Function Structure Diagram


A Function Structure Diagrams (FSD) is a graphical representation of the functions a product performs on its inputs and outputs. In a FSD, the overall function is broken down into elemental or atomic sub-functions. Each sub-funtion can not be broken down further and is solution neutral.
The sub-functions are connected by “flows” on which they operate. Flows are materials, energy or information that is used by or effects the product.
FSD’s are used for many tasks in the design process. Most importantly they can help break down a complicated design problem into manageable chunks. Solutions for each chunk can be found and then an engineering concept assembled from a group of solutions for each chunk. Remember FSD’s place the emphasis on what has to be accomplished rather than how.


Before attempting to build a FSD you should have a Design Brief, House of Quality and Engineering Specifications completed for the product.

Step 1 Understanding Functions

A function is the operation that the product performs on a flow or a set of flows to transform it from its input state to its output state. A flow is material, energy or signal that is used by or affects the product. In this context energy is the ability to make something happen. Examples of an energy flow are electrical energy, potential energy, kinetic energy, magnetic energy and heat. For example a George Foreman Grilling product transforms electrical energy into heat. An information flow is a signal provided to device or data that device acts on. For example an “on/off” switch provides a signal to a device. Finally a material flow is any physical entity that the device transforms. For example a coffee machine transforms ground coffee and water into coffee.

A functional description is a combination of a function (verb) acting on a flow (object). Examples of functional descriptions include:

Cook Food – The function is cook and the flow is food.

Deposit Lead – The function is deposit and the flow is lead.

Transport People – The function is transport and the flow is people.

What common products can accomplish these functions?

Note that each functional description tells what the product does not how the product performs the function. For instance the function “Transport People” could be accomplished with a bicycle, a car, a bus or a plane. Since the functional description does not tell how the function is accomplished the functional description is solution neutral. Remember functional descriptions must be solution neutral because we do not want to begin focusing on how a product accomplishes a function until we completely understand what the product must do.

Step 2 Creating the Black Box Model

The next step is to create a Black Box Model of the product. A Black Box Model consists of the overall function of the product, the flows into the product and the flows out of the product. A diagram of a generic Black Box Model is shown in Figure 1.

Figure 1. A generic black box diagram.

Figure 2 shows a typical pneumatic nail gun. These guns use compressed air to drive nails into a variety of materials.

Figure 2. A pneumatic nailer.

Therefore if we were constructing a black box diagram for the product the input flows would be compressed air, nails and trip signal. Compressed air is a form of energy, nails are a material and the trip signal is information. The products overall function is “Drive Nails”. The desired flow from the product is a driven nail. Unfortunately there are some other, undesirable flows out of the product. These undesirable flows include noise and heat. As designers we may want to minimize these undesirable outputs. The full black box diagram is shown in figure 3.

Figure 3. The black box diagram for a pneumatic Nailer.

Step 3 Tracing Flows

Next the input flows are traced through the system as they are transformed by the functions. The input flows for the nailer are nails, compressed air and a trip signal. We will start with the nails. First the nails are accepted and stored. Then one nail is isolated and finally Kinetic energy is applied to drive the nail. The tracing of the nails is shown in figure 4.

Figure 4. Tracing of the flow "Nails" through the product.

Figure 4. Tracing of the flow “Nails” through the product.

Figure 5 shows the trace for the flow “Compressed Air”.

Figure 5. Tracing of the flow "Compressed Air" through the product.

Figure 5. Tracing of the flow “Compressed Air” through the product.

Figure 6 shows the trace for the flow “Trip Signal”.

Figure 6. Tracing of the flow "Trip Signal" through the product.

Figure 6. Tracing of the flow “Trip Signal” through the product.

Step 4 Assembling the Traces

To assemble the traces functions in more than one trace must be combined. For instance the “Apply K.E. (Kinetic Energy) function is in the “C. Air” (Compressed Air) and the “Nails” traces. Occasionally extra sub-functions must be added. Iterations are always required. Figure 7 shows the assembled traces.

Figure 7. Traces assembled.

Figure 7. Traces assembled.

Step 5 Selecting the Boundary

Finally devices boundary must be defined. The boundary separates the Functions that the device must perform from the functions that the operator or another device must perform. Figure 8 shows the boundary for the pneumatic nailer.

FSD of Nailer finalFigure 8. The final FSD for the nailer.

The boundary should agree with the scope of the project described in the design brief. For instance, the inclusion of the the function “Compress Air” would radically change the required design! Think of the consequences of inclusion of the function “Load Nails”.

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 nailer example, the input “Compressed Air” assumes that the nailer will be powered by compressed air.

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. Other nailers are powered by electricity or natural gas.

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

Yes, heat and noise are included in the design brief. Kickback, shock or vibration should be considered; especially if users complain about these outputs.

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?

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 if the nail was driven completely should be added. Again customer feedback on this idea should be used to determine if it is included in the product.


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