Function Structure Diagram Example

A coffee roaster FSD will be constructed as another illustrative example. A few companies have been trying to cash in on the rise of interest in quality coffee. No company has designed and marketed a completely satisfactory roaster for the home market. As part of an effort to develop a coffee roaster to be used in a home, a FSD will be developed.

Step 1 Understanding Functions


Step 2 Creating the Black Box Model

Product’s Basic Function:┬áRoast Beans

Input Flows

  • Green Coffee Beans
  • Electricity
  • Desired Roast Level
  • Start Signal

The desired roast level is how dark the user would like the roasted beans. Different roast levels will produce coffee with different flavors.

Output Flows

  • Roasted Coffee Beans
  • Chaff
  • Unpleasant Odor
  • Noise
  • Heat

Chaff is a thin skin that comes off of the coffee bean during roasting. The chaff must be separated from the roasted beans before the beans are ground. Roasting beans produces an unpleasant odor that must be contained or eliminated. Grinding coffee produces the pleasant odor that many people associate with coffee houses.

Black Box Diagram
The black box diagram for the roaster is shown in figure 9.

blackbox diagram or coffee roasterFigure 9. The black box diagram for the roaster.

Step 3 Tracing the Input Flows

Trace for Green Beans
Figure 10. Trace for the input flow “Green Beans”.

Note that the functions “Heat Beans” and “Agitate Beans” are shown in parallel because they must happen simultaneously. If the beans are not agitated while they are heated they will burn.

trace for electricity
Figure 11. Trace for the input flow “Electricity”.
trace for information
Figure 12. Trace for the input flow “Start Signal” and “Roast level”.

The Process of stitching the traces together is not straight forward and requires thought and iterations. Some functions may need to be added. In this case, the functions “Separate Chaff”, “Store Chaff” and “Export Chaff” had to be added to the FSD. In other cases similar functions may need to be combined. Always check your final FSD against your original black box model.

Step 5 Assembling the traces and Selecting the Boundary

Function Structure Diagram for a roast, grind and brew coffee machine.

Figure 15. The traces assembled with boundary.

Step 6 Reviewing and Revising Your FSD

Do the inputs and outputs of your final FSD agree with the inputs and outputs of your original black box diagram?


What assumptions were made by your choice of input flows output flows and system boundaries?

In the Roaster example, the input “Electricity” assumes that the roaster will plug into a household outlet.

Are these assumptions appropriate for your project? Are they documented in your design brief?

The answer would depend on the project and the design brief.

Are all of the harmful, unintended or unwanted output flows documented?

Yes, heat, noise and chaff are included in the FSD.

Do your assumptions align with your design brief?

All assumptions should be explicitly documented in the design brief.

Does the boundary of the FSD communicate the scope of the project described in the design brief?

The answer depends on the design brief.

Are all Functions solution neutral?

Yes, all functions describe what must be done not how the function will be accomplished.

Can any of the functions be broken down into simpler functions?

No, each function is atomic.

Would the user benefit from output signals that confirm correct operation of the device?

Perhaps a signal that indicates when the ground coffee is ready to be removed from the machine would be useful. Customer feedback on this idea should be used to determine if it is included in the product.

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