GB2029006A - Displacement sensing device and liquid gas tank gas level indicating system - Google Patents

Abstract

A gas tank gas level indicator utilises a float 2 which rotates a shaft 7 via gears. The shaft 7 drives a shaft 10 outside the tank through a ss magnetic coupling 8 and 11. An optical pick off utilises a light emitting diode 13 and a cadmium selenide photoresistor 14 with mutually compensating temperature characteristics. A variably opaque rotary disc 12 (also Fig 2, not shown) on the shaft has five discrete sectors of which one is white, one is black and the remaining three are of graded shades of uniform grey in effect, the "grey" being produced by finely drawn alternate lines and spaces whose relative widths vary from one grey sector to the next. The light detector has a relatively wide field of view and, over much of the range of rotation of the disc, the light detector is receiving light through parts of one or other pair of adjacent sectors, the relative proportions of the parts of the sectors through which the light passes being progressively variable as the disc rotates, so as to give a sloped characteristic. A signal processing circuit is disclosed (Fig 3 not shown). <IMAGE>

Description

SPECIFICATION Displacement sensing device and liquid gas tank gas level indicating system This invention relates to a displacement sensing device and to a liquid gas tank gas level indicating system.

A particular application of the invention is to a liquid gas tank gas level indicating system comprising a displacement sensing device for sensing the liquid gas level inside a liquid gas tank of an automobile which is adapted either to run only on liquid gas or to run selectively on liquid gas or petrol.

Because a liquid gas tank is required to be hermetically sealed, it is not possible to extend a rotary shaft or other movable member through a seal in the tank wall for the purpose of operating a gas level indicating device. If the tank wall is made of non-magnetic material, a magnetic coupling may be used to cause a "slave" shaft outside the tank to follow rotary motion of a "master" shaft inside the tank, but the torque-transmitting capability of a moderately sized magnetic coupling are somewhat limited. Accordingly, any device for sensing the rotational position of the slave shaft should preferably impose little or no load on the slave shaft. An optical device would be almost ideal, if a way could be found of standardising the characteristics for a production series.Problems with such standardisation are encountered with variations in the luminosity of light sources, sensitivities of light detectors and variations in the characteristics of different light-modifying members.

According to a first aspect of the invention there is provided a displacement sensing device comprising a light source, a light detector and a light-modifying member, the light-modifying member being relatively movable over a range of relative movement relative to the light source and light detector and having a plurality of discrete side-by-side light-modifying portions having graded light-modifying effects, each individual said portion having a uniform light-modiWfng effect and the light-detector having a sufficiently wide field of view so that relative movement of the light-modifying member over substantial parts of said range results in progressive variation in the relative proportions at any instant I of two or more of said portions modifying the light received by the detector.

According to a second aspect of the invention there is provided a liquid gas tank gas level indicating system comprising means inside a liquid gas tank for sensing liquid gas level and turning one magnetic member of a magnetic coupl;ng to a corresponding rotational position inside the tank and a device according to the first aspect of the invention connected to another magnetic member located outside the tank of the magnetic coupling, said light detector controlling an electric meter for indicating the gas level.

The invention will be described by way of example with reference to the accompanying drawings, wherein: Figure 1 is a partly sectioned illustration of a liquid gas tank gas level indicating system embodying the second aspect of the invention; Figure 2 illustrates a circular disc forming a light-modifying member of a displacement sensing device embodying the first aspect of the invention, in the system of Figure 1; Figure 3 is a diagram of an electrical circuit in the system of Figure t; Figure 4 is a graph illustrating the overall temperature characteristics of a light source and a light detector in the displacement sensing device in the system of Figure 1; and Figure 5 is a graph iilustrating the output characteristic of the circuit of Figure 3.

Referring to the drawings, the illustrated liquid gas tank gas level indicating system 1 of Figure 1 comprises a float 2 at one end of an arm 3 which is pivoted on a shaft 4 and which carries a counterweight 5 at its other end. The shaft 4 rotates to and fro as the float 2 rises and falls with the level of the liquid gas and drives a bevel gear 6 which is fast with the shaft 4. The bevel gear 6 drives another bevel gear (not shown) to rotate a vertical shaft 7 to and fro as the gas level rises and' falls. At the top end of the shaft 7 is a first magnetic member 8 of a magnetic coupling.

The float 2, arm 3, horizontal shaft 4, counterweight 5, bevel gear 6, vertical shaft 7 and first magnetic member 8 are all inside the hermetically sealed, non-ferromagnetic, wall 9 of a liquid gas tank.

