Sensing microgripper with pid control systems

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VIETNAM NATIONAL UNIVERSITY, HANOI COLLEGE OF TECHNOLOGY Phan Huu Phu SENSING MICROGRIPPER WITH PID CONTROL SYSTEMS MASTER THESIS Hanoi - 2008 VIETNAM NATIONAL UNIVERSITY, HANOI COLLEGE OF TECHNLOGY Phan Huu Phu SENSING MICROGRIPPER WITH PID CONTROL SYSTEMS Major: Electronics and Telecommunications Technology Concentration: Electronics Engineering Code: 60 52 70 MASTER THESIS SUPERVISOR: Dr. Chu Duc Trinh Hanoi - 2008 2 TABLE OF CONTENTS DECLARATION......................................................................................................................1 ACKNOWLEDGEMENTS.....................................................................................................4 ABSTRACT..............................................................................................................................5 LIST OF ABBREVIATIONS .................................................................................................6 CHAPTER 1 INTRODUCTION ............................................................................................7 1.1 MANIPULATION IN MICRO-WORLD ................................................................................................................7 1.2 MICRO-GRIPER FOR MICRO-MANIPULATION ..................................................................................................7 1.2.1 Electrostatic microgripper ...................................................................................................................7 1.2.2 Piezoelectric microgripper...................................................................................................................8 1.2.3 Electrothermal microgripper................................................................................................................8 1.2.4 Polymeric electrothermal microgripper .............................................................................................10 1.3 MICRO-MANIPULATION WITH A FEEDBACK SYSTEM ....................................................................................11 1.3.1 Force sensor .......................................................................................................................................11 1.3.2 Sensing microgripper .........................................................................................................................14 CHAPTER 2 SENSING MICROGRIPPER .......................................................................17 2.1 INTRODUCTION ...........................................................................................................................................17 2.2 FORCE-SENSING CANTILEVER......................................................................................................................17 2.3 SILICON-POLYMER ELECTROTHERMAL MICROGRIPPER................................................................................18 2.4 SENSING MICROGRIPPER ..............................................................................................................................20 2.5 THE SENSING MICROGRIPPER CHARACTERISTICS .........................................................................................22 2.5.1 Electrothermal actuator characteristics.............................................................................................22 2.5.2 Sensing cantilever beam characteristics ............................................................................................25 2.5.3 Response frequency of the sensing microgripper ...............................................................................27 CHAPTER 3 BUILDING PID CONTROL FUNCTION ..................................................29 3.1 FEEDBACK LOOP CONTROL ..........................................................................................................................29 3.2 BUILDING A PID TRANSFER FUNCTION FOR THE SENSING MICROGRIPPER SYSTEM ......................................30 3.2.1 Transfer function of sensing microgripper .........................................................................................30 3.2.2 Transfer function of driver circuit ......................................................................................................30 3.2.3 Open-loop control ..............................................................................................................................31 3.2.4 Proportional control...........................................................................................................................32 3.2.5 Proportional – Integral control..........................................................................................................32 3.2.6 Proportional – Derivative control......................................................................................................33 3.2.7 Proportional – Derivative – Integral control .....................................................................................34 CHAPTER 4 ELECTRICAL DESIGN ...............................................................................35 4.1 INTRODUCTION ...........................................................................................................................................35 4.2 PROCESS SELECTION AND SIMULATION .......................................................................................................35 4.2.1 Process selection ................................................................................................................................35 4.2.2 Device modeling .................................................................................................................................36 4.2.3 Silicon-level simulation ......................................................................................................................36 4.2.4 Analog-only simulation ......................................................................................................................38 4.2.5 Mixed Analog/Digital simulation .......................................................................................................38 4.3 SYSTEM BLOCK DIAGRAM ...........................................................................................................................39 4.4 CELLS DESIGN .............................................................................................................................................40 4.4.1 Voltage reference generator...............................................................................................................40 4.4.2 Internal regulator ...............................................................................................................................45 4.4.2.1 The regulator:............................................................................................................................................... 45 4.4.2.2 The high temperature detector: .................................................................................................................... 46 4.4.2.3 The UVLO................................................................................................................................................... 47 4.4.2.4 Bias current generator.................................................................................................................................. 47 4.4.3 Digital to Analog converter (DAC) ....................................................................................................52 4.4.4 Buffer..................................................................................................................................................56 4.4.5 PID Controller ...................................................................................................................................56 4.4.6 Other cells ..........................................................................................................................................60 4.5 FULLY SCHEMATIC OF SYSTEM AND SIMULATION RESULTS .........................................................................62 3 4.5.1 Schematic ...........................................................................................................................................62 4.5.2 Simulation results ...............................................................................................................................62 CHAPTER 5 PHYSICAL DESIGN .....................................................................................65 5.1 INTRODUCTION OF LAYOUT ........................................................................................................................65 5.1.1 Matching concepts..............................................................................................................................65 5.1.2 MOS transistor layout ........................................................................................................................66 5.1.3 Resistor layout....................................................................................................................................68 5.1.4 Capacitor layout.................................................................................................................................69 5.1.5 Layout rules........................................................................................................................................70 5.2 SYSTEM FLOOR PLAN ..................................................................................................................................71 5.3 SYSTEM LAYOUT .........................................................................................................................................72 5.3.1 Sensing microgripper .........................................................................................................................72 5.3.2 Electrical circuits ...............................................................................................................................72 CHAPTER 6 CONCLUSION & FUTURE WORKS.........................................................74 