GB2217013A - An ultrasonic proximity device for the control of light emission from a display unit - Google Patents

Abstract

A light emitting display such as a cathode ray tube is controlled by sensing an object e.g. an observers head, at a predetermined position relative to the display. The object is sensed using an ultrasonic transmitter and detector (10, 11), the output of the detector being arranged to control the display such that it is active only when the presence of the object is sensed. The apparatus can be used in photographic darkrooms where there is a need to minimise light emission. The display is made visible only when an observer's head is in a viewing position. <IMAGE>

Description

CONTROL OF LIGHT EMISSION FROM DEVICES SUCH AS CATHODE RAY TUBES.

This invention relates to the control of light emission from devices such as cathode ray tubes. The invention has particular application to the control of such devices in environments where there is a requirement to maintain the emission of light at a minimum. Such a need exists, for example, in photographic darkrooms, which may be equipped with TV monitors or visual display units. The present invention provides a means by which such monitors or display units can be controlled automatically in accordance with a certain sensed condition or conditions, so that their display is actuated or energised to emit light only in response to that sensed condition. In one form the invention performs its function by sensing a particular condition, e.g.

the presence of an object such as an observer's head, in a given region by sensing ultrasonic radiation reflected from that region.

According to one aspect cf the present invention there is provided apparatus for controlling operation of a light emitting display means comprising an ultrasonic transmitter and a receiver so arranged that the transmitter can transmit periodic bursts of ultrasonic energy towards a selected region and the receiver can sense ultrasonic energy reflected from that region, and control circuitry associated with the transmitter and receiver and arranged to produce a control signal or signals for operating the light emitting display means when it senses that the receiver is receiving bursts of ultrasonic energy which occur in a preselected time relationship to the transmitted bursts.

The preselected time relationship may be arranged to correspond to the detection of an obj: within a predetermined range of the transmitter.

The light emitting display means may be a cathode ray tube or visual display unit and the circuitry may be arranged to activate such display means when it senses said received bursts.

The circuitry may include an oscillator for energising said transmitter.

The circuitry may include a gate coupled to receive signals derived from the output of said receiver, said gate being arranged to open to transmit signals for a predetermined period of time which begins a given time after the transmission of an energy burst. The control signal for opening said gate may be derived from said oscillator. The control signal may be fed from the oscillator to said gate via a delay circuit.

The output of the gate may be coupled to a comparator for producing an output in response to receipt of signals above a preselected threshold. The output of the comparator may be arranged to energise a relay in response to sensed ultrasonic energy bursts.

In an arrangement which employs more than one transmitter and receiver arrangement in the same vicinity, each oscillator can be arranged to transmit its bursts simultaneously. To achieve this each transmitter can be triggered by pulses derived from the mains power supply.

A further delay circuit may be provided between the comparator and relay.

According to another aspect of the present invention there is provided a method for controlling operation of a light emitting display means comprising monitoring a predetermined condition in a region by scanning the region using a radiation transmitter and receiver arrangement and controlling the operation of the light emitting display means on the basis of the monitored condition. The monitored condition may be the presence or absence of an object in that region and the radiation may be ultrasonic radiation.

According to a further aspect of the present invention there is provided apparatus for sensing an object within a predetermined range of the apparatus which comprises an ultrasonic transmitter and receiver arrangement, so arranged that the transmitter can transmit periodic bursts of ultrasonic energy towards the sensed region and the receiver can receive ultrasonic energy reflected from the region, and control circuitry associated with the transmitter and receiver for sensing when the receiver senses bursts of ultrasonic energy which bear a predetermined time relationship with the transmitted bursts.

In one embodiment of the invention pulses of ultrasonic energy are emitted towards a region to be monitored. Ultrasonic energy reflected from the region is sensed by a receiving transducer whose output is amplified. Received signals falling in a preselected narrow time interval are fed to a comparator which produces an output for actuating a TV display if the signals are above a given threshold.

