GB2026270A - Weighing scales with digital indication - Google Patents

Abstract

The deflection of a cantilever beam under the load to be measured is converted to an electrical signal by strain gauges 22 and the electrical signal is digitised using a dual ramp digital voltmeter. <IMAGE>

Description

SPECIFICATION Improvements in scales This invention relates to scales for domestic use and in particular scales in which the weight to be measured is converted to an electrical signal which is then passed to a number display.

Scales commonly used in the bathroom and kitchen are of the mechanical type in that, both the sensing of the weight and the indication of the weight are performed mechanically. The accuracy of such scales is low as is the reproducibility of the result.

It is therefore an object of the present invention to provide scales in which the weight is sensed and presented electrically.

According to the invention, there is provided scales comprising a weight sensor in the form of a beam which is supported as a centileverfor deflection under the effect of the load to be measured, at least one strain gauge attached to the beam whose electrical resistance varies according to the extent of deflection of the beam, means for providing an analogue electrical signal related to the change in resistance of the strain gauge, means for amplifying that analogue signal, a counterto which the digital signal from an oscillator is passed, means for causing the counter to stop counting a number proportional to the amplified analogue signal and a display, preferably light emitting diodes, for presenting the number counter by the counter.

By using electronic circuitry and components in scales according to the invention, one can be assured of accuracy andreproducibility of measurements.

Preferably there are four strain gauges which are attached to the beam in the region of its greater deflection under load, two of the strain gauges being positioned in the region of compression and two in the region of tension under load. The strain gauges are electrically joined in the form of a bridge so that the bridge is in balance when no load is applied to the scales but becomes unbalanced when the beam is deflected by a load. When an excitation voltage is applied across the bridge, the output resulting from the unbalanced condition can constitute the analogue electrical signal. By arranging four strain gauges in this way one can avoid problems of thermal expansion and compression of the beam.

Preferably the amplified analogue signal is fed to integrator in which the analogue signal is compared to a reference voltage, the integrator ramping up or down depending upon whether the voltage of the analogue signal is below or above, respectively, the reference voltage. The integrator provides an output which depends upon the result of the ramping up or down. The output from the integrator can then be used to stop the counters from further counting of the oscillator signals.

Preferably, the control of ramping up or down is effected by the use of a dual D flip flop and analogue switches.

The electronic circuit is preferably operated by dry batteries and, in the event that these start to fail, it is desirable to provide a further electronic circuit which measures their voltage and when this voltage becomes lower than a preselected figure, to provide a signal to the display to indicate a low battery tondi tion.

The beam is supported on a base for the scales at one of its ends and arrangements are made to apply the load to be measured to the other end of the beam which is free to deflect downwardly under the effect of the load. Preferably the beam is relatively long so that it deflects to a significant extent under even low loads. By an appropriate arrangement of levers, it is possible to apply the load from a weighing table to that outer end of the beam which need not be positioned near the centre of the base but can extend most of the way across the base, these levers having one end resting on the free end of the beam, being supported near their other ends by the base and receiving load from the weighing table at a position intermediate their ends.

Scales in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is an upright section through the scales; Figure 2 is a plan view of the scales; Figure 3 is a block diagram of the electronic circuitry for measuring the load; and Figure 4 is a more detailed circuit diagram of the electronic circuitry.

The scales 10 shown in Figures 1 and 2 of the drawings include a base 12 supporting a weighing panel 14.The base 12 has feet 16 and a beam 18 is attached at one of its ends to the base 12. This beam is therefore supported for cantilever deflection under the effect of loads placed on the weighing table 14.

The base also carries electronic circuitry 19 and a light emitting diode number display 20 giving the output reading which is visible through a transparent panel 21 in the table 14.

Near its end attached to the base 12, a number of strain gauges 22 are bonded to the beam 18. Loads from the weighing table 14 is applied to the outer end 18a of the beam 18 by four levers 24 to 30. Each of these levers has one end resting on the end 18a of the beam whilst its other end is pivotally supported by a pin 32 upstanding from and joined to the base 12. At a position intermediate its two ends, the table 14 rests on each lever 24 to 30 by means of downwardly directed pins 34.

With this arrangement, it is possible for the beam to be relatively long and so be capable of relatively large deflections under relatively low loads. Despite this however, all loads placed on the table 14 can still be transmitted via the levers 24 to 30 to the outer end 188 of that beam.