6.1 CONCLUSION ...............................................................................................................................................74 6.2 FUTURE WORKS ...........................................................................................................................................74 6.2.1 Finishing the system layout ................................................................................................................74 6.2.2 Process establishment ........................................................................................................................74 6.2.3 Layout verification .............................................................................................................................75 6.2.4 Sample fabrication .............................................................................................................................75 6.2.5 Characterization.................................................................................................................................75 BIBLIOGRAPHY ..................................................................................................................76 6 List of Abbreviations AFM: Atomic Force Microscope AlN: Aluminum Nitride Bi-CMOS: Bipolar junction transistors and CMOS technology CAD: Computer Aid Design CMOS: Complementary Metal-Oxide-Semiconductor CTE: Thermal expansion coefficient DAC: Digital-to-Analog Converter DC: Direct Current DIMES: Delft Institute of Microsystems and Nanoelectronics DOFs: Degrees(s) of Freedom DRC: Design Rules Checking ERC: Electrical Rules Check ESD: Electro Static Discharge FIB-cut: Focused ion beam-cut GDSII: Graphic Data System II IC: Integrated Circuits LSB: Least Significant Bit LSI: Large-Scale Integration LVS: Layout versus Schematic MEMS: Micro-Electro-Mechanical systems MIS: Minimally Invasive Surgery MOSFET: Metal-Oxide Semiconductor Field-Effect Transistor NMOS: N-channel Metal-Oxide Semiconductor Field-Effect Transistor PID: Proportional Integral Derivative PLI: Photolithographic Invariance PMOS: P-channel Metal-Oxide Semiconductor Field-Effect Transistor PSRR: Power supply rejection ratio PTAT: Proportional to Absolute Temperature PVDF: Polyvinylidene Flouride PZT: Lead Zirconate Titanate SEM: Scanning Electron Microscope SOI: Silicon on Insulator SPICE: Simulation Program Integrated Circuit Emphasis TC: Temperature Coefficients UVLO: Under Voltage Lock-Out ZnO2: Zinc peroxide 7 Chapter 1 Introduction 1.1 Manipulation in micro-world The dominant physical principles in the micro-world can be quite difference from those of the macro-world. When the size of the object is less than 1mm, adhesive forces between the manipulation tool and the object such as surface tension, electrostatic and van der Waals forces can be significant compared to the gravitational force [23]. Manipulation of micro-particles can be done using several physical principles and methods. For manipulation of a micro-structure under specific ambient conditions or liquid; suction, cryogenic, electrostatic, and friction are the most often considered methods. Friction principle is chosen for addressing the micro-manipulation of small objects presented in this thesis. Like the human hand, friction manipulation uses at least two fingers applies on two sides of a clamped object. A friction manipulator can old an object due to the friction force between the tools and the object surfaces. This friction method is the most widely used in micro-manipulation because of size, cost, reliability, and fabrication aspect in particular. 1.2 Micro-griper for micro-manipulation In the recent years, microgrippers have been widely researched as they are in great demand in many research and application areas, such as advanced micro-assembly, micromanipulation, micro-robotic, minimally invasive and living cell surgery. For the development of microgrippers fabricated using integrated circuits (IC) or IC compatible technology, electrostatic, piezoelectric and electro-thermal actuation are generally used. 1.2.1 Electrostatic microgripper The electrostatic principle is based on the distance change between a fixed electrode and a suspended one when the voltage applied to these two electrodes changes. The first successful electrostatic microgripper based on bulk and surface silicon micromachining techniques was presented in 1992 (see Fig. 1.1) [3]. The 12 µm thick and 1500 µm long polysilicon microgripper is overhanging from a supporting silicon cantilever. The microgripper jaw displacement is 10 µm at an applied voltage of 45 V, with a basic frequency of 5 kHz. A monolithically fabricated electrostatic microgripper has been recently presented [7]. A lateral comb drive has been chosen to actuate this gripper. This microgripper can manipulate glass or copolymer spheres of size ranging from 20 to 90 µm with an applied force up to 380 µ at an applied voltage of 140 V. The main limitation of this device is the high voltage, the large size and the complicated electronic circuit typical of the electrostatic method. 