GB2250600A - Inclinometer - Google Patents

Abstract

An inclinometer has a vessel or cell comprising a sealed container (1) filled partly with an electrically conductive fluid (2) defining an interface surface (3) with a gaseous or lighter liquid medium (4) which is non-conductive. The cell houses three electrodes of which the centre one (5) is always immersed in the fluid (2) and the outer ones (6 and 7) are positioned just above the interface surface (3) when the cell is level. An electronic circuit comprising an audio oscillator is NAND gated by a pulse circuit of which the pulse length is varied according to the conductivity between electrode (5 and 6 or 5 and 7). When the cell is level the pulse length is zero giving a continuous audio output.

Description

TITLE Inclinometer This invention relates to apparatus for measuring inclination or slope or level or plumb.

According to this invention there is provided an inclinometer comprising a vessel partly filled with an electrically conductive fluid and defining an interface surface, at least two electrodes the conductivity between which is changed by the fluid in response to an inclinal change in the vessel, circuit means connected with the electrodes to detect the conductivity change and output means connected with the circuit means to generate a signal according to the detected conductivity.

Preferably the vessel includes at least three electrodes of which one electrode, at least, remains in contact with the fluid during normal measurement of inclinal changes.

In an embodiment all the electrodes remain immersed in the fluid during normal measurement use.

In an embodiment of vessel having three electrodes, a centre electrode remains in contact with the fluid with the other electrodes being positioned just above the fluid surface when in a level or reference position.

In an alternative arrangement, with three electrodes, the two other (outer) electrodes are submerged in the fluid when the vessel is level. The current flow in each outer electrode is monitored and a comparison between the two is determined. This version enables accuracy to be improved and the device to be electronically calibrated.

In an embodiment the vessel comprises an elongate, preferably oblong, tank containing an electrically conducting fluid and three electrodes; one electrode being mounted centrally in the tank such that when the tank is inclined within the normal operating range the electrode is fully immersed in the liquid; two needleshaped electrodes, one at each end of the tank, being mounted such that with the tank level, the points are just clear of the liquid surface. All electrodes are mounted on a top closure lid of the tank When the tank is level, there is an open circuit between the centre electrode and both of the needle electrodes because the needle points will be clear of the liquid surface and thus no current flow occurs through the electrolyte.If the left hand side of the tank is raised a small amount, then the needle on the right side will dip into the electrolyte and there will be a change in resistivity between this needle and the central electrode. Similarly, if the right hand side of the tank is lifted, the needle on the left hand end of the tank will dip into the electrolyte and a resistivity will be present between this needle and the central electrode. The greater the degree of tilt, the lower the resistivity between the appropriate needle and the central electrode due to a greater surface area of the needle's body being in contact with the electrolyte.

Preferably the circuit means connected with the electrodes comprises an oscillator whereby the resistance between the electrodes effects a change in frequency being a function of the inclination of the vessel. The frequency preferably is within the audible range.

The apparatus according to this invention may be used as a substitute for a conventional spirit level and a particular advantage is that a remote indication of inclination is available.

The circuit of a preferred arrangement is designed so that if the surface is level (+/-0.3 degrees) then the device emits a continuous 3KHZ tone from a piezo transducer. If the device is tilted slightly off level, then it emits an infrequent 3KHZ "bleep" (approximately one per second). As the degree of the slope increases, the bleeps are emitted at a faster rate. At a slope of about ten degrees, the bleeps are emitted at the rate of around ten per second.

This invention is further described and illustrated with reference to the accompanying drawings showing an embodiment and modification thereof by way of example. In the drawings: Figures la to ic show the inclinometer vessel in cross-section and in three positions, Figure 2 shows a circuit providing output signals proportional to inclination, Figure 3 shows a construction schematically in side elevation Figures 4a to 4d show two modified inclinometer vessels, and Figure 5 shows a second circuit for a vessel wherein all the electrodes are immersed.

Referring to Figure 1 this shows the inclinometer vessel or cell comprising a sealed container 1 filled partly with an electrically conductive fluid 2 defining an interface surface 3 with a gaseous or lighter liquid medium 4 which is non-conductive. The cell houses three electrodes of which the centre one 5 is always immersed in the fluid 2 and the outer ones 6 and 7 are positioned just above the interface surface 3 when the cell is level. The ends.of electrodes 6 and 7 will have needle points.

