GB2045978A - Temperature Monitoring - Google Patents

Abstract

A temperature monitoring and control system, for an infant incubator, comprises a pair of temperature sensitive skin probes 14 (only one shown) adapted to be attached to the skin of the patient, each probe having associated comparative circuit 15, 16, 17 & 18 adapted to sense a high or low skin temperature and to activate corresponding indicators 20 or alarms 11, and an associated comparative circuit 25 adapted to monitor probe integrity and to activate a corresponding indicator 26 or alarm 11 whenever a probe failure condition is sensed. The two probes are coupled to comparative circuitry (not shown) designed to check the performances of the probes one against the other and to activate a corresponding indicator or alarm should the probe outputs differ by an unacceptable degree. A heater element is powered in dependence upon the output of one of the probes. A power monitoring circuit monitors the current through the heater element and is arranged to trip an alarm 11 should the power to the heater exceed a predetermined level for longer than a predetermined time period. <IMAGE>

Description

SPECIFICATION Improvements Relating to Electromedical Apparatus This invention concerns improvements relating to electromedical apparatus and particularly, though not exclusively, concerns incubators for infants, especially those born prematurely who require intensive care for survival. Whilst in the following the invention will be considered and described with particular reference to infant incubators, the wider electromedical application of the invention to other fields of medical care, such as burn therapy for example, will be appreciated.

Infant incubators are known which are adapted for the provision of an intensive care environment for the neonate. One aspect of the neonate's environment which requires special attention is the temperature, and infant incubators are known (for example from British Patent Specification No.

929665) in which the neonate's skin temperature is monitored and serves to control the supply of heat to the neonate from a source of radiant heat and (for example from British Patent Specifications Nos. 1 092 164 and 1 240 693) in which both the incubator air temperature and the neonate's skin temperature are monitored to control the heat supply. In all of these known incubators protective arrangements are made to a greater or lesser extent to ensure that the neonate is not subjected to a potentially hazards environment by virtue of equipment malfunction.

The present invention, like the systems of the British Patent Specifications abovementioned, is predominantly concerned with the temperature of the neonate's environment, with temperature monitoring and control, and with the provision of protective arrangements to ensure the safety of the neonate. The object of the invention is to provide an improved temperature monitoring and control system which is particularly suitable for use with infant incubators, but could be used in other medical care situations where the temperature of the patient and his environment are vital, and which makes use of modern electronic circuit techniques to achieve secure and accurate performance.

In accordance with the present invention a temperature monitoring and control system, particularly though not exclusively for an infant incubator, comprises a pair of temperature sensitive skin probes adapted to be attached to the skin of the neonate or other patient and to be sensitive to the skin temperature. Each of the skin probes has individually associated comparative circuitry adapted to sense a high or low skin temperature and to activate corresponding indicators or alarms, and individually associated comparative circuitry adapted to monitor probe integrity and to activate a corresponding indicator or alarm whenever a probe failure condition is sensed.Furthermore, the two probes are coupled to comparative circuitry designed to check the performances of the probes one against the other and to activate a corresponding indicator or alarm should the probe outputs differ by an unacceptable degree. A heater element, which can be of the radiant type or an alternative type, is powered in dependence upon the output of one of the probes, and a power monitoring circuit monitors the current through the heater element and is arranged to trip an alarm should the power to the heater exceed a predetermined level for longer than a predetermined time period.

Other features of the invention reside in (i) the connection of the probes, constituted by thermistors, in an intrinsically safe circuit configuration which precludes the possibility of the neonate or other patient being exposed to a hazardous electrical potential, (ii) the powering of all indicator and alarm systems from a rechargeable nickel/cadmium battery such that the integrity of the indicator and alarm systems is ensured even in the event of a failure of the mains supply, (iii) the complete isolation by transformer decoupling and through optoelectronic isolators of the mains supplied heater unit from the patient monitoring systems thereby enhancing patient safety, and (iv) indicator and alarm system interlocks precluding disabling of an alarm condition indication without attention to the condition responsible for initiating the alarm.

As applied to an infant incubator, the invention would of course also include other features more or less well known in the infant incubator art such as for example incubator atmosphere humidity and gas constituent control systems, respirator control systems, infant attitude controls, etc.

These features form no part of the temperature monitoring and control system which constituted the principal inventive system disclosed herein, and therefore will not be described herein. Such ancillary features can be seen in the incubators manufactured and sold by Ohio Medical Products division of Airco Inc., by Vickers Electromedical, by Draeger of Germany, and by Air-Shields Inc.

amongst others.

