GB2418733A

GB2418733A - Calibrating the zero setting of an ultrasonic depth sensor

Info

Publication number: GB2418733A
Application number: GB0519896A
Authority: GB
Inventors: Bryan Frederick Davenport; Nigel James Davenport
Original assignee: Individual
Current assignee: Individual
Priority date: 2004-10-02
Filing date: 2005-09-30
Publication date: 2006-04-05
Also published as: GB0421944D0; GB0519896D0

Abstract

A ultrasonic sensor 11 is mounted in a housing 10 which is located in proximity to the surface 21 of a non-gaseous medium 22, which may be flowing in an open channel. A datum reference plate 16 is located on an accessible part of the housing 10. The distance f between the datum reference plate 16 and the bottom of the channel or tank 23 is predetermined and known. A target plate 18 is mounted in the housing 10 at a fixed distance g from the datum reference plate 16. The target reference plate 18 is movable between a first position in which is located in the path of the ultrasonic signal and a second position in which it does not impede the ultrasonic signal. In the first position the distance d from the sensor 11 to the target 18 can be measured. The measured target distance d can be used to determine the zero setting a, i.e. the distance from the sensor 11 to the bottom of the tank or channel 23, as a = f-g+d.

Description

24 1 8733 - 1 -

AN APPARATUS AND METHOD FOR CALIBRATING

A SENSOR WITHIN AN HYDRAULIC SYSTEM

The present invention relates to an apparatus and method for use in the calibration of a sensor within an hydraulic system and, in particular, for measuring the 'zero setting' of the sensor or for checking the calibration of the system thereafter.

The rate of flow of a non-gaseous medium in an open channel, gully or lo trough is dependent on the mean velocity of the medium and the crosssectional area of the flow. Hence, the rate of flow is dependent on the depth of the flow. Conventional flow measuring apparatus use this fact and operate by measuring the velocity of the flow and its depth in a channel of predetermined width so that the rate of flow can be calculated. The depth of the medium is typically measured using an ultrasonic sensor.

A more sophisticated way of measuring the rate of flow is to use a flume, weir or other apparatus which incorporates a restricted channel, called a throat. The throat generates a predetermined, accurate cross- o sectional area of flow which ensures that the mean velocity of the medium is solely dependent on the head created immediately upstream of the throat regardless of any downstream condition. It is then only necessary to measure a single variable, namely the depth of the head created immediately upstream of the throat, in order to determine the rate of flow through the apparatus. Such apparatus are widely used in the water industry so that rates of water flow can be measured for various purposes including water supply, effluent control and the like. These apparatus typically incorporate an ultrasonic sensor to make the depth measurement.

so The present invention has been devised for particular use in such apparatus but it will be appreciated that it is not restricted to such usage and could find application in many types of hydraulic systems which require ultrasonic measurements to be made. - 2 -

As shown in Fig. I, an ultrasonic sensor operates by transmitting a sound wave at an ultrasonic frequency towards the surface 3 of a liquid or nongaseous medium 4 in a tank or channel whose depth is to be measured.

The surface 3 of the medium 4 reflects the wave back to the sensor and, as the speed of sound through air is relatively constant, the time interval between the transmission of the wave and receipt of its reflection by the sensor can be used to determine the distance b from the sensor to the surface 3. This measurement is then subtracted from the distance a between o the sensor and the bottom 5 of the tank or channel at the hydraulic zero to give depth c of the medium 4, i.e. c=a-b. The distance a is 'the zero setting' and is usually measured accurately when the ultrasonic sensor is installed and thereafter stored, for example by being programmed into the electronics associated with the operation of the sensor and the ultrasonic system as a whole. It should be appreciated that the hydraulic zero may not be the bottom of the tank or channel in all cases. If the tank or channel has an outlet spaced above the bottom, then there will be a dead space below it and the zero setting is then the distance to the bottom of the outlet, which may not be directly measurable by the sensor after installation.

Hence, the accuracy of the depth measurement is dependent on two parameters.

I. The accuracy of the distance measurement between the sensor 2 and the surface or the liquid or non-gaseous medium.

a. The accuracy of the zero setting.

