ELECTROSTATIC SENSING AND ACTUATION || MODULE 2 || KTU MEMS || MEMS KTU || LECTURE 14

Hi all, today we are discussing about electrostatic sensing and actuation For this purpose, we are using capacitors A capacitor is broadly defined as two conductors that can hold opposite charges How can we define a capacitor? A capacitor is two conductors that can hold opposite charges Apart from that, what do we see between these two conducting plates? We see a dielectric medium We have studied all these things We are using this capacitor as a sensor and actuator First, let us look at capacitive or electrostatic sensing When we apply a stimulus, the relative position or distance of these two conductors will change The capacitance will also change Next, let us look at electrostatic actuation When we apply a stimulus, the relative position or distance of these two conductors will change The capacitance will also change This is how we can sense a stimulus Next, let us look at electrostatic actuation When we apply a voltage across these two conductors, an electrostatic phase will form This is how electrostatic actuation occurs An example of electrostatic actuation is a micromotor We have discussed this in the first module In a micromotor, we see a rotor and a stator A rotor is a fixed electrode The rotor is attached to the substrate with a hub We have grouped the stators in this We have grouped them together Four electrodes are simultaneously biased in each group These are the three groups In this figure, you can see an arrow mark for the biased electrodes When we bias the stator electrodes, an electric field is generated An electric field is generated between the stator electrode and the rotor As a result, an attractive force is generated The attractive force tries to align the rotor with the stator This is what we see in Figure B Then, the bias is transferred to the next group of stator electrodes This is what we see in Figure C The arrow mark is transferred to the next group of stator electrodes This is how the angular displacement occurs and the next alignment occurs This is how the next group of stator electrodes is activated This is how the continuous movement occurs This is how a micromotor functions The motor consists of a rotor that is attached to the substrate with a hub and a set of fixed electrodes on the periphery called stators The stators are grouped together such that each group of four electrodes is electrically biased simultaneously Three such groups are present for the motor shown in Figure and are identified by different field patterns Consider the rotor at an arbitrary angular rest position that is shown in Figure A One group of stator electrode is first biased The electrodes that are biased at a particular stage are identified by the arrows placed next to them An in-plane electric field develops between any given stator electrode in this group and closest rotor tooth next to it This generates an electrostatic attractive force that aligns the set tooth with the set stator electrode The torque values are on the order of pico nanometer for voltages of the order of 100 volt large enough to overcome static friction A small angular movement of the rotor is made to reach a new configuration which is shown in Figure B The electric bias is then shifted to a next group of stator electrodes that is shown in Figure C resulting in another small angular movement in the same direction Continuous motion of the rotor can be achieved by activating the stator electrodes by the groups in succession In this way, continuous motion of the rotor can be achieved by activating the stator electrodes by the groups in succession Now let's see the advantages of electrostatic sensing and actuation The first one is simplicity The sensing and actuation principles are relatively easy to implement Two conducting surfaces are enough There is no need of any special functioning materials The second one is low power That means electrostatic actuation relies on differential voltage rather than current Also energy efficient for low frequency application The third one is fast response It offers high dynamic response speed The transition speed is the charging and discharging time The charging and discharging time is constant It is of low value in the case of good conductors The main disadvantage is high voltage required for static actuator High voltage is needed for static actuator The problem with high voltage is the compatibility issues There will be compatibility issues when connected to other components Increasing the electronics complexity When we connect electrodes with mechanical insulators, the charges accumulate in DC operation modes. The trapped charges change the operation characteristics. This is the main disadvantage. We can classify capacitive sensors and actuators based on electrode geometry. One is parallel