GB2488423A

GB2488423A - Temperature-monitoring system for a heating element using a temperature model

Info

Publication number: GB2488423A
Application number: GB1202953.4A
Authority: GB
Inventors: Holger Vogel; Thomas Hermann; Wolfgang Sippach
Original assignee: Carl Zeiss Optronics GmbH
Current assignee: Hensoldt Optronics GmbH
Priority date: 2011-02-21
Filing date: 2012-02-21
Publication date: 2012-08-29
Also published as: DE102011004448A1; GB201202953D0

Abstract

The invention relates to a temperature-monitoring process for a heating element in particular a heating target 109 used for calibrating at least one thermographic camera 103a, 103b. The heating element has at least one predeterminable upper temperature limit (2, Fig 2). A temperature model for the heating element is derived, which includes temperature characteristics (1, Fig 2) for several starting temperatures, depending on the level and/or duration of the supplied energy input for a heat-up phase and for a cooling phase of the heating element. The current temperature of the heating element is ascertained continuously on the basis of the temperature model and by means of a past-related examination, whereby a switching-off of the energy input takes place if the current temperature exceeds the temperature limit (2, Fig 2) and whereby the energy input is switched on again if the current temperature again falls below the temperature limit (2, Fig 2).

Description

Description:

Temperature-monitoring system for a heating element subjected to an input of energy The invention relates to a temperature-monitoring system for a heating element subjected to an input of energy.

Optical measuring systems can be utilised for the purpose of determining physical measured guantities such as, for example, the length of an object. In the course of an optical measuring procedure, as a rule an object scene which is present is imaged by an optical system onto a radiation-sensitive detector, the object scene being constituted by a three-dimensional space with differing objects. Frequently quantitative statements are also required in the course of the measuring procedure, so the deviations of the measured values from the physical quantities of the measured object scene should be as small as possible. For the purpose of determining these deviations a calibration is required, whereby, particularly in the case of optical measuring processes, a range of parameters may be calibrated. These include, for example, photometric quantities and also lateral and axial spacing quantities. In the case where use is made of line-scan cameras or area-scan cameras, as a rule many measured values arise which are arranged in a one-dimensional or two-dimensional array, respectively. With the line-scan cameras and area-scan cameras, parts of the object scene are registered and are changed by means of the individual pixels into discrete values with respect to location and intensity. Particularly in the case of optical measuring processes for determining axial spacing quantities, the camera signals received are, as a rule, subjected to further processing by an evaluating algorithm, whereby, for example, points in a spatial ooordinate system may also arise by way of result of measurement. For an assignment of the measured values to the imaged objeots of the measured object scene and for the purpose of registering the deviations arising in the process, calibration procedures are reguired. When carrying out the calibration, use is generally made of calibration objects having known properties. If the optical measuring system is to be used for the purpose of determining length data, the geometrical properties -such as, for example, the shape, position, attitude and size of structures applied to the calibration object -are crucial. The design of the structures is often matched to the application and the measuring task. In addition, the calibration objects may be self-luminous or may be Illuminated by an additional radiation-source. If thermographic cameras are being employed, care has to be taken in the course of the optical imaging to ensure a sufficient intensity contrast in the corresponding electromagnetic spectral region, in order that the geometrical structures of the calibration object can be registered and evaluated. By way of calibration objects for thermographic cameras, use is therefore freguently made, for example, of so-called heating targets or, to be more exact, heatable reference targets in which a locally differing heat distribution is present and which ensure a sufficient contrast in the image. Electrically heatable wires, for example, may also be employed by way of calibration objects, in which case various demands are then often made of the heat-up procedure, such as, for example, also a short heat-up time, in particular by using a high heating current. In the case where the heating current is applied for too long a time this may result in a destruction of the heating target. Furthermore, in the case of heat-up phases following one another at short intervals possible thermal preloads can only be taken into account with difficulty solely by means of the heat-up time. For the purpose of limiting the maximal temperature, temperature sensors may have been provided, by means of which a temporal regulation is made possible. The integration of an additionai temperature sensor on the heating target may, however, be costly, and reguires additional electrical switching circuits and control circuits. A control via a fixed heating-time may result, for example in the case of heating-up regulated via the image impression, in premature switching-off of the heating current. Since in the case of the heating target it is a question of an optical component, an additional temperature sensor on the calibration object may also have a perturbing effect on the optical beam path.