The accuracy of the distance measurement is dependent on air temperature and ultrasonic systems usually incorporate a thermometer.

so However, there can be a temperature gradient between the sensor 1 and the surface 3. Similarly, a temperature gradient can also affect an initial measurement of the zero setting when it is usual, if possible, to measure the distance a between the sensor and the bottom 5 of the tank or channel when no medium is present. The accuracy of this measurement can also be affected by the fact that the small target area at the bottom 5 of the tank or channel directly beneath the sensor may not represent the true hydraulic zero of the system. Other problems can arise if the position of the sensor is accidentally changed without this being realized. For example it may not be replaced in exactly the same place after servicing or it may have suffered an accidental knock that alters its position slightly.

In order to overcome these problems, conventionally a target is used lo in the measurement of the zero setting, as shown in Fig. z. Here a target in the form of a plate 6 is set at a known height e above the hydraulic zero which is representative of a particular depth of medium. The sensor is then used to measure the distance d to the target 6 and this distance is added to the height e of the target above the hydraulic zero, i.e. a=d+e. The target 6 :5 can then be used to check the calibration of the system by comparison with the measurements indicated in Fig. as distance a should also equal b+ c. At any given time, therefore, it can be determined if the position of the sensor has changed. A further advantage is that if the target plate 6 is set at 50% of the calibration range, it can be used as a means to set the zero setting and in 2 o addition to check the calibration.

However, such an arrangement also has several disadvantages, as follows.

I. The use of the same target plate to check the zero setting and the calibration can result in a 'double error'.

a. It can be difficult to determine the true height of a target which simply hangs in position.

3. If the target is removable, it is difficult to ensure that it is always positioned in exactly the same position every time it is located in o the system.

4. A removable target may be accidentally replaced with a target from a different system. - 4

5. In some systems the target is suspended from an integral part of the fixing bracket for the sensor. If this bracket is accidentally moved or damaged, it may not be apparent that the whole assembly has been moved.

6. If a target is mounted some distance from an ultrasonic sensor then a temperature gradient between the sensor and the plate can result in measurement errors.

The object of the present invention is to provide a method and o apparatus for checking the calibration of a sensor within an hydraulic system that overcomes or substantially mitigates all of the aforementioned disadvantages.

According to a first aspect of the present invention there is provided an apparatus for use in the calibration of a sensor within an hydraulic system comprising a housing for location in proximity to the surface of a non-gaseous medium in which the sensor can be mounted; a datum reference plate located on an accessible part of the housing; o and a target plate mounted within the housing at a fixed distance (9) from the datum reference plate, the target plate being movable between a first position wherein it is located in the path of a signal emitted by the ultrasonic sensor and a second position wherein it does not impede the signal emitted by the sensor.

Hence, when the apparatus is in use in order to measure the zero- setting (a) of the sensor, the target plate is located in its first position and the distance (d) between the target plate and the sensor is measured for use so in determining or checking the zero setting. When in its second position the target plate does not impede measurements by the sensor of its distance (b) from the surface of the medium. As the distance f between the datum reference plate and the hydraulic zero of the system is fixed on installation of

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the apparatus, and the distance g between the datum reference plate and the target plate is factory set on manufacture of the apparatus and therefore known, then the height e of the target plate above the hydraulic zero is also known as e=f-g. Hence, as before, the measured target distance d between the sensor and the target plate can be added to the height e of the target plate to determine the zero setting a i.e. a=d+ e. However, the advantage of the apparatus is that the provision of the datum reference plate a fixed distance from the target plate means that the zero setting can be measured with considerably more accuracy than hitherto.

Preferably, the housing has an inverted box-like structure with closed side and top faces and an open bottom.

Preferably, the side faces of the structure are of sufficient depth that s they cover the sensor when it is mounted in housing.

Preferably also, the housing is provided with an adjustable mounting means in order that its level and therefore that of the datum reference plate can be adjusted.

Preferably also, the datum reference plate is located on the top face of the structure.

Preferably also, the target plate is mounted within the housing on a shaft which is rotatably mounted to and which protrudes from the housing.

Preferably also, the other end of the shaft protruding through the housing is provided with a handle to enable the target plate to be rotated between its first and second positions.

