Detector

The invention relates to a detector for detecting the presence of an object and in particular but not limited to a proximity detector for detecting objects entering the proximity of a lift/elevator doorway. The invention also relates to a method of detecting the presence of an object.

The use of proximity detectors in lift (elevator) systems for the detection of obstructions such as people, items of luggage, personal effects or the like in the lift doorway are well known.

One such detector, for example, comprises what is commonly referred to as a 'balanced bridge' proximity sensor. Balanced bridge detectors typically include two metal strip electrodes, each about 1 metre long, configured into respective arms of a capacitive 'Wheatstone bridge' type circuit. Each of the remaining two arms of the bridge generally comprise a variable trimmer capacitor to allow the bridge to be properly balanced, by a user, so that there is only negligible output from the bridge when the system was undisturbed by external signal sources.

A power oscillator (typically operating at -100kHz) provides a signal source and is connected such that the bridge is 'hot' at the operating frequency with respect to ground. The other pole of the oscillator is grounded, for example, to the doors and/or car frame of the lift. In operation, an 'earthy' body entering the proximity of one of the strips causes a capacitive imbalance between the two electrodes, and an associated signal level difference. Hence the bridge produces an oscillating output, at the operating frequency, with amplitude dependent on the proximity of the object to the electrode. A detection circuit monitors the output from the bridge and when the amplitude of the output signal exceeds a predetermined threshold level the object is detected.

However, capacitive bridge detectors of this type are known to have thermal stability issues and are prone to drift out of balance over time. Hence, such detectors require regular maintenance from a skilled and experienced lift fitter to tune the bridge circuit in a relatively complex setup procedure. This has led to a decline in the popularity of such systems in favour of, for example, infrared systems.

According to the present invention there is provided an improved detector and an improved method for detecting the presence of an object.

According to one aspect of the present invention there is provided a detector for detecting the presence of an object, the detector comprising: antenna means; means for applying a signal to each antenna means to induce a field emanating therefrom; means for feeding said signal back from said antenna means wherein entry of said

object to said field causes a change (e.g. a reduction) in said fed back signal; and means for detecting the change (e.g. a reduction) in the signal from the antenna means thereby to detect the presence of said object.

Using a reduction in the returned signal rather than an increase can assist in reducing the power requirements of the detector (and in particular of the oscillator).

Furthermore, detectors configured this way can potentially have a reduced sensitivity to variations in ground quality and external interference which otherwise may trigger the detector undesirably. The arrangement such that the applied signal is reduced

('absorbed') by the presence a grounded body in the range of an electrode is particularly advantageous because it can also help to avoid the problems associated with the 'hot' electrical configuration and also allows the electronics to be grounded in a conventional way.

According to another aspect of the present invention there is provided a detector for detecting the presence of an object, the detector comprising: at least four antenna means; means for applying a signal to said antenna means to induce a field emanating therefrom; means for feeding said signal back from said antenna means wherein entry of said object to said field causes a change in said fed back signal; and means for detecting said change thereby to detect the presence of said object.

A detector of this type which has an arrangement of at least four antenna means can allow for greater discrimination between large and small objects. In the case of lift cars, for example, is can be used to improve discrimination between a lift door closing on a metal 'slam post' of the car (which should be ignored) and a hand becoming trapped (which should be detected).

The signal is preferably an alternating signal and said change (e.g. a reduction) occurs in an amplitude of said signal. The field is preferably an electric field and entry of said object to said field preferably causes said change (e.g. a reduction) in said fed back signal capacitively. The magnitude of said change (e.g. a reduction) may be dependent on the proximity of said object to said antenna means.

The antenna means preferably comprises an antenna electrode and may comprise at least one bootstrap electrode arranged parallel to the antenna electrode. The antenna electrode and the or at least one bootstrap electrode may be arranged in parallel planes.

