Viewpoint
Haptics describes the sense of touch and movement. An engineer tends to describe
haptics in terms of forces, elongations, frequencies, mechanical tensions and shearforces.
This of course makes sense and is important for the technical design process.
However haptics is more than that. Haptic perceptions range from minor interactions
in everyday life, e.g., drinking from a glass or writing this text, to a means of social
communication, e.g. shaking hands or giving someone a pat on the shoulder,
and very personal and private interpersonal experiences. This chapter deals with
the spectrum and influence haptics has on the human being beyond technological
descriptions. It is also a hint for the development engineer, to be responsible and
conscious when considering the capabilities to fool the haptic sense.
1.1.1 Haptics as a Physical Being’s Boundary
Haptics is derived from the Greek term “haptios” and describes “something which
can be touched”. In fact the consciousness about and understanding of the haptic
sense has changed many times in the history of humanity. ARISTOTELES puts the
sense of touch in the last place when naming the five senses:
1. sight
2. hearing
3. smell
4. taste
5. touch
Nevertheless he attests this sense a high importance concerning its indispensability
[3]:
The social estimation of the sense of touch experienced all imaginable phases.
Frequently it was afflicted with the blemish of squalor, as lust is transmitted by it:
“Sight differs from touch by its virginity, such as hearing differs from smell and
taste: and in the same way their lust-sensation differs [289].”
It was called the sense of excess [78] . In a general subdivision between lower
and higher senses, touch was almost constantly ranged within the lower class. In
western civilization the church once stigmatized this sense as forbidden due to the
pleasure which can be gained by it. However in the 18th century the public opinion
changed and KANT is cited with the following statement [126]:
“This sense is the only one with an immediate exterior perception; due to this it is
the most important and the most teaching one, but also the roughest. Without this
sensing organ we would be able to grasp our physical shape, whose perception the
other two first class senses (sight and hearing) have to be refered to, to generate
some knowledge from experience.”
KANT thus emphasizes the central function of the sense of touch. It is capable of
teaching the spatial perception of our environment. Only touch enables us to feel and
classify impressions collected with the help of other senses, put them into context
and understand spatial concepts. Although stereoscopic vision and hearing develop
early, the first-time interpretation of what we see and hear, requires the connection
between both impressions perceived independently and information about distances
between objects. This can only be provided by a sense, which can bridge the space
between a being and an object. Such a sense is the sense of touch. The skin, being a
part of this sense, covers a human’s complete surface and defines his or her physical
boundary, the physical being.
Wearing glasses is another fascinating example of the effect of the relationship between
distance and perception. Short- sightedness requires glasses that demagnify
the picture of the environment on the retina due to the distance between eyeball and
lenses. Shortsighted people have a different view of size, e.g. concerning their own
body height, dependent on whether they wear glasses or contact lenses. At every
change between both optical aids the perception of their body has to adapt. Dependent
on a person’s kind of defective vision this is a consciously perceivable process.
It can be performed within seconds by using the well known references of one’s own
arms which touch things or one’s legs which walk
Especially in the 20th century art deals with the sense of touch and plays with
its meaning. Drastically the furry-cup (fig. 1.1) makes you aware of the significance
of haptic texture for the perception of surfaces and surface structures. Whereas the
general form of the cup remains visible and recognizable, the originally plane ceramic
surface is covered by fur. “Fighting the mud” (fig. 1.2) remembers you that
not only hands and fingers are relevant for haptic perception, but that the whole body
surface is able to touch and feel. In 1968 the “Pad- and Touch-Cinema” (fig. 1.3)
allowed visitors to touch VALIE EXPORT’s naked skin for 12 seconds through a box
being covered by a curtain all the time. According to the artist this was the only
valid approach to experience sexuality without the aspect of voyeurism [70]. These
are just a few examples of how art and artists played with the various aspects of
haptic perception
artistry. This is repeatedly demonstrated by expositions during the Worldhaptics
Conferences. At the same time, Prof. ISHII from MIT Media Laboratory or the
Graduate School of Systems and the Information Engineering group of the University
of Tsukuba of Prof. IWATA demonstrate startling exhibits (fig. 1.4) of “tangible
user interfaces” (TUI). These interfaces couple visual displays with haptically reconfigurable
objects to provide intuitive human-machine interfaces. There is much
more to find when the senses are sharpened to search for it.
