Patent Publication Number: US-2019198748-A1

Title: Self-sensing bending actuator

Description:
TECHNICAL FIELD 
     The present disclosure relates generally to actuators and, more particularly to, a self-sensing bending actuator. 
     BACKGROUND 
     In the past few decades, there has been a tremendous interest in the use of piezoelectric sensors and actuators for control and sensing applications. Piezoelectric bending actuators amplify the small strains in the piezoelectric material to usable displacement at the tip which can be of the order of millimeters. The large tip displacement of the bending actuators could be leveraged to develop complex actuating mechanisms. Piezoelectric elements have been used successfully in the closed loop control of a variety of active structures including beams, plates, shafts, trusses, etc. Some of the attributes, which have made piezoelectric actuators particularly attractive for active control, include the large useful bandwidth, the efficient conversion of electrical to mechanical energy, the ability to perform shape control, and the mechanical simplicity of the piezoelectric actuator. 
     Conventionally, actuators need external sensors to estimate displacement or stiffness of external load, which adds to the cost, and in some cases may have poor results or are difficult to implement due to space constraints. For instance, available force based sensors are afflicted with low resolution and low bandwidth; strain gauge based sensors have low frequency bandwidth and need alteration of host structure, and laser based sensors are expensive, bulky and need additional space as well as mounting system to incorporate. With the advancement in piezoelectric actuators, there has been a growing interest in development of piezoelectric element in which the actuator and sensor are combined into a single unit, sometimes known as a self-sensing actuator. A self-sensing actuator is a special kind of actuator that has the inherent capability to measure its own state (displacement, force, speed etc.) without the need for an external sensor. The in-built sensing is due to the inherent property of the material used in making the actuator or an independent sensing mechanism that is integrated with the main body of the actuator. 
     Another benefit of a self-sensing actuator is that the sensor and actuator are truly collocated. Collocated control has been shown to have a number of advantages relating to the closed loop stability of the structure. For instance, in the absence of actuator and sensor dynamics, structures controlled with collocated velocity feedback are unconditionally stable at all frequencies. However, existing self-sensing piezoelectric actuators mostly utilize the same piezoelectric layer(s) for actuation as well as sensing which requires complex electronic circuitry to decode the sensing and actuation signals or have one dedicated sensor layer along with only one dedicated actuator layer that, usually, results in low displacement output. 
     In light of the foregoing discussion, there exists a need to overcome the aforementioned drawbacks of conventional self-sensing piezoelectric actuator. 
     SUMMARY 
     Various embodiments of the present disclosure provide a self-sensing bending actuator. 
     In one aspect of the present disclosure, a self-sensing bending actuator is disclosed. The self-sensing bending actuator includes at least two layers of bending elements. The self-sensing bending actuator also includes a metallic layer disposed between each of the layers of bending elements. The self-sensing bending actuator further includes an insulating layer disposed on at least one of the layers of bending elements. The self-sensing bending actuator further includes a sensing element disposed on the insulating layer. 
     In another aspect of the present disclosure, a self-sensing bending actuator is disclosed. The self-sensing bending actuator includes one or more layers of piezoelectric bending elements constructed of PZT-5H material. The piezoelectric bending elements are configured to generate a strain therein resulting in displacement of the beam. The self-sensing bending actuator also includes an insulating layer disposed on at least one of the layers of piezoelectric bending elements. The insulating layer is composed of a polyimide material. The self-sensing bending actuator further includes a sensing element disposed on the insulating layer. The sensing element is constructed of one of more of polyvinylidene fluoride and polyvinylidene difluoride (PVDF) material. The sensing element is configured to estimate the tip displacement therein. 
     In yet another aspect of the present disclosure, a self-sensing bending actuator is disclosed. The self-sensing bending actuator includes a support body. The self-sensing bending actuator further includes a stack of layers cantilevered on the support body. The stack of layers includes a first piezoelectric bending element and a second piezoelectric bending element. The stack of layers further includes a metallic layer disposed between the first piezoelectric bending element and the second piezoelectric bending element. The metallic layer is electrically coupled to the first piezoelectric bending element and the second piezoelectric bending element. The stack of layers further includes an insulating layer disposed on the second piezoelectric bending element. The stack of layers further includes a sensing element disposed on the insulating layer. 
