Robustness P-Diagram :: Overview Click here to go to the homepage
What is it?
The Parameter Diagram (P-Diagram) takes the inputs from a system/ customer and relates those inputs to desired outputs of a design that the engineer is creating also considering non-controllable outside influences. The P-Diagram, is a useful tool in brainstorming and documenting:
  Signal Factor(s)
  Response Variable or Ideal Function
  Control Factors
  Noise Factors
  Error States (or the failure modes)
P- Diagram is essentially a schematic diagram that encompasses signal factor, control factor, noise factor and response variable. Click here for a blank P-Diagram form in MS Excel.

Signal Factor (inputs) pass through the design of the product and is output into measured Response Variable (also called the Ideal Function). Signal Factor is transformed via the Control Factors to convert the input to the desired output.
Control Factors are typically elements such as design, materials and processes that the engineer has 'control' over.
Error States are the failure modes or effects of failure as defined by an end user when using the product.
Noise Factors are things that can influence the design but are not under the control of the engineer, such as environmental factors, customer usage, interfaces with other systems, degradation over time, piece-to-piece variation, among others. These Noise Factors, if not protected for, can make the design useless and it can be said that the design is not robust against the expected noise factors.
Why is it important?
The P-Diagram is based on the concept of converting 100% of input energy (input signal) into 100% of the ideal function. 
signal/noise = ideal function/error states
Any engineered system reaches its "ideal function" when all of its applied energy (input) is transformed efficiently into creating desired output energy. In reality, nothing functions like this. Every system is less than 100% efficient in its energy transformation. This loss goes to creating unintended functions, or error states.
When to use it?
Development of the P-Diagram should begin alongside development of the FMEA.
How to use it?
1. At initial design phase or at beginning of problem solving effort determine the overall objective the system.
2. Identify the cross-functional team, include members who can provide expertise in the following areas:
a) knowledge of systems approach
b) measurement know-how
c) information on how the system will be used
d) knowledge regarding sources of input variability
e) working knowledge of quality and testing methods
f) persons responsible of the design and/or manufacturing of the system being optimized
g) knowledge of statistical methods
3. Review components and their functionality - get all members of the cross-functional team familiar with the system or sub-system.
4. Boundaries of the system must be clearly defined to more easily identify the input and outputs.
5. Determine the Ideal Function
a) Understand the ideal relationship of the system which exists between the input signal and the functional response.
6. Determine Control and Noise Factors
a) Identify parameters that affect the function of the system and classify them into control factors and noise factors
b) Control Factors - specified by the engineering team
c) Noise Factors - parameters related to the usage, environmental and/or manufacturing variabilities that the engineer does not want to change or cannot change.
7. To continue with robust design, layout the test plan
a) Create a comprehensive testing and data collection plan that is designed to maximize research and development efficiency through the use of DOE.
b) Experimental samples are created either by building experimental hardware or by analytical modeling to simulate the test combinations
c) Calculations are made to determine which factors are important in reducing the variability of the response and adjusting the mean toward target.
d) Perform a test with samples configured at the optimum design levels to confirm the results of the DOE.
8. Implement results and plan for further improvements.
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