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Drive Technique Comparison
One of the first decisions that must be made to undertake the project is what type of
actuator will be used to drive the motion of the servo. Many aspects such as size, cost,
heat generation, necessary control scheme and potential acceleration are evaluated. The
following is a comparison of four different types of actuation: magnetostrictive, Lorentz
force, variable reluctance (electromagnetic), and piezoelectric.
Magnetostrictive
Magnetostrictive materials expand when exposed to a magnetic field. Conversely, a
strain in these materials will result in a changed magnetic field (also called the Villari
effect). This property is caused by tiny magnetic domains inside a material. When
exposed to a magnetic field, the randomly oriented domains align, causing a strain in the
material. This effect can be optimized by controlling the initial configuration of the
domains through processes such as thermal annealing and cold working. Some common
magnetostrictive materials include iron, nickel, cobalt, and terfenol.
According to NASA, these materials can operate at higher temperatures and generally
undergo higher strains with lower input voltages than piezoelectric materials. However,
limited work has been done with magnetostrictive materials. Eda developed a
magnetostrictive actuator with a 2 m stroke [1], although no frequency response was
provided for this device. Liu et al created a 50 m stroke magnetostrictively actuated
tool holder [2]. This system had a natural frequency of 1.5 KHz, but no closed loop
bandwidth was specified. Other problems associated with these materials are the fact
that they are not easily integrated into a control system and also have a hysteresis effect.
Lorentz Force
Another possibility for diamond turning applications is a fast tool servo driven by a
Lorentz force actuator (Trumper-moving-magnet galvanometer). One application of this
drive technique is a rotary servo. This type of servo is especially useful for machining
spherical workpieces. The tool holder is placed on a rotational axis attached to the base
structure. This enables the tool to access a spherical piece at its pole as well as its
equator, as shown in the figure below.
Figure : An example of a rotary fast tool servo set up for operation on a two-axis
lathe. Side view (left). A top view (right) shows the servo’s ability to engage the
workpiece at its pole and equator by rotation about the B-axis.
A rotary servo offers other attractive qualities as well. The reaction forces created during
the cutting process are significantly reduced when compared to a more traditional linear
servo. A cleverly balanced rotary design can essentially eliminate reaction forces. Also,
it is claimed that rotary servos have higher achievable accelerations, lower cost, and a
more compact size than a linear FTS (Ludwick).
Much literature exists on this type of actuation. Douglass developed a linear slide fast
tool servo [3]. It was powered by a large voice coil motor. The stroke was 500 m and it
has a 100 Hz bandwidth.
Greene and Shinstock also developed a linear voice coil based fast tool servo [4]. This
FTS had a very large stroke of 6mm. However, the bandwidth was only 100 Hz. Due to
the low bandwidth and low mass, the dynamic system of the system was so low that the
design was not feasible for cutting applications.
Todd and Cuttino built a long range FTS with a stroke of 1 mm at 20 Hz [5]. It was
driven through a rotary motor through a steel ribbon.
Ludwick and Trumper developed a rotary FTS with a bandwidth of 200 Hz, 50 G peak
acceleration, and a stroke of 3 cm. This servo was driven by a commercial brushless
motor and had a resolution on the micrometer scale. Additionally, the use of a balanced
rotary design cancelled reaction forces.
Although designs using Lorentz force actuators have had success in the past, there are
some problems associated with this type of drive technique. The maximum achievable
acceleration of the motor is severely limited by the heat generated and the magnetic flux
density. The acceleration of this type of actuator falls below 100 G’s in the literature.
This value is considerably less than the necessary 1000 G’s needed for operation at 10
KHz with a 5 m stroke.
Variable Reluctance
Another design that has been introduced is that of a variable reluctance
(electromagnetically driven) fast tool servo. This design is based around an armature that
is push-pull driven by two high frequency solenoids. These solenoids are powered by a
high bandwidth linear power amplifier. Attached to the armature is a very light moving
mass and cutting tool, and position feedback is obtained with a capacitance gauge
observing the back of the armature. A general schematic of the design is shown below.
Figure : A schematic for an electromagnetically driven FTS
Not many researches have focused on variable reluctance actuators, most likely because
of the inherent non-linearity in such a system.
Gutierrez and Ro [6] developed a magnetically driven fast tool servo with a stroke of 800
m and a bandwidth of 100 Hz. However, the control scheme used to improve the
tracking performance had a negative effect on the resolution.
Currently, a prototype is being developed by Liu et al with proposed specifications of a
50 m stroke and 20 KHz bandwidth [7].
There is no doubt that very high accelerations are achievable with the use of
electromagnetic actuators. However, because of the inherent nonlinearity of the actuating
force (proportional to the current squared, inversely proportional to the air gap squared),
the system is very hard to control. Another problem with this method is the induced
Eddy currents by the high frequency magnetic field. These currents may reduce the force
density, limiting the bandwidth. It is therefore necessary to use materials with low
conductivity properties in this design.
Piezoelectric
Piezoelectric materials are those that deform when subjected to an applied voltage and,
conversely, produce a voltage when subjected to a mechanical stress. In this way,
piezoelectric materials are similar to the magnetostrictive materials described earlier.
Because of their high stiffness and high achievable bandwidth and acceleration, most
FTS’s use piezoelectric actuators.
Much research has been done in this area. Patterson and Magrab designed a FTS with a
2.5 m and 660 Hz bandwidth [8]. The natural frequency of this design was above 1
KHz. The basis of this servo was a moving cylindrical shell that held in line by two
diaphragm flexures.
Rasmussen and Tsao used a piezoelectrically actuated FTS for asymmetric turning
purposes [9]. The tool was driven with the help of a lever assembly and had a 50 m
stroke and 200 Hz bandwidth.
Okazaki developed a fast tool servo with 15m stroke, 2.5 KHz bandwidth and 2 nm
resolution [10]. A 19 mm long stacked ring piezoelectric actuator was used to achieve a
primary resonant frequency of 10 KHz.
Piezoelectric actuators do have some undesirable qualities. One of these is the heat
generated due to dielectric loss in the material, also known as hysteresis. Often, a cooling
system must be implemented to account for this heat generation. Another problem with
piezoelectric actuators is that expensive high-voltage amplifiers must be used to drive the
system. However, new innovations in the piezoelectric industry include much smaller
piezo stack sizes as well as low voltage/low heat generation materials. These discoveries
are very promising for the possibility of creating a FTS with a much higher bandwidth
than before.
Conclusion
After considering all possible drive techniques, it appears that a piezoelectric actuator
will best fit this application. The very small sizes that are newly available (5 X 5 X 9
mm) as well as the development of the low voltage piezo stacks weighed heavily in this
decision. This new type of actuator will lead to a very compact design in which cooling
will likely not be a major issue. Additionally, piezoelectric actuators can be controlled
through fairly conventional methods, unlike an electromagnetically driven system.
The following section shows preliminary calculations that indicate promise for the use of
a piezoelectric actuator.