Vibration order identification with no speed signal

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Steve Smith
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Joined: Sun Aug 25, 2013 7:22 am

Vibration order identification with no speed signal

Post by Steve Smith »

Following on from our Audio Import forum post here viewtopic.php?p=93381#p93381 we did touch on vibration order identification if our customer can provide some key pointers about the audio recording.

For example, our customer mentions “The recording was taken at 30 mph in 3rd gear”. Armed with this information and our product knowledge we can then begin to calculate several known frequencies and so determine their origin (Vibration order identification)

Typically, vibration orders are identified in our NVH software (E1, P1 & T1) because we obtain Engine RPM and Vehicle Speed via the OBD connector.

E1 calculation
The NVH software will convert Engine RPM into Hz simply by dividing the obtained OBD engine speed by 60 and so display E1 (3000 rpm / 60 = 50 Hz)

T1 calculation

NVH will convert the obtained OBD vehicle speed into Hz thanks to the entered tyre size information
E.g., Vehicle speed approx. 67 mph (108 km/h) with a tyre size of 205/55/R16
Vehicle speed / Tyre Circumference
Circumference = Diameter (0.6 m) x Pi (3.14) = 1.884 m
Wheel and Tyre frequency = 108 km/h X 1000 m / Tire circumference x 3600 seconds
108000/6782 = 15.92 Hz (T1)

P1 calculation
Once we know T1, the world is our oyster!
NVH will multiply T1 by the entered differential ratio (RWD, AWD & 4WD only)
E.g., T1 (15.92 Hz) x 2.7:1 (Rear Diff Ratio) = 42.984 (P1) the speed of our propshaft

As you can see from the above, a combination of OBD speed inputs and data entry (tyre size and diff ratio) are vital for NVH to display vibration order identifiers.

So, what happens when there is no speed input into NVH? (Electric vehicles are a prime example as emission related data is longer required at the OBD connector)

Basically, the answer is you have raw data with no vibration order identification!

In the example below we have just this scenario where a microphone and accelerometer (mounted in the cabin) have captured raw data during a road test to diagnose a complaint of vibration
Image 1
Image 1

How on earth can we diagnose anything from the data above?

The answer lies in the application of a cabin mount microphone in conjunction with driver narration.

Please refer to the set up below (3 axis plus single channel mode) where a mic is also included inside the cabin in order to record the drivers’ comments (Note also the limited data entered into our NVH software)
Image 2
Image 2
Using quality “over-the-ear” headphones to listed to the driver narration, we can begin to plot several key areas within the recording thanks to the announcements. We also obtain clues such as engine boom to indicate load/acceleration and “ticking” turn signal indicators to suggest a steering input may be imminent

Please have a listen to the file below:
NVH File
(15.09 MiB) Downloaded 523 times
Based on the drivers’ comments, we can add “makers” to divide the signal history into key areas of interest. To add markers, use the marker button or right click on the signal history; use the “View” menu to add notes to describe what the markers indicate (Remember to resave this file or your makers and notes will be lost upon closing NVH)
Image 3
Image 3
With the signal history broken up into key areas of interest, the driver clearly mentions a strong vibration at 55 mph which is backed up with a noticeable vibration peak at 11.7 Hz (See below)
Image 4
Image 4
What could this vibration be as we have no vibration order identifier?

Let us start with E1; the driver mentions an engine speed of approx. 1350 rpm between markers 1 and 2. 1350 / 60 = 22.5 Hz, this suggests our vibration peak is not a first order engine vibration

Now let us try T1; we know the vehicle is fitted with different tyre sizes front and rear as this information was entered into the software 225/40R19 front and 255/35R19 rear. (If this information had not been entered, we could refer to product knowledge and tyre options for the vehicle under test.

T1 = Vehicle speed (km/h) x 1000 / 2 Pi x Tyre Radius x 3600 (seconds)
Vehicle speed: 55 mph = 88.51 km/h (Multiply MPH by 1.609)
Tyre Radius: NVH displays the tyre diameter as 66.26 cm (front) and 66.11 cm (rear)
Add these together and divide by 2 to find the average as we are not concerned with identifying front or rear tyre at this stage
66.26 + 66.11 / 2 = 66.185 cm (0.662 m) Divide by 2 for the radius = 0.331 m
(88.51 x 1000) / (2 x 3.14 x 0.331 x 3600)
88510 / 7,483.248
11.83 Hz

I think we can agree this is close enough to be our offending vibration at 11.7 Hz and can be considered a T1 first order tyre vibration (Remember, not necessarily a tyre, possibly a component rotating at the same frequency as a tyre)

Earlier on I mentioned “when you have T1 the world is your oyster” ………………

From T1 we can now calculate P1 as the differential ratio for the above vehicle is 2.81:1

T1 @ 11.83 x 2.81 = 33.24 HZ (P1)

Given we can now locate E1, T1 & P1, can we identify the gear position? Yes.

