Fingertip Force and Contact Position and Orientation Sensor

Home , Orientation (geometry)

2011 IEEE International Conference on Robotics and Automation Shanghai International Conference Center May 913, 2011, Shanghai, China

Fingertip Force and Contact Position and OrientationSensor

Yu Sun

Abstract— This paper presents a novel integrated system and control algorithms have beendeveloped with inspirations that is composed of a fingerprint sensor and a force sensor from humangrasping studies. Many encouraging results have to measure contact position and orientation ofthe fingertip been achieved. However, moreand more researchers have along with thecontactforce. The system uses fingerprints from the fingerprint sensor to identify thecontact position realized the limitation of only including contact force and and orientation with fingerprintfeaturessuch as core point contact position on the object. Many researchers have sought and ridge orientations. Thecontact position and orientation for a way to measure thecontact position on the fingertip are represented in a fingerpad coordinate system for grasping tobetter model and understand more sophisticatedgrasping studies. An experiment has beendesigned to evaluate the behaviors like rolling and regrasping [4]. proposed system in terms of accuracyand resolution with three subjects. The proposed system canbe used in human grasping Commercialtactile sensorssuch as FingerTPS from Pres- studies to characterize the fingerpad contact. sure Profile Systems [15] and flexible tactile sensors from Tekscan [23] have been successfully embedded on many I. INTRODUCTION robotic fingers toprovide rich spatial grasping information Roboticists have learned a great deal from studying human for grasping control. However due to their stiffness, those grasping as the humanhand exhibitssome of the most tactile sensors cannot deform along with the human finger complex human motor skills. Many studies ([22], [12], [5], tissue topreserve theelastic profile or precisely measure [9]) have proposedvarious taxonomies of humangrasping. thecontact spatial features, which is notidealtobe used A basic distinction exists between a power grasp, where the to in many humangrasping studies as wearable sensors goalis to stabilize an objectto resist external forces, and on fingertips. Most studies attach the tactile sensors on the a precision grasp, which is an essential skill to manipulate force sensors other than on fingertips, whichprovides local objects including stable grasps on objects, changing the contact spatial profiles around thecontact point rather than grasps (re-grasp), and maintaining the grasps during tasks. thecontact position on the fingertip. Recently, a number of A preponderance ofresearchhas focused on precision flexible tactile sensors have beendeveloped [7], which could grasps and on instrumentation to measure the fingertip forces be worn on fingertips as a part of a glove or with a thimble and position on the object during contact. A rich experimen- setup. However, the glove or thimble setup may measure tal paradigm for the study of humangrasping was devised differentlybetween trials if the tactile sensor is not aligned by Johansson and Westling [10], consisting of parallel force- with the user’s fingertip consistently. sensing pads for opposition grasps between the index finger This paper presents a novel fingertip force and posi- and thumb. This approachhas beengeneralized to add more tion/orientation (FFPO) sensor system that uses a regu- touchpads to include the middle, ring, or little fingers. In larforce/torque sensor to measure fingertip force and a general, instrumented objects are typically created to incor- fingerprint sensor mounted on the force/torque sensor to porate miniature 6-axis force/torque sensors at predefined measurecontact position and orientation on the fingertip grasp points (e.g., [20], [21], [16], [19]). Special purpose simultaneously. The fingerprintimage obtained from the objects were fabricated to incorporate force sensors. fingerprint sensor is processed and registered to a precon- Besides thecomplexhand postures and forces during structed fingerprint map with fingerprint core point and ridge grasping, the great variance in whichparts of the fingertip orientations. The center position of the registered fingerprint contact an object makes the grasping studies even more on the fingerprint map in the fingerprint coordinate system complex. To understand humandexterity, it is very important is defined as thecontact position on the fingertip. Since to fully study the local mechanicalinteraction between the thecontactlocal spatial features are measured from the fingerpad and an object. Many studies have been carried fingerprint, the obtained contact position and orientation are outto characterize the mechanical response profile in the always consistent between trials. fingerpad responding to force. Serinaet al. [17], and Pawluk Fingerprints have beenused in Neuroscience literature and Howe [14] haveexamined the dynamic force response to characterize the skindeformation level. Westling and of the fingerpad to indentors indifferent sizes and shapes. Johansson [25] and Birznieks et al. [2] have measured the Withvarious kinds offorce sensors, the fingertip force and area of the fingerprintleft on a grasping pointto indicate the thecontact points on the object were measured indifferent skindeformation related togasping force. In the study, the experiments by many research teams to study humandexter- fingertip wasstained with ink to enable the measurement. ous grasping and manipulation. Many robot graspplanning In robotics literature, Levesqueand Hayward [11] have used Yu Sun is with the Department of Computer Science and Engineering, fingerprint features to study the stretch and compression of Universityof South Florida. [email protected] the human fingerpad skin during tactileexploration. Kurita

9781612843803/11/$26.00 ©2011 IEEE 1114 et al. [26] wereable to inferred contact force by observing Fingerprint Development Kit and connected to the PC with fingerprint changes atthe point of contactthrough a glass a USB cable. The fingerprintimage is collected at 14 fps. plate by a camera to estimate the incipient slip movement The fingerprint sensor has a scanning window size 12.8x18 between the fingertip and the glass. From our best knowl- mm and image resolution is 256 x 360 pixel. The fingerprint edge, theapproachof using fingerprints to measurecontact sensor has weight at 6grams with the size of 20.4(L) x positions has not beenproposedbefore. 27(W) x3.5(H) mm [24].

