Pulse Driven Induction Electrostatic Motor



Pulse Driven Induction Electrostatic Motor



Movie
Motor motion (QuickTime, 337KB)

Some applications (QuickTime, 453KB)


Introduction(See
this
page.)


Principle



Figure 1 shows a photo of Pulse Driven Induction
Electrostatic Motor[1]. This motor consists of two thin
films, slider and stator. Stator film contains 3-phase parallel
electrodes, and its surface are covered by isolation layer. On the other
hand, slider has a thin layer that has small conductivity, and does not
have any electrodes nor tooth structure.






Figure 1: Pulse Driven Induction Electrostatic
Motor



Figure 2 shows the principle of operation. To
operate this motor, place slider film onto stator film, and:

Induce charges on slider surface,
Drive slider by chaging the stator voltage, and,
Recharge on slider surface, and move to the second step. By
repeating this sequence, slider is driven by step motion. The following is
detail descriptions of the sequence.


(Initial charge step)At first, slider has no electric charge. As
shown in figure 2 (1a), apply (+,-,0) voltages on three poles of stator
electrodes, and induce electric charges on slider surface. Each electric
charge on slider surface have reverse polarity of the voltage of facing
stator electrode. By this operation, the voltage pattern is copied on
slider surface as electric charge pattern. The time constant of this
charge operation is determined by both capacitance between stator and
slider, and surface resistivity of slider. For example, the time
constance is 1 second, in case of figure 1 motor.After finishing
this charge step (figure 2 (1b)), slider is attracted to stator by
electrostatic force, and is held strongly by friction force.
(Drive step)As shown in figure 2 (2a), switch stator voltage
pattern to (-,+,-). By this switching, electric charges on stator
electrodes can change instantly, but electric charges on slider can not.
As same as the initial charge step, it takes some time for electric
charges on slider to reach new equilibrium. Therefore, right after
switching, the charge and voltage distribution shown in figure 2 (2a)
appears.At this time, both slider electric charge and facing stator
electric charge have same polarity, and therefore, slider gets repulsive
force from stator. At the same time, slider gets thrust force toward
right by electrostatic forces from electric charges on side. As a
result, slider is driven one electrode pitch toward right (figure 2
(2b)).
(Recharge step)Some electric charges on slider surface will be
lost during drive step. If we repeat the drive step, thrust force will
decrease. Therefore, we recharge slider electric charge after drive step
by using the voltage pattern shown in figure 2 (3). Since the lost
electric charges are relatively small, recharge time is far shorter than
initial charge step.






Figure 2: Operation
principle




Figure 3 shows one of prototypes whose electrode
pitch is 240µm. The stator film is fabricated using Flexible Printed
Circuitry (FPC) board. The slider film is made of PET whose thickness is
12µm, and is coated by high-resistivity layer, which consists of
carbon-black and polyurethane resin. The surface resistivity of the slider
is around 1014[ohm]. Weight of stator and of slider are 0.32g
and 0.03g, respectively.
We measured thrust force of this motor by adding some weight to be
pulled up by slider, and obtained thrust force of 0.1N. During measurment,
glass beads of 10µm diameter are inserted between stator and slider to
reduce fricition force, and gap between stator and slider are filled with
dielectric liquid (3M Fluorinert FC-77) to prevent electric breakdown of
air gap. The dielectric breakdown voltage of the stator film was 800V,
and breakdown occured in adhesive layer by applying higher voltage.







Figure 3: 240µm electrode pitch
model


FeaturesThe following lists are some features of this motor.


Slider does not have tooth structure. Charge pattern required for
operation is copied from stator electrodes by charge process. This gives us
large tolerance for electrodes alignment of stator and for assembling of
motor. However, stable high-resistivity material is required.
Since electrostatic repulsive force decreases friction between stator and
slider while movement, this motor does not require bearings (or linear guides)
to keep gap between stator and slider. Therefore, motor structure can be very
simple. On the other hand, when stopping, the slider is held strongly by
electrostatic attractive force.
Simple structure and ease of assembling allows us to make stacked motors
described in the next section.


