system
The core of an accurate measurement of intensity using a single p-p probe is to devise a method to accurately rotate the probe, maintaining its acoustic centre. The first attempts were made without a cradle system and the results were impractical and not satisfactory see Figure 4.3.
9 http://www.gras.dk/50vx-intensity-probe.html [Accessed on May 4th 2013]. 10 http://www.gras.dk/media/docs/files/items/p/d/pd_50vi_ver_06_08_02.pdf
Figure 4.3: The first attempts to measure a 2-D sound intensity vector using the face-to- face configuration and the p-p probe with its existing holder.
The design of the custom-made rotation cradle had three development phases. The first one was accomplished in the mechanical workshop of the University of Salford see
Figure 4.4. The original idea was to adapt the original holder provided by Brüel and Kjaer p-p intensity probe to a device that could rotate accurately 90º from its acoustic centre. This device was unreliable for the task. This fixture without a cradle was not accurate enough to obtain reliable results. The procedure used the laser cross to maintain the geometric centre of the probe, however the orthogonal rotation of the probe was time consuming and unreliable because there were too many degrees of freedom to control. In
Figure 4.4: The first rotation cradle prototype was made in the mechanical workshop at the University of Salford, however it was not accurate enough to perform orthogonal
rotations and it needed a Manfrotto head to align it vertically.
The design of the second prototype was done with the help of Minalum de México
company. (Please refer to Figure 4.5Figure 4.5: Second prototype consisted on a “C”
and a “L” shape machined in CNC Vertical Machine centre using high- grade hardened
aluminium in Minalum de México company. The work piece was machined first by the upper side and later by the downside in order to make a single piece prototype without any bending or soldering to achieve high accuracy. The right photo of the same figure has a detail of the machining pieces “L” and “C” shape. The dark hole at the right is used to align the work piece after it is turned over by 180 degrees by accurately measuring its
centre and align the x-axis. The next step was to use the wire cut EDM machine to cut
two octagon shaped holes on the “L” shaped piece. The idea behind the octagons was to be able to rotate at 45º steps instead of 90º in order to resolve simultaneous arrivals. The
diameter of the “C” and “L” pieces was chosen to be φ =8.6 mm in order to not disturb
λ 2<8.6 mm @ f = 20 kHz) and it was planned to be round in order to create the same effect on all directions of incoming sound. The shape of this design was based on the
custom cradle designed by (Abdou, 1994), (please refer to Figure 4.6). Unfortunately,
this design did not comply with the required accuracy because the C and L shape were
machined using two different procedures based on CNC. The octagonal components
(male and female) were cut with a wire electro-discharge machine ( EDM ). Hence, a
subestimation in the design provided no guarantee of obtaining an accurate rotation with the geometrical centre of the device. Additionally, the mounting system did not overlapped with the centre of the microphone base, which was a good option for locating the receiver when performing measurements at any given coordinate.
Figure 4.5: Second prototype consisted on a “C” and a “L” shape machined in CNC
Vertical Machine centre using high- grade hardened aluminium in Minalum de México company.
Figure 4.6: The second prototype for rotation cradle was tested for accuracy on the measurements of intensity. Unfortunately, it had a design problem where its geometric centre was not accurate for rotation. The best solution was to redesign a new prototype with an error proof rotation mechanism.
4.3.1.1 Design details of p-p intensity probe’s custom-made rotation cradle
It was later concluded that the best way to design a rotation cradle was by only using
Delcam Power SHAPE and PowerMILL CAD-CAM software and CNC and a vertical
machining centre ( VMC) in the facilities of Minalum de México company. The third
attempt was done using only CNC machined parts, which were done with only one
alignment on the piece to work. That realization guaranteed the precision required by using small machining steps and slow rate of removing of material. The third prototype introduced a number of new features that ensure accuracy on the setting rotation and repeatability on the measurements. The “O” carcass first was planned to be round in order to provide uniform scattering of sound and superior rigidity to be deformed with accidental misuse. In the last design, it ended as a squared shape to maximise the distance. The diameter of the shapes became slightly smaller than in the second prototype and the joints were round accordingly to the best acoustic fixtures.
Figure 4.7: Detail of the assembly of the rotation cradle machined in CNC.
Figure 4.8: Assembly parts used in the rotation cradle using Allen 1/8” diameter type screws. Octagonal profile Octagonal profile Allen Screws to tight the cover 1/4” diameter hole to hold the p-p
Figure 4.9: The third prototype of rotation cradle guaranteed the accuracy of the positioning of the face-to-face p-p probe configuration with the highest accuracy, the rotation of the p-p probe at 90 degrees.
Figure 4.10: Custom rotation cradle aligned with its geometric centre with a laser cross and the plumb bob to align the vertical line with the laser to obtain an accuracy of about ± 1 mm.
4.3.1.2 The p-u Microflown custom-made aligner device
One of the problems encountered using a Microflown 3-D probe is how to measure the
origin of the probe as a reference and how to assure that the device is accurately aligned to the measurement axis. The inner transducers of this device are quite small and in its carcass only has some printed points on its body to locate the 3-D axis. There is uncertainty of correct alignment and a custom-aligner is needed to ensure proper alignments. This device can be used in conjunction with the laser cross device to align
the Microflown p-u probe. Since the company that manufactures the probe has not
produced any alignment system, there was an opportunity to improve this measurement system.
The next design that was manufactured in Minalum de México company consists of a custom alignment system (please refer to Figure 4.11). This is a tool for the correct
measurement of directional information when using the p-u Microflown USP intensity
probe. One of the required features when performing a precision measurement aligning the probe with the orthogonal axis is to be able measure the accuracy of the setup at the exact centre position of the anemometer wires. This action is crucial because it determines the precise coordinate of the receiver. This task can be done with the aid of the dual laser cross system described in section 5.1.5 . This device helps to achieve the accurate position of the receiver with tolerances of ±1 mm. More importantly, it helps to align the axis of the probe along the axis used externally in the measurement setup. It works in the following manner:
The lower component of the system attaches the probe to the stand and aligns the centre of the probe with the centre of the microphone stand. The upper part of the system consists of a squared surface, which has a slit in the centre of it. The purpose of the slit is to make visible the anemometer wires and the pressure microphone at the centre of them. By securing this upper component with the Allen screw to the probe, both pieces can rotate together against the lower component. Doing this is possible to align the external axis along the external axis dictated by the laser cross system. Once this alignment procedure is finished, the squared upper component needs to be removed in order to
perform accurate acoustic directional measurements. It is crucial to avoid movement of the probe while performing this task to ensure that the coordinate measured with the laser cross is valid.
Figure 4.11: Custom co-lineal probe holder and alignment system for the p-u Microflown
USP intensity probe.