The above vehicle is fitted with the ZF 8-speed transmission with the following ratios:
Image 5
Image 5
With E1 at 22.5 Hz (transmission input) and P1 at 33.24 (transmission output) we clearly have an over-drive condition as our output is faster than our input

Input / Output = Gear ratio

22.5 / 33.24 = 0.676 which I think we can agree was 8th gear (note vehicle was at a fixed 55 mph based on the narration and so torque convertor would be locked)

We have used the above gear ratio calculation here viewtopic.php?p=103173#p103173 looking at engine power evaluation with our TA143 accelerometer

Below we have added our “calculated” vibration order identifiers based on the driver’s narration and math’s (which as we all know “Math’s is cool”) Note, I have hidden channel D audio waveform from view for clarity (Right click in frequency view and click on “Channel in view”)
Image 6
Image 6
Notice above how once you have found your fundamentals (E1, P1 & T1) you simply multiply by 2, 3 or 4 respectively to find their associated orders/harmonics (More on harmonics here viewtopic.php?p=88781#p88781)

For example, to locate our engine order linked to combustion (E2 for this 4-Cylinder engine) we multiply E1 @ 22.4 x 2 = 44.8 Hz (E2) To find E3 we multiply E1 x 3 and so on.

Please note that the above is no substitute for an OBD (or other) speed inputs as this technique works for fixed speed as narrated by the driver; for constantly changing speeds it will be difficult to categorically identify peak vibrations without graphing engine or vehicle speed

With that said, identifying “expected” vibrations such as E2 under acceleration and deceleration is not too challenging. E2 will be the predominant vibration during acceleration due to combustion (you can hear this acceleration during the recording)

E2 will instantly disappear when decelerating (as clearly indicated by the driver’s narration) These inherent characteristics will help you identify E2, and remember E1 is simply E2 /2 and so you also detect engine speed (E1)

Below we have E2 as the predominant vibration during acceleration and E1 at 33.1 Hz x 60 = 1986 rpm
Image 7
Image 7
Below, E2 has vanished due to deceleration, over-run fuel cut and hence no combustion
Image 8
Image 8
Coming back to T1 and the world being your oyster, once we know the road wheel frequency, we can calculate the tooth contact frequencies of components such as differentials, transmissions, and EV drive trains

Let us say for example we have a whine from the drivetrain at 55 mph and a peak in our audio waveform (Channel D cabin mic) that appears above all other frequencies when this whine is present (they are therefore related)

To identity this “unknown peak” we can start with T1 as we know where this order resides in the spectrum, at 55 mph (11.7 Hz)
With knowledge of the tooth count for our crown-wheel & pinion in the differential (e.g., 45/16 to obtain a differential ratio of 2.81), any differential whine attributed to crown wheel tooth contact (meshing noise) will appear at T1 x 45 teeth (526.5 Hz) @ 55 mph (see below)
Image 9
Image 9
Taking this one step further, imagine now an EV drive train containing an electric motor, reduction gear and a differential as the entire transmission assembly (very common approach)

Once again, if you know T1 for a given vehicle speed and have knowledge of the tooth counts of all gears within the drive train you can calculate which is your offending gear within the transmission that is responsible for the customer complaint

Having this knowledge before dismantling the drivetrain is paramount and the diagnosis involved to obtain this invaluable data was as non-intrusive as it gets. (I.e., the strategic mounting of microphones and accelerometers)

Below is a one such example where T1 helped locate the offending gear responsible for a customer complaint of cabin whine at low road speed. (Hybrid vehicle)

Note, RPM and Vehicle speed were present via OBD, however, using this example we can see the principle of using T1 to locate offending components)
Image 10
Image 10
Based on the offending 1 kHz frequency above, we conclude the offending component below using T1 combined with product knowledge and technical data
Image 11
Image 11
The above case study is discussed at length with Frank Massey in our NVH series here ... Automotive (9:53 minutes onwards)

Staying with EV drivetrains for a moment longer, EV motor speed (like engine RPM) is an invaluable input for NVH diagnosis, but as mentioned earlier, not available or implausible at the OBD connector

Once again, T1 to rescue; if you have knowledge of the EV drive train and more importantly the “Total Ratio” you can find Motor Speed (M1)

Below is an example using our E-Golf where the following drivetrain calculations apply:

The transmission of the E-Golf contains a single gear ratio of 2.704:1 and a final drive ratio of 3.608, our total overall gear ratio is equal to 2.704 x 3.608 = 9.756 :1

Therefore, T1 x 9.756 = Motor Speed (M1)
Image 12
Image 12
Note above how we can use the “Add Vibration” feature to place a bespoke vibration order identifier (M1) to denote E-Motor speed. This was possible because we are graphing vehicle speed via the OBD. The keen eyed would have also notice the implausible rpm values in the signal history! Again, another example where the VM no longer has to provide emission related data at the OBD connector, (On this vehicle we were lucky to get vehicle speed)

The following case study from Maróti Könyvkereskedés Kft. in Hungary uses this technique to locate a very intriguing EV fault here ... Automotive

I hope the above is of some help when you cannot obtain OBD data; remember, we also have alternative RPM inputs using a signal on channel D (e.g., crank sensor or optical pick up) and a static rpm value if all else fails

Take care……Steve

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