PARKING SPACE MONITOR

This invention relates to a device for monitoring vehicle parking, particularly, but not exclusively, the parking of cars in designated parking spaces.

In many locations it is desirable to allocate parking spaces to specific individuals or groups. In such circumstances unauthorised parking is not only a nuisance, but can deny access necessary for say disabled persons. Various methods of preventing unauthorised parking are known, but these can be expensive and not entirely effective.

For example, post-type barriers are often used, normally situated towards the centre of a designated space, adjacent the side used for access. These are locked in an upright position preventing unauthorised parking, and may be released, using a key, and laid horizontal thereby allowing an authorised person to park. However, these posts are expensive to install, usually requiring digging a hole in the tarmac or concrete surface of the car park and locating the posts in a cement block. Furthermore, the posts can easily be damaged and misaligned, and in some cases people can park between adjacent raised posts. The posts can also accidentally be left lowered allowing unauthorised parking.

According to a first aspect of the invention we provide a parking space monitor unit comprising detecting means adapted to detect the presence of a vehicle at a parking space, alarm means adapted to produce an alarm, authorisation means adapted to determine whether a predetermined signal has been received by or input to the unit indicative of an allowable user or vehicle, and control means adapted to control the operation of the alarm means in response to the output of the authorisation means.

Preferably signal-receiving means is provided.

The unit is preferably provided with mounting means adapted, in use, to attach the unit to the ground in the vicinity of the parking space.

We can simply attach the unit to an existing ground surface, without extensive preparation of the surface, and indeed without necessarily any preparation of the surface. This is cheaper and quicker than digging holes for posts, and the psychological effect of an alarm sounding upon unauthorised parking will deter unauthorised parking. Preferably the mounting means comprises one or more mechanical fasteners, such as bolts, adapted to be introduced into the ground, or one or more locating formations adapted to receive such mechanical fasteners.

This is a particularly convenient and cheap way to attach the unit to the ground. Preferably the unit is mounted within the space that the vehicle will occupy, preferably under the vehicle in use. This avoids the problem of pedestrians falling over it.

Preferably the unit is provided with a housing in which the detecting means, alarm means, authorisation means, and control means are provided. The housing may comprise a base member, and there is also preferably a cover member which extends, in use, over the base member. The cover member may be secured to the ground and overlies the base member. ' The base member may rest on the ground, but may not itself be fastened to the ground.

Preferably the mechanical fasteners comprise tamper-resistant bolts. These can preferably be released with special tools to demount the unit from the ground, or to gain access to the interior of the housing. The authorisation means and the control means may be provided in the same circuit, as may the detecting means.

Preferably the unit does not project above the ground by more than about 150mm, and preferably not by more than about 100mm. This is usually low enough to avoid the unit fouling on the underside of vehicles as they drive over it.

Preferably the unit has at least one (and most preferably, a second) sloping ramp surface adapted, in use, to facilitate the driving of a wheel of a vehicle over the unit.

In its most preferred form the unit will be self-contained and has a battery inside it. In the preferred embodiment the unit is light enough and small enough to be transported by a single person picking it up and carrying it around. Thus in practice we would unload a device from a van, or from the back of a car, drill some holes in the tarmac of the car park, and bolt the self-contained unit down. One person, with little skill or training, could do this quickly.

We also prefer to provide the unit with display means adapted to display the identity of, or category of, allowable people or vehicles which the unit will recognise as being authorised to park in the space. The display means may be a panel adapted to receive a nameplate or numberplate, or it may be a display with an adjustable visible output (eg LCD or LED display) .

Thus the unit can perform the dual function of guarding the parking space and identifying the correct space or place that a user should park in. It can be used in car parks, at the side of the road .on people's drives, and even in multi-storey car parks.

Preferably the unit is adapted to be driven over by a vehicle using the parking space, the detecting means detecting the presence of, or passage of, the vehicle. The detecting means preferably comprises induction means, such as an induction coil. These are cheap and reliable.

We could theoretically program into the unit a preset level or frequency from the induction coil which would indicate that there was a lot of metal around perturbing the magnetic field near the coil

(implying the presence of a car) . However, we have found that because the height of cars vary, and the magnetic field is also influenced by temperature changes (probably expansion of the coil effects) , it is difficult to set an absolute value in the factory above which a vehicle is present. We have, however, discovered that it is better, and cheaper, to set the detecting means and control means up such that the change of signal, or rate of change of signal, from the induction means, can be used to indicate a change in the magnetic environment, and thereby infer that a vehicle has entered or left the parking space. This could either be by looking at the rate of change of signal in real time, or by comparing a signal at one time with a signal from a predetermined earlier time and seeing if there had been a significant change.