The invention, together with other features and advantages thereof, will become apparent from the following description of an exemplary temperature monitoring and control system embodying the invention, which system is illustrated in the accompanying drawing to which reference will be made in the following and which comprises a schematic electrical circuit diagram of the system. Detailed electrical explanations such as would from consideration of the accompanying drawing in light of the following description be readily apparent to one vested with appropriate skills and understanding in and of electrical circuits will not be provided in the following; rather, explanation will be provided in terms of the function of the circuitry depicted.

Referring to the drawing, it is first to be noted that the circuit portion to the left hand side of the broken line A-A is largely duplicated in the inventive embodiment. The circuit portion shown is associated with and includes one skin temperature probe only, and in reality a second identical probe with substantially identical associated circuitry also is provided. The second probe and associated circuitry has been omitted from the drawing for the sake of clarity.

Connection points referenced B, C and D are shown in the drawing where the duplicate probe circuitry (not shown) connects into the illustrated circuit.

The circuit is mains powered, the L and N mains connections being shown towards the lower right hand side of the drawing, and is adapted to supply a load, constituted by a radiant or other heater source, which is series connected with a triac 1 across the L and N mains connections. The triac 1 is controlled by application to its gate electrode of control potentials derived by a zero voltage switch 2 constructed in conventional manner around an L121 integrated circuit. The zero voltage switch 2 ensures that the triac 1 is switched at zero crossover points in the applied mains waveform so as to avoid the generation of switching transients which otherwise might occur. The zero voltage switch 2 is controlled by gating impulses from the phototransistor side of an optoelectronic isolator 3 whereof the photodiode connections will be described later.The optoelectronic isolator 3 together with a further optoelectronic isolator 4 to be described hereinafter provide signal path isolation between the mains operated circuitry associated with the heating element and the remainder of the circuit which, as will be seen, is low voltage d.c. powered. A mains step-down transformer 5 is coupled to a bridge rectifier 6 which, via smoothing circuits and a voltage regulator 7, provides a stabilized 8 volt d.c. supply to the remainder of the circuit. It will be seen that the mains transformer 5 completes the electrical isolation of the mains operated circuitry.

Associated with the heating element load and the triac 1 is a power monitoring circuit which includes, as shown, a resistance in series with the triac 1 to develop a potential in proportion to the triac or load current. This potential serves to drive the photodiode side of optoelectronic isolator 4 which develops a corresponding signal in its phototransistor side. The phototransistor supplies its signal to an integrated timer 8 which is coupled via NAND gates 9 to a light emitting diode (LED) 10 and a buzzer or other audible alarm 1 The logical arrangement of the NAND gates 9 is such that the LED 10 will be driven to provide a visible warning and the audible alarm 11 energized to provide an audible warning in the event that the power drawn by the heater element load exceeds a predetermined level for longer than a predetermined time period.The timer 8 is arranged to be triggered to initiate a timing operation when the phototransistor side of the optoelectric isolator 4 provides a low signal to the number 2 terminal of the timer in response to the phototransistor being switched on as the result of an increase in the current through the heating element (load). The signal at the number 3 terminal of the timer 8 is high whilst the timer is timing and goes low when the timer times out after a predetermined time period. An alarm condition is detected in response to the existence of a low on terminal 2 of the timer 8 when terminal 3 goes low when the timer times out.

Other logical situations resulting in operation of LED 10 and buzzer 11 clearly could be provided either alternatively or additionally to the one described. A timer reset switch 12 is shown.

At this juncture, the nickel/cadmium rechargeable battery supply to the system alarms can be mentioned. This is shown at 13 in the drawing. A switch in series with the battery 1 3 is ganged with the mains on-off switch of the instrument and it is arranged that the battery 1 3 will be in a charge mode so long as the mains supply is connected.

Referring now to the left hand side of the drawing that one of the two probes which serves inter alia for controlling the energization of the heating element (load) is to be seen at 14. The probe 14 is commonly constituted by a thermistor which is connected (a) with other components including a zener diode coupled to the 0 volts d.c.

power supply rail intended to render the probe intrinsically safe to the patient such that no voltage can be applied through the probe to the patient in the event of probe insulation failure for example such as would be harmful to the patient, and (b) in a resistive bridge circuit having fixed and variable resistance arms providing inputs to a preamplifier 1 5. The preamplifier 1 5 provides an output signal representative of probe resistance, and correspondingly of probe temperature, to one input of a comparator 1 6 which takes a second input from a temperature set control mounted on the instrument front panel. The output of comparator 1 6 thus represents the deviation of the probe temperature from a predetermined set temperature.This output of comparator 1 6 is then supplied to inputs of "high" and "low" comparator 1 7 and 1 8 respectively which take their other, reference, inputs from controls provided in the machine and adjustable only by means of a special tool. These reference inputs to comparators 17 and 18 define high and low temperature deviation thresholds above which and below which, respectively, the probe temperature deviation from standard can go only with operation of corresponding alarm condition indicators. To this end, each comparator 17, 1 8 feeds an output to logic circuitry 1 9 designed inter alia to provide a drive to illuminate LED 20 and to energize buzzer 11 when correspondingly signalled from the comparators 17, 18. It is to be noted that whilst a single LED 20 provides an alarm indication for both high and low temperature deviations, this being currently considered sufficient, it could be arranged for separate LED's to perform these two functions.

