<aside> 🚩 A Subsystem Identification Matrix (SIM) is a tool that helps to identify and document the subsystems in a system, based on established Requirements.

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Overview

You have a blank page on which to draw out your system. You know the inputs and outputs, but you have no idea what subsystems are needed, and how they might have to interact. A System Identification Matrix (SIM) is a tool you can use to structure your thinking and document your results.

You will build up the collection of subsystems by considering each Requirement function in turn and what systems you can use to manifest that function.

The next section provides a detailed example of how to develop a SIM.

Generating Subsystem Identification Matrices

Consider the design of an intervention to lift people and cargo between floors in a building, in which passengers are expected to be able to see out of the intervention while they are being lifted or lowered.

Step 1: Extract the functions from your Requirements

To create a SIM, we need all and only the functions that are in your Requirements.

For our example, assume the following functions are known to the designers of an elevator that must provide a view to its users (like the ones in the CN Tower):

• protect passengers: this includes keeping them safe and comfortable, as well as accommodating emergency situations.
• lift a mass: and lower it of course.
• provide a view: as stipulated in the design problem.
• interact with passengers: to accept commands and provide appropriate feedback to human users.
• control the machinery: the elevator will doubtlessly have various subsystems; the whole thing must behave in a controllable way; this includes internal self-control (noticing what it should do under, for instance, emergency situations) and external control (for safety, security, maintenance, etc).

Step 2: Set up a SIM table

Create a table similar to the one shown to the right.

In the first column list all the functions and only the functions in your Requirements.

Step 3: Treat the first function listed

Examine the first function and identify a subsystem or subsystems that will satisfy it, or at least contribute to it significantly. Create a column for that subsystem and add an appropriate mark in the corresponding cell.

In the table to the right, the first FR is Protect passengers. The CAB subsystem is specified for this function. The X indicates that the subsystem provides the function.

Note that we've used a very concrete noun, CAB, to label the subsystem that contains people/cargo to be moved by the elevator. This is acceptable, so long as it remains clear that the label is just an abbreviation and does not necessarily mean the subsystem will be implemented as typically found in other elevators (or, worse, taxicabs!)

Step 4: Treat the second function

The second function is Lift mass. So we will obviously need a LIFTING SYSTEM to do that.

We will place an X to indicate that the LIFTING SYSTEM provides the function of Lift mass.

However, we must also place an X to indicate that the CAB also contributes to providing the lifting function. After all, the LIFTING SYSTEM lifts the CAB which holds the passengers; that is, the CAB also contributes to deliver the function Lift mass.

We will have a table as shown to the right.

Note: it may be that you do not need to add a new system for every function. Take advantage, if you can, of the fact that one system can provide multiple functions.