Preferably the detector comprises a plurality of said antenna wherein for each antenna means said feed back means feeds back an associated signal therefrom. The feed back means preferably comprises signal conditioning means for converting the or each fed back signal into a DC signal. The conditioning means may comprise, for example, at least one synchronous rectifier configured to use the signal applied to

the antenna means as a clock signal. The conditioning means may comprise a plurality of analogue switches.

The detector may further comprise multiplexing means for sampling the or each signal fed back from the or each antenna means and for multiplexing the or each sample into a multiplexed output signal and may multiplex the fed back signal after intermediate processing, for example by signal conditioning means. It will be appreciated that the term 'fed back signal' may refer to the signal fed back after any stage of processing (e.g. rectification, multiplexing, amplification, filtering, etc..)

The detection means preferably comprises means for amplifying and/or filtering the multiplexed signal, or the or each fed back signal prior to measurement and/or analysis.

The detection means may be configured to measure a magnitude of the change (e.g. a reduction) in the or each fed back signal and to initiate a response if appropriate. The response may be initiated if the magnitude of the or at least one change (e.g. a reduction) is above a predetermined threshold and/or may be initiated in dependence on a comparison of the change (e.g. a reduction) in the signal for at least one of said antenna means with the change (e.g. a reduction) in the signal fed back from at least one other or all the antenna means.

Alternatively or additionally the detection means may be configured to measure the magnitude of each change (e.g. a reduction) over a plurality of time intervals and the response may be initiated if a change (e.g. a reduction) occurs for each of a plurality of consecutive time intervals.

The detector preferably comprises four of said antenna means thereby allowing objects to be detected independently by one or more of said antenna means. The antenna means may comprise between four and twelve antenna means and more specifically may comprise eight antenna means.

According to another aspect of the invention there is provided a method of detecting the presence of an object, the method comprising: applying a signal to antenna means to induce a field emanating therefrom; feeding said signal back from said antenna means wherein entry of said object to said field causes a change (e.g. a reduction) in said fed back signal; and detecting the change (e.g. a reduction) in the signal thereby to detect the presence of said object.

According to another aspect of the invention there is provided a method of detecting the presence of an object, the method comprising: applying a signal to at least four antenna means to induce a field emanating therefrom; feeding said signal back from said antenna means wherein entry of said object to said field causes a change in

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said fed back signal; and detecting the change in the signal thereby to detect the presence of said object.

The signal applied in the applying step may be an alternating signal and the change (e.g. a reduction) may occur in an amplitude of the signal. The field induced in the applying step is preferably an electric field and entry of the object to the field preferably causes a capacitive change (e.g. a reduction) in the fed back signal. The magnitude of the change (e.g. a reduction) may be dependent on the proximity of the object to the antenna means.

The antenna means preferably comprises an antenna electrode and the applying step preferably comprises applying the signal to the antenna electrode. The antenna means may comprise at least one bootstrap electrode arranged parallel to the antenna electrode. The antenna electrode and the or at least one bootstrap electrode may be arranged in parallel planes.

The applying step preferably comprises applying the signal to a plurality of the antenna means and the feed back step preferably comprises feeding back a signal from each of the antenna means.

The method preferably comprises converting the or each fed back signal into a corresponding DC signal preferably using signal conditioning means. The conditioning means may comprise at least one synchronous rectifier and the method may comprise using the signal applied to the antenna means as a clock signal for the rectifier. The conditioning means may comprise a plurality of analogue switches.

The method may further comprise sampling each signal fed back from each antenna means and may comprise multiplexing the or each sample into a multiplexed output signal. The method may comprise multiplexing the or each fed back signal after intermediate processing (for example by signal conditioning means). The method may comprise amplifying and/or filtering the multiplexed signal, or the or each fed back signal.

The method may comprise measuring a magnitude of the change (e.g. a reduction) in the or each fed back signal and may comprise initiating a response in dependence on a condition. The response may be initiated if the magnitude of the or at least one change (e.g. a reduction) is above a predetermined threshold. In the applying step the signal may be applied to a plurality of antenna means and the response may be initiated in dependence on a comparison of a change (e.g. a reduction) in the signal fed back from at least one of the antenna means with a change (e.g. a reduction) in the signal fed back from at least one other or all the antenna means.

