Abstract:
Systems for providing electro-mechanical sensors are provided. In some embodiments, a system for providing an electro-mechanical sensor comprising: a flexible material forming at least a first channel and a second channel, wherein the first channel includes a first plate region and the second channel forms a second plate region that is substantially aligned with the first plate region; and an electrically conductive fluid that fills the first channel and the second channel.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Patent Application No. 61/591,881, filed Jan. 28, 2012, which is hereby incorporated by reference herein in its entirety. 
    
    
     TECHNICAL FIELD 
     Systems for providing electro-mechanical sensors are provided. 
     BACKGROUND 
     Electro-mechanical sensors are devices for converting mechanical stimulus into electrical signals so that the stimulus can be detected or measured. An example of an electro-mechanical sensor is a capacitive sensor. A capacitive sensor typically includes a pair of opposing plates whose capacitance increases as the distance between the opposing plates decreases, or the permittivity of a dielectric medium between the plates increases, due to mechanical stimulus. Capacitive sensors offer advantages such as high sensitivity, tunable spatial resolution when used in an array configuration, and a simple, well-known governing equation. 
     One application of electro-mechanical sensors is tactile sensing. Tactile sensing is a field of great interest due to its potential impact in robotic sensing applications such as robot-assisted surgery and robotic grasp and manipulation, among other applications. In many cases, visual feedback and acoustic feedback alone do not provide the information necessary for decision making in robotic sensing applications. A classic case is that of an amputee who accidentally crushes or drops an object with his prosthetic hand due to inadequate tactile information about the hand-object interaction. 
     A difficulty in implementing tactile sensors in robotic applications is that robotic applications often require robust tactile sensing capabilities on curved surfaces, such as artificial fingertips. Such sensing capabilities can be difficult to implement with existing electro-mechanical sensors. 
     Accordingly, it is desirable to provide new electro-mechanical sensors. 
     SUMMARY 
     Systems for providing electro-mechanical sensors are provided. In accordance with some embodiments, systems for providing electro-mechanical sensors are provided, the systems comprising: a flexible material forming at least a first channel and a second channel, wherein the first channel includes a first plate region and the second channel forms a second plate region that is substantially aligned with the first plate region; and an electrically conductive fluid that fills the first channel and the second channel. 
     In some embodiments, systems for providing an electro-mechanical sensor are provided, the systems comprising: a means for forming a flexible material with at least a first channel and a second channel, wherein the first channel includes a first plate region and the second channel forms a second plate region that is substantially aligned with the first plate region; and a means for filling the first channel and the second channel with an electrically conductive fluid. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top view of an electro-mechanical sensor in accordance with some embodiments. 
         FIG. 2  is a cross-section view of an electro-mechanical sensor showing layers of the sensor in accordance with some embodiments. 
         FIG. 3  is a top view of a layout of an electro-mechanical sensor in accordance with some embodiments. 
         FIG. 4  is a cross-section view of an outer layer of an electro-mechanical sensor in accordance with some embodiments. 
         FIG. 5  is a cross-section view of an outer layer of an electro-mechanical sensor with holes punched in the layer in accordance with some embodiments. 
         FIG. 6  is a cross-section view of an inner layer of an electro-mechanical sensor in accordance with some embodiments. 
         FIG. 7  is a cross-section view of a combination of an outer layer and an inner layer of electro-mechanical sensor in accordance with some embodiments. 
         FIG. 8  is a cross-section view of a combination of an outer layer and an inner layer of electro-mechanical sensor with Galinstan inserted into the combination in accordance with some embodiments. 
         FIG. 9  is diagram of a mechanism for testing and/or calibrating an electro-mechanical sensor in accordance with some embodiments. 
         FIG. 10  is a schematic diagram of circuitry for receiving, amplifying, and processing a signal from a sensor in accordance with some embodiments. 