The electrodes 5,6,7 may be carried on a top closure plate 8 and may be made adjustable to achieve accurate positioning relative to the surface 3.

Figures ib and Ic show the conditions in the cell when an inclinal change 8 occurs and the degree of inclination causes one or other of the electrodes 6 or 7 to dip into the fluid 2 to a greater or lesser degree, thus the resistance measured between an electrode 6 or 7 and 5 provides an indication of inclination.

Figure 2 shows one circuit arrangement for providing an output representative of the cell inclination.

Referring to Figure 2 the centre electrode 5 is connected to CE, the electrode 6 is connected to LE and electrode 7 is connected to RE.

ICld is used to form a split supply for the op-amps and R1 and R2 form a potential divider which biases the non-inverting input of the op-amp at half supply voltage.

The amplifier is arranged as a voltage follower and so the output is a low impedance voltage source.

IClb is arranged as a low frequency (nominally 1HZ) oscillator, with frequency determined by C1 in combination with the cell resistance. The lower this resistance (more tilt), the higher the frequency of ICla.

The output of ICla is used to gate the 3KHZ oscillator formed by IClc. This gating forms a series of bleeps.

The frequency of this oscillator is determined by R21 and C3. 3KHZ was chosen, as this is the correct frequency for most low-cost piezo transducers, but in any case, the frequency of this oscillator should be adjusted to suit the resonant frequency of the piezo transducer eventually selected.

IClb is arranged as a comparator, the output of which is held low by the fact that pulses from the low frequency oscillator (ICla) charge up C2 and hold the inverting input higher than the ground referenced noninverting input. However, ICla only oscillates when the cell 1 is not level. When the cell is level the cell resistance is infinite and ICla stops oscillating.

The problem is that the output can be either high or low when oscillation ceases, and it is this randomness which the comparator prevents.

It is assumed that ICla stops oscillating with its output in the high state; then C2 will be charged and the output of the comparator (pin 7) will be low. Diodes D4 and D5 are reverse biased and so the 3KHZ oscillator is not affected and it oscillates continuously. Dead level gives a continuous tone. However, if oscillator ICla stops oscillating with its output low, then C2 discharges through R20 (time constant about one second) and the output of the comparator goes high, thereby preventing the low output on pin 1 from disabling the 3KHZ oscillator via resistor R19. Again, the 3KHZ oscillator runs continuously.

If the device is tilted off-level, then ICla oscillates, the comparator output is low, and now the 3KHZ oscillator is periodically disabled every time pin 1 goes low since connecting pin 9 to -ve via D5 and R19 disables the oscillator. Two further comparators IC2a and IC2b are used to detect when either the left or right end electrodes are immersed in the liquid. Considering one comparator only, when the oscillator capacitor is charging through the electrode resistance the current flowing causes a voltage to develop across R3. The comparator accordingly gives a low output and LED D1 is turned on. When the oscillator capacitor begins to discharge, the voltage across R3 is reversed, the comparator changes state, and the LED Dl is switched off.

thus the LED is made to flash on and off at a frequency determined by the degree of inclination of the level.

The overall accuracy of the level will be determined by the following factors: i) the length of the cell 1 as the longer the cell, then the greater the rise in the height of liquid at an end since: Change in liquid height at an end = (Length of a cell) x (Tan o ). (Where 0 is the angle of the tilt.) ii) The accuracy of the electrodes: Positioning tolerance of either end electrode will determine the accuracy to which the level position can be measured.

Any misalignment in the positioning of the electrode tips will produce a similar error in the measurement of the horizontal position.

iii) Cell fluid level: The tank is filled with electrolyte fluid to a level set just below the two outer electrodes. The accuracy to which this can be achieved will determine the degree of tilt required before the liquid makes contact with an electrode.

Adjustment of the fluid level in the tank can be achieved by means of a threaded plug which can be inserted or withdrawn from the tank to raise or lower the liquid level by displacement. The plug can serve a dual purpose in that it can also be used at the filling point for the cell.

iv) Width: The cell needs to be wide enough to accommodate a sensible volume of fluid in order to minimise the effects of volume depletion due to small amounts of electrolyte clinging to the side walls after the cell has been agitated. A sensible volume also makes production filling less critical and reduces the effects of lateral tilt.

Furthermore, if the device were unduly narrow, meniscus effects would result in unacceptable hysterisis.