The sense of touch can be a lot of things, e.g. a limitation of the physical being,
which helps to assess distances and calibrate other senses like vision, as well as a
means of social communication and a mediator of very personal experiences .Additionally
it is - like all the other senses - a target of art which makes us aware of
the importance of haptic experiences by fooling, distorting and emphasizing them.
. Besides these facets of the haptic sense, its function and its dynamic mechanical
Fig. 1.2 KAZUO SHIRAGA: Doro ni idomu (Fighting the mud) 1955 [70][217].
Fig. 1.3 VALIE EXPORT: Pad- and Touch-Cinema 1968 [70].
properties are also very impressive. Haptic perception in all its aspects is presented
in the following section.
Formation of the Sense of Touch
As shown in the prior section, the sense of touch has numerous functions. The
knowledge of these functions enables the engineer to formulate demands on the
technical system. It is helpful to consider the whole range of purposes the haptic
sense serves. However, at this point we do not yet choose an approach by measuring
its characteristics, but observe the properties of objects discriminated by it.
Fig. 1.4 Example for “Tangible Bits”, with different data streams accessible by opening bottles.
In this case single instrumental voices are combined to a trio [109] .
The sense of touch is not only specialized on the perception of the physical
boundaries of the body, as said before, but also on the analysis of surface properties.
Human beings and their predecessors had to be able to discriminate e.g. the
structure of fruits and leaves by touch, in order to identify their ripeness or whether
they were eatable or not, like e.g. a furry berry among smooth ones. The haptic
sense enables us to identify a potentially harming structure, like e.g. a spiny seed,
and to be careful when touching it,in order to obtain its content despite its dangerous
needles. For this reason, the sense of touch has been optimized for the perception
and discrimination of surface properties like e.g. roughness. Surface properties may
range from smooth ceramic like or lacquered surfaces with structural widths in the
area of low μm, to somewhat structured surfaces like coated tables and rough surfaces
like coarsely woven cord textiles with mesh apertures in the range of several
millimeters. Humans have developed a very typical way how to interact with theses
surfaces enabling them to draw conclusions based on the underlying perception
mechanism. A human being moves his or her finger along the surface (fig. 1.5),
allowing shear forces to be coupled to the skin. The level of the shear forces is dependent
on the quality of the frictional coupling between the object surface and the
skin. It is a summary of the tangential elasticity of the skin depending on the normal
pre-load resulting from the touch Fnorm and the velocity Fexplr of the movement and
the quality of the coupling factor μ.
Everyone who has ever designed a technical frictional coupling mechanism
knows that without additional structures or adhesive materials viscous friction between
two surfaces can hardly reach a factor of μr = 0.1. Nevertheless nature, in
order to be able to couple shearing force more efficiently into the skin, has “in
vented” a special structure at the most important body-part for touching and exploration:
the fingerprint. The epidermal ridges couple shearing forces efficiently to the
skin, as by the bars a bending moment is transmitted into its upper layers. Additionally
these bars allow form closures within structural widths of similar size, which
means nothing else but canting between the object handled and the hand’s skin. At
first glance this is a surprising function of this structure. When one looks again, it
just reminds you of the fact that nature does not introduce any structure without
a deeper purpose. Two practical facts result from this knowledge: First of all the
understanding of shear-forces’ coupling to the skin has come into focus of current
research [65] and has resulted in an improvement of the design process of tactile
devices. Secondly, this knowledge can be applied to improve the measuring accuracy
of commercial force sensors by building ridge-like structures [275]. Additional
details of the biological basics of tactile perception are given in chapter 3.
Consequently the sense of touch, as said before, has been developed for the discrimination
of surface structures. Although the skin may be our most sensitive organ,
it is still not the only haptically relevant one. Additional receptors are located
within muscles and joints, which enable us to get an impression of acting forces.
