Firm foundation in the main hci principles, the book provides a working

Figure 3.16 Left: Various actuators used for tactile feedback: (a) miniature speaker, (b) minia-
ture electromagnet/latch, (c) piezoelectric strip, (d) microvibratory motors. Right: tactile array with 
multiple actuators. (From KU Leuven, Tactile Feedback, 2010, https://www.mech.kuleuven.be/en/
pma/research/ras/researchtopics/tactfb.html [21].)

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51
H U M A N FA C T O R S A S H C I T H E O R I E S
Using a similar control structure, body-worn devices transfer force with 
its mechanism directly attached to the body.
Important haptic display parameters are (a) the degrees of freedom 
(the number of directions in which force or torque be can displayed), 
(b) the force range (should be at least greater than 0.5mN), (c) operating/
interaction range (how much movement is allowed through the device), 
and (d) stability (how stable the supplied force is felt to be). Stability 
is in fact a by-product of the proper sampling period, which refers to 
the time taken to sense the current amount of force at the interaction 
point and then determine whether the target value has been reached 
and reinforce it (a process that repeats until a target equilibrium force 
is reached at the interaction point). The ideal sampling period is about 
1000 Hz, and when the sampling period falls under a certain value, 
the robotic mechanism exhibits instability (e.g., exhibited in the form 
of vibration) and thus lower usability. The dilemma is that provid-
ing a high sampling rate requires a heavy computation load, not only 
in updating the output force, but also in physical simulation (e.g., to 
check if the 3-D cursor has hit any virtual object). Therefore, a careful 
“satisficing” solution is needed to balance the level of the haptic device 
performance and the user experience (Figure 3.18).
In general, due to their mechanical nature, “robotic” haptic devices 
are not yet very practical. They tend to be heavy, clunky, dangerous, 
and take up a large volume. The cost is very high, often with only a 
small operating range, force range, or limited degrees of freedom. In 
many cases, simpler devices, such as one-directional latches or vibra-
tors, are used in combination with visual and aural feedback to enrich 
the user experience.
(a)
(b)