Taguchi's Robust Design Principles and Automotive NVH

For all NVH engineers, the main goal is to optimize the vehicle design for the best possible sound quality and the least vibrations perceived by its passengers. However, in real-world scenarios, variations in materials, manufacturing processes, ageing, customer usage or environmental conditions can lead to some field complaints regarding higher NVH levels, though many other customers are happy.

With the famous Taguchi's approach, we can design the vehicle- systems or components such that its NVH would be robust to the above-mentioned variations. This can be achieved by focusing on non-linear relationship to be explored from the overall Transfer Function as shown below:

By designing the system to operate within a Robust zone, its performance would remain relatively stable even in the face of external variations. Taguchi's methods typically involve conducting Design of Experiments (DOE) to identify the optimal settings for the factors involved in the system.

A good example could be a Radiator cooling Fan vibrations of a passenger car. If its imbalance goes beyond a set limit (20 gm mm), then it would induce mid-frequency vibrations at the steering wheel.

Here, the first natural frequency in bending of the steering column as assembled in the trimmed body with an air-bag at the steering-wheel generally lies between 28 and 38 Hz whereas the Fan RPM ranges from 1800 to 2400 (first order ranging from 30 to 40 Hz)!

In this case, in order to achieve a six-sigma quality of the car-NVH where 99.9999966 % customers will not have the resonance of their steering wheel vibrations when the cooling Fan reaches its low or high speed, either of 3 control parameters need to be set as follows:

1. in order to assure frequency-separation between the natural frequency the steering wheel system and the first fan order of rotation > 2 Hz, either the Fan speeds need be chosen out of the range or the steering column bending natural frequency need be increased

2. a tuned mass damper needs to be placed at the steering wheel

3. a cooling Fan should be mounted directly on the power-train whose rubber mounts on the chassis must assure > 95 % vibration isolation efficiency at the Fan-order.

Another example could be the Power-train rubber mount stiffness variations in production (from +/- 10 % to +/-15 %) which seriously influence the NVH of vehicles especially at idling when the IC engine-RPM may also fluctuate or in case of multi-cylinder engines, half order excitation forces might get generated due to non-uniform combustion forces.

Here, the mount transmissibility of vibrations can come in close proximity of its amplification zone.

The Robust performance will be then assured by adding a high damping element into the system in a form of a Hydra-mount whose frequency will be tuned close to the idle RPM of the engine.

Often encountered brake squeal problems can be resolved by designing a brake pad such that its natural frequencies in the range of 1k to 4k Hz do not come close to those of the rotor/disc/wheel-rim or of the Caliper assembly. Modal decoupling should be also assured for in-plane modes of vibrations of the brake-disc for avoiding the high frequency squeal above 5k Hz.

If this is difficult, then use of tuned masses or a constrained shim-damper or anti-squeal sprays on the brake pads will be inevitable to make the system insensitive against the squeal issue.

There could be many more such examples where engineers have to fight for the optimal setting of those "control" design parameters in a Robust zone to aspire for the 6-sigma status of NVH of production vehicles!