GB2111684A - Transducers giving variable frequency outputs - Google Patents

Abstract

A transducer comprises a resonant circuit 11, 12, the resonant frequency of which is affected by changes in a variable input, the output comprising a variable frequency. The resonant circuit can comprise an inductance 11, if desired in the form of a temperature- compensated coil spring, a change in length due to a change in applied force, bringing about a change in frequency. The transducer can be used in a weighing arrangement, or for responding to changes in temperatures or relatively humidity. <IMAGE>

Description

SPECIFICATION Transducers This invention relates to transducers giving an electrical output indicative of a variable input.

A typical transducer for giving an electrical indication of applied force is a strain gauge, which comprises a metal filament which is subjected to an applied force to be measured so that its electrical properties are altered, the extent of the alteration being measured for example by incorporating the filament in a bridge circuit.

Generally speaking, the more sensitive or accurate the transducer, the more precisely must it be manufactured and the higher the cost. A delicate filament must be protected against overloading, and calibration of transducers in general use is often a fairly complicated matter.

The present invention provides a sensitive and accurate transducer, however, which can be manufactured in quantity at relatively low cost and which is, moreover, robust and, if desired, easily calibratable.

The invention comprises a transducer giving an electrical output indicative of a variable input comprising a resonant circuit, the resonant frequency of which is affected by changes in the variable input, the output comprising a variable frequency.

The resonant circuit may comprise an inductance, which may comprise a coil spring.

The length of a coil spring depends on an applied force (either a compression or an extension) and a change in length brings about a change in the inductance of the coil.

a change in inductance gives rise to a change in the resonant frequency of the resonant circuit.

A typical coil spring such as is already used in commercially available scales and balances can be incorporated as the inductance of an oscillator having a frequency of the order of 20MHz. It may then be arranged that changing the length of the spring by an applied force over its working range brings about a frequency change of the order of 1 MHz. It is found that with such an arrangement, the response is substantially linear.

The output frequency may be measured by counting the number of cycles over a given time interval. A microprocessor can be programmed during an initial calibration to take account of any non-linearity, and can provide for easy zeroing or taring prior to any weighing or other measuring operation.

Of course, variables other than force can affect the resonant frequency of a circuit. Two such variables are temperature and relative humidity, which might affect any capacitance in the circuit. On the one hand, the effect of these other variables should be reduced to an insignificant level if an accurate indication of force is required. This can be taken care of for example by using a temperature compensated spring, such as is common in scales and balances, or by suitable programming of the microprocessor and careful design of the electronics generally to avoid thermal drift. On the other hand, such other variables may themselves be measured by observing the change in resonant frequency of any circuit they affect.

One embodiment of a transducer according to the invention will now be described with reference to the accompanying drawing in which the single Figure is a diagrammatic illustration of a weighing scale incorporating the transducer.

The weighing scale illustrated diagrammatically in the Figure comprises a coil spring 11 to which a force equal or proportional to the weight to be measured is applied as shown by the arrow. This can be done in any conventional way, as by suspending the weight from the spring which is itself suspended from a fixed support, or by compressing the spring by a lever system.

The spring 11 is included in a resonant circuit which includes a capacitance 1 2 and a suitable transistor driving circuit 1 3. The frequency of the resonant circuit is measured electronically by a unit 1 4 which includes a fast counter and a clock and which counts the number of cycles of oscillation of the resonant circuit in a given time interval determined by the clock. The unit 14 also includes a microprocessor which converts the frequency thus measured into a weight indicated on a digital read-out 1 5.

Naturally, the microprocessor can also be adapted to perform other operations, such, for example, as multiplying the weight by a unit price, so as to give a price for the articles weighed, as is nowadays customary in scales for greengrocers and other traders.

It has been found in trials using a coil spring such as is commonly used in weigh scales that the response was subtantially linear as a load of 50 grammes was applied, the frequency changing by about 1.1 MHz from a lowest value of 29.4MHz, though above this loading there was a noticeable departure from linearity. Clearly, any system can be designed to operate in a linear part of the load/extension curve of the spring, or any non-linearity can be calibrated out in the microprocessor.

Zeroing or taring can also be accomodated in the microprocessor by providing, for example, a tare key that is operated to reduce the indicated load to zero whatever load is applied initially, further loading then resulting in tne added load only being displayed. This could be useful in industrial applications where goods are often loaded into containers of widely different weights.

The coil spring 11 will also oscillate mechanically, and this can be damped in what ever ways are customary in weighing applications. This notwithstanding, the time interval over which the electric oscillations of the resonant circuit are counted can be selected, having regard to the frequency of these mechanical oscillations, to maintain the digital display stable for observation of the applied load.

Claims (11)

1. A transducer giving an electrical output indicative of a variable input comprising a resonant circuit, the resonant frequency of which is affected by changes in the variable input, the output comprising a variable frequency.

2. A transducer according to claiml, in which the resonant circuit comprises an inductance.

3. A transducer according to claim 2, in which the inductance comprises a coil spring.

4. A transducer according to claim 3, in which the coil spring is incorporated as the inductance of an oscillator having a frequency of the order of 20MHz.

5. A transducer according to claim 4, in which it is arranged that changing the length of the coil spring brings about a frequency change of the order of 1 MHz.

6. A transducer according to any one of claims 1 to 5, in which the output frequency is measured by counting the number of cycles over a given time interval.

7. A transducer according to any one of claims 1 to 5, having electric zeroing or taring means.

8. A transducer according to any one of claims 1 to 7, adapted to measure force such as weight.

9. A transducer according to any one of claims 1 to 7, to measure a variable other than force which affects the resonant frequency of a circuit.

1 0. A transducer according to claim 9, adapted to measure temperature.

11. A transducer according to claim 9, adapted to measure relative humidity.

1 2. A transducer according to any one of claims 1 to 11, comprising a temperature compensated spring as an inductance in a resonant circuit.

1 3. A transducer substantially as hereinbefore described with reference to the accompanying drawings.