Biomedical Engineering Reference
In-Depth Information
electrostatic forces depends on the conditions. If electric field
E
is constant, the
force is:
S
2
F
A
¼
F
B
:
(8.52)
The properties of materials allow an increase in the electric field in accordance
with equation [
4
]:
E
B
p
:
E
A
¼
(8.53)
In this case:
F
A
¼
S
F
B
:
(8.54)
Trimmer analyzed the possibility of using magnetic and electrostatic forces to
create micromotors. The scaling of electrostatic forces (equation (8.54)) allows an
increase of the volumetric power, diminishing the micromotor size. But experience
with MEMS micromotors demonstrates that, in the micrometric range, the electro-
static electromotors still do not have sufficient power for use in micromachine tools
and micromanipulators. In this sense, they cannot compete with micromotors based,
for example, on piezoelectric forces.
The magnetic forces, which scale in accordance with equation (8.51), in princi-
ple, can be used for micromotor creation, but the micromotor efficiency
decreases
in accordance with the equation:
A
¼
S
B
:
(8.55)
If these motors are to be used in microfactories, the consumption of volumetric
energy will grow linearly with the diminution of the micromachine tool sizes,
making it necessary to search for other forces for micromotor creation. In MEMS,
many different principles of motor operation were tested, such as piezoelectric
motors, thermal motors, motors based on shape memory alloys (SMAs), pneumatic
motors, and hydraulic motors. The piezoelectric motors have many problems with
the wearing of their components. The thermal motors and SMA motors have low
efficiency,
. The pneumatic and hydraulic motors have great volumetric power and
can have high efficiency, so it is interesting to consider their scaling.
8.3.6 Viscosity and Velocity of Flow
8.3.6.1 Pneumatic and hydraulic forces
Let us consider two hydraulic or pneumatic cylinders
A
and
B
with pistons. The size
of cylinder
A
is
S
times larger than cylinder
B
.