     Other aspects and example embodiments are provided in the drawings and the detailed description that follows. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       For a more complete understanding of example embodiments of the present technology, reference is now made to the following descriptions taken in connection with the accompanying drawings in which: 
         FIG. 1  is a diagrammatic view of a self-sensing bending actuator, in accordance with an example embodiment; 
         FIG. 2  is a cross-sectional view showing the arrangement of various layers in the self-sensing bending actuator, in accordance with an example embodiment; 
         FIG. 3  is a cross-sectional view showing the arrangement of various layers in the self-sensing bending actuator, in accordance with another example embodiment; and 
         FIG. 4  is a plot showing a relationship between a tip displacement against an actuator thickness ratio for the self-sensing bending actuator, in accordance with an example embodiment. 
     
    
    
     The drawings referred to in this description are not to be understood as being drawn to scale except if specifically noted, and such drawings are only exemplary in nature. 
     DETAILED DESCRIPTION 
     In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present disclosure. It will be apparent, however, to one skilled in the art that the present disclosure can be practiced without these specific details. 
     Reference in this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present disclosure. The appearance of the phrase “in an embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. Moreover, various features are described which may be exhibited by some embodiments and not by others. Similarly, various requirements are described, which may be requirements for some embodiments but not for other embodiments. 
     Moreover, although the following description contains many specifics for the purposes of illustration, anyone skilled in the art will appreciate that many variations and/or alterations to said details are within the scope of the present disclosure. Similarly, although many of the features of the present disclosure are described in terms of each other, or in conjunction with each other, one skilled in the art will appreciate that many of these features can be provided independently of other features. Accordingly, this description of the present disclosure is set forth without any loss of generality to, and without imposing limitations upon, the present disclosure. 
     Referring now to the drawings,  FIG. 1  illustrates a diagrammatic view of a self-sensing bending actuator (generally referenced by the numeral  100 ). In accordance with an embodiment of the present disclosure, the self-sensing bending actuator  100  is a piezoelectric bending actuator. Hereinafter, the terms “self-sensing bending actuator,” “bending actuator,” “piezoelectric bending actuator,” and “piezoelectric actuator” have been used interchangeably without any limitations. As may be understood, an actuator is a device that converts some form of energy to mechanical motion. A piezoelectric actuator, in particular, converts electrical energy into mechanical motion using principle of the inverse piezoelectric effect. The term “piezoelectric effect” refers to the conversion of mechanical energy to electrical energy, and this effect is used in the construction of piezoelectric sensors and energy harvesters; whereas the term “inverse piezoelectric effect” refers to the conversion of electrical energy to mechanical energy, and this effect is used in the construction of piezoelectric actuators. The inverse piezoelectric effect causes a change in length in these types of materials when an electrical voltage is applied. The self-sensing bending actuator  100  is designed to amplify the small strain of piezoelectric materials that results from the inverse piezoelectric effect in order to achieve a usable displacement. The generated mechanical motion may be used to drive a mechanism and to accomplish a task like opening or closing a camera shutter, moving a piston rod to control the flow of materials in an assembly line or to move a relay mechanism in a switching electric circuit, to name a few examples. 