The narrow time interval is arranged to correspond to the sensing of an object within about 2 to 12 inches from the viewing head aperture of a TV monitor so that only objects such as a viewer's head in a viewing position cause the display to become visible. At all other times the display is dark.

The invention will be described now by way of example only, with particular reference to the accompanying drawings. In the drawings: Figure 1 is a block schematic diagram of apparatus for controlling the operation of a TV monitor in a photographic darkroom or similar environment; Figure 2 shows a series of waveforms illustrating the operation of the apparatus of Figure 1; Figure 3 is a detailed circuit diagram illustrating the elements which make up the blocks of Figure 1; Figure 4 shows a modification of the arrangement of Figure 3; and Figure 5 shows another modification of the arrangement of Figure 3.

Referring now to Figure 1 apparatus for controlling operation of a TV monitor within a photographic darkroom comprises an ultrasonic transmitter and receiving arrangement which includes an ultrasonic transmitting transducer 10 and an ultrasonic receiving transducer 11. The transducer 10 is arranged, when energised, to transmit ultrasonic energy towards a preselected region and the receiver 11 is arranged to receive ultrasonic energy reflected from that region. The region is typically that which would be occupied by the head of a person viewing the TV monitor.

The transducer 10 is energised by a circuit which includes a pulse repetition frequency oscillator 14 which generates a train of short pulses at a repetition rate about 70 Hz. These pulses are fed to an oscillator 15 which causes the transducer 10 to transmit bursts of ultrasonic energy towards the selected region. The frequency of the ultrasonic radiation in each burst is 40 Khz.

The receiver 11 is arranged to produce electrical signals representative of the ultrasonic energy which it receives. These electrical signals are fed to an amplifier 18, which amplifies the signals sufficiently for an envelope detector 19, which is connected to the output of the amplifier, to function. The output of the envelope detector 19 is fed to an analogue gate 20. It will be seen that the analogue gate has a control input 21 which can receive the pulse output of the pulse repetition frequency oscillator 14 via a delay arrangement which comprises a cascaded pair of monostable multivibrators 22, 24.

The two multivibrators 22 and 23 operate to produce a train of pulses of the same repetition frequency as that produced from the oscillator 14, but at a controlled delay relative to the output of the oscillator 14. The pulses from the output of the monostable 23 are applied to the control input of the gate 20, so that that gate opens only for the duration of each such pulse. The delay is so selected that the gate 20 will be open at a time which corresponds to the arrival at its input of a signal derived from the reflection of ultrasonic radiation from an object at a predetermined position relative to the transducer 11.

Any pulses transmitted by the gate 20 are fed to a comparator 25, which is arranged to produce an output only in response to the receipt of pulses having an amplitude greater than a preselected threshold as determined by a threshold setting means 26. The output of the comparator 25 is fed to an amplifier 28 and then to a relay driver 30. The contacts of the relay are connected in a circuit which controls a disabling bias for the cathode ray tube of the TV monitor.

In operation the transducer 10 generates ultrasonic pulses which are transmitted towards the monitored region which is that occupied by an observer's head when the observer is viewing the TV monitor. If the observer is not present, then essentially no reflected signal is received by the receiving transducer 11 during the period for which the receiver is sensitive and no signals above the preselected threshold are fed to the comparator 25. In this situation the relay contacts of the relay 30 remain in a condition which maintains the disabling bias for the cathode ray tube, i.e. no display is displayed by the cathode ray tube. If the observer moves to a position for viewing the TV monitor then ultrasonic radiation is reflected from his head to the receiving transducer 11.The received ultrasonic bursts are converted to electrical signals by the transducer, amplified by the amplifier 18 and fed to the analogue gate 20. If the observer's head is within a preselected range relative to the receiving transducer 11, then the signals from the envelope detector 19 will arrive at the gate 20 at a time when the gate has been opened by control signals on the control input 21. The received signals are thus fed to the threshold detector 25 and assuming they are above the preselected threshold, are amplified by the amplifier 28 and fed to energise the relay 30.