Using a cell of fluid in this manner means that there is a certain settling time if the level is violently disturbed. This is normally about two or three seconds, but the effects of this are masked by the protocol chosen for the output bleeps. If the cell were to be simply connected across an audio oscillator, then the frequency hunting would be obvious.

It is possible to reduce the settling time of the fluid by inserting baffle plates widthways across the tank. This is more applicable to larger cell sizes and baffle plates must be designed to allow free passage of the liquid through them.

The shape of the central electrode is not critical but to achieve reliable performance the surface area immersed in the liquid is designed to be as large as possible. A straight wire electrode, terminating in a flattened hook has been found to be suitable for most applications.

The end electrodes should ideally taper to a sharp point. The profile of a darning needle has been found to be suitable because this gives the correct logarithmic resistance response to achieve the maximum change of resistance a degree or two either side of the level and thereafter a more gradual change. The sharper the point of the needle, the lower the surface tension effects at the tip. The needle point rests just above the liquid surface when the cell is level, but in reality, the needle point must be raised slightly higher than this or surface tension will bridge the gap between the point and the surface.

The end electrodes should be positioned approximately 5mm in from each end of the cell in order to avoid meniscus end-effects. Also they should be positioned dead centre of the width dimension to minimise the effects of lateral tilt of the cell.

Any conducting liquid can be used for the electrolyte but practically, corrosive, volatile or high viscosity liquids are not suitable. For a production unit a dilute solution of acetic acid has been found to be suitable. The strength of the solution is determined by the size of the tank and the frequency of oscillation required when one of the outer electrodes is just touching the liquid surface. A feature of the level cell is that the currents are AC, because the cell resistance is used both to charge and discharge the same capacitor, hence electrolysis and plating effects are greatly minimized.

Figure 3 shows one practical construction with the tank 1 located on a rotating dial 10 which can be set any any angle between 0 and 180 degrees in relation to the reference edge of the case 11. With the dial set at 0 degrees the unit can be used for level measurement, at 90 degrees for plumb and at 180 degrees for inverse level or overhead measurement. Any intermediate angle setting can be achieved by referencing a pointer 12 on the dial 10 to an angle scale 13 on the main body 14 of the case.

The body 14 also houses the circuit board 15, a battery 16 and has an on-off switch 17.

Figure 4 shows a cell unit formed from two individual vessels at right angles. The cells are similar to the arrangement shown in Figure 1.

By providing dual cells, one for level and one for plumb, automatic switching between the two cells can be achieved as described below.

The liquid level in the Level Cell is arranged (along with the geometry of the cell) such that if the cell is rotated through 90 degrees then the electrolyte level settles underneath the central plate electrode, thereby ensuring an open circuit on all electrodes of the level cell when the device is measuring plumb. A similar situation occurs in the plumb cell when measuring level.

In order to measure plumb, a second cell of identical construction, but somewhat smaller dimensions (50 x 50 x 22) is mounted at right angles to the first cell. The two cells together have four resistances, and these can be arranged in parallel in the RC oscillator circuit as only one of them is active at any one time (the rest being open circuit). In this way, all cell elements share the same electronics, thereby reducing costs.

Referring to Figure 5, this circuit is intended for use with a cell such as that shown in Figures 4c and 4d having the electrodes immersed wherein the cell is filled to a preset depth and the two outer electrodes are adjusted to contact the liquid surface when the tank is level. In practice, the electrode tip can protrude below the surface by a small amount (see Figure 4d).

A circuit diagram is shown in Figure 5 and with reference to this diagram IC2a provides a mid-rail reference supply voltage for the detector circuits.

IC2b in conjunction with the three electrodes (left, centre and right) forms an oscillator.

lOla and IClb measures the current flowing in the left and right hand electrodes respectively.

IClc and ICld act as comparators which provide a signal output when the electrode current exceeds a certain value. The signal from these comparators are XOR d by IC3a to provide a difference signal, i.e. when the current in the electrodes is different, the output (LEV) is high, when the current is the same the output is low. Thus 'as the tank is moved from a positive inclination through level to a negative inclination, the pulse width of the LEV signal decreases, passes through zero and then increases.

