ABSTRACT
Interfaces are essential to ensure that various entities (ie, systems, subsystems, and components) of a complex product can work together to enable the product to possess the attributes derived from the needs of its customers Interfaces are links or joints between entities The interfaces can perform many functions Physical interfaces allow two entities to be attached to each other, whereas other interfaces such as electrical connections allow the transfer of electrical power and signals (for information and control) required for functioning of the product Since the complex products have many entities and the number of interfaces between the entities is usually very large, the task of managing these interfaces is very complex and time-consuming Engineers involved in designing the complex products spend a lot of time in meetings with other engineers and designers from all the interfacing entities to understand the requirements, functions, and trade-offs between different attributes related to the interfaces In this chapter, we will study many aspects of the interface design-in terms of design considerations and management of various tasks (Pimmler and Eppinger, 1994; Sacka, 2008)
An interface can be defined as a “joint” where two (or more) entities (eg, systems, subsystems, or components) are linked together to serve certain functions Thus, the interface affects the design of both the entities and the parameters defining the link (ie, configuration of connecting elements at the interface) The link or the interface between the two entities must be compatible; that is, the values of the parameters (eg, dimensions) of the two interfacing entities defining their capabilities must match An interface can involve (1) physical connection, (2) sharing of space (ie, packaged close to each other), (3) exchange of energy (eg, transfer of mechanical or electric power, air, or fluid under positive or negative pressure), (4) exchange of material (eg, oil, coolant, and gases), and/or (5) exchange of data (eg, digital and/or analog signals)
Knowing the type of interface and its characteristics is important to ensure that the two interfacing entities work with each other to perform certain functions During the early design phases of the product, as the requirements are allocated and the systems are identified, the interfaces between different entities and their parameters must be identified As the design progresses further, the parameters that define each interface in terms of its characteristics (eg, their dimensions, strength of physical
attachment forces, amount of current or data flow passing through the interface) and their level of strength must be analyzed and controlled during subsequent detailed design activities The engineers involved in designing both the interfacing entities must know how the two entities work with each other and how and what the interface must exchange, communicate, or share to get the two entities to work together and perform their intended functions
It should be realized that since each system in a product performs one or more functions and all systems in a product must work together for the product to function, the interfaces must be carefully designed to ensure that they are compatible with both the interfacing systems
Interfaces between systems, subsystems, or components of a product and other external systems that affect the operation of the product and their components (eg, parts, subassemblies, human operators, and software) need to be studied and designed to ensure that the product can be used by its customers Interfaces can be categorized by considering many engineering characteristics and user needs of the product (Lalli et al, 1997) Some commonly considered types of interfaces are described in the following
1 Mechanical or physical interface: This type of interface ensures that any two interfacing components can be physically joined together (eg, by use of bolts, rivets, threads, couplings, welds, adhesives, and linkages that can allow movements and transmission of forces between entities such as a spring, damper, or frictional element) and have the required strength, transfer capabilities (eg, for materials, heat, and forces), and durability (ability to work under many work cycles involving loads, vibrations, temperatures, etc)
2 Fluidic or material transfer interface: A fluidic or material transfer interface (for transfer of fluids, gases, or powdered or granular materials) can be considered as a different type of interface or it can be considered as a mechanical interface involving pipes, tubes, hoses, ducts, seals, and so on The fluidic interface will enable flow of fluids, gases, or powdered/ granular materials with their characteristics such as flow rates, purity, pressures, temperatures, insulation, sealing, and corrosion resistance
3 Packaging interfacing entities: Physical space is required to package or to accommodate the two interfacing entities The required space can be determined from (1) the sizes/volumes and shape of spaces (ie, three-dimensional envelopes) occupied by the two interfacing entities and their interfaces; (2) clearance spaces required around the entities to account for vibrations, movements of parts/linkages, air passages for cooling, hand/ finger or tool access space for assembly/service/repair; and (3) consideration of minimum and maximum separation distances required for their operations
4 Functional interface: In some cases, depending on certain needs to provide one or more functions, one or more of the preceding types of interfaces