     The self-sensing bending actuator  100  includes a stack of layers (collectively referred to by the numeral  102 ) and having an actuator potion  102   a  and a sensing potion  102   b  defined therein, as will be discussed in detail in the subsequent paragraphs. As illustrated in  FIG. 1 , the self-sensing bending actuator  100  may include a support body  104  which supports the various elements therein. In the illustrated example, the stack of layers  102  is cantilevered on the support body  104 , i.e. the stack of layers  102  is supported in the form of a cantilever beam. The support body  104  may be constructed of any suitable insulating material, such as, but not limited to, plastic, fiberglass, asbestos, Teflon®, rubber, or any other electrically insulating polymer including polyurethane, polystyrene, etc. The support body  104  could also be made of stainless steel or any other metal. The support body  104  may be a rigid structure in order to hold the various functional material layers  102  of the self-sensing bending actuator  100 . Although, the support body  104  is shown to have a generally cuboidal shape, it may be understood that the support body  104  may have any other suitable shape based on the application of the self-sensing bending actuator  100 . In the present example, where the self-sensing bending actuator  100  is shown in the form of a cantilever beam, the support body  104  may include a groove or the like (not shown) formed in one of the end surfaces, such as surface  105  to support the stack of layers  102  of the self-sensing bending actuator  100 . 
     In the self-sensing bending actuator  100  of the present disclosure, the stack of layers  102  includes one or more bending elements  106  (also sometimes referred to as actuator layers  106 ), a metallic layer  108  disposed between the bending elements  106  for providing support and electrical contact, a sensing element  110 , and an insulating layer  112  separating the sensing element  110  from the immediate bending element  106 . In the present example, the bending elements  106  and the metallic layer  108  therebetween define the actuator portion  102   a  of the self-sensing bending actuator  100 , and the sensing element  110  herein along with the insulating layer  112  define the sensing portion  102   b  of the self-sensing bending actuator  100 . This combination of the actuator portion  102   a  and the sensing portion  102   b  within the single unit makes the present device a self-sensing bending actuator. 
     In an embodiment of the present disclosure, the self-sensing bending actuator  100  is a 5-layered self-sensing actuator. In such arrangement, the self-sensing bending actuator  100  includes two layers of the bending elements  106  (bimorph structure), as shown in more detail in  FIG. 2 . In alternate embodiments, the self-sensing bending actuator  100  may include one layer of the bending element  106  (unimorph structure) as shown in  FIG. 3 . Further, the self-sensing bending actuator  100  may have more than two layers of the bending elements  106 , i.e. the self-sensing bending actuator  100  may be a multi-layered cantilever beam, without departing from the scope of the present disclosure. In  FIG. 2 , the actuator portion  102   a  of the self-sensing bending actuator  100  has been shown to include two layers of the bending elements  106 , a first bending element  106   a  (positioned at top as shown in  FIG. 2 ) and a second bending element  106   b  (positioned lower as shown in  FIG. 2 ). Each of the one or more bending elements  106  is a smart material beam that bends in response to an applied electrical signal. In the present embodiment, the bending elements  106  are piezoelectric layers, and the two terms have been interchangeably used in the present disclosure. Although the self-sensing bending actuator  100  has been described in terms of using piezoelectric layers as bending elements  106 , it may be contemplated by a person skilled in the art that the self-sensing bending actuator  100  may be realized with any other smart material, such as, but not limited to, magnetostrictive or shape memory materials. 