II. SYSTEM SETUPS

(A)(B)

Fig. 3. (A) The second design of the proposed integrated FFPO sensor system that a ATI Nano 17 force sensor and a solid-state TouchChip TCS1C Fingerprint Sensor(B)

The ATI Nano 17 sensor has beenused in many studies Fig. 1. A schematic drawing of the FFPO sensor system: it is composed of a fingerprint sensor and a force sensor as a standard instrumentto measure fingertip force during grasping [1]. Its weightis around 9 grams with the size As illustrated in the schematic drawing Figure 1, the of 15 mm height and 17 mm diameter. Compared with the proposed FFPO sensor system is composedof a force sensor dimensions of the solid-state fingerprint sensor, the weight and a fingerprint sensor thatis rigidly mounted on the top increase is not significant, considering in many studies those of the force sensor. The center of the fingerprintimaging is ATI Nano 17 sensors are mounted on aluminum plates. The aligned with the center of the force sensor, and the short side solid-state fingerprint sensor is very thin and light weight, and the long side of the fingerprint sensor are parallelto the which doesn’t affectthe propertyof the force sensor as much. x-axis and the y-axis of the force sensorrespectively. The For most grasping studies that use the ATI Nano 17, the small contact force between the user’s finger and the FFPO sensor dimension increase will not affect its usage. Even the optical system is measured with the force sensor and thecontact fingerprint sensor is larger, it still couldbe useable in many spatial properties is characterized with the fingerprint sensor. applications. Figure 2 and 3 demonstrate twodesigns with twodifferent Compared withhigh sampling rate of the force sensor, types of fingerprint sensors: an optical fingerprint sensor and thecurrent off-the-shelf fingerprint sensors haveavery a solid-state fingerprint sensor. In Figure 2-A, a Futronic FS low sampling rate. However, as thecontact position in the 90 optical fingerprint sensor(Figure 2-C) is mounted on the fingertip doesn’t change very fast for many cases, the relative tool face of an ATI Nano 17 6-axis force sensor(Figure 2-B). low sampling rate will unlikely posealimitation of its use The force readings arecollected from a Sensoray 626 data for typical grasping studies. For high temporal resolution acquisition board and sampled at 1000 Hz. The Futronic FS requirement, a high speed fingerprint sensor couldbe used. 90 fingerprint sensor is connected to a PC with a USB cable. A Windows PC was used to collectthe data from the force The fingerprintimage is taken at 10 frame-per-second (fps). sensor and the fingerprint sensor. A Sensoray 626 data acqui- The fingerprint sensor has a scanning window size 15x22 sition card reads the force data from the ATI Nano 17 force mm and image resolution is 440x300 pixel. Its weightis 60 sensor at 1000 Hz sampling rate. A software was developed grams with the size of 43(L) x 24(W) x 25(H) mm. It uses to read the ATI Nano 17 force sensor through a Sensoray 626 infrared LEDs to illuminate the fingertip. data acquisition card at 1000 Hzand the fingerprintimage through a USB port at 10 fps. To ensure performance, the fingerprintimages are processed with an imageenhancement algorithm [8] to reduce noiseand improve theclarityof the ridgeand valley.

III. FINGERPRINT COORDINATE SYSTEM (A)(B)(C) To locate thecontact area on a fingertip from a fingerprint, theentire fingerprint of the fingertipneeds tobe obtained and Fig. 2. (A) One design of the proposed integrated system FFPO sensor acoordinate system shouldbe defined to uniquely represent system thatis composed with an ATI Nano 17 force sensor(B) and a Futronic FS 90 optical fingerprint sensor(C). the position of the fingerprint with a set of numbers. The curved surface of the fingertip canbe represented on a flat In Figure 3-A, a solid-state TouchChip TCS1C Fingerprint 2D surface of a map with acontinue mapping function. Sensor(Figure 3-B) is used. Itis connected to a TI C5515 In this paper, a set of fingerprintimages are taken and

1115 stitched together to createaflatten fingerprint map that covers theentire fingertip. A coordinate system is defined on the fingerprint map.

A. Mosaicing Fingerprints If a fingerprint sensor has a sufficient scanning window, a fingerprint map canbeconstructed from a sequence of fingerprints over a rolling fingertip on the fingerprint sensor. As the finger is rolled on the sensor, an image sequence over (A)(B) the rolling time canbe acquired and stacked together without registration or alignment. If a fingerprint sensor has a small Fig. 5. (A) The fingerprint map merged from 200 fingerprintimages; (B) sensing area, only a small portion of the fingerprint canbe Theenhanced fingerprint map captured at each timeand the fingerprints between frames have tobealigned. Many techniques have beendeveloped to combine multiple partially overlapping fingerprintimages into a large fingerprintimage [3]. Our setup has a sufficiently large fingerprint, and we instructed subjects to roll their fingertips on the fingerprint sensor without slippage, so that the images canbeconsideredpre-aligned. A large fingerprint sensor canbe used to build a fingerprint coordinate system and a small fingerprint sensor canbe integrated with the force sensor to archive a compact setup.