Energisation of the relay then causes the disabling bias to be removed from the cathode ray tube of the TV monitor so that the observer sees a display on that monitor. In this way the apparatus of Figure 1 operates to energise the display of the TV monitor only when the observer is in a position for viewing the monitor. Thus, the light emitted by the TV monitor is minimised and only occurs when the observer wishes to view the monitor. Furthermore, the apparatus operates automatically and the observer does not need to operate manually any switches to energise the display, which could prove to be be difficult in darkroom conditions. Moreover the switching on of the display as the observer approaches helps him to locate it thereby avoiding a collision with the display.

For correct operation of the arrangement the pulse repetition frequency and pulse widths have to be chosen correctly. The principal constraints are the speed of sound in air, the required resolution and range, and the need that the equipment be relatively insensitive to echoes from the walls of the room.

Given that the speed of sound in air is about 1200 feet per second, this corresponds to 1.2 feet per millisecond or about 36 centimetres per millisecond.

The maximum range is determined by the interval between successive transmitted pulses. An interval of 14 milliseconds is chosen which gives a maximum range of 14 x 36 cm = 2.52 metres (about 8 feet).

2 It should be noted that the "2" in the expression above arises because the radiation travels over a path which is twice the range since there is a transmission and a reflection path.

A pulse width of 0.3 milliseconds is chosen as a compromise between sensitivity, which is better with a wider pulse, and resolution, which depends on the pulse length in air, and is better with shorter pulses. The length of the pulse in air is given by 0.3 x 36 cm = 10.8 cm (about 4 inches), which is the approximate limiting range resolution.

-The minimum range is that which gives no overlap between transmitted and received pulses and is thus 0.3 x 36 cm = 5.4 cm (about 2 inches) 2 It will be appreciated that these values have been arrived at for a particular application and can be altered to suit changed circumstances without affecting the principle of operation of the circuitry.

The waveforms which appear at various parts of the circuitry are shown in Figure 2. Waveform A represents the bursts of ultrasonic energy emitted by the transmitter 11 and waveform B represents the reflected bursts. It will be noted that waveform B shows multiple room reflections. Waveform C is the detected envelope which appears at the output of the envelope detector 19. Waveform D represents the delayed gate pulse applied to the control input 21 of the gate 20. Waveform E represents that part of the detected envelope transmitted by the gate 20 during the duration of the pulse applied to the input 21.

Figure 3 is a detailed circuit diagram showing the elements which make up the blocks of Figure 1.

This circuit is not described in detail, but the elements which make up each individual block of Figure 1 are shown by corresponding reference numerals.

The circuitry described above has been designed specifically for use with an installation comprising a TV picture monitor in a photographic darkroom which is used for slitting and packing photographic materials.

The monitor can, for example, show the working area seen by an infra-red sensitive TV camera or it can display digital or analogue data generated by a computer. Other types of display can also be controlled.

It will be appreciated that the apparatus can also be used in non-darkroom situations, but in enviroments where light emission is to be minimised.

For example, in systems which carry out surveillance by night, or where a control device has a limited life which must be conserved. A further application is in portable or battery-operated equipment where power must be conserved, e.g. where light emitting diode displays are used.

The arrangement as described above, with reference to Figures 1 to 3, has a single transmitter and receiver. In some situations it may be necessary to use several installations within the same vicinity.

In this situation there is a possibility of interferenence between ultrasonic systems. This means that a TV display can sometimes turn on when no operator is near and is due to the occasional detection of a received signal at the correct time, but due to a transmitted pulse from another system.

There are various ways in which this problem can be alleviated such as operation of several systems at different ultrasonic carrier frequencies. This is unsatisfactory however, because the bandwidths required are wide compared to the range of suitable operating frequencies and it is unlikely that sufficient discrimination can be obtained and furthermore the flexibility and interchangeability is lost if all the units cannot be identical.

A better solution is to synchronise the pulse transmitters of the systems so that they all emit simultaneously. The first echo received can then only be from the system's own transmitters, echoes from other transmitters appearing to be at greater ranges.