Anyone who has ever lifted a four pound weight (e.g. a well filled pitcher) with
an outstretched arm in a horizontal position, will have little recollection of the tactile
surface properties of the handle. The much more impressive experience of such
an experiment is the tensing up of the muscles, their slowly increasing fatigue and
the resulting change in the lifting angles of the joints. This is called “kinaesthetic
perception”. Whereas tactile perception describes forces (≈ 5mN..5N) and elongations
between skin and object which are low in amplitudes (≈1μm..1mm) and high
in frequencies (≈ 10Hz..1000Hz), kinaesthetic perception happens within muscles
and joints at higher forces but with lower dynamics (≈ static..10Hz). This enables
the human being and every other biological system with a firm supportive structure
- may it be bones or shells of chitin - to perform coordinated movements and targeted
interactions with its environment. While tactile perception generates similar
impressions during passive (e.g. a relative movement between a static finger-tip and
a moving surface) and active (e.g. a relative movement between a static surface and
a moving finger-tip) movement, kinaesthetic perception is more complex and influenced by additional factors. The human being is able to change deliberately his or
her mechanical properties. A handshake of the same person can be firm and rigid,
but it may be also loose and amicable. The coupling between muscles, joint position
and perception enables us to consciously influence the kinaesthetics of ourselves,
and to influence the intensity of our kinaesthetic perception in one and the same
situation. This makes us capable of blocking a blow with the same hands we use to
rock a baby to sleep. It gives us the ability to touch a structure carefully before we
grasp it firmly. The borders between action and reaction, active and passive become
blurred in the perspective of kinaesthetics. The awareness of this fact is important
for the requirements on systems with closed-loop control, which are important for
the design of haptic devices (chapter 5). At the same time this adaptability of the
human being and the connected ambiguity of the system’s borders are a significant
challenge for the design of a technical device.
Special Aspects of the Design Process
The design of any technical system always includes a long chain of compromises.
The achievement of the engineer lies in the selection of those compromises which
ensure that existing requirements are still fulfilled. Often these compromises are financially
motivated - a product should be inexpensive during the production process
without losing performance. Concerning these demands, an optimization of systems
with interfaces to other purely technical systems is often elegantly possible. The
technical systems are quite exactly known as to their characteristics and a technical
design can anticipate these characteristics with a certain security margin. Thus the
interpretation of a sensor capturing the rotation of a wheel, e.g. a speedometer, is
a relatively clear task. The necessary speeds are known, and disturbance variables
like temperature areas as well as humidity. can be identified Alternatively they can
be measured with high exactness. It is also relatively easy to identify the requirement
of measuring a two-dimensional movement of a human operated device on a
level surface - e.g. a computer mouse. The temperature range of the appliance is
known; the disturbance variables are limited to the optical measurement path and
the mechanical surface state and can easily be investigated. Only the speed is not
given as precisely as by a technical system. It results from the consideration about
the maximum speeds a human hand can reach. Here uncertainties become evident,
soon. Although the dynamics of human movement can be measured - technically,
a high variance between different people will be observed. This variance also concerns
the technical requirements of any object used by humans, and may it only be
the physical dimensions of tables and chairs. Dealing with such variances, matching
measuring methods and statistical analysis methods have entered anthropometric
modeling up to ergonomic design of work-places [153] as well as ergonomic standardization
ISO norms 9241/DIN 33 402 1. The science of anthropometrics applies to static (lengths, dimensions) and dynamic (speeds) cases. As a matter of fact: Every
human’s-applicable characteristic value is affected by such a wide variance that
with the information of ergonomic or anthropometric data only a proportional estimation
can be made. These estimations are called percentiles (fig. 1.6). A percentile
is a percentage of the totality of the data subject to analysis (e.g. European female
children between 10 and 15 years) and, depending on the context, encloses all people
who exceed or are below the percentage.
Fig. 1.6 Anthropometric design for sitting and standing work places considering the 5% and the
95% percentile according to DIN 33406.
percentiles introduced above is well established, as it fits quite well the natural variance
of people.With regard to the description of senses and their performances average
values are more common, e.g. when using a threshold 2. Thresholds themselves
are a key parameter in finding physical values to quantify human performance. Derived
from such values the technical system’s requirements like amplitude, amplitude
change or dynamics can be employed for deceiving a human sense and for
generating a “realistic” or “sufficient” haptic impression. The choice of words already
shows that requirements seldom comprise a concrete verifiable measurement.