plate capacitors and the other is interdigitated finger capacitors. It is also known as combo drive capacitors. We can classify capacitive sensors and actuators based on electrode geometry. One is parallel plate capacitors and the other is interdigitated finger capacitors. We have studied these two in the first module. So you know the introduction. The static capacitance value of parallel plate capacitors and combo drives is in the picofarad range. We use electrostatic attraction in capacitive actuators. Let's see the parallel plate capacitor. This is a basic configuration of parallel plate capacitors. There are two plates inside. Both sides are parallel to each other. The overlapping area inside the figure is represented by AUC. The spacing between the two plates is called D. There is a dielectric medium between the two plates. The dielectric constant or relative electrical permittivity is represented by epsilon r. The permittivity of the media is epsilon 0 and epsilon r. Epsilon 0 is the permittivity of the vacuum. The value of epsilon 0 is 8.85 into 10 raised to minus 14. The value of the capacitance between the two plates is C is equal to Q divided by V. Q is the amount of stored charge and V is the electrostatic potential. We use U to denote the electric energy stored in the capacitor. We have studied the energy stored in a capacitor. U is equal to half Cv square. C is equal to Q by V. V is equal to Q by C. Q square by C square is half C into Q square by C square. Half Q square divided by C. In the case of a parallel plate capacitor, the electric field lines are parallel to each other. The field lines are perpendicular to the surface of the plate. There are some fringe electric fields outside the boundary of this electric field. If we are going to do a rigorous design analysis, we need to consider this. We have studied Gauss's law. We have studied an equation relating the primary electric field to Q. E is equal to Q divided by epsilon. Voltage is the electric field times the distance between the plates. Voltage V is equal to E into D. Let us take the equation of a parallel plate capacitor. C is equal to Q divided by V. V is equal to E into D. E is equal to Q divided by epsilon. C is equal to epsilon A divided by D. From this equation, we can say that the capacitance value is proportional to A and inversely proportional to D. Also, it is a function of electric permittivity epsilon. This parallel plate can be moved in two ways. One is by normal displacement. The other is by parallel sliding displacement. Now let us see how this parallel plate capacitor works as a sensor. This parallel plate capacitor can be used as a physical sensor, chemical sensor, biological sensor etc. So here, by measuring the capacitance value of the parallel capacitor, one can sense the changes of permittivity, area or distance. The permittivity changes are based on the temperature and humidity of the capacitor media. The permittivity changes can be categorized as liquid, air or biological particles. Next, let us see how a parallel plate capacitor can be used as an actuator. How can we use a capacitor as an actuator? We can use it as an actuator. For this, we need a pair of electrode plates. One plate is firmly anchored. The other plate is suspended in a mechanical spring. If we apply a differential voltage between these two plates, an electrostatic attraction force is developed. The magnitude of this force is equal to the stored electric energy. So, the expression for the magnitude of the force is given by F is equal to dou u divided by dou x. Here, u is the energy. So, we can write, u is equal to half cv square, right? That is, dou by dou x of half cv square. Since half is outside, it is a constant. So, half dou c divided by dou x into v square. Here, x is the dimensional variable of interest. If the plate moves along their normal axis, the gap between the electrode changes. Instead of c, we can write epsilon a by d. So, the equation for the force is dou u divided by dou d. It is equal to half epsilon a divided by d square into v square. Half epsilon a v square divided by d square. That means, half cv square divided by d. So, under constant biasing voltage, the magnitude of electrostatic force decreases drastically with increasing gap. Electrostatic force is a short range force. Because, as we said earlier, when the gap between two plates increases, the electrostatic force generated there decreases. So, we can say that electrostatic force is a short range force. It is most effective when the gap is only a few micrometers. The voltage we apply here has an upper limit. The upper limit is called the breakdown voltage of the dielectric medium. Similarly, if the dielectric medium is air, the breakdown voltage is predicted by using a passion curve. It is a relation of how the breakdown voltage increases with