Preferably also, the handle is mounted with respect to the housing such that its position mirrors that of the target plate within the housing. 6 -

Preferably also, the housing defines an aperture through which a measuring device can be inserted.

Preferably also, the sensor is an ultrasonic sensor.

According to a second aspect of the present invention there is provided a method of measuring the zero-setting (a) of a sensor forming part of an hydraulic system comprising the steps of mounting a datum reference plate a fixed height (9 above an o hydraulic zero of the system; mounting a target plate at a fixed distance (9) from the datum reference plate; calculating the height (e) of the target plate from the hydraulic zero by subtracting the fixed distance (y) from the fixed height (,0; measuring the distance (d) between the target plate and the ultrasonic sensor; and adding the distance (d) between the target plate and the ultrasonic sensor to the calculated height (e) of the target plate from the hydraulic zero to give the zero-setting (a).

The present invention will now be described by way of example with reference to the accompanying drawings, in which: Figs. and are schematic side elevations of hydraulic systems with conventional ultrasonic measuring apparatus for determining the depth of a non-gaseous medium of the system; and Fig. 3 is a schematic side elevation of an hydraulic system incorporating an embodiment of apparatus according to the present 3 o invention.

As shown in Fig. 3, an embodiment of apparatus in accordance with the present invention comprises a housing to that essentially comprises a - 7 - mounting bracket for an ultrasonic sensor As. The housing has an inverted box-like structure with closed side faces 1, a top face 3 and an open bottom 14. The side faces 1 of the housing are of sufficient depth that they cover the sensor 11 when it is mounted therein in order to shield it from direct sunlight. The housing therefore comprises a temperature shroud. The housing 10 is also provided with adjustable mounting means 15 so that its level and can be adjusted in situ.

The top face 3 of the housing 10 either forms or has mounted thereon o a datum reference plate 16. It is important that this is level in both horizontal directions on installation of the apparatus. The location of the plate 16 in this position also enables its level in both directions to be checked immediately prior to each measurement by the sensor As. It is highly unlikely that any accidental movement or damage to the housing so or its fixing would retain s the datum reference plate 6 level in both directions so that any variation in level is indicative of movement of the housing having taken place.

In addition to the foregoing, the top face 13 of the housing lo defines an aperture 17 through which a measuring device such as a vernier can be 2 o inserted for a reason as described below.

Mounted within the housing 10 is a target plate 18 which is mounted within the housing at a fixed distance g from and parallel to the datum reference plate 16. The distance g is factory set and therefore accurately s known. However, it can be checked at any time by inserting a measuring device such as a vernier through the aperture t7 to measure directly the distance between the datum reference plate 16 and the target plate 18 when it is located beneath the sensor 11.

so The target plate 18 is attached to one end of a vertical shaft 19 that is rotatably mounted through the top face 13 of the housing, the other end of the shaft 19 protruding from the housing 10 being provided with a handle no. The handle no allows the shaft 19 to be rotated to move the plate 18 between 8 - a first position wherein it is located in the path of an ultrasonic signal emitted by the ultrasonic sensor 1 and a second position wherein it does not impede the ultrasonic signal emitted by the sensor 1. In order to facilitate appropriate location of the plate 6 in these positions, the handle 20 is mounted such that its position mirrors that of the target plate 8 at the other end of the shaft 19. It is preferable that the target plate 18 is located as close as is practical to the sensor 1 in order to minimize adverse temperature effects, such as temperature gradients, during measurements.

