GB1577941A - Glass break defectors - Google Patents

Description

(54) GLASS BREAK DETECTORS (71) We, RCA CORPORATION, a corporation organized under the laws of the State of Delaware, United States of America, of 30 Rockefeller Plaza, City and State of New York, 10020, United States of America, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement.

This invention relates to glass break detectors for detecting the breakage of a sheet of glass for example window.

An object of the invention is to provide a system which gives an alarm if a sheet of glass is broken, for example to detect the deliberate breakage of a window by an intruder attempting to gain entry through the window, while not responding to taps on the glass or cuts on its.

According to the invention, a glass break detector consists of a piezo-eleqtric or other transducer adapted to be mounted on a sheet of glass, and a detector circuit coupled to the transducer and arranged to give an alarm signal in response to simultaneous high and low frequency signals from the transducer, each at above a respective predetermined signal level.

The transducer may be attached to a sheet of glass directly or to a nearby surface.

The detector circuit may include respective parallel paths each including a band pass filter, one filter passing high frequencies and another passing low frequencies, and an energy level sensor. The high pass filter may, for example, allow a frequency band centred on at least 200 KHz to pass while the low pass filter may allow a band centred on a frequency no more than 50 KHz, e.g. 500 Hz, to pass.

Preferably, signals from all paths are fed to different inputs of a gate which may be arranged to energise an alarm when it receives a signal on all inputs simultaneously.

In this way the danger of a false alarm is minimised because disturbances to the glass such as an accidental collision, a scrape or even heavy nearby traffic will not result in high amplitude wide frequency band vibrations.

There are four main sources of excitation of the transducer; a tap, a cut, a break, and radio frequency interference. The following table shows the energy level of short term energy in the transducer output for all four sources at low and high frequencies.

Excitation Low Frequency High Frequency Tap High Low Cut Low Medium Break High High R.F.I. Low-Medium High Therefore by monitoring short term energy levels at a high and low frequency, a break can be uniquely defined by these levels being above predetermined levels and present simultaneously.

Figure I of the accompanying drawings shows the probability of vibrations being generated at various amplitudes at two different frequencies Figure IA is for frequencies in a band centred on 35 KHz and Figure IB is for frequencies in a band centred on 275 KHz. The values are as measured on the detector in Figure 3.

The chain lines are the curves for a break in a sheet of glass, the dashed lines are the curves for a tap; and the continous lines for a cut or score which is not a complete break.

In pactice the frequency bands may be considered as low frequency up to 50 KHz t 30 KHz, and as high frequency if they are above 100 KHz and probably at 200-600 KHz.

The invention may be carried into practice in various ways and one embodiment will now be described by way of example with reference to the accompanying drawings of which; Figure 1 shows curves as described above; Figure 2 is a block diagram of the circuit of the invention; and Figure 3 is a more detailed circuit diagram corresponding to Figure 2.

Referring to Figures 2 and 3, a detector consists of a transducer 11 attached, possibly by an adhesive, to a sheet of glass, a signal amplifier 12 and a detector circuit 21 connected to an alarm device. The transducer is a piezoelectric device able to generate an electrical signal when subjected to mechanical stress, as for example when the glass is disturbed. when glass breaks, a wide frequency band of high amplotude vibrations is emitted, and that is characteristic of a break. Thus, when the glass is broken, a wide frequency band, high amplitude signal is generated.

The signal is amplified by the amplifier 12, and fed into two parallel paths which are similar except that each includes a band pass filter of a different mid-frequency. The path 32 includes a high frequency band pass filter 13. This filter allows a frequency band centred on 2()0 KHz to pass to an energy sensor 15 which compares the energy of the signal with a set value. If the signal energy exceeds this set value, the sensor 15 transmits a signal along line 34 to one input of a gate 17.

The second path 33 includes a low frequency band pass filter 14, which allows frequencies in a band below 50 KHz to pass to an energy sensor 16 which compares the energy of the signal with a set value. If the signal energy exceeds this set value the sensor 16 transmits a signal along line 35 to the other input of the gate 17. There is a band of frequencies between the pass bands of the two filters in which band at least one filter does not pass high energy sigals.