They mostly represent a well-known structure, so that a group of people - or just
the superior or the board of directors - is content with its haptic impression. For the
design engineer this is an unsatisfying benchmark. Alternatives will be discussed to
a large extent. in the course of this book and especially in chapter 6.
The Significance of Haptics in Everyday Professional Life
The importance of haptics for professional life differs dependent on the profession
considered. In handcraft or manual trades the word ‘hand’ already implies the
relevance of haptics for performing these jobs. No bricklayer, carpenter, butcher,
plumber or barber would be able to do his job, if the sense of touch did not give
then important information about the material they work on. May that be the hair
they hold between their fingers, the humidity of the wall (as a change of heat transmission),
the cable core within the insulation, the difference between tendons and
muscles, the graining of pine and beech trees, the consistency of mortar. Even with
today’s state-of-the-art technology the involvement of man increases with the required
complexity and carefulness of a manual work. With this involvement and
the use of sense of touch the tools usually become less complex. Whereas during
archaeological excavation a first layer of earth is removed with an excavator, when
approaching a hidden structure a shovel will be used or maybe a spatula or for precision
work a brush or even the bare hands. However even in handcraft jobs machines
of increased flexibility made people turn away from the workpiece and its haptic
properties. Today master craftsmen criticize apprentices either for not having anymore
a sensation for materials and their properties or for lacking the information
-based technological know-how for the control of machines. By optimizing the interface
between manual work and machine-programming, engineers try to overcome
this gap. But in other areas of professional life, not only in jobs carrying the word
“manual” in their name, the loss of the sense of touch for everyday work has already
taken place.
The Sense of Touch in Everyday Medical Life
In many medical disciplines high manual skills are required. The capabilities of the
sense of touch are necessary for diagnostics and therapy, be it for the identification
of skin diseases, the diagnosis of joints, and the palpation of inner organs from the
outside or via natural openings; or for a direct surgical application like the transplantation
of a heart, the sawing of the cranium or the punctuation of the spinal
cord. The sense of touch transmits a plurality of information about texture, elasticity
and temperature to the medical professional - information which would either
be inaccessible or not so easily accessible in other ways. Nevertheless, in certain
situations it is necessary to substitute the sense of touch in diagnosis and therapy.
Via magnetic resonance imaging e.g. tendons and menisci of the knee can be visualized.
Thus a demanding manual examination of the joints’ movement range is not
necessary; especially as performing the procedure and interpreting the haptically
felt data requires experience and still leaves room for misinterpretation. Additionally
the results of a manual investigation are harder to explain to the patient than
the distinctiveness of a real image. However, when comparing the expenses of both diagnostic procedures, the precedence should be given to the haptic diagnosis. A
compromise can be seen in devices like the “Wristalyzer” [77]. This device either
puts varying loads on a moving joint - the wrist - or actively moves it, while dynamically
measuring the angle vs. displacement curves. Additionally it acquires a
complete electro-myography of the muscles. Besides for diagnosis, devices of this
kind are already planned for therapy. By actively generating forces and torques,
they can be used for the training of all joints of our extremities, of the cervical spine
and of the pelvis. Considering all these factors, there seems to be a tendency for the
mechanization in diagnostics and therapy. In orthopedic areas there is, however, still
some room to discuss its necessity, whereas in surgery there is an urgent need for
mechanization which, however, leads to a loss of haptic impressions. After surgical
interventions like e.g. an appendectomy, the wish for small wounds and scars for
medical and cosmetic reasons has therefore led to the design of laparoscopic instruments
(fig. 1.7). Simply by their length, mass and stiffness they also resemble a filter
for the haptic information. This decoupling between patient and surgeon has found
its temporary climax in the DaVinci system (fig. 1.8) - a laparoscopic telemanipulation
system without force feedback. This loss of the sense of touch during surgical
(or any other internistic) interventions is obvious and regrettable. As a result numerous
research projects were and still are focusing on an adequate substitute for the
direct haptic interaction by alternative technologies [73] or improved instruments
with integrated force-feedback [209] (fig. 1.9).
