GB2493196A

GB2493196A - Optical Magnetic Position Sensor

Info

Publication number: GB2493196A
Application number: GB201112991A
Authority: GB
Inventors: Jonathan James Roberts
Original assignee: Individual
Current assignee: Individual
Priority date: 2011-07-28
Filing date: 2011-07-28
Publication date: 2013-01-30
Anticipated expiration: 2031-07-28
Also published as: GB201112991D0; GB2493196B

Abstract

This invention relates to an optical sensor 2 that uses the rotation of polarised light passing through an optical rod to measure displacement. A magnet 3 that moves along the rod rotates the polarised light using the Faraday Effect. A tapered or unevenly slotted metal sheath moderates the flux entering the rod. An interface unit 5 comprises a polarized beam splitter 6, a laser 7 and a photodiode 8 linked to electronic interface amplifier 9 such that the rotation of polarized light may be measured which represents the position of the magnet in relationship to the rod. The embodiment used here 1 describes a practical use of the invention where a tank fluid level 10 is measured.

Description

Optical Magnetic Position Sensor This invention relates to an optical sensor that uses the rotation of polarised light passing through an optical rod to measure displacement. A magnet that moves along the rod rotates the polarised light using the Faraday Effect. A tapered or unevenly slotted metal sheath moderates the flux entering the rod. By this means and with a suitable optical circuit, the rotation of polarized light indicates the position of the magnet in relationship to the rod.

The invention maybe used to measure the fluid height inside a tank. The following embodiment uses this application for the following embodiment. This shows an industrial use for this invention and best describes the invention.

Level sensors employ many methods to measure fluid height inside a tank. A float may use a mechanical coupling to operate an electrical transducer. A float may incorporate a magnet that operates a series of relays built into a tube spanning the length of the sensor.

Other level sensors may use ultrasound or light pulses where the time difference between the outgoing pulse and the reflective pulse references the height of the fluid.

If the liquid is non-conductive, the dielectric strength of the liquid relative to air may be used to measure the height of the fluid. This type of level sensor typically uses two concentric metal cylinders partially immersed within the fluid which together behaves as a capacitor the value of which changes with height of the fluid.

These level sensors use electricity in their operation in or near the fluid being measured.

Electrical devices are limited to a maximum continuous working temperature of 125°C which render them unsuitable for high temperature environments as found in engine compartments or in some processing plants.

Electrical devices may be subjected to interference from external electro-magnetic sources resulting in spurious readings.

Electrical devices can be a source of ignition resulting in an explosion or fire when used in environments that contain flammable liquids or vapours.

An optical level sensor would remove these disadvantages. Also an optical level sensor proposed here would simplify the construction and improve reliability.

Many changes, modifications and substitutions may be made to the following embodiment without departing from the spirit and scope of the invention. The invention is not limited by the following embodiments and their descriptions outlined in the following drawings.

Figure 1 shows the system layout of the optical level system I comprising sensor cylinder 2, a magnetic float 3 and the interface unit 5.

Figure 2 shows a cross section of the sensing cylinder with a high Verdet optical rod 11 and an outer tapered metal sheath 13. Figure 2 only shows the top and bottom of the sensing cylinder side by side to show construction details at each end. The sensing cylinder may have length to diameter ratios exceeding 50.

The optical level sensor system 1 shown in figure 1 comprises a sensing cyHnder 2, magnetic float 3, polarizing maintaining (PM) optical fibre 4 and an interface unit 5.

The sensor cylinder 2 and a magnetic float 3 combine to make a single assembly.

This assembly is housed inside the tank 10 with a suitable flange that allows for vertical mounting from the top of the tank 10.

The sensing cylinder 2 passes through the middle of the hollow cylinder shaped magnetic float 3. This allows the float to ride up and down the length of the cylinder with positive buoyancy as the fluid level changes within the tank 10. The magnetic poles sit on the top and bottom face of the float's magnet.