The electronic circuitry 19 which will now be described for converting signal from the strain gauges 22 and presenting an output reading at the number display 20 is accommodated within the body of the scales although it is not shown in detail. Similarly, dry batteries (not shown) for operating the electronic circuitry are also accommodated therein.

Referring now to Figures 3 and 4, a voltage is generated due to deflection of the beam and developed by the strain gauge sensor 50. The four strain gauges 22 are arranged in bridge formation and a voltage is applied across X and Y. The bridge is balanced by adjusting resistors R1 and R2 so that, with no load on the table 14, there is no voltage across or between N and S. However, upon deflection of the beam 14, the voltage between N and S is applied through resistors R5 and R6 to the inputs at pin 2 and pin 3 of an amplifier 52, and referred to as IC1 in Figure 4 and the difference is amplified. C1 and C2 are filtering capacitors for input lead noise.Capacitor C3 sets the frequency response of an integrated circuit IC1 and filters off any power supply noise. Resistors R3 and R4 and together with the control by resistors R1 and R2 constitute a balancing circuit which compensates for unbalance in the input circuit The analogue signal given out by circuit IC1 has to be converted into digital signal through the use of an analogue to digital converter. The conversion is in four basic stages, namely, an integrator 54, a comparator 56, an oscillator 58 and counters 60 and 62 using the dual slope technique. The control of ramping up and down is effected through the use of dual 'D' flip-flops 64 and 66 and analogue switches 68 and 70.

The integrator 54 has two inputs which control its output stage. One input receives the input voltage while the other input is a fixed reference voltage.

Resistor R9 and capacitor C5 constitute the time constant for the integrator. Resistor R10 damps the overshoot at the switching instant without sacrifying the linearity of the integrator. When the input voltage is higher than the reference voltage, the output ramps down, and vice versa.

Integrated circuit IC3A with its external components make up the comparator 56. The output is either at a high or low state depending upon whether the input voltage is greater or less than the reference voltage by an amount controlled by the feedback resistor R14.

The oscillator consists of a comparator circuit IC3B and several timing components. The oscillator is a free running clock which provides a continuous series of pulses to the decade counter 60 formed by integrated circuit IC6 and the four digits counter 62 formed by integrated circuit IC7. The frequency is set by capacitorC7 and resistor R20, R21, R22, and R23.

The circuits iC6 and IC7 make up a five decade counter. Only half of the dual decade counter circuit IC6 is used. The counter circuittlC6 functions as a divider and its output transition at counts 9-0 is fed to counter 62 (circuit 1C7) as a clock and the '0' flipflop 64 constituted by circuit IC4 as a synchronization control.

The counter circuit IC7 provides multiplexed decoded outputs for display 20 and driving circuit 76.

The display 20 consists of an array of four lights emitting diodes which is designed to accept mult plexed inputs and four driving transistors Q2 to Q5 for digit selection. The digit selection control Is provided by the counter circuit IC7 internally.

When a load is applied to the table 14, the beam 18 deflects and a change in the resistances of the gauges 22 cause an unbalance in the bridge configuration. The voltage generated across N and S is applied to the inputs of the amplifier IC1. The voltage difference is amplified and is applied to pin 3 of switch IC5B. At this time, switch IC5B is closed whilst switch IC5A is open. The amplified voltage passes through the switch IC5B and goes to the input at pin 2 of the integrator 54. The heavier the weight applied to the scale, the more positive the voltage will be.

Since the voltage at pin 2 of the integrator is more positive than the reference voltage applied at pin 3, the output starts to ramp down at a constant slope.

The oscillator 58 feeds a continuous series of pulses into counters formed by circuits IC6 and IC7.

At the moment that the output of integrator formed by circuit IC2 starts to ramp down, the counter starts count at 0. By the time the counters reach 59999 to 60000 transition, a positive control at pin 14 of circuit IC7 is coupled through capacitor C10 to pin 8 of the dual 'D' flip-flop constituted by circuit IC4A. The O output goes high thus closing switch IC5A and at the same instant the amplified output is disconnected from the input of the integrator circuit IC2. The input at pin 2 of the integrator circuit is now at a lower potential than the reference voltage, the integrator output then starts to ramp up.

