ME330: Engineering Experimentation

EXAMINATION II

Fall 2006

 

This examination is closed book and closed notes. You are to work only on the sheets provided, and only on one side of the paper. If you need additional worksheets or require assistance, please notify your instructor. The use of calculators is not allowed. Note that there is a formula sheet at the end of the exam.  Please check to verify that your name is on every sheet prior to turning in your exam paper.

 

 

 

 

GOOD LUCK!

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Do not write in this space

 

 

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Problem 1(13). Check the following statements as either true or false.

a.       8 bits of digital data is termed a byte.                                                                                   T(  ) F(  )

 

b.      A ratiometric device is one whose gain varies inversely with the excitation voltage.                T(  ) F(  )

 

c.       A 5volt 8-bit D/A converter has a resolution of 0.195 volts.                                                 T(  ) F(  )

 

d.      In serial communications, data bits are transmitted sequentially.                                            T(  ) F(  )

 

e.       Sending an ASCII character across a serial line with one start bit and one

stop bit requires 12 bits total.                                                                                              T(  ) F(  )

 

f.        Fourier analysis can be used to evaluate the frequency content of any real-life

time-varying signal.                                                                                                             T(  ) F(  )

 

g.       A thermocouple is a First Order System.                                                                             T(  ) F(  )

 

h.       All Wheatstone Bridge circuits are temperature compensated.                                              T(  ) F(  )

 

i.         A 12-bit A/D converter has four times the resolution of a 10-bit A/D.                                  T(  ) F(  )

 

j.        The A/D converter of the PICaxe08M is a 10-bit device.                                                    T(  ) F(  )

 

k.      What are three conditions on a signal that allow us to use Fourier analysis?

 

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Problem 2(15).  You are to estimate the damping ratio and natural frequency from the data for the second order system shown in Figure 1.  Show your work clearly.

Figure 1.  Accelerometer response to a harmonic input.

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Problem 3(27).  Consider the microprocessor-based measurement system shown in Figure 2.  You are to add to the given program to accomplish the following:

a.          Wait for push switch SW1 to be depressed.

b.         After SW1 is pushed, turn power on to the temp sensor (apply vdc as an output from leg 5).

c.          Wait 765 milliseconds for the sensor to stabilize and then read an 8-bit A/D conversion of the sensor voltage.

d.         Send this data out the serial interface (using the sertxd command) as an ASCII string.

e.          Turn off the sensor power after this is completed.

f.           Repeat the above steps indefinitely.

Please add comments to your program to explain what you are doing.

Figure 2.  PICaxe08M setup (pin numbers in parentheses next to leg numbers).

 

symbol green   =  2                             ; name output2 ‘green’

symbol CR      = 13                            ; ASCII 13 = Carriage Return

symbol LF       = 10                            ; ASCII 10 = Line Feed

symbol SW1    =  3                             ; Define pin3 as SW1 input

 

 

main:                                                   ; make a label called main

if SW1 = 0 then main             ; loop until switch depressed

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Problem 3 (Continued)

 

 

 

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Problem 4(20). Consider the instrumentation system shown in Figure 3, where a signal S(t) is the input to the system and the output is the recorded signal T(t). The gain and phase shift of the system are documented in Figure 4.

Figure 3. A system with corruption of data caused by H(t).

When the output signal of the above measurement system, T(t), was analyzed and represented by a Fourier Series, the following result was obtained.  All units in the equation are either in volts or radians.

One specific term arose which was attributed to a nearby noise source.  It is the last term.

Figure 4. Gain response data for the instrumentation system.

(a)    What is the frequency of this noise source in Hz?

(b)   Find the magnitude of the original noise signal, N(t).

(c)    What is the phase shift of the noise relative to S(t)?

(d)   What is the DC level of S(t)?

Show your analysis methods and units clearly if you want to get a grade for this problem.

 

 

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Problem 4 (Continued):
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Problem 5(15).   For the bridge circuit in Figure 5, it can be shown that

Eqn.3
 

We seek to build a tension transducer where both active arms experience positive strains in tension.

a.       There are to be two active arms.  If R1 is one of the active arms as shown, circle the other arm on Figure 5 that provides an output when both active arms are R = R +DR and the inactive arms are of value R.

b.      Based on your expert knowledge, write down an expression for Eo/Ei in terms of DR/R.

 

 

 

 

c.       Would the bridge output voltage change if the temperature changes (ie, is this bridge temperature compensated)?  Explain.

Yes  (     )               No   (     )

 

Figure 5.  A Wheatstone bridge circuit.

 

 

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Problem 6(10).   Answer the following.

 

(a)   What are the input bits if the output has a logic value of one?

 

 

 

(b)   Complete the Truth Table for the following digital logic OR-gate:

 


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Problem  (    ):

 

 

 

Useful Formulae

 

First Order Systems

 

Common PICaxe Commands

 

for b = a to b

go to addr

gosub addr

high p

low p

if expr then addr

next b

pause xxx

readadc p, b

readadc10 p, w

return

sertxd (“string”, x, #y, z)

wait sss
 
Second Order System

 

 

 

  and

 

 

 

 

Fourier Series

 

  where

  and