Force measurement systems

Gas dynamics Assignment

Balaji Sankar

March 4,2011

Force measurement techniques used in 0.3 m, .55m,1.5m and base flow tunnel are described.

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Table of ContentsMechanical balances:...........................................................................................................................3

Strain gauge balances:...........................................................................................................................3

Sting mounted balances....................................................................................................................3

Floor mounted balances:...................................................................................................................3

Measurement of forces using strain gauge:......................................................................................4

How Strain gauges work?.............................................................................................................4

What are major types of circuit arrangements?...........................................................................5

How is strain due to temperature compensated...........................................................................5

Why are there two pairs on each location?...................................................................................6

Why are strain gauges are on top and bottom surface the sting?................................................6

Why are strain gauges are on two locations on the sting?............................................................7

How to measure side force and yawing moment?........................................................................7

Why are strain gauges weakened in torsion..................................................................................7

How to measure axial forces?........................................................................................................8

Caliberation:..................................................................................................................................8

1.5 metre tunnel:...............................................................................................................................8

Data acquisition system:................................................................................................................9

Base flow tunnel:.............................................................................................................................10

0.55 metre tunnel:...........................................................................................................................10

0.3 metre tunnel:.............................................................................................................................10

List of figures:STING MOUNTED BALANCE...........................................................................................................................................3FLOOR MOUNTED BALANCE SETUP.................................................................................................................................4STRAIN GAUGE WORKING PRINCIPLE...............................................................................................................................5STRAIN GAUGE CIRCUIT- MAJOR TYPES...........................................................................................................................5LOCATION OF STRAIN GAUGES ON TOP SURFACE...............................................................................................................6TEMPERATURE COMPENSTAION.....................................................................................................................................6FINDING NORMAL FORCE AND MOMENT.........................................................................................................................7AXIAL FORCE SENSING MECHANISM................................................................................................................................8SCXI 1521 CARD.......................................................................................................................................................9

Mechanical balances:Mechanical balances measure the aerodynamic forces applied on the model by balancing the moments due to the forces by moments generated by forces due to the counter weights. The counterweights or the location of the counter weights can be varied to balance the aerodynamic moments. In automated measurement systems , the location of the weights is automatically varied by a stepper motor to balance the aerodynamic moments. These mechanical balances become very complicated as the number of components to be measured increases as it requires complicated arrangements to decouple the three forces and three moments. It is not used in NAL and is not further explained.

Strain gauge balances:Strain gauge balances used in NAL are of two types.

Sting mounted balances :

Sting mounted balances are mounted on the sting that supports the balance. Major advantages of sting mounted balance are

(1) It is compact(2) Causes no additional flow interference other than the basic sting.(3) Rolling and pitching of the model is convenient.

Disadvantage is that when wall to wall models are used,it is very inconvenient to use sting type balance.

Sting type balances are of further two types:(a) Floating type sting balance.(b) Integral sting mounted balance.

Floor mounted balances:Floor mounted balance is located at the base of the sting outside the wind tunnel. It is mainly used in

1. half-model testing2. industrial aerodynamics (where forces on building models and automobile models

need to be measured accurately).3. rotor and propeller testing

Figure 1 Sting mounted balance

As these models are resting on the tunnel floor, it is convenient to use floor mounted balance. It is very advantages in testing of airfoils that span from wall to wall, vertically. However , in the following figure , model is shown to span from left to right wall for clarity.

The wall/floor balance is often equipped with decoupling devices so that high pressure air piping can pass through it without effect on the measurements. For rotor and propeller testing a drive shaft must pass through the balance again without affecting the balance results. The structure of these balance is rigid, the forces are measured by six different dynamometers which are very well decoupled.

Measurement of forces using strain gauge:Strain gauges are used in measuring the forces using balances. Principle of strain gauges are explained here.

How Strain gauges work?If a strip of conductive metal is stretched, it will become skinnier and longer, both changes resulting in an increase of electrical resistance end-to-end. Conversely, if a strip of conductive metal is placed under compressive force (without buckling), it will broaden and

Figure 2 Floor mounted balance setup

shorten. If these stresses are kept within the elastic limit of the metal strip (so that the strip does not permanently deform), the strip can be used as a measuring element for physical force, the amount of applied force inferred from measuring the change in its resistance.

Figure 3 Strain gauge working principle.

What are major types of circuit arrangements?If only one of the four gauges is pasted on the specimen, its a quarter bridgecircuit. If all four are pasted, its a full bride circuit.

Figure 4 Strain gauge circuit- Major types

Besides these, temperature compensated strain gauge circuits are also widely used.

How is strain due to temperature compensated?

In a quarter bridge circuit, by keeping the resistance R2 also in the same stressed location as R4, whatever change in R4 comes, will also come in R2. Hence the ratio is unaffected. Thus temperature can be compensated .

