How to Test an Ultrasonic Probe?

Introduction

Ultrasonic probes are electronic devices that convert energy from one form to another through the process of transduction. Ultrasonic probes are commonly found in various devices like loudspeakers, microphones, and antennas. They are also used in control systems, measuring devices, and ultrasound machines.

However, before using a transducer, proper testing of the device is necessary to ensure data accuracy and prevent undesirable results.

In this article, we will take a look at various characteristics of transducers and the main methods of ultrasonic transducer testing.

What is Transducer Testing?

Testing a transducer is necessary to ensure that it is functioning properly. Transducers are tested before installation and also at periodic intervals. Each transducer has a datasheet that is used to check the performance parameters.

Note, no transducer is 100 percent efficient. Some of the energy is lost during the conversion process in the form of heat. Even so, some high-quality transducers approach the 100 percent efficiency mark.

For ultrasonic testing, transducers convert electrical energy into ultrasound energy and vice versa. The ultrasound waves have frequencies ranging above 20KHz and are beyond the human hearing range.

These sound waves are directed to a particular object. Once the waves strike the object’s surface, they are reflected back to the transducer. The transducer converts this reflected echo back to an electrical signal. This way, they can be used for measuring distances and loads.

In some applications, the process of through-transmission testing is used. Here two transducers are used as an emitter and a receiver. The emitter emits the ultrasound waves while the receiver gathers the waves from the other side of the object.

A major advantage of the process is the ultrasound waves are not affected by any background interference. Besides, they remain unaffected by dust, dirt, and humidity. Thus, these transducers work as sound-related sensors in various distance-measuring applications and medical applications.

Another common use of ultrasonic transducers is in the field of non-destructive testing. In this process, a particular piece of material is tested and evaluated for flaws, discontinuities, or differences in the material properties. Most ultrasonic testing is carried out within the frequency band of 1 MHz and 10 MHz.

There are a wide variety of transducers based on the arrangement of the piezoelectric crystals, frequency, and size. For ultrasonic testing, the common types of transducers used are angle beam transducers, contact transducers, delay line transducers, and immersion transducers.

What are the Applications of Ultrasonic Transducer Testing?

An ultrasonic transducer can be used to test a wide variety of materials including metals, ceramics, plastics, composites, and concrete. It is widely used in industries like aerospace, automotive, manufacturing, and construction.

Some of the major applications of Ultrasonic transducer testing are listed below.

They are used to check for defects in metals, Carbon Fiber Reinforced Polymer (CFRP), Glass Fiber Reinforced Polymer (GFRP), and other composite materials. These materials are effectively used as protective armors.
Detecting any irregular parts in various industrial equipment like hoppers and feeder bowls.
Detection of web breaks in materials like transparent films, glossy adhesive labels, and black parchment.
Testing vehicle components like ball bearings, steering columns, pistons, axles, shafts, and valves for defects like shrink-holes, cracks, and inclusions.
Thickness measurement to assess corrosion in pipework and other metal structures.

What are the Types of Ultrasonic Transducer Testing?

So, how to test an ultrasonic transducer?

Basically, transducer characteristics are carefully analyzed through waveform and spectrum data to ensure an accurate performance. In addition, there are some other forms of transducer testing methods that are followed.

Beam Alignment Measurements

These measurements provide information on the degree of alignment between the transducer housing and the axis of the sound beam. The data is useful for applications that need a high level of accuracy regarding the focusing of the beam on a mechanical surface.

Beam Profiles

Generally, a transducer is scanned across a surface placed at a fixed distance from the transducer face to generate a transverse beam profile. This profile can provide detailed information about the sound field characteristics of the transducer. It is also useful in determining the level of beam symmetry and focal spot size.

Beyond that, axial beam profiles are generated by recording the pulse-echo amplitude of the sound field. This profile is a function of the distance of the target object from the transducer face. The data gathered is useful in studying the depth of field and beam focal length.

Electrical Impedance Plots

This is an advanced method of transducer testing and is also termed as Bode Plots. Here, the impedance and frequency are plotted against each other. This provides details about the design and process performance of the transducer. A transducer with good performance conforms to the linear pattern of an ideal model generated from Finite Element Analysis.

How do you Test Ultrasonic Transducer?

The following are some of the common methods for ultrasonic transducer testing.

An initial check of the physical dimensions of the transducer.
Electrical measurements are taken to check the capacitance and loss of the device after assembly. In addition, tests for continuity, opens, shorts, etc. are also conducted.
Next, functional testing is conducted through pulse-echo measurement. Various aspects like ring-down, bandwidth, and sensitivity are recorded and checked at this stage by using a calibration block. In some cases, the Total Acoustic Power output (TAP) is also measured.
The generated pulse and the corresponding return signal are viewed through an oscilloscope to check the overall quality of the waveform. Numerical measurements are also done for a thorough analysis.

What Characteristics are used to Evaluate Ultrasonic Transducer Testing?

The acoustic and electrical characteristics used to evaluate transducers are guided by the ASTM E1065- Standard Guide for Evaluating Characteristics of Ultrasonic Transducers.

Take a look at the main parameters listed below:

Frequency Response

Frequency response is an effective way to measure the accuracy of transducers, especially pressure transducers. It measures how fast the transducer can respond to pressure changes. Generally, it is measured by using one of the two processes- shock excitation and sinusoidal burst.

Relative Pulse-Echo Sensitivity

Sensitivity is a vital parameter that throws light on the energy conversion efficiency and the overall performance of a transducer. The method used is a sinusoidal burst procedure to generate a specific value from two parameters. These are the amplitude of the voltage applied to the transducer and the amplitude of the pulse-echo signal reflected from a specific target.

Time Response

This characteristic describes the radio frequency response of the waveform generated from a specific target. It is measured by a pulse-echo procedure. A fast response time indicates good performance.

Complex Electrical Impedance

The electrical impedance of a transducer affects various parameters like noise performance, driving response, bandwidth, and sensitivity. It is usually measured by using commercial impedance measuring instruments under laboratory conditions. In some cases, a method using the sine wave correlation for signal amplitude and phase extraction is also used.

Sound Field Measurements

These measurements are taken to establish the on-axis and transverse sound beam profiles of a transducer. One common process for measuring this is by using the point reflector and hydrophone measurement method. In this case, the sound fields are scanned and mapped in three-dimensional space. Another method is the use of laser interferometry to measure the radiating surface motion of the transducer.

Conclusion

Ultrasonic transducers can be used to test or measure a wide variety of materials. Moreover, the results are not affected by environmental conditions. Transducers also deliver highly accurate results that make them perfect for precision jobs.

To get reliable results, proper ultrasonic transducer testing and periodical calibration of the transducers are necessary. For this reason, understanding the primary characteristics of transducers and their testing is extremely important in all relevant industries.