GB2177196A - Improvements in means for detection and/or compensation for changes in the optical gain of a pyrometer - Google Patents

Abstract

An apparatus for the optical measurement of emitted radiation comprises an optical system (1,4) for channelling radiation from a target area of which the temperature is to be measured to two separate radiation sensors (2,3) having different spectral sensitivities. A means (6,7,8,9,10) is provided for processing the output of the radiation sensors to make a comparison of the relation between the output signals of the radiation sensors with a predetermined relationship established during a calibration of the apparatus. A feedback control means is provided for varying the gain factor of the respective measuring channels containing the sensors (2,3) in response to a detected difference in the compared relationships, until the relationship between the respective signals is restored to that which was established during the calibration step. Preferably an output signal representing the measured temperature is provided by one of the measuring channels. <IMAGE>

Description

SPECIFICATION Improvements in means for detection and/or compensation for changes in the optical gain of a pyrometer The present invention concerns improvements in means for detection of and/or compensation for changes in optical gain of a pyrometer or pyrometer system.

Such a change in gain may be due to contamination, for example. When using infrared and optical pyrometers in dirty environments, errors may occur due to such contamination and obscuration of the optical system. In the case of a single colour pyrometer, which determines surface temperature by measuring the amount of radiation emitted from a target, such obscuration will result in a temperature indication which is less than the actual surface temperature. In some applications when the pyrometer is used for temperature control, for example of gas turbine blades or processes, such errors can be catastrophic.

Similar errors may also occur due to change in the optical gain of the pyrometer for reasons other than contamination, for example as a result of damage to or changes in the adjustment of the elements of the optical system. Even the removal and replacement of an optical system of a pyrometer can result in such a change in gain, and for this reason pyrometers for some applications are made with non-exchangeable optical systems. It would be an advantage to provide for the simple exchange and/or replacement of optical systems in such pyrometers.

A two colour pyrometer can be used to reduce the problem of optical contamination.

By detecting radiation from the same target area, or two equivalent target areas, over two different radiation wavelength intervals a two colour pyrometer can be used to detect the spectral distribution as well as the intensity of the emitted radiation. It is a well established technique to process the outputs of such a pyrometer by dividing one output by the other. Such a device is known as a ratio pyrometer.

However, a disadvantage of the ratio pyrometer is that the dynamic range of the instrument is much less than a single colour pyrometer so that the effect of any noise level is correspondingly greater.

Another disadvantage is that although the effect of optical contamination is in some cases eliminated, the extent of contamination is not known. Excessive contamination will further increase the effect of noise levels in the processed signals.

In another prior proposal, see GB 1,387,060, there is provided a system utilising two radiation detectors of different spectral sensitivity to which radiation is channelled from a target area by means of a common optical system, and the respective outputs of the radiation detectors are compared in order to detect a variation in the relationship between their output signals, caused by interference or optical contamination. However, in this arrangement it is required that the error characteristics of the two detectors due to interference or optical contamination have a constant predetermined relationship over the range of temperature to be measured, such a relationship being difficult to achieve in many applications. Moreover, in order to obtain a true temperature measurement, it is necessary to combine the output signals of both detectors.

It is an object of the present invention to provide a pyrometer system, wherein the advantageous characteristics of a single colour pyrometer are obtained, whilst also providing means for the compensation and/or detection of changes in the optical gain of the system.

In accordance with the present invention there is provided an apparatus for the optical measurement of emitted radiation, comprising an optical system for channelling radiation from a target area, or two equivalent target areas, to two separate radiation sensors respectively having different spectral sensitivity, and means for processing the outputs of said radiation sensors to obtain a comparison of the relationship between respective output signals of said sensors with a predetermined relationship established during a calibration of the apparatus and feedback control means for varying the gain factor of both of said sensors in response to a detected difference in said compared relationships in order to restore the output signals of said sensors to said predetermined relationship.

