Connections and Interfaces

Interfaces control the flow of energy, material, and signal or information between components. They should be designed to consider equilibrium and equivalency of flows. For example, the interface between a power system and power using component should be specified to a consistent power level (voltage and current). Additionally, the same type of flow must be consistent. It may seem obvious that if a liquid exits one component it should not be considered a thermal energy by the next component. However, heat exchangers do this exact behavior. The concept of primary and secondary flow can be useful to ensure that designers are considering all aspects. Using this example, the interfaces must consider both a flow rate, pressure, and thermal energy carried by that flow. Failure to consider all properties of the flow can lead to unexpected failure.

Interfaces and the connections between components is the course of much of the complexity and required design hours for a product. There are a few principles that can help reduce the time and risk of needed redesign.

• Work from the outside in. It is generally best to begin with external interfaces and work your way into the details of the product. Consider the input and output flows of material, energy and information. Things like the power to a product as well as any human interaction interfaces. Next, it is helpful to consider primary flows or primary functions. These are the flows or functions that you as the designer deem are most critical.
• Be prepared for the generation of new functions and requirements. When specifying an interface, it can often lead to identifying new functions and requirements. For example, excess heat or vibration from two mating parts needs to be controlled – a new function introduced by the interface.
• Leverage interface design to support functional independence of components and assemblies. By keeping assemblies functionally independent, designers reduce the risk of change propagation. That is, if a flow must travel between to component or assemblies to perform a single function, then consider redesigning the component or assembly.

Consider how the complexity of a product grows with the interactions between components. We can describe these interactions using a design structure matrix. This is a special matrix with all of the components listed in both the columns and rows. This is also called an adjacency matrix in domain of math known as graph theory. The most complexity is when every component is connected with every other component. Imagine a densely interconnect graph or network. Ideally, we want to minimize the number of connections and implied interfaces. One approach to doing this comes from a design theory known as Axiomatic Design.

Axiomatic Design

Axiomatic Design is a systems design methodology that uses matrix methods to systematically analyze the transformation of customer needs into functional requirements, design parameters, and process variables. It is based on two design principles or axioms that govern the analysis and decision-making process in developing high quality product or system designs. The two axioms are:

Axiom 1: The Independence Axiom.  Maintain the independence of the functional requirements (FRs).

Axiom 2: The Information Axiom. – Minimize the information content of the design.

The Independence Axiom states that the FRs should be satisfied independently of each other by the corresponding design parameters (DPs). This ensures that changing one DP does not affect multiple FRs, which would cause coupling or interference among them. The Information Axiom states that the design should have the minimum probability of not satisfying the FRs, which is measured by the information content of the design. This ensures that the design is robust and reliable under various conditions.

Types of Physical Interfaces

A physical interface is the way that two or more components are connected or attached to each other. There are different types of physical interfaces depending on the function and design of the components.

– Fixed and non-adjustable interfaces are used when the components need to be rigidly attached and aligned, and there is no need for adjustment or movement. For example, a bolt and nut can create a fixed and non-adjustable interface between two metal plates.

– Adjustable interfaces are used when the components need to have some degree of flexibility or variability in their position or orientation. For example, a screw and a slot can create an adjustable interface between a bracket and a wall, allowing the bracket to slide along the slot.

– Locator interfaces are used when the components need to be positioned or aligned in a specific way, but not necessarily attached. For example, a pin and a hole can create a locator interface between a lid and a box, ensuring that the lid fits snugly on the box.

– Hinged or pivoting interfaces are used when the components need to have some degree of rotation or angular movement. For example, a hinge can create a hinged or pivoting interface between a door and a frame, allowing the door to swing open and close.

The Principle of Exact Constraints

The principle of exact constraints states that a component should be designed with the minimum number of constraints necessary to ensure its proper functioning. This means that the product should have no redundant or overconstrained elements that could cause stress, deformation, or failure. For example, a table with four legs is exactly constrained, as each leg supports one corner of the table. A table with five legs is overconstrained, as one leg is redundant and could create instability or unevenness. A table with three legs is underconstrained, as it could wobble or tip over.