5.0 Actuator Power Systems

  In this project, the purpose of the actuator is to sweep the wing, pitch the stabilator,
and oscillate the control surfaces. The hydraulic actuator power system built by Bolding is
no longer functional and complications have arisen in the repair of the outdated system.
The decision has been made to discard the restoration of the outdated system and
investigate new technology in actuator power systems. Further, Bolding's wing-stabilator
model required an output force of 300 lb to sweep the wing, and maximum frequency
response of 56Hz to oscillate the pitch of the stabilator. The larger frequency response
range that is required to actuate active control surfaces induced concern for AWT. The
concern led AWT to investigate multiple types of actuator power systems. The three types
of actuator power systems considered were a modern hydraulic system, a piezoelectric
system, and a magnetic shape memory based system.

5.1 Hydraulic System

  A modern hydraulic power supply system was considered first as one of the possible
replacements for Bolding's system. AWT's initial idea was that a modern hydraulic power
supply could provide the design specifications that Bolding's outdated power supply could
not. The fundamentals of the hydraulic power supply system are explained in this section
along with a brief review of the hydraulic system built by Bolding.
  The basic idea behind the hydraulic system is Pascal's theorem. The theorem states
that when a pressure is applied to a liquid in a sealed container, the pressure is distributed
across the liquid, and the pressure is the same everywhere throughout the liquid. For
example, the basic application of Pascal's theorem is explained in figure 14. Two cylinders
are connected, and filled with a liquid. The cylinders differ in the cross sectional area; the
ratio of the cross sectional area is 1 to 10. When a force of 10 N is applied to the left
cylinder, the right cylinder experiences a force of 100 N. The piston in the right cylinder
achieves a large force while a small force is applied to the piston in the left cylinder.
However, it is important to note that the conservation of energy still applies. The 100 N
forces acts through a shorter distance than the 10 N force.


Figure 14. Pascal's Theorem [10]

  Any hydraulic systems can be divided into four logical segments: a power input
segment, a control valve segment, a power output segment, and a power transmission
system. The first segment is the power input segment. The power input segment may be
considered the power supply of the entire hydraulic system. The job of power input segment
is to raise the pressure of the operating fluid in the hydraulic system. Pumps and
accumulators are the common types of power source used in hydraulic systems. Pumps
convert work done by rotating or translating shafts into flow energy. Accumulators convert
power generated in a gaseous medium or energy stored in a gaseous or mechanical spring
into flow energy.
  Any type of pump operates by allowing oil to flow into a pumping cavity and then
forcing the oil out into another chamber. A gear pump is shown in Figure 15. A gear pump
adds kinetic energy to the flow by turning the gears in the path of the flow. Advantages of
the gear pumps are: it is a relatively simple structure, has less structural failure, and it is a
small inexpensive design. Disadvantages are the noise and vibration generated, the limited
pressure handling capabilities; and it cannot deal with a lot of amount of oil flow.


Figure 15. Pumps [11]

Figure 16 shows types of accumulators commonly used in the power input portion of
hydraulic systems. Some accumulators employ gaseous medium, and some employ
mechanical spring to store the pressure energy in them.


Figure 16. Accumulator [11]

A picture of a control valve is given in figure 17. Control valves control the flow of the
pressurized oil from the power input devices. While the job of the power input section of the
hydraulic system is to increase the pressure of the operating fluid, the function of the
control valves is to regulate the flow of the pressurized fluid to the actuating devices. In
general, a six-valve system comprises the control valve portion of the hydraulic system.


Figure 17. Control Valves [11]

A hydraulic actuator is shown below in figure 18. The actuator converts the energy stored
in the pressure of the operating fluid to a mechanical force used to prefrom a task. The
control valves determine the amount of force output by the actuator indirectly.


Figure 18. Actuator [11]

Power transmission system of a hydraulic system refers to piping through which the control
valves, the power input section, and the power output section communicate. Pipes transmit
oil from the pumps or accumulators to control devices. Design of these pipes is based on a
cost benefit analysis between energy loss and weight. For example, a large pipe transmits
oil flow with less energy losses however it will cost and weigh more.
Modern type of pipes is designed to transmit the oil flow with an average velocity of 15 feet-
per-second.

