AU683678B2

AU683678B2 - Weight sensitive detection

Info

Publication number: AU683678B2
Application number: AU72770/94A
Authority: AU
Inventors: Clyde Raymond Mitchell
Original assignee: Harding Electronic Systems Ltd
Current assignee: Harding Electronic Systems Ltd
Priority date: 1993-07-27
Filing date: 1994-07-27
Publication date: 1997-11-20
Anticipated expiration: 2014-07-27
Also published as: WO1995004291A1; GB2294329A; GB2294329B; AU7277094A; GB9600041D0

Description

WEIGHT SENSΓΠVE DETECTION

FIELD OF THE INVENTION

This invention relates to the detection of people, vehicles or other objects at given locations, in particular but not solely to a sensor for detecting the presence of pedestrians at traffic signal controlled intersections. BACKGROUND OF THE INVENTION

Various detector systems are known for controlling the_flow of people and traffic through intersections, pedestrian crossings, automatic doors and the like. Some employ a sensor loop embedded in a road surface and connected to a remote oscillator circuit. An object passing overhead changes the inductance of the loop by compression or through having an appreciable magnetic susceptibility. A car for example, thereby changes the resonant frequency of the oscillator and is readily detected.

Other systems may use piezoelectric sensors in weight sensitive road or pavement pads. Still others use heat or movement sensors mounted above the traffic. Most are either expensive or difficult to install, of limited sensitivity, or prove unreliable in their normally harsh environments. In particular, sensors which rely on compression to detect the weight of an object overhead have generally been unsatisfactory for use at pedestrian crossings and other locations where the presence or absence of a person must be detected over a relatively wide area. One sensor which uses compression of an inductive system to detect vehicular traffic is described in German patent specification no. 3418588. This involves a hollow strip of resilient material embedded in a roadway, having a vertically or horizontally oriented sensor loop. In the former orientation the loop itself is distorted by weight of the traffic to provide a change of induction, while in the latter a strip of metal or other magnetically susceptible material is depressed towards the loop through an appreciable distance. In each case the distortion is very noticeable and the strip is not suitable for properly monitoring an area of several square metres on which a person may be standing.

By way of example, the sensor strips of DE 3418588 are not suitable for use in traffic or lift control systems. Traffic is often interrupted at signal lights to allow a pedestrian to cross, when in fact the pedestrian has already departed. Similarly, multiple lift stops are sometimes triggered by pranksters who have already disembarked. A wide area monitoring system is required to cancel the unnecessary interruptions or stops under these circumstances.

The overall detector systems generally require sophisticated electronic circuitry to monitor output from their respective sensors, such as the induction detectors disclosed in British patent specification nos. 2125598, 2130777 and 2203876. The primary difficulties which the circuitry must meet relate to drift and dynamic range in the sensor output. Various experimental systems are described in Research Report 364 from the Transport Research Laboratory in the Department of Transport in the United Kingdom. SUMMARY OF THE INVENTION It is an object of the present invention to provide for improved weight sensitive detection of pedestrians in traffic control systems, or at least to provide the public with a useful choice.

The invention broadly contemplates a weight sensitive pad for use with an induction detector, comprising an inductive system disposed across the pad, wherein action of a weight on the pad changes the inductance of the system to be sensed by the detector. A "pad" in this sense has horizontal dimensions appreciably greater than its depth.

Preferably the inductive system comprises an inductor such as a flat coil of wire arranged adjacent to a body of magnetically susceptible material such as a sheet of metal. The weight of a person is sufficient to distort the system and produce the change of inductance, normally by a slight displacement of one or other component.