A push button switch 21 is provided to latch the logic circuitry 19, once energized to sound the buzzer 11, into a condition where the buzzer is silenced but a corresponding LED 22 is illuminated.

The temperature deviation signal at the output of comparator 16, which signal it will be recalled is derived by comparison of the probe temperature signal with a set temperature signal, is also used to control the energization of the heater (load) as aforementioned. To this end, the output of comparator 1 6 is applied to one input of an amplifier 23 having gain set facilities enabling the level of the amplifier output voltage to be set for correspondence with the level of control required for accommodating differently rated heater elements (loads), that is for setting the proportional control bandwidth. A further amplifier 24 serves a similar function in enabling power offset adjustment to avoid hunting of the control system.Series connected coarse and fine controls are provided in the output of amplifier 24 and the output from these serves to drive the photodiode side of the aforementioned optoelectronic isolator 3. In operation of this arm of the system, as will be readily appreciated, as the probe temperature deviation from the standard varies so the energization of the load will be varied by appropriate switching of the triac 1 in a sense to reduce the probe temperature deviation, the precise transfer function for this operation being determined by the settings of the amplifiers 23 and 24 and the associated controls.

Probe integrity is monitored by a comparator 25 which takes the probe output signal from preamplifier 15, combines it with a reference, and looks at the resultant to determine whether or not it corresponds to the level which might be expected were the probe operating normally. For a predetermined probe inoperative condition, such as the probe going open circuit for example, an output will be provided from the comparator 25 such as to illuminate an indicator LED 26. This condition also puts a signal into logic circuit 19 such as to cause buzzer 11 to be energized.A connection is also provided from the output of comparator 25 to an input of amplifier 24 in the signal path to the optoelectronic isolator 3 which controls the supply of power through the heating element (load) for the purpose of clamping the potential of the said amplifier input to such a level in the event of damage to either probe being sensed by the probe integrity comparator 25 that the resulting output of amplifier 24 causes optoelectronic isolator 3 to shut off the supply of power to the heating element (load). A similar connection exists from the logic circuitry 9 for shutting off power to the heating element in the event that the power supplied to the heating element exceeds a predetermined level for longer than a predetermined period.If desired, a similar arrangement can be provided for shutting off power to the heating element in the event that the "high" temperature comparator 1 7 provides a positive output indicative of an excessively high temperature probe condition.

As previously mentioned, probe 14 together with preamplifier 1 5 and comparators 1 6, 1 7, 18 and 25 are duplicated together with their associated components. The second probe duplicates all of the functions of the first probe except that it does not have any involvement in the control of the power supply to the heating element (load). As aforementioned, the connections of the duplicated (not shown) circuitry to the circuitry shown in the drawing are shown at B, C and D; to B a comparator duplicating comparator 18 is connected, to C a comparator duplicating comparator 17 is connected, and to D a comparator duplicating comparator 25 is connected.

Not shown in the drawing, but advantageously provided, is an arrangement for monitoring probe performance by comparison of the output of one probe with the output of the other probe. To this end, an output is taken at E from each probe preamplifier 1 5 and is applied as one of two inputs to a differential amplifier, the other input being from the other probe. The output of the differential amplifier will represent the difference between the two probe signals and, should this exceed a preset level, then an alarm condition will be realized resulting in the activation of a visible and/or audible alarm and, if desired, in the shutting off of the power supply to the heater (load).

There has thus been described a temperature monitoring and control system which incorporates a comprehensive range of protective and safety features. Probe integrity and integrity of the temperature monitoring and alarm circuitry associated with the probes is assured in part by duplication, in part by comparison of the output of each probe with a norm related to the probe output anticipated under normal operating conditions, and in part by direct comparison of the output of one probe with that of the other probe.

The supply of power to the heating element (load) is controlled in dependence upon the condition of one of the probes by use of electronic circuit techniques enabling total electrical isolation of the mains powered part of the system from the low voltage d.c. powered signal part. A power monitoring technique is utilized to detect and respond to excessive power drain by the heating element. All alarm systems are duplicated, giving rise to visible as well as audible alarm indications.

Various improvements and modifications may occur to those skilled in the art upon perusal of the foregoing description and of the accompanying drawing. Such improvements and modifications as do not materially depart from the spirit and scope of the invention disclosed herein and of equivalent thereof are intended to be embraced by the invention. The hereinbefore described system is to be regarded only as exemplifying the invention and is not to be regarded in a limiting sense.