may be combined and defined as a functional interface For example, an automotive suspension system forms a unique functional interface (involving physical links and their relationships with relative movements) between sprung and unsprung masses of the vehicle
5 Electrical interface: An electric interface ensures that two interfacing entities can form an electrical connection/coupling (eg, with connectors, pins, screws, soldering, and spring-loaded contacts/brushes) that can carry required electrical current or signals, provide necessary insulation protection, transfer data, and may have other characteristics such as resistance, capacitance, electromagnetic fields, and interferences
6 Software interface: A software interface ensures that when data are transferred from an entity (with a software system) to another, the format and transmission characteristics of the coded data are compatible to facilitate the required amount and rate of the data transfer
7 Magnetic interface: A magnetic interface generates the required magnetic fields for operation of devices such as solenoids/relays, electric machines (motors and generators), and levitation devices
8 Optical interface: An optical interface (eg, fiber optics, light paths, light guides, light piping, mirrors or reflecting surfaces, lenses, prisms, and filters) allows transfer of light energy between adjoining entities through luminous or nonluminous (eg, infrared [IR]) energy transmission, reflection and by shielding, baffling/blocking, or filtering of unwanted radiated energy
9 Wireless interface: This type of interface can communicate signals or data without wires through radio frequency communication, microwave communication (eg, long-range line-of-sight through highly directional antennas, or short-range communication), and IR short-range communication The interface applications may involve point-to-point communication, pointto-multipoint communication, broadcasting, cellular networks, and other wireless networks
10 Sensor or actuator interface: A sensor has a unique interface that converts certain sensed energy (eg, light, motion, touch, distance, or proximity to certain objects, pressure, and temperature) into an electrical output or signal Whereas, an actuator produces an output (eg, movement of a control or mechanical links) by converting an input from one type of modality to a different output modality For example, a steeper motor produces a precise angular movement for each electrical pulse input Similarly, floats or floating sensor devices can sense fluid levels and convert into electrical signals
11 Human interface: When a human operator is involved in operating, monitoring, controlling, or maintaining a product, the human-machine or human-computer interface (commonly referred as the HMI or HCI, respectively) will include devices such as human accommodating or positioning devices (eg, chairs, seats, armrests, cockpits, standing platforms, steps, foot rests, handles, and access doors), controls (eg, switches, buttons, touch controls, stalks, levers, joy sticks, pedals, and voice controls), tools (eg, hand tools and powered tools), and displays (eg, visual displays, auditory displays, tactile displays, and olfactory displays)
To design an interface, an engineer must first understand the overall requirements on the product, allocated functions, and characteristics of both the entities attached or linked at the interface The requirements on the interface should specify the following: (1) the functional performance of both the entities, (2) configuration of the entities, (3) available space to create the interface, (4) environmental conditions for the operation of the product and comfort of the human operators, (5) durability (minimum number of operational cycles the product must function), (6) reliability and safety in performing the required functions, (7) human needs (eg, viewing and reading needs, hearing needs [sound frequencies and levels], lighting and climate control needs, and product operating needs), (8) electromagnetic interference, and so on In addition, the requirements should include any other special constraints (eg, weight requirements, aerodynamic considerations, and operating temperature ranges) that must be met
Steps involved in the interface requirements development process generally use an iterative approach (with a series of steps and loops as shown in Figure 22) unless a previously developed requirements document (or a standard) is available The series of steps typically involve the following:
1 Gather information to understand how the interfacing entities work, fit into the product, and support the overall functionality, performance, and requirements of the product (eg, review existing system design documents and standards) Draw an interface diagram (see section entitled “Interface Diagram”) Meet with the design team members of the interfacing entities (eg, core engineering functions such as body engineering, powertrain engineering, electrical engineering, and climate control engineering-for an automotive product) and product design teams to understand issues and trade-offs considerations with the product attributes (eg, packaging space, safety, maintenance, and costs)
2 Document all design considerations such as inputs, outputs, constraints, and trade-offs associated with the interface and its effects on other entities (eg, develop a cause-and-effect diagram, see Chapter 14; conduct a FEMA, see Chapter 13)
3 Study existing designs of similar interfaces and compare them by benchmarking the competitors’ products (see Chapter 13 for benchmarking technique)