8 Figure 1.1: The schematic design of a polysilicon electrostatic microgripper (adapted from [3]) 1.2.2 Piezoelectric microgripper Piezoelectric materials (such as PZT, AIN, and ZnO2) are capable of producing stress and/or strain when an electric field is applied. Piezoceramic elements have been used to build microgrippers (see Fig. 1.2) [22]. However, piezoelectric actuator fabrication processes are generally not IC compatible and are difficult to miniaturize. Piezoelectric actuators also require a high actuation voltage and produce small displacements. Figure 1.2: Schematic drawing of a piezoelectric microgripper [22]. 1.2.3 Electrothermal microgripper A thermostat consist a bi-metallic strip, which is made of two thin metallic pieces of difference materials that are bonded together. As the temperature of the strip changes, two pieces change length at difference rates, forcing the strip to bend. Based on the 9 Timoshenko’s bi-metal thermostat theory, an electrothermal microgripper (see Fig. 1.3) is fabricated using doped silicon and a special bonding technique [9]. This structure consists of a silicon cantilever beam with a doped layer on top. Out of plane bending is obtained when a current is induced though the doped layer. Figure 1.3: Schematic drawing of a typical bi-material cantilever actuator. The doped-silicon expands when applying a current, therefore the cantilever bends downwards (adapted from [9]). Figure 1.4: Schematic drawing of a typical flexure actuator: (a) single hot arm configuration and (b) two hot arms configuration 10 Figure 1.5: A microgripper product of Zyvex company: (a) the entire device is about 650 µm long, 270 µm wide, 50 µm thick. The initial gap between the two jaws is 36 µm and the maximum opening is 80 µm; [(b) and (c)] the grippers being used to manipulate a FIB-cut coupon [13]. Since the well-known flexure thermal actuator (see Fig. 1.4(a)) was introduced in 1992 [10], the actuator has received wide interest as they can produce a large displacement, a large force and use IC compatible fabrication process. The flexure thermal actuator consists of a thin arm with higher electrical resistance than its thick arm. The thin arm (hot arm) gets more heat than the thick one and consequently elongates more than the thick arm and in-plane bending occurs. A configuration more efficient in terms of power consumption uses two hot arm thermal actuators (see Fig. 1.4(b)). The electrical current just passes through two hot arms. As there is no electrical current in cold arm and flexure, the efficiency of power consumption is improved compared with the single hot arm structure. This principle is also used in the Zyvex gripper shown in Fig. 1.5. The limitations of these devices are the extremely high operating temperature and high power consumption. 1.2.4 Polymeric electrothermal microgripper Recently, polymeric electrothermal microgrippers have been extensively researched as they are capable of producing large displacements at a lower drive voltage and operating temperature [19]. Based on the above-mentioned flexure thermal actuator, polymeric microgrippers are developed using a polymer layer with a thin metal heater on top [19]. The structures have a large displacement at low operational temperature and low power consumption due to the large thermal expansion coefficient (CTE) of the polymer. Fig. 1.6 shows the schematic design of a developed polymeric microgripper using SU8 with a thin metal layer (Cr/Au) on top as a heater [19]. The microgripper jaw displacement is 12 µm at an applied voltage of about 2 V and an operational temperature of less than 100 0C. This microgripper has been designed to manipulate cells in air and also in fluid solutions. 11 In these polymeric microgrippers the metal heater is deposited on top of a high thermal expansion coefficient polymer layer. The interface between the heat source and the polymer layer is limited by the surface area of the metal layer, and the heat transfer along the vertical dimension is not effective. Since the polymer layers have low thermal conductivity, these devices can generate limited movement. Moreover, the unintentional vertical movement couples and interferes with the desired lateral movement [19]. Figure 1.7: Schematic drawing of a fabricated SU8 microgripper (adapted from [19]). 1.3 Micro-manipulation with a feedback system When manipulating micro-objects, operating on living cells or in minimally invasive surgery (MIS), enhanced dexterity, accuracy and speed are considerably improved when the force on the objects can be sensed and controlled in real-time. The development of miniaturized manipulators with force control is also of great interest in micro-robotics and micro-assembly. Manipulation of micro-objects with traditional microgrippers without a built in force sensor normally requires a camera inserted into the system to obtain visual feedback. This approach results in a two-dimensional image. Depth perception of the contact between the manipulating tool and the object being manipulated is lost, making it difficult to identify the position of the tool [16]. Moreover only displacement, and not force, can be detected. A microgripper with a built-in force sensor can address this limitation and is therefore suitable for holding objects firmly, whilst avoiding any squeezing of delicate objects. 