There are also frequency hands below the pass band of the filter 14 and above the pass band of the filter 13 in which neither filter passes high energy signals.

The gate 17 produces a signal along line 36 to trip an alarm when it receives a signal from both sensors simultaneously. This will only occur when the filters 13 and 14 each allow a large energy signal to pass which in turn will occur when the transducer generates high amplitude signals at frequencies in the pass bands of both filters which indicates that the glass has been broken.

Figure 3 shows that the amplifier 12 includes a field effect transistor and that each of the filters 13 and 14 comprises three operational amplifiers connected in series each with its own feed back path, an arrangement which gives good selectivity with stability under different operating conditions. The resistance and capacitor values in the various stages are chosen to give the desired overall band pass characteristics of the two filters 13 and 14.

The energy sensors 15 and 16 include precision rectifiers and are supplied with a stabilised voltage 'V' for the respective inputs of an operational amplifier 43 and 44. The amplifier outputs are normally 'l' but change to 'ü' at '34' or '35' if the band pass filter passes a signal with a level above the level set by the detector circuit 15 or 16.

The lines 34 and 35 are connected to the two inputs of the gate 17 which is a 'NOR' gate which produces a '1' output only when both its inputs are '0'. The '1' output is fed to '1' input of a pair of 'NOR gates connected as a latch which serves to set the signal at 36 at '1' if a '1' is received at the output of the gate 17 and is thereafter held at '1' even if the '1' at the output of 17 is lost. The latch can however be reset by means of a push button 46 to supply a '1' to one input of the second 'NOR' gate.

Once a 'I' appears at the output of the latch at 36. the alarm 41 is switched on by means of a relay with its coil in the output of a transistor amplifier, 47.

There is also an arrangement for giving a signal if an attempt is made to temper with the glass break detector by applying heat to the glass to be protected. A potential divider 51 connected across the supply includes a fixed resistor 52 and a thermistor 53 whose resistance varies with its temperature, the thermistor being positioned in close relationship with the glass so that if a source of heat is applied to the glass the potential at the junction 54 of the two resistors will go down. That can then provide an output signal at 55 from a comparator 56 including an operational amplifier having one input connected to the junction 54, and the other to a fixed tapping in a potential divider connected across the stabilised supply. A signal at 55 latches on on a mono-stable circuit 56 whose output is supplied to an amplifier 57 similar to the amplifier 47 for operation a tamper relay 58.

WHAT WE CLAIM IS: 1. A glass break detector comprising a transducer adapted to be mounted on a sheet of glass. and a detector circuit coupled to the transducer and arranged to give an alarm signal

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (7)