         FIG. 11  is a cross section view of a sensor system in accordance with some embodiments. 
         FIGS. 12A and 12B  are cross section views of another sensor system in accordance with some embodiments. 
         FIG. 13  is a cross section view of yet another sensor system in accordance with some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Systems for providing electro-mechanical sensors are provided. In accordance with some embodiments, capacitive electro-mechanical sensors are provided. In some embodiments, these capacitive electro-mechanical sensors can be used as tactile sensors. In some embodiments, these tactile sensors can be used for robotic tactile sensing applications. 
     For example, in accordance with some embodiments, a flexible and multilayer capacitive microfluidic normal force sensor with a 5×5 tactile sensor element (“taxel”) array can be provided. The sensor can use microfluidic channels filled with an electrically conductive fluid as capacitive plates and conductive interconnects. The sensor can be microfabricated using soft lithography microfabrication techniques and can include multiple layers of polydimethylsiloxane (PDMS) microchannels filled with an electrically conductive fluid, such as Galinstan (for example) and air pockets that modify the mechanical and electrical properties of the sensor. Galinstan is a fairly conductive fluid created by Geratherm Medical AG of Geschwenda, Germany for use in thermometers as a nontoxic substitute for mercury. Galinstan is a eutectic metal alloy composed of gallium, indium, and tin. 
     In accordance with some embodiments, such a flexible tactile sensor can be conformally wrapped around curved digits of a robotic hand for tactile sensing and can enhance grip by cushioning impacts and increasing the effective contact area during grasp. 
     A single taxel of such a sensor can be calibrated for normal forces ranging from 0-2.5 N in accordance with some embodiments. The sensor can have a spatial resolution on the order of 0.5 mm and perform reliably even when wrapped around a curved surface. The deformable liquid capacitive plates and heterogeneous PDMS-air dielectric medium can be designed to tune the sensor&#39;s sensitivity and range. 
     Turning to  FIG. 1 , an example of a capacitive, microfluidic sensor  100  in accordance with some embodiments is shown. As illustrated, sensor  100  can be flexible enough to conform to the curvature of a human finger. In some embodiments, the sensor can be fabricated using soft lithography and include a flexible elastomer  102  to mimic the mechanical properties of human skin and an electrically conductive fluid to serve as flexible plates  104  and interconnects  106  for the capacitive sensing units. 
     As shown in cross-section in  FIG. 2 , a sensor  200  in accordance with some embodiments can include four layers of PDMS  202 ,  204 ,  206 , and  208 . The two outermost PDMS layers  202  and  208  can contain microfluidic channels  210  filled with Galinstan and the two inner layers  204  and  206  can seal the microfluidic layers and together form an array of square pockets  212  to time the overall sensor&#39;s mechanical and electrical properties. Pockets  212  can be filled with any suitable gas (such as air), solid, gel, etc., or can contain a vacuum, in some embodiments. 
     As shown in  FIG. 3 , microfluidic channels  302  and  304  can form a 5×5 array  306  of taxels  308  connected by in-plane paths formed by channels  302  and  304  (lengthways paths formed by channels  302  can be in the top layer and transverse paths formed by channels  304  can be in the bottom layer). The paths formed by channels  302  and  304  can have any suitable dimensions. For example, in some embodiments, the paths can be 125 μm thick. As shown, each of these paths can pass through and connect five 0.5 mm×0.5 mm (or any other suitable size) taxel plates, each of which can be separated from the next plate by 0.5 mm (or any other suitable spacing). 
     In some embodiments, as shown in  FIG. 2 , a 5×5 array of square air pockets  212  formed by layers  204  and  206  can have the same layout and dimensions as 5×5 array of plates  306  in the microfluidic channel layers  202  and  208 . 
     An example of a process for making a sensor in accordance with some embodiments is now illustrated in connection with  FIGS. 4-8 . 
     In some embodiments, PDMS masters for layers  202 ,  204 ,  206 , and  208  can be made as follows. PDMS masters for the microfluidic layers  202  and  208  can be fabricated by patterning 40 μm of SU-8 2015 photoresist  402  (available from Microchem of Newton, MA) onto 4″ silicon wafers  404  as shown in  FIG. 4 . PDMS masters for the air pocket layers  204  and  206  can be fabricated by patterning of 18 μm thick SU-8 2010 photoresist  602  onto 4″ silicon wafers  604  as shown in  FIG. 6 . These masters can be soft baked at 95 degrees Celsius for five minutes and then exposed to 22.5 mW/cm 2  UV light for 16 seconds using mylar masks. After a five minute post-exposure bake on a hot plate at 95 degrees Celsius, the wafer can be developed and then hard baked in an oven at 140 degrees Celsius for five minutes. The thicknesses of the masters can be measured using a profilometer (e.g., a Dektak IIA available from Sloan of Scotia, N.Y.). 
