Bichop R.H. (Ed.) Mechatronic Systems, Sensors, and Actuators: Fundamentals and Modeling

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20-144 Mechatronic Systems, Sensors, and Actuators

offers important advantages. An examplary surface micromachining process is described in the following

paragraph.

The fabrication process for a surface micromachined pressure sensor is shown in Figure 20.117. It starts

with a silicon substrate (a) with the front side polished. A local thermal oxidation process is performed

first to form a silicon oxide well with a typical thickenss of 1.3

m. The thermal oxide is part of the sacrificial

layer that will be removed at a later stage. Using a process called low-pressure chemical vapor deposition

(LPCVD), a thin layer of oxide is again deposited over the wafer surface. This oxide layer is patterned

using the photolithography method (c). The entire wafer is coated with a silicon nitride thin film deposited

by LPCVD technique as well (d). The silicon nitride film is patterned and etched to produce an access

hole on top of the underlying oxide layer (e). Through this access hole, hydrofluoric acid removes the

oxide materials inside the cavity. The etch rate of the acid on silicon nitride is negligible (f). After the

cavity is emptied and dried, another layer of LPCVD silicon nitride is deposited to seal the opening in

the original silicon nitride layer (g). Following this step, polycrystalline silicon with suitable doping

concentration is deposited on top of the wafer and patterned to form the piezoresistors (h).

It should be noted that piezoresistive sensing, though dominant in the methods reviewed, is not the

only sensing mechanism available. Capacitive sensing and piezoelectric sensing are also feasible and have

been demonstrated in the past. However, discussions of these methods are beyond the scope of this text.

FIGURE 20.117 Schematic diagram of major steps for making a surface micromachined pressure sensor with silicon

nitride diaphragm.

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

Local thermal oxide

Deposited oxide film

Deposited silicon nitride film

Piezoresistors

Seal cavity

Starting silicon wafer

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Pressure Sensors Made of Non-Silicon Materials

For certain applications such as monitoring of internal combustion engine, pressure sensors are required

to sustain high temperature of operation. In such cases, silicon is not the optimal material because high

temperature causes doped silicon junctions to fail.

Work has also been done to implement polymer materials for pressure sensors. Though such devices

are relatively few, they represent an important development trend for future sensors.

20.9.2.3 Accelerometers

Bulk Micromachined Accelerometers

Acceleration sensors (or so-called inertial measurement units, IMU) are important for monitoring

acceleration and vibration experienced by a subject, such as an automobile, a machine, or a building.

Low-cost accelerometers used in automobile airbag deployment systems can reduce the costs and enhance

driver safety. Micromachined sensors can be made small and sufficiently low-cost to be used in smart

projectiles, for example, concrete penetrating bombs. Small, multi-axial accelerometers can also be applied

in writing instruments (smart pens) for handwriting recognition.

A representative bulk micromachined accelerometer is illustrated in Figure 20.118. A SEM micrograph

of a prototype sensor is shown in Figure 20.119. A silicon proof mass is attached to the end of a cantilever

beam. At the base of the cantilever beam lies a piezoresistive element. Supposing the mass of the proof

mass is m, and the magnitude of the acceleration is a, one can estimate the sensor output following a

few simple analysis steps. First, a concentrated force with a magnitude of F = ma is applied in the center

of the proof mass according to Newton’s first law. Secondly, the force translates into a torque loading at

the base of the cantilever with the magnitude being

The magnitude of the strain experienced at the surface of the cantilever beam, where the piezoresistors

are located, is

FIGURE 20.118 Schematic diagram of a bulk micromachined accelerometer.

(a) Top

view

(b) Side view

(zero acceleration)

(finite acceleration)

Tensile stress

Proof mass (m)

F = ma

Piezoresistor

Metal

leads

MFl

---

+ ma l

---

+==

2EI

--------

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20-146 Mechatronic Systems, Sensors, and Actuators

Here, the term t is the thickness of the beam, E is the modulus of elasticity of the cantilever beam material,

and I is the momentum of inertia associated with the beam cross section. Supposing the cross section

of the cantilever beam is a rectangle with a width w and a thickness t, the moment of inertia is

Note that the moment of inertia is strongly related to the thickness of the beam. If the thickness of beam

is reduced to half, the magnitude of I is reduced by eight times, and the sensitivity of the sensor increases

by eight fold.

Surface Micromachined Accelerometers

Surface micromachined accelerometers offer the potential advantage of ready integration with signal

processing circuits. As a result, various types of surface micromachined versions have been made in the

past decade. A successful commercial product has been made by analog devices for sensing automobile

acceleration to deploy airbags in the events of collision. The structure, operational principle, and fabri-

cation process for such a sensor is briefly discussed in this section.

