Electronics patent of the month: Inductance sensing circuit monitors scrap metal

Inductance sensing circuit for detecting any disturbance in a conductor

Michael Jaeger, patent attorney at leading UK patent and trade mark attorneys Withers & Rogers LLP, writes:

GB Patent Number: GB2495373
Granted to: Cresatech Limited

A couple of years ago some friends of ours were looking forward to moving into a house just down the road from us, but first they had to undertake some alterations and improvements. One night while this building work was taking place they had a visit from some uninvited handymen, who removed all of the copper piping from the house, no doubt to be sold on as scrap.

This was a distressing time for our friends, not least due to the damage caused and the cost of replacing the piping (this time around with plastic pipes) but also because of the delay it caused to their eventual moving in date.

We have all probably heard of similar stories. Metal theft has become widespread in recent years due to the rise in commodity prices.

In addition, the audacity and shamelessness of some thieves has been astounding; causing disruption to rail networks by removing valuable power lines and even desecrating war memorials. The value of metals is also affecting mobile networks in Africa, where telecoms operators have struggled to maintain wired networks against the plundering endeavours of shovel-carrying metal pirates.

No doubt this situation is something which motivated Cresatech Limited to file a patent application in 2012 for a metallic conductor disturbance detector device, which was granted on 23 October 2013.

In addition to detecting the removal of a metallic part, Cresatech’s invention also provides the benefit of being able to monitor the degradation of conductors due to corrosion, as well as detecting any accidental damage which may occur.

Cresatech’s invention includes an inductance sensing circuit for detecting any disturbance in a conductor, a filter circuit at the sensing circuit’s output and an alarm circuit.

A tuneable capacitor, forming part of the tuning circuit of the sensing circuit, is used to regulate or couple the device to the metallic conductor to be monitored. Additionally, a large-value resistor may be incorporated to add stability to the device in order to shunt a portion of the impressed signal to ground. This helps to limit the initial gain of the sensing circuit and prevent it from becoming saturated. Finally, transistors are used as amplifiers and oscillation circuits for the sensing circuit.

In use, an incoming signal of a given frequency and amplitude is mixed with a local oscillation produced by the transistor circuit. After further amplification the signal is fed into a pot with the wiper of the pot being connected to an LED. This is then used to give a visual indication of the operational status of the device. The device is set up so that the driver transistor is held about halfway between full on and off, thereby providing a null condition. By adjusting the tuneable capacitor of the tuning circuit and the gain of the pot, the device can be tuned to ensure it is in a sensitive state.

For a metal to be monitored it must have its own LC&R (inductance, capacitance and resistance), hence the conductor can then be considered to be a tuned circuit. When the metallic conductor is disturbed, either by damage, corrosion or removal, the change in voltage and amplitude of the impressed frequency can be detected, causing the alarm to be triggered.

Regrettably for Cresatech, however, other factors have caught up with its technological innovation. Firstly, the price of metals has fallen over recent months, making metal theft less common.

Secondly, as of 1 October 2013 the UK government has outlawed buying scrap metal for cash, thereby introducing traceability of every metal transaction so as to provide an audit trail and put off would-be metal thieves.

Furthermore, recent UK legislation specifies that scrap metal dealers must verify the identity of the person who is selling the metal, which will further hamper illegal activity. Nevertheless, since Cresatech’s invention can also be used to detect corrosion and damage of metals then no doubt their innovation will be rewarded in the future.

Michael Jaeger is a patent attorney at leading UK patent and trade mark attorneys, Withers & Rogers LLP.

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Tags: patent

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