IC4a and IC4b forms a detector which senses the LEV pulse. When the LEV pulse occurs IC4a is triggered and the output is used to reset IC4b. Upon completion of the timing period the reset is released and IC4b can then be triggered by LEV. The output is then used to interrupt the oscillator IC3d/3c which drives the piezo transducer (speaker). As the unit is moved towards level the width of the LEV pulse decreases until it eventually becomes shorter than the timed output pulse from IC4a. When this occurs IC4b is not triggered and the oscillator gives a continuous tone output indicating level.

The LED1 and LED2 signals are controlled from the LEV output. As the level is moved towards horizontal the LED on period will decrease until both LEDs are off.

When the unit is set on a level surface the LEV signal pulse width should be zero. This relies on the current through both electrodes being equal. In order to adjust this, there are two possible methods:a) One electrode is raised or lowered by adjustable means such as a threading until the pulse width is minimised. This leaves the problem of tightening the terminals and sealing the thread until after final test.

b) A fixed resistor, R3 is added in series with the left electrode and potentiometer, UR1, is added in series with the right-hand electrode. Adjustment can then be made electronically allowing the electrodes to be fully inserted and sealed during production.

This is determined by the minimum pulse width of the LEV signal which can be resolved which is set by the timing resistor on IC4a.

Tapered electrodes, ending in a point, give the best performance (see Figure 4d), the reason being that a greater resistance change occurs with the taper than on straight electrodes.

The difference gradient must be maximised when the system is level to maintain the accuracy. To achieve this, the tips of both electrodes should protrude below the surface of the liquid by the minimum amount when the tank is level. Ideally this distance should be kept to less than 0.5mm to maximise the accuracy. In production, this can readily be achieved by setting the probes to a fixed position, and filling the tank with a known measure of liquid.

Claims (17)

1. An inclinometer comprising a vessel partly filled with an electrically conductive fluid and defining an interface surface, at least two electrodes the conductivity between which is changed by the fluid in response to an inclinal change in the vessel, circuit means connected with the electrodes to detect the conductivity change and output means connected with the circuit means to generate a signal according to the detected conductivity.

2. An inclinometer according to Claim 1, including at least three electrodes of which one electrode, at least, remains in contact with the fluid during normal measurement of inclinal changes.

3. An inclinometer according to Claim 1 or 2, wherein all the electrodes remain immersed in the fluid during normal measurement use.

4. An inclinometer according to Claim 1 or 2, wherein a centre electrode at least remains in contact with the fluid during normal measurement use, at least the outermost of the other electrodes being positioned just above the fluid surface when in a level or reference position.

5. An inclinometer according to Claim 1, 2 or 3, having three electrodes, the two other (outer) electrodes being submerged in the fluid when the vessel is level.

6. An inclinometer according to Claim 5, wherein the current flow in each outer electrode is monitored and a comparison between the two is determined.

7. An inclinometer according to Claim 4 wherein the vessel comprises an elongate, preferably oblongs tank containing an electrically conducting fluid and three electrodes; one electrode being mounted centrally in the tank such that when the tank is inclined within the normal operating range the electrode is fully immersed in the liquid; two needle-shaped electrodes, one at each end of the tank, being mounted such that with the tank level, the points are just clear of the liquid surface.

8. An inclinometer according to any preceding claim, wherein the circuit means connected with the electrodes comprises an oscillator whereby the resistance between the electrodes effects a change in frequency being a function of the inclination of the vessel.

9. An inclinometer according to any preceding claim, wherein the circuit means includes an audio oscillator producing an output signal having an audible change in character according to conductivity change between the two electrodes.

10. An inclinometer according to Claim 9, wherein said character change is in audible frequency.

11. An inclinometer according to Claim 9, wherein the output signal is at audible frequency, said character change comprising a change in amplitude.

12. An inclinometer according to Claim 9, wherein the output signal is at audible frequency and is amplitude modulated, pulsed or pulsating, said character change comprising repetition rate, or pulse width.

13. An inclinometer according to Claim 12, wherein the repetition rate is zero when the vessel is at reference level".

14. An inclinometer according to Claim 13, wherein the pulse amplitude is maximum (on) when the vessel is at reference "level'.

15. An inclinometer according to Claim 12, wherein the pulse suppresses the output signal, the pulse width being zero at reference level' and increasing with a change in inclination from said level position.

16. An inclinometer according to any preceding claim1 wherein the output signal has a different characteristic for inclination in one or other directions from reference level .

17. An inclinometer constructed and arranged to function substantially as described herein with reference to the drawings.