Vibration Manual 01 TIMES SERIES The time series view displays the acceleration vs. time values for each of the three accelerometers. This is also the view where the sample process is initiated.
SAMPLE Pressing the sample button will start the data acquisition process. If the data length is 128 or 256 you will see the time series graph update in real time. For longer time series only a progress bar is visible until the acquisition is complete, After each acquisition the app compares the requested sample length with the actual recorded sample length. If there is more than a 5% error in the timebase a warning alert is displayed. This cause of the error are typically background processes such as mail, iPod, notifications etc. taking up too much CPU time. The problem can be resolved by shutting down and restarting the iPhone or iPod.
ABORT The abort button cancels any data acquisition, or sample delay which is currently in progress. This button is only active during acquisition.
SNAPSHOT The snapshot button captures an image of the time series and frequency display screens and saves them to the Saved Photos album. These images will be synced to your computer the next time you connect allowing you to document your measurements
SEND CSV BUTTON The send button is used to create an email containing CSV formatted amplitude and frequency data as an attachment. The email also has screen captures of both the time series and the frequency displays. As of version 1.61 there is an option to email .WAV files of each channel. These files can be used to “Play” the noise recorded by the accelerometers. Be aware that these are very low frequencies and may not be audible without really big speakers and lots of audio power.
SETTINGS BUTTON The Settings button displays the time series settings screen allowing adjustment of the data acquisition and display parameters.
ZOOMING IN FOR A BETTER VIEW You can zoom in 2x or 3x for a better view by double tapping with one finger on the screen. Zoom back out by double tapping again or by tapping with two fingers. Set the default zoom value in the Settings app. Note that 3x zoom requires more memory and will be disabled automatically if a memory warning occurs. You can re-enable 3x zoom by quitting Vibration and turning it on in the Settings app.
INSPECTING THE DATA You can inspect the data by touching the screen. A cursor will appear under your finger and the time and amplitude values for all three channels will be reported. These values will update live as you move your the cursor. This feature also works in the zoom mode.
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Vibration Manual 02 TIME SERIES SETTINGS
SAMPLE RATE The sample rate slider sets the accelerometer sample rate. It is adjustable from 10Hz to 100Hz. The upper sample rate is limited by the hardware in the device. The total sample time is then computed from the sample rate and the data length.
SAMPLE DELAY The sample delay slider allows the start of sampling to be delayed from 0-20 seconds after pressing the Sample button. This allows sample button to be pressed and the iPhone to be positioned for the best sampling before acquisition begins. The remaining delay time is displayed on the screen as the countdown progresses.
DATA LENGTH The data length control sets the number of sample points used during acquisition. Selection of more points will result in a longer data acquisition time as well as the ability to measure lower frequencies. If this value is changed any existing data will be cleared.
TIME SERIES VERTICAL SCALE The vertical scale control sets the scaling of the time series graph. The selected value will be one division on the graph. The scaling can be changed after acquisition and the result will be immediately reflected in the time series graph.
DC REMOVE SWITCH The DC remove switch is used to control the automatic removal of the average value. The switch is typically used to remove the mean gravity vector from the data.
CALIBRATE BUTTON Switches to the calibration screen to calibrate the accelerometers
TRIGGER BUTTON Switches to the trigger set up screen to enable and configure level triggering ? BUTTON The ? icon opens Safari to the Vibration help page table of contents. Diffraction Limited Design LLC Southington, CT USA • 06489 Phone: (860) 276-0450 Fax: (860) 620-9026 • Contact: Bruce Truax dld-llc.com
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Vibration Manual 03 HOW TO CALIBRATE When you press the Calibrate button from the Time Series Settings screen you will be taken to the calibration page where you will see a start and cancel button. When you are ready to calibrate press the Start button and orient the device as shown by the graphics on the screen. As soon as the calibration is complete for an orientation the device will beep. At this point change the orientation to the next position. The calibration process consists of 6 orientations. The last one being face down. If at any time during the calibration process you wish to stop, press the abort button. The calibration value for any axes which have been completed will be saved.