The method may comprise measuring the magnitude of each change (e.g. a reduction) over a plurality of time intervals. The response may be initiated if a change

(e.g. a reduction) occurs for each of a plurality of consecutive time intervals. The applying step may comprise applying the signal to at least four antenna means.

The applying step may comprise applying the signal to between four and twelve antenna means. According to another aspect of the present invention there is provided an obstruction detection system for a doorway the system comprising: a powered door for closing the doorway; and a detector according to any aspect configured with said antenna means arranged along an edge of said door.

The obstruction detection system preferably comprises a further powered door or slam post; and a further detector according to any aspect configured with said antenna means arranged along an edge of said further door or slam post; wherein each said detector operates at a different frequency.

The or at least one powered door preferably further comprises a mechanical system for initiating door opening in the event that said powered door collides with said object.

The invention extends to methods and/or apparatus substantially as herein described with reference to the accompanying drawings.

Any feature in one aspect of the invention may be applied to other aspects of the invention, in any appropriate combination. In particular, method aspects may be applied to apparatus aspects, and vice versa.

Furthermore, features implemented in hardware may be implemented in software, and vice versa. Any reference to software and hardware features herein should be construed accordingly.

The invention will now be described, by way of example only, with reference to the attached figures in which:

Figure 1 is a simplified block diagram of a detector according to an embodiment of the invention;

Figures 2a, 2b, and 2c is a circuit diagram of a detector according to an embodiment of the invention; and Figure 3a and 3b show respective faces of an antenna board for the detector of Figures 1 to 2c.

In Figures 1 , 2a, 2b, and 2c a detector for a lift/elevator doorway is shown generally at 10. The detector is a capacitive proximity detector comprising a main processing unit 12, a multiplexer unit 14, a signal generation unit 16, a switching unit 18, and

eight sensor branches 20 each comprising a rectifier portion 22, a buffer portion 24, and antenna means 26.

It will be appreciated that although eight sensor branches 20 are described and shown any suitable number of detector branches may be provided. In operation the antenna means 26 of each branch 20 is driven with an alternating signal provided by the signal generation unit 16, thereby generating an associated oscillatory electric field. Each signal is fed back into the system, from the antenna means 26, via the corresponding buffer 24 and rectifier portion 22 to the multiplexer unit 14 for subsequent processing and analysis in the main processing unit. When an 'earthy' object such as a person's hand or the like enters the field of a particular branch 20 the object capacitively couples with the associated antenna means, which results in a corresponding reduction in the amplitude of the fed back signal. In effect, the object 'absorbs' some of the transmitted signal. The reduction is detected and analysed and if appropriate a corresponding response initiated (via the switching unit 18) in dependence on the magnitude of the reduction and/or any branch to branch variation in the reduction. For example, a trapped hand or the like will result in a different branch to branch variation from, and will require a different response to, the closure of a lift door onto a metal 'slam post' or the like.

The signal generation unit 16 is configured to produce a square wave having an appropriate operating frequency (typically 16kHz) and amplitude (typically 4V to 5V). The oscillation signal is derived from the output of a crystal operating in a high speed mode (HS). The crystal output is tapped by a divider (IC5) which divides it down to the required frequency (i.e. 16kHz). The crystal typically oscillates at 4.0MHz, although any suitable crystal may be used. In centre-opening lift systems a pair of such detectors (each with a plurality antenna branches) may be used, one for each door. In such systems each paired detector is configured to oscillate at a different frequency to avoid interference between the detectors when the doors are closing or closed. Typically, for example, one of the detectors has a 4.0MHz crystal (for providing a 16kHz signal to the antenna electrodes) and the other a 4.19MHz crystal (for providing 16.76kHz signal to the antenna electrodes) thereby avoiding production of a low frequency 'beat tone' when the detectors are close to one another.

