Whilst NVH diagnosis marches on, a feature not yet discussed at any length is the Function Generator
I have been experimenting with the potential of this feature and have to say it is going to prove invaluable when homing in on noises and vibration.
To activate the Function Generator an NVH licenced PicoScope is required to be connected to your PC before selecting the NVH test option, or loading a previously captured NVH file.
Once connected (NVH selected) click on the Options tab and select Function Generator.
The Function Generator user interface will open where you can enter a frequency of interest and playback a fixed audio frequency (Sine or Square wave) at your selected frequency.
In the example below a transmission whine is detected at 1250 Hz and so 1250 Hz is entered as the frequency of interest. (Either by typing the specific value, or using the slide control)
Clicking on the speaker symbol (item 3) 1250 Hz will now be played back via the PC speakers or “audio out” headphone socket.
With a continuous 1250 Hz audio signal now played back, select the play button beneath the signal history to playback your NVH recording simultaneously.
Using headphones you can make a subjective comparison between both audio signals (function generated and NVH capture).
Be aware here as the transmission whine is far from a fixed, continuous audio signal at 1250 Hz (more like a continually shifting signal from 700 Hz to 1250 Hz depending on transmission speed including harmonics)
In this scenario with a shifting frequency (700-1250 Hz) we could use the Frequency Sweep function of the Function Generator.
Given our transmission whine commences at 700 Hz approx. and increases to 1250 Hz within 5 seconds we can use the Frequency Sweep function to simulate this exact scenario over the same time period.
Click on the Options tab and select Function Generator, select Sweep Mode (item 3) and enter the Start/End frequencies of interest (700-1250 Hz).
Enter the required time period of the audio sweep and the direction of the sweep.
Sweep up from low frequencies (700 Hz) to high frequency (1250 Hz)
This is the exact behaviour of our captured transmission whine that increases in frequency with road speed over 5 seconds (possibly less)
Clicking on the speaker symbol (item 6) will now play the frequency sweep via the PC speakers or “audio out” headphone socket.
Select the play button beneath the signal history to playback your NVH recording simultaneously for direct comparisons
Remember the Function Generator output can be played back via the In Car Audio system using the audio out of your PC, via Bluetooth or via a Frequency Modulator! (So how could this help with NVH?)
Customer’s complaining of interior rattles or trim noises can assist the technician using the Function Generator. The Function Generator can be used to playback continuous audio frequencies via the In Car Audio system in an attempt to reproduce trim rattles without road-tests and assumption of the customers complaint. (This is ideal for intermittent rattles)
The customer can identify and point out the rattles to the technician in order to remove the guess work and reduce repair time.
Be careful here as the Function Generator may introduce trim rattles the customer was not aware off!
The Function Generator can be used to playback very high frequencies via the In Car Audio System whilst searching for cabin air leaks or wind noise.
Playing back a fixed high frequency audio signal (around 20 kHz) whilst using the NVH Mic to “Sniff” about the vehicle exterior trim and monitoring the frequency display for high amplitudes at 20 kHz will indicate the source of the cabin air leak.
The Function Generator has the ability to playback an audio Square Wave not only a fundamental Sine Wave. With a fixed frequency audio square wave played back via the “audio out” headphone socket it is possible to drive a strobe light or high speed LED device.
I have used a strobe light here at Pico where the “audio out” passes through a "pick up coil" (no doubt with some clever circuitry involved) and a conventional inductive timing light connected about the "pick up coil" to pulse at the relevant frequency set by the Frequency Generator.
In a scenario where we suspect a vibration to be present from a cooling fan or tensioner pulley at 66 Hz (discovered in the NVH Frequency Display) we place a white mark on our fan blade or tensioner pulley, select a fixed Frequency 66 Hz Square Wave at the Function Generator user interface, where the timing light will now pulse at this exact frequency based on the signal present at the “audio out” pick up coil.
Aiming the strobe light at the offending fan or tensioner will give the illusion the fan is stationary, confirming the frequency/speed of the fan or tensioner so revealing the possible cause of our vibration.
Whilst this “strobe technique” has immense value when looking for unknown vibrations, Pico do not manufacture such a “pick up coil” for the PC “audio out” at present.
The NVH software has the capability for individuals to use the PC “audio out” to your advantage but at your own risk.
Please don’t feel as though I am teaching users to suck eggs but I know I will have to write the next line in the interest of health and safety.
Never look directly into a strobe light or use such devices if you suffer from epilepsy or are knowingly photosensitive.
Likewise, when aiming a strobe light at rotating engine components, they may appear stationary, please please do not attempt touch such parts.
I hope this is of some help and no doubt I will add more here as our use of the Function Generator grows