4 Study available and new technologies that could be implemented to improve the interface
5 Create an interface matrix (see section entitled “Interface Matrix and N-Squared Diagram”) to understand the types of interfaces and their characteristics
6 Create a preliminary set of requirements 7 Translate requirements into design specifications (use of the quality function
deployment technique can help in this and the next step, see Chapter 13) 8 Brainstorm possible verification tests (or obtain available test methods from
existing standards) that need to be performed to demonstrate compliance to the preliminary requirements
9 Develop alternate interface concepts/ideas 10 Review alternate concepts and ideas with subject matter technical experts 11 Select a leading design by analyzing all other entities that are functionally
linked to the entities associated with the interface (develop a Pugh diagram to aid in decision making, see Chapter 13)
12 Modify and refine interface diagram and interface matrix 13 Iterate the preceding steps until an acceptable interface design is found
The iterative workload described in the preceding process can be reduced if an internal (company) design guide or standard for the entities being interfaced can be used as a starting document along with the product-level requirements Experts and other knowledgeable people in the organization can provide information on valuable lessons learned during the development of similar interfaces from the past product programs
An interface between any two entities (which could be systems, subsystems, or components) can be represented by use of a simple arrow diagram as shown in Figure 51
The arrow indicates a link (or relationship) between the two entities, namely, entity A and entity B The arrow representing the link can denote any of the following (see Figure 51):
1 Output of entity A is an input to entity B 2 Entity A is mechanically attached to entity B 3 Entity A is functionally attached to entity B (ie, function of A is required
by B to perform its function) 4 Entity A provides information to entity B 5 Entity A provides energy to entity B 6 Entity A transmits or sends signals, data, or material (eg, fluids, gases) to
entity B
For example, in an automobile, an engine mount (a rubber coupling) between the engine and the chassis is a physical coupling that serves as a physical interface between the engine and the engine mount and another physical interface between the engine mount and the chassis frame The function of the engine mount is to position the engine, physically attach it to the chassis, and also to isolate the vibrations transmitted from the engine to the chassis system (see Figure 52) (Note: Letter “P” placed above the arrows in Figure 52 indicates “physical” connection)
An interface diagram is a flow (or an arrow) diagram showing how different systems, subsystems, and components of a product are interfaced (ie, joined or linked) It provides a visual representation of the product or a portion of the product showing where the interfaces occur It also should show the type of each interface (by use of letter codes, eg, “P” for a physical connection, “E” for an electrical connection, “M” for material/fluid transfer, and “D” for data transfer, placed next to the arrow)
An interface diagram is a useful tool in understanding how various systems, subsystems, and components are interfaced with each other The diagram can be created at any level, that is, at the product level showing all the systems of the product, at a system level showing all the subsystems of the system, at a subsystem level showing all components of the subsystem, or at mixed levels showing a system, its subsystems, and also showing other major systems of the product Two examples of the interface diagram are shown in Figures 56 and 58 a later section entitled “Examples of Interface Diagrams and Interface Matrices” of this chapter (Figure 56 presents an interface diagram for a laptop computer and Figure 58 presents an interface diagram for an automotive fuel system)
An interface matrix and N-squared diagram are two commonly used methods to illustrate existence and types of interfaces between different entities (systems, subsystems, or components) Both are formatted in a matrix type arrangement with entities described by rows and columns of the matrix The interface matrix shows names of all the entities as headings for both rows (on the left) and columns (on the top) of the matrix, whereas the N-squared diagram shows names of all the entities in the diagonal cells of the matrix The N-squared diagram has been used extensively to develop data interfaces, primarily in the software development field However, it can also be used to develop hardware interfaces Both methods present descriptions of the interfaces in the cells of the matrix A cell is defined by the intersection of the row and column represented by the two interfacing entities The description of the interface is shown by a letter code to indicate the type of the interface An interface diagram, thus, shows the following: (1) it captures the existence of all interfaces, (2) it shows output-to-input relationships between any two entities (see Figure 53), and (3) it presents type(s) of interface between any two entities