In addition to driving the antenna means 26, the square wave is provided as a reference signal for the rectifier portion 22 of each branch 20. With reference to Figures 3a and 3b the antenna means 26 of each branch 20 comprises an antenna electrode 30, a pair of 'bootstrap' traces 32, and a bootstrap (or 'feedback') plane 34, all fabricated (for example by printing) onto an appropriate

substrate (for example a fibreglass printed circuit board 'PCB'). As seen in Figures 3a and 3b the antenna means 26 are provided on each substrate in pairs. Hence, for the eight sensor branches four substrate boards are used. It will be appreciated, however, that any suitable arrangement may be used, for example, a separate substrate may be provided for each branch individually.

As seen in Figure 3a each antenna electrode 30 comprises a rectangular strip of conductive material (typically copper) provided a first face the substrate board. The antenna electrodes may be any suitable dimensions, for example about 225mm long and 6mm wide. The bootstrap traces 32 are provided parallel to respective opposing long sides of the corresponding antenna electrode 30.

The bootstrap plane 34 is provided on a second face of the substrate opposite the first, as seen in Figure 3b, and has substantially the same size and shape as (and is aligned with) the antenna electrode 30.

The bootstrap plane 34 effectively acts a shielding track which is driven, in operation, via the buffer 24. The bootstrap traces 32 are similarly connected. The bootstrap configuration assists in reducing unwanted capacitance between the antenna electrode and any nearby door metalwork or the like, thereby improving the detector sensitivity to objects such as human bodies in the lift doorway.

When installed in a lift doorway the antenna boards of the detector system 10 are arranged in vertical array, extending substantially from floor level towards (typically about 1.8m) the top of the doorway. The boards are enclosed within a tubular plastic housing which is preferably shaped such that it can be substituted for the safety edge of a micro-switch based 'mechanical bumper' type system that older lift doors are typically fitted with. Such an arrangement has the advantage that it provides a mechanical backup to the proximity sensor should the object being sensed be of such a material that is not picked up by the electronic detector.

As seen in Figures 2a, 2b, and 2c, in operation, each antenna electrode 30 is driven by the square wave signal via a high value resistor 40 (typically 3.3MOhm) which is selected to maximize impedance. Maximising impedance increases sensitivity of the signal amplitude and phase to the small capacitance which arises between the antenna electrode and an object entering the associated field radiated from it.

Referring to Figure 2a in particular, each buffer 24 comprises a (PNP) bipolar transistor in an emitter follower configuration. The base of the transistor is connected to the antenna electrode and hence is driven through the drive resistor 40, by the generated square wave signal.

The output of the emitter follower (i.e. the transistor emitter) is connected both to drive the bootstrap plane 34 and to feed the output signal back to the associated rectifier portion 22.

Each rectifier portion 22 comprises a synchronous rectifier which includes a pair of analogue switches arranged to rectify the signal fed back from the antenna means

26. In operation, the switches connect the signal from the antenna means directly to the rectifier output when the drive signal is high and disconnect it when the signal is low. A smoothing capacitor CP3, CP4 is provided at the output of the rectifier for each branch for smoothing the associated output signal into a substantially smooth DC signal.

The synchronous rectifier 22 of each of a plurality of branches is typically integrated into a single integrated circuit ('IC) chip as seen in Figure 2a (i.e. three ICs for the eight branches). Any suitable chip may be used, for example a triple 2-channel analogue multiplexer chip (IC1 to IC3 in Figure 2a). Referring now to Figure 2b, the processed feedback signal (smoothed DC output) output from each rectifier 22 is input to the multiplexer unit 14. The multiplexer unit 14 comprises a multiplexer IC4 and a signal processing arrangement 50. The multiplexer IC4 is configured to sample the output from each branch in turn, and to provide the resulting output signal to the signa I processing arrangement 50. The multiplexer may comprise any suitable multiplexing means, for example, an eight- channel analogue multiplexer chip or the like.

