GB2191281A - Thermal imaging systems - Google Patents

Abstract

A thermal imaging system has a camera 1 which can scan a field and produce multiple outputs 5 which give information as to the operation of the camera as well as thermal characteristics of the scanned field. The camera is connected by an interface arrangement 6, 8 to a processor 9 which controls operation of a graphic display device. The interface arrangement has a first interface 6 in the camera which converts the multiple camera outputs to a single multiplexed output, and a second interface 8 in the processor which digitalises the received multiplexed output. <IMAGE>

Description

SPECIFICATION Thermal imaging systems This invention relates to a thermal imaging system for producing a graphic display representative of thermal characteristics of a scanned field.

Thermal cameras are known which are capable of producing output signals representative of thermal characteristics of a scanned field. An object of the present invention is to provide a system with which a graphic display can be produced from such output signals in a simple and convenient manner.

According to the invention therefore there is provided a thermal imaging system comprising a camera capable of scanning a field so as to produce multiple outputs providing information as to the operation of the camera and the thermal characteristics of the scanned field, a graphic display device, a processing device for controlling operation of said graphic display device to produce a graphic display as determined by control data fed to the processing device, and an interface arrangement interconnecting said camera and said processing device to produce said data from said outputs of said camera, characterised in that said interface arrangement comprises a first device associated with said camera and arranged to convert said multiple outputs to a single multiplexed output, and a second device associated with said processing device and arranged to receive and digitalise said multiplexed output to produce therefrom said control data.

The system of the invention may be used in any suitable context, particularly although not necessarily for industrial inspection, e.g. for inspecting the output from rolling mills (such as steel or aluminium strip), rotating kilns (cement), plastics containers and other heatsealed or welded items on production lines.

The camera may be of any suitable kind. In a preferred embodiment the camera comprises within a housing a cooled infra-red sensor which scans the desired field through a window in the housing e.g. via a rotating prism or the like. The camera may also include one or more reference black bodies which are also scanned to give calibration data.

The first interface device may be mounted within the camera and preferably is arranged to produce said multiplexed output in the form of an analogue time-based thermal intensity signal superimposed on a train of clock pulses together with one or more synchronisation signals identifying one or more stages in the scanning procedure.

The second interface device may be mounted within the processing device which conveniently may comprise a microcomputer.

Where the multiplexed output is as described in the preceding paragraph, the digitalisation may be effected on the analogue thermal intensity signal between clock pulses on a sampling basis. To achieve synchronisation with the clock pulses a phase lock loop may be used. The clock pulses preferably have a small mark/space ratio to minimise interference with the clock pulses. The resulting digital control data may be stored directly in the processing device memory i.e. not in memory in the second interface device.

The invention will now be described further by way of example only and with reference to the accompanying drawings in which: Figure 1 is a schematic representation of one form of an imaging system according to the invention; Figure 2 is a circuit diagram of an interface device of the system of Fig. 1; and Figure 3 shows input and output signals of the interface device of Fig. 2.

The imaging system of Fig. 1 comprises a proprietary infra-red camera 1 sold under the trade name AGEMA THP 5. The camera has a cooled infra-red sensor inside a housing 2.

Within the housing there are two black bodies maintained at reference temperatures, respectively 75"C and 250"C. The housing 2 contains a window 3 and the sensor repeatedly scans a 90" field (as a line scan) through the window via a rotating prism within the housing. The scan rate is typically siow (less than 50Hz). The 250"C and 75"C black bodies are scanned respectively before and after scanning of the 90" field.

The camera is connected to a power supply 4 and includes electronic circuitry connected to the sensor, to the prism drive and to the black bodies, and output signals are produced at multiple outputs 5 of the camera circuitry as the prism rotates.

These multiple camera outputs 5 are connected to an interface device 6 in the form of a printed circuit card which in practice is mounted inside the camera housing 2 and is powered from the camera power supply 4. As can be seen from Fig. 2, seven outputs 5 from the camera are connected to inputs of the interface device and these outputs represent respectively: a 'ready' signal for the 75"C black body, a 'ready' signal for the 250"C black body, a synchronisation signal when the 75"C black body is scanned, a synchronisation signal when the 250"C black body is scanned, a re-set pulse (one per revolution of the prism), a series of square wave clock pulses (200 per revoluttion of the prism), an analogue a.c. coupled 'video' signal of magnitude determined by the intensity of infra-red radiation received by the camera sensor.

As shown in Fig. 3, the interface device 6 produces a single multiplexed output which comprises the analogue video signal imposed as an envelope on a train of pulses which correspond to the clock pulses but are of shorter duration, with an inserted start pulse which corresponds to the re-set pulse but is of extended duration, and with inserted pulses corresponding to ready signals for the 250"C and 75"C black bodies.

The multiplexed output of the interface device 6 is fed via a coaxial cable (or fibre optic or other cable) 7 to an input of a further interface device 8 in the form of a printed circuit card which is in practice incorporated within a microcomputer 9 having a vdu and/or printer with graphics display capability. The microcomputer may be a proprietary microcomputer such as an IBM PC/XT/AT.

