What Is a Coil Actuator?

MEMS actuators are MEMS devices that convert electrical signals into micro-actions or micro-operations. Typical MEMS actuators include: micro motors, micro switches, micro clamps, etc .; digital micro mirrors and various micro optical switches in optical MEMS devices; RF micro switches in RF MEMS devices; mixers in micro fluidic MEMS devices , Valves, pumps, etc.

Chinese name: MEMS actuator

Types of: A MEMS device

MEMS actuator implementation

MEMS actuator electrostatic execution

The electrostatic execution method is to inject oppositely charged charges on the two electrodes, and use the attractive force between the two oppositely charged electrode plates to control the relative movement of the electrode plates by controlling the amount of charges. In the MEMS system, the specific forms of this implementation include cantilever beam type, comb electrode type, and twist type. ^[1]

The electrostatic deflection cantilever structure is shown in Figure 1-1.

According to engineering mechanics theory, the width is

Of a cantilever beam at the end of the beam when a concentrated load is applied from the fixed end x

for

Where the electrostatic force q (x) from the fixed end x of the beam is

Figure 1-2 Structural principle The comb-shaped electrostatic execution method uses a large number of comb teeth and is driven by applying a voltage U between them. Its structure is shown in Figure 1-2. Unlike cantilever beams and torsional actuators, in comb actuators, the capacitance is changed by changing the area rather than the plate spacing. Since capacitance is linear with area, displacement will be proportional to the square of the applied voltage.

The advantages of electrostatic execution are low power consumption, short response time, and it is suitable to complete higher frequency driving, and the structure of electrostatic driving is relatively easy to implement, so there are many applications in this area.

MEMS actuator piezoelectric implementation

When some material applies pressure or tension in a certain direction, polarization occurs, and the two surfaces of this material will generate charges with opposite signs. This effect of the surface charge of a material due to mechanical forces is called the piezoelectric effect. Conversely, if a certain voltage is applied across the piezoelectric material, the material will show a certain deformation (elongation or contraction). This opposite process is called the inverse piezoelectric effect.

Dielectrics with a piezoelectric effect are called piezoelectric materials. In nature, most crystals have a piezoelectric effect, but most crystals have very weak piezoelectric effects. With the in-depth study of piezoelectric materials, it was found that artificial piezoelectric ceramics such as quartz crystal, barium titanate, and lead zirconate titanate are excellent piezoelectric materials, see Table 1-3.

Table 1-3 Related characteristics of piezoelectric materials ^[1]

material	Types of	Piezoelectric constant d / (PC / N)	Relative permittivity
quartz	Single crystal	d ₃₃ = 2.33 [2,3]	5 [2] 4.0 [3]
Polyvinylidene fluoride (PVDF)	Aggregate	d ₃₁ = 20 d ₃₂ = 2 d ₃₃ = -30 [2] d ₃₁ = 23 [1] d ₃₃ = 1.59 [3]	12 [1,2]
Barium titanate (BaTiO ₃ )	Ceramic products (perovskite crystals)	d ₃₁ = 78 [1,2] d ₃₃ = 190 [3]	1700 [1,2] 4100 [3]
Lead Zirconate Titanate (PZT)	Ceramic products	d ₃₁ = 110 [1,2] d ₃₃ = 370 [3]	1200 [1] 300 ~ 3000 [3]
Zinc oxide (ZnO)	Metal oxide	d ₃₃ = 246 [4]	1400 [4]

The inverse piezoelectric effect of piezoelectric materials can be used to convert electrical energy into mechanical energy. Applied voltage

Can generate the corresponding force

And cause size changes

. In general, to obtain a displacement on the order of micrometers, a voltage exceeding 1000V is often required. In order to withstand high voltages, the film must have a sufficient thickness.

A significant feature of the piezoelectric MEMS structure is that through proper structural design, the dual functions of sensors and actuators can be combined on one unit, and there are also large differences in specific performance and applications. However, since piezoelectric micromachines need to use piezoelectric materials that are different from the original microelectronic materials, a lot of research work is still needed on the mechanical structure design, material preparation technology, material processing technology, and process compatibility of the sensor.

MEMS actuator thermal execution

The thermal execution method is based on that when the driving structure obtains a certain amount of thermal energy, the corresponding deformation is generated under the driving of thermal stress to complete the driving. In principle, thermal execution methods can be divided into two types: thermal expansion and thermopneumatic.

The thermal expansion type usually uses different thermal expansion coefficients of two bonding materials to form a thermal dual-chip actuator. That is, a heater is sandwiched between two layers of "movable" materials. After power-on, the two layers of materials produce different expansion coefficients, thereby achieving the purpose. .

Bimetal execution is another implementation that uses thermal expansion. When the actuator constituted by this execution mode is heated, the temperature of the driving element itself rises, thermal stress is generated inside the structure, and a linear strain is generated in the film, thereby achieving the driving purpose. The bimetal thermal actuation method has the characteristics of low driving voltage, large driving force, large stroke, linear displacement-energy relationship, simple structure and manufacturing process (relative to thermo-pneumatic methods), easy implementation of driving energy, and easy integration. Therefore, it has a broad application prospect.

A typical method of thermopneumatic method is to form a cavity with a sealed fluid (such as air, water vapor, and liquid water, etc.). When the fluid in the cavity is heated, it expands and the pressure increases, thereby pushing the film to move. Common are wave tube micro-actuators, piston actuators and so on. The corrugated tube micro-actuator has a ring-shaped folded film structure. When the internal pressure is applied, the corrugated tube is deformed to achieve driving. An expanding gas-driven piston actuator moves in parallel along the plane on which the substrate is located. Under the action of the polysilicon heater, bubbles of water vapor are formed and expand in the piston cavity, pushing the piston outward. When the heating stops, the bubbles in the piston cavity burst, and the piston returns to its original position.