     In an embodiment, the bending elements  106  are constructed of PZT-5H layers (where PZT stands for lead zirconate titanate). PZT-5H is chosen as the actuator material due to its high piezoelectric coefficient, so that the piezoelectric layers  106  provides a high bending or tip displacement as well as blocked force, where blocked force is defined as the maximum force output of a bending piezoelectric actuator at a given voltage when the displacement is completely blocked. Further, ease of availability as well as low price of PZT-5H compared to other commercial piezoelectric materials makes it a suitable choice. Alternatively, the bending elements  106  may be composed of any appropriate material such as lead magnesium niobate-lead titanate solid solutions, strontium lead titanate, quartz silica, piezoelectric ceramic lead zirconate and titanate (PZT), piezoceramic-polymer fiber composites, and the like. Further, the metallic layer  108  is composed of brass material. Brass is chosen primarily because of its relatively good electrical conductivity for providing electrical contact between the two piezoelectric layers  106   a ,  106   b , and low stiffness for allowing maximum displacement of the self-sensing bending actuator  100 , and further for its ability to be formed into thin sheets. Further, the sensing element  110  is constructed of polyvinylidene fluoride, or polyvinylidene difluoride, (PVDF) layer. PVDF is chosen for the sensing element  110  due to its high sensing resolution, ability to be formed into micrometer-sized thin sheets and low stiffness so that it offers as little resistance as possible to the motion induced by the piezoelectric layers  106 . Further, the insulating layer is composed of Kapton® layer which is placed between the PZT-5H layer  106   b  and the PVDF layer  110 . Kapton is a polyimide film with the chemical name poly (4,4′-oxydiphenylene-pyromellitimide), and with its good dielectric qualities, large range of temperature stability and its availability as thin sheets have made it a preferred material for use as insulating material in the present configuration. Further, layers of adhesive compositions may be employed to adhere the various layers with each other. It may be understood that the mentioned materials for various layers  102  are preferred materials for the self-sensing bending actuator  100 ; however, these materials may be replaced with other suitable materials of substantially similar properties and thus shall not be construed as limiting to the present disclosure. 
     In the self-sensing bending actuator  100 , the two piezoelectric layers  106   a ,  106   b  act as the actuator layers and are responsible for generating a displacement as well as force output on the application of an electrical voltage. For this purpose, the piezoelectric layers  106   a ,  106   b  are connected via an electric circuit  114 , as shown schematically in  FIG. 2 . Specifically, in the electric circuit  114 , the two outer surfaces of the piezoelectric layers  106   a ,  106   b  are connected together electrically via a conductive wire or the like. Also, the inner surfaces of the piezoelectric layers  106   a ,  106   b  are already disposed in electrical contact via the conductive brass layer  108 . As illustrated, in the electric circuit  114 , a pair of wire leads are attached to the brass layer  108  and to at least one of the outer surfaces of the piezoelectric layers  106   a ,  106   b  in order to provide the potential difference between the two surfaces of the corresponding piezoelectric layers  106   a ,  106   b . To actuate the bending actuator, a voltage is applied to these wire leads which results in the same voltage being applied across each of the two piezoelectric layers  106   a ,  106   b . It may be contemplated that the electrical signal is generated by a computer or a function generator and is fed to the piezoelectric layers  106   a ,  106   b  after amplification. The design and configuration of such circuitry is well known in the art, and thus have not been described herein for the brevity of the present disclosure. 
     In the self-sensing bending actuator  100 , the two piezoelectric layers  106   a ,  106   b  are polarized in the same direction. Each of the piezoelectric layers  106   a ,  106   b  has orthotropic symmetry with respect to its material properties such that the material properties are the same along X and Y directions and different along Z direction. The piezoelectric layers  106   a ,  106   b  are polarized in the thickness direction, i.e. along the Z direction. When a voltage is applied across the thickness direction of each layer, it either expands or contracts. In the electric circuit  114 , the wiring is done such that the inner surfaces of the two piezoelectric layers  106   a ,  106   b  are at a same first potential while the outer surfaces are at a same second potential. This ensures that the field directions are opposite to each other in the two piezoelectric layers  106   a ,  106   b . When a positive voltage is applied, the electric field is aligned with the direction of polarization with the first piezoelectric layer  106   a , whereas the electric field and polarization are in opposite directions for the second piezoelectric layer  106   b . This causes the first piezoelectric layer  106   a  to expand and the second piezoelectric layer  106   b  to contract resulting in the bending or up-down movement of the cantilever beam as formed by the stack of layers  102 , along the Z direction (as shown by means of a double-sided arrow in  FIG. 1 ). 
