Uncommon sense

A sensor that will allow doctors to measure the electrical activity of patients' hearts without touching them could also be used for more accurate non-destructive testing of carbon composite aircraft parts and microchips. Engineers from the

have developed prototypes for these applications using electric potential sensors (EPSs).

Similar devices, which measure magnetic fields, already exist in medicine and industry. However, the EPS offers a non-invasive way of measuring lesser-explored electrical fields, present wherever there is electrical activity. It works like a remote electrocardiogram, which is used to record the electrical activity of a patient's heart.

The system consists of a single sensor, a three-axis computer position controlling scanning system and signal processing hardware. It measures the electrical conductivity above the surface of a material sample by the application of a small AC current. The sensor scans the surface to measure the conductivity and location of possible defaults. If fibres in the material are damaged, the sensor will detect an electrical resistance in that area and a colour change will be displayed on the computer image screen.

The researchers have proved the EPS method can differentiate between damaged and undamaged parts of a carbon fibre sample and provide spatial information about the location of the damage. They believe their method will be complementary to thermography and acoustic methods, developed for use with composites such as those in aircraft parts.

In most cases, damage to composites is the result of a heavy impact or excessive loading, which leads to delamination and reduced conductivity in the damaged area.

The researchers say the sensors can detect resistance even through an insulating surface layer. This, they claim, makes EPS ideal for carbon composite materials, where the carbon mat is embedded in an insulating matrix. In tests, the system has proved capable of yielding local conductivity information for a range of materials less well suited to magnetic field measuring techniques because of their poor electrical conductivity.

Magnetic methods such as eddy current testing work well with good conductors such as metals but are less effective for materials with lower electrical conductivity.

Other testing techniques such as ultrasonics, radiological and surface inspection methods also have drawbacks. Ultrasonics, which use a pulse echo technique to monitor reflected signals, normally require a liquid coupling medium between the sensor and the sample. Radiological methods can give good information but are hazardous and contrast can be poor. Surface inspection techniques, which include infra-red thermography, are restricted to near-surface fault detection.

Non-contact electric field sensing is only recently being applied to non-destructive testing of materials because inventors have spent 10 years developing a sensor appropriate for the task.

lead researcher Robert Prance said EPS technology will soon be ready for commercial applications such as testing faults in microchip circuitry, stainless steel, carbon fibre composites and aircraft parts. It could also soon help enhance MRI scanning techniques in hospitals.

'More aircraft components are being made of composite materials like carbon fibre, and the number of methods you can use to test carbon fibre are fewer than metal structures because we haven't been doing it for so long,' said Prance. 'Our sensors can now be used for that purpose. We're testing them in the laboratory with carbon fibre samples. It's in an early stage but it's looking quite promising.'

Prance said the researchers are working on small sensor arrays but can scale them up for large applications like the testing of aircraft parts. All they need is a commercial partner.