Figure 53 presents the output-to-input relationships between the two entities for each cell of a 6 × 6 interface matrix (except for the cells in the diagonal) The entities are labeled as E1 to E6 The outputs of the entities are labeled as O1 to O6 and the inputs are shown as I1 to I6The arrow shown in each cell indicates that the output of an entity defined by its row is used as an input to an entity defined by the column of the matrix For example, the cell in first row and second column shows that O1 is
the output of entity E1, and it is interfaced with entity E2 shown as input I2 received by entity E2 Similarly, Figure 54 presents an N-squared diagram illustrating the relationships between the entities E1 through E6 shown in the diagonal of the matrix
The interface matrix (also called as an interaction matrix in some organizations) and the N-squared diagram are useful tools in understanding and displaying
interfaces The contents of the cells typically present coded descriptors of the types of interfaces between the outputting entities and the entities receiving the inputs The codes typically include P = physical interface, S = spatial-packaging interface, E = electrical interface, M = material flow, I = information or data flow, and 0 (or a blank cell) = no relationship
The interface diagram and interface matrix are both very useful tools in visualizing relationships and documenting presence of the interfaces (Sacka, 2008) These tools make the design team realize the presence of many interfaces and types of these interfaces in the product The next step is to understand the connection configuration details and requirements of these interfaces to ensure that the interfacing entities can be designed to work together to perform their allocated functions
Based on the systems, subsystems, and components of a laptop computer presented in Table 21, an interface diagram of the systems in the laptop computer is presented in Figure 55 The letters defining interface types are placed next to each arrow showing the interfaces between different systems The chassis system provides space for packaging all other systems, and the size of the chassis system depends on sizes of other systems Furthermore, all the systems are physically attached to the chassis systems Thus, the arrows indicating “output of” other systems to “input to” the chassis system are indicated as of types “P” and “S” The power system provides electrical energy to all other systems except the chassis system Thus, the arrows from the power system to all other systems (except the chassis system) are identified as of type “E” The cooling system is assumed to provide cooling air (material flow “M” interface) to cool only the electronic processing system The electronic processing systems takes data (information) outputs (type “I” interface) from the input system (ie, keyboard and mouse), the wireless system, and the memory system Whereas, the electronic processing system delivers its outputs (type “I” interface) to the display system, the audio system, the memory system, the wireless system, and the cooling system The cooling system is also functionally attached to the chassis system to assure proper airflow (type “M” interface) through inlets and outlets provided through the chassis system
Figure 56 provides the interface matrix for the laptop computer and provides information contained in the interface diagram presented in Figure 55 The matrix shows that the chassis system, electronic processing system, and power system are interfaced with all systems of the laptop computer The chassis system contains and packages all other systems in the laptop The electronic processing system is the heart of all information processing and the input system controls the information processing activities of the computer The systems with least interfaces are the wireless and cooling systems Thus, these systems can be designed independent of most other systems in the laptop
An automotive fuel system is considered here to include the following three subsystems: (1) fuel delivery system, (2) air induction system, and (3) electronic fuel control system The components of the three subsystems are presented in Table 51
Figure 57 presents an interface diagram of the automotive fuel system The interface diagram shows interfaces (links) within the fuel system, its subsystems, and other systems in the vehicle The three subsystems of the fuel system are placed
within the three separate boxes shown in the dotted lines in Figure 57 Other automotive systems, namely, electrical system and powertrain system, are placed outside the dotted-lined boxes The fuel delivery system supplies fuel (type M interfaces) to the air induction system, and the air induction system provides fuel monitored through the electronic control subsystem to the engine subsystem The electrical system through the wiring systems (type E interfaces) in the electronic monitoring system provides power to the electronic control unit, the air flow meter, and the fuel pump
Figure 58 provides the interface matrix for the automotive fuel system It includes interfaces between various subsystems of the fuel system and other vehicle systems (ie, electrical, powertrain, body, and instrument panel systems) The cells where there is no relationship (or interface) are left blank to reduce clutter in the matrix
The interface diagram and interface matrix are important tools because they provide basic information by identifying all the interfaces and thus help the engineers realize the complex interfacing issues and tasks during the development of any system For example, the engineer designing the fuel pump realizes that the fuel pump has an electrical interface to receive power for its operation; it has to be physically
TABLE 5.1 Components in the Three Subsystems of the Automotive Fuel System