Thus our unit can, in the preferred embodiment, effectively self-calibrate itself by noting its background state and identifying changes, as opposed to the absolute value, of the coil frequency.

Preferably the unit automatically re-sets itself when a vehicle leaves its parking space/place so as to be operational for deterring subsequent unauthorised parkers. To do this it preferably detects the departure of the vehicle, or that the space is unoccupied and re-sets itself.

Preferably the unit includes guard means adapted to detect the unauthorised removal of a parked vehicle from the parking space, and removal permittance means adapted to indicate to the unit that the removal of the vehicle is permitted. Preferably when the guard means identifies the unauthorised removal of a vehicle from the parking space the control means activates the alarm means.

Most preferably the guard means comprises the same detecting means as is used to detect an unauthorised parking. Most preferably it is induction means of the detecting means which is adapted to produce a signal indicative of the removal of the vehicle.

Preferably this signal indicative of the removal of the vehicle is the rapid rate of change of the signal from the induction means (compared with changes due to other effects, eg temperature variation) .

Preferably the removal permittance means and the authorisation means both comprise the same identification means adapted to identify the inputting of an allowed identification signal.

Preferably the identification means, upon identifying an allowed vehicle, causes the control means not to activate the alarm means, or deactivates the alarm means, or deactivates it for a predetermined time.

Preferably the control means does not operate the detecting means continuously but instead conserves power by turning the detecting means on and off at intervals. This conserves power, which is important if the unit is battery operated.

The detecting means may not be powered up (sleep mode) for periods that are longer than the periods for which it is powered up and operational.

Preferably the detecting means is adapted to be cyclically turned on and off, the ratio of inoperational phase to operational phase being at least 10:1, and preferably at least 20:1, and most preferably at least 30:1 or 40:1. Preferably the period for which the detecting means remains inoperational is between 1 and 8 seconds, and preferably between 2 and 6 seconds, and most preferably about 3 or 4 seconds.

In the preferred embodiment when the induction means is energised in a powered up, operational cycle, the signal from it is compared with one or more previous signals from previous cycles (most preferably with the signal from the previous cycle) and if it has changed by more than an allowable amount this is used by the control means as an indication that a vehicle has either entered or left the parking space.

Preferably a change of status of the occupancy of the parking space from unoccupied to occupied is detected and the control means looks to see whether the unit has received an allowable vehicle identification signal.

The allowable identification signal may come from an electromagnetic radiation signal emitted by the vehicle or user-held or operated device, or from a key, or card, or entry of a number code via buttons, or any suitable source.

Preferably when an unauthorised car is detected in the parking space the unit is adapted to emit a less distressing and noticeable (eg quieter) alarm signal for an initial period and then, unless deactivated, emit a more distressing and noticeable (eg louder) alarm signal.

According to a second aspect of the invention we provide a parking space comprising an area of ground (or floor) upon which a vehicle is intended to be parked and a parking space monitor unit according to the first aspect of the invention.

According to a third aspect of the invention we provide a vehicle park, for example a car park, comprising a plurality of parking spaces and associated parking space monitor units which are in accordance with the first aspect of the invention. The vehicle park may have further parking spaces which do not have an associated unit in accordance with the first aspect of the invention but instead have a dummy unit, or unit cover, of identical or similar appearance.

Another way of looking at the invention is that we provide a parking space monitor comprising detecting means for detecting a vehicle, alarm means for providing a warning and sensing means for arming and disarming the monitor responsive to an authorising signal, the monitor being mounted such that it is in the proximity of the vehicle when parked.

The monitor is preferably completely enclosed in a casing making it preferably water-proof, and most preferably damage and vandal proof. It is simple and easy to install.

The monitor has preferably at least two modes of operation, the so-called standby and enabled modes. In the standby mode the monitor is disarmed, and any vehicle entering its vicinity will be detected but will not cause commencement of an alarm sequence. In the enabled mode, a vehicle is detected upon which an alarm sequence may be started, which may be stopped and the monitor placed in the standby mode on receipt of an authorising signal. Alternatively, the monitor may remain in the enabled mode or the monitor may be placed in a third mode, the guard mode, where a warning will be issued should the vehicle be removed from the vicinity of the monitor without a signal being received indicating that the removal of the vehicle is permitted.