DE 10 2008 058 798 Al relates to a stereo camera device with at least two adjusted thermographic cameras arranged at a defined spacing from one another and aligned, which is provided with a calibration device for the ongoing automatic calibration thereof.

The task underlying the invention is to make available a temperature-monitoring system of the type mentioned in the introduction which effectively prevents a destruction of the heating target during the entire heating procedure, in particular without using a temperature sensor, and in which, in particular, a short heat-up time can be realised.

In accordance with the invention this task is achieved by a temperature-monitoring process for at least one heating element subjected to an input of energy, in particular for use by way of heating target in the course of the calibration of at least one thermographic camera, with at least one predeterminable upper temperature limit and with access to a temperature model for the heating element, which includes temperature characteristics for several starting temperatures, depending on the level and/or duration of the supplied energy input for a heat-up phase and for a cooling phase of the heating element, whereby the current temperature of the heating element is ascertained continuously on the basis of the temperature model and by means of a past-related examination, whereby a switching-off of the energy input takes place if the current temperature exceeds the temperature limit and whereby the energy input is switched on again if the current temperature again falls below the temperature limit.

The temperature-monitoring process according to the invention takes into account the level of the energy input and the heat-up time of a heating element or heating target, in order to realise an automatic limitation of the heat-up time without a premature switching-off thereby taking place. Consequently, in advantageous manner a heating element that has been heated for a relatively long time can also be made available without using a temperature sensor.

The temperature model may be determined numerically, analytically, experimentally or from empirical values.

Use may be made of a preferentially pulse-width-modulated heating current by way of energy input. As a result, the arithmetic mean of heating current or heating power in the case of a known supply voltage is substantially proportional to the duty ratio of the pulse-width-modulated signal. Alternatively, a pulse-frequency modulation (pulse-position modulation) could also be employed, for

example.

For the past-related examination during the heat-up phase, values characterising the input of energy to the heating element can be added up at temporally equidistant intervals. To this end, a duty-ratio value of the pulse-width-modulated heating current during the heat-up phase can be added up at temporal in particular equidistant, intervals, in particular in the manner of Riemannian partial sums. By means of a linearisation of such a type, in advantageous manner a computationally intensive utilisation of the processor can be avoided. For the past-related examination during the cooling phase, a value taking the cooling-time constant of the heating element into account can be subtracted at temporal, in particular equidistant, intervals.

If the sum exceeds a configurable limit, for example the maximally permissible static heating temperature of the heating element, the electronics automatically switch off the heating circuit in software-controlled manner also in the heat-up phase. After this, the cooling-time constant of the heating element is taken into account by subtraction of an appropriate value. In the case where the limit is fallen below, the heating circuit is reactivated again with the heating power currently being applied. Consequently, in a manner similar to that in the case of a two-point controller, a thermal equilibrium about the fixed limit can be set. By means of the subtraction, stated above, of the value of the cooling-time constant, the residual heat is automatically taken into account.

In the course of the determination of the current temperature an interpolation may be carried out.

In the temperature model a radiation of energy of the heating element can be taken into account.

Advantageous oonfigurations and further developments of the invention arise out of the dependent olaims. An embodiment of the invention is speoified in principle in the following on the basis of the drawing.

Shown are: Figure 1 a simplified diagram of a pulse-width-modulated signal of a heating current; Figure 2 a simplified diagram of temperature characteristics of a temperature model for a heating element in the heat-up phase; Figure 3 a simplified diagram of temperature characteristics of a temperature model for a heating element in the cooling phase; and Figure 4 a schematic representation of a stereo camera device with two thermographic cameras.

In the present embodiment at least one thermographic camera 103a, 103b (see Figure 4) is assumed, and the camera or cameras may, for example, also have been implemented in an optical measuring system and may, for example, take the form of line-scan cameras or area-scan cameras. A thermographic camera 103a, 103b of such a type can be calibrated by means of a heating target 109 by way of heating element.

Figure 1 shows a diagram of an input of energy into the heating target 109, the diagram having been divided up along the time axis into fixed time-segments or periods D. The parameter P may be the heating current, the heating power or another guantity correlated with the energy input of the heating target 109. In the present embodiment, use is made of a preferentially pulse-width-modulated heating current by way of energy input. Within the period D, which is defined as the difference derived from t(n+1)tn a time-interval W results by way of pulse width, in which the parameter P is switched, i.e. active, so that during the pulse width W a heating current, for example, is flowing.

