GB2027900A - Loop metal body detector assemblies - Google Patents

Abstract

To enable selectivity as to the detection of metal objects such as vehicles by a loop detector assembly, a relay 1 connected to a loop detector circuit 2 is operated when a vehicle is detected at low sensitivity over a buried conductive loop 3, operation of the relay causing sensitivity to increase. Thus, the sensitivity of the assembly is automatically increased from a low to a high value after detection of a vehicle at low sensitivity. The sensitivity of the arrangement automatically reverts to the low value after the vehicle has ceased to be detected. The assembly may be operated at maximum sensitivity by operating a switch 4 in parallel with relay 1. Alternatively minimum sensitivity can be obtained by operating switch 5. The assembly can be provided with a time delay so that relatively fast moving objects are not detected. <IMAGE>

Description

SPECIFICATION Loop detector assemblies THIS INVENTION relates to the sensitivity of loop detectors which are used for detecting metal bodies and, generally, motor vehicles.

Loop detectors are widely used for the detection of the presence of a motor vehicle in a roadway, entrance to a parking or restricted area, or the like. The sensitivity of such loop detectors is critical to the effective operation thereof and depends upon the metal bodies which are to be recognized and those which are to be ignored by the loop detector. Thus, for example, in many applications pedal cycles and supermarket trolleys or other small metal articles are generally to be ignored by the loop detector whilst vehicles are to be recognised. Vehicles having a peculiar shape such as a high bed in the central region thereof, or alternatively vehicles connected together by means of a tow bar assembly, must not be capable of breaking the recognition when, for example, the tow bar or high bed centre of the vehicle is located over the loop detector.

In such cases the combination of the vehicle or vehicle itself is to be recognised as a single unit since it may happen that a barrier for example will close in the middle of the passage of a vehicle over the loop detector.

Also, at different times of the day or week it may be required to recognize all metal bodies, for example, for security reasons, and also at least sometimes it may be desirable to have a delay in the vehicle recognition action in order to allow small fast moving bodies to pass over the loop detector without being recognised.

It is the object of this invention to provide for sensitivity variation of a loop detector in different ways so that the above problems can be obviated as and where required.

In accordance with one aspect of this invention there is provided a loop detector assembly wherein the sensitivity of the detector assembly is adapted to automatically increase to a high sensitivity value immediately after detection of the presence of a metal body at a relatively low detection sensitivity, the sensitivity of the detector arrangement being adapted to automatically revert to a relatively low sensitivity after the presence of a metal body has ceased to be detected.

In accordance with another aspect of this invention a loop detector arrangement has a variable sensitivity arrangement as defined above and embodies switching means for switching from such variable sensitivity condition to either of a continuously low sensitivity or a continuously high sensitivity condition.

In accordance with a still further aspect of the invention there is provided a loop detector arrangement wherein delay means are embodied in the detector circuitry such that a metal body detected by the loop is only recognised by the circuitry after a pre-determined time delay.

Preferably a loop detector arrangement will embody all three of the above aspects of the invention and a single selector switch may be provided for selecting variable sensitivity, continuously high sensitivity, continuously low sensitivity or delayed recognition. The delayed recognition could be provided for in all three different sensitivity situations in which case a separate switch could be provided for either switching in or out the time delay arrangement.

The sensitivity of a loop detector arrangement according to this invention is illustrated graphically in Fig. 1 of the accompanying drawings whereas Fig. 2 illustrates the device in block diagram form. As shown, the sensitivity is selectable between a minimum (min) and maximum (max) and when the detector arrangement is switched to detect at minimum sensitivity but then to automatically change to maximum sensitivity the sensitivity changes from minimum to maximum as indicated by line number 1 in Fig. 1. The loop detector then continues to recognize the presence of a vehicle even if only a tow bar or a high bed portion of the vehicle is located over the detector loop the maximum sensitivity being chosen to ensure this behaviour. This ensures that cycles and supermarket trolleys can be passed over the detector loop without being recognized when it is on minimum sensitivity.

However, once the frontal mass of a vehicle has been recognized the sensitivity changes to maximum in which case the presence of the vehicle continues to be detected until it has passed completely over the detector loop.

Immediately thereafter the sensitivity reverts to minimum.

The iatter can conveniently be achieved by the simple operation of a relay 1 connected to the loop detector circuit 2 such that the relay is operated when a vehicle is detected at low sensitivity over the buried conductive loop 3, and such operation of the relay causes the sensitivity to change to high sensitivity (max).

The loop detector circuit continues at a high sensitivity until the vehicle and any metal part associated therewith has passed over the loop.

A selector switch assembly as mentioned above is provided for selecting either continuously minimum or continuously maximum sensitivity as may be required for certain purposes or for use at certain times. For example, maximum sensitivity may be required during night times and over weekends when the loop detector arrangement can be used as a security device and for such occasion a switch 4 in parallel with the relay 1 could be operated.

Minimum sensitivity on a continuous basis could be selected by operating a different switch 5 which seves to isolate both the relay and parallel switch 4.

In addition the loop detector arrangement can be provided with a time delay capable of being switched in or out as may be required to enable metal bodies to pass over the detector loop rapidly without being recognised.

Such a situation may exist where large supermarket trolleys are not to be recognized and are wheeled over the detector loop relatively rapidly. Thus the time delay could be of the order of one second (illustrated by 'x' in Fig.

1) and this would be sufficient time to enable a trolley to be wheeled over the detector loop without being recognized.

It will be understood that the above described loop detector assembly can be used to positively exclude pedal cycles and other small metal bodies but to recognise large multi-axled vehicles and combination units each as a single unit.

It will be understood that the above described invention can be embodied in a loop detector arrangement easily by a skilled electronics engineer and that the remainder of the loop detector assembly can be any conventional or other arrangement.

Claims (7)

1. A loop detector assembly wherein the sensitivity of the detector assembly is adapted to automatically increase to a high sensitivity value immediately after detection of the presence of a metal body at a relatively low detection sensitivity, the sensitivity of the detector arrangement being adapted to automatically revert to a relatively low sensitivity after the presence of a metal body has ceased to be detected.

2. A loop detector assembly as claimed in claim 1 in which there is provided switching means for enabling the sensitivity of the detector to be switched to a continuously high sensitivity selectivity from said variable sensitivity condition.

3. A loop detector assembly as claimed in either of claims 1 or 2 in which further switching means are provided for enabling the sensitivity of the detector to be switched to a continuously low sensitivity selectively from said variable sensitivity condition.

4. A loop detector assembly as claimed in any one of the preceding claims in which time delay means are embodied in the detector circuitry such that a metal body detected by the loop is only recognized by the circuitry after a predetermined time delay.

5. A loop detector assembly as claimed in any one of the preceding claims in which sensitivity variation from low to high sensitivity is achieved by the actuation of a switch to alter the condition thereof upon detection of a metal body at a low sensitivity.

6. A loop detector assembly as claimed in claim 5 in which the switch is arranged to be automatically held in the actuated condition by the detector circuit operatively detecting a metal body at a high sensitivity but to revert to the pre-actuation condition when the presence of such metal body ceases.

7. A loop detector assembly as claimed in claim 1 and substantially as herein described or exemplified with reference to the accompanying drawings.