mounted inside the fuel tank, and the pumped fuel has to pass through the filter to the fuel delivery pipe Thus, the fuel pump engineer needs to communicate with the electrical systems engineers and other fuel systems engineers to make sure that his fuel pump can physically connect and operate with the interfacing components In addition, the engineer must understand the bigger picture of how the rest of the fuel system works with the powertrain system, electrical system, body system, and the instrument panel (which displays status of the fuel and other systems) of the vehicle (product level)
This section provides an illustration of an interface diagram created to show interfaces between an infotainment system and other systems in an automotive product An infotainment system consists of the driver information and entertainment system Let us assume that the interface matrix presented in Figure 59 was developed by an automotive instrument panel engineer to understand the tasks of designing the infotainment system and making it functional by interfacing it with the rest of the vehicle systems The notations of the types of interfaces shown the interface matrix are as follows: F = functional, E = energy transfer, I = information transfer, P = physical connection, M = material transfer, S = spatial packaging space, and 0 = none
The infotainment system is an integrated driver information and entertainment system and it is assumed to contain the following subsystems: (1) climate controls, (2) a radio (with AM/FM bands and satellite radio), (3) a CD player, (4) a navigation system, and (5) a vehicle systems monitoring system (which checks and provides status of different vehicle systems including fuel consumption, trip statistics, and distance to empty) In addition to understanding the customer needs for the infotainment system and engineering requirements of each of the subsystems, the engineer would need to understand the characteristics and requirements of the other interfacing systems
Using the information provided in the interface matrix given in Figure 59, the tasks of the instrumental panel engineer assigned to develop the infotainment system for a passenger car can be analyzed as follows:
1 The body system provides information on when the doors and/or trunk lid are ajar The body system also provides space for packaging the infotainment system
2 The powertrain system provides information on vehicle speed, engine speed, engine temperature, and fuel consumption to the infotainment system
3 The chassis system provides information on state of brake pads (wear and temperatures) to the infotainment system
4 The fuel system provides information on the fuel situation (fuel used, fuel consumption, and distance-to-empty) to the infotainment system
5 The electrical system provides electrical energy and the information on the state of electrical system (current flow and voltage) to the infotainment system
6 The climate control system provides information on the exterior tempera-
ture and settings of different climate controls to the infotainment system 7 The vehicle safety system provides information on the status of readiness
of the safety subsystems such as braking system temperature, brake fluid level, tire pressure warning system, airbags, and night vision system to the infotainment system
8 The infotainment system needs space inside the instrument panel (a subsystem of vehicle body system) and it is packaged in the center stack with a high-mounted screen with touch, voice, and other hand-operated controls
9 The infotainment system provides information on the driver-selected modes to the powertrain system (eg, sporty, economy, manual, or auto transmission settings), the climate control system (eg, selected interior temperature, a/c unit on/off, and air distribution mode), electrical system (eg, status of driver-selected modes for power demand computations), and the chassis system (eg, to adjust brake pads for wear)
The tasks of the infotainment engineer would be to understand the preceding inputs and outputs and provide necessary software and hardware systems to create user-friendly controls and display interface To obtain the necessary information, the engineer will need to meet and communicate with all engineers from the interfacing systems to ensure that his infotainment system can perform all the preceding communication, control, and status display functions
The interface diagrams and interface matrices are also very useful in visualizing configuration and links between entities in a system The usefulness of the techniques can be better understood by illustration of an example of clustering given in the following discussion
Let us assume that a mechanical system involving 16 components is being analyzed to determine its configuration and packaging into a self-contained unit The components are defined as C1, C3, C3, …, C16 Figure 510 presents an interface diagram of the system showing all the mechanical interfaces between the
components The diagram shows that components C4, C10, C11, C12, C13, and C14 are not connected to any other components in the system The lack of interfaces to these six components suggests that these components are performing functions that are independent of other remaining components Thus, they must be serving other systems but are merely included in the mechanical system because of some other reasons (eg, designer needed space to locate these components purely for packaging convenience) The rest of the components are connected and have several connections to other components The interface matrix of the system is shown in Figure 511