Advantageously, the output signals from said radiation sensors are initially linearised and controlled in amplitude so that, at the time of calibration, the respective output signals are equal to one another over the desired measurement range. Thus, the said predetermined relationship at the calibration stage is a condition of equality, and the presence of optical contamination can thus be sensed simply in terms of a difference between the respective linearised output signals.

By the provision of an automatic feedback gaincontrol of the response of the optical sensors, it can be ensured that each output signal individually represents a true temperature measurement, and thus a temperature measurement can be derived from a single one of the channels, without the need to process both signals.

The invention is illustrated by way of example in the accompanying drawings, in which: Figure 1 is a block diagram illustrating the general construction of a pyrometer system in accordance with the invention, and Figure 2 is a flow chart illustrating the prin ciple of operation of the device of Fig. 1.

In Fig. 1, there is shown in diagrammatic form an optical pyrometer system comprising a common optical system 1 for collecting radiation from a given target area, and a pair of radiation detectors 2 and 3, to which the radiation is directed by means of a bifurcated canalisation device 4, for example a fibre optic guide. The detectors 2 and 3 are arranged to have different spectral sensitivities, which may be achieved by the use of two detectors of different characteristics, or the use of one or more optical filters, indicated diagrammatically at 5. The output signals from the detectors 2 and 3 are passed via amplifiers 6 and 7 and analogue to digital convertors 8 and 9 to a processor 10 for comparison of the respective output signals.

The principal of operation of the device shown in Fig. 1 is as follows. Since the two detectors 2 and 3 have different spectral sensitivities, the response of their output signals over a given range of temperature measurement will differ from one another, but, in the absence of any variation in the gain of the optical system 1, will bear a relationship to one another that will be constant at any given temperature, but may vary over the temperature range. Accordingly, by processing the output signals in order to establish that, for any given output signal of, for example the detector 2, representative of a certain temperature, the corresponding output signal from sensor 3 that would also be expected at the same temperature is in fact present.If the required relationship between the two signals does not exist, then this can be taken as an indication of a variation in the gain of the optical system 1, for example due to contamination, damage or misadjustment. Furthermore, since, in the case of contamination, it may be assumed that contamination of the optical, system 1 is of neutral density, and thus does not affect the relationship between the spectral response of the detectors 2 and 3, the effective attenuation of the radiation by any such contamination will be the same for both detectors, and can be compensated by providing a corresponding gain adjustment equally in both signal paths of the two detectors. Such gain adjustment can thus be effected in a simple manner by means of a feed-back control until the predetermined relationship between the output signals corresponding to the absence of contamination is reached.The output signals of the detectors will then provide a true measure of the radiation temperature.

Fig. 2 shows one example of the manner in which the signals from analogue to digital convertors 8 and 9 may be processed, the inputs indicated at channel 1 and channel 2 corresponding to the outputs from analogue to digital convertors 8 and 9 respectively. Channel 1 is used for temperature indication. The signal in this channel is processed by applying a suitable gain (to be explained), and linearisation to provide a temperature output.

The level of gain is determined by comparison with channel 2 or by a predetermined gain which may be applied at switch on. The signal from channel 2 is processed by a stage variable gain which is equal to the variable gain in channell . After linearisation, if conditions are acceptable, the temperature indications from channel 1 and channel 2 are compared. If the tempature levels in each channel differ by more than a preset amount then the variable gain stage in each channel is adjusted to bring the two channels into agreement. The level of the gain setting will indicate the amount of alttenuation occurring in the optical system in comparison with an initial state of calibration of the apparatus at which the linearised signals from channels 1 and 2 were set to be equal to one another.

Whilst the above example illustrates one method of processing, it will be appreciated that, using microprocessor control, a variety of possible methods of comparison of the signals of the respective channels may be utilised. For example, in place of linearising the two signals prior to comparison and making a comparison on the assumption that the two values should be equal, a comparison between nonlinearised signals could be made on the basis of stored values of output signals entered during the calibration process, the stored values being recalled from memory by the processor system, using a look-up table, for the purpose of comparison.