In addition, there is a way to implement shape memory alloys. Shape memory alloy (SMA) is an alloy that undergoes a significant change in length (shrinkage) when heated. Alloys that are deformed by mechanical forces return to their pre-deformation state when heated. Because they are electrically conductive, they can be heated by electric current. Deformation causes the material to change from one crystal orientation to another. This process can be reversed by heating. Ti-Ni alloy can produce

The above resilience and 2% recovery strain, while its another biggest advantage is non-toxic to human body, very suitable for medical applications.

MEMS actuator electromagnetic execution

Electromagnetic drive is also a common way of implementing actuators. The working principle of the electromagnetic actuator is to use the principle of electromagnetic induction. The actuator generates a mechanical action under the action of a magnetic field. When the microstructured coil is energized, due to the influence of the magnetic field, the electrons are affected by the Lorentz force, which makes the coil mechanical. Movement to complete the conversion of electrical energy to mechanical energy. The advantage of electromagnetic execution is that it has a high output torque, and can be attracted or repelled. The disadvantage is that the power consumption is generally high, the torque is limited by the number of coil turns, and the generated magnetic field will affect nearby objects, such as moving charged particles or affecting magnetic data storage media. In addition, the micromechanical processing technology is complex and difficult, and the compatibility with the existing microelectronic process is very poor. There are still major problems in the application of microelectronic mechanical systems.

Of course, in addition to the four driving methods described above, there are other types of driving methods that can be applied to MEMS actuators, such as light driving and fluid driving.

MEMS MEMS actuator typical MEMS actuator

MEMS actuator micromotor

Micro-motors are the most commonly used MEMS actuators, which are characterized by miniaturization, diversification and integration. There are currently six types of micromotors, namely electrostatic micromotors, piezoelectric micromotors, ultrasonic micromotors, electromagnetic micromotors, resonant micromotors, and biological micromotors, of which electrostatic micromotors are the focus of research.

Figure 1-4 The first electrostatic micromotor The electrostatic micromotor uses an electrostatic drive motor to realize the rotation of the motor. This motor structure has a freely rotatable intermediate rotor, which is surrounded by capacitor plates and driven at a suitable phase, so that the rotor can be rotated to obtain a relatively high speed. A considerable number of scientific and technological workers have carried out research on the modeling and design of such structural motors. ^[1]

In 1988, UCBekeley in the United States successfully developed the first electrostatic rotating micromotor using surface sacrificial layer technology, which marked the development of MEMS technology into a new era. Figure 1-4 is the first rotary micromotor that uses electrostatic drive. The micromotor uses 6 fixed electrodes and 8 rotor electrodes. The rotor diameter is 120um, the thickness is 1um, and the gap between the stator and the rotor is 2um, at 350V. Under three-phase voltage driving, the maximum speed reaches 500r / min.

MEMS actuator electromagnetically driven microswitch

Figure 1-5 Prototype of electromagnetic microswitch developed by Microlab and UIUC The magnitude and distance of the electromagnetic force are non-linear. When approaching the contact, the electromagnetic force increases exponentially and finally achieves stable contact, so it is very suitable for driving switches. At the same time, there is no delay between the generation of electromagnetic force and the establishment of the magnetic field, so this driving method can provide faster switching time. In terms of electromagnetically driven microswitches, the electromagnetically driven microswitches developed by Microlab and UIUC, as shown in Figure 1-5, can generate a suction force of 10uN ~ 1mN under the control of an excitation current of about 20mA ~ 500mA, making the microelectrode Generates a displacement of 10um ~ 20um, its on-resistance is 50m, and it can pass a 1.2A current signal.

In addition, many scientific research groups have demonstrated the prototype of electromagnetically driven microswitches using the technology of tiny coils, cantilever beams of soft magnets, and secondary bonding of permanent magnets. Among them, the 8 * 8 array switch made by Hiroshi HOSAKA and others of Japan is a typical example. The prototype prototype of the switch consists of small coils of traditional technology, soft-processed iron-nickel alloy soft magnet electrodes and permanent magnets. The volume is 14mm * 15mm * 25mm, the switching time is 0.2ms, and the on-resistance is less than 100m.

MEMS actuator shape memory alloy micro clamp

Figure 1-6 Shape memory alloy micro clamp Han Zhang et al. Used micro-clamps developed by SMA thin plates, as shown in Figure 1-6. The micro-clamp is made of a single piece of shape memory alloy (Ni-Ti-Cu) and uses laser annealing technology to make the local position of the active finger side (B side) have shape memory function, while the remaining part is in the cold working state. With shape memory function. The partially annealed part will be deformed after heating, deforming a finger, closing the claws, and the parallelogram elastic structure as a bias spring. After cooling, the finger is restored to the open state.

Figure 1-7 Shape memory alloy thin-film silicon micro-clamp The Lawrence Livermore National Laboratory (LLNL) in the United States has developed a micro-clamp driven by an SMA film, as shown in Figure 1-7. Bulk silicon MEMS processed silicon wafers are aligned, eutectic-coupled at selected locations, and then Ni-Ti-Cu thin films are deposited by sputtering on the upper and lower sides, the films are heat treated, and then cut into micro-clamps. The Ni-Ti-Cu films deposited on the upper and lower sides of the micro-clamp can generate a large driving force (500 MPa) in a relatively low temperature range (30 ° C to 70 ° C). The external dimensions of the micro-clamp are 0.9mm * 0.25mm * 0.25mm. The stress generated by the film can cause deformation of the clamp end by 55um, and the entire clamp can be opened by 110um.