     Furthermore, the self-sensing bending actuator  100  of the present disclosure also has self-sensing characteristics. That is, the present self-sensing bending actuator  100  is capable of estimating its own deflection without using any external sensor. For this purpose, the PVDF layer  110  acts as the sensor to estimate the tip displacement of the self-sensing bending actuator  100 , in response to the applied voltage. For example, when a voltage is applied as input across the actuator layers  106   a ,  106   b , the applied voltage results in bending of the cantilever beam structure of the bending actuator  100 . The strain developed in the PVDF layer  110  due to the bending generates a charge across the PVDF layer  110  because of the piezoelectric effect. A pair of leads are connected across the PVDF layer  110  to fed the charge generated in response to the tip displacement into a convertor circuit  116  (as shown in  FIG. 2 ). Sometimes, the generated charge from the PVDF layer  110  is amplified using the charge amplifier. The convertor circuit  116  provides a voltage output based on the fed charge which is proportional to the tip displacement and may be read using any available means, such as an analog or a digital meter, a display, etc. known in the art. For a given applied voltage, the tip displacement is related to the charge generated across the PVDF layer as: 
     
       
      
       U 
       tip 
       =k 
       ss 
       *V 
       PVDF  
      
     
     Wherein, U tip  is the predicted tip displacement, k ss  is the sensor constant that relates the tip displacement and V PVDF  is the voltage generated across the PVDF layer  110 . The sensor constant is experimentally determined by linear curve fit of the PVDF layer  110  voltage and the displacement measured using an external sensor. 
     It may be understood that the thickness of the various layers  102  affects the tip displacement achieved by the self-sensing bending actuator  100 . Although increasing the thickness of the piezoelectric bending elements  106   a,    106   b  should generally lead to more bending movement, but it may be noted that such increase in thickness may also increase the stiffness of the structure which adversely affects overall achieved tip displacement. Therefore, beyond a certain increase in thickness, there may be an actual drop in tip displacement; and thus the thickness of various layers may need to be optimized. It has been seen that the tip displacement generally increases as the total thickness is reduced demonstrating the inverse-square relationship between the tip displacement and thickness. Further, it may be understood that that the piezoelectric bending elements  106   a,    106   b  have their maximum displacement at resonance. It is known that increasing the thickness of the piezoelectric bending elements  106   a ,  106   b  increases its natural frequency. Therefore, by tuning the frequency of the piezoelectric bending elements  106   a ,  106   b  to a desired value the achieved tip displacements may be varied depending on the type of application of the self-sensing bending actuator  100 . Other parameters like length and width can be similarly modified to optimize certain geometrical parameters for the required frequency response, and thereby to achieve the desired performance objectives. 
       FIG. 4  shows a plot  400  of the tip displacement against the actuator thickness ratio for an exemplary input loading of ‘1 V’ for various actuator thickness ratios of the piezoelectric bending elements  106   a ,  106   b , keeping the total thickness of the self-sensing bending actuator  100  constant. Herein, the actuator thickness ratio is defined as the ratio of the combined thickness of the two piezoelectric bending elements  106   a ,  106   b  and the total thickness of the self-sensing bending actuator  100 . The plot  400  suggests the existence of an optimum thickness of the piezoelectric bending elements  106   a ,  106   b  that maximizes the tip displacement. The initial increase in tip displacement with increasing actuator thickness ratio may be due to the greater energy needed to overcome the resistance offered by the passive layers, i.e. the brass layer  108 , the sensing element  110  and the insulating layer  112 . As may be seen from  FIG. 4 , beyond a certain thickness fraction of about 0.20, increasing thickness of the actuator layers  106   a ,  106   b  also increases the overall stiffness and hence the tip displacement drops. 
     Based on the above, the present self-sensing bending actuator  100  may have any appropriate combination of dimensions. Further, the dimensions of various layers  102  may be non-uniform, e.g., the self-sensing bending actuator  100  may have a tapered configuration or the like without any limitations. In an exemplary embodiment, for a self-sensing bending actuator  100  with a length (L) of about 40 mm and a width of about 10 mm, the optimum thickness for various layers for achieving high tip displacement and high blocked force may be: PZT-5H thickness (t pzt ) of about 0.2 mm, Brass thickness (t br ) about 0.1 mm (making the overall thickness of the bimorph structure of the actuator portion  102   a  to be about 0.5 mm), Kapton thickness (t k ) of about 0.28 mm, and PVDF thickness (t s ) of about 28 μm. In one example, the PVDF layer  110  may be larger in length (L) than the other layers, for example, the PVDF layer  110  may be about 55 mm in length (L) while the other layers are about 40 mm in length (L). It may be understood that the given dimensions are exemplary only, and shall not be construed as limiting to the present disclosure in any manner. 