Why are there two pairs on each location?Both the gauges on top of the specimen are under tension. So if diagonally opposite gauges are pasted side by side, change in resistance due to bending measured by the gauge circuit will be higher because resistance change due to the two gauges add up. So we see pairs of gauges on top and bottom.

Figure 6 Location of strain gauges on top surface

Moreover full bridge circuits have linear response with respect to strain. Quarter bridge circuits only are approximately linear.

Why are strain gauges are on top and bottom surface the sting?Strain gauges are located at places/ planes of maximum strain. Since stress is maximum at the outer surface of the sting in bending, at maximum distance from neutral axis, strain is also maximum. So gauges are placed there.

Figure 5 Temperature compenstaion

MEASURE STRAIN AT LOCATION

1

COMPUTE THE CORRESPONDI

NG STRESS FROM

MATERIAL PROPERTIES

COMPUTE THE MOMENT

(P*Y1) , THAT WILL CAUSE

THAT STRESS AT THAT LOCATION

Why are strain gauges are on two locations on the sting?Strain gauges measure forces(normal) and moments by measuring the strains at a location. They work on the following process.

By measuring the strains at another location , using the same process, we get another equation.

By solving the two equations , we get both the load and its location, thus giving the moment that can be scaled to any reference location.

Thus we need two locations where strain gauges are mounted in pairs.

How to measure side force and yawing moment?By turning the gauge by 900, we can measure side force and yawing moment the same way we measure pitching moment and force.

Why are strain gauges weakened in torsion?

Strain gauges have high torsional rigidity . they give very less strain for the applied torsional moment. To increase the torsional strain, they are weakened to a cruciform section, that has lesser torsional rigidity.

Figure 7 Finding normal force and moment.

How to measure axial forces?Axial forces are measured by converting them to bending moments by using an arrangement similar to the one shown below.

Figure 8Axial force sensing mechanism

Using the above arrangement, once the forces have been transferred as bendin moments to the gauges, they can be measured as pitching moments are measured.

Caliberation:The gauges are caliberated by applying known loadings in the weighing pan and measuring the voltage signals. A typical three component balance has nine caliberation constants of which six are interaction coefficients. They characterise the interaction between various loads , like response in axial force sensor to applied normal force.

1.5 metre tunnel:1.5 metre tunnel has two half model balances and 4 sting type balances. Of the 4 sting type balances, 2 can measure all 6 components .Specification of the sting type(5 component) balance that has 38.08 mm diameter is given below:

Component Max valueAxial force -Side force 4 kgNormal force 8 kgPitching moment 0.9 kg metre Yawing moment 0.45 kg metre Rolling moment 0.05 kg metre

Table 1 Balance specificcation, 5 components, sting type.

Specification of the wall mounted( half model) balance is given below:

Component Max valueAxial force 8kg

Side force 20 kgNormal force 80 kgPitching moment 8 kg metre Yawing moment 2 kg metre Rolling moment 20 kg metre

Table 2 Balance specificcation, 6 components, floor mounted type.

Data acquisition system:Data acquisition system used at the 1.5 metre tunnel was scxi1521, a card from National instruments. The salient features of the card are:

24 analog input channels 120 Ω quarter-bridge completion resistor per channel Programmable excitation (0 to 5 V) per channel Lowpass filter (10 Hz) per channel NI-DAQmx measurement services software.

There are eight lead out wires coming out from the strain gauge. 6 correspond to strain data of 6 components1 supplies the excitation voltage and one senses the applied voltage at the sensor input. This is because applied voltage may not actually reach the sensor due to losses and we need to know what is the voltage input actually got by the sensor.

Base flow tunnel:Annular force balances(strain gauge based) that allow the free passage of the inner jet flow are used in the base flow tunnel. The specifications of the six component type annular force balance is given below:

Figure 9 SCXI 1521 card

Component Max valueAxial force 80 kg Side force 80 kgNormal force 40 kgPitching moment 2 kg metre Yawing moment 2 kg metre Rolling moment 2 kg metre

Table 3 Balance specificcation, 6 components, Annular type.

0.55 metre tunnel:0.55 metre tunnel has equipments similar to 1.5 metre tunnel.

0.3 metre tunnel:0.3 metre tunnel has 3 component , 5 component and 6 component balances. Diameter of the balance is 16mm. 0.3 metre tunnel also has a dummy balance to check and set the alignment of the model when it is mounted on the sting. Specification of the 6 component is given below.

Component Max valueAxial force 27 kgSide force 9.1 kgNormal force 182 kgPitching moment 6430 kg metre Yawing moment 3465 kg metre Rolling moment 700 kg metre

Table 4 Balance specificcation, 6 components, sting type.