lo When the apparatus according to the invention is installed in a hydraulic system, the housing 10 should preferably be located as close as possible to the highest surface 21 of the non-gaseous medium 22 of the system and the ultrasonic sensor 11 mounted within it. As indicated above, it should also be ensured that the datum reference plate 16 is level in both s horizontal directions. Thereafter, initial calibration measurements can be taken and stored, namely the fixed height f of the datum reference plate 6 above the hydraulic zero of the channel or tank, say in this example the bottom 23 of the channel or tank. Such a measurement can be made using conventional height/distance measuring means. The height e of the target to plate 18 from the hydraulic zero can then be calculated by subtracting the fixed distance y from the fixed heightf Thereafter, the target plate 18 should be rotated beneath the sensor 11 and the distance d between the target plate and the ultrasonic sensor 1 measured. It will be appreciated that this distance d can be added to the calculated height e of the target plate from the hydraulic zero to give the zero-setting a. After this measurement, the target plate 18 can be rotated out of the path of the ultrasonic signal emitted by the sensor 11 to enable measurements of the surface level 21 of the medium 22 to be taken. As indicated above, measurement of the distance b from the sensor 1 to the surface 21 and knowledge of the zero-setting a enable the depth c of so the non-gaseous medium at any given time to be determined and thence the rate of flow of the medium through the system to be calculated.

It is also possible to motorize rotation of the target plate 8 between its first and second positions and to install a sensor which comprises a transducer enabling readings to be taken remotely, for example by the provision of a radio link, or similar, to external apparatus. Once installed and calibrated, therefore, the apparatus may never need to be accessed directly except for maintenance purposes.

The apparatus is preferably prefabricated in a factory or workshop and may be shipped to site with part of the calibration data already to measured because when fitted with reasonable care the dimensions and tolerances of the apparatus will not vary. Thereafter, initial measurements on site can supply the rest of the calibration data. Thus the invention provides an apparatus which substantially overcomes the problems described above. In particular, the target plate 8 is not used both to set the s initial zero setting a and to check the readings of the system. The setting of the zero setting a is determined by the position of the datum reference plate 6 and the location of the housing lo. The target plate 8 is then only used to check the zero setting. Also, the target plate 8 does not simply hang within the housing to and neither is it readily removable therefrom. It is mounted to such that it always adopts the same first position when rotated in this direction. .

Claims

I. An apparatus for use in the calibration of a sensor within an hydraulic system comprising a housing for location in proximity to the surface of a non gaseous medium in which the ultrasonic sensor can be mounted; a datum reference plate located on an accessible part of the housing; and a target plate mounted within the housing at a fixed distance 0 from the datum reference plate, the target plate being movable between a first position wherein it is located in the path of an ultrasonic signal emitted by the ultrasonic sensor and a second position wherein it does not impede the ultrasonic signal emitted by the sensor.

a. An apparatus as claimed in Claim I, wherein the housing has an inverted box-like structure with closed side and top faces and an open bottom.

to 3. An apparatus as claimed in Claim I, wherein the side faces of the structure are of sufficient depth that they cover the sensor when it is mounted in housing.

4. An apparatus as claimed in any of Claims to 3, wherein the housing : is provided with an adjustable mounting means in order that its level and therefore that of the datum reference plate can be adjusted.

5. An apparatus as claimed in any of Claims to 4, wherein the datum reference plate is located on the top of the housing.

6. An apparatus as claimed in any of Claims to 5, wherein the target plate is mounted within the housing on a shaft which is rotatably mounted to and which protrudes from the housing. - 11

7. An apparatus as claimed in Claim 6, wherein the other end of the shaft protruding through the housing is provided with a handle to enable the target plate to be rotated between its first and second positions.

8. An apparatus as claimed in Claim 7, wherein the handle is mounted with respect to the housing such that its position mirrors that of the target plate within the housing.

9. An apparatus as claimed in any of Claims to 8, wherein the housing defines an aperture through which a measuring device can be inserted.

to. An apparatus as claimed in any of Claims to 9, wherein the sensor is an ultrasonic sensor.

At. An apparatus for use in the calibration of a sensor within an hydraulic system substantially as described herein with reference to Fig. 3.

An. A method of measuring the zero-setting (a) of an ultrasonic sensor forming part of an hydraulic system comprising the steps of mounting a datum reference plate a fixed height (fl above an hydraulic zero of an ultrasonic sensor; mounting a target plate at a fixed distance (g) from the datum reference plate; calculating the height (e) of the target plate from the hydraulic zero by subtracting the fixed distance (g) from the fixed height (9; measuring the distance (d) between the target plate and the 3 o ultrasonic sensor; and adding the distance (d) between the target plate and the ultrasonic sensor to the calculated height (e) of the target plate from the hydraulic zero to give the zero-setting (a).