Claims (20)

Claims

1. A temperature monitoring system for electromedical apparatus comprising first and second temperature sensitive probes for attachment to the skin of a patient to be sensitive to the skin temperature, means associated individually with each of said probes for responding to an abnormal temperature condition sensed thereby to provide a corresponding output, means associated individually with each of said probes for monitoring probe integrity and providing a corresponding output in response to a probe failure condition, and means associated collectively with both said probes for comparing the performances of the probes one against the other and for providing a corresponding output if the probe performances differ by more than a predetermined amount.

2. A system as claimed in claim 1 wherein means are provided for controlling a heater element in dependence upon the temperature of one of said probes.

3. A system as claimed in claim 2 wherein means are provided associated with said heater element for monitoring the output thereof.

4. A system as claimed in claim 3 wherein said means for monitoring the output of the heater element is arranged to monitor the current through the heater element and to provide a corresponding output should the power to the heater element exceed a predetermined level for longer than a predetermined time period.

5. A system as claimed in claim 2 or 3 or 4 wherein the heater element is mains powered and wherein the mains powered heater element and its associated control and/or monitoring systems are electrically isolated from the probes and their associated systems for enhancing patient safety.

6. A system as claimed in claim 5 wherein said electrical isolation is achieved by means of transformer decoupling and/or optoelectronic isolation.

7. A system as claimed in claim 6 wherein the heater element is series connected with a controlled rectifier device between the main supply terminals, a zero-voltage switch has an output coupled to the gate electrode of said controlled rectifier device for determining the state of conduction thereof, and the state of said zero-voltage switch is determined in dependence upon a signal output derived from one of said probes, an optoelectronic isolator being provided in the signal path from said one probe to the zerovoltage switch.

8. A system as claimed in all of the preceding claims wherein a current sensing resistor is connected in series with said controlled rectifier device for deriving a signal representative of the current through the heater element, and an optoelectronic isolator is provided in the signal path from said current sensing resistor to circuitry for monitoring the level of said current and for responding to an excessive current level persisting for longer than said predetermined time period.

9. A system as claimed in-claim 4 or in any of claims 5 to 8 as dependent upon claim 4 wherein a circuit connection is provided whereby, in response to said output occurring when the power to the heater element exceeds said predetermined level for longer than said predetermined time period, a corresponding input is provided to said means for controlling the heater element for reducing the heat output thereof.

10. A system as claimed in any of the preceding claims wherein each of the probes comprises a temperature sensitive electrical element connected in an intrinsically safe circuit configuration such as to protect a patient from exposure to potentially hazardous electrical potentials.

11. A system as claimed in claim 10 wherein each probe comprises a thermistor.

12. A system as claimed in any of the preceding claims wherein each of the probes is coupled to a first comparative circuit arranged for comparing the probe output, representative of sensed temperature, with a preset temperature level and deriving a corresponding deviation signal, and second and third comparative circuits are provided for comparing said deviation signal with respective preset high and low deviation levels and providing corresponding outputs when said deviation signal is excessively high and excessively low respectively.

13. A system as claimed in claim 12 as dependent upon claim 2 wherein the said deviation signal derived from the first comparator associated with one of the probes is employed for so controlling the heater element as to tend to reduce the deviation signal.

14. A system as claimed in claim 13 wherein a circuit connection is provided from the output of said second comparative circuit associated with one or other or both of said probes for effecting a reduction in the heat output of said heater element in the event of an excessively high temperature deviation signal being sensed.

1 5. A system as claimed in claim 12 or 13 or 14 wherein an indicator is arranged to be energized in response to an output from one or other or both of said second and third comparative circuits.

1 6. A system as claimed in any of the proceding claims wherein said means for monitoring probe integrity comprises a comparative circuit arranged to compare an output signal derived from the respective probe with a reference signal representative of a predetermined inoperative probe condition.

17. A system as claimed in claim 16 as dependent upon claim 2 wherein a circuit connection is provided from the output of each said probe integrity comparator for effecting a reduction in the heat output of said heater element in the event of loss of probe integrity.

18. A system as claimed in any of the preceding claims wherein indicator and/or alarm systems are provided for giving visual and/or audible indications in response to various predetermined situations, and an auxiliary rechargeable battery supply is provided for powering such systems even in the event of failure of the main supply, said battery supply being arranged so as to be in a charge mode so long as the mains supply is available.

1 9. A temperature monitoring and control system substantially as herein described with reference to the accompanying drawing.

20. Electromedical apparatus, such as an incubator for infants, incorporating a temperature monitoring system as claimed in any of the preceding claims.