     Once the masters are complete, layers  202 ,  204 ,  206 , and  208  can be manufactured as follows. As described above, in some embodiments, these layers can be made from PDMS, and any suitable PDMS composition and curing process can be used. For example, in some embodiments, PDMS with a 10:1 A:B ratio (e.g., RTV615 available from Momentive of Columbus, Ohio) can be used. 
     Each of the two 300 μm thick microfluidic channel layers  202  and  208  can be fabricated by (1) spin coating PDMS onto the microfluidic channel mold at 500 rpm for 30 seconds and curing it in an oven at 80 degrees Celsius for an hour to produce a 150 μm thick layer, and then (2) repeating this process a second time to produce 300 μm thick PDMS films  406 . The resulting layer can appear as shown in  FIG. 4 . 
     Next, PDMS layer  406  can be removed from wafer  404 . Any suitable technique for removing the layer can be used in some embodiments. For example, PDMS layer  406  can be removed from wafer  404  by careful peeling the layer from the wafer by gloved hands in some embodiments. 
     As shown in  FIG. 5 , the two ends  502  and  504  of each wire-plate path in layers  202  and  208  can next be punched with a 700 μm diameter stainless steel TiN-coated round punch (available from Technical Innovations of Angleton, Tex.) to create through-holes that serve as inlets and outlets for the injection of Galinstan. Although an outlet is shown in  FIG. 5  and described herein, in some embodiments, the outlet can be omitted. 
     The 25 μm thick air pocket layers  204  and  206  can be created by spinning PDMS  606  onto the corresponding master at 3000 rpm for 30 seconds and curing it in an oven at 80 degrees Celsius for an hour. The resulting layer can appear as shown in  FIG. 6 . 
     Next, as shown in  FIG. 7 , microfluidic channel layers  202  and  208  can be bound to an air pocket layer  204  and  206 , respectively, after oxygen plasma treatment (using, for example, PDC-001, available from Harrick Plasma of Ithaca, N.Y.). In some embodiments, during binding, isopropanol (IPA) can be used to wet each of layers  202  and  204  (or  206  and  208 ) and these layers can be aligned under a microscope to ensure accurate alignment of the 5×5 arrays of taxel plates and air gaps. Each of the two-layer sandwiches  702  (i.e., formed from layers  202  and  204 , or layers  206  and  208 ) can then be placed on a hot plate at 80 degrees Celsius for one hour. 
     Next, two-layer sandwiches  702  can be removed from wafer  604 . Any suitable technique for removing sandwiches  702  can be used in some embodiments. For example, two-layer sandwiches  702  can be removed from wafer  604  by careful peeling the sandwiches from the wafer by gloved hands in some embodiments. 
     As shown in  FIG. 8 , Galinstan  802  can then be injected into each arm of the five paths in each of the layers  202  and  208  using a syringe with a 700 μm diameter stainless steel tube attached. Next, rigid, insulated 500 μm diameter (or any other suitable diameter) wires  804  can be positioned in the inlet and outlet holes  502  and  504  and uncured PDMS  806  can be poured over the holes. The system can be placed in an oven for 2 hours at 80 degrees Celsius to cure the PDMS applied to the channels&#39; inlet and outlet holes. 
     In some embodiments, electrical continuity and resistance of 1.5-2.5Ω between the inlet and outlet of each path can be verified with a multimeter. 
     Finally, an O 2  plasma-IPA alignment and bonding technique can be used to position and bond the two halves of the sensor perpendicular to one another in order to obtain a functional sensor as shown in  FIG. 2 . Note that for clarity, the wires at the end of the paths of the bottom layer of Galinstan-filled channels are not shown in  FIG. 2 . 
     As shown in  FIG. 9 , in some embodiments, in order to test and calibrate a sensor  902 , a single taxel of the sensor can be loaded by a uniaxial, point-load using a 1.5 mm×1.5 mm rectangular-shaped tip  904 . Double-sided sticky mylar tape can be used to affix sensor  902  to a rigid, flat support plate  906  affixed to a six degree-of-freedom force/torque transducer (e.g., Nano-17 available from ATI Industrial Automation of Apex, N.C.)  908  having resolutions of 1/80 N and 1/16 N-mm for force and torque, respectively. 
