensed, including an integrating circuit, having a first input for receiving said analog signal and a second input for receiving a reference input, responsive to a difference between said first and second inputs for providing a voltage proportional to that difference, a pulse generating circuit for producing pulses of constant width and amplitude, each said pulse provided in response to a predetermined output level of said integrator, and a feedback circuit for introducing each said pulse to one of said first and second inputs to reduce the difference between said first and second inputs and reset said integrating circuit, and means for counting said pulses for a predetermined period of time for providing an indication of the temperature being sensed.

2. An electronic thermometer system comprising:
a temperature sensing circuit for providing an analog signal representative of the temperature being sensed, a voltage to rate converter circuit responsive to said analog signal for providing constant width and amplitude pulses at a repetition rate which is proportional to the temperature being sensed, including an integrating circuit, having a first input for receiving said analog signal and a second input for receiving a reference input, responsive to a difference between said first and second inputs for providing a voltage proportional to that difference, a pulse generating circuit for producing pulses of constant width and amplitude, each said pulse produced in response to a predetermined output level of said integrator, and a feedback circuit for introduc-ing each said pulse to one of said first and second inputs to reduce the difference between said first and second inputs and reset said integrating circuit; including an RC network which includes capacitive and resistive impedance components interconnecting said temperature sensing circuit and said voltage to rate converter circuit, which has a current characteristic inverse, and similar to, the thermal time con-stant of said temperature sensing circuit, and which is res-ponsive to said analog signal representative of the temperature being sensed to increase the current flow to said voltage to rate converter circuit for driving the system to indicate the final value temperature before that temperature is actually measured.