The synchronisation can be achieved very simply, as illustrated in Figure 4 which shows a modification to the part of the circuit of Figure 3 which includes the oscillator 10. In this modification, each oscillator 10 is energised from and synchronised to the 50 Hz mains power supply, suitably transformed. In order to achieve this it is necessary to increase the pulse repetition period of each oscillator 10 from 14 to 20 microseconds. This results in a proportionate, but insignificant increase in the maxirrum range of the system. It will be appreciated that in countries which have a different power supply frequency, then the pulse repetition period will be different, e.g.

for 60 Hz frequencies the pulse repetition period becomes 16.6 milliseconds.

A further modification which is desirable in the multiple transmitter and receiver arrangements is a delayed turn-off to prevent flickering of the display, in the event that the operator moves his head in a manner which produces standing wave effects. A modification which can overcome this problem is illustrated in Figure 5. This shows a modification to the output stage of the circuit of Figure 3 and comprises a delay element which is inserted between the output of the amplifier 28 and the relay 30. it includes a 555 timer 35 which operates as a delay to delay operation of the relay coil. A delay of 10 to 20 seconds has been found to be satisfactory.

Claims (16)

CLAIMS:

1. Apparatus for controlling operation of a light emitting display means comprising an ultrasonic transmitter and a receiver so arranged that the transmitter can transmit periodic bursts of ultrasonic energy towards a selected region and the receiver can sense ultrasonic energy reflected from that region, and control circuitry associated with the transmitter and receiver and arranged to produce a control signal or signals for operating the light emitting display means when it senses that the receiver is receiving bursts of ultrasonic energy which occur in a preselected time relationship to the transmitted bursts.

2. Apparatus as claimed in claim 1, wherein the preselected time relationship is arranged to correspond to the detection of an object within a predetermined range of the transmitter.

3. Apparatus as claimed in claim 1 or claim 2, wherein the light emitting display means is a cathode ray tube or a visual display unit and the circuitry is arranged to activate such display means when it senses said received bursts.

4. Apparatus as claimed in any preceding claim, wherein the circuitry includes an oscillator for energising said transmitter.

5. Apparatus as claimed in claim 4, wherein the circuitry includes a gate coupled to receive signals derived from the output of said receiver, said gate being arranged to open to transmit signals for a predetermined period of time which begins a given time after the transmission of an energy burst.

6. Apparatus as claimed in claim 5, wherein the control signal for opening said gate is derived from said oscillator.

7. Apparatus as claimed in claim 6, wherein the control signal is fed from the oscillator to said gate via a delay circuit.

8. Apparatus as claimed in claim 7, wherein the output of the gate is coupled to a comparator for producing an output in response to receipt of signals above a preselected threshold.

9. Apparatus as claimed in claim 8, wherein the output of the comparator is arranged to energise a relay in response to sensed ultrasonic energy bursts.

10. Apparatus as claimed in any preceding claim including more than one transmitter and receiver arrangement in the same vicinity and wherein each oscillator can be arranged to transmit its bursts simultaneously.

11. Apparatus as claimed in claim 9, including a further delay circuit provided between the comparator and relay.

12. A method for controlling operation of a light emitting display means comprising monitoring a predetermined condition in a region by scanning the region using a radiation transmitter and receiver arrangement and controlling the operation of the light emitting display means on the basis of the monitored condition. The monitored condition may be the presence or absence of an object in that region and the radiation may be ultrasonic radiation.

13. Apparatus for sensing an object within a predetermined range of the apparatus which comprises an ultrasonic transmitter and receiver arrangement, so arranged that the transmitter can transmit periodic bursts of ultrasonic energy towards the sensed region and the receiver can receive ultrasonic energy reflected from the region, and control circuitry associated with the transmitter and receiver for sensing when the receiver senses bursts of ultrasonic energy which bear a predetermined time relationship with the transmitted bursts.

14. Apparatus for controlling operation of a light emitting display means substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

15. A method for controlling operation of a light emitting display means substantially as hereinbefore described.

16. Apparatus for sensing an object within a predetermined range substantially as hereinbefore described with reference to and as shown in the accompanying drawings.