GB2051375A - Measuring speed of moving webs - Google Patents

Abstract

A signal representative of the speed of a moving web e.g. conveyor belt comprising a material (e.g. PVC) which polarises when subjected to an electric potential is produced by passing the belt between two electrodes (3a, 3b) positioned on opposite sides of the web; and producing a signal representative of the capacitance formed by the electrodes and the web. An A.C. potential (7) of e.g. 2.5-5 Hz. is applied between the electrodes which form part of an A.C. bridge circuit 3, 4, 5, 6 which is adjusted so that under static conditions of the belt the bridge output is a minimum. As the belt moves the bridge output increases and is amplified and detected at 8. Alternatively the phase of the output of a suitable bridge circuit incorporating the plates 3a, 3b may be detected. <IMAGE>

Description

SPECIFICATION Speed measurement This invention relates to the measurement of the speed of moving webs. The invention finds particular application in the measurment of the speed of conveyor belts.

Existing means of measuring the speed of conveyor belts are of two types: contacting and non-contacting. Accurate and repeatable speed measurement is frequently not possible with the contacting type due to slip between the measuring device and the belt and with the non-contacting type due to the difficulty of "marking" the belt in some way, e.g. magnetically or optically.

The environent and the type of material used in the construction of the belt also introduce further problems, particularly where materials such as rock, coal, gypsum, etc. are transported on the belt. Such materials themselves tend to contaminate contacting type measuring means and cause malfunctioning and frequently physical damage.

One solution to this problem would be to introduce small metallic members into the belt at regular intervals during manufacture, which could in use of the belt be sensed by suitable metal detectors. Such a solution, however, could not easily be applied to belts already installed.

The present invention seeks to provide a method and an apparatus for use in measuring the speed of a moving web, e.g. a conveyor belt, wherein the above mentioned disadvantages may be overcome where the web consists of certain material.

According to a first aspect of the present invention a method of producing a signal representative of the speed of a moving web comprising a material which polarises when subjected to an electric potential comprises: forming a capacitance using part of the belt as dielectric; applying a potential across the capacitance; and producing a signal representative of the capacitance.

According to a second aspect of the present invention an apparatus for producing a signal representative of the speed of a moving web comprising a material which polarises when subjected to an electric potential comprises: two electrodes arranged ta be positioned on opposite sides of the web; and signal producing means for applying an electric potential between the electrodes and for producing a signal representative df the capacitance formed by the electrodes and the web.

Preferably the signal producing means comprises a bridge circuit having two resistances and two capacitances, one of which capacitances is formed by the electrodes and the web, two opposite terminals of the bridge being connected to a source of electric potential and the output of the signal producing means being derived from between the other two terminals of the bridge.

Preferably the electric potential is an alternating potential.

Where the web material is PVC or a material having similar polarising characteristics, the alternating potential suitably has a frequency in the range of substantially 2.5-5Hz.

Preferably the apparatus further comprises detector means arranged to recive the signal representative of the capacitance formed by the electrodes and the web and to detect the speed of the web.

Preferably the apparatus further comprises indicator means connected to the detector means to indicate the speed of the web.

The invention lies in the inventor's appreciation that a hitherto undesirably regarded property of PVC, from which material many conveyor belts are made, to polarise when subjected to an electric potential, may be used in measuring the speed of a web of such material. If a web of the material is subjected, at some fixed point along its path, to an electric potential the material will tend to polarise.

The degree of polarisation will depend, for a given potential, on the amount of time for which the material is subjected to the potential, i.e. will depend on the speed of the web. If the material is then used as the dielectric of a capacitance, the value of the capacitance will vary with the speed of the web. Hence a signal which is representative of the capacitance will be representative of the speed of the web.

One method and apparatus for measuring the speed of a PVC conveyor belt in accordance with the invention will now be described with reference to the accompanying drawings in which Figure 1 shows a perspective view from above of part of the apparatus is use with a PVC conveyor belt; Figure 2 shows a cross-sectional view along the line X-X in Figure l:and Figure 3 shows the apparatus in diagrammatic form.

Referring firstly to Figures 1 and 2, a conveyor belt 1 made from PVC is moved by rollers 2.

Between two adjacent rollers on one side of the belt there are arranged two metal plates 3a, 3b.

The plates are located one above the other so as to "sandwich" between them part of the edge of the conveyor belt 1. The plates 3a, 3b, are, of course, stationary and are spaced from the belt 1 so as not to impede the movement thereof.

Referring now also to Figure 3, the plates 3a, 3b, form the electrodes of a capacitance 3 with the belt 1 therebetween. This capacitance is connected in a bridge circuit with a reference capacitance 4 and two resistances 5 and 6, the resistance 6 being adjustable to allow the bridge to be balanced. To two opposite terminals of the bridge an alternating voltage source 7 of low frequency (i.e. a fixed frequency in the range of, for example, 2.5 to 5 Hz) is connected. An output signal derived from between the other two terminals of the bridge is applied to amplifier/detector means 8 which drives an indicating device 9.

It will be appreciated that as the conveyor belt 1 begins to move, the value of the capacitance 3 will increase, since the mean time for which a given portion of the PVC material of the belt 1 remains between the electrodes 3a, 3b will decrease, thus decreasing the time for polarisation. Hence the faster the belt moves the nearer the dielectric constant of the PVC material between the electrodes 3a and 3b will be to its unpolarised value.

The bridge circuit is adjusted so that under static belt conditions, the amplitude of the output signal from the circuit, which signal is representative of the value of the capacitance 3 and so representative of the speed of the conveyor belt 1 , is minimum. As the conveyor belt 1 moves, the capacitance 3 changes and the amplitude of the output signal from the birdge circuit increases.

The amplifier/detector means 8 produces a d.c.

signal representative of the amplitude of the output signal from the birdge circuit, and hence of the speed of the conveyor belt 1. The indicating device 9 provides a visual indication of the magnitude of the d.c. signal and hence of the speed of the conveyor belt 1.

In other arrangements in accordance with the invention belt speed may be detected by monitoring the phase of an output signal produced by a suitable bridge or other network incorporating the capacitance provided by the plates 3a and 3b and the belt 1.

Claims (9)

1. A method of producing a signal representative of the speed of a moving web comprising a material which polarises when subjected to an electric potential, the method comprising: forming a capacitance using part of the belt as dielectric; applying a potential across the capacitance; and producing a signal representative of the capacitance.

2. An apparatus for producing a signal representative of the speed of a moving web comprising a material which polarises when subjected to an electric potential, the apparatus comprising: two electrodes arranged to be positioned on opposite sides of the web; and signal producing means for applying an electric potential between the electrodes and for producing a signal representative of the capacitance formed by the electrodes and the web.

3. An apparatus according to claim 2 wherein the signal producing means comprises a bridge circuit having two resistances and two capacitances, one of which capacitances is formed by the electrodes and the web, two opposite terminals of the bridge being connected to a source of electric potential and the output of the signal producing means being derived from between the other two terminals of the bridge.

4. An apparatus according to claim 2 or 3 wherein the electric potential is an alternating potential.

5. An apparatus according to claim 4 wherein the alternating potential has a frequency in the range of substantially 2.5-5 Hz.

6. An apparatus according to any one of claims 2 to 5 further comprising detector means arranged to receive the signal representative of the capacitance formed by the electrodes and the web and to detect the speed of the web.

7. An apparatus according to claim 6 further comprising indicator means connected to the detector means to indicate the speed of the web.

8. A method substantially as hereinbefore described with reference to the accompanying drawings.

9. An apparatus substantially as hereinbefore described with reference to the accompanying drawings.