The period D may also have been temporally discretised, i.e. the individual pulse widths W are present as, for example, 8-bit values. The quotient W/D is designated as the duty ratio. As evident from Figure 1, the parameter P within the pulse width W is constant (constant heating current or supply voltage) . However, there are periods D in which the parameter P amounts to zero. By means of a so-called pulse-width modulation, whereby in the case where the period D is fixed the pulse width W is varied, a control of the input of energy into the heating target 109 is possible. In further embodiments which are not represented, this control could alternatively or additionally also be effected by a change of the parameter P. Represented in Figure 2 is a diagram with several different temperature characteristics 1 or, to be more exact, temperature/time curves of a temperature model for the heat-up phase, whereby instead of the characteristics a look-up table or such like may also have been saved which, however, is not represented here. In the case of the curves 1 which are shown, the duty ratio is varied in each instance. In the diagram, the time has been plotted horizontally, and the temperature vertically. The maximally permissible static heating temperature is indicated by the dashed line 2.

Figure 3, on the other handy shows temperature characteristics 3 for the cooling phase after heating up with the corresponding duty ratios to approximately 80 °C or 100 °C, and after subsequent cooling with energy input switched off.

If a pulse width W and a parameter value P of, in each instance, greater than zero are present within a period D and the period D is substantially longer than a cooling-time constant of the heating target, in the entire period D a heating procedure takes place and the temperature/time curves 1 for the heating phase from Figure 2, for example, are to be used. An effective cooling within a whole period D occurs when the temperature of the heating element lies above the ambient temperature and the effect of the cooling is greater than the energy Input or, to be more exact, if the following holds: (1 -W/D) cooling-time constant > (W/D) . energy input.

In this case the cooling curves 3 from Figure 3 are to be used for the computation of the temperature obtaining towards the end of the respective period.

The diagrams represented in Figures 2 and 3 can be determined, for example, by experiments, tabular values or by a modelling of the heating and cooling phases. There should always be a dependence on the duty ratio W/D or on the pulse width W. The variation of the parameter P is optional, since, for example, the same heating current can always flow through the heating target, and hence the value P within the pulse width w is constant. The temperature model is consequently determined numerically, analytically, experimentally or from empirical values.

In accordance with the invention a temperature-monitoring process is now employed for the heating element subjected to an input of energy for use by way of heating target 109 in the course of the calibration of at least one thermographic camera 103a, 103b, with the predeterminable upper temperature limit 2 and with access to the temperature model for the heating target 109, which includes the temperature characteristics 1, 3 for several starting temperatures, depending on the level and/or duration of the supplied energy input for a heat-up phase and for a cooling phase of the heating element, the current temperature of the heating target 109 being ascertained continuously on the basis of the temperature model and by means of a past-related examination, whereby a switching-off of the energy input takes place if the current temperature exceeds the temperature limit 2 and whereby the energy input is switched on again if the current temperature again falls below the temperature limit 2.

For the past-related examination during the heat-up phase, values characterising the input of energy to the heating target 109 are added up at temporally eguidistant intervals, and during the cooling phase a value taking the cooling-time constant of the heating target 109 into account is subtracted at temporally equidistant intervals.

Moreover, in the temperature model a radiation of energy of the heating target 109 is taken into account.

To begin with, the starting-point is the first heat-up phase, in the course of which the ambient temperature and, in particular, also the temperature of the heating target 109 are known. The temperature arising initially may, for example, also be determined by a temperature sensor which is not represented and which is located in the vicinity of the heating target 109 but is not integrated on the latter.

In the course of the heating of the calibration object, a high heating current and a large pulse width W can be chosen, for example, if a short heat-up time is desired.

Depending on the ohosen duty ratio W/D, a oertain temperature/time ourve is present; the latter either results directly on the basis of the saved temperature characteristic 1 or may, for example, emerge from an interpolation of intermediate values. If a sufficient computing capacity is available, then from the individual energy inputs (heating power pulse width W) and from the cooling phases the resulting temperature of the heating target 109 in each instance towards the end of the pulse width VI, or, to be more exact, of the period D, can be determined very exactly by means of the exponential function.

Moreover, it is also possible to integrate a time-out monitoring into the control so that in the event of a loss of communication a switching-off or limitation of the supply of energy likewise takes place.