     The self-sensing bending actuator  100  of the present disclosure may be manufactured by stacking layers of various materials (as described above) in a suitable manner. First, the actuator portion  102   a  with the bimorph structure of the bending elements  106  is prepared by adhering the brass layer  108  to the first PZT-5H layer  106   a  and then the second PZT-5H layer  106   b  is pasted thereon, using a conductive epoxy. Further, a Kapton film  112  is attached to the formed bimorph structure by using a two-part epoxy adhesive and is left for about 24 hours to dry. To the free surface of Kapton film  112 , one or more PVDF sheets  110  are attached using a high-shear adhesive, such as, for example, Loctite-380. Since, in one example, the PVDF sheets  110  are longer than the other layers, part of these may be free hanging. Herein, copper tapes with conductive adhesive are attached on each of these surfaces of the free hanging part. Further, in such case, a few layers of paper tape are attached to the inner surface of the copper tapes to prevent the two surfaces of the PVDF sheets  110  from shorting. 
     The self-sensing bending actuator  100  of the present disclosure has a number of desirable properties not easily achieved with a separate piezoelectric sensor and actuator. The self-sensing bending actuator  100  is more accurate due to the collocated arrangement of sensor and actuator elements. The modular design of the present bending actuator  100  allows for multiple bending actuators  100  to be stacked on top of each other in order to achieve higher tip displacement and/or force output. Thus, the self-sensing bending actuator  100  may be employed in many application areas, including micromanipulation, robotic end-effectors, deformable mirrors, high speed switches, tattoo machines, shakers and vibration based energy harvesting, to name a few. Furthermore, it may be contemplated by a person skilled in the art that the self-sensing bending actuator  100  of the present disclosure may conversely also be used to distinguish between objects of varying stiffness. Such feature may particularly be helpful in medical settings, in particular surgical applications and the like. 
     The materials selected from the present bending actuator  100  also provide several advantages. For example, PZT smart materials are chemically inert to most common chemicals, rigid, have higher Curie point, are capable of generating large tip displacements due to their high piezoelectric coupling coefficients, and are readily available at very competitive rates. Their dynamic range can extend up to a few kilohertz (kHz) which is sufficient for most needs. The electronic circuitry needed to drive and analyze the signals from piezoelectric materials requires a simple signal generator and a linear amplifier for the actuator mode and a charge amplifier for the sensor mode which are also inexpensive compared to the requirements for other smart materials. For instance, on the other hand, other possible materials for actuators, like EAP smart materials need a very high actuation voltage of the order of kV which again drives up the cost of the overall device. They are also costlier to manufacture compared to piezoelectric devices with no advantages over piezoelectric materials. Magnetostrictive materials work analogous to piezoelectric materials but the coupling is between magnetic and mechanical domains instead of electrical and mechanical domains as is the case for piezoelectric materials. Besides posing a high risk of injury due to the use of high magnetic fields, for example, during a surgical procedure where other magnetic devices may be in use, the technology has not been demonstrated experimentally and hence its applications is uncertain. Moreover, extremely high magnetic fields are needed to generate high force and displacement output which are impossible without sophisticated cooling technology and electromagnet coil design. This would drive up the production cost as well as maintenance cost of a device using magnetostrictive materials. 