The interface device 8 as shown in Fig. 1 acts as an analogue to digital converter and digitises the output received on the coaxial cable 7 using a sampling technique with a phase lock loop which synchronises with the shortened clock pulses. That is, the video intensity is measured in selected gaps between the shortened clock pulses and the resulting intensity values are fed as digital signals to the microcomputer to be stored using the computer's Direct Memory Access; no memory is used in the interface device 8 for this purpose. At the same time, synchronisation signals marking the beginning of the scan and various stages in the scan are separated and utilised in the interface device 8.

The stored digital signals are processed within the computer 9 to produce a graphic display in accordance with software with which the computer is programmed. The software contains routines for setting up the appropriate parameters within the microcomputer, for Direct Memory Access transfer of data from the interface device 8 to the computer's memory, for processing the transferred data, and for simultaneously displaying the results graphically on the computer vdu and/or printer.

The system works essentially one sweep (one line scan) at a time, digitising and transferring to the microprocessor memory only that video data which is appropriate to the measurement process i.e. the thermal data collected in the 90" scan plus thermal data relating to the reference black bodies, whereby internal microprocessor bus loading is minimised. Other signals and information carried by the multiplexed output are utilised for control purposes within the interface device.

The interface device is capable of digitalising from 450 to 800 points per scan, adjustable under software control. The digital display may be in colour or black and white and may comprise a graph of temperature against time, the thermal data from the reference black bodies being used to provide quantitative calibration for the graph. The thermal information from the two reference bodies at the start and end of the scan, as identified by the 'ready' signals, is measured at a number of separate points and dynamically averaged over time to improve stability and accuracy. The rate of averaging is independent of the scan digitisation procedure itself. The interval between the end of reference scanning and the commencement of the field scanning is ignored to optimise resolution.With regard to the data pertaining to the scanned field, the signal is dynamically integrated in multiples of two (2,4,8,16,32,64,256) to average successive scans and reduce noise, interference or temporary interruption of the view of the subject under inspection. The periodicity of the scan is adjustable to provide interval updating of the subject under inspection.

Where a vdu display (using an integral vdu or separate monitor) is used, the screen format may be adjustable to give a single horizontal and vertical axis graph or a split screen format to display multiple graphs. Horizontal and vertical cursors may be provided for measurement purposes. Information e.g. alarm information may be provided in screen 'windows'. In addition to display in line scan mode, the information may be displayed as an envelope where successive scans are overwritten onto the display without erasing the previous trace, or 'relief' where a pseudo three-dimensional display is used. A two-dimensional coloured image mode may be used in which each point is displayed as a colour, the selected colour representing magnitude.

Successive scans can be displayed as consecutive iines vertically up the screen with the colour of each point being representative of its magnitude. Produced scans may be stored in computer RAM or saved on a storage medium.

In addition to displaying thermal data, the system may respond to an exceeded threshold level to trigger an audible and/or visual alarm. The system may also be used for direct process control in that remedial action is automatically taken in an alarm condition and/or closed loop process control is implemented.

As shown in Fig. 1, the device 8 may incorporate isolated reed switches 10 which provide alarm outputs. There is also a 20ma current loop which enabies scan start and stop to be synchronised with external events as desired.

If desired multiple interface devices 8 may be installed in the same microcomputer 9 to enable multiple cameras (with installed devices 6) to be controlled by the same computer.

It is of course to be understood that the invention is not intended to be restricted to the details of the above embodiment which are described by way of example only.

Claims (10)

1. A thermal imaging system comprising a camera capable of scanning a field so as to produce multiple outputs providing information as to the operation of the camera and the thermal characteristics of the scanned field, a graphic display device, a processing device for controlling operation of said graphic display device to produce a graphic display as determined by control data fed to the processing device, and an interface arrangement interconnecting said camera and said processing device to produce said data from said outputs of said camera, characterised in that said interface arrangement comprises a first device associated with said camera and arranged to convert said multiple outputs to a single multiplexed output, and a second device associated with said processing device and arranged to receive and digitalise said multiplexed output to produce therefrom said control data.

2. A system according to claim 1, characterised in that the camera comprises within a housing a cooled infra-red sensor which scans the field through a window in the housing.

3. A system according to claim 1 or 2, characterised in that the camera also includes one or more reference black bodies which are also scanned to give calibration data.

4. A system according to any one of claims 1 to 3, characterised in that the first interface device is mounted within the camera.

5. A system according to any one of claims 1 to 4, characterised in that the first interface device is arranged to produce said multiplexed output in the form of an analogue time-based thermal intensity signal superimposed on a train of clock pulses together with one or more synchronisation signals identifying one or more stages in the scanning procedure.

6. A system according to any one of claims 1 to 5, characterised in that the second interface device is mounted within the processing device.

7. A system according to claim 5, characterised in that the said digitalisation is effected on the analogue thermal intensity signal between clock pulses on a sampling basis.

8. A system according to claim 7, characterised in that a phase lock loop is used to achieve synchronisation with the clock pulses.

9. A system according to any one of claims 1 to 8, characterised in that the said digital control data is stored directly in the processing device memory.

10. A system according to claim 1, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.