Various alternative constructions are possible, such as use of a printed circuit board to form the inductor, and use of a second such inductor in place of the magnetically susceptible material to form a mutually inductive combination. In each such construction the pad should be sensitive to the weight of a person across a reasonably wide area. BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment will be described as an example of the invention with respect to the drawings of which:

Figure 1 indicates a coil embedded in a plastics material for use in a weight sensitive pad, Figure 2 gives an underside view of the plastics material containing the coil,

Figure 3 is a vertical section through the coil placed in a metallic tray,

Figure 4 is a detailed section through the plastics material containing the coil,

Figure 5 is a side view of two weight sensitive pads in use under a pavement surface, Figure 6 shows an array of weight sensitive pads in use at a pedestrian crossing, and

Figure 7 indicates a coil sandwiched between two sheets of metal in an alternative weight sensitive pad. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to these drawings it will be appreciated that a weight sensitive pad according to the invention can take many forms and is not limited to uses in the field of traffic detection or control. For example, it may be useful under industrial conditions to monitor passage of vehicles or people in factories, under commercial conditions for lift control or security, or even domestically in relation to pets or other animals. These uses become possible through a robust general purpose construction of the pad.

Figure 1 schematically shows a sheet 10 of suitable material which forms part of the pad, having at least one loop of an electrical conductor 11. The material is preferably a flexible and slightly compressible plastic which is cheaply and readily moulded. The loop forms an inductor coil 12 having several turns, in the. form of a flat spiral for example. Each end of the conductor extends normally sideways from the sheet to join a neighbouring pad or a detector system. Figure 2 shows some structure on the undersurface of a prototype sheet 10 of which the corresponding upper surface would normally be flat. A conductor 11 is embedded or enclosed as coil 12 in an annular region 20, with a central disk region 21 which helps prevent the sheet from sagging. The sheet has a surrounding side portion 22 with thickened portions 23. Various nodule portions 24 may also support the sheet in its central region. Shaping of this kind is the result of an attempt to reduce the weight of plastic in an otherwise uniform block of material while providing a robust compressibility for weight detection.

Figure 3 is a vertical section through a pad formed by placing the sheet of Figure 2 in a tray 30. The sheet rests on portions 22, 23 and 24, and extends above side walls 31 of the tray. This arrangement supports coil 12 adjacent to a sheet of magnetically susceptible material 32 forming the base of the tray. The inductance of the coil is determined by various geometrical factors, including the number of turns and their diameter, and proximity to any susceptible material of this kind. An inverted configuration having the coil positioned below the tray would also be feasible. A load L in Figure 3 distorts the geometry, usually minutely, with a microscopic displacement of the coil towards material 32, to produce a detectable change of inductance. In general the distortion is spread over an appreciable area and a person stepping on to the pad would not ordinarily be aware of any downwards displacement. Movement may be a feature in some versions however, to provide tactile feedback. Figure 4 is a detailed vertical section through annular region 20 shown in Figures 2 and 3. This shows eleven loops of conductor 11 encased in an annular recess by an annular cap 50. Edge portions 52 of the cap engage rims 51 of the recess to hold the coil firmly in place. Simply embedding the coil in plastic as shown in Figure 3 will not usually be satisfactory due to differential cooling rates of the conductor and plastics material during manufacture. In prototype pads recently tested for use in a footpath to detect pedestrians, the plastics sheets were formed from a urethane compound about 300 x 300 x 15 mm in size placed in an aluminium or stainless steel tray. The coil was formed from high quality wire suitable for long life under harsh conditions and had an inductance of about 10 to 15 μH. An approximately 0.5% change in inductance was recorded for 10 kg weight increments although the output was generally nonlinear over the full range of normal bodyweight.

Other pads have been considered but not yet tested. For example, a pad in which the inductor is formed on a printed circuit board and arranged adjacent to a sheet of_ metal, or even another inductor having a similar structure. Using two parallel flat inductors perhaps separated by a sheet of plastics material would dispense with the need for a sheet of magnetically susceptible material. A change in mutual inductance then provides a signal to be sensed by the induction detector.