decreasing gap. We don't need it. Now, let's see the parallelogram. It is a relation of how the breakdown voltage increases with decreasing gap. We don't need it. Now, let's see the main applications of parallel plate actuators. First one is called inertia sensor. We use parallel plate capacitors to sense the inertia. Inertia force modifies the capacity and electrode spacing. So, we can sense the inertia. Second application is pressure sensor. We use pressure sensor in automotive system, industrial process control, medical diagnosis, environmental monitoring etc. The primary factor of a pressure sensor is the membrane thickness. In case of MEMS, we consider the membrane to be very thin. We use this membrane based pressure sensors for parallel capacitive sensing. The main advantages of this pressure sensor are capacity based, greater pressure sensitivity, lower temperature sensitivity and reduced power consumption compared with piezo resistive pressure sensor. These are the advantages of this pressure sensor. Next application is flow sensor. This is used to find the fluid speed and volume of flow rate. Advantages of micro machined flow sensors are, Small size, low disturbance to flow field of interest, high sensitivity and uniform performance. Next one is tactile sensor. Tactile sensor is a critical component in robotic applications. Fifth application is parallel plate. Next one is interdigitated finger capacitors. Interdigitated finger capacitors are a different class of capacitors, take the advantage of capacitance generated from the side walls of the electrodes. Such capacitors provide alternative fabrication and operation modes compared with parallel plate capacitors. They involve interdigitated fingers to increase the edge coupling length. Two sets of electrodes are placed parallel to the substrate. One set of finger electrodes is fixed on the chip. Second set of electrode is suspended and can be moved in the axis. Interdigitated finger capacitors are like the teeth of a chip. We can call it as comb drive device. Two sets of electrodes are placed parallel to each other. One is movable and other is fixed. Overlapping distance is denoted by L0. Finger length is denoted by Lc. Thickness is denoted by T and width is denoted by Wc. Distance between fixed and movable finger is denoted by D. Capacitance is denoted by Epsilon0, EpsilonR into A by D or Epsilon A by D. Area is denoted by I0 into T. C is equal to EpsilonR, Epsilon0 into I0T divided by D. Capacitance value depends on thickness and distance. The greater the thickness, the larger the capacitance effect. The distance between gaps should be as small as possible. Finite element method is used to estimate fringe capacitance. Transverse Comb Drive and Longitudinal Comb Drive are classified. This is the diagram of Transverse Comb Drive. In the case of Transverse Comb Drive, there will be a transverse moment or axis of motion. The moment will be in the x-axis. This is called Soft in X and Stiff in Y. In Longitudinal Comb Drive, the moment will be in the y-axis. This is called Soft in Y and Stiff in X. In the case of Transverse Comb Drive, the moment will be in the x-axis. This is called Soft in Y and Stiff in X. There are two major capacitance associated with each finger pair. The left hand side of the finger is called CSL. The right hand side of the finger electrode is called CSR. CSL is equal to CSR is equal to Epsilon0 L0 T divided by X0. Epsilon0 A divided by D is equal to Epsilon0 A L0 T divided by X0. In the case of free fingers, the moment will be in the x-axis. CSL is equal to Epsilon0 L0 T divided by X0 minus X. CSR is equal to Epsilon0 L0 T divided by X0 plus X. The value of total capacitance is CSL plus CSR plus CF. We can use the transverse comb drive as a sensor or an actuator. If we use the transverse comb drive as a sensor, the displacement sensitivity Sx will be dou C total divided by dou X. If we use the transverse comb drive as an actuator, the magnitude of force can be calculated by taking the derivative of total storage energy with respect to X. Force Fx is equal to dou U divided by dou X. U is equal to half C V square. In the longitudinal comb drive, the moment will be in the y-direction. Capacitance associated with CSL and CSR is equal to Epsilon0 L0 T divided by L0 minus Y. Capacitance associated with CSL and CSR is equal to Epsilon0 L0 minus Y into T divided by X0. The displacement sensitivity here is dou C total divided by dou Y. This is the diagram of the longitudinal comb drive. The displacement is here. Lateral moment is here. L0 minus Y is here. Capacitance is here. Epsilon0 L0 minus Y into T divided by X0. The applications of comb drive devices are used as inertia sensors and actuators. That's all for today. Electrostatic Sensing and Actuation. I hope everyone understood. See you in the next lecture. Thank you.