It will be appreciated that processing of the output signals can be made sufficiently intelligent to make the appropriate comparisons only in advantageous conditions, for example at high signal levels or in the absence of interfering radiation. The output signals may also be compared after an integration step to provide a mean value with respect to time, in order to eliminate the effect of transient conditions in the target area of the pyrometer system that are to be ignored. Moreover, a warning signal could be provided when the level of gain required to bring the signals into the required relationship is such as to indicate excessive contamination of the system.

The arrangement in accordance with the invention has the advantage that, since an effective temperature measurement can be made on the basis of an output signal from a single one of the respective channels of the system, the system has the benefits of a single colour pyrometer, while at the same time correcting and/or detecting vartiation in optical gain.

Although one preferred embodiment of the invention has been described above, it will be appreciated that various alterations and modifications to the system described may be made by one skilled in the art without departing from the underlying concept of the invention as set out in the accompanying claims, or, alternatively, any other underlying concept that would be recognised by one skilled in the art and possessed of a full knowledge of the state of the art as constituting an inventive step. It is to be understood that the applicants reserve rights to any such additional invention whether or not it forms the subject of any accompanying claims.

Variations of the above described embodiment that are contemplated include the following: (a) The replacement of the common optical system 1 by two equivalent optical systems serving the separate channels of the pyrometer device, in conditions such that any variation in the gain of the two separate systems will in practice be identical.

(b) The routing to the two separate channels of the optical system radiance information derived from two equivalent target areas, instead of a common target area. Such target areas might, for example, be two immediately adjacent areas of a target object that may be assumed necessarily to be of the same temperature. For example, in the case of a nonrandomised fibre optic guide 4 served by the lens system 1, the radiation canalised in the respective limbs of the bifurcated guide will be derived from separate portions of the image area covered by the optical system 1.

Claims (8)

1. An apparatus for the optical measurement of emitted radiation, comprising an optical system for channelling radiation from a target area, or two equivalent target areas, to two separate radiation sensors respectively having different spectral sensitivity, and means for processing the outputs of said radiation sensors to obtain a comparison of the relationship between respective output signals of said sensors with a predetermined relationship established during a calibration of the apparatus and feedback control means for varying the gain factor of both of said sensors in response to a detected difference in said compared relationships in order to restore the output signals of said sensors to said predetermined relationship.

2. An apparatus as claimed in Claim 1, wherein said optical system includes a common objective for collecting radiation to be channelled to said radiation sensors.

3. An apparatus as claimed in Claim 2, wherein the respective optical channels of said system are defined by a bifurcated fibre-optic light guide.

4. An appratus as claimed in Claim 3, wherein the optical fibres of said light guide are randomised so that radiation collected by said objective from the same target area is distributed uniformly between the said optical channels.

5. An apparatus as claimed in any one of Claims 1 to 4, wherein the said processing means comprises means for linearising each of the output signals of said radiation sensors, the system having been calibrated in a condition wherein the linearised output signals are equal to one another over a desired range of temperature measurement, and means for comparing the two output signals and providing to said feed back control means a corresponding difference signal.

6. An apparatus as claimed in any one of Claims 1 to 4, wherein the said processing means comprises a storage means containing the values of the output signals of the respective radiation sensors obtained during a calibration step, means for obtaining from said storage means in response to an output signal from one of said radiation sensors the corresponding calibrated signal value for the other sensor, and means for comparing the actual value of the output signal from said other sensor, with said calibrated value and providing to said feed back control means a corresponding difference signal.

7. An apparatus as claimed in Claim 5 or 6, wherein the said feedback control means is arranged to vary the gain factor of both of said sensors in an identical manner.

8. An apparatus for the optical measurement of emitted radiation, substantially as described herein with reference to the accompanying drawings.