Other modes of operation may be used and an override may be provided to place the monitor in the standby mode if necessary (or alternatively to place it in the enabled or guard mode at the choice of the user) .

Means such as LED's may be fitted to the monitor indicating the current operation mode.

An authorising signal which arms and disarms the monitor may be provided by various trigger means. These could be operated manually, for example by the person in the vehicle or by a person in a remote station, or automatically, for example by means located on the vehicle. A number of combinations of manual/automatic arming and disarming of the monitor may be used.

The authorising signal could comprise radio waves, infra red radiation or a signal from a key, all of which would be coupled with an appropriate sensing unit in the monitor. In all cases, the signal may be coded, and each monitor may be set to recognise only one particular code, or perhaps a limited number of different codes.

Using appropriate encoding techniques such as bit coding, a large number of codes may be available, and each monitor may be assigned an individual code increasing the security of the device.

Alternatively, a number of monitors may be set to recognise the same code, providing parking for groups, for example a household, or disabled persons. The driver of a vehicle may then have a choice of parking spaces.

Preferably the monitors recognising the same code would allow a vehicle to be parked within say 30 seconds. Preferably only the monitor at which the vehicle is parked changes state to recognise that a vehicle has been parked. Therefore the remaining devices sharing the same code may still protect spaces they are positioned in. Similarly, if a user operates his radio or infra-red (e.m.) identification device its signal will be received by several units at several parking spaces. All of the units that receive the identification signal, and recognise it as allowable, may be de-activated temporarily (eg for 30 seconds) , but only the unit that, after the de-activation period, noted a change in occupancy state would alter its own operational state (eg from unauthorised use operation to guard operation the other units returning to their previous state if there were no change in occupancy. Thus parking next to a vehicle in an adjacent space does not permanently de-activate that adjacent unit.

Alternatively, only the unit that notes a change in occupancy state within say 30 seconds before or 30 seconds after receipt of an indentification signal may be set to change its operational state. Other time sequences may be used. Alternatively, in a situation with a group of units a user may drive into one parking space and then activate his identification signal. Again only the unit that has had a change in occupancy would have its status changed (the unchanged occupancy units could be arranged so as not to alter their status upon receipt of a signal) .

Override codes may also be provided which may be used in emergencies or for maintenance. Alternatively, override switches, for example a magnet-operated reed switch, may be provided preferably internally of the monitor.

The alarm unit may issue any suitable warning, which could be visual, such as flashing lights or signs, or audible, such as a sequence of beeps or a synthesised message. The warning could also, in combination with the above or alone, be transmitted to a remote station, for example to a parking attendant or to a house, informing the occupant of unauthorised parking or, when in the guard mode, unauthorised removal.

Various alarm sequences may be used. For example, a warning may be issued immediately a vehicle is detected, which warning may start in an unobtrusive manner, getting progressively more obtrusive as it continues. Alternatively, a delay may be introduced between detection of a vehicle and giving a warning. This would allow an authorised person time to disarm the monitor. The alarm unit may be set to automatically deactivate on any of a number of conditions being satisfied, for example an authorising signal is received, the unauthorised vehicle is removed or the alarm has been issued for more than a preset time.

Preferably the monitor unit may re-activate once an authorised vehicle is removed. Preferably the monitor unit issues a signal as or shortly after the vehicle is removed. This may indicate that it has reactivated.

The detection unit within the monitor may operate by a variety of means such as optical. magnetic, capacitative, induction, radar, audible or the like. In the preferred embodiment, the detection unit comprises an induction loop.

The monitor may be powered by any suitable means such as mains power or solar energy, but is preferably battery powered. When this is so, it is desirable to operate the monitor in a power saver mode, where at least high-power components of the unit are intermittently powered up.

When the monitor is mains powered, this power could be at full voltage with a transformer stage provided within the monitor. Alternatively, one or more transformers could be used to provide voltage at any suitable level to one or more monitors. The transformers could be internal or external to the monitors. A general transformer could be used to provide power to all the monitors. Where a monitor or monitors require lower voltage than mains voltage, a general transformer would allow the use of low voltage cables to supply the monitors, increasing the safety.

The monitor may further comprise a CPU (comprising the control means) , which controls the operation of the unit. The CPU may also be used to store data such as the time of arrival and departure of a vehicle, which information could be relayed to a parking attendant, and could even be used to provide for automatic billing. This system could also be adapted to monitor the location of vehicles and therefore goods around the country, transmitting this to a central control centre. (If vehicles are allocated their own unique code.)

