What is Robust Quality Engineering(RQE)?
Qulaity Loss Function:
Most of the graphics here are copyrighted by ASI Consulting Group. LLC.
Quality loss function is a mathematical expression of the social loss caused by product or process variability, based on the deviation from the target value.
Robust quality engineering (RQE, Robust Engineering, Taguchi Method) aims to minimize societal losses. The definition of Quality by Genichi Taguchi is given below.
- The quality of goods and services is the loss to society after they are shipped to the market. the quality is evaluated in terms of the loss to society.
- This loss does not include the loss caused by the fuction itself (e.g., alchohol) but includes price, cost of use, maintenance, and pollution.
The Kano model has been globally recognized as the model for customer wants and needs. RQE deals with the basic and performance needs, while a hands-down performance can be categorized as an excitement need.
What can poor engineering quality lead to?
Poor engineered quality causes failure modes and quality problems. If it is during the warranty period, the producer bears the repair cost. After the warranty period is over, the customer pays the price. Regardless of who pays the cost, there will be a loss. Losses are not limited to directcosts such as repair costs. If we look at society as a whole from a long-term perspective, the short-term & long-term, the direct and indirect, the tangible & intangible, losses will occur.
The content of Quality Loss can include the following.
▸Cost and time spent by customers and users on repair, maintenance, and replacements.
▸Customer/user dissatisfaction, discomfort, anxiety, injury, and loss of life
▸Low resale value.
▸Waste of global resources.
▸Warranty and Recall (Short-term loss for producers).
▸Decrease in repeat customers (Long term loss for producers)
▸Damage to corporate image (Long term loss for producers)
▸Decline in competitiveness and market share (Long-term loss for producers)
▸And more!!
Traditional quality assessment represented by inspection evaluates whether requirements are met. It is a Go/No-Go approach and is more of an accounting mentality. Taguchi's loss function evaluates the loss in a continuous fashion. For example, the differences in evaluating a characteristic with its target value m are as follows.
・Conventional quality assessment is usually a "go/no-go" or "pass/fail" as to whether it meets the requirements.
・This evaluation is not engineering, it's just accounting!
・When the characteristic value deviates from the ideal or target value, a loss occurs. The larger the deviation, the greater the loss.
・Engineering should make this assessment.
QLF is a monetary evaluation of quality and performance.
・Taguchi used this idea as the basis for the QLF, an equation for estimating loss.
・The graphics above show the case of a Nominal-the-Best response which has a target value. The loss function is also derived for the STB, LTB and Dynamic responses.
・The question is not whether the requirement is met or not, but how far it is from the ideal.
・The basic concept is that the loss is proportional to the square of the distance from the ideal.
・Being able to estimate the loss in monetary values allows us to apply it in situations where we want to balance cost and quality.
Application of Quality Loss Function
▸Tolerance design for investment decision making. Example: What if we buy a $20k measuring instrument that halves the measurement error?
▸Multi-factor tolerance design using an orthogonal array. Example: Which of the 14 parts should be upgraded to meet the requirement?
▸Setting tolerances and specifications (this is the same as determining the safety factors, called tolerancing) Example: ▸How to determine the specification of brake stopping distance?
▸Optimization of Process Control Scheme. Example: How do you determine the frequency of process checks, adjustment limits, and preventive failures in process control? What to do and how much to do? Where to put the costs?