GB2130730A

GB2130730A - Movement sensing apparatus

Info

Publication number: GB2130730A
Application number: GB08328270A
Authority: GB
Inventors: Ronald W Schaller
Original assignee: SELTRUST ENG Ltd
Current assignee: SELTRUST ENG Ltd
Priority date: 1982-10-21
Filing date: 1983-10-21
Publication date: 1984-06-06
Also published as: GB2130730B; GB8328270D0

Abstract

A linear Hall effect chip 6, that is, a chip giving an output voltage which varies in direct relation to the magnetic field, is located between a magnet 10 and a detection region. A current is passed through the chip and a Hall voltage arises, perpendicular to the magnetic field and the current. When a magnetic or paramagnetic material such as a truck wheel 20 on rail 4 passes through the detecting region the magnetic flux distribution through the element changes and the Hall voltage is increased. The Hall voltage is fed into suitable control circuitry, which includes a DC amplifier, and comparison means which compares the Hall voltage with a reference voltage and provides an output signal only when the Hall voltage deviates from the reference voltage by a predetermined value. Thus signals from small objects may be rejected. <IMAGE>

Description

SPECIFICATION Movement sensing apparatus This invention relates to movement sensing apparatus incorporating a Hall effect element, with particular application to the railway and mining industries.

According to the present invention there is provided apparatus for detecting the passage of ferromagnetic or paramagnetic objects including a magnet and a Hall effect device, the magnet providing a magnetic field through which the objects to be detected pass, the Hall effect device comprising a Hall effect element disposed in the magnetic field and circuitry such that the output of the device varies in direct relation to changes in the magnetic field through the element caused by the passage of objects to be detected, and a comparator for comparing the output of the device with a reference level and delivering a signal while the output of the device differs by a predetermined amount from the reference level and thus indicates the presence of an object to be detected.

Thus small objects may pass undetected, the change they cause in the magnetic field through the element not causing a sufficient change in the output of the device to enable the comparator to deliver a signal.

Preferably the magnet has an end face at one pole, the Hall effect element being disposed adjacent the end face so that the magnetic flux between the poles of the magnet passes from the end face through the Hall effect element and the region through which the objects to be detected pass.

The Hall effect element and the magnet may be encapsulated in a cylindrical casing, one end of the casing, past which end objects to be detected pass, being closed by a flux-concentrating end piece of magnetic material. Preferably the rest of the casing is of non-magnetic material.

Switching means responsive to the signal delivered by the comparator may be used to provide a connection to an external control circuit, which may control track equipment such as points and lights and, in mines, equipment such as track arrestors and ventilation doors. Furthermore, two sensors located close together may be used for sensing the speed of a body, in conjunction with suitable timing equipment. Three sensors may be used for determining direction.

The preliminary control circuitry, that is, the comparator and switching means, may be housed in a control box. The power supply unit, to provide the required 12 volt DC supply from the normal AC supply may also be housed in the control box.

However, in hazardous conditions a separate flameproof power supply unit may be required.

The control box may contain circuits controlling a number of adjacent track sensors.

The invention will be further described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a partial cross-section of a probe housed in a mine track rail, and Fig. 2 is a block diagram of preliminary control circuitry associated with the probe of Fig. 1.

The probe 2 is located in a vertical bore in a rail 4, with the top surface of the probe flush with the top surface of the rail. The probe 2 comprises a linear Hall effect chip 6 fixed in a plastics block 8.

The chip 6 is a fully integrated circuit having a Hall effect element and signal conditioning and thermal stability circuits "on chip". A suitable chip is the UGN 3501 T manufactured by the Sprague Electric Company of the United States. The output of this chip is closely proportional to the magnetic field through the Hall effect element over a wide range of output, and the thermal stability circuits are claimed to stabilise the output over a temperature range from 0 to 700 C.

The plane of the Hall effect element is parallel to the upper surface of the rail 4. Beneath the chip 6 and partially embedded in the plastics block 8 is a magnet 10. The magnet 10 is a bar magnet whose axis is perpendicular to the plane of the chip 6. Thus, the end faces of the magnet, at its north and south poles respectively, are parallel to the plane of the chip 6. The top end face 11 of the magnet is positioned close to the chip 6. The magnet and chip are chosen and juxtaposed to achieve a sensing distance of 1 50 to 200 mm.

The probe itself is an encapsulated unit. The casing of the probe comprises a tubular brass member 12, closed at the detection end of the probe, that is, between the chip 6 and the detection region, by a mild steel cap 14, which acts as a protective surface for the probe and as a magnetic flux concentrator, and at the other end by a plastics composition 1 5.