The monitor may be used to detect a range of vehicles such as goods vehicles, motor bikes, boats or aeroplanes. In a preferred embodiment the device is used to monitor the parking of cars. It is suitably located within, or perhaps adjacent, to a parking space, and is firmly mounted on the ground. For ease of activation of an induction loop, it is preferable that the monitor should be mounted within the parking space, and be of such dimensions that the car may be driven over it. The casing may be formed of a resiliently deformable material such as plastics material thereby preventing damage to the monitor should the car come into collision with it.

According to another aspect of the invention, a monitor is provided which acts as a theft alarm. The monitor is situated in any suitable place where protection for a vehicle is required, for example in the drive of a house. The vehicle is parked within the vicinity of the monitor, which is placed in the guard mode. An alarm unit is provided in a monitoring station for example inside the house. This may include audible or visual alarm means or preferably a combination of both. The alarm unit may further be provided with a reset facility and is preferably powered by mains electricity.

Should the vehicle be removed without permission, an alarm sequence is activated within the monitor which emits a signal to the in-house alarm unit, which in turn emits a warning. This informs the occupants that the vehicle is being removed without alerting the thief. Further, should the alarm be inadvertently activated this will not cause disturbance to the surrounding houses. If desired, however, a warning could also be emitted by the monitor.

A warning could be emitted for a predetermined time, or could be stopped and the monitor reset using the reset facility provided on the alarm unit.

It will be appreciated that such a monitor need not have the parking space unauthorised use protection facility.

According to a further aspect of the invention, a parking space monitor is provided comprising a detection unit and an alarm unit, such that on detection of a vehicle, an alarm sequence is started which emits a warning after a preset time has elapsed. The monitor may include a display unit or units which indicate the time elapsed since detection of a vehicle. This could be in addition to or instead of the alarm unit.

Preferably visual display units are provided on both sides of the monitor so as to be visible even when a vehicle is parked.

Such a monitor would be particularly useful in situations where parking is restricted to a limited time only, for example in airports or stations.

The detection, display and alarm units may comprise any suitable means as outlined above, and the monitor may be powered in a number of ways.

Clearly, if after the preset time the monitor noted that the parking space was empty it could be arranged to cancel displaying the time elapsed and/or not to sound its alarm. Alternatively or additionally the monitor could detect the removal of the vehicle and reset itself to a standby, no alarm, zero display, condition upon noting an empty parking space.

The monitors provided are reliable in use, and versatile in operation providing a number of optional features to enhance the protection provided. They are simple to install and resistant to damage, and provide an effective means for monitoring parking spaces.

According to another aspect of the invention we provide a method of deterring unauthorised parking comprising attaching to the ground (or floor) a surface-mounted parking space monitor unit in the vicinity of the parking space, the monitor being adapted to produce an alarm when an unauthorised vehicle parks in the space.

Another way of looking at the invention is as a cheaper way of installing an electronic parking space monitor comprising fixing a unit, preferably stand-alone unit, onto the surface of the ground (or floor) instead of embedding the unit under the surface.

We have appreciated that there is no need to bury induction coils, especially with our new technique of looking at changes in induction coil signal instead of absolute values. Burying the coil reduces the perturbation on the coil due to temperature variations and also means that differences in vehicle to ground heights do not matter so much because when buried the coil to vehicle distance is larger anyway, and the variations less significant than if the coil were on the surface. These factors point away from surface mounting, but we have found a way to live with surface mounting by looking at changes in induction coil signal instead of its absolute value. Surface mounting is cheaper to install but has hitherto been thought unworkable.

A further advantage of surface mounted units is that if we have a car park we can alter the layout of the units in the car park much more easily than we can with partially buried posts, or subterranean coils. Indeed, if our mechanical fasteners (eg tamper bolts) are removable we could even have a car park with some real monitor units and some dummy monitor units (which would be much cheaper than real units) and swap the real and dummy units around from time to time. This would have the psychological effect of a user not knowing whether he could "cheat" or not. This possibility is facilitated by the specific embodiment where the cover bears the user identification (eg name of person to park in the space) and the base the operational electronics just the bases can be swapped, leaving the original covers behind. In cases where the cover fully or substantially obscures the base we expect this to be effective. Visitor car parking spaces could simply have dummy units or covers.