Fig. 1.7 Rigid laparoscopic instrument by Karl Storz.
The Sense of Touch in the Cockpit
Besides the aim of getting information which is already mechanically available
(elasticity, surface structure, etc.), there is the necessity to provide artificially generated
tactile data in addition to overloaded visual or auditory senses in information
Fig. 1.8 Surgical telemanipulator DaVinci R by Intuitive Surgical, installation in Munich.
Fig. 1.9 Functional muster of a hand-held laparoscopic telemanipulator with increased number of
degrees of freedom at the instrument’s tip, such as a prepared intracorporal force measurement
with haptic feedback on the control unit [209].
loaded working places. Such working places can be found in control stations where
the human has to make time critical and responsible decisions, e.g. within a jet, airplane
or at the steering wheel of a common car. The designers of a cockpit typically
choose between visual, acoustic and haptic transmission paths. Even the choice of
a scroll-wheel with hard stops instead of a pure incremental sensor is influenced by
the knowledge that a selection within a certain range can be much faster done if
the limits of this range are explicitly given [14]. Control knobs like the i-drive in
a BMW allow a reconfiguration of its haptic properties during operation. Warning
signals are already given via vibrating motors or so called “tactons”. Especially in
the military area a complex spatial orientation based on vibrating clothing (fig. 1.10)
for marines and flight-personnel is subject to actual research [267, 115], whereas
active sidesticks in military and civil airplanes and vibrating braking assistance or
in-lane guidance in cars are already established.
Fig. 1.10 West equipped with vibrators for the spatial coding of positioning and bearing data
(TNO, Netherlands) [267].
The Sense of Touch at the Desk
There is hardly any other job where the sense of touch has lost so much of its significance
than in the office. Just a few decades ago the use of paper, pens in a large
variety, rulers, folders and files was a joyful source of haptic information for the
sense of touch. Today the haptic interface to an office working place is defined by
a keyboard and a mouse. Due to this extreme focus on a single type of haptic interface
for a variety of things, the ergonomics of a keyboard is of extraordinarily
high importance. Besides the switching characteristics of the key itself, the surface
structure and the tactile markers on the letters F and J (fig. 1.11) and the size of
the key are necessary and considerable design criteria. ISO 9241-400 defines clear decision paths for both, the designer and the buyer of keyboards. Nevertheless it is
beyond doubt that major ergonomic improvements are not done by the optimization
of keyboard and mouse, but by improvements of office software ergonomics. Contrary
to many cases where the term “interface” refers only to the graphical interface,
RASKIN’s “The Humane Interface” [203] is a decided and enjoyable collection of
software with unergonomic graphical interfaces offering methods and design criteria
for their improvement.
The Sense of Touch in Music
If regarded from an abstract standpoint, haptic sense and acoustic perception have
multifarious parallels. Both are sensitive to the perception of mechanical oscillations
and cover a comparable frequency range. Thereby the haptic sense rather perceives
frequencies covering two decades below 1 kHz, whereas the acoustic sense rather
perceives frequencies up to two decades above 100 Hz. Music quite often makes
use of these parallels which may be used to perceive the oscillations of the string of
a valuable violin or harp; or to touch the soft vibration of a wind instrument giving
a low A. They are even to be found in studio technology. Devices like the “ButtKicker”
(fig. 1.12) from The Guitammer Company are electrodynamic actuators
which are used as tactile feedback devices during concerts. They transmit the lower
frequency range to the drummer giving the rhythm of the band without drowning his
own instrument. Additionally the acoustic pressure for the musicians is reduced, as
they may not necessarily want to be exposed to the same loudness as their excited
audience. These kinds of actuators are also suitable for e.g. the couch in a home cinema or chairs in front of gaming PCs to increase the perception of bass-intense
effects. Here again, the tactile effect is of similar intensity as the perception of a
bass impulse, connected with the advantage that little acoustic pressure is emanated
resulting in almost no disturbing noise for people around.
Fig. 1.12 Electrodynamic actuator “ButtKicker” for generating low-frequency oscillations on a
drum-stool.
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