The signal processing arrangement 50 comprises a plurality of amplifier/filter stages for amplification, active and passive filtering, each based around an associated operational amplifier IC6B to IC6D (see Figures 2b and 2c). The stages include a low pass filter based around IC6C, the output of which is provided to a level shifter which in combination with IC6B provides an appropriate input to the processing unit 12. In typical operation, for example, the output for a particular branch is approximately 4V when the doors are open and no objects are present in the field radiated from the associated antenna electrode. If an object enters the field radiated from the antenna, the output drops as the object approaches, typically to approximately 2V when the object is substantially in contact with the antenna cover.

The main processing unit 12 comprises an appropriate processor (for example a PIC processor such as a PIC16F88) for carrying out multiplexing, measurement and analysis functions. The processor is also configured to generate clock signals for setting the sampling and scanning periods of the multiplexing unit. Any suitable periods may be used, for example a 5ms sampling period for each branch which equates to a 40ms period for a full scan of all eight (or ~50ms taking into account additional processing time). The processor has three outputs connected to

respective channel selection inputs of the multiplexer for selecting the each antenna branch in turn for sampling.

The processor includes a 10 bit analogue to digital converter which is programmed to measure the output from each of the antenna branches at appropriate intervals. The digitised results of the measurements are stored in first in first out ('FIFO') buffers for subsequent reference and analysis.

The processor is configured to analyse the measured data and, if appropriate, to trigger an associated response, for example, to open the door(s) of a lift car when an obstruction is detected. In operation, the processor maintains a substantially continuous stream of pulses from a trigger output RBO to indicate an untriggered condition. When the processor is required to trigger a response (i.e. door opening) the pulse stream is interrupted and RBO maintained in either a high or a low state.

Door opening is triggered via the switching unit 18 which comprises a relay RL1 and associated circuitry. The switching unit circuitry includes a (MOS) transistor TR1 arranged for allowing current to flow through the relay coil when in on-state (drawing the relay switch in) and for preventing current flow when in an off-state (allowing the relay to drop out). A parallel resistor-capacitor R17-C8 configuration is provided across the gate-source junction of transistor TR1 , and the gate is capacitively coupled to the trigger output from the microprocessor. The configuration is such that while the pulse stream is maintained from the trigger output C8 remains charged and TR1 is maintained in the on-state. However, when the pulse stream is interrupted and the output goes either low or high, C8 discharges, TR1 switches off, and the relay drops out thereby initiating door opening. This arrangement is advantageous because if power to the circuit is interrupted the relay automatically enters a triggered state.

The processor is configured to operate two triggering modes in parallel: a presence sensing (or 'static') triggering mode and a motion sensing (or 'dynamic') triggering mode. In the presence sensing mode, the output from the antenna branches is monitored for a large reduction in the voltage (above a predetermined 'static' trigger threshold) from a particular branch, or a selection of branches when compared to a mean signal for all the branches. A localised change such as this is indicative of an object (for example, a hand) in very close proximity to the associated antenna electrodes. Comparison with the mean signal is advantageous because the mean signal is generally representative of the local environment and allows changes in local conditions (e.g. temperature) to be taken into account automatically.

The large change in signal is thus easily detectable and unlikely to be the result of drift. The processor is configured to respond to detection of such a signal by generating a 'static' or semi-permanent door open (trigger) signal. Hence, the doors may be 'held' open by a hand or the like without repeated attempts being made to close the doors.

A large signal reduction will also occur when the door closes and the antenna array is close to a lift slam post (or opposite detector in a centre closing system). However, in this case the signal drop occurs in all of the antenna branches simultaneously. Hence, by comparing the signal reduction in all branches with the mean signal, the processor automatically identifies a door closure event to be as one in which door triggering is not required.

The processor is also configured to detect a large drop, above the (or possibly a different) static trigger threshold, across all electrodes to identify the approach of full door closure and respond accordingly. Initially the dynamic trigger mode is disabled, then as the detector gets nearer (typically 20mm from) the slam post (or opposite door detector) all triggering is disabled until the re-opening.

While the doors are fully closed (for example as the lift moves between floors) there is no risk of an external object entering the field of the antennas. Thus identification of full door closure may be used by the processor to initiate a reduction in the range of the detector circuitry until a door opening event occurs.