According to another aspect of the invention we provide a parking space unit which comprises detecting means to detect when the space is occupied with a vehicle and when it is unoccupied, alarm means, and control means, the control means being arranged such that it activates the alarm means upon removal of the vehicle from the space unless it is deactivated by the input of an authorised deactivating signal.

Thus this effectively comprises a vehicle alarm provided in the parking space, instead of in the vehicle itself, the alarm being activated upon unauthorised removal (eg theft) of the vehicle.

It will be appreciated that the "guard" mode of the specific embodiment could be used in a device on its own, without the unauthorised use of the parking space facility - it may be a separate invention.

When both parking space monitoring and vehicle removal alarm are provided our system does not cost much more, if any more, than providing just one facility. The detector induction coil is the same, it is just the control unit that operates differently.

Preferably the vehicle removal alarm uses a comparison of old and new signals from the detecting device to look for sudden changes to indicate the removal of the vehicle. This idea may itself lead to another invention - a way of reducing the cost and/or sophistication of the electronics needed to monitor the presence or absence of a vehicle using induction means comprising comparing new signals with relatively recent old signals and identifying rapid large changes.

Of course, we would not want to compare new signals with signals from six months ago because the temperature may have changed, a new man-hole may have been installed, etc. But we do want to compare new signals with signals from the recent past (eg last signal check, or last few seconds).

It will be appreciated that in the space- guarding mode the user authorisation signal may effectively re-set and/or de-activate the alarm means for a predetermined time (eg 30 seconds). Thus if the alarm goes off loudly only upon a change in status of parking space occupancy (after possibly an initial delay) , and the device is re-set after the vehicle has parked but before the alarm goes off loudly, the alarm will not go off (at least loudly) for that authorised user. However, when that user drives away the unit is still active in protecting the space and/or active in sounding a car theft alarm.

An embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a plan view of part of a car park; Figure 2 is a side view showing a car approaching a monitor unit;

Figure 3 is a plan view of the monitor unit shown in Figure 2, with the outer casing removed;

Figure 4 is a circuit diagram of the circuit board shown in Figure 3; and

Figure 5 shows a perspective view of the monitor unit of Figure 1 , and a hand-held trigger unit.

There now follows a general description of the device followed by a more detailed description of the circuit.

Figure 1 illustrates a portion of a car park in which individual parking spaces 1 are defined by lines 2. Each parking space 1 is provided with a monitor 3 which is securely mounted on the ground using tamper-proof bolts (shown referenced as 50 in Figure 5) . The monitors are located within the parking spaces adjacent and substantially parallel to normal access sides 4 (alternatively they could be towards the other end of the parking spaces, or even in the middle region) . The monitors are of each a frustoconical shape with sloping sides 5, substantially parallel to the sides 4 of the parking spaces 1. Each monitor comprises a hollow casing 6 which completely covers in use a base unit 6' which contains the operational components. The casing 6 and the base unit 6' are moulded from synthetic rubber, or any other suitable, resiliently deformable material which could for instance be fibreglass. They are shorter than a typical distance between wheels on an axle.

Figure 2 shows a side view of a car 7 approaching one of the monitors 3. The car is driven over the monitor and parked. The wheels normally pass either side of the monitor since the dimensions of the monitor are such that it fits easily between the opposite wheels and underneath the chassis of the car. Should, however, the car come into collision with the monitor, its sloping sides provide ramps which enable the car to travel over the monitor in either direction. The material and shape of the monitor ensures that it will not be damaged. The vehicle may be parked so as to have the monitor completely or substantially under it, or the monitor may be outside of the plan projection of the vehicle.

In Figure 3, a plan view of the monitor 3 is shown with the outer casing 6 removed. An induction coil 8 is firmly located by means of ties 9 in a groove 10 formed in the base 6'. The groove 10 surrounds a hollow 12 which contains a circuit board 13, details of which are shown in Figure 4. A second hollow 14 in the base 6' defines a battery compartment, which contains four batteries 15, of 996 lantern type which form two independent 12V supplies. The batteries are connected to the circuit board, and thence to the remainder of the monitor, by means not shown. Diode protection is provided on the board to prevent one battery discharging into another, or accidental reversal of the battery connections.

The monitor is operated in a power-saver mode which is controlled by a slow running oscillator. This intermittently activates a CPU which controls the other units of the monitor. In this mode the monitor is active for 200ms in every 4s, which helps to maintain the battery life.

The induction coil 8 is driven by an oscillator and the CPU monitors the frequency of oscillation in the coil and uses this to determine the change of frequency of oscillation.

