GB2154730A

GB2154730A - Liquid level sensing

Info

Publication number: GB2154730A
Application number: GB08404539A
Authority: GB
Inventors: Gillies David Pitt; Philip Extance; Michael Victor Verrells
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1984-02-21
Filing date: 1984-02-21
Publication date: 1985-09-11
Also published as: GB8404539D0

Abstract

An arrangement for liquid level sensing uses a light source (Tx) from which light is transmitted in pulsed form towards the surface whose level is to be sensed. Two or more receivers are used, the signals received being compared, and ratios of the signals received at the receivers used to determine the distance of the surface from the receiver. <IMAGE>

Description

SPECIFICATION Liquid level sensing This invention relates to apparatus for sensing the level of a material, especially (but not solely) a liquid.

According to the present invention, there is provided apparatus for sensing the level of a material, which includes an optical source from which light is transmitted in beam form towards the surface whose level is to be sensed, the light thus transmitted being pulsed, receiving means so located as to receive light reflected from the surface of the material whose level is to be sensed, and means associated with the source and the receiving means to assess from the relation between the light transmitted from the source and the light received by the receiving means the level of said surface, wherein the receiving means includes two or more spaced receiving devices, the distance of the surface from the receiving means being determined on the basis of the ratios of the signals received at the receiver.

Embodiments of the invention will now be described with reference to the accompanying Figs. 1 to 6.

In the arrangement shown in Fig. 1, there is a single transmitter 1, which includes a light-emitting diode or a semiconductor laser, and two laterally-spaced receivers 2 and 3, all equidistent from the surface 4 to be detected.

The transmitter is a 940nm diode, pulsed, and producing an infra-red beam with a dispersion of + 5 . Each receiver is a BP104, sold by Siemens, sensitive only to infra-red radiation. The signals received at the receivers are amplified and averaged, and by comparison with the signals sent out by the transmitter give a measure of the distance of the measuring means from the surface 1.

The determination of the distance h is effected by taking the ratios of the received signals.

The above described arrangement works satisfactorily for a clean still surface. However, if the surface is not still, as is often the case for a liquid like water, the ratio between the received signals falls, and is also noisy. This is because the received signal can vary due to changes in height, due to ripples, and also due to changes in surface reflectivity, e.g. due to surface depositants.

In Fig. 1 it is also possible to use a collimated beam from the transmitter and check for the peak output from the first receiver 2-then the surface is locally flat. Then the other receiver is accessed and the results statistically averaged. This is done using a microprocessor to get the relatively long time constants (of the order of minutes) needed, using digital filtering.

Fig. 2 shows an arrangement in which there are two receivers, as in Fig. 1, but a number, five in this case, of laterally-spaced transmitters. Here the beam dispersions used, are + 10 , with the transmitters spaced over 25cm. This arrangement integrates over a complete wave (ripple) on the liquid surface, which provides some improvement over the arrangement of Fig. 1. However, the signals received at the receivers tend to become less independent. Hence the signals received are in general more averaged. This unfortunately reduces sensitivity.

In Fig. 3 we have two receivers mounted at different distances h and (h + d) from the liquid surface. The relative positions of the devices in plan can be seen from the inset to Fig. 3, where the rings labelled 1 and 2 correspond to the receivers and the ring labelled T corresponds to the transmitter. The beam dispersion is + 5 .

The two receivers are not quite verticaily aligned, so that the lower receiver does not shadow the upper one. For larger ranges, d is considerably less than h, and the ratio of the signals at the receivers is such that very accurate measurement is needed. The basic principle is the use of the inverse square law of intensity with distance. This arrangement works quite well for a still surface, but there are still difficulties with a surface subject to ripples. Again the inverse square law does not quite hold as the transmitter beam is not uniformly intense across its width, and the beam axis does not pass through the receivers. Further, the signal ratio changes when the surface is waving as reflection from any point, e.g. X, hits the two receivers at significantly different angles.

Where the surface to be located is rippled, see Fig. 4, a collimated beam is used, with one transmitter and four photo-diodes. Thus each receiver R X 1 and R x 2 has two photodiodes in parallel, arranged in plan in cruciform relation with the beam centrally located, and accurately vertical (to within 5m rad.).