HINTS FOR SUCCESSFUL CALIBRATION Vibration watches what you are doing during calibration and tries to assure that only excellent quality data is used. The software checks to make sure the device is in the proper orientation and that it is very stable. If you do not satisfy these requirements within 10 seconds Vibration will abort the calibration process and present you with an Alert box explaining that calibration failed. The best way to achieve a successful calibration is to hold the device on against a level horizontal surface, preferably braced against a vertical wall. The calibration will fail if any of these conditions are not satisfied: •The uncalibrated, measured acceleration must be within 10% of the expected value of 1. •The device must be within 6 degrees of vertical (corresponding to a 0.5% error in calibration). •The measured rms acceleration must be less than 0.15g for 1 second.
HOW ACCURATE IS THE CALIBRATION? The devices I have tested are typically much better than 5% prior to calibration. On my iPod the scale factors are within 2.5% and the 0 point offset are less than 0.04g. With careful calibration the residual offset errors are less than 0.01 and the scale factors are good to better than 0.5%.
CAN I SEE THE CALIBRATION VALUES? You sure can! Open the settings app and navigate down to the bottom until you see Vibration. Touch Vibration and you will be taken to the Vibration settings. The calibration values are at the bottom of this table. You can even edit these values if you desire. If you want to return to factory calibration simply enter 0.0 for the offset values and 1.0 for the scale correction values. NOTE: These calibration values are only used by Vibration. They will not affect or improve the performance of any other apps.
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Vibration Manual 04
HOW TO SETUP TRIGGERING
Return to Table of Contents Sometimes you want to start data acquisition after a specific event occurs such as an impulse on one or more axes. The trigger setup view allows you to turn on triggering for one or more axes as well as microphone input and set a level above which the trigger will trip and data acquisition will start. The triggers are level triggers and they are tripped when the level on the selected channel(s) are either greater than (in the event a positive level is set or the microphone is selected) or less than (in the event a negative level is set) the selected value.
HOW DOES DC REMOVE AFFECT TRIGGERING? If DC remove is ON then the triggers on the accelerometer channels are relative to the first sample. In other words, the first sample after the delay timer is used as a baseline. This baseline is subtracted from all future samples. The trigger will trip when the value after baseline subtraction exceeds the trigger level. If DC remove is off the trigger levels are absolute. Remember gravity! One or more channel(s) will have a significant bias due to gravity and you must take this into account when setting your trigger levels. Diffraction Limited Design LLC Southington, CT USA • 06489 Phone: (860) 276-0450 Fax: (860) 620-9026 • Contact: Bruce Truax dld-llc.com
HOW DO TRIGGERING AND SAMPLE DELAYS WORK TOGETHER? If a non-zero sample delay is selected the sample delay will occur first. When the delay expires Vibration will look for the trigger. This allows you to set a sample delay and a trigger and put the device into position before the trigger becomes active
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Vibration Manual 05 FREQUENCY DISPLAY The frequency display shows amplitude vs. frequency display for the three accelerometer channels. It is not possible to initiate data sampling or capture screen images from this tab. To initiate these functions it is necessary to return to the Time Series tab.
SETTINGS BUTTON The Settings button displays the frequency display settings screen allowing adjustment of the parameters which control the display of the frequency data.
ZOOMING IN FOR A BETTER VIEW You can zoom in 2x or 3x for a better view by double tapping with one finger on the screen. Zoom back out by double tapping again or by tapping with two fingers. Set the default zoom value in the Settings app. Note that 3x zoom requires more memory and will be disabled automatically if a memory warning occurs. You can re-enable 3x zoom by quitting Vibration and turning it on in the Settings app.
