Cell Phone Vibration Experiment
Objective
Most cell phones are designed to vibrate. But at what frequency do they vibrate? With an accelerometer, data acquisition and signal analysis the vibration frequency of cell phones can be determined. This is the objective of this experiment.
At the end of this lab students should understand, how the accelerometer is calibrated, how to interpret its signal, data acquisition with LabVIEW, conversion of the raw data signal to engineering units and signal analysis of the acceleration signal using FFT.
Apparatus
1. Personal computer with LABVIEW software
2. National Instruments (8 slot) cDAQ NI9178 data acquisition 3. Crossbow Accelerometer and Signal Conditioner
4. Cell phone supplied by student to generate vibration signal (or data downloaded from website)
Experiment
1. SENSORS AND SIGNALS:
Review the spec sheet for the Crossbow accelerometer; it is online.
Identify the range and sensitivity of the accelerometer.
ME 4600:483 – Lab Manual Revised August 2015
DATA ACQUISITON SETUP:
1. Build a simple VI to collect analog input data (as voltage)
1.1. Open a new VI. On the Block diagram right click and select Express → Input → DAQ Assist 1.2. Acquire signals → Analog Input → Voltage → NI 9205 → ai0
1.3. Set the voltage range from 0 to 5 volts
1.4. Set the timing to collect 1024 samples at 1024Hz from this channel
What is the Nyquist frequency?
What is the smallest frequency that can be detected?
2. Add a Waveform Graph on the front panel to display the data and wire it to the DAQ assist on the block diagram
2.1. Change “Amplitude” on the y-axis to “Voltage [V]”
2.2. Change “Time” to “Time [s]”
2.3. Test the VI by pressing the Run button or Ctrl-R.
You must hold in the red button on the signal conditioner as you collect data.
The accelerometer voltage signal should now be displayed in the graph window.
2.3. Move the accelerometer or turn it upside down to confirm that a signal proportional to acceleration is being recorded.
3. Perform a basic calibration check
3.1. Use the VI to measure the gravity signal with accelerometer pointing up (+1g) and down (-1g). 3.2. Determine sensitivity from voltage change versus acceleration change.
g V Sensitivty
3.3. Compare the sensitivity of the accelerometer to the spec sheet value. 4. Plot the frequency spectrum
4.1. On the Front Panel add two numeric controls: Numeric → Numeric Control Rename one “Number of Samples” and the other “Sampling Rate”
4.2 Add an XY Graph
Rename the graph “Frequency Spectrum”
ME 4600:483 – Lab Manual Revised August 2015
4.5 Convert the data into an array of rms voltages of the detectable frequencies.
Signal Processing → Spectral → Amplitude and Phase Spectrum
Wire from the signal input to the data output of the DAQ Assistant. The data will be automatically converted to an array.
4.6 We have an array storing the rms voltages of the data at certain frequencies. We now need to create another array that will store the corresponding frequencies.
4.6.1 Create a for loop: Structures → For Loop
4.6.2 Outside of the loop add a divide operation: Numeric → Divide
Wire this to divide the Number of Samples by 2.
To place the 2: Numeric → Numeric Constant
Wire the quotient to the Loop Count of the Loop. 4.6.3 Create another divide operation outside of the loop.
Divide the Sampling Rate by the Number of Samples.
4.7 The corresponding amplitude at zero frequency is the average offset from zero of the signal. Because this value is often much larger than the amplitudes of the frequencies detected, we will remove it by deleting the zero index from each array.
4.7.1 Make a Delete From Array function for each array: Array → Delete From Array
4.7.2 Wire the Array input of the delete function into the Amp Spectrum Mag (Vrms) output of the amplitude array.
4.7.3 Wire the other Array input of the other delete function to the product of the multiplication from the loop (the frequency array.)
4.7.4 Create a numeric constant of zero and wire it to the index input for each delete operation. 4.8 Bundle the arrays together and wire the output to the frequency spectrum graph.
Cluster, Class, & Variant → Bundle
ME 4600:483 – Lab Manual Revised August 2015
5. Alter the VI to display the predominate frequency
5.1 Create an Array Max and Min function: Array → Max & Min 5.2 Wire the amplitude array as the input.
5.3 Create an Index Array function: Array → Index Array
5.4 Use the frequency array as the array input and the max index from the amplitude array as the index input.
5.5 Wire the output to the Predominate Frequency indicator.
6. Save the VI
7. Collect Vibration Data
7.1. On the Front Panel, set the Number of Samples and the Sampling Rate numerical indicators both to 1024. You can change these settings later if it is necessary.
7.2. Use rubber bands to attach the accelerometer to a cell phone. Use paper to separate the accelerometer from the phone to avoid scratches. 7.3. Put the cell phone on a foam pad.
7.4. Be ready to collect data (Ctrl-R). Repeatedly press Run until a good vibration signal is captured.
Make sure you hold the red button on the signal conditioner while recording data.
7.5. Either call the phone from another phone or make the phone vibrate by setting an alarm.
ME 4600:483 – Lab Manual Revised August 2015
8. Record the results
Show your results to the TA. Discuss any open questions you may have.
9. WRAP UP DISCUSSION WITH TEAM
(write each response neatly in your lab notebook)
9.1 What could be done to improve this experiment? Where do you see potential sources for measurement or signal errors?
9.2 How accurate is the frequency measurement? What could be affecting it?
9.3 How accurate is the vibration amplitude measurement? What could be affecting it? Do you have a concern using this accelerometer for this frequency?
9.4 Address the following questions and write down your ideas for improving this measurement/experiment.
Sensor Calibration
Sensor Frequency Range
Sensor Mass
Signal Conditioning
Signal Analysis