Outside of the wall 9 of the tank is another vertical shaft 10, coaxial with the shaft 7, and carrying a second magnetic member 11 of the magnetic coupling at its lower end, so that the shaft 10 follows any rotational movement of the shaft 7. Mounted on the shaft 10 is an optical disc 12 (see Fig. 2) which is keyed to the shaft 10 so as to follow its rotational movement exactly.

A light source 13, in the form of a light-emitting diode, and a light detector 14, in the form of a cadmium-selenide photoresistor, are fixed inside a support housing 1 5 as shown, with the disc 1 2 passing between the light source 1 3 and light detector 14 so as to control the amount of light reaching the light detector 14.

A second disc 1 6 is mounted at the top of the shaft 10 and can be observed through a window 1 7 in the top of the casing 1 5 to give a direct visual indication of the rotary position of the shaft 10 and hence the rotary position of the shaft 7 and the level of gas inside the tank.

Referring now more particularly to Figure 2, the disc 1 2 has five discrete side-by-side light modifying sectors 1 2a to 1 2e having graded light modifying effects. A first sector 1 2a is substantially transparent and subtends 900 (at the centre of the disc 12) and transmits nominally 100% of the light from the light source 13. The second sector 1 2b subtends 700 and transmits 30% of the light falling onto it from the light source 13. The third sector 1 2c subtends 500 and transmits 10% of the lightfalling on it. The fourth sector 12d subtends 700 and transmits 3% of the light falling on it, whilst the fifth sector 1 2e subtends 800 and transmits between 0 and 1% of the light falling on it.The second, third and fourth sectors 1 2b, 1 2c and 1 2d are formed by substantially transparent strips alternating with substantially completely opaque lines of different proportions for the different sectors. By way of example, the second sector 1 2b may have lines of .0467 inch thickness and clear strips of .02 inch thickness, the third sector 1 2c lines of .18 inch thickness and transparent strips of .02 inch thickness and the fourth sector 1 2d lines of .647 inch thickness and transparent strips of .02 inch thickness.Although on a micro scale these sectors are not uniform, the light detector 14 has a relatively wide field of view and cannot distinguish at all between the individual lines and the clear strips, so that each of the sectors 1 2b, 1 2c and 12d appear effectively to be each of a uniform grey, graded between the transparency of the first sector 1 2a and the substantially total opaqueness of the fifth sector 1 2e.

Referring now to Figure 3, the electrical circuit is adapted to be supplied with 12 volts from an ordinary car battery between terminals T1 and T2, terminals1 being positive and terminal T2 being earthed as is conventional for modern automobile electrical systems. Axener diode D1 maintains a constant potential difference of about five volts between lines L1 and L2, being connected across them directly. Terminal T1 is connected directly to line L1 , whilst terminal T2 is connected through a ballast resistance R1 to line L2.An indicator lamp in the form of a light-emitting diode D2 is connected via a ballast resistance R2 to terminal T1 and via a first pair of contacts of a switch SW1 to terminal T2, the function of the lamp D2 being to indicate when the automobile is running on liquid gas (and not petrol). The light source 13 and light detector 14 are both connected via a common terminal c to terminal T1. Light source 1 3 is connected via a ballast resistance R3 and a terminal a to line L2, whilst light detector 14 is connected via a terminal b and voltage-dividing resistances R4 and R5 to line L2. The junction between resistances R4 and R5 is connected to the non-inverting input 3 of an operational amplifier 101. Terminals T1 and T2 are connected to inputs 7 and 4 respectively of the operational amplifier 101.The output 6 of operational amplifier IC1 is connected directly to its inverting input 2, in order to supply negative feedback for purposes of stability. A smoothing capacitance C1 is connected between line L2 and the non-inverting input 3 to amplifier IC1, to smooth fluctuations caused by surging of the liquid gas. The output 6 of amplifier IC1 is connected via a load resistance R6 to the positive input terminal of an electrical meter M, the negative input terminal of which is connected to line L2.Finaily, a second pair of contacts of the switch SW1 are ganged to the first pair of contacts thereof and, in use, control petrol solenoid and a gas solenoid for the purposes of shutting off the petrol when the sutomobile is required to run on gas and alternatively shutting off the gas when the automobile is required to run on petrol, according to the setting of switch SW1.