**WARNING** start of CLMS field may overlap end of DESC **. The chain lines are the curves for a break in a sheet of glass, the dashed lines are the curves for a tap; and the continous lines for a cut or score which is not a complete break. In pactice the frequency bands may be considered as low frequency up to 50 KHz t 30 KHz, and as high frequency if they are above 100 KHz and probably at 200-600 KHz. The invention may be carried into practice in various ways and one embodiment will now be described by way of example with reference to the accompanying drawings of which; Figure 1 shows curves as described above; Figure 2 is a block diagram of the circuit of the invention; and Figure 3 is a more detailed circuit diagram corresponding to Figure 2. Referring to Figures 2 and 3, a detector consists of a transducer 11 attached, possibly by an adhesive, to a sheet of glass, a signal amplifier 12 and a detector circuit 21 connected to an alarm device. The transducer is a piezoelectric device able to generate an electrical signal when subjected to mechanical stress, as for example when the glass is disturbed. when glass breaks, a wide frequency band of high amplotude vibrations is emitted, and that is characteristic of a break. Thus, when the glass is broken, a wide frequency band, high amplitude signal is generated. The signal is amplified by the amplifier 12, and fed into two parallel paths which are similar except that each includes a band pass filter of a different mid-frequency. The path 32 includes a high frequency band pass filter 13. This filter allows a frequency band centred on 2()0 KHz to pass to an energy sensor 15 which compares the energy of the signal with a set value. If the signal energy exceeds this set value, the sensor 15 transmits a signal along line 34 to one input of a gate 17. The second path 33 includes a low frequency band pass filter 14, which allows frequencies in a band below 50 KHz to pass to an energy sensor 16 which compares the energy of the signal with a set value. If the signal energy exceeds this set value the sensor 16 transmits a signal along line 35 to the other input of the gate 17. There is a band of frequencies between the pass bands of the two filters in which band at least one filter does not pass high energy sigals. There are also frequency hands below the pass band of the filter 14 and above the pass band of the filter 13 in which neither filter passes high energy signals. The gate 17 produces a signal along line 36 to trip an alarm when it receives a signal from both sensors simultaneously. This will only occur when the filters 13 and 14 each allow a large energy signal to pass which in turn will occur when the transducer generates high amplitude signals at frequencies in the pass bands of both filters which indicates that the glass has been broken. Figure 3 shows that the amplifier 12 includes a field effect transistor and that each of the filters 13 and 14 comprises three operational amplifiers connected in series each with its own feed back path, an arrangement which gives good selectivity with stability under different operating conditions. The resistance and capacitor values in the various stages are chosen to give the desired overall band pass characteristics of the two filters 13 and 14. The energy sensors 15 and 16 include precision rectifiers and are supplied with a stabilised voltage 'V' for the respective inputs of an operational amplifier 43 and 44. The amplifier outputs are normally 'l' but change to 'ü' at '34' or '35' if the band pass filter passes a signal with a level above the level set by the detector circuit 15 or 16. The lines 34 and 35 are connected to the two inputs of the gate 17 which is a 'NOR' gate which produces a '1' output only when both its inputs are '0'. The '1' output is fed to '1' input of a pair of 'NOR gates connected as a latch which serves to set the signal at 36 at '1' if a '1' is received at the output of the gate 17 and is thereafter held at '1' even if the '1' at the output of 17 is lost. The latch can however be reset by means of a push button 46 to supply a '1' to one input of the second 'NOR' gate. Once a 'I' appears at the output of the latch at 36. the alarm 41 is switched on by means of a relay with its coil in the output of a transistor amplifier, 47. There is also an arrangement for giving a signal if an attempt is made to temper with the glass break detector by applying heat to the glass to be protected. A potential divider 51 connected across the supply includes a fixed resistor 52 and a thermistor 53 whose resistance varies with its temperature, the thermistor being positioned in close relationship with the glass so that if a source of heat is applied to the glass the potential at the junction 54 of the two resistors will go down. That can then provide an output signal at 55 from a comparator 56 including an operational amplifier having one input connected to the junction 54, and the other to a fixed tapping in a potential divider connected across the stabilised supply. A signal at 55 latches on on a mono-stable circuit 56 whose output is supplied to an amplifier 57 similar to the amplifier 47 for operation a tamper relay 58. WHAT WE CLAIM IS:

1. A glass break detector comprising a transducer adapted to be mounted on a sheet of glass. and a detector circuit coupled to the transducer and arranged to give an alarm signal

in response to simultaneous high and low frequency signals from the transducer each at above respective predetermined signal levels.

2. A glass break detector as claimed in Claim 1 in which the transducer is a piezoelectric device.

3. A glass break detector as claimed in either of the preceding claims including two parallel paths each including a band pass filter and an energy level sensor.

4. A glass break detector as claimed in Claim 3 in which the two band pass filters have pass bands centered respectively on a frequency of at least 100 KHz and no more than 80 KHz.

5. A glass break detector as claimed in Claim 4 wherein the two centre frequencies are at least 200 KHz and no more than 50 KHz.

6. A glass break detector as claimed in claim 5 wherein the two centre frequencies are 275 KHz and 35 KHz.

7. A glass break detector arranged substantially as herein specifically described with reference to Figure 2 or Figure 3 of the accompanying drawings.