GB2056080A

GB2056080A - Capacitance method for measuring movement

Info

Publication number: GB2056080A
Application number: GB8023293A
Authority: GB
Original assignee: OPTIBAL HOLDINGS Ltd
Current assignee: OPTIBAL HOLDINGS Ltd
Priority date: 1979-07-18
Filing date: 1980-07-16
Publication date: 1981-03-11

Abstract

A capacitance method for measuring movement in which the weight of a load is balanced against a reference weight comprising transmission of an electric signal along a wire, introducing a signal to a second wire, introducing a variable gap through which the signal passes, passing the signals to a bridge circuit and comparing the signals and visually indicating the variation in width of the variable gap. <IMAGE>

Description

SPECIFICATION A capacitance method of measuring movement This invention relates to a capacitance method of measuring movement and is particularly applicable for weighing apparatus.

Mechanical designs have long been known for the transmission of force applied to give a linear movement to a mechanical structure. This mechanical movement for weighing mechanism is then displayed on an indicator or chart using mechanical, optical, or electronic means and calibrated so that a given amount of movement corresponds to a value expressed in units of weight as a usual display.

Although the basic method of obtaining a mechanical response to a force applied has been developed to the point where great accuracies are possible it has often proved difficult to convert this movement accurately into a satisfactory display of weight. Weighing mechanisms are now available which will respond with great accuracy and precision to the weight applied and modern technology makes it desirable to express this force applied in terms of weight indicated by modern digital display. The means of converting the mechanical movement to an electronic signal is conventionally achieved using Linear Differential Transformers, Strain Gauge Devices or via optical systems converted to electrical signals. Other systems are based upon tension changes in vibrating wire strings and giroscopic principles etc.

The object of the invention is a method of converting movement into an electrical signal which is both accurate and cheap to produce. As far as is known the principle is not being used commercially in any field and although it has many possible areas of application the technique will be described with reference to the field of weighing apparatus.

According to the invention a capacitance method for measuring movement in which the weight of a load is balanced against a reference weight comprises transmission of an electric signal along a wire, introducing a fixed gap through which the signal passes, transmitting a signal to a second wire, introducing a variable gap through which the signal passes, passing the signals to a bridge circuit and comparing the signals and visually indicating the variation in width of the variable gap.

A simplified explanation of the principle can be set out as follows: an electrical signal is transmitted along a wire into which has been introduced a fixed gap across which the signal passes. This signal is taken as a reference signal.

From the same source a signal is transmitted along a second wire and passes across another gap which is variable. The two signals, one which has passed across the fixed gap and one which has passed across the variable gap are then compared and the difference in value obtained is directly proportional to the variance introduced across the moveable gap. The fixed gap is provided by means of a fixed capacitor valve and the moveable gap is taken from the difference in distance between two metallic plates of certain geometrical dimensions according to the particular weighing apparatus and its capacity and the movement of which is governed by the response of a weighing mechanism apparatus with respect to the force applied.

The invention will be described with reference to the accompanying drawings.

Fig. 1 is a diagrammatic circuit diagram of a simplified circuit.

Fig. 2 is a circuit diagram of a bridge circuit.

Fig. 3 is a schematic diagram of the circuit.

Fig. 3a is a detail view showing the balance diagrammatically.

As shown in Fig. 3 a known reference weight A is balanced against a load B, on the weighing mechanism load B is attached to a plate b' directly above a similar stationary plate b fixed to the base of the weighing mechanism with an air gap between the two plates. These two plates form a capacitor which is connected as one capacitance leg CV of a Scherering bridge (C). The other capacitance leg of the bridge is formed by a predetermined value fixed capacitor CF.

The size of the air gap between b and b' is adjusted so that the two capacitance values of the bridge are equal.

Any change in load B will give a turning movement which will alter the gap between b and b' therefore altering its capacitance value.

This will cause the phase angle of the oscillations WO to change at W1.

The phase angle and frequency of WO and W1 are compared by the phase phase comparator P of the phased locked loop (hereinafter referred to as PLL) which gives a DC output, linear to the difference WO--W1. This DC voltage drives the weight display D which can be calibrated to read the unit values of weight applied by means of which the gap distance is changed. It also feeds back to the control input of the VCO which alters WO, which brings WO and W1 back into synchronization.

The basic concept is a modified form of Scherering bridge which was originally devised in order to measure capacitance by a comparison method. This principle was established by Scherering many years ago but has not been applied commercially as far as we are aware.

Modern chip technology enables the circuit to be built economically and a more detailed description is shown by the diagram given as Fig. 2.

The method of detecting the difference between the two signals is also of prime importance and is based upon the Phased Locked Loop Principle.

The basic principle of PLL operation can be briefly explained as follows: -- with no signal input applied to the system, the error voltage Vd is equal to zero. The voltage control oscillator operates at a set frequency WO, which is known as the free-running frequency. When an input signal is applied to the system, the phase comparator P compares the phase and the frequency of the input with the VCO frequency and generates an error voltage Ve(t) that is related to the phase and the frequency difference between the two signals. This error voltage is then filtered, amplified and applied to the control terminal of the VCO. In this manner, the control voltage Vd(t) forces the VCO frequency to vary in a direction that reduces the frequency difference between WO and the input signal. If the input frequency W1 is sufficiently close to WO, the feedback nature of the PLL causes the VCO to synchronize or lock with the incoming signal.

To summarize, the Scherering principle is not very well known and where applied is known as a method of measuring capacitance. We believe that the uniqueness of our application is in using the Scherering principle as a means of measuring a variable gap CV. Aithough the PLL system is well established and known we believe that the use of the method to measure a change in capacitance is also unique in the context of a weighing application.

Claims

1. A capacitance method for measuring movement in which the weight of a load is balanced against a reference weight comprises transmission of an electric signal along a wire, introducing a fixed gap through which the signal passes, transmitting a signal to a second wire, introducing a variable gap through which the signal passes, passing the sigrials to a bridge circuit and comparing the signals and visually indicating the variation in width of the variable gap.

2. A method as in Claim 1 in which a phase locked loop principle (PLL) compares the electrical angle of the variable signal and expresses the difference as a D.C. output proportional to the difference in angle.

3. A method as in Claim 1 in which the Scherering bridge is combined with the PLL system to measure a change in capacitance.

4. A capacitance weight measuring apparatus for carrying out the method of Claim 1 comprising a reference weight balanced against a load, the resistance of the capacitance of the legs of a Scherering bridge being balanced at zero when the weights are equal.

5. A method as hereinbefore described for measuring movement.

6. A capacitance weight measuring apparatus pad with reference to the accompanying drawings.