Airborne sound measurements

Distance measurements with airborne sound use the time-of-flight of the echos pulses reflected from the surface to determine the distance to the surface. Due to the fact, that the sound velocity is very small, even resolutions of 0.001mm are achievable with moderate technical effort. Normally, environmental conditions prevent the conversion of this resolution into accuracy. To reach a high accuracy a careful analysis of the measurment problem is necessary, as well as the application of error-preventing devices.

For thickness measurements both sides of an object have to be taken into consideration. There are two different methods. A distance measurement is made to the two surfaces of the test object (upper and lower), this value is then subtracted from the distance between the two opposing sensors (the reference distance without test object) thus giving the thickness value. This method is used by the OP-US 2. The other method measures the height of the test object relative to the support plate (e.g. a conveyor belt) in two successive measurements using one sensor. This method is used by the OP-US 1 .



Measuring thickness with OP-US 1

In the picture above an example of such a measurement is sketched (see example) . As the reference surface the conveyor belt is selected on which, for example, boards are transported. As long as no board is in the measuring area, the distance to the conveyor belt is measured and stored as the reference value. If a board appears, detected by a step in the measured values, the instrument switches to height measurements. The new distance is reference distance minus thickness. By subtraction of the stored reference value the thickness is computed, displayed and transmitted, if necessary, to an external computer. With these values the control of successive machines is updated, sorting processes are startet or protocolls for quality control are serviced. The number of individual measurements, which are averaged to one transmitted value, can be adjusted. The process restarts, when the board has left the measuring area.

- The sound field at the reflection area, i.e. the measuring point, has a diameter of approx. 20 mm. Over this area the distance is averaged, but dust, chips, fibres or grain on one side and pittings, small holes or cracks on the other side are strongly suppressed in the averaging process. This is due to the fact that the wave length of airborne sound is approximatly 3 mm and for all reflectors with a dimension below one wavelength the crossection for the reflection is very small. Therefore the ultrasound method has advantages over a measurement using mechanical contact in an industrial environment.

- Instrument measurements are contact-free and made from positions remote from the automatic production process. However, if mechanical damage occurs, or if the system has been dismounted, any deviation influences both the distance value and the reference value. By taking the difference to compute the thickness, the deviation is cancelled. The continuous reference measurements effect good long term stability. Recalibrations are normally not necessary.

- The measuring range extends from approx. 1.5 mm to the distance of the sensor. The accuracy is constant in this range.

- The surface of the sensor slightly vibrates. This prevents the sensor from becoming dirty.

- A small additional reference echo is generated in the sensor to remove temperature or other environmental effects.



This simple measurement of thickness gives precise values only if the complete board is seated on the conveyor belt Any gap between board and conveyor belt produces a sytematic deviation. Elastic belts, which equalize local error sources are advantageous. We recommend the previously described method, using two sensors, if deformated boards are to be measured.



The described thickness measuring system has a accuracy of approximatly 0.1 mm and a measuring range of up to 150 mm. To achieve higher accuracies, (accuracies up to 0.01mm are feasable), additional technical devices should be used.



Thickness with the OP-US2

The thickness measurement method used by the OP-US 2 for boards and other components is a known method for thickness gauging. Two sensors, fixed in a known distance, each measure its distance to the surface of the object. The difference of the fixed distance minus the sum of both individual measurements gives the thickness of the object. (see example)





Using airborn ultrasound offers some unique features:

- The method is contact-free. Mechanical damage of the surface of the product is therefore excluded. Problematic surfaces (see last page foto) or structured surfaces can be measured at high production velocities. And damage by the measuring device in the production process is widely excluded. - It will make no difference, where the board passes the measureing gap, because its thickness is the same every-where in the gap. Imperfect conveyor systems, vibrations up to very high frequencies and amplitudes (in fact we never found a vibration impact on the measured values) or nonplanar objects will not affect the measured thickness. This feature is due to the excellent measuring range of airborne ultrasound

. - The interface airborn sound field / surface of the board is advantageous for industrial measurements. In all reflection processes, reflectors with dimensions smaller than one wavelength of the sensor are suppressed by the reflection physics. The smooth surface will dominate effects due to dust or chips, because the wavelength of the ultrasound, we normaly use, is 3 mm

. - Ultrasound is generated by membrane oscillations. Even in dirty environments there is normally no decrease in sensor efficency.

