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.