     In some embodiments, testing and calibration of the sensor can be performed with the sensor and its electrical circuit inside a Faraday cage for shielding from external electromagnetic noise. For example, in some embodiments, an electromagnetic noise shielding film (e.g., such as films available from Tatsuta System Electronics Co., Ltd. of Osaka, Japan) can be used for testing and/or production use. In some embodiments, a metallic deposition layer of such a film can be connected to a common ground of a tactile sensor skin circuit using any suitable mechanism, such as a conductive adhesive. 
     In some embodiments, sensor data can be collected with the sensor at rest in an unloaded state. The tip of the load platform can then be carefully centered over a single taxel with no overlap of adjacent taxel units. Calibrated masses can then be added to the load platform to gradually achieve a total of 250 g (2.45 N). The actual transmitted load can be determined by the force transducer. The masses and load platform can be removed in reverse order (and with different load increments) until the sensor is completely unloaded. The sensor can be allowed to equilibrate after each change in external load before data are collected for a 0.1 sec interval. 
     In some embodiments, low frequency dynamic loads can be applied to the sensor as in  FIG. 9  with a load that is raised and lowered against the load platform. In some embodiments, the sensor can follow the loading and unloading phases of the dynamic stimulus with no significant lag. 
     In some embodiments, any suitable circuit can be used to measure the capacitance of individual taxels. For example, as illustrated in circuit  1000  of  FIG. 10 , in some embodiments, a charge amplifier circuit can be used to detect the capacitance value of a first taxel (illustrated as C in    1005 ) and provide a corresponding output voltage. This output voltage can then be converted to digital form by an analog-to-digital converter  1022 , and processed by a hardware processor  1024 . The detected taxel can be switched by a pair of analog switches (or analog multiplexers)  1020  and  1021  under the control of hardware processor  1024 . Within or after hardware processor  1024 , the digitized sensor signal (or any signals based on this signal) can be used for any suitable purpose, such as detecting force or pressure on a robotic finger. Components  1020 ,  1022 , and  1024  can be combined and/or replaced with any other suitable components in some embodiments. 
     More particularly, in some embodiments, the charge amplifier circuit of circuit  1000  can be implemented as follows. As illustrated, an AC input signal  1004  can be applied to a first side of an analog switch  1020 . A second side of switch  1020  can be coupled to a plurality of rows of taxel plates. For example, these rows of taxel plates can be on a top layer of a sensor. One side of a taxel  1005  (e.g., the top side) can be coupled to second row of taxel plates, and therefore to a row  2  connection on the second side of switch  1020 . The other side of taxel  1005  (e.g., the bottom side) can be coupled to a second column of taxel plates, and therefore to a column  2  connection on a first side of an analog switch  1021 . Other columns of taxel plates can be coupled to other connections on the first side of switch  1021 . A second side of switch  1021  can then be coupled to an inverting input  1006  of an operational amplifier  1008 . 
     A non-inverting input  1010  of operational amplifier  1008  can be connected to ground. An external feedback capacitor  1012  and a resistor  1014  can be connected across the operational amplifier&#39;s inverting input  1006  and output  1016 . While the AC input voltage remains constant, changes in taxel capacitance produce changes in charge, which translates to changes in the operational amplifier&#39;s output voltage amplitude. Thus, the gain in amplitude of the AC input signal depends on the ratio of capacitance between the constant external capacitor and the variable capacitive taxel. 
     In some embodiments, under the assumption of an ideal operational amplifier, nodal analysis can be performed on the charge amplifier circuit to obtain: 
                       V   out     =     -       V   in     ⁡     (       jω   ⁢           ⁢     R   out     ⁢     C   in           jω   ⁢           ⁢     R   out     ⁢     C   out       +   1       )           ,           (   1   )               
where V out  is the output voltage amplitude of the operational amplifier, V in  is the input voltage amplitude, ω is the excitation frequency of the input signal, R out  is the external feedback resistance, C in  is the capacitance of a single taxel (connected to the operational amplifier&#39;s inverting input), and C out  is the external feedback capacitance. If ωR out C out &gt;&gt;1, then equation (1) simplifies to:
 