When a car is within the proximity of the coil, the frequency of the coil oscillations is changed. This is detected by the CPU, which compares the new rate of change of frequency with the previous value. The difference in the case of a car is marked and this activates the alarm unit. Since a car drives in and out of the space in a matter of minutes the change of consecutive signals is of a very different order of magnitude than that which might occur due to the sun beginning to shine on the unit.

Other features such as temperature and ground capacitance will also cause changes in the frequency of oscillation in the coil. However, as discussed above, the rate of change of frequency caused by these factors will be small, and this will not activate the parking monitor (consecutive signals will still be practically the same) .

The CPU intermittently (every 3.5 seconds) monitors the frequency of the coil oscillations, and compares the new frequency with its previous value which has been stored. Attempts to simulate a car's presence, for example by sliding a metal plate over the monitor, will result in a change of frequency which being detected and compared with the previous value will activate the alarm, (unless by some chance the vehicle is driven off and a plate which just happens to have the same effect on the magnetic field is put in its place all within the time that the CPU has deactivated the coil, which is unlikely) .

The monitor possesses two main modes of operation - standby and enabled. In the standby mode the unit is disarmed and a vehicle may be detected but no alarm will be given. An LED (shown in Figure 5) connected externally of the monitor will also be deactivated. In the enabled mode, power is intermittently supplied to activate the detection coil, and the LED flashes for a 1/40th of a second every 4s.

The sensor unit, which includes a radio receiver (or some other signal input means if some other authorisation signal is used), switches between the modes in response to an authorising signal from a hand-held trigger unit (shown in Figure 5) . The trigger unit is powered by a battery 16 and operated manually by pressing a button 17 emitting a radio signal with a 10m range. The duration of the signal is greater than the period for which the monitor is not capable of detecting a vehicle (in this example 4s) to ensure that the sensor unit is active for at least part of the signal's lifetime. The signal is coded using bit switches 18 in the trigger unit, and the sensor unit is correspondingly coded also using bit switches. This ensures activation of the monitor when in receipt of an authorising signal only. It should be appreciated that the bit switches may be set, or indeed reset, by the user of the device.

When no vehicle is present, pressing the button 17 whilst the monitor is in the standby mode will place it in the enabled mode, and the alarm will issue one beep to indicate this has taken place. Pressing the button again will place the monitor in standby mode, and the alarm will issue three beeps. This disarms the unit, allowing unrestricted access. Thus the unit could be used to say, prevent unauthorised parking during the week, and to allow unrestricted parking at the weekends.

The trigger unit could also be activated automatically, on receipt of an activating signal from the sensor unit.

When not in use, the monitor is normally left in the enabled mode. When a car approaches and is driven into the parking space, it is detected and the alarm unit is automatically activated which starts an alarm sequence. A short, and relatively quiet, warning beep is emitted every 4s for 20s, this then changes to one long loud beep and three short loud beeps repeated for 4 mins. The alarm then stops for 20s and restarts after a further 20s continuing this sequence for a further 6 mins, after which it stops. The entire alarm sequence is emitted unless a signal is received from the trigger unit, which stops the alarm sequence leaving the unit in enabled mode or alternatively placing it in standby mode. In enabled mode the monitor periodically checks that the vehicle has not left the parking space (every 4 seconds) and that the monitor does not therefore need to re-set itself. Alternatively, the monitor may be placed in another mode - the guard mode. Here, a guard alarm sequence is activated, consisting of one beep per second for 10 mins if the car is removed from the parking space without first pressing the button on the trigger unit. Pressing the button whilst in guard mode will place the monitor in enabled mode. A time delay (eg 30 seconds) may be introduced between changing from one mode to another.

The monitor in this example automatically places itself in the guard mode when an authorised vehicle is parked in its space.

Thus in practice an authorised parker drives into his parking space with the unit in enabled mode, gets out of his car, and hears the quiet bleeps of the initial alarm sequence. This reminds him to cancel the full alarm sequence using his hand-held trigger. The unit then automatically puts itself into guard mode. When the user leaves he presses his hand-held trigger again to put the unit back from guard mode to enabled mode and the user drives out without the alarm sounding.

In an alternative version, without the guard mode feature, when a user stops the alarm sequence using his trigger the unit is left in enabled mode. When the user returns to his car he simply drives off, the unit detecting the absence of the vehicle and automatically resets itself, measuring the frequency of the coil oscillations caused by the surroundings. Reset may take place immediately or within a time interval to allow for the departure of the vehicle eg after 15 seconds has elapsed.

