IEEE Robotics & Automation Magazine - September 2022 - 54

through pipes with small diameters. Capillarity enables the
system to achieve linear damping through a laminar flow.
Orifice Dampers
Orifice dampers are systems in which a fluid is forced to flow
through a hole between two chambers. As evident in Table 3,
row 2, they generate a damping action that is quadratically
proportional to speed [56]. The devices described in [46],
[49], [57], and [58] are examples of this technology. The first
three use a silicone tube filled with a viscous fluid that is
forced to move to achieve a damping effect. The fourth
achieves variable damping by pushing a viscous fluid through
a variable-diameter orifice operated by a motor, by all terms, a
valve. Variable-orifice viscous dampers find wide use in other
fields, particularly the automotive industry.
Variable-Rheology Fluid Dampers
These damping mechanisms rely on a particular class of fluids
that vary their rheological behavior when subjected to a magnetic
or an electric field. They can change from free-flowing
viscous liquids to semisolids, with a yield strength that
depends on the magnitude of the externally applied field
(magnetic or electric) [59]. The natural application of electrorheological
(ER) and MR fluids is in semiactive damping
systems, where their intrinsically variable behavior finds its
best use. Usually, ER and MR fluids consist of microscopic
particles suspended in a carrier medium. These particles can
be polarized electrically or magnetically (for ER and MR,
respectively), so when an external electric or magnetic field is
applied, they align along the field lines and act as a barrier to
the flow of the carrier. An extension of the Bingham plastic
model can describe the behavior of an ER or MR fluid subject
to an electric or magnetic field, as shown in Table 3, row 3.
Further information about ER and MR fluids and their applications
can be found in [60]. Three approaches are possible to
design a damping system based on variable-rheology fluids
[61]: squeeze mode, shear mode, and valve mode.
Squeeze Mode Dampers
In squeeze mode dampers (Table 3, row 3) a volume contains
the variable-rheology fluid that is squeezed away. An ER
damper that uses squeeze mode is in [62], and an MR example
is in [63].
Shear Mode Dampers
In shear mode dampers (Table 3, row 3), plates in relative
motion are used to generate shear strain in the fluid. MR
examples are in [64]-[69].
Valve Mode Dampers
Finally, in valve mode dampers (Table 3, row 3), the fluid is
forced to flow through a narrow orifice, where the external field
is applied. An ER damper that works in valve mode is reported
in [61], and an MR version is in [70]. Rheology dampers have
proved to be advantageous in terms of fast control response
and simple mechanics since they do not require additional
54 * IEEE ROBOTICS & AUTOMATION MAGAZINE * SEPTEMBER 2022
movers to vary their properties. They also present disadvantages.
MR fluids tend to settle through time without frequent
mixing. Particles are liable to stick together due to residual
magnetization, making redispersion difficult and forming a
hard layer [71]. ER fluids, on the other hand, have a tendency
to chemically and mechanically react with diverse materials,
necessitating precise damper component selection [72].
Electromagnetism-Based Dampers
This class of mechanisms relies on passive electromagnetic
forces that resist the relative motion between a conductive
object and magnetic field. They divide into two families. In
the first, that of eddy current dampers (ECDs), the conductive
body has a generic shape. In the second, motor-like electromagnetic
dampers, the conductive body comes in the form of
one or more wires that make a circuit. Electromagnetismbased
dampers offer the advantage of avoiding friction,
extending component longevity. Moreover, they have fast and
precise dynamics. Their primary drawback is a low energy
dissipation density [73].
ECDs
ECDs are passive electromagnetic devices composed of a conductive
material moving through a magnetic field (Table 3,
row 4). They exploit the Foucault currents induced inside a
plate moving in a magnetic field. Examples of this technology
can be seen in [74]-[77]. Permanent magnets and electromagnets
can be used to realize ECDs. The most straightforward
implementations use permanent magnets to produce passive
damping [75]. Nevertheless, semiactive damping can rely on
electromagnets as well as permanent magnets [74]. When
using electromagnets, the electrically induced magnetic field
controls the magnitude of the damping action. On the other
hand, when using permanent magnets, it is possible to change
the amount of damping by modifying the system geometry.
Motor-Like Electromagnetic Dampers
These systems exploit the same effect as those described previously
but rely on a geometry that is similar to that of electric
motors. In them, examples of which are in [6], [78], and [79],
the conductive body forms a set of coils similar to a rotor,
while a component comparable to a stator generates a magnetic
field that can be constant if the component is built using
permanent magnets or variable if the device uses other coils.
Table 3, row 5, describes the physics [53] of a single coil rotating
in the magnetic field. The many coils of the system are
then short-circuited, possibly through a variable resistor, to
generate a circulating current and back electromotive force,
which induces damping. Passive damping can be obtained
through permanent magnets on the stator [6], whereas it is
possible to design semiactive damping by using electromagnets
in the stator [78].
Friction-Based Dampers
This family of damping systems relies on forces that naturally
resist the relative motion of solid surfaces. Such surfaces can
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