     
       
         
           
             
               
                 
                   
                     V 
                     out 
                   
                   = 
                   
                     - 
                     
                       
                         
                           V 
                           in 
                         
                         ⁡ 
                         
                           ( 
                           
                             
                               C 
                               in 
                             
                             
                               C 
                               out 
                             
                           
                           ) 
                         
                       
                       . 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     By setting the input signal frequency ω to 62832 rad/s, R out  to 200 MΩ, and C out  to 1 pF, the expression ωR out C out  has a value of 12.566 and allows the use of equation (2). In response to mechanical deformation under load, changes in taxel capacitance C in  can be measured through changes in output voltage amplitude V out . 
     In some embodiments, data acquisition boards (e.g., NI-6255 and NI-6211 available from National Instruments, Austin, Tex.) can be used to collect data from the load cell at 1 kHz and a single sensor taxel at 200 kHz. The amplifier circuit input signal can be sinusoidal with a peak-to-peak voltage of 1.0 V and a frequency of 10 kHz. Sensor taxel data can be collected at 20 times the input signal frequency in order to obtain accurate amplitudes from the output signal. 
     Post-processing of the raw load cell and capacitive sensor signals can be performed in Matlab (available from Mathworks of Natick, Mass.). The amplitude of the capacitive sensor output voltage can be determined for each cycle (using the maximum and minimum value for each wave). The mean load cell readings and mean taxel output amplitudes can be computed for each 0.1 sec interval of data. Assuming constant V in  and C out  values, the changes in taxel capacitance C in  can be directly reflected by changes in output voltage amplitude V out  using equation (2). The relative percent change in output voltage amplitude, % ΔV, can be calculated as: 
                       %   ⁢           ⁢   Δ   ⁢           ⁢   V     =           V     out   ,   loaded       -     V     out   ,   unloaded           V     out   ,   unloaded         *   100   ⁢   %       ,           (   3   )               
where the output voltage amplitude V out  is a function of load.
 
     In some embodiments, a power-law curve can be fit to the force values as a function of the calculated % ΔV using nonlinear regression analysis. For example, in some embodiments, the final regression model can be given by:
 
 F   fit =0.0455(% Δ V ) 1.73 −0.00976(% Δ V ) 2.14   , R   2 =0.982,   (4)
 
where F fit  is the force calculated by the curve fit.
 
     In addition to being implemented as normal force tactile sensors as illustrated above, in some embodiments, the capacitive electro-mechanical sensors described herein can additionally or alternatively be implemented as other types of sensors. For example, as illustrated in  FIG. 11 , a sensor system can include a normal force tactile sensor (e.g., like illustrated above), a vibration sensor  1104 , and a shear force sensor  1106 . Like sensor  1102 , sensors  1104  and  1106  can be implemented using channels formed in PDMS that are filled with an electrically conductive fluid, such as Galinstan. As illustrated in  FIG. 11 , the PDMS can have different stiffnesses in different areas in order to facilitate detecting vibration and shear forces. As with the pairs of plates in sensor  1102 , the pairs of plates in sensors  1104  and  1106  can be coupled to a charge amplifier circuit (such as that illustrated in  FIG. 10 ) and the signals processed to detect any suitable mechanical stimulus on sensor system  1100 . 
     Turning to  FIGS. 12A and 12B , another example of a shear force sensor  1202  that can be used in some embodiments is shown. As illustrated in  FIG. 12A , when sensor  1202  is not under shear force, the top plate  1204  and the bottom plate  1206  of the sensor can be substantially aligned as shown by region  1208 . As illustrated in  FIG. 12B , however, when a shear force  1210  is applied to the sensor, the alignment of plates  1204  and  1206  changes, as shown by region  1212 , resulting in a change in capacitance of the sensor. In some embodiments,  FIG. 12A  could illustrate the sensor when force is applied and  FIG. 12B  could illustrate the sensor when the force is removed. Reference numerals  1214  and  1216  illustrate the air pocket and the PDMS, respectively, that can be used in the sensor in some embodiments. 
       FIG. 13  illustrates yet another example of a shear force sensor  1302  that can be used in some embodiments. As shown, this sensor can include a rough surface  1304  that can be used to improve the friction between the sensor and a surface (not shown) applying a shear force to the sensor. 
     In some embodiments, the sensor&#39;s multilayer design can enable nonlinear tuning of the sensitivity over a wide range of forces which can be used to tailor the sensor response to the application of interest. The multilayer design utilizing PDMS and air sub-layers can allow for the tuning of mechanical and electrical properties, particularly for the heterogeneous, deformable dielectric medium in some embodiments. 
     Although the invention has been described and illustrated in the foregoing illustrative embodiments, it is understood that the present disclosure has been made only by way of example, and that numerous changes in the details of implementation of the invention can be made without departing from the spirit and scope of the invention, which is limited only by the claims which follow. Features of the disclosed embodiments can be combined and rearranged in various ways.