Thus a substantial proportion of the flux between the poles of the magnet passes from the end face 11 adjacent the chip 6, through the chip 6 and the end cap 14, and into the detection region.

Three connections are made to the chip namely, positive and ground power supply connections, and an output connection. The wires are indicated at 16, and lead to a three core cable 18. A continuous DC current of around 10 mA, at 1 2 V supply, passes through the chip between the input and ground connections. The interaction of this current with the magnetic flux through the chip gives rise to a Hall voltage between the output and the ground connections. When a ferrous truck wheel 20 passes over the probe, and thus through the detection region, the magnetic flux distribution through the chip 6 changes and the Hall voltage is increased.The Hall chip is biased at near saturation conditions so that even when an austentic high manganese truck wheel passes through the detection region the flux distribution change through the Hall chip is sufficient to cause a detectable change in the Hall voltage.

The three core cable 1 8 terminates in a local connecting box. A three cored armoured signal cable leads from the connecting box to the control box, which houses preliminary control circuitry, to be described below. The control box is of mild steel, and has a lockable and hinged front access panel. It is provided with glands for the track sensor cable, incoming power supply and external control circuit.

Referring now to Fig. 2, when a wheel passes the signal from the probe 2 is received by a DC amplifier 22. The use of DC amplifier ensures that all signals are amplified. AC amplifiers previously used downstream of earlier, relatively crude Hall sensors have been prone to filter out wide pulses generated by slowly moving objects due to the lower limit of their bandwidth.

The amplified signal is received by an adjustable window comparator 24, which ensures that signals below a certain value are rejected. The sensor sensitivity is set on a variable potentiometer 26, whose signal is received by the comparator 24. A signal is then delivered by the comparator to a second DC amplifier 28, and thence, via an LED 30 to confirm correct sensor operation, to a mechanical relay 32. On energization of the relay 32 an external control circuit is completed.

A linear Hall effect chip provides an output signal whose value varies in direct relation to the magnetic field through it, and therefore, in this application, in practical terms, upon the mass of magnetic material passing over it. This feature enables the apparatus to accurately reject signals from extraneous magnetic objects. The probe and control equipment are sensitive enough to discriminate between, for example, different sizes of wheel on a track.

The apparatus may be used in various applications in the railway and mining industries concerned for example with the detection of the passage of vehicles and lifts. If railway regulations prohibit the location of the probe in a bore in a rail head it will be appreciated that the probe can conveniently be mounted through the web of the rail. Conveniently the probe may be housed in a protective block, which is fixed through the web, the end cap 14 of the probe being flush with a face of the block, adjacent which face the flanges of railway wheels pass.

Claims

1. Apparatus for detecting the passage of ferromagnetic or paramagnetic objects including a magnet and a Hall effect device, the magnet providing a magnetic field through which the objects to be detected pass, the Hall effect device comprising a Hall effect element disposed in the magnetic field and circuitry such that the output of the device varies in direct relation to changes in the magnetic field through the element caused by the passage of objects to be detected, and a comparator for comparing the output of the device with a reference level and delivering a signal while the output of the device differs by a predetermined amount from the reference level and thus indicates the presence of an object to be detected.

2. Apparatus according to claim 1, wherein the Hall effect device is an integrated circuit.

3. Apparatus according to claim 1 or 2 further comprising a DC amplifier between the Hall effect element and the comparator.

4. Apparatus according to any preceding claim, wherein the magnet has an end face at one pole, the Hall effect element being disposed adjacent the end face so that the magnetic flux between the poles of the magnet passes from said end face through the Hall effect element and the region through which the objects to be detected pass.

5. Apparatus according to claim 4 wherein the Hall effect element and the magnet are encapsulated in a cylindrical casing, one end of the casing, past which end objects to be detected pass, being closed by a flux concentrating end piece of magnetic material.

6. Apparatus according to claim 5 wherein the casing, other than the end piece, is of nonmagnetic material.

7. Apparatus according to claim 5 or 6 wherein the casing is located in a bore in a guide or rail along which the bodies to be detected pass, the end piece of magnetic material being substantially level with the surface of the guide or rail.

8. Apparatus according to any preceding claim, further comprising switching means downstream of the comparator, the switching means responding to a signal from the comparator by completing an external control circuit.

9. Apparatus substantially as hereinbefore described with reference to the accompanying drawings.