INSPECTING THE DATA Beginning with version 1.3 you can inspect the data by touching the screen. A cursor will appear under your finger and the frequency and amplitude values for all three channels will be reported in a heads-up display window. These values will update live as you move your the cursor. This feature also works in the zoom mode.
INTEGRATION The frequency data can be converted to velocity (touch integrate once) or displacement ( touch integrate again). The values reported are in mm/sec (in/sec) for velocity and mm (in) for displacement. NOTE: In velocity and displacement mode the frequency graphics are automatically switched to log-log mode. This automatic conversion is done because the large dynamic range make a linear Y scale useless. When you differentiate back to acceleration your selected display mode will be restored.
DIFFERENTIATION You can return from displacement to velocity or from velocity to acceleration by pressing the derivative button
UNITS English units can be selected for velocity and displacement displays. The units are set in the Settings App.
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Vibration Manual 06 FREQUENCY DISPLAY SETTINGS
AXIS DISPLAY SWITCHES The three axis display switches allow the user to turn individual frequency plots on and off in order to see more detail eliminating the clutter of the other plots. These switches are only visible when at least one of the two axes is in Log mode. When both switches are in Linear mode the three plots are automatically plotted separately in a stacked plot.
LOG PLOT SWITCHES The two log plot switches determine if the respective axis is plotted in log or linear form. When both switches are off the three plots are staggered on the display, otherwise they are overlaid on top of one another.
HZ/RPM SWITCH Select the units for the frequency axis. The selections are Hz (cycles/second) and rpm (revolutions/minute). RPM was added for those users who are using Vibration to balance machine tools.
FREQUENCY SCALE The frequency scale control sets the vertical scale of the frequency plots when the Y axis Log Switch is off and the Y plot is in linear mode. When the Y axis Log Switch is on this control is hidden. This control is a live control so when it is changed the current frequency data is updated with the new scale.
HAMMING WINDOW SWITCH This Hamming window switch determines if a Hamming window is applied to the data prior to the computation of the Fourier Transform. This switch is a live control so when it is changed the frequency plot is immediately updated.
EMAIL BUTTON The email icon can be used to send an email to the author of the program in the event of problems or questions.
? BUTTON The ? icon opens Safari to the Vibration help page table of contents.
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Vibration Manual 07 APPLICATION SETTINGS There are some settings for Vibration which are changed infrequently. Rather than complicate the Vibration application itself Apple provides a nice place to locate these settings in the Settings app. The settings app is typically found on the first page of your device
When you open Settings scroll to the bottom of the page and you will see the Vibration settings.
This page will allow you to set a default email address for your CSV data, turn on sounds which indicate the start and stop of acquisition and the countdown of the delay timer, switch from left handed to right handed coordinate systems and set the default zoom value. At the very bottom of the page are the accelerometer calibration constants. If you have not calibrated you accelerometers these values will be set to zero for the offsets and 1 for the scale factors. Once you have calibrated the device the actual calibration constants will be displayed. If you want to fine tune the calibration you can manually edit these values. Please be aware that these values are only used by Vibration and they do not affect the calibration of other apps which use the accelerometers.
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Vibration Manual 08 LOW MEMORY WARNING Some users have reported seeing low memory warnings when running Vibration. The iPhone operating system sends warning messages to running applications when memory is running low. Vibration warns the user if these memory warnings occur. Each window which is displayed receives this error message so you may find that you need to dismiss this message 4 separate times. This will be fixed in the 1.2 release.
SHOULD I WORRY IF I SEE THIS MESSAGE? The lower memory warning is just that, a warning. By the time these messages are shown the application is typically fully initialized and as long as the program is running there should not be a problem. If memory becomes critically low the operating system will force the application to quit. I have seen this happen on my iPod, typically when switching to the frequency display
WILL LOW MEMORY AFFECT MY DATA ? This is a very good question and I do not know the answer. When I have seen this error I have not noticed an affect on acquisition. But if memory is low the operating system may need to swap information from RAM to non-volatile memory. This could slow down data acquisition which will degrade the accuracy of the time base.