The static triggering mode requires a significant drop in signal and therefore has a limited range of detection (typically 15mm from the face of the detector).

The motion sensing mode is used for long range detection where much smaller voltage drops are exhibited. Small changes can be the result of undesirable effects associated with interference spikes, vibration etc. Hence, the motion sensing mode is configured to mitigate against spurious effects by comparison of current signal levels with data stored in the FIFO buffers to identify a progressive fall in the outputs of the antenna branches over a series of sampling periods. For example, when a door moves towards an obstructing object (e.g. a person) the signals from the affected branches fall exponentially over successive sampling periods. Noise spikes and vibration, on the other hand, tend to fluctuate randomly and can therefore be discriminated against.

The history of each signal is stored for an appropriate number (e.g. eight) of scanning cycles. Typically, however, detection over a succession of three falling voltages from any antenna is sufficient to detect a distant, but approaching, obstruction with good reliability.

As with the presence sensing, motion sensing compares the signals from all eight branches, thereby allowing characteristics of an obstructing object to be identified. A human body, for example, will typically result in a large reduction in the signals from two or three branches (because of its irregular outline) compared to the signals from the others.

Motion sensing only responds to rapid changes, within a few scan cycles. Thus the motion sensing mode is very tolerant to slow drifts/ changes over several cycles, and therefore has a relatively high sensitivity.

Each antenna is affected differently by environmental conditions, component tolerances, distance to metal doors etc. Hence, the associated outputs vary between antenna branches. To compensate for these offsets, a calibration function is provided. Under quiescent conditions, the outputs from the eight antenna branches are measured and stored as offsets in a reference array (in an EEPROM of the processor or the like). Hence, in operation, after each measurement is taken the stored offset is subtracted to normalise the result from each branch. An additional fixed offset may also be subtracted to avoid mathematical difficulties in performing calculations at near to the zero point.

Generation of the calibration reference array occurs shortly (typically 0.5 seconds) after power on. Any drift in antenna characteristics is compensated for by automatically regenerating the array at appropriate intervals (for example, approximately every 24 hours). Regeneration of the reference is triggered by a rapid rise in received signal, which is indicative of a door reopening cycle. When such a rise is detected, the regeneration is initiated after an appropriate delay (for example about 2 or 3 seconds) selected to ensure that no obstructions are likely to be near the door edge when the measurements are made. The processor is also configured to allow array generation to be triggered manually by grounding an associated input for a predetermined period (typically between 1 and 30 seconds).

Watchdog circuitry 60 is also provided for resetting the processor in the event of software malfunction. The watchdog circuitry 60 has an input capacitively (AC) coupled to one of the channel selection outputs of the processor and an associated output connected to a reset (or clear) pin of the processor. The watchdog 60 includes an inverting buffer IC8A having an input connected via parallel connected capacitor C6 and (MOS) transistor TR2 to ground, and an output resistively connected to the reset pin. The circuit is configured such that while the channel selection output is a series of pulses, C6 continually discharges through TR2 to ground to prevent the inverter from oscillating. In the event of a software malfunction leading to a permanent loop, however, TR2 remains switched off, C6 charges and the inverter

begins to oscillate. When the inverter output goes high the reset pin follows and the processor resets.

Thus, the digital proximity detector disclosed advantageously removes the need for extensive manual adjustment during set-up and/or recalibration. The advantageous design allows for a multi-electrode configuration which enhances the flexibility of the system to detect objects having different characteristics and in particular different sizes. The arrangement such that the applied signal is reduced ('absorbed') by the presence a grounded body in the range of an electrode is particularly advantageous because it helps to avoid the problems associated with the 'hot' electrical configuration and also allows the electronics to be grounded in a conventional way

Each feature disclosed in this specification (which term includes the claims) and/or shown in the drawings may be incorporated in the invention independently of other disclosed and/or illustrated features. In particular but without limitation the features of any of the claims dependent from a particular independent claim may be introduced into that independent claim in any combination.