The sensor consists of two sets of interdigited comb-finger-shaped electrodes as shown in Figure 20.120.

One set of fingers is stationary and fixed to the substrate. Another set if suspended by cantilever springs

to the substrate. Capacitors are formed between each pair of comb-like fingers. When an external

acceleration is applied along the horizontal axis, an inertia force is applied to the moving set of fingers

and causes the moving fingers to displace. The amount of displacement is related to the magnitude of

the acceleration and the force constant of the supporting springs. The relative motion of the two sets of

fingers result in changes of the overall capacitance value between the two sets of fingers. The minute

capacitance change is sensed and processed by a signal-processing circuit consisting of an Σ − ∆ A/D

conversion stage [10].

The fabrication process for such a sensor according to the A

–

A cross-section is illustrated in Figure 20.121.

First, transistors for signal processing circuits are first made on a silicon substrate (a). A sacrificial silicon

dioxide layer is deposited onto the wafer surface (b), followed by the deposition of a polycrystalline silicon

FIGURE 20.119 A SEM micrograph of a prototype bulk micromachined pressure sensor (Junjun Li).

--------

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Sensors 20-147

layer (c). The polycrystalline silicon is patterned and etched to form the comb fingers (e). Subsequently, the

oxide layer is removed by using a web etchant (hydrofluoric acid) that etches polycrystalline with negligible

rates. In areas where the polysilicon is anchored to the substrate, a via hole is patterned and etched in the

sacrificial layer before step (c).

20.9.2.4 Tactile Sensors

Tactile sensors are most widely used for robotics applications to provide tactile sensations for object

handling. The sensor density on a human fingertip is on the order of 100/cm

. Such a high sensor density

can be achieved using microfabrication technology.

An arrayed tactile sensor is illustrated in Figure 20.122. A two-dimensional array of individual sensor

elements provides two-dimensional mapping of contact force and shear force. The schematic cross-

sectional diagram of an array in contact with an arbitrary object is shown in Figure 20.122b. As an

object contacts a sensor beam, the amount of displacement corresponds to the contact force as well as

the surface topology.

The fabrication process of the tactile sensor is discussed in the following and illustrated in

Figure 20.123. Starting with a silicon wafer (a), a local ion implantation is first conducted to produce

piezoresistors (b). A thermal oxide film is grown to provide passivation to the entire wafer. The oxide

layer on the bottom of the wafer is patterned and etched to expose silicon substrates (c). An anisotropic

silicon etch is performed to remove silicon from the backside of the wafer (d). The oxide film on the

front of the wafer is then patterned and etched using plasma anisotropic etch to create free-standing

cantilever beams (e and f). Metal thin film is then deposited and patterned to provide lead wires (g).

FIGURE 20.120 Schematic diagram illustrating the operation principle of a surface micromachined accelerometer.

Side

view

Movable

fingers

Movable

fingers

Fixed

fingers

Equilibrium

(without acceleration)

Under acceleration

To p

view

AA’

Fixed

fingers

Spring

support

Spring

support

Spring

support

SubstrateSubstrate

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20.9.2.5 Flow Sensors

Sensors for monitoring the flow rate of fluid (air or liquid) and for measuring the drag force exerted on

an object moving in a fluid have important applications in robotics applications. Existing flow sensors

are based on a number of principles, notably thermal and momentum transfer principles.

FIGURE 20.121 Schematic diagram of the fabrication process for a surface micromachined accelerometer illustrated

in the previous figure.

FIGURE 20.122 Schematic diagram of an array tactile sensor: (a) perspective view, (b) cross-sectional view.

(a)

(b)

(c)

(d)

(e)

Transistors

Sacricicial oxide

Polysilicon

Transistors

(a)

(b)

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Flow Sensors Based on Heat Transfer Principles

For sensors based on thermal transfer principles, a heated element is used with temperature slightly

above the temperature of the ambient fluid (Figure 20.124). The heat is generally created by passing

current through a resistive element. An ideal element to serve as the heating element is doped

polysilicon resistor. The resistivity is generally lower than what can be achieved using metal resistors

of the same dimension, hence the resistance value is greater and the heating element can be made

smaller.

The movement of the fluid creates velocity-dependant forced convection of heat, thereby reducing the

temperature of the heated element accordingly. The temperature of the element is therefore used to

provide information about the flow rate and direction. Such sensors are commonly referred to as hot-

wire anemometers. Micromachined hot-wire anemometers have been demonstrated by several groups

[11,12].

FIGURE 20.123 Schematic diagram of the microfabrication process for realizing a tactile sensor.

FIGURE 20.124 Perspective diagram of a thermal-

transfer based flow sensor (anemometer).

