<aside> 🚩 The Human-Machine Interaction Loop (HMIL) is a model of a single “interaction” between a user and a thing being used, and helps focus attention on all the HF aspects that must be attended to during design.

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What is an Interaction Loop?

When a human interacts with any machine, the general set of activities that happen is always the same. These activities can be modelled in various ways; one of the most well-known is the Human-Machine Interaction Loop (HMIL).

A single HMIL represents a single task, like raising the volume of a radio, or grasping a blender on an upper kitchen cabinet, or setting the speed of a lathe. Every task can be represented with a HMIL.

One classic model is that of Chapanis Cha76 shown in Figure 1.

In this model, both the human and the machine are represented, albeit only implicitly, by the same general three-part grouping: sensing, processing, and actuating.

Figure 1: The HMIL per Chapanis (1976).

Actuation by one of either the human or the machine produces a flow of mass, energy, or information that is sensed and ultimately acted upon by the other.

While Chapanis's model was intended to treat information flows only, we can through systems thinking see that the model works just fine for flows of mass and energy too.

A (slightly) better model for our purposes is shown in Figure 2.

• Colours help identify the three types of elements, high-lighting the similarities.
• All inputs and outputs have to pass through some kind of interface, so the machine “inputs” and “outputs” from outside the human-machine system are now directed through the interface elements.

Figure 2: A more sophisticated model of an HMIL.

• Each of the human and machine are more readily identified as of similar general structure (they're both just systems).
• We clearly mark a boundary between not only the human and the machine, but between each of them and their environment as well.
• We also clearly indicate the importance of inputs and outputs from/to the environment in which both the human and the machine exist. These environmental inputs and outputs can have significant impact on the effectiveness and efficiency of the HMIL in any particular case.

The Human-Machine Interaction Loop (HMIL) shown in Figure 2 is the one to which we will refer subsequently.

Example: Adjusting the volume on a radio

Assume there is a human, and a radio, and that the human wishes to adjust the volume of the radio, and that the human knows how to do that.

Consider the specific activities involved with respect to the six elements of the HMIL (Figure 2).

1. In this case, the feedback has two components: the music coming from the radio, and the current position of the volume control.
2. The human observes this feedback; they hear the music and see the volume control setting.
   • At this point, we can already begin to consider some relevant human factors.
     • How is the human’s hearing?
     • How is the human’s vision?
     • Even if their vision is good, is there enough light, coming from the right direction, to see the volume control setting?
     • How much other/ambient noise is there that the radio has to compete with to be heard by the human?
3. The human’s brain performs some cognition to process the observations.
   • The human decides if the sound from the radio is too loud or too soft.
   • The human decides whether to raise or lower the volume.
4. The human actuates the volume control to change the volume.
   • More human factors apply here:
     • Is the human able to operate the control?
     • Is the control too far away, or too stiff to operate, or too close to other controls that block access by the human’s hand?
     • How much attention does the human have to apply to operate the control?
     • Is the “fineness” of the control too fine for the human’s hand to make minute adjustments?
5. The new volume control setting is used by the radio to change its loudness.
   • Now that there is a new volume level and a new volume control setting, the human returns to step 1 above. This loop repeats until they find the most suitable radio volume.