A Four Degree of Freedom MEMS Microgripper with Novel

A Four Degree of Freedom MEMS Microgripper with Novel
A Four Degree of Freedom MEMS Microgripper with Novel
Bi-Directional Thermal Actuators
Michael A. Greminger
A. Serdar Sezen
Department of Mechanical Engineering
University of Minnesota
Minneapolis, MN 55455
{grem,ssezen}@me.umn.edu
Abstract— A four degree of freedom thermally actuated
MEMS microgripper is presented in this paper. Each jaw of
the microgripper has independent x and y degrees of freedom.
These extra degrees of freedom give the gripper added dexterity
for manipulating microparts. The motion of each gripper jaw
is provided by a two degree of freedom compliant mechanism
which is based on a five bar rigid link mechanism. This gripper
also introduces a novel thermal microactuator design that is
capable of bi-directional actuation giving it a greater range of
motion than previous thermal actuator designs. The actuator
provides a total range of motion of 12.7 µm and a maximum
force of 1.9 mN. Also, since this microgripper is based on a
compliant mechanism, deformable object tracking can be used
to provide force as well as position feedback for the gripper. This
combination of an increased number of degrees of freedom and
increased sensory feedback provides a level of dexterity that has
not been previously available in microassembly.
Index Terms— Microgripper, microassembly, microrobotics,
MEMS, compliant mechanism.
I. I NTRODUCTION
Microrobotic systems are currently used for many applications in microassembly [3] [15] and in biomanipulation
[2] [13]. Microrobotic systems hold much promise. However,
the capabilities of these systems are limited when compared
to their macroscale counterparts. For example, microgrippers
typically are limited to a single degree of freedom for both
actuation and feedback. Greater end effector dexterity and
sensory feedback is required to produce more advanced
microrobotic systems. This paper presents technology that
will help to increase capabilities of microrobotic systems
by presenting a microgripper that both provides increased
dexterity and the potential for increased sensory feedback
when compared to previous designs.
In this paper, a four degree of freedom bulk micromachined
MEMS microgripper is presented. Each jaw of the microgripper is a compliant mechanism with both an x and a y
degree of freedom. A bi-directional bending thermal actuator
is used for each degree of freedom. The thermal actuator is
able to bend in either direction because the electric circuit
is completed through the kinematic chain of the compliant
mechanism. Therefore, the location of heating within the
mechanism can be controlled remotely by applying electrical
potential at the appropriate contacts.
Bradley J. Nelson
Institute of Robotics and Intelligent Systems
Swiss Federal Institute of Technology (ETH)
CH-8092 Zurich, Switzerland
[email protected]
Force feedback can also be provided for the microgripper
design presented in this paper. Since compliant mechanisms
are used to provide the motion of the gripper jaws, the
deformations of these compliant mechanisms can be tracked
visually to provide force feedback through the use of visionbased force measurement. Force feedback can be provided
for each degree of freedom of the gripper. It has been
shown that vision-based force measurement provides a robust
and non-contact approach for providing force feedback in
microsystems [4] [5] [14].
This paper is organized as follows. First the design of
the microgripper is discussed. The design of both the compliant mechanism and the design of the thermal actuators
are addressed. Next, the fabrication and instrumentation of
the microgripper is presented. Finally, the characteristics and
performance of the devices and their actuators are presented.
II. D ESIGN
The design goal for this microgripper is for each jaw
of the gripper to have both x and y degrees of freedom.
Typically, MEMS microgrippers only possess a single degree
of freedom which opens and closes the gripper. The addition
of these three extra degrees of freedom gives the gripper
added dexterity.
A compliant mechanism is used for each jaw of the
microgripper to give it the necessary degrees of freedom. The
compliant mechanism design is based off of a five bar rigid
link mechanism. The design of the thermal actuators used to
actuate the device is also discussed in this section.
A. Mechanism Design
The design of the compliant mechanism for each jaw of
the gripper is based off of a five bar mechanism design. A
five bar mechanism has two degrees of freedom. The input
to the mechanism is the rotation angle of each of the links
attached to ground as shown in Figure 1. The position of the
entire mechanism is completely determined by the position
of these two input links.