(a)

(b)

(c)

(d)

(e)

(f)

(g)

Doped strain gauge

Thermally-grown oxide

Lead wire

Starting silicon substrate

Support

Heating

element/

sensor

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20-150 Mechatronic Systems, Sensors, and Actuators

Flow Sensors Based on Momentum Transfer Principles

For sensors based on momentum transfer principles, a mechanical member is bent by the momentum

imparted by a moving fluid (Figure 20.125). The amount of the bending is used to decipher the strength

of the fluid flow. The schematic diagram of an exemplary flow sensor is shown in the figure below. It

consists of a vertical shaft attached to the end of a cantilever beam (a). When an external flow is

exerted, it will apply a distributed force onto the vertical shaft, hence causing the cantilever to bend.

The extent of the bending, as sensed by the embedded piezoresistor, is proportional to the average

flow rate.

The fabrication process is similar to the tactile sensor except for the attachment of the integrated

vertical shaft. A number of techniques for assembling three-dimensional microstructures using efficient

integrated processes have been developed in the past. For example, three-dimensional structures can be

realized using hinged microstructures and using solder joints or polymer joints. Recently, a process called

the plastic deformation magnetic assembly (PDMA) has been developed. In the following paragraph the

PDMA process is briefly discussed.

The PDMA technique is discussed using a simple surface micromachined cantilever as an example.

As shown in the diagram below (Figures 20.126 and 20.127), a single-clamped cantilever made of a

ductile metal (e.g., gold or aluminum) is suspended from the substrate. A piece of Permalloy, a

ferromagnetic alloy made by electrodeposition, is attached to the cantilever. When a magnetic field

is applied from underneath the wafer, the magnetic piece will be magnetized and will experience a

magnetic torque M. The torque lifts the cantilever beam away from the substrate. If the amount of

bending is significant, the ductile metal will be displaced permanently due to plastic deformation at

the hinge region.

FIGURE 20.125 Schematic diagram illustrating the operation principle of a momentum-transfer based flow sensor.

FIGURE 20.126 Schematic diagram of the plastic deformation magnetic assembly process.

(b) With flow(a) Without flow

Silicon

substrate

Substrate Substrate

Electroplated permalloy

Metal cantilever

Plastic

deformation

region

(a) (b)

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20.9.3 Future Development Trends

Miniaturization and integration of circuits has resulted in revolution in the society so far. It drastically

reduces the costs and increased the performance of circuits. Without the integrated circuits technology,

the information age would not have dawned on the human society.

It is conjectured that integrated microsensors are likely to produce as broad and deep an impact on

the society as the integrated circuits. Sensors can be used for robotics sensing, smart buildings, smart

FIGURE 20.127 SEM micrographs of cantilever beams (a) while in plane, and (b) after PDMA assembly.

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20-152 Mechatronic Systems, Sensors, and Actuators

toys, automotive safety and control, and industrial control. However, in order to realize the advantages

of integrated sensors, a number of technical barriers must be overcome. Two important barriers are (1)

high R&D costs of integrated sensors and (2) reliable and robust packaging of sensors.

The development of microintegrated sensors involves high development costs and long time-to-

market. The surface micromachined accelerometer developed by analog devices costs tens of millions of

dollars and took more than 5 years to produce. Why do integrated sensors cost so much to build? Sensors

are developed using a group-up approach. The development cycle of a sensor begins at the level of physical

principles. The cost of sensor development includes expertise for material selection, design generation,

prototype process development, and characterization.

Such a development cost and speed is not tolerable in applications where only a small amount of

custom sensors is required. Standard sensing modules, low-cost, flexible foundry fabrication processes,

and advanced computer simulation and prototyping tools are required to advance the state-of-the-art

of microintegrated sensors.

Future sensors will involve more non-silicon materials. For example, polymer materials can be used

to reduce the costs while high-temperature materials maybe used for high-temperature sensing applica-

tions (e.g., monitoring of conditions in engines).

20.9.4 Conclusions

A brief historical overview of the development of microfabrication technology and microintegrated

sensors is presented. Common sensing principles, including capacitive, piezoresistive, and piezoelectric

sensing, are discussed. Four important case studies of sensors are undertaken. For each type of sensor

applications—pressure sensors, acceleration sensors, tactile sensors, and flow sensors—the sensor archi-

tectures and fabrication processes are reviewed. Interested readers may find more in-depth information

in the references provided in this section.

References

1. Nathanson, H.C., Newell, W.E., Wickstrom, R.A., and Davis J.R. Jr., “The resonant gate transistor,”

IEEE Transactions on Electron Devices, Vol. ED-14, No. 3, pp. 117–113, March 1967.