5.1.1 Bolding's Hydraulic System

The hydraulic system designed by Bolding, as shown in figure 19, was comprised of three
major assemblies: the hydraulic power supply module, the servo control module, and the
model hydraulic actuator system. The operating fluid of the old system was an oil-like fluid.
"The hydraulic power supply module is built around and an air-driven hydraulic pump. Air
for the pump enters the model from a regulated air supply through a quick-disconnect
fitting and a filter." [1] The air was used to pressurize the operating fluid of the system. In
Bolding's design, there are many safety features to prevent the catastrophic failure of the
system casing in the event of over pressurization. Future groups wishing to design a
hydraulic system will benefit from reviewing Bolding's design.


Figure 19 Model Control System [1]

The servo control module, see figure 19 (b), is the brains of the hydraulic system. The servo
control module regulates the pressure from the power supply to the actuators. Bolding had
two actuators connected to the hydraulic system. However, the system could only operate
one at a time. The control electronics were used to determine which actuator was to be
given energy by the hydraulic system.

5.2 Piezoelectric system

The piezoelectric system is another possible replacement for the outdated hydraulic
system. In this section the fundamentals of a piezoelectric power system are discussed.
Advantages and disadvantages of the piezoelectric system are also given.
The piezoelectric system is based on piezoelectric effect. When a piezoelectric material is
forced to deform in compression or tension, it generates an electrical current.
"The effect, discovered by Pierre Curie in 1883, is exhibited by certain crystals, for
example, quartz and Rochelle salt, and ceramic materials."[12] To use a piezoelectric
material in an actuator, the reverse piezoelectric effect must be utilized. The reverse
piezoelectric effect is the opposite phenomenon; when an electrical current is applied onto
the opposite sides the piezoelectric material, it elongates. (See figure 20.) When the
piezoelectric material is placed inside an actuator, the deformation of the material provides
the driving force.


Figure 20. Piezoelectric Material [13]

"A Piezoelectric actuator is an electric type actuator, which converts electrical input
energy into an output such as a displacement or generated force." Generally, the
piezoelectric actuator has advantages over a hydraulic actuator system. For example, the
piezoelectric system has an "extremely rapid response of 0.01 milliseconds, ultra-minute
movements of 0.01 microns, and amazing power that exceeds 3 kilograms per square
millimeter."[14=n6] On the other hand, a disadvantage of the piezoelectric actuator is that
its limited range of movement it can achieve. The limitation is on the order of 0.05 mm
[14]. Therefore, its application is also limited for only several fields, and the system is not
suitable for AWT's project. Furthermore, the piezoelectric material is brittle, care needs to
be taken during use. "High pulling or shear forces must be avoided and this is usually
achieved through the design of the actuator and associated mechanical system" [14].

5.3 Magnetic shape memory material

A magnetic shape memory (MSM) material based actuator is the final type of actuator that
AWT consider. MSM material is similar to the piezoelectric material. The difference is that
the MSM material requires a magnetic field to deform it. As seen in figure 21 a magnetic
field causes the MSM material to deform. The concept of the MSM actuator is the same as
the piezoelectric actuator. MSM material is placed inside an actuator casing and a magnetic
field is applied. Deformation of the material, while inside the casing, provides the force
output of the actuator.


Figure 21. Magnetic Moment [15]

The MSM material may achieve large elastic deformations when an external magnetic field
is applied to it. "The largest magnetic field induced deformations have been observed in Ni-
Mn-Ga alloys close to the stoichiometric composition Ni2MnGa, where strains up to 6 % are
obtained in the field of 0.6 T."[15]. "In addition to the higher strain, they possess higher
energy density, higher coupling factor and they operate at low magnetic field, and
complicated shape changes can be produced." [16] Furthermore, large frequency response
characteristics of the MSM actuator may be achieved by a rapidly changing magnetic field.
The MSM mechanism has been patented by AdaptaMat Ltd. According to the company, an
MSM actuator can achieve a piston stroke of up to 5 mm and forces up to 1.5kN. However,
AdaptaMat said that the system would cost over $5000 to meet AWT's requirement of
specifications. An AdaptaMat Ltd. MSM based actuator is shown in figure 22.


Figure 22. A Schematic of a Magnetic Shape Memory Material [15]









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