In another version, when the user stops the alarm sequence using his trigger, the unit is placed in standby mode. The unit should then be placed in enabled mode before leaving the vicinity of the monitor, by pressing the button on the trigger again. Alternatively the unit may automatically change to enabled mode on detecting the departure of the car. The same signal from the trigger means may be used to place the unit in standby mode as that to place the unit in enabled mode.

There now follows a more detailed description of the circuit.

The processing is performed by an 87C51 processor, U5 which has its own internal RAM and ROM. The processor U5 is clocked at 4MHz which is provided by the crystal X1. The 4MHz clock frequency is also used to drive other frequencies for the circuit as explained hereinafter.

Switches SU1 , SU2 and SU3 connect to the processor, U5 and provide a means for setting the radio trigger code and the modes in which the circuit will perform. That is the switches SU1 , SU2 and SU3 provide inputs to the processor, U5, tying various input pins to ground. However means other than switches SU1 , SU2 and SU3 may be used to configure the device and radio trigger code.

The integrated circuit U2 is a dual 4 bit binary counter. Only one of the counters U2:A is used in the circuit. U2:A can be enabled as required by the 87C51. The counter U2:A is clocked by the 4MHz clock signal applied to the 87C51. The counter U2:A is used to divide the 4MHz clock signal by 8, providing a 500 kHz signal on pin 5 of the counter U2:A.

An RLC tuned circuit is provided which is tuned to oscillate at a frequency of 512kHz, with no metallic objects in the vicinity of the inductors.

The 500kHz square wave from U2:A and the 512 kHz sine wave are input to a phase comparator. The phase comparator is constructed from 2 X-OR grates U1 :B and U1 :D. The gate U1 :D allows the phase comparator to be enabled at the same time as the counter U2:A is enabled.

The output from the phase comparator is connected to a second-order filter formed using the Exclusive-OR gate U1 :A. The output of the filter is connected to an input to the processor U5.

A low frequency oscillator is provided to control the timing of the sleep modes which are discussed herein before. The oscillator is constructed using a Schmitt triggered NAND gate U3:C, with the necessary capacitors and resistors, and has a period of 3 to 4 seconds. There is derived from this low frequency oscillator a watchdog signal which is input to the processor U5. The watchdog signal is 1kHz with a mark to space ratio of 1:1.

A 12 to 5 volt regulator U4 is provided which powers the electronics. The regulator U4 is of the low dropout type. Diodes D2 and D7 are provided to protect the circuit and the batteries. The diodes prevent one battery pack from charging the other and protect the circuit if the batteries are incorrectly inserted.

Two transistors Q2 and Q4 are provided. These can be switched on by the processor U5. Q4 is provided to control the radio receiver, and Q2 is provided to control the phase comparator and the counter U2:A.

The Darlington transistor Q3 is provided to drive the alarm. A Darlington transistor is used to ensure that the high peak currents which can be drawn by the alarm can be met. The transistor Q3 can be switched on by an output from the processor U5.

The logic gates and counters are provided on integrated circuits which are from the 4000 CMOS family. This logic family has been used due to its very low power consumption properties, which are essential for battery powered applications. Also the family offers a good operating voltage range and good noise immunity which are also essential for this application.

A reed switch is provided on the device. The reed switch provides an input to the processor U5.

The processor U5 controls the circuit. It determines whether a car is present in the parking space and takes appropriate action.

The processor U5 may be in a processing mode or a sleep mode. It switches between these two modes in order to save battery power. The processor is in processing mode for a short period in which it must determine whether a car is present and whether the device has been signalled by the trigger unit. After the processor has performed these two tasks, and decided that there are no further actions, it enters sleep mode. It is awakened from sleep mode by the low frequency oscillator which provides a pulse to the reset pin of the processor U5, every 3 to 4 seconds.

When the processor is in processing mode it must use the RX module to determine whether the device has been signalled by the triggering device. The processor must use a counter U2:A and phase comparator U1 :D & U1 :B to determine whether a car is present. These two tasks cannot in this embodiment be performed simultaneously as the high frequencies generated by the counter and associated circuitry will interfere with the RX module.

The processor, therefore, activates the RX module and the counter and phase comparator alternately. This is achieved by switching on the transistors Q2 and Q7 as required, but not at the same time.