2. Petersen, K.E., “Silicon as a mechanical material,” Proceedings of the IEEE, Vol. 70, No. 5, pp. 420–457,

May 1982.

3. Angell, J.B., Terry, S.C., and Barth, P.W., “Silicon micromechanical devices,” Scientific American, Vo l.,

248, pp. 44–55, April 1983.

4. Siewell, G.L., Boucher, W.R., and McClelland, P.H., “The ThinkJet orifice plate: a part with many

functions,” Hewlett-Packard Journal, May 1985, pp. 33–37.

5. Allen, R.R., Meyer, J.D., and Knight, W.R., “Thermodynamics and hydrodynamics of thermal ink

jets,” Hewlett-Packard Journal, May 1985, pp. 21–27.

6. Williams, K.R., and Muller, R.S., “Etch rates for micromachining processing,” Journal of Microelec-

tromechanical Systems, Vol. 5, No. 4, pp. 256–268, December 1996.

7. Kovacs, G.T.A., Micromachined transducers sourcebook, McGraw-Hill, 1998.

8. Trimmer, W.S., Micromechanics and MEMS––Classis and seminal papers to 1990, IEEE Press, 1997.

9. WWW site http://mems.isi.edu.

10 Yun, W., Howe, R.T., and Gray, P.R., “Surface micromachined, digitally force-balanced accelerometer

with integrated CMOS detection circuitry,” Technical D igest, IEEE Solid-State Sensor and Actuator

Workshop, pp. 21–25, Hilton Head, SC, June 1992.

11. Jiang, F., Tai, Y.C., Ho, C.M., Karan, R., and Garstenauer, M., “Theoretical and experimental studies

of micromachined hot-wire anemometers,” Technical Digest, International Electron Devices Meeting

1994, San Francisco, CA, pp. 139–142, December 1994.

12. Ebefors, T., Kalvesten, E., and Stemme, G., “Three dimensional silicon triple-hot-wire anemometer

based on polyimide joints,” Proceedings of the 11th International Workshop on MEMS, pp. 93–98,

January 1998.

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21-1

Actuators

21.1 Electromechanical Actuators ............................... 21-1

Introduction

•

Type of Electromechanical

Actuators—Operating Principles

•

Power Amplification

and Modulation—Switching Power Electronics

•

References

21.2 Electrical Machines .................................................. 21-33

DC Motor

•

Armature Electromotive Force (emf)

•

Armature Torque

•

Terminal Voltage

•

Methods of

Connection

•

Starting DC Motors

•

Speed Control of DC

Motors

•

Efficiency of DC Machines

•

Machines

•

Motor Selection

•

References

21.3 Piezoelectric Actuators ............................................ 21-51

Piezoeffect Phenomenon

•

Constitutive Equations

•

Piezomaterials

•

Piezoactuating Elements

•

Application

Areas

•

Piezomotors (Ultrasonic Motors)

•

Piezoelectric

Devices with Several Degrees of Freedom

•

References

21.4 Hydraulic and Pneumatic Actuation Systems ........ 21-63

Introduction

•

Fluid Actuation Systems

•

Hydraulic

Actuation Systems

•

Modeling of a Hydraulic Servosystem

for Position Control

•

Pneumatic Actuation

Systems

•

Modeling a Pneumatic Servosystem

•

References

21.5 MEMS: Microtransducers Analysis,

Design, and Fabrication .......................................... 21-97

Introduction

•

Design and Fabrication

•

Analysis of

Translational Microtransducers

•

Single-Phase Reluctance

Micromotors: Microfabrication, Modeling, and

Analysis

•

Three-Phase Synchronous Reluctance

Micromotors: Modeling and Analysis

•

Microfabrication

Aspects

•

Magnetization Dynamics of Thin

Films

•

Microstructures and Microtransducers with

Permanent Magnets: Micromirror Actuator

•

Micromachined Polycrystalline Silicon Carbide

Micromotors

•

Axial Electromagnetic

Micromotors

•

Conclusions

•

References

21.1 Electromechanical Actuators

George T.-C. Chiu

21.1.1 Introduction

As summarized in the previous chapter, a mechatronics system can be partitioned into function blocks

illustrated in Figure 21.1. In this chapter, we will focus on the actuator portion of the system. Specifically,

we will present a general discussion of the types of electromechanical actuators and their interaction

George T.-C. Chiu

Purdue University

Charles J. Fraser

University of Abertay Dundee

Habil Ramutis Bansevicius

Rymantas Tadas Tolocka

Kaunas University of Technology

Massimo Sorli

Stefano Pastorelli

Politecnico di Torino

Sergey Edward Lyshevski

University of Rochester

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