The relationships represented in a matrix diagram can be rearranged using a number of clustering and sequencing methods to help grouping or clustering of components (eg, defining content of modules in a family of products), systems, or functions and to create product configurations (eg, sequencing of steps and/or determining product layouts) to reduce or combine interfaces Design structure matrix (DSM) approach is useful in “product architecturing” (Yassine, 2004; Zakarian, 2008)
Pimmler and Eppinger (1994) showed that using the DSM approach, the components can be rearranged from the initial arrangement in the matrix in Figure 511 to the matrix in Figure 512 so that they could be packaged into three modules (clusters) The rearranged matrix simply places the rows and columns of the original matrix in a different order selected by a clustering technique The three modules can be easily seen as three clusters around the diagonal in the rearranged interface matrix shown in Figure 512 The three clusters, identified as modules #1, 2, and 3, are also outlined with darker solid and dotted lines around the clusters of cells in the interface matrix in Figure 512 Figure 513 also shows the clusters identified as modules #1, 2, and 3 in the interface diagram Component C5 is common or connecting component between modules # 1 and 2 Similarly, component C8 is common or connecting component between modules # 2 and 3
The DSM and clustering techniques thus are useful in determining alternate configurations and modularization of components or subsystems The modularization generally reduces complexity and costs by sharing of common modules across various product models
Each engineer responsible for delivering a system (with all its subsystems and components) must have design expertise on functioning details, manufacturing processes, and trade-offs between the characteristics of the system and its entities (ie, its subsystems and their components) The engineer must also have knowledge about how his system fits, connects, and works within a bigger (parent) system involving the complex product and its operating conditions A thorough understanding of the requirements on the product is necessary to ensure that these higher-level requirements can be cascaded down to the lower levels of systems in a complex product
Knowing the functions of the parent system, each of its subsystem, and each component can be allocated and analyzed to determine how each subsystem, its components, and interfaces need to be designed Problems are identified iteratively and modifications (or even major redesigns) are undertaken If the responsibilities to
design and deliver subsystems or components are assigned to different individuals (teams or suppliers), then they will need to understand how their deliverables are affected by other components or subsystems that interface with their entity
Constant checks by independent experts, structured walkthroughs, design reviews, or even simulation models (or prototyping) of the interface operations are useful in ensuring that all the interfaces are designed to meet their requirements and allocated functions Interface control process and documents (described in the section on “Establishment of Interface Control”) help monitor and control the complex interface development work
The basic steps involved in establishment of an interface control process during the product concept definition typically involve (1) assign basic functional areas of responsibility to different functional teams; (2) define design responsibilities of each team; (3) identify and categorize all interfaces; (4) define interfaces to be controlled; (5) establish formal interface control procedures; (6) disseminate scheme, framework, and traceability of requirements and allocated functions to the teams; and (7) monitor the team outputs to ensure that interface control procedures are followed (NASA, 2007)
The interface control process suggested for NASA projects is described in a report by Lalli et al (1997) The purpose of interface control is to define interface requirements so as to ensure compatibility between interrelated pieces of equipment and to provide an authoritative means of controlling the design of interfaces Interface design can be controlled by an interface control document (ICD) These documents are useful for the following: (1) control the interface design of the equipment to prevent any changes to characteristics that would affect compatibility with other equipment, (2) define and illustrate physical and functional characteristics of every piece of equipment in sufficient detail to ensure compatibility of the interface, so that this compatibility can be determined from the information in the ICD alone, (3) identify missing interface data and control the submission of these data, (4) communicate coordinated design decisions and design changes to program participants, and (5) identify the source of each interface component
A complex product cannot be created without interfaces Interfaces are very important because they allow connections and transfer of materials, energy, and signals between different entities Engineers and designers must understand the functions of the entities, characteristics of the interfaces, and trade-offs between various parameters of the interfaces Interface diagrams and interface matrices are simple but effective techniques in visualizing and understanding the interfaces and issues related to their design and operation