Stacked motorBecause of its simple structure and ease of
assembling, this motor can easily be stacked to get higher power. Figure 4 shows
an example of stacked motors. The motor in figure 4 has 40 layers. Figure 5
shows the internal structure of the stacked motor. Stator films and slider films
are stacked together with spacer films, and their edges are held by metal pins
and acrylic plates. Interval between each stator film (or each slider film) is
0.35mm. To utilize both surface of stator films, each layer consists of one
stator film and two slider films as shown in the figure. So, the total number of
stator films are 40, whereas the number of sliders are 80.
The metal pins for stator works also as electric feed. Stator electrodes are
connected to those metal pins via washer made of conductive rubber.
Whole motor (figure 4) are held in plastic case and immersed in dielectric
liquid. Both side of slider are connected to output wires, which lead to outside
of the case. Thrust force is transmitted through these wires.
The total weight of the motor, including case and dielectric liquid, is 110g.











Figure 4: Stacked motor






Figure 5: Structure of the stacked motor

Results of measurements of thrust force and power are described in the
following.
Figure 6 shows a relationship between voltage and thrust force when the motor
is driven at 10Hz. The motor could operate over 100V applied voltage, and its
thrust force was proportional to square of applied voltage. The maximum
applicable voltage was limited under 800V due to dielectric breakdown of stator
film.
Figure 7 shows characteristics of thrust force and power against operation
frequency. The plot was obtained at 800V applied voltage. We obtained larger
thrust force at lower frequency, and maximum thrust force was 8N. Power was
larger at higher frequency, and maximum power was 0.5W.










Figure 6: Thrust force of the stacked motor







Figure 7: Power of the stacked motor



Applications2-DOF motor[2], Disk type rotary
motor[3]
By utilizing various electrodes instead of parallel electrodes, the motor
works as a 2-DOF (Degree of Freedom) motor or as a rotary motor.
Figure 8 shows 2-DOF motor. This motor utilizes matrix type electrodes. 9
electrode elements (3 by 3) makes one set of electrodes. By expanding the
operation principle as shown in figure 2, we can drive slider for any direction
on the stator.
Figure 9 shows disk type rotary motor. This motor utilizes radial electrodes
instead of parallel electrodes. Figure 9 shows only stator. We can use any shape
of slider.










Figure 8: 2-DOF motor






Figure 9: Disk type rotary motor
Transparent
motor[4]
ITO (Indium Tin Oxide) is called transparent conductor, and is utilized in,
for example, LCD (Liquid Crystal Display) as electrodes. Figure 10 shows
transparent electrostatic motor,which utilized ITO electrodes.
Stator is made of PET film of 125 µm thickness and ITO electrodes of 25nm
thickness. The visible electrodes on side are feed line made of silver.
The slider is made of chloroethene to equip transparency.
Figure 11 shows cylindrical transparent motor. In tihs motor, the transparent
stator and slider are rapped around transparent acrylic pipe.










Figure 10: Transparent motor







Figure 11: Cylindrical transparent motor
Electrostatic
paper feeder[5]
Pulse Driven Induction Electrostatic Motor can drive such sliders that have
surface resistivity of from 1013 to 1015. Most papers have
such surface resistivity at low humidity (their surface resistivity is sensitive
to the humidity). Therefore, at low humidity, we can directly drive papers as
sliders of the motor. Utilizing this drive, we can realize electrostatic paper
feeder, which is more compact than conventional ones.


See also

Dual
excitation multipahse electrostatic drive
Electrostatic
paper feeder
Electrostatic
non-contact suspension of thin plate
Particle
handling by electric field


References[1]S.Egawa, T.Niino, and T.Higuchi, "Film
actuators: Planar Electrostatic Surface-Drive Actuators", Proc. 1991 IEEE
Workshop on Micro Electro Mechanical Systems, pp. 9-14
(1991)[2]T.Higuchi, S.Egawa, T.Niino, and Nishiguchi, "Trial fabrication
of Planar 2-DOF electrostatic actuator", JSPE 1992 spring meeting, pp.
329-330 (1992) (in Japanese)[3]T.Niino, S.Egawa, T.Higuchi, and
Nishiguchi, "Trial Fabrication of Disk Type Electrostatic Film Motor",
JSME Robotics and Mechatronics Meeting '91, Vol. B, pp. 165-166 (1991) (in
Japanese)[4]S.Konno, Takada, Aida, T.Niino, S.Egawa, and T.Higuchi,
"Development of Transparent Film Motor", JSME Robotics and Mechatronics
Meeting '91, Vol. A, pp. 73-74 (1991) (in Japanese)[5]T.Niino,
S.Egawa, and T.Higuchi, "Electrostatic Paper Feeder", Journal of JSPE,
Vol. 60, No. 12, pp. 1761-1765 (1994) (in Japanese)


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