The design objectives for the compliant mechanism are to
decouple each of the degrees of freedom and to amplify the
motion of the actuator inputs. The mechanism design that is
used for each jaw of the gripper is shown in Figure 2(a).
Fig. 1.
(a)
Fig. 2.
Five bar rigid link mechanism.
(b)
(a)
(b)
(c)
(d)
(c)
A compliant mechanism (c) based on a rigid link mechanism (a).
The paths followed by the a gripper jaw for each degree of
freedom are shown in Figures 3(a) and (b). It can be seen
that the degrees of freedom are nearly completely decoupled.
A compliant mechanism is created from the rigid mechanism by using a compliant member to approximate each
pivot joint in the rigid link design. Each compliant member
is a flexible beam with the center point of the beam lying
at the location of the pivot it replaces. The length of each
of the compliant members affects how closely the motion of
the compliant mechanism approximates that of the rigid link
mechanism. The shorter the compliant members, the closer
the motion of the compliant mechanism matches that of the
rigid link mechanism. The drawback of short compliant links
is that they experience greater internal stresses for the same
level of gripper motion as compared to longer compliant
members. Finite element simulations of the compliant mechanism were used to evaluate the kinematic performance of the
compliant mechanism as well as to insure that the compliant
members will not fail under normal operating conditions.
Simulation results for each degree of freedom are shown in
Figures 3(c) and (d).
B. Actuator Design
Both electrostatic actuators and thermal actuators were considered as the source of actuation for each degree of freedom
of the microgripper. Thermal actuation was chosen for two
reasons. The first is that electrostatic actuators could not provide the forces necessary to actuate the compliant mechanism
without requiring excessive device area or extremely high
actuation voltages. Second, electrostatic actuators are most
effective at providing linear actuation whereas the gripper’s
compliant mechanism requires a rotational input. Bending
thermal actuators naturally provide a rotational motion.
Fig. 3. Degrees of freedom for the rigid link mechanism (a)(b) and the
associated compliant mechanism (c)(d).
All thermal actuators deflect as the result of an applied
thermal stress where heating is generally provided by Joule
heating. Thermal stresses arise in three situations: when
there is a nonhomogeneous temperature distribution in a
structure; when there is a nonhomogeneous coefficient of
thermal expansion and the temperature changes; and when a
structure is overly constrained and the temperature is changed
thus causing buckling. Thermal actuators using each of these
mechanisms are currently implemented in MEMS devices.
The commonly used MEMS thermal actuators are shown in
Figure 4. Figure 4(a) shows a thermal actuator that relies
on nonuniform temperature distribution [6] [7], Figure 4(b)
shows a thermal actuator based on the buckling principle [9]
[12], and Figure 4(c) shows a thermal actuator that relies on
a nonhomogeneous coefficient of thermal expansion [1] [10].
The limitation of each of these designs is that they can only
be actuated in a single direction.
The thermal actuator design that is proposed here is shown
in Figure 5. As can be seen from the figure, depending on
which contacts electrical potential is applied to, the actuator
can be actuated in either direction. This is possible because
the compliant mechanism itself is used to close the electrical
path, thus providing a return path for the current used to
heat the hot arm of the actuator. In order to control this type
of an actuator with a computer, an interface which converts
the signal level voltages from the computer to currents has
been designed. This circuit is discussed in Section III-B. The
bending is [8]:
eEAα
(T2 − T1 )
(1)
2
where A is the cross sectional area of each beam, T1 and
T2 are the temperatures of beams 1 and 2 respectively, e is
the distance between the centers of the beams, and α is the
coefficient of thermal expansion for both of the beams. The
moment M is the moment that would have to be applied to
the end of the undeflected actuator to cause it to deflect the
same magnitude as when the temperatures of beam 1 and 2
are T1 and T2 respectively. The radius of curvature ρ of the
actuator is related to the bending moment by the following:
M=
(a)
(b)
(c)
Fig. 4. Common thermal actuator designs: (a) bending thermal actuator, (b)
buckling beam thermal actuator, and (c) bimorph thermal actuator.