     Due to utilization of two piezoelectric actuator layers  106   a ,  106   b , the present bending actuator  100  generally provides a higher tip displacement as well as the blocked force output for the same applied voltage compared to an actuator utilizing a single PZT-5H layer as the actuator. This is because the extra actuator layer of PZT-5H provides an increase in displacement while the softer sensing layer adds sensing capabilities without significantly increasing the stiffness of the overall structure of the bending actuator  100 . Furthermore, the geometry of the present bending actuator  100  is preferred over a conventional piezoelectric based actuator due to higher force and displacement output per unit input voltage as well as a higher PVDF sensor output per unit displacement. For example, in comparison to a self-sensing actuator design in which both the actuator and the sensor layers are PZT-5H such that a top PZT-5H layer is connected to the input voltage and acts as the actuator while a bottom piezoelectric layer generates a voltage in response to the displacement and acts as a passive sensor, the displacement and force output of such design would be lower than present bending actuator  100  for the same applied voltage. This is due to the fact that there is only one actuating layer in contrast with two actuating layers in the present bending actuator  100  and furthermore the PZT-5H layer would have a higher stiffness and hence such actuator would have a higher resistance to work against compared to the present bending actuator  100  with the sensing element  110  having relatively low elastic stiffness. Additionally, in comparison to a conventional 3-layered self-sensing actuator in which the actuator is made of PZT-5H and the sensor is made of PVDF such that a top PZT-5H layer is connected to the input voltage and acts as an actuator while the bottom PVDF piezoelectric layer generates a voltage in response to the displacement, the displacement and force output of such an actuator would be lower than the present self-sensing bending actuator  100  since there is only one actuating layer compared to two actuating layers in the present actuator  100 . 
     Again, for example, comparing the present design of the bending actuator  100  against a conventional actuator with both the top as well as bottom layers acting as actuator as well as sensor layers, the present design would provide better overall results despite the conventional design expected to have a higher force and displacement output (since the Kapton and the PVDF layers are the additional layers against which the present design has to work against). In such conventional design, since both the actuation and sensor signals are read and/or generated through the same set of wires, they are prone to high noise. Using bridge circuits may provide a solution, but it needs an additional dummy sensor otherwise an accurate estimation of the capacitance of the piezoelectric layers is hard to achieve due to its variation with environmental factors. In contrast, the present design uses the same amplifier as the other designs and a charge amplifier to convert the PVDF sensor signals to usable voltage. Furthermore, using Kapton and PVDF materials for the insulating layer  112  and the sensing element  110 , respectively, the overall stiffness of the present design is not substantially increased because PVDF is available in very thin sheets of up to 28 μm and has a low elastic stiffness while Kapton is an excellent insulator with very low elastic stiffness, thereby generally offsetting any disadvantage over the conventional design. 
     Embodiments of the present disclosure substantially eliminate or at least partially address the aforementioned problems in the background, and provide an improved self-sensing bending actuator. The unique geometry makes this design achieve a larger force and displacement output compared to conventional bending actuators with one actuating layer and one sensing layer. For example, the self-sensing bending actuator  100  is capable of generating a tip displacement of about 3.26 mm, and possibly more, at its resonant frequency and has a predicted blocked force output of about 1.6 N at its optimal thickness ratio of actuator layers. The choice of materials ensures that the device can be used in a medical setting without any risk of injury to patients in contrast to magnetic field based devices. The estimated tip displacement using its self-sensing capability (by measuring the PVDF sensor layer charge output) has higher signal-to-noise ratio than such ratio achieved by any external sensor, in addition to providing the benefit of collocated design. 
     The embodiments illustrated and described herein as well as embodiments not specifically described herein but within the scope of aspects of the invention constitute exemplary self-sensing bending actuator. 
     The benefits and advantages described above may relate to one embodiment or may relate to several embodiments. The embodiments are not limited to those that solve any or all of the stated problems or those that have any or all of the stated benefits and advantages. 
     The above description is given by way of example only and various modifications may be made by those skilled in the art. The above specification, examples and data provide a complete description of the structure and use of exemplary embodiments. Although various embodiments have been described above with a certain degree of particularity, or with reference to one or more individual embodiments, those skilled in the art could make numerous alterations to the disclosed embodiments without departing from the spirit or scope of this specification.