NVH- Function Generator

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NVH- Function Generator

Postby Steve Smith » Mon Aug 01, 2016 3:00 pm

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.

1. OPEN FUNCTION GENERATOR.jpg
Open Function Generator


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

2. FREQUENCY SWEEP MODE.jpg
Frequency Sweep mode


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.

3. SQUARE WAVE-STROBE OUPUT.jpg
Square waveform output


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

Take care………Steve
Steve Smith
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Re: NVH- Function Generator

Postby Steve Smith » Thu Aug 11, 2016 9:09 am

I was going to add this to the initial post but thought an explanation was in order before waffling on about another feature of the Function Generator…………..

In addition to the items above, the Function Generator can be used to simulate “Beating noise” or “Cyclic noises” such as those generated by tyres or engine components.

Beating or phasing occurs when two vibrations or sounds with slightly different frequencies exist in the same area.
A typical example would be tyre noise described as “Wow Wow” noise (Try describing a noise without making the sound!)

Think of two, equal size tyres one new, one worn:
Their circumferences will most certainly differ resulting in different rotational speeds/frequencies at a given road speed.
There will be occasions where the frequency/noise produced from each tyre will momentarily coincide with one another (in phase) so increasing the noise level.
Likewise there will be times when the tyre frequencies will momentarily shift (out of phase) so decreasing the noise level.

This is the same principle as noise cancelling headphones where external noise/frequencies are captured and the exact inverse of these captured frequencies are played back into the ear so cancelling out the external noise/frequencies.
The images below will assist with the above explanation.

4. BEATING NOISE.jpg
Beating noise explanation


Returning to tyre “Wow Wow” noise generated thanks to slightly differing frequencies, add into the mix different tyre tread patterns and we truly have a recipe for entertaining noises. I am thinking here of known quality branded tyres fitted to one side of the vehicle and “Ditch Finder” tyres fitted to the other!
I have seen transmissions, bearings, driveshaft’s and differentials replaced when the root cause of the customer complaint has been tyre noise!

What about the potential from the engine bay for beating/cyclic noises?

E2 is a second order engine vibration typically found in all four stroke 4 cylinder engines. (2 x combustion shocks directed onto the crankshaft per engine revolution)

Think now about imbalanced auxiliaries driven at near twice engine speed:
For Example:
Engine speed 3000 rpm divide by 60 = 50 Hz (E1 = 50 Hz)
E2 therefore = 50 Hz X 2 = 100 Hz (E1 X 2)
Crankshaft pulley 150 mm Water pump pulley 78.50 mm
150 / 78.50 = 1.910. Water Pump speed = 1.910 x E1 (Engine speed) = 95.50 Hz

An imbalanced water pump pulley running at near twice engine speed (E.g. 95 Hz) is very close to E2 (100 Hz) The water pump vibration may peak around 3000 rpm / 50 Hz (engine speed/frequency) and momentarily coincide with the characteristic second order vibration from the engine (E2). As they coincide (in phase) the noise/vibration will increase and as they “shift” the noise will decrease.

The frequency of the beating/cyclic noise will be equal to the difference between both frequencies at the point of maximum noise/vibration:
Engine (E2) 100 Hz Water pump 95 Hz 100 - 95 = 5 Hz.
5 Hz will be the frequency of the beating/cyclic noise

Therefore our frequency display (spectrum) would indicate two peaks in close proximity to one another. (E2 and WP)

Positioning the frequency markers at both peaks will reveal their individual frequencies and the difference between the two (Delta frequency value……….Our Beating Frequency)

So how does this all fit with the Function Generator?

In the simulated example below, a 4 cylinder engine running at a static engine speed of 3000 rpm (50 Hz) has been entered into the “Vehicle Information” section of the NVH software.

A water pump pulley diameter of 78.50 mm has also been added under the “Advanced” section within “Vehicle Information” driven by a crank pulley of 150 mm diameter.

5 BEATING SIMULATION.jpg
Beating Simulation


Once again, 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 wave) at your selected frequency.

95 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) 95 Hz will now be played back via the PC speakers or “audio out” headphone socket.

With a continuous 95 Hz audio signal now played back, select the play button beneath the signal history to playback your NVH recording simultaneously.

Both frequencies played back (E2 at 100 Hz and WP at 95 Hz) will produce a beating frequency at 5 Hz.
This will be exceptionally useful when playing back audio to customers or colleagues who have experienced or complained about a specific noise but find it difficult to describe or relay.

Reproduction of the beating frequency and sound pitch (acknowledged/confirmed by the customer) will confirm your discovery to be relevant to the diagnosis.

In addition to the Fixed Frequency option as the “Source” of the Function Generator we can select a specific vibration as our “Source” and then apply a correction factor in order to track a specific frequency such as E2 or WP throughout the entire engine speed/frequency range.

In the example below E1 (50 Hz) has been used as the “Source” vibration and a correction factor of 1.910 applied to E1.

6 TRACKING E1.jpg
Tracking a specific frequency


The "Source" of the Function Generator is now E1 (50 Hz)
The correction factor is the speed relationship between engine speed (E1) and water pump speed (WP)
150 mm crank pulley / 78.50 mm water pump pulley = 1.910
1.910 x 50 Hz (E1) = 95.54 Hz (water pump speed / frequency)

Now, rather than a Fixed Frequency output from the Function Generator, the Source is now E1 with a multiplication factor of 1.910.

The Function Generator will now output a frequency relevant to E1 x 1.910 allowing the output to track E1 throughout the entire engine speed / range

For example E1 now at 60 Hz (3600 rpm)
60 (E1) x 1.910 (Correction Factor) = 114.60.
The output from the Function Generator will be at 114.60 Hz

The tracking feature of the function generator will initially be available whilst recording live only and not during playback of saved results.

A future New Feature Request will be filed for the tracking feature to be made operation during playback of saved results

The PD data file used in the example above will be posted as soon as the new beta build is available

Take care………Steve
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Re: NVH- Function Generator

Postby Mark Dalton » Sun Aug 14, 2016 11:01 pm

Hi Steve,
These new features look very cool, any outlook on the next Beta release?
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