PDP — Design Verification


Design verification is the process by which we check whether what we designed (design output) matches what we asked for (design input). The concept is simple to state (Figure 1), but it can be incredibly challenging in practice, partly because of the sheer volume of checks that need to be made.

fc-Basic Verification Process

Figure 1. The basic concept of design verification: Did we get what we want?

In my experience, the design effort results in a drawing of the product which can then be used to make a physical prototype (Figure 2). So in verifying the design, we are checking whether the drawing or the prototype meet all the requirements we specified. The processes used for verifying the design range from simple visual inspection to elaborate tests.

FC--PDP

Figure 2. One possible product development process showing where design verification fits in.

Methods of Verification

Visual Inspection It is possible to confirm whether a design contains the required physical attributes through simple visual inspection. Physical attributes include:

  • Material
  • Size
    • Dimensions
      • Length
      • Radius
    • Aspect ratios
  • Shape
  • Color
  • Count of features
  • Surface finish
  • Surface coating

Table 1. The table shows one possible approach to design verification of physical characteristics. Design inputs are specified in the Engineering Requirements column. Design output in this example is the drawing titled B6-32.DWG. The evidence of the verification of a particular characteristic is given by the page number and grid location on the drawing.

t-design verification

Analysis Properties of the designed product may be calculated using physical laws (from Newtonian Mechanics)

  • Weight may be calculated using the design’s volume and material density
  • Deflection may be calculated using methods from Statics/Mechanics of Materials
  • Stress under given loads may be calculated using methods from Statics/Mechanics of Materials
    • Computational modeling such as finite element analysis (FEA) may be used, provided certain requirements are met (see the FDA’s guidance on Reporting of Computational Modeling Studies in Medical Device Submissions)
  • Tolerance Stack Analyses (TSA) may be performed for sub-assemblies and assemblies to determine whether the design conforms to the size requirements
  • TSA’s may similarly be used to determine whether the components of a sub-assembly or assembly fit together

Table 2. The table shows one possible approach to design verification of a property of the design. Design input is specified in the Engineering Requirements column. An engineering analysis report was created to show the calculation of the weight. The evidence of the verification is given by the page and section number of the report.

t-design verification 2

Testing Some aspects of the design will require testing. These include:

  • Biocompatibility
  • Package integrity

Table 3. The table shows a second possible approach to design verification of a property of the design. Design input is specified in the Engineering Requirements column. A prototype was built. The test was performed on it, and a test report was created to show the results. The evidence of the verification is given by the page and section number of the report.

t-design verification 3

Design Verification by Assembly Hierarchy

Now let’s take a step back and look at design verification from a different perspective: that of design hierarchy (Figure 3). Product design can be any combination of components, sub-assemblies, or assemblies. Verification should occur at each hierarchy of the design.

Figure 3. Overall product design can be any combination of components and sub-assemblies.

For example, in the case of an assembly made up of a bolt, a nut, and a washer, my approach to verification would be the following:

Component

Bolt

  • Perform a visual inspection of the bolt drawing to verify the bolt’s physical attributes e.g. size, material, color, etc.
  • Do the engineering calculations using the bolt’s design to demonstrate the bolt’s functional attributes e.g. torsional strength, bending strength, etc.

Nut

  • Perform a visual inspection of the nut drawing to verify the nut’s physical attributes e.g. size, material, color, etc.
  • Do the engineering calculations using the nut’s design to demonstrate the nut’s functional attributes

Washer

  • Perform a visual inspection of the washer drawing to verify the washer’s physical functional attributes
  • Do the engineering calculations using the washer’s design to demonstrate the washer’s performance attributes

Assembly

Nut-Washer-Bolt

  • Perform a visual inspection of the assembly drawing to verify the assembly’s physical attributes:
    • Are the correct bolt, nut, and washers specified?
    • Does the drawing specify the number of each component to be used in a single assembly?
    • Does it show how these components are to be assembled?
  • Perform a tolerance stack analysis of the components to show components will fit together, or build prototypes of each component and perform a test of assembly.

Different aspects of the design are verified at each level of the assembly hierarchy.

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