I NEED TO TAKE CRITICAL DATA, HOW CAN I AVOID A PROBLEM? The iPhone operating system only allows one user program to run at a time but some Apple programs such as Safari do stay running in the background. Also, some apps which have quit can leave a memory footprint, perhaps for quicker launches later. If this memory is needed by another application the operating system will make it available, but this takes some time. There is no CLEAR MEMORY button but you can use an App such as FreeMemory to free up some memory. FreeMemory appears to do this by force quitting Safari. In my experience this does free more than enough memory for Vibration to run with no warnings. An alternate method is to shut down and restart your iPhone/iPod.
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Vibration Manual 09 FREQUENTLY ASKED QUESTIONS WHAT CAN I DO IF I FIND A BUG? If you encounter a bug in Vibration please use one of the email buttons on the settings pages to send an email with a description of the problem. Describe what caused the problem and how it can be repeated. Efforts will be made to fix it as soon as possible. THE TIME BASE SEEMS WRONG I have tested Vibration on both a first generation iPod Touch and an iPhone 3Gs. and I have noticed periodically at times the data acquisition on the iPod going very slowly. A 100Hz acquisition of 128 samples which should take 1.28 seconds taking over 3 seconds. After a lot of debugging I determined there was some Apple task running in the background which was grabbing a lot of CPU cycles. So, for any of you that suspect that the timebase is running slow, turn your device off and back on and the problem should resolve itself, at least for a little while until the offending processes relaunch.
WHERE IS THE “SETTINGS.APP”? To set your email address, turn on sounds, set RH coordinates and view calibration values, quit Vibration and run the “Settings” app typically found on the first page of your device. Scroll to the bottom and you will see Vibration. Touch the name and you will see the settings information.
WHAT DO I DO IF I GET MEMORY WARNING? There is an entire page devoted to memory issues located here.
WHY IS ONE AXIS NOT WORKING ? If one or more axis there are two possible reasons. First, when I got my new iPhone and first tested Vibration I noticed that the Y and Z axes did not work. If you see this, before you call Apple and complain that your phone does not work, please go to Settings.app and navigate to the settings for Vibration. Make sure all of the axes scale factors are set to 1.0 and the offsets are set to 0.0. Quit the settings app and try Vibration once again. Second, if you have done what is recommended in the previous paragraph and one or more axis is still not working this could mean that there is a problem with the accelerometer in your iPod or iPhone. Download one of the free bubble level programs and see if you have a problem with the same axis. If you do the device may need repair. This is a function of the way the graphs are drawn. Each time a new point is added the entire graph is redrawn up to the last point. On longer data sets the redraw process can take too long causing a slow down in the sampling rate. It is hoped that in the future the graphics code will be improved to allow live plotting for all data set lengths.
WHY DON’T YOU PLOT THE LONGER DATA SAMPLES LIVE LIKE YOU DO THE 128 AND 256 POINT SAMPLES? This is a function of the way the graphs are drawn. Each time a new point is added the entire graph is redrawn up to the last point. On longer data sets the redraw process can take too long causing a slow down in the sampling rate. It is hoped that in the future the graphics code will be improved to allow live plotting for all data set lengths.
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DO YOU PLAN TO ADD A CONTINUOUS MODE? Some reviewers have requested this functionality. It may be possible to implement it for the 128 and 256 sample lengths. I plan to investigate this option if there is enough demand. In this case the frequency analysis would not be computed until sampling is halted.
HOW ACCURATE IS THE TIME BASE? I have been investigating the time base accuracy and it is probably good to about 10-12%. Typically at the 100Hz sample rate it runs slow by about 10Hz. If something happens in the background on your device it there could be a couple of points which have abnormally long sample times (I have seen as high as 0.05sec at 100Hz sampling for one point in the series). I am working on a reliable method to calibrate the time base and I hope to include it in a future release.
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