Instrumentation Engineering Assignment
1. An experiment is conducted to measure the flex (amount of bending) in a thin composite beam when placed under a known load (force). Smaller beams made of the same material will then be used as springs to protect electronics.
The manufacturer needs the beam to flex on average 5.0 mm, with 95 % of all samples having a flex between 4.8 mm and 5.2 mm.
After testing the flex of 20 samples, the following numbers are found:
i=1∑20xi = 99.23 i=1∑20xi2 = 493.03
Each sample has the flex measured to the nearest 0.01 mm using electronic calipers.
(a) What is the mean flex found using 20 samples?
(b) What is the sample standard deviation of flex found using 20 samples?
(c) What is the 99 % confidence interval for an individual sample? (Not true mean this time!) That is, in what range would we expect the flex of 99 % of the samples to be found?
(d) Does the set of samples used satisfy the desired failure rate? Explain the reasoning behind your answer.
(e) What percent difference is there between the actual average flex and the desired flex? Treat the desired flex as the "true" value.
(f) Discuss the accuracy and precision of the experiment performed.
(g) Write a conclusion for the experiment performed.
2. Sketch a Venturi flow meter, labelling all relevant components, dimensions and flows. Assume a U-tube manometer is attached for measuring the pressure difference.
3. A signal is expected to have a maximum frequency of fm ≈ 80 Hz.
(a) What minimum sampling frequency is required in order to accurately measure this signal?
(b) What frequency would be measured if the signal were sampled at 150 Hz? Show the folding diagram relevant to this problem.
(c) Explain the concept of aliasing. Use a simple diagram of a signal and sampling points to support your explanation.
4. A Wheatstone bridge circuit is to be constructed to measure strain. The bridge must operate in the deflection mode. The sensor must be R1.
(a) Sketch the Wheatstone bridge circuit required. Label all important components.
(b) The strain gauge has a resistance of R1 = 120.0 Ω. All other resistors are therefore chosen to have resistance R = 120.00 Ω. Explain why.
Include equations where they may assist your answer (a long derivation is not required).
Hint: You are NOT deriving the ugly equation for δV.
(c) The strain E can be determined from the deflection voltage δ6 using
∈ = 4 δV / GF Vs
where: GF = 2.10 ± 5 % (5 % of 2.10)
Vs = 10.0 ± 0.05 V
Find an expression for the design stage uncertainty in the strain Δ∈.
Simplify as much as possible (think about what is happening/not happening in the design stage!)
(d) What maximum uncertainty can there be in the deflection voltage Δ (δV) in order to measure the strain to the nearest ue?
That is, get the uncertainty in the strain below Δ∈ = 0.5 x 10-6?
5. A system is designed to measure the pressure difference across a Pitot tube so that airspeed in a wind tunnel can be determined.
A FAQ is used to capture the voltage output from the electronic differential pressure transducer.
A LabVIEW VI is required that will:
(i) take measurements for 0.1 s at 1000 Hz
(ii) average the results to eliminate noise due to turbulence (or low pass filter the signal)
(iii) apply an equation to convert the output voltage to airspeed
(iv) write data to a measurement file
(v) display data on a graph
(vi) repeat the measurement 11 times with a 6 second delay between measurements (this allows the total measurement time to span 1 minute!)
(vii) allow the user to stop the program at any time as well.
Draw the Block Diagram of the required VI. Make sure it is clear to me what each part does!