Page 648 - Mechatronics with Experiments

634 MECHATRONICS

Plunger
Current is constant
Force i) Flat
Stop

ii) Conical
i)

iii) Stepped
conical
ii)
iii)
FIGURE 8.18: Solenoid force versus
plunger displacement under constant
coil current condition. As the plunger
closes the air gap between itself and
the stopper, the force capacity
increases. This is called the holding
force, which is generally higher than
Stroke the average pull-push force.

The operating principle of solenoids is based on the tendency of the ferromagnetic
plunger and coil generated magnetic flux to seek the minimum reluctance point. As a
result, when the coil is energized, the plunger is pulled in towards the stopper. The higher
the magnetic field strength (n ⋅ i, number of turns in the coil times current) and the
coil
better the magnetic permeability of the medium that guides the flux to the plunger, the
higher the force generated. The plunger works on the pull principle. However, by mechanical
design we can obtain pull or push motion from the solenoid (Figure 8.17). The mechanical
connection between the plunger, made of a ferromagnetic material, and the tool must be via
a nonmagnetic material. For instance, in the case of a push-type solenoid (Figure 8.17b),
the push-pin is made of a nonmagnetic material. The quality of the magnetic circuit and
its ability to guide magnetic flux lines depends on the permeability of the coil frame, the
air gap between the plunger and coil (fixed gap), and the air gap between the plunger and
stopper (variable gap). For a given current, the force generated by the solenoid varies as
a function of the air gap between the plunger and the stopper. The smaller this gap is, the
smaller the effective reluctance of the magnetic flux path, and hence the higher the force
generated. The force as a function of plunger displacement under constant current varies as
shown in Figure 8.18. Notice that the shape of the force-displacement curve for a constant
current can be affected by the shape of the plunger and stopper head. For high performance
applications, in order to reduce the eddy current losses in the solenoid, the iron core of the
winding and the plunger may be constructed from laminated sheets of iron that are insulated.
Sometimes, solenoids are referred as single acting type and double acting types. A
solenoid is a single acting type device: that is, when current is applied the plunger moves
in one direction in order to minimize the reluctance, regardless of the direction of the
current. Force is always generated in one direction. A double acting solenoid can move in
both directions by generating force in both directions, both pull and push directions, by
using two solenoids in one package. Therefore, a double acting solenoid is basically two
solenoids with one plunger, two coils, and two stops.
A directional flow control valve may have one or two solenoids to position it in
two or three discrete positions. For instance a double acting solenoid (two solenoids in one
package) can have three positions: center position when both the solenoids are deenergized,
left position when the left solenoid is energized, and right position when the right solenoid
is energized. If the current in each solenoid is controlled proportionally, instead of fully