GB2306035A - Differential weight security alarm - Google Patents

Abstract

The differential weight security alarm consists of a rigid surface A bearing on piezo-electric sensors C under deflection which provide an output signal as a result of the rate of change of weight experienced by the placement on or removal of an object from the surface. This signal is sent to an electronic unit (Fig. 7) which filters, amplifies and produces an output signal as well as resetting the apparatus in preparation for a subsequent event. The phase of the output signal may also be determined to tell whether an item has been placed on or removed from the surface.

Description

Differential weight security alarm.

The apparatus described is particularly effective as a security alarm input device where a large number of shelves or surfaces are to be monitored which can then effectively protect many hundreds of individual objects from pilferage or by alerting staff that an object has been removed.

The alarm is activated by a change of weight, either by adding or subtracting weight about a steady state weight datum condition. The datum operating point is the steady state condition regardless of the total weight of the surface plus objects. A maximum safe operating weight limit depends on the application in hand and the choice of materials.

The apparatus described does not need to allow for the absolute weight as voltage is only generated when the amount of deflection is changing. This is distinct from capacitive, piezo-resistive, strain-guage spring-balance and load cell arrangements where the absolute weight determines their actual value.

The overall weight of the objects to be protected and the surface on which they are placed is supported by a sensor or number of sensors on which the total weight is bearing.

In practise it is convenient to use 4 sensors per surface but any number of sensors connected together could be used to achieve the desired result.

It is the nature of the sensors, which are piezoelectric, and their subsequent electronic signal handling processes which give the arrangement its specific importance.

In practise it has been found to provide a very sensitive characteristic together with a good degree of immunity to unwanted events such as vibration and external noise.

The construction and principle of operation of the sensor or sensors on which the weight bears is consistent and important to the correct performance of the system.

For the purposes of the description, the example of a shelf is used. It should be appreciated that the shelf could equally be a tray, a table or any surface whether flat or irregular in nature which could be arranged so that the total weight of the surface and objects on the surface bear on the sensors.

In the description the shelf is supported by 4 sensors so that the weight is evenly distributed amongst them and that the sensors in turn rest on fixed shelf supports.

With reference to Fig 1 and Fig 4 and 5 The shelf A rests directly on the resilient mount B which acts as a controlling op position to the piezoelectric transducer C. A bearing pad D fits between C and the shelf support E. This bearing pad may be fixed to C by adhesive or similar means.

The bearing pad D must be different in diameter from B in order for the piezoelectric sensor C to deflect. If B and D were of the same physical diameter then the piezoelectric transducer C would be subject to compression and not deflection and little or no output would result. Connections from the Transducer consist of wires F & G which convey the signals generated by the rate of change of deflection of C to the electronic circuitry.

The weight of the shelf and objects on the shelf causes C to physically deflect from its unladen state, the amount of deflection depends on the amount of weight and the coefficient of stiffness of the resilient mounts B & D from which the mounts are made. In practise the resilient mount B is made from a rubber with a suitable characteristic of stiffness and recovery from compression. Thus the sensitivity of the system, or the voltage generated per unit of weight change is dependent on the coefficient of stiffness of the resilient mount B and the bearing pad D. In practise, D is made from a hard rubber material but could be made from a rigid material such as nylon.

When weight is placed on, or removed from the shelf the characteristic of the piezoelectric transducer under these conditions is to generate a linear voltage proportional to the rate of change of displacement of the piezo crystal profile from its previous profile. After the event has occurred the sensor takes up a new physical position, and although it may be subject to deflection the state is steady and the output falls zero.

A second factor which determines the sensitivity is the relative areas of the resilient pad B and the bearing pad D. The greater the difference in relative areas, the greater the sensitivity. In practise a ratio of 2:1 results in good sensitivity together with good mechanical stability.

Fig 3 shows an alternative arrangement where the shelf A rests directly on the bearing pad D and the resilient pad B sits on the fixed support E. This arrangement is functionally identical to the one described above.

It is a characteristic of Piezoelectric transducers that the polarity of the voltage generated by the deflection depends on the direction of deflection. Thus it is possible to identify whether an object has been placed on, or removed from the surface. The polarity, sometimes referred to as the phase of the signal generated will indicate whether the overall weight has been increased or decreased by the event.

Figure 5 shows the effect of weight bearing on the shelf and although exaggerated in the drawing, shows the deflection moment acting on the sensor C The electronic circuitry must perform an integration function on the sensor output in order to interpret the event as genuine. ie Vo =df dt Where + = the change due to deflection and Vo = voltage produced by the sensor.

Change in profile = Vo x t = Volts x time after integration.