During assembly, the light source 13, light detector 14 and resistance R3 initially form a separate sub-circuit, before incorporation in the rest of the circuit of Figure 3. The resistance R3 is adjusted so that a given voltage across terminals a and c (of, say, 5 volts produces a given resistance (of light detector 14) between terminals b and c when light source 13 and light detector 14 are in view of each other through e.g. sector 12c. In this way, any given unit consisting of light source 13, light detector 14 and resistance R3 may be quickly exchanged for another similar unit, without need for recalibration. Similarly, the resistance R6 is adjusted for a given resistance between terminals b and c to produce a given output reading on the meter M.

Referring to Figure 4, the light source 13 and light detector 14 have mutually compensating temperature characteristics. More particularly, curve 1 7 indicates the actual variation in the light output of the light source 13 with variation in temperature if a constant voltage is supplied to the light source. Curve 1 8 indicates the actual variation in resistance of light detector 14 with variation in temperature for a constant light flux.

Curve 19 is a kind of "mirror image" of curve 1 7, coinciding therewith at the 100% point (at a temperature of approximately 240) and indicates the effect on the light detector resistance (if the light detector were maintained at constant temperature) of variation inthe light output of the light source. Because the light detector 1 4 has a negative temperature coefficient, as indicated by curve 1 8, the final resultant variation of resistance of the light detector 14 is only 6%, as shown by curve 20, over a range of 200 to 500C, if the light source 13 and light detector 14 remain at substantially the same temperature as each other at all times.

Referring now to Figure 5, in combination with Figures 1 and 2, the light detector 14, being a cadmium selenide cell, has a relatively wide field of view. Consequently, over a substantial proportion of the range of rotation of the disc 12, the light detector 14 is receiving light from the light source 1 3 simultaneously through two adjacent sectors 1 2a and 12b, or 1 2b and 12c, or 12c and 12d or 12d and 12e of the disc 12. As the disc 1 2 rotates, the relative proportions change progressively of the said pair of sectors through which the light passes, hence producing the relatively steeply sloped portions 21a, 21c, 21d and 21 f of curve 21. There are intermediate portions 21 b and 21 e of curve 21 where the characteristic is almost flat. Here, the light from the source 1 3 is reaching the detector 14 through a single sector which, being uniformly shaded, does not vary the amount of light reaching the detector 1 4 with rotation of the disc 1 2. However, given that the meter M is not required to give an accurate indication of gas level, but only an approximate indication, the curve 21 follows the ideal straight line 22 closely enough for practical purposes.

Claims (10)

1. A displacement sensing device comprising a light source, a light detector and a light-modifying member, the light-modifying member being relatively movable over a range of relative movement relative to the light source and light detector and having a plurality of discrete side-byside light-modifying portions having graded lightmodifying effects, each individual said portion having a uniform light-modifying effect and the light-detector having a sufficiently wide field of view so that relative movement of the lightmodifying member over substantial parts of said range results in progressive variation in the relative proportions at any instant of two or more of said portions modifying the light received by the detector.

2. A displacement sensing device as claimed in claim 1 wherein the light source and light detector are fixed to a support whilst the light-modifying member is movable.

3. A displacement sensing device as claimed in claim 1 or 2 wherein the light source and the light detector have mutually compensating temperature characteristics.

4. A displacement sensing device as claimed in claim 3 wherein the light source is a light-emitting diode and the light detector is a cadmium setenide photoresistor.

5. A displacement sensing device as claimed in any preceding claim wherein the light-modifying member modifies the light by varying the proportion of light transmitted through said member.

6. A displacement sensing device as claimed in any preceding claim wherein the relative movement is relative rotation.

7. A displacement sensing device as claimed in claim 6 read as appended to claim 5 wherein said member is a disc and said portions are sectors of the disc.

8. A displacement sensing device as claimed in claim 7 wherein said disc has a first substantially transparent sector of 900, a second 30% lighttransmitting sector of 700, a third 10% lighttransmitting sector of 500, a fourth 3% iighttransmitting sector of 700 and a fifth between 0% and 1% light-transmitting sector of 800.

9. A liquid gas tank gas level indicating system comprising means inside a liquid gas tank for sensing liquid gas level and turning one magnetic member of a magnetic coupling to a corresponding rotational position inside the tank and a device as claimed in claim 6, 7 or 8 connected to another magnetic member located outside the tank of the magnetic coupling, said light detector controlling an electric meter for indicating the gas level.

10. A system as claimed in claim 9 wherein a gas level indicator is directly mechanically driven by said other magnetic member of the magnetic coupling.