M
eAα
1
=
=
(T2 − T1 )
ρ
EI
2I
(2)
where I is the moment of inertia of both beams together and
is defined as:
3
bh
Ae2
I=2
+
(3)
12
4
where h is the height of each beam and b is the depth of each
beam. From the curvature of the beam, the total deflection of
the actuator can be calculated using the following equation
[16]:
eAαl2
l2
=
(T2 − T1 )
(4)
δ=
2ρ
4I
Fig. 5.
The thermal actuator design introduced in this paper.
limitation of this type of actuator is that it can only be used
in a situation where the actuator is connected to a closed
kinematic chain. However, when a closed kinematic chain is
being used, this design is superior to the previous actuator
technologies because it can be actuated in two directions.
The displacement of this actuator design can be estimated
by modeling the actuator as two parallel beams separated
by an air gap. The beams are clamped on both ends as
shown in Figure 6(a). The properties of each of the beams
are identical with the only difference being the temperature.
This temperature difference induces a stress which results in
a deflection of the beam. The actuator is in a state of pure
bending when the beams are at different temperatures. The
effective bending moment associated with this state of pure
where l is the length of the actuator. It can be seen from
the above equation that the deflection of the actuator is
proportional to the temperature difference between the two
beams. The maximum actuation force can also be calculated
in terms of the temperature difference between the beams. The
maximum force that the actuator can supply is equivalent to
the force F required to keep the end of the actuator from
moving when a moment M of magnitude (1) is applied to
the end of the beam (see Figure 6(c)). The equation for F is
[16]:
3eEAα
3EI
=
(T2 − T1 )
(5)
F =
2lρ
4l
This is the force that the actuator can generate at zero
deflection which is referred to as the blocking force. The
force that the actuator can generate decreases as the actuator
travels through its range of motion.
III. FABRICATION AND I NSTRUMENTATION
The microgripper is bulk micromachined from 100 µm
thick <100> silicon with less than 0.01 Ohm-cm resistivity.
This 100 µm silicon layer makes up the device layer of
a silicon-on-oxide (SOI) wafer which has a 2 µm silicondioxide box layer and a 250 µm thick silicon handle layer.
The oxide layer serves both as an etch stop layer during
fabrication and as an electrical insulator for the final device.
Deep reactive ion etching (DRIE) is used to create the
microgripper’s features.
After the microgripper is fabricated it needs to be interfaced
with a circuit to provide current to actuate the thermal
actuators. An interface circuit was designed so that voltages
(a)
(b)
Fig. 8. Interface circuit used to control the gripper with signal level voltage
inputs.
(c)
Fig. 6. Model used to calculate the displacement of the actuator and the
maximum force of the actuator.
The device layer that remains after this etching step forms the
geometry of the gripper jaws, the compliant mechanisms, and
the thermal actuators. Finally, the devices are released from
the wafer by etching away the exposed oxide (see Figure
7(e)). The oxide is etched using a plasma etcher.
B. Instrumentation
(a)
(b)
(c)
(d)
(e)
Fig. 7.
Fabrication sequence used to fabricate the microgripper.
from a digital to analog converter can be used to control the
displacement of the actuators.
A. Fabrication Process
The fabrication sequence is outlined in Figure 7. The SOI
wafer that forms the starting point is shown in Figure 7(a).
The first step is to etch the handle layer of the wafer using
DRIE (see Figure 7(b)). This step forms the frame and the
support structure for the final device. Figure 7(c) shows the
next step which is to pattern the aluminum contacts that are
used to electrically connect the device to its control circuit.
The next step is to etch the device layer (see Figure 7(d)).
An interface circuit was implemented so that voltages from
a digital to analog converter can be used to control the motion
of the gripper. The direction of actuation and which actuator
are actuated depend on the path the electrical current takes
through the device. Transistors are used as gates to control
this path. The state of the transistors determines which of
the two actuators is activated and the direction of actuation.
