Low Voltage Coded Excitation for Intrinsically Safe Environments
By G Asfis
Ultrasonic testing (UT) is an important non-destructive testing (NDT) method, that is widely used in nearly all industrial sectors. However, traditional high-voltage UT equipment cannot be used in some hazardous flammable and explosive atmospheres as it cannot achieve the relevant certification (ATEX). The main reason conventional UT equipment is unsuitable for these environments is the high voltage used by the pulsing circuitry, which has the potential to create a spark in case of a failure, and enough energy to ignite a gas. A higher pulsing voltage improves the capability of the inspection technique by increasing the signal to noise ratio (SNR) and the penetration depth of the acoustic waves. Attempts to reduce the pulsing voltage lead to noisy and unusable results since the resulting ultrasonic energy inserted by a single pulse is very low and the signal attenuates very quickly.
This report investigates the use of long, complementary code excitation pulse trains to improve the SNR of low voltage ultrasonic testing. Coded excitation refers to the technique of using a series of pulses following a calculated width and pause structure. The result of such an excitation is a long pulse train whose reflections look like noise and must be "decoded" or reconstructed in order to get a meaningful signal.
The long duration of the pulse trains allows a significant amount of ultrasonic energy to be transmitted into the material. Thus, if the pulsing voltage remains unchanged, this technique is suitable for testing highly attenuative materials, since the maximum amount of energy is not limited by the amplitude of the pulse.
The technique also has drawbacks. The complexity of the electronics required is higher than that which exists in conventional NDT equipment. Very few commercial off-the-shelf systems exist. At the time of writing only one system is available for sale, and its cost and size limit it to laboratory applications only. Additionally, because longer pulses are used, the transducer dead zone is increased, which may require the use of longer waveguides. Most importantly, the maximum achievable resolution is of significant interest, since this may affect defect detection capabilities.
- A low voltage 4-bit coded excitation can match the performance of a high voltage unipolar pulser. Similar performance can be achieved with even lower voltages if higher bit lengths are used.
- A 24dB increase in receiver signal amplitude was achieved using a 20-bit Golay pulse pair. Further improvement in SNR is expected with higher bit lengths.
- The impact on resolution is small. Use of a 16-bit code only doubled the width of the processed received pulse.
Golay code pairs with 4-bit length.
Four-bit 10ns pulse train width suitable for a 50MHz ultrasonic wave.