The counter circuits IC6 and IC7 continue to count up from 60000 until the ramp output of the integrator circuit IC2 crosses the threshold at pin 4 of comparator IC3A. A positive pulse is then coupled to pin 10 of 'D' flip-flop circuit 1C4, thus switching the switches back to the original states. At the same time, the number of pulses counted in circuit IC7 are available at the output pins of circuit 1C7 and are held and displayed using a time-sharing (multiplexing) principle. The digit driving transistors Q2 and Q5 are aiternatively switched on and off by the control sign als from circuit IC7. The scanning speed is so high that the display appears to be stable.These displayed numbers are directly proportional to the input voltage at pin 2 of integrator circuit IC2.

The positive pulse from comparator circuit IC3A is also coupled through capacitor C8 to reset the flipflop circuit IC4B. This flip-flop circuit synchronises the output of the comparator circuit IC3A with the output pulses at pin 14 of the decade counter circuit IC6. The output Oat pin 1 of flip-flop circuit IC4B cannot go high until the clock goes high. The positive edge of the synchronised output at pin 1 of flipflop circuit IC4B is coupled to the latch enabling input of the counter circuit IC7.

The negative going portion of the output pulse of the comparator circuit IC3A is coupled through a capacitor C6 to open the switch circuit IC5C momentarily. This allows a positive reset pulse to clear the counter circuits IC6 and 1C7.

The strobe and reset pulse are of such short durations as compared to the display update rate, that the output of the integrator circuit IC2 hardly starts to ramp down before the counters are re-set.

As soon as the counters are reset, the output of integrator circuit IC2 starts to ramp down, and the cycle starts again.

When there is no weight on the scale, the above procedure is repeated. But in this case, a positive pulse is generated at the output of comparator circuit IC3A at the instant the counters count up to a value between 00000 and 00009. This can be achieved by control resistors R2 and R1 Adjustable resistor. R1 is used for fine control whereas adjustable resistor R2 is for rough zero control. As a result, the display 20 can be adjusted to show 000.0.

It is desirable to provide an indication of a low or failing battery condition since accurate results can only be given by the circuit described above in the eventthatthe reference is kept stable.

The circuit IC3C compares the battery voltage with the reference voltage set by the resistors R28 and R29. When the battery voltage becomes low enough, such that the voltage present at pin 6 is lower than the reference voltage, the output of the comparator circuit IC3C will go high, thus switching on the transistor Q6. This allows a larger current than normal to pass along this line 80 to the pin 10 of the display 20 and arranges in turn to switch on all the decimal points of the display 20.

The power supply is obtained from six 'C' batteries. The input voltage is stepped down to 5 volt by a regulator circuit IC8, which provides the power for the analogue to digital conversion. The output from regulator circuit IC8 serves as a base control to a transistor 01, from which the power to digital display 20 is obtained. The capacitor C11, C12, C13 and C14 are all filtering capacitors to stabilize the power supply since it is important that the 5 volt output which is used as the reference voltage be kept very stable.

Suitable integrated circuits for use as the circuits IC1 to IC8 are readily obtainable on the market from a range of suppliers and precise details of their design are believed to be well known in the art.

Claims (7)

1. Scales comprising a weight sensor in the form of a beam which is supported as a cantilever for deflection under the effect of the load to be measured, at least one strain gauge attached to the beam whose electrical resistance varies according to the extent of deflection of the beam, means for providing an analogue electrical signal related to the change in resistance of the strain gauge, means for amplifying that analogue signal, a counter to which the digital signal from an oscillator is passed, means for causing the counter to stop counting at a number proportional to the amplified analogue signal and a display for presenting the number counted by the counter.

2. Scales as claimed in Claim iin which there are four strain gauges two on each side of the beam, which are electrically joined in a bridge such that deflection of the beam whilst an excitation voltage is applied across the bridge, causes the bridge to assume an unbalance condition and provide a voltage output constituting the analogue electrical signal.

3. Scales as claimed in Claim 1 or Claim 2, in which the amplified analogue signal is fed to an integrator in which the analogue signal is compared to a reference voltage, the integrator ramping up to down upon whether the voltage of the analogue signal is below or above, respectively, the reference voltage, and the integrator providing an output which depends upon the result of the ramping up or down.

4. Scales as claimed in Claim 3, in which the output from the integrator is used to stop the counters from further counting of the oscillator signals.

5. Scales as claimed in any preceding claim, in which a number of levers are used to transmit the load from the weighing table to the end of the beam, the levers being supported at or near their ends by the beam and a base of the scales and receiving the load from the weighing table at a position intermediate their ends.

6. Scales substantially as herein described with reference to Figures 1 and 2 of the accompanying drawings.

7. Scales substantially as herein described with reference to Figures 3 and 4 of the accompanying drawings.