1.3.1 Force sensor The contact forces between living cells in a laboratory or between micro-particles and a manipulator are generally in the nano-Newton to mili-Newton range. Cantilever force sensors are generally used to measure force in this range. 12 The bending of the cantilever is related to the applied force. By monitoring the deflection of the beam, the amplitude of the applied force can be detected. Several force-sensing methods, such as capacitive, piezoelectric, optical laser detection, and piezoresistive can be used [30]. The capacitive method is based on the capacitance change which occurs when the structure is deformed, and is widely used in micro-accelerometers and harsh environment sensors. An example of a capacitive force sensor (see Fig. 1.8) is shown in [29]. It has two degrees of freedom (2-DOF) and uses silicon on insulator (SOI) wafers. This sensor is capable of measuring forces up to 490 µN with a resolution of 0.01 µN along the x -axis and up to 900 µN with resolution of 0.24 µN along the yaxis. Complete isolation between the two electrodes is a limitation of capacitive force sensors, so normally SOI wafers are used. Alternatively, trench isolation can be used. However, it is difficult to control the etching area to obtain a completely isolated structure. Moreover, the capacitive method requires a complicated fabrication process and complex electronic circuitry. Figure 1.8: Schematic drawing of a capacitive force sensor in two dimensions (adapted from [29]). A two-dimensional piezoelectric force sensor is presented in [28] (see Fig. 1.9). It consists of two perpendicular pieces of polyvinylidene fluoride (PVDF) material. This structure is symmetric in the vertical and lateral dimensions, with resolution and sensitivity in the µN range. However, the PVDF cannot be patterned optically. The two pieces have to be glued perpendicularly to each other, resulting in a rather large sensor structure. With this approach, the sensor cannot be miniaturized and the fabrication process is not IC-compatible. The piezoelectric method also requires complicated electronic circuits for processing the signal. Cantilevers based on optical force measurement are often used in atomic force microscopy (AFM) and high-resolution measurement. With this principle, the deflection of a cantilever is amplified by a laser beam reflected on to the cantilever tip, and monitored using a split photodiode detector. This principle is very powerful for 13 measuring small displacements but requires high accuracy of optical alignment and adjustment. The cantilever surface must be reflective and larger than the laser beam spot. It is not easy to create a lateral and multidimensional force sensor using an optical method. Moreover, the lasers required are relatively large, and so it is impossible to miniaturize the entire force-sensing system down to the micrometer size range. Piezoresistive transducers translate a force into a change in the value of a resistor. They are widely used as sensing elements in pressure sensors, accelerometers, and AFM cantilevers [15]. Recently, developments in piezoresistive cantilever fabrication have led to submicrometer cantilevers with a resolution of pN and even fN. Most of the previously developed high-sensitivity force sensors use SOI wafers and vertical structures. The sidewall-doping technique is normally used in lateral force-sensing. Independent detection of a vertical and lateral forces sensor is shown in [25] (see Fig. 1.10). Separate piezoresistor circuits serve the triangular probe and the two inner ribs, enabling independent detection of vertical and lateral forces. However, oblique implantation, a rather special technique, is required in order to produce resistors on the vertical sidewalls of the cantilever. Figure 1.9: (a) Force cantilever using piezoelectric polymer PVDF and (b) a two dimensional force sensor is based on the two perpendicular glued pieces [28]. 14 Figure 1.10: Dual-axis AFM cantilevers with orthogonal axes of compliance. Oblique ion implants are used to form electrical elements on vertical sidewalls and horizontal surfaces simultaneously [25]. 1.3.2 Sensing microgripper In recent years, several designs of microgripper with force feedback have been demonstrated. A force-sensing microgripper for MIS application is shown in [18] (see Fig. 1.11). This device uses piezoelectric actuation with a strain gauge sensor on the sidewall of the structure. It is capable of actuating at high frequency (hundreds Hz) with very high drive voltage. In [5], a similar device is shown. Figure 1.11: Scheme of the microgripper based on PZT actuator with the location of the strain gauge sensors [18]. 