Fig. 6. A spherical coordinate system thatis used to representthecontact position on the ﬁngertip

B. Fingerprint Coordinate to Fingertip Coordinate Mapping To express thecontact position on the fingertip, a spherical coordinate is used. Considering thatthe shape of the fingerpad is very close to a quarter of ellipsoid, a point on the fingertip canbe represented with u = asinφ cosθ v = bsinφ sinθ w = ccosφ. (1) For each fingerpad, a, b, and care fixed and the pair (θ, φ) can sufficiently represent any point on the fingerpad with −π/2 < θ < π/2 and 0 < φ < π/2, as illustrated in Figure 6. φ is thecolatitude, or zenith, and θ is the longitude, or azimuth. To furtherreduce thecomplexity, weassume the Fig. 4. A fingerpad is rolled on the fingerprint sensor, a set of fingerprint images are taken to construct a fingerprint map. fingerpad is a spheroid, which indicates a = b = r and a point on the fingertrip canbeexpressed as Figure 4 shows a fingerprint sequence captured during a u = r sinφ cosθ fingerpad rolling of one subject. The fingerprint foregrounds v = r sinφ sinθ are segmented from the background and then stacked to- w = ccosφ. (2) gether. The pixel values at overlapping areas arecomputed by averaging the pixel values of the overlapping foregrounds On the other hand, thecontact position canbe identified [18]. Figure 5 shows thecomposed fingerprint map from on the fingerprint map thatis an unwrapped mapof the the fingerprint sequence in Figure 4-A. Theenhancement fingerpad surface. We define the origin (0, 0) of fingerprint algorithm proposed in Ref. [8] is used to remove noise map with thecore point of the fingerprint, the x axis and and increase robustness. An enhanced fingerprint map is the y axis along the short and long side of the fingerprint displayed in Figure 5-B. respectively as illustrated in Figure 7.

1116 Figure 8-A shows a fingerprintimage from the fingerprint sensor. The fingerprint area canbeextracted from the background with imageclosing and erosion asshown in Figure 8-B. The centroidof the fingerprint area in the image can becomputed and used as thecontact position relative to the force sensor.

Fig. 7. The deﬁned coordinate system on the ﬁngerprint map

When weconstructthe fingerprint map, as the long side of the fingerprint sensor is aligned with the W axis in the fignertip coordinate (Figure 6), the rolling behavior on roll direction relates theangle θ to the latitude on the fingerprint map with Fig. 8. (A) A fingerprintimage. (B) Thearea of the fingerprintin the image. x = 2 π r θ, (3) and the rolling behavior on the pitchdirection related the B. Contact position and Orientation on the Fingertip colatitudeangle φ1 to the y axis in the fingerprint map with theelliptic integral of the second kind The fingerprint obtained with the fingerprint sensor can be registered onto the fingerprint mapdescribed in Section φ1 2 − 2 III-B with many different registration approached [3]. Then − c r 2 y = c 1 2 sin φ dφ, (4) !0 " c thecontact position and orientation on the fingerprint can be obtained from the registration result. Howeverregular as y is the lengthof theelliptical arc with φ1 degree. registration approaches require featureextraction and corre- For acontact position (x,y) located in the fingerprint lation matching that request a long computation time. In this map, its θ and φ in the fingerpad coordinate system can paper, we utilize the fingerprint features, core point and local becomputed with Equations 3 and 4. The parameter c and ridge orientation around thecore point, todevelop a simple r are calibrated with θ = 0, θ = π/2, φ = 0, and φ = π/2. approach to estimate the fingerprint position and orientation. Since it is verydifficult to compute thecolatitudeangle φ 1 Thecore pointin a fingerprintis defined as the point of from an arc lengthof an ellipse, we retrieve the φ from a maximum ridge linecurvature. After fingerprint enhance- φ − y table pre-computed with numericalintegration [6]. ment, an automaticcore point extracting approachusing acomplex filter[13] is implemented and applied on the IV. CONTACT CHARACTERIZATION fingerprintimage to obtain the position of thecore point and When a fingertip contacts a force sensor, thereare number itsspatial orientation. Asshown in Figure 9, thecore point of variables. Most research studies have only included the in the fingerprint map and an arbitrary fingerprintimage is force and torque without considering thecontact position detected. For example thecore point of the fingerprint map or orientation on the force sensor or on the fingertip. With a is at (167,149) and core point of thearbitrary fingerprintis fingerprint sensor, the FFPO sensor system can measure three at (143,111) from the left-bottom corner. additional contact characters:thecontact position relative to The local ridge orientation at a pixelis defined as theangle the force sensor, thