Advantageous characteristic features of airborne
- contact-free measurements
- no limits on the linear range, favourable measuring error to distance relation
- possibilities for self-calibration
- supression of dust effects on the surface or in the transmission gap
- non-complicated and easy to use
Some basic physics on airborne ultrasound.
Ultrasound is similar to the well known sound, except for its frequency, which is usually above 20 kHz. Therefore ultrasound can only be handled by using ultrasound sensors, which transform the pressure vibrations into electrical signals and vice versa. By nature, the laws for ultrasound are the same as the laws of electromagnetic waves, except its propagation velocity is millions of times smaller. Because of this small sound velocity, time-of-flight measurements lead to high space resolution using only moderate electronic effort. In one microsecond airborne sound propagates 0.33 mm. If you transmit an ultrasound pulse which is reflected by a wall, the same as a mirror, the distance to the wall can be measured by measuring the time-of-flight. In the following, electronic instruments are described which measure the distance with a resolution of 0.01 mm or better. Due to the small sound velocity, interference effects are already seen at larger dimension than we would expect from optics. Interference effects lead to deviations from the laws of geometrical optics. Airborne sound with a frequency of 100 kHz has a wavelength of 3.3 mm. Our sound generators have a diameter of approx. 9 wavelengths. Therefore the sound is distributed not only in the forward direction normal to the surface, but also in a cone of approx. 13 degrees.
Interference effects also limit the possibility to focus the ultrasound to
a spot size smaller than half of a wavelength (this is approx. 2 mm at the above
frequency). Reflectors with a diameter smaller than one wavelength reflect ultrasound
waves with smaller amplitudes than they should corresponding to their area.
Once again, this is a diffraction effect which has an advantageous influence
on the ultrasound distance measurement. Small particles, such as dust, chips
, sand , droplets on a surface are suppressed in the averageing reflection mechanisme
which produces the reflection echo. You will find that distance measurements
with ultrasound produce more reliable information, than contacting measurements
in an industrial environment.
Generation of ultrasonic waves
Air as a medium has a very low mass and a very high compressibility. High
pressure amplitudes on the one side and high detection sensitivity on the other
side for high frequencies can only be achieved by using a matched sensors with
small masses. In our sensors we use thin metalized plastic foils which are excitated
by electrostatic attraction. Incoming ultrasonic pressure waves deform the foil,
poduce a capacity change and generate the electric signal. To keep the sensor
free of additional masses, the membran should be prevented from splash water
or oil by using deviation mirrors, front tubes or fans. The active transmitting
area of the sensors has a diameter of 30 mm (50 mm). The diameter of the sound
field, which corresponds to the measuring spot of the sensors, is approx. 20
mm (33 mm) and increases according the diffraction laws.
. Near the sensor, the sound field can be focussed with the help of reflection paraboloids to smaller diameters. With a focused transducer smaller parts or structures can be measured.
The sound velocity in air depends on different parameters, whereby the temperature dependence at 2 % per 10 degrees temperature change is the most important. Because this fully affects the distance measurement precise measurement precautions have to be made.
The best method to compensate temperature effects is to use a reference measurement
having a fixed and known distance and with environmental conditions as close
as possible to the distance measurement. Using this reference, nearly all error
sources can be eliminated. The sensor OP-US S1 has an internal reflector (approx.
20 mm distance) which can be used for this compensation. If long distances are
measured, the extrapolation from the internal reference to the conditions of
the long distance will become doubtful. In this case we recomend to use far
distant references, e.g. the support or the conveyor. Using thickness meters
it is possible to use the distance of the sensors as a reference. This can be
directly measured during the time no object is between the sensors. Both reference
measurements are supported by the software and will give, if used periodically,
long-time stability. For precision measurements it is recomended to homogenize
the air in the transmission path, using fans or compressed air. The diameter
of dust or particles in the transmission path is usually smaller than one wavelength.
It will not affect the measurement. External ultrasound sources in an industrial
environment will normally be excluded from the measurement because they are
not correlated with the measuring process. These random echos will be suppressed
by tolerance considerations.
Due to diffraction the diameter of the airborne sound spot increases with increasing distance. The receiver diameter remains constant, therefore the received signal decreases with increasing reflector distance. This limits the range of distance measurments. Using a higher frequency improves directivity, however absorption losses increase making the application of higher frequencies impractical. In fact for long distances a longer wavelength is neccessary.
At low frequencies, bad directivity can be compensated by using a larger diameter
sensor. Summarizing: for standard application at large distances you need sensors
with low frequencies and large diameters and for small distances sensors with
high frequencies and a small diameters. A reflector, which is smaller than the
ultrasound spot or has high absorption, will reflect an echo, which is reduced
and thus will limit the range. The selection of the right sensor parameter needs
a lot of consideration and should be done very carefully.
There is always a large impedance step at the air/non-porous material interface so that no ultrasound can penetrate into the material. All surfaces act as perfect mirrors, even liquid ones. Porous materials or materials with deep structured surfaces need additional considerations: Normally a small reflector is suppressed, e.g. a hair on a board or the surface of a flees on a board, which will not be measured. (By increasing the gain of the ultrasound receiver, this can, by special adjustment, of course be changed.) If the density of hairs increases, then the measuring point will move to the tip of the hairs. The standard gain adjustment in the instrument corresponds to a quarter of the area of the measuring spot, i.e. a protuding structure on the object is not included in the measuring result, if its area is less than a quarter of the spot size. If the area of the protuding structure is larger than this value, especially structures with a height of 0.8 mm (1/4 wavelength) can make problems by interference effects. In this case the frequency of the sensors should be changed. Contrary to this, at surfaces with holes or pits, there the signal of the bottom of the hole is always suppressed by the plane echo of the intact surface. If holes are to be measured, focused transducers have to be used. For normal plain surfaces, the measured distance corresponds to the averaged value over the spot aerea. Summarizing surface effects: - The colour of the surface has no influence. - Small particles, dust or chips on even surfaces are suppressed in the evaluation. - Structures on surfaces are suppressed, if their area is less than a quarter of the spot size of the ultrasound beam. - Structures over the surface with a depth smaller than 0.8 mm are averaged. Misalignement of the surface is averaged. - Structures over the surface with a constant depth of approximatly 0.8 mm need special care. - Pits, holes or structures over the surface with a depth larger than 0.8 mm are again suppressed. If they should be measured, focused transducers have to be used. - Porous materials need special care. - Hot surfaces need fans to stabilize the air between surface and sensor.
The physical effect that shortwave rays are stronger affected by scattering from dust and particles than longwave rays is responsable for similar beautiful things as the colour effects of blue sky, the sun rise and sun dawn.
Oscillations at thickness measurements
There are no oscillation effects to measured readings ranging up to very high amplitudes and frequencies when using the OP-US 2 tickness meter. Due to the high measuring rate, effects are expected only if the oscillations generate sound beyond the feeling threshold. Because of this, the transportation velocity of the product is not important
Vibrating planes are sources of airborne sound waves. This results in noise
or error echos in distance measurements. Normally these echos have small amplitudes
or can be eliminated by various noise reduction algorithms, as in the sensor
OP-US S2. But there are applications, where these sound waves are used to detect
vibrating parts (example: a nonperfect laser weldpoint) or other sources of
airborne sound waves. A high effective sound generator is a leackage in a high
pressure pipe or an electric discharge. These application need microfonetype
sensors as the sensor OP-US S3.
The effect is well known when making measurements with a normal gauge: the measured thick-ness is to high, if the gauge is not exactly norma-lized to the surface. The correct value is the minimum achievable thickness. A stationary measuring system in an industrial environment will always display deviations due to this effect, because of guide tolerances. In our ultrasound measuring systems we deduct the declination of the reflector from the echo shape and compute corrections. This correction is limited to small deviations, whereby the result is improved. A good guidance and a normal adjustment is always working better. A declined surface reflects echos with smaller amplitudes than a normalized one. This effect is compensated by the gain control to some extent. If the problems due to declined surfaces become too severe, focussing paraboloids should be used, which extend the accepted angle.
Compared to other contactfree distance measurements, ultrasonic measurements use the direct way . All other methods have to use triangulation methods, just as we do it with our 3-dimensional distance estimation in the daily live. For us it is not so important that the accuracy decreases at large distances.
In technical measuring tasks this may be a problem, because in most cases the object to be measured is sitting on a transporting belt or a similar support. That means, that a smaller object is measured at farer dixstance, though it should be measured with higher accuracy. Ultrasonic measurement show a more balanced behaviour in accuracy versus distance. Additionally, the dynamic range of the measurements extends farer.
The accuracy mostly depends on the achieved stability of the air in the measuring gap. Depending on the technical requirement and the properties of the measured objects, accuracies between 0.1% and 0.01% are achieved at small distances. For example, at a distance of 100 mm an accuracy of 0.01 mm can be obtained. The software algorithmes contribute to measurement accuracy in addition to the sensor and the electronics. An important feature in this context is the possibility to match the timing of the measuring process to the timing and the poduction process periods.
Airborne ultrasound has been proven to be a robust and cost effective means of measurment in industrial environment. Call us and we will gladly advise you.