Another usable arrangement uses four detectors arranged symmetrically but set at 45 .

That is two sets of detectors as shown but the two sets rotated with respect to each other.

An arrangement (such as shown in Fig. 4) has a very well collimated transmitter beam, and as can be seen the transmitter is symmetrically located with respect to all the receiver photo-diodes. With a relatively lengthy time constant, e.g. one to two minutes, the receiver outputs can be statistically averaged. In such a case, due to the ripples, the reflected spot rapidly traverses the diodes from time to time, producing a signal. As the receiver output in thus a series of bursts, the statistics of these bursts in the time domain may limit the system accuracy. It is also possible, and may be better to monitor for peak output from each of the photo-diode detectors independently and take the ratio of the peaks. This would recessitate the use of a microprocessor to process the results of the monitoring.

Such an arrangement does not work satisfactorily with a still surface since the reflected light is still very well collimated, and so does not impinge on any diodes. If the beam is decollimated such that the reflected beam from a still surface covers all receiver diodes, then this enables the arrangement to work for a still surface. In this case some of the difficulties which are referred to in respect of the arrangement of Fig. 3.

In this arrangement further receivers can be included above or below the ones shown, which improves resolution.

Figs. 5 and 6 show how a wide parallel beam can be produced, in Fig. 5 using a concave mirror behine the light source and in Fig. 6 using a convex lens in front of the light source.

Claims

1. Apparatus for sensing the level of a material, which includes an optical source from which light is transmitted in beam form towards the surface whose level is to be sensed, the light thus transmitted being pulsed, receiving means so located as to receive light reflected from the surface of the material whose level is to be sensed, and means associated with the source and the receiving means to assess from the relation between the light transmitted from the source and the light received by the receiving means the level of said surface, wherein the receiving means includes two or more spaced receiving devices, the distance of the surface from the receiving means being determined on the basis of the ratios of the signals received at the receiver.

2. Apparatus as claimed in claim 1, and wherein there are a plurality of laterallyspaced transmitters, such that if the surface whose level is being monitored is subject to ripples integration over a complete wave of a ripple can be effected.

3. Apparatus as claimed in claim 1, wherein the receivers are mounted at different distances from the surface.

4. Apparatus as claimed in claim 3, wherein each said receiver consists of two or more photo-diodes in parallel.

5. Apparatus as claimed in claims 3 or 4, wherein the beam from the transmitter is a narrow collimated beam.

6. Apparatus as claimed in claim 1, 2, 3, 4 or 5, wherein the ratios of the signals received by the receivers are determined by a processing means such as a microprocessor which assesses from the said ratios the location of the liquid level.

7. Apparatus as claimed in claim 6, wherein the detection of the ratios is effected on the basis of correlated peak detection for the signals.

8. Apparatus for sensing the level of a material, substantially as described with reference to Fig. 1, Fig. 2, Fig. 3, Fig. 4, Fig. 5 or Fig. 6 of the accompanying drawings.

CLAIMS (2 July 1984) New or amended claims:- 9-11

9. Apparatus for sensing the level of a material, such as the surface of a body of a liquid, which includes an optical source from which light is transmitted in beam form towards the surface whose level is to be sensed, the light thus transmitted being pulsed, receiving means so located as to receive light reflected from the surface of the material whose level is to be sensed and means associated with the source and the receiving means to assess from the relation between the light transmitted from the source and the light received by the receiving means the level of the surface, wherein the receiving means includes two or more spaced receiving devices, wherein the said receiving devices are spaced both in a plane parallel to the surface and in a direction normal to the surface, and wherein the distance of the surface from the receiving means is determined on the basis of the ratios of the signals received at the receiving devices.

10. Apparatus as claimed in claim 9, and wherein there are two receiving devices one of which is adjacent to the optical source and one of which is between the plane of the optical source and the surface to be sensed.

11. Apparatus as claimed in claim 9, wherein there are four receiving devices arranged in two pairs, which pairs are at different distances from the surface to be sensed.