15 It uses electromagnetic actuation and piezoelectric force sensing. It generates large displacement at low voltage and a linear sensing output. However, the main limitations of the above-mentioned devices are a fabrication process that is not compatible with CMOS technology and the rather large dimensions which are not suitable for the manipulation of micro-objects. In Fig. 1.12, another type of sensing microgripper is shown [12]. The actuator design is based on three doped silicon beams connected by an end bar. The two inner beams are electrically connected in parallel. When a voltage is applied between the outer beams and the two connected inner beams, the current in the outer beam is twice the current in the inner one. Therefore, the outer beam functions as the hot arm of an electrothermal actuator. The displacement of the actuator is detected by monitoring the resistance change of the two inner beams. The microgripper displacement is due to the thermal expansion of silicon, a material with a low CTE. Therefore, the microgripper displacement and sensor output is quite small. Figure 1.12: Piezoresistive feedback microgripper: (a) optical image of the device; (b) a typical Wheatstone bridge resistor configuration; (c) gripper made with two actuators in the three-beam configuration, which are connected as a Wheatstone bridge circuit; and (d) gripper configuration with three actuators consisting of the three-beam structure connected to avoid thermal stress influencing the force detection [12]. 16 Figure 1.13: Solid model of the electrostatic microgripper with integrated force sensor: one comb capacitor is used as actuator and the other one is the sensing part [7]. An electrostatic microgripper with an integrated capacitive force sensor is shown in [18] (see Fig. 1.13). It consists of a lateral comb drive for actuating the gripper and another for force sensing. This device is capable of motion of up to 100 µm with force sensitivity of 4.41 kV/m, and corresponding 70 nN force-sensing resolution. However, this device requires high drive voltage, large dimensions and a complicated electronic circuit. Another sensing microgripper is shown in [17] (see Fig. 1.14). This device combines a CMOS photo-detector and a polymeric electrothermal actuator based on the hot-andcold-arm design [19]. By using an external light source, this device is capable of detecting the object present between two gripper jaws. However, this method does not offer the possibility of monitoring the contact force between the manipulator tip and the object grasped. Figure 1.14: (a) Schematic of a simple 2 x 2 CMOS micro-machined gripper array with on-chip optical detection and (b) gripper with a polystyrene bead [17]. 17 Chapter 2 Sensing Microgripper 2.1 Introduction For many of the application areas of micromanipulators force control is highly desirable as dexterity, accuracy and speed are considerably improved by a feedback system [16]. A microgripper with a built-in force sensor is suitable for holding objects firmly, whilst avoiding any squeezing of delicate objects. As discussed in chapter 1, several designs of microgrippers with force feedback have been demonstrated in recent years [5, 7, 9, 12, 18]. However, these devices suffer from some limitations: either they are large in size and thus not suitable for micromanipulation; or require a fabrication process that is not CMOS-compatible and/or need a complicated electronic circuit. In this chapter a novel sensing microgripper based on silicon-polymer electrothermal actuators and a piezoresistive force-sensing cantilever beam is introduced. 2.2 Force-sensing cantilever A novel two-dimensional nano-Newton force-sensing piezoresistive cantilever is presented with providing nano-Newton sensitivity, a wide force measurement range, and the possibility of being combined with handling tools. Instead of using sidewall implantation, two separate sensing piezoresistors are located on each sides of the cantilever. By using two switches to change the Wheatstone bridge configuration, the piezoresistive cantilever is capable of detecting both the lateral and vertical bending independently. Consequently this force-sensing cantilever can detect the applied forces in both parallel and perpendicular direction to the wafer surface. The cantilever is made on regular silicon wafers with a fabrication process compatible with CMOS technology. In Fig. 2.1 a schematic drawing of the two-dimensional piezoresistive force sensing cantilever is shown. The four piezoresistors are located on the surface of the structure. The piezoresistors are aligned along the [110] direction of the (001) crystal plane of a silicon wafer. The resistor pairs located on the cantilever are stress-sensing resistors (the resistance changes when the cantilever is deflected). Two other resistors are outside the cantilever. They are not subjected to stress and therefore they are used for common-mode signal compensation in a Wheatstone bridge arrangement [4]. 18 Figure 2.1: Schematic drawing of the two-dimensional piezoresistive force-sensing cantilever with geometric symbols and orientations used [4]. When a force F is applied to the tip of the cantilever the cantilever bends. On the yz plane, perpendicular to the longitudinal x -axis of the cantilever, the applied force F can be determined as two components: Fz is the lateral and Fy is the vertical component (see Fig. 2.1). By monitoring Fz and Fy we can obtain both amplitude and direction of the applied force [4]. 