Fig 2 shows the arrangement where the shelf A is supported by 4 resilient mounts B and these supports are of the type illustrated by Fig 1.

The 4 sensors are connected together and in the same phase by wires F & G this wire consisting of 2 or more cores and conveys the combined output signals as an input to the electronic unit. Additional cores may be used as cut-cable supervision.

When more than one shelf is used, the sensors on alternate shelves may be connected in the opposite phase. This has the advantage of cancelling the effects of temperature change to which all the shelves in the system are subjected. Temperature changes can produce small deflections as the shelf, and the sensors themselves expand and contract. The disadvantage of reversing the phase of alternate shelves is that, if the sensors are all connected to a common input point then the electronics can no longer determine whether the phase of the event signal is as a result of an object being removed from one shelf or being placed on a shelf of opposite phase. In practise it is necessary to determine the requirements of the application and whether phase indication is required.

The arrangement described allows 20 or more shelves to be collectively connected to an electronic unit and a sensitivity of 20grams in 20Kg can be achieved per shelf or 0.1% of the total weight.

Fig 6 shows the block diagram of the electronic unit up to the point where the event has been detected. It can be appreciated that once an event has been detected then standard practises apply to the electronics which receive this signal.

The outputs from the sensors 1 are all connected in parallel from n shelves.

These are applied to an input filter 2 which removes the effect of high frequency vibration signals. The cut off frequency of this filter is in the order of 2 Hz This filter is also effective in improving the immunity to unwanted signals such as noise spikes and mains hum which may be induced into the sensor cables.

The output of the sensors must be integrated from Rate of Change of sensor profile or deflection to actual change of profile or deflection and this is achieved in block 3 Therefore the output from the integrator is proportional to the change in profile of the sensors subjected to the event, ie change in weight.

A second filter in block 4 further removes unwanted vibration and short term small weight disturbances. The output of this filter is fed to block 5 which amplifies the signal to a level which may be compared to a preset threshold.

The comparator 6 as shown will generate an output signal at 12 for an event which represents a placement of an object on, or removal of an object from a surface. If required, the phase of the event is available at 13. Preset voltages at 10 and 11 determine the operating parameters of the comparator.

After a valid event detection, a time delay circuit 7 is initiated. After a preset period, in practise approximately 1 second is suitable, the timer initiates a reset action. The reset is provided by the circuit 8 and discharges the stored charge in the integrator and filter circuits in preparation for a subsequent event. The action of resetting, is to restore the steady state condition.

An additional circuit 9 is arranged to detect disconnections or short circuits or non characteristic changes in the input circuitry which would indicate faults or tampering.

Fig 7 shows the waveforms associated with the block diagram. It shows the steady state condition follwed by an event equal to an object being removed from a surface. This generates the voltage waveform shown at 1 which is fed to the integrator 3. The integrator sums the rate of change of voltage in time and generates the signal at 3. Providing a preset level is exceeded, the comparator triggers producing the output signal at 12. The phase line at 13 is arranged in the example to go high if an item is placed on the surface. After a preset period the reset circuit 8 resets the charge in the Integrator and filters to datum steady state.

Fig 7 then shows the effect of placing an object on the surface, the polarity of the signal at 1 is reversed, and the integrator again responds in the negative phase this in turn generates a phase output at 13 which reflects the placing of an object on the surface. Finally the second reset occurs and the datum is restored.

Claims (3)

Claims

1) A shelf or similar surface is partially or wholly supported by one or more piezoelectric sensors subjected to deflection by the overall weight of the shelf or similar surface and the object or objects thereon. The piezoelectric devices are mounted between two pads of different diameters, at least one of which has a resilient nature to allow the piezoelectric device to be deflected by the weight to which it is subjected. Any object or objects placed on or removed from the surface will cause a change of deflection in the sensors and an output voltage proportional to the rate of change of weight will occur. At all other times a steady state exists regardless of the overall weight to which the sensors are subjected up to a preset operational limit.

This voltage is then integrated filtered and amplified to produce a signal which represents placement on or removal of the object or objects. More than one surface may be used and their associated sensors may be connected collectively to the electronics input. The sensitivity of the apparatus is determined by the relative diameters and stiffness of two pads which support the piezoelectric sensors and allow the deflection of the piezoelectric sensors to take place together with the parameters of the electronic unit. The application allows a large number of objects to be collectively and individually protected in a single or multi surface environment.

2) The effect of temperature change experienced by a series of surfaces referred to in 1 may be greatly reduced by reversing the polarity of alternate surfaces in a multi surface system.

3) Following the detection of an object being placed on or removed from a surface the electronics resets the integrator and filter circuitry to the original steady state so that the system is ready for a subsequent event. This eliminates the effects of any previous activity.