- The measurment range of ultrasound from the sensor is unlimited. Therefore a favourable feature is, that reference measurements can be taken, if no board is in the measuring gap. By this reference measurement, the distance of the sensors i.e. the base for the thickness measurement is determined. By periodic reference measurements all error sources due to temperature, other environmental effects or mechanical deviations are eliminated, leading to long term stability of the system. This periodic update of the sensor distance is supported by OP-US 2 software.

For an optimal set-up of a thickness measurement the use of the instrument depends not only on then momentary measuring effectivness, but also on the handling of compensation procedures, timing matching, noise suppression and correction measurements. Most of the achievable accuracy and stability depends on these internal procedures. A lot of the computer power is spent on accompanying procedures to stabailize against statistical and systematical error sources.  

The measured values in this set-up are the air gap distances between sensor and surface. The achievable accuracy depends of course on the length of this distance. A high accuracy can only be expected with small air gaps. Contrary to dry dust, drops of oil are a problem for the sensor efficency and should be kept away from sensors by using fans and/or mirrors. Normaly the protective filter has to be changed twice a year if the sensors are used where there is a lot of oil dust as in metal-working industry and where there is no filtered air flow. A certain amount of sensor degradation is electronicaly compensated and is tolerable.

With air gaps of 100 mm, the achievable accuracy ist 0.1 mm without any additional precautions under normal industrial conditions. Higher accuracies need improved guidance of the measured objects and controlled air gaps.



Thickness of steel coils

Measuring the thickness of steel coils is not only a measurement made under laboratory conditions but made in an environment with high mechanical stress, high electrical noise and high production throughput. Additional mechanical elements, electronic parts and additional software are neccessary to operate a user-friendly measuring system. In the production, the steel strip is continuously guided through the gap of the thickness measuring system. To have a two dimensional grid of measured points over the coil you have to use a scanner for the lateral dimension and a running wheel, or an external transducer for the longitudinal coordinate.(see examplel)

If necessary, the measuring system can be protected by a screen, triggered by software or a photo-electric barrier. This is advisable at the end of the coil where the steel can be bend by internal stresses to such an extent, that it would damage the sensors. The ultrasound electronic and software is similar to the thickness meter OP-US 2. Again reference values are continuouesly measured by scanning beyond the steel plate. The scanner is controled by a PC and can be operated fully automatically or manually via a menu system. The job description of future jobs is stored in a database. If a new coil is mounted, the scanner is checked and started. The actual thickness cross section is displayed on a monitor and evaluated in a tolerance monitor, according the job description.

The tolerance monitor consists of 32 individual channels (width, upper-, lower limit) covering the width of the strip. For each channel in the tolerance system a trend-plot can be displayed or printed for documentation.The measuring process is supported by a lot of automatic software programs such as adjustment procedures, learning functions, log-book...

Advantageous features
- The measured value is independent of the position of the steel strip in the measuring gap. Rapid variations of the position, due to high process velocities or strongly bended material, will not affect the result.
- The measured value is indepent of the surface structure and does not depend on the material.

Airborne ultrasound is also reflected from oil surfaces. Normaly oil films are less than 0.01 mm and will not affect the result.





Microfone for ultrasonic frequencies

The sensor OP-US S3 can be used to survey high pressure pipes and to monitor the beginning of a leackage. The emergent air or liquid of a leackage is always turbulent and a source for pressure waves in the air. The frequency of these sound waves is predominant in the ultrasonic area, because a beginning leackage is always very small. (intuitive : the sound of a pipe is as higher as the smaller the pipe is.)

In an industrial environment there exist always airborne sound sources due to mechanical contacts everywhere. Different to a leackage, these sources generate sound only for a short period and are not correlated in time. For this reason the sensor OP-US S3 is sensitive to correlated waves and suppresses statistic generated noise from the industrial environment.

The sensor scans the frequency interval between 20 kHz and 100 kHz and reports alarm, if a airborne sound source is detected repeatedly.

A test transmitter with variable output is provided to check the functional integrity of this surveyance system.



Force inmpact with ultrasonic vibrations

For tribological investigations of wear and for the characterization of surface and surface layers, the US-Tester is used. A hard metal ball is exited to vibrations and increases the wear of the mechanical contact. Because the additional impact increases with frequency, the frequency of the vibration is favourable in the ultrasonic region. Investigations with additional vibration loaded tools are closer to normal use in industry.


The dramatic influence of vibrations on the mechanical contact of the hard metal ball with a high technologic hard coated substrate is seen in the microscope pictures.

(- no ultrasonic vibration)

(- with ultrasonic vibrations).