A polarized maintaining (PM) optical fibre 4 enables remote connection between the sensor assembly and the interface unit 5. This allows the electronic interface unit 5 to be housed in a benign environment away from heat, various contaminates, corrosive material and explosive atmosphere. The type of optical fibre 4 required needs to maintain the polarization angle through its length irrespective of fibre stresses caused by heat and bending. The angle of the polarized light contains information with regard to the fluid level inside the tank and therefore must be maintained to avoid inaccuracies.

The interface unit 5 comprises a polarized beam splitter 6 a laser 7and a photodiode B linked to an electronic interface amplifier 9.

The laser 7 generates the light and sends it through the beam splitter. The beam splitter 6 allows the vertical polarized components to be transmitted but reflects the horizontal polarized components away at 45 °. The remaining vertical polarized light travels along the PM optical fibre 4 to the sensing cylinder. Here the magnetism from the float 3 causes the polarized light to rotate by a controlled amount depending on the position of the float by using the magnetic property of the metal sheath. A mirror 16 at the base of the sensing cylinder reflects the light back through the rod and into the optical fibre.

When the reflected polarized light returns, it has been rotated and thus comprises both horizontal and vertical components. The beam splitter 6 again reflects the horizontal polarized components away at 45 but this time into a photodiode that converts the light strength into a useful electronic signal. The strength of the horizontal component relates to the fluid level in the tank 10.

The sensing cylinder 2 shown in detail in figure 2 comprises a single optical rod 11 with a high Verdet constant and a tapered wall metal sheath 13 with a high magnetic permeability. A equally tapered but opposite sheath 12 manufactured from a material with a low magnetic permeability sits inside the metal sheath 13 as a support between it at the optical rod. It may, with careful design, act as a thermal barrier limiting thermal shock at the optical rod 11.

A suitable lens 14 focuses the light between the fibre optic 4 and the optical rod 11.

A top housing 15 secures the lens into position and allows the optical connector of the optical fibre 4 to mate with the sensing cylinder.

A mirror 16 reflects the light backup the optical cylinder. A bottom housing 17 secures the mirror into position.

The focused polarized light emitted from the optical fibre 4 travels down the optical rod and reflected back by the mirror 16. Due to the Faraday Effect, the magnetism from the float rotates the polarized light twice: once travelling towards the mirror 16 and once from the mirror 16.

The metal sheath 13 limits the amount of magnetism entering the optical rod. The permeability of metal provides a path for the flux to flow between the poles away from the optical rod 11. At saturation the flux density reaches a limit inside the metal.

This aliows the remaining fiux to pass through the optical rod 11.

Saturation depends on the metal type and the cross sectional area through which the flux passes. The metal sheath 13 has a cross sectional area that various from a maximum to a minimum along the longitudinal axis of the rod 11. This maybe achieved by either varying the sheath's wall thickness, by having slots with varying width or a combination of both.

A position of the magnet 3 along the length of the rod 11 coincides to a specific cross sectional area of the sheath at that point.

The specific cross sectional area defines to a flux saturation limit in the metal sheath 13.

The specific flux saturation limit directs a specific amount of flux to enter the optical rod.

A specific amount of flux entering the optical rod relates to a specific angle of rotation of the polarized light.

Therefore the above establishes a direct relationship between the position of the float 3, or in other words the fluid level, and the polarized light angle returning from the sensing cylinder 2 which can be measured using the polarized beam splitter 6 and the diode 8 linked to the electronic amplifier 9.

Claims

<claim-text>Claims The following claims: 1. An optical sensor comprising an optical rod surrounded by a longitudinally asymmetric sheath made from magnetic permeable material, a magnetic source that rotates polarized light passing through the rod by means of the Faraday Effect and a means to measure the angle of rotation where the magnetic source moves along the longitudinal axis of the optical rod and where the means to measure the angle of rotation ot the polarised light indicates the longitudinal position of the magnetic source to the optical rod.</claim-text> <claim-text>2. An optical sensor in accordance with claim 2 where the magnet is fitted to a float to measure the fluid height inside a man made or natural structure.</claim-text>