A simplified version of the interface circuit is is shown in
Figure 8 for one jaw of the microgripper. The input to the
circuit is the gate voltage to each of the transistors. The input
voltage is boosted using noninverting summing amplifiers for
the transistors on the high voltage side of the circuit because
the gate voltage required for these transistors is above signal
levels. The supply voltage to the circuit is 80 volts. The
voltage required to actuate the device to full scale deflection
is 40 volts. The circuit runs at above 40 volts to account for
voltage drops at the gate transistors.
Shunt resisters are placed in series with each electrical
contact of the device. The voltage drop across each of these
resisters is measured by a differential amplifier to provide a
voltage proportional to the current entering the device. These
current measurements are the output of the interface circuit
and are used for two purposes. The first is to provide feedback
for the user of the device. The second is to provide feedback
for a current limiting circuit that is not shown. The current
limiting circuit limits the maximum amount of current that
can be run through the device. The current limiting circuit
protects the device from being damaged by overheating. This
Fig. 9.
Fig. 10.
SEM image of the fabricated microgripper. This device is 6mm by 6mm in size. The insets show a closeup of one of the thermal actuators.
Image of fabricated microgripper. This device is 3 mm by 3 mm.
limiting is important because in order to achieve the most
performance out of the thermal actuators it is desirable to
operate them as hot as possible without overheating them.
Any control input can be sent to the device and the current
limiting circuit insures that the device will not be overheated.
degrees of freedom. The opening and closing range of motion
for each gripper jaw is 38.4 µm, and the orthogonal range of
motion for each gripper jaw is 11.6 µm.
The motion of a thermal actuator is shown in Figure 12.
The total range of motion for this actuator is 12.7 µm. The
actuator shown is 400 µm long, 100 µm deep, has beams that
are 4 µm thick, and has a 10 µm gap between the beams.
Using (4), the amplitude of deflection shown corresponds to
a temperature difference of 578 ◦ C between the hot and cold
beams. The maximum force that this actuator can produce is
1.9 mN, which is calculated using (5).
For the results given here, the achievable actuator deflection
was limited because the compliant members of the compliant
mechanism overheated before the actuator could reach its
maximum temperature. This is due to increased Joule heating
of the compliant members as compared to the thermal actuator
beams. This occurs even though the actuator beams and the
compliant members were designed to have the same thickness.
The compliant members are thinner in the final device because
of the ununiform etching rate of DRIE. The problem of
compliant member heating can be solved in future designs
by patterning a metal conductor onto the compliant members.
This fix would allow the actuators to reach their maximum
temperature without concern that the compliant members will
overheat.
V. C ONCLUSIONS AND D ISCUSSION
IV. R ESULTS
Figure 9 shows an SEM image of a fabricated device. A
microscope image of another device is shown in Figure 10.
This device has an overall size of 3 mm by 3 mm and is used
for the results presented in this paper. Figure 11 shows the
range of motion of the microgripper jaws for each of their
A new gripper design has been presented that offers greater
dexterity then is currently available. It also introduces a novel
thermal actuator design that takes advantage of the closed
kinematic chain design of the compliant mechanism to allow
actuation in two directions. It was also discussed how visionbased force measurement can be used to provide position and
force feedback for the microgripper which will aid in the
micromanipulation task.
Improving the dexterity and the sensing capabilities of
micromanipulators will greatly enhance the cababilities of
micromanipulation and biomanipulation systems. These capabilities will allow the algorithms and techniques used in
macromanipulation to be applied to micromanipulation.
ACKNOWLEDGMENTS
This research was supported in part by the National Science
Foundation through Grant Numbers IIS-9996061 and IIS0208564. Michael Greminger is supported by the Computational Science Graduate Fellowship (CSGF) from the
Department of Energy.
(a)
(b)
(c)
(d)
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(e)
Fig. 11.
Range of motion for the microgripper jaws. The undeformed
microgripper is shown in (a).
(a)
(b)
(c)
Fig. 12. Thermal actuator motion: (a) actuator deflecting upwards, (b) actuator undeflected, and (c) actuator deflecting downwards. A total displacement
of 12.7 µm is shown.
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