Figure 5 shows two weight sensitive pads 40, 41 installed in series under corresponding tiles 42, 43. The pads are placed on sand or in a similar foundation material 44 so that the tiles are flush with the surrounding ground or floor surface 45. The tiles will generally have a rough undersurface and each pad advantageously has a slightly defoπnable upper surface, as a result of the plastics material 10 or a similar construction, so that the tiles are more readily levelled. Spaces between the tiles may be filled with a waterproof silicon sealer or flexible bitumen depending on the surrounding surface.

Figure 6 shows several weight sensitive pads combined to form a broad pedestrian monitoring area 60 of about 2100 x 600 mm. The surface of individual tiles 63 has a relief pattern to provide traction. The coils in the pads are generally connected in series to a roadside monitoring unit 62 which detects changes of inductance due to the arrival or departure of pedestrians waiting at a road crossing. The unit may also be used to monitor inductance loops beneath the road itself.

Although the inductance of a pad drifts slowly due to temperature and other factors, the monitoring unit 62 is easily able to determine the ongoing presence of one or more people. The coils will generally be connected in an alternating pattern to reduce noise effects, so that a magnetic field generated by the coil in one pad is reversed in relation to its nearest neighbour pads.

On arriving at the road crossing a pedestrian will generally push a button control 62 and wait for the automatic traffic control system to interrupt the flow of vehicles. The system will confirm the pedestrian's presence by detecting their weight on the array of pads. Should the pedestrian cross early or depart in another direction, with no further pedestrians having arrived in the meantime, the pad array 60 will detect the absence of any person waiting and will cancel the interrupt signal.

Figure 7 schematically shows an alternative weight sensitive pad which may be useful in some circumstances. A number of turns of a coil 70, typically ten turns, are embedded in a sheet or slab of urethane 71, sandwiched between two sheets of metal 72, 73. A weight on the pad applied to either of the metal sheets causes a slight distortion of the overall geometry in relation to the coil and consequently a fractional change of inductance. A pad of this kind may be heavier and more expensive to manufacture but is an example of the variations which may be envisaged for this invention.

Claims

- -CLAIMS:

1. A weight sensitive pad for an induction detector, comprising an inductor arranged adjacent to a sheet of magnetically susceptible material, wherein action of a weight on the pad distorts the said arrangement and produces a change of inductance in the inductor to be sensed by the detector.

2. A weight sensitive pad according to claim 1 wherein the inductor comprises a plurality of turns of an electric conductor supported by a plastics material and arranged substantially parallel to a sheet of metal.

3. A weight sensitive pad according to claim 1 wherein the pad comprises a coil of about ten turns of wire supported in a shaped urethane sheet and placed in a shallow aluminium tray.

4. A weight sensitive pad according to claim 1 wherein the pad comprises a coil of about ten turns of wire embedded in urethane and sandwiched between two sheets of metal.

5. A method of detecting arrival or departure of a body on a given ground or floor area, comprising arranging a substantially flat inductor under the area, adjacent to a respective sheet of magnetically susceptible material, forming a weight sensitive pad, and monitoring changes of inductance in the inductor caused by the action of weight from the body in distorting the said arrangement.

6. A method according to claim 5 wherein the inductor is a coil of wire supported in a plastics material and placed above the sheet of magnetically susceptible material, with both being installed below a substantially rigid tile.

7. A method according to claim 5 wherein a plurality of the pads are placed side by side with their inductors connected together and monitored by a single detector system.

8. A sheet of flexible plastics material supporting a coil of electrical conductor, adapted to be arranged adjacent to a sheet of relatively rigid magnetically susceptible material, whereby action of a weight on the plastics material distorts the said arrangement to produce a change in inductance of the coil.

9. A sheet of flexible plastics material according to claim 8 wherein the coil is in the form of a flat spiral which in use lies substantially parallel to a sheet of an appropriate metal.

10. A weight sensitive pad for an induction detector, comprising an inductive system disposed across the pad, wherein action of a weight on the pad produces a change of inductance in the system to be sensed by the detector.