NORTHERN ILLINOIS UNIVERSITY Department of Mechanical Engineering
MEE 390 EXPERIMENTAL METHODS IN MECHANICAL ENGINEERING
©1990-2001 M. Kostic
(Your Scope may be newer model: Check with TA- Tektronix-TDS 200-Series Digital Oscilloscopes *Programming-TDS 200-Series

Objective: To learn how to set and use an oscilloscope.

Procedure :
1. Connect the apparatus to Power supply.
2. Setup the Oscilloscope as given below

Standard Initial setup
Horizontal magnification to X1
All adjustable knobs(VOLTS/DIV and SEC/DIV) to CAL position (click sound)
Storage buttons OUT

Prevent Overloads
Amplifiers to minimum magnification = 20 VOLTS/DIV (if 10X probe)
Channel 1 and 2 inputs "grounded" (GND)

Prepare to display a trace
Power on, wait for 15 seconds (If horizontal line is not visible, press and hold BEAM FIND, and adjust Channel 1 VERTICAL POSITION, and also HORIZONTAL POSITION until trace is centered. On releasing BEAM FIND, trace should be visible (if not, increase intensity and try again).
Adjust FOCUS for sharpness. Lower intensity to prevent screen damage.

Display a signal
Connect a voltage signal to Channel 1 (CH 1)input. A voltage signal can be a Function Generator whose output is connected to the input Channel 1 of oscilloscope.

Settings for the Function Generator are as follows.

1. Connect it to power supply and switch the Function Generator ON.
2. Choose a range by pressing one of the buttons where your interested frequency is.
3. Choose the type of function by pressing one of the buttons under the function option, either SQUARE, TRIANGULAR or SINE wave (represented by graphical symbol).
4. The frequency of signal output can be read on the LED display. To fine tune the frequency value, rotate the knob beneath the LED clockwise or counter-clockwise until the desired frequency is reached.
5. The amplitude of the signal, if to be varied, can be done by turning the AMPL knob. Once the desired frequency is set, let us now move back to the oscilloscope.

Display a signal (continue):

1. Change ground (set to AC, unless average voltage is comparable to signal ® DC)
2. Increase amplification (eg. 1V/div) until signal is visible, but not bigger than screen
3. Switch trigger source to Channel 1 (to synchronize display with signal)
4. Adjust sweep rate and Ch. 1 gain to vertical position, to view signal as desired.

If signal is repetitive and stable, setup is complete. However, if it is irregular or occasional, you will want to STORE a SINGLE SWEEP in memory. Because we are using a Function Generator, the signal is repetitive and will be stable. For example, if we are going to read the response of a starting motor or dynamic response of a thermocouple, we will be going for STORE or SINGLE SWEEP option, because the signal is going to start from a stable point, reach a maximum value and is going to be stable again. If we are not storing the jump from zero to the maximum value, the following (stable) signals are going to overwrite on the old one which we are not interested in.

Triggering:
For a merely irregular signal this is easy, but for a very occasional signal it is rather hard. To make life easier for an occasional signal, there is a TRIGGER option which will is discussed next. Set triggering mode (this is for an occasional signal like a hammer striking a plate) to SINGLE SWEEP, and press STORE. (The stored signal you see is irrelevant- old data). Your aim is to make the trace cross the screen, by making the TRIGGER fire. Press RESET- the ready light will glow until triggering. If nothing happens, fiddle with trigger level and trigger slope until a trace appears (READY light will go off). To repeat, press RESET again -- if trigger level is correctly adjusted, the oscilloscope will wait until the signal level is as you set, then sweep just once across the screen. Although hard to adjust, this feature is invaluable for capturing a rare voltage spike or event - the trigger will wait all day for the signal to be correct. By first pressing 50% PRE-TRIG, the scope can also be made to show the signal just before the event.

Your many further options include:

Apart from using the Function Generator, students can use the motor start-up and dynamic response of a thermocouple (especially) to use the trigger option.

STORE button on the right top captures the signal digitally, i.e. the trace of the beam is stored on the screen up to that it completes one movement from left to right. Once it reaches the right end, it again over traces on the old one. SAVE button is used to save the current trace of the beam, i.e. no more readings taken up to that the SAVE button is released. SAVE is useful when reading the dynamic response of a starting motor. The motor is started when the beam has just passed the left end of the screen. Once the motor reaches the maximum speed, SAVE button is pressed so that the initial response of the motor is saved on the screen. The difference between STORE and SAVE is that STORE stores the trace of the beam up to the next cycle of display. But SAVE saves it and no more recordings until that SAVE button is released.

USING THE TRIGGER FUNCTION AND MEASURING THE FREQUENCY OF VIBRATION OF A CANTILEVER BEAM

1. In this part of the experiment, we will be measuring the frequency of vibration of a cantilever beam (see Strain Gage Lab handout for more data and using the TRIGGER function as explaind above).
2. Setup the Strain gage using the setup instructions given in the Strain Gage Lab handout for the "quarter bridge" setup.
3. Connect the oscilloscope to the output terminal of the Strain indicator.
4. Set triggering mode to SINGLE SWEEP and press STORE. The signal displayed on the screen is old data. Press RESET and the ready light above the switch will glow until that triggering occurs. Set the TRIGGER SLOPE to +ve slope and set the TRIGGER LEVEL knob to middle position.
5. Press the 50% pre-trigger switch on. Set the AC/GND/DC switch of appropriate channel to AC.
6. Tap the cantilever beam with finger (gently will less force) and the signal will be displayed on the screen. If the signal is too close or too small, play with CH VOLTS/DIV switch and SEC/DIV switch so that the display is good on the screen.
7. Measure the time (X axis) for ONE complete period (you could measure time for more periods and then divide with the number of periods for which the time is measured). This is the period of vibration.
8. Repeat the experiment at least 5 times.
9. Calculate the theoretical frequency of vibration. Compare and comment.

First Natural Frequency of vibration of cantilever beam (Average of the above 5 readings) =

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