Page 46 - Design for Six Sigma a Roadmap for Product Development

24 Chapter Two

Angular speed

Lateral
acceleration Turning
radius

Steering Wheel
Figure 2.3 Vehicle steering system.

For example, Z could be environmental variations, user conditions, or
manufacturing variations. The output Y is usually a function of design
parameters X and noise factors Z.
Y f (X, Z) (2.1)

A good example of a P-diagram is given by Taguchi and Wu (1986).
Example 2.1. Automobile Steering System (Taguchi and Wu 1986) In an
automobile steering system (see Fig. 2.3) when the driver turns the steering
wheel, the vehicle’s direction should change according to the degree of turn
and how quickly the driver is turning the wheel.
The inputs, process, and outputs of the steering system are as follows:
Inputs. User intent: steering wheel turning angular speed; energy: mechan-
ical energy
Process. Vehicle steering system
Design parameters X. Column design parameters, linkage design parame-
ters, material properties, and so on
Noise factors Z. Road conditions, tire air pressure, tire wear, load in vehicle,
load position, and so on
Outputs. Turning radius, lateral acceleration of the vehicle
Figure 2.4 shows the P-diagram of the steering system.
Another useful process model is the supplier-input-process-output-
customer (SIPOC) diagram (Fig. 2.5).
A SIPOC diagram is one of the most useful models for business and ser-
vice processes. It can also be used as a model for a manufacturing process.
The acronym SIPOC derives from the five elements in the diagram.