2.3 Silicon-polymer electrothermal microgripper A novel silicon-polymer laterally stacked electrothermal in-plane microactuator is proposed. The device is composed of three materials: a metal heating layer, a silicon frame with high heat conductivity and a polymer with a high thermal expansion coefficient (CTE). During actuation, heat is efficiently transferred from the heater to the polymer thanks to the high thermal conduction of the deep silicon structures which have a large interface with the surrounding polymer. Moreover, the polymer layer is constrained between two silicon plates. The thermal expansion of the constrained polymer is three times larger than the no constraint one. Fig. 2.2 shows the sketch of a silicon polymer block. It consists of laterally stacked silicon-polymer segments. Each segment is formed by a rectangular plate of polymer which is bonded between two silicon plates. The metal heater is on the top of the silicon. Instead of transferring heat directly from the heater to the polymer as in the conventional developed designs [19], the efficient distribution of the heat to the polymer occurs through the large interface areas between the silicon plate and the polymer. Polymer layers are bonded between two silicon plates which constrain the expansion of the polymer layer in the direction parallel to the plate surfaces. Therefore, the thermal expansion of the polymer plate is enhanced in the perpendicular direction. Sketches of the microgripper based on a silicon comb structure are shown in Fig. 2.3(a). The microgripper is designed for normal open operation mode. Each actuator 19 has a silicon comb finger structure with the aluminum heater on the top. A thin layer of silicon nitride is used as the electrical isolation between aluminum structure and silicon substrate. The gaps between comb fingers are filled with SU8 polymer. Each actuator consists of 41 silicon fingers and SU8 layers in between. The polymer comb fingers are 3 µm wide, 75 µm long and 30 µm thick. The ratio between the width of the polymer layer and the dimensions of its bonded surface with silicon rigid plate are 25 and 10, respectively. These ratio values satisfy the prerequisite for the maximum constraint effect. When the heater is activated, the generated heat is efficiently transferred to the surrounding polymer though the deep silicon comb finger structures which have a large interface area with the polymer layer. The polymer layers expand along the lateral direction causing bending displacement of the actuator arm [4]. Figure 2.2: (a) Scheme of the silicon polymer laterally stacked segments; (b) Side cross-section of a single segment depicting the constrained polymer between two rigid silicon plates [4]. Figure 2.3: a) Sketch of the silicon-polymer electrothermal microgripper based on the silicon comb structure, b) top and cross-section view of the silicon-polymer electrothermal microgripper based on the silicon comb structure [4]. 20 2.4 Sensing microgripper The sensing microgripper is based on the combination of a silicon-polymer electrothermal microactuator and a piezoresistive lateral force-sensing cantilever beam. A schematic drawing is shown in Fig. 2.4. When the electrothermal actuator is activated, the microgripper’s arm and also the sensing cantilever are bent. This causes a difference in the longitudinal stress on the opposite sides of the cantilever. This changes the resistance value of the sensing piezoresistors on the cantilever changes. The displacement of the microgripper jaws can be monitored by the output voltage of the Wheatstone bridge of the piezoresistive sensing cantilever beam. The contact force between the microgripper jaws and clamped object is then determines based on displacement and stiffness of microgripper arm [4]. Figure 2.4: a) Schematic drawing of the sensing microgripper, b) Top and cross-section view of a sensing microgripper arm with geometry symbols and parameters. The Wheatstone bridge configuration is also shown [4]. The sensing microgripper is designed for normal open operating mode, using the silicon-polymer electrothermal microgripper based on the silicon comb structure. This configuration is more rigid than the one based on the silicon serpentine and therefore preferred. The force sensor is designed based on the lateral force-sensing piezoresistive cantilever beam as presented in 2.2 paragraphs. The four piezoresistors are located on the cantilever beam structure and connected to create a Wheatstone bridge. The piezoresistors are aligned along the [110] direction in the (001) crystal plane of the silicon wafer. The resistor pairs located on the cantilever are the stress sensing resistors. When the electrothermal actuator is activated, the cantilever beam is bent parallel to the wafer surface. Therefore, the differential change of resistance occurs on the two resistors RS1 and RS2. 21 Figure 2.5: SEM picture of (a) the sensing microgripper and close-ups of (b) the piezo-resistors; (c) the jaws, and (d) a section of the thermal actuator [4].
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