Using a similar selection methodology as for the sensing bars, the strut is obtained by cutting an alu- minium tube with30mmof external diameter and22mmof internal diameter to the desired length. In this case, however, we have to consider the existence of two testing scenarios that require the use of two struts with different heights, one with400mmand another with130mm. As they present the same external and internal diameters, they can be obtained from the same initial tube. As it occurs for the sensing bars, this tube can be purchased in hardware stores or in aluminium suppliers, provided this is a standard cross-section dimension.
Having defined the metallic components that compose the force balance and presenting their man- ufacturing or selection process, it is necessary to present some considerations regarding the electronic devices that have to be used in order to make it fully operational.
6.3 Mechanical Assembly
To keep all the components in place and guarantee the force balance integrity, it is necessary to carefully detail the way the components are linked to each other. As it can be seen in table 6.1, the main mech- anism used to connect the components is the bolted connection. The table at the bottom of the whole structure is connected to a platform by means of 8 M8 bolted connections that is attached to the ground.
Above this table, the rotary collar is also linked to the table using, in this case, 6 M5 bolts with flat head.
The flat head is used in this case due to the lack of space between the table, the fixed platform and the rotary collar. Similarly, to connect the rotary collar to the fixed platform, 6 M4 bolts with flat heads will be used. To link both fixed and moving platform to the couplers, a total of 24 M6 bolted connections are necessary. As for the flange, 3 M6 bolted connectors will be used to attach it to the moving platform and 3 M5 bolts will be used to tightly secure the strut. Considering the mechanism to adjust the angle of attack at the top of the strut, a M10 bolted connection will be used to tighten both components together after selecting the desired angle of attack.
The connection between the rod end bearings and the couplers is made of dowel pins and the respective breaks. The dowel pins can be manufactured to fit the desired dimensions. One is interested in having pins with8mmof diameter that can be obtained by cutting steel bars with this same diameter into pins of around40mm. This dimension corresponds to the distance between the two8mmholes of the coupler (24mm) plus an extra length to both sides, necessary to link the brakes that keep the mechanism in place. In order to guarantee that the rod end bearing does not slip in the pin, two additional brakes will be placed at each side of the rod end bearing.
6.4 Instrumentation
In order to guarantee the higher accuracy on the strain gauges measurements, the attachment of the strain gauges to the sensing bars requires high level of detail. As referred previously in section 2.3.1, the strain gauges will be connected as full bridges. In this case, whenever two of the strain gauges are in tension, the other are in compression. Given this, it is necessary to guarantee the correct alignment of the strain gauges, as they have to be attached in two perfectly perpendicular pairs. In order to guarantee a correct attachment of the strain gauges to the aluminium bars, a set of tips, provided by the manufacturer of the strain gauges (HBM), can be presented:
• Proper preparationIt is necessary to guarantee that all the materials are previously prepared and organized to guarantee that everything goes as expected;
• Bonding surface preparationThe regions of the metal where the strain gauges are to be attached have to be treated in order to obtain a smooth surface without pores, notches and oxides. This process is usually carried out by using a sandblaster or, more simply, a handheld sander;
• Surface cleaningAfter the surface is softened, it has to be completely cleaned to eliminate grease and dust. It can be used a proper cleaning agent, preferably, an organic solvent. Clean pads should be used to wipe down the area;
• Measuring pointsIn this case, four measuring points have to be marked in each bar, correspond- ing to the center of each strain gauge. This operation must be done with a ball pen as it avoids the creation of any further notches;
• Adhesive selection To attach the strain gauges to the bars, an adequate adhesive has to be selected being the most common a super-glue;
• Strain gauge handlingThe handling of the strain gauges has to be very careful and, since these components are very small, tweezers must be used.
After the strain gauges are carefully attached to the bars, a wiring procedure must be carried out.
First, wires have to be welded to the strain gauges leads in order to form the full bridge circuit that was presented in section 2.3.1. After the wiring process is completed, it is possible to connect the full bridges to the data acquisition device, as discussed in section 5.3.
Further consideration on the strain gauge installation process can be found in literature, however, in this case, the most relevant is the one provided by the strain gauge manufacturer (HBM) [37].
6.5 Cost Breakdown
Considering the bill of materials presented before, it is possible to come up with an estimated budget for the force balance. Regarding off-the-shelf components, not only mechanical but also electronic, it was possible to research and find the best offers. Concerning all made-to-specification components, several requests of quotation to specialized companies had to be made, in order to obtain the best offers. Table 6.2 sums up the final cost of each component and the estimated cost of the force balance.
Table 6.2: Expected cost for force balance components
Component Quantity Price (e) %
Mechanical
Table 1 121.52 3.1
Rotating collar 1 57.58 1.5
Fixed platform 1 117.86 3.0
Coupler 12 210.18 5.3
Rod end bearing (INA GAL(R)8-UK) 12 231.60 5.8
Sensing bar 6 8.00 0.2
Moving platform 1 − −
Flange1 1 15.00 0.4
Strut1 1 8.00 0.2
αadjustment mechanism 2 139.09 3.5
Bolts, nuts and other connectors 106 19.25 0.5
Subtotal 928.08 23.4
Electronic Strain gauges (HBM 1-LY13-6/350) 24 253.65 6.4
DAQ NI9237 2 2782.00 70.2
Subtotal 3035.65 76.6
Total 3963.73 100
1This indicates that the cost is estimated and is based on similar parts.
It is possible to conclude that, in terms of cost percentage, the electronic components of the force balance constitute the main slice of the whole project as the data acquisition devices needed to decode the output signals of the strain gauges represent more than70%of the total cost of the force balance.
Overall, it is possible to conclude that this is an expensive project and, for this reason, the assembling and calibration procedures have to be carefully carried out so that the force balance measurements can be as precise as possible.