Furthermore, by virtue of a configurable delay-time, for example between 0 s and 255 s, the heating target 109 can be brought into the optical registration zone of the camera or cameras or swivelled into the beam path only when a certain temperature or excess temperature in relation to the environment or a thermal equilibrium has been attained.

To this end, the delay-time can be chosen so that, given full drive of the target, i.e. given a rise in temperature that is as steep as possible, the significant excess temperature is attained. After expiration of the delay-time -that is to say, after the excess temperature has been attained -the heating target 109 is swivelled into the beam path, and the drive of the heating target 109 is reduced, to the effect that a thermal equilibrium of the heating target sets in. Given a delay-time with the value 0 s, a running waiting-time is terminated and the heating target 109 is swivelled into position at once. This procedure may, for example, be triggered by an evaluation of the image impression by an image-processing unit 107 connected to the cameras lO3a, 103b (see Figure 4), in which case the attaining of a certain temperature of the heating target 109 is not used by way of triggering criterion.

Figure 4 shows a stereo camera device 101 as part of a monitoring apparatus, which is not represented in any detail, for take-off runways and landing runways and/or air corridors of airports with a stereoscopic registration of approaching birds 106 or flocks of birds, whereby parameters such as flying height, direction of flight, flying speed and type/size of the birds 106 or of the flocks of birds can be ascertained. One or more stereo camera devices 101 of such a type is/are arranged in the region of the take-off runways and landing runways and/or air corridors and exhibit(s) at least two thermographic cameras 103a, 103b arranged with respect to one another at a defined and adapted spacing and running synchronously during recording. The recording-times of the thermographic cameras 103a, lO3b are at least approximately identical, and the respective viewing-fields lO4a, 104b thereof exhibit an overlapping region 105. In the overlapping region 105 a bird 106 is registered by way of object. The two thermographic cameras lO3a, 103b are adjusted and calibrated with respect to one another. For the thermographic cameras 103a, lO3b, thermographic regions such as LWIR, MWIR, VIJWIR, FIR as well as SWIR, NIR enter into consideration.

The stereo camera device 101 exhibits the image-processing device 107 which is provided for the purpose of processing the image data recorded with the two thermographic cameras 103a, 103b.

The stereo oamera devioe 101 oan oommunioate with higher-ranking systems, in partioular with air-traffio control systems. The stereo camera device 101 operates autonomously. The information as well as the recordings are consequently available outside the individual station.

These data are chiefly transmitted to the air-traffic control system.

In the image-processing device 107 of the stereo camera device 1 there runs, inter alia, a monitoring process for take-off runways and landing runways and/or air corridors of airports, with which approaching birds 6 or flocks of birds are registered stereoscopically by means of the monitoring apparatus or, to be more exact, the stereo camera device 1, whereby parameters such as flying height, direction of flight, flying speed and type/size of the birds 106 or of the flocks of birds or rather the flock density thereof are ascertained. The parameters are determined by means of a stereo evaluation. In this connection, by virtue of the at least two angles of view onto the region 105 recorded by the at least two thermographic cameras 103a, 103b of the stereo camera device 101 absolute points in space of the birds 106 or flocks of birds to be registered are determined. The flying speed of the birds 106 or of the flocks of birds is determined by an examination over an appropriate time-interval. Birds 106 or flocks of birds at a greater distance can also be registered, in which case an appropriately longer focal length is used for the two thermographic cameras 103a, 103b. In addition, flying objects such as model aircraft, kites or such like (not represented) can also be registered by the stereo camera device 101. On the basis of the parameters an assessment is carried out and, where appropriate, a corresponding warning message is output.

As is further evident from Fig. 4, for the purpose of ongoing automatic calibration the stereo camera device 101 is provided with a reference beam path 108 (indicated in dot-dashed manner) and with a heating target 109 having a reference structure, by means of which a reference image is imaged onto the respective thermographic camera 103a, 103b.

The temperature-monitoring process according to the invention may, for example, be employed in the image-processing device 107 for the purpose of monitoring the heating target 109. In the present embodiment the heating target 109 is driven (indicated by a dashed line) by the image-processing device 107.

List of Reference Symbols 1 temperature characteristics, heat-up phase 2 maximally permissible static heating temperature 3 temperature characteristics, cooling phase 101 stereo camera device 103a thermographic camera 103b thermographic camera

104a viewing-field

104b viewing-field

overlapping region 106 bird 107 image-processing device 108 reference beam path 109 heating target W pulse width D period P parameter I temperature t time