When the counter and phase comparator are active a 12 kHz signal (which is the difference in the output frequency of the counter U2:A and the RLC circuit) is applied to the second order filter U1 :A. The filtered 12kHz signal is input to the processor U5. Software running on the processor is able to determine whether a car is present by detecting changes in the filtered 12 kHz signal.

If a car is present over the device the frequency of oscillation of the tuned circuit will vary. A complex algorithm runs on the processor to determine whether a car is present. Part of the algorithm requires a change of at least a predetermined magnitude in 12 kHz signal for it to be decided a car is present.

If a car is present the alarm can be activated/de-activated by switching on/off the transistor Q3 and thus switching on/off the alarm. The alarm is made to sound as described herein before.

The processor will also monitor the watchdog signal. If the watchdog signal fails or the frequency varies the processor will be alerted to the fact that there is a fault in the low frequency oscillator.

The current process of the processor U5 may be overridden by placing a magnet on the outside of the device in such a position so as to close the reed switch. Thus an input is provided to the processor U5. The processor U5 may then stop its current activity such as sounding the alarm.

Due to the manufacturing tolerances of electronic components such as inductors, resistors and capacitors the tuned circuit may be set to the correct frequency. This is achieved with the variable capacitor VC1 , and possibly a shunt capacitor C2A. If the variable capacitor VC1 cannot be altered to achieve the correct frequency, the shunt capacitor C2A can be inserted.

Setting the correct frequency of the tuned circuit is part of a test sequence. For such a test sequence the switches SU1 , SU2 and SU3 are set to a known configuration, and a test link is inserted. The processor U5 is thus alerted that a test is being performed.

To perform the test the circuit must be complete and batteries inserted into the device. Also all metal objects must be removed from the vicinity of the device. These steps are necessary due to the sensitivity of the device to its surroundings, particularly metallic objects in the surroundings. That is the surroundings, in particular metallic objects, will affect how the device operates.

A range of checks can then be performed which ensure that the device is operating correctly. These checks include setting the frequency of the tuned circuit and measuring that current drain is within known limits as the device performs various operations. The circuit will be tuned and tested at the factory, usually manually, before the monitor unit is sold.

It will be appreciated that the slow running oscillator U3C performs two functions; a) it provides the sleep mode/processing mode change signal, and b) it provides the watchdog signal pulses. This avoids the need to have two oscillators and reduces cost.

In other embodiments of the invention we envisage that there may be detection means other than an induction coil. The other detection means may be for instance infra-red, ultrasonic, or CCD camera, these are only examples and this list is not exhaustive.

In a further embodiment the alarm may be replaced with a means providing a synthesised voice or means illuminating a sign. The device may activate the sign or voice or alarm by a wire connection or by a radio link.

In a further embodiment the device may have one or more displays showing the elapsed time since a vehicle was parked in the space. The displays may be analogue or digital. The device may record the time a vehicle has been present, allowing the vehicle to be billed for its parking. It should be appreciated that record may mean the transmission of data to a remote receiver or other means.

In a further embodiment the trigger for the device may be provided on a particular class of vehicle, operating the device automatically for that class of vehicle. For instance the class of vehicle may be for cars with disabled owners allowing them to park in their designated spaces.

In another embodiment the device may be used to provide a space for advertisements, or other information such as endorsements which may be useful, to people viewing the device. Having the device carry advertisements may appeal to some customers.

In yet another embodiment, different versions of the device may be provided which perform different functions. For instance one device may be provided for guarding a parking space. Another version of the device may be provided for guarding a car parked in the parking space. Of course other versions may be provided.

In the preferred embodiment the car is parked near to the device and the alarm sounds. The driver of the car, or other person, then switches off the device's alarm. In an alternative embodiment the driver, or other person, may disable the alarm before the car is parked near to the device.

In still another embodiment the device may be connected to a display or similar device, such as a computer, within an office or a control centre.

In a further embodiment a network of devices may monitor vehicles as they move around the country or further afield. The device may communicate to a central station that a known vehicle has been detected at the device. Thus the central station will be able to track the vehicle as it visits a series of devices.

It will be appreciated that the authorisation signal for the space protecting operation and/or the vehicle guarding signal may effectively be simply a re-setting of the alarm sequence, the alarm sequence being triggered by a change in occupancy of the parking space.

The device may be used to monitor vehicle arrival and departure. This information could be stored, and could be retrieved from each monitor, or could be relayed to a remote station. Such information may be used for a variety of purposes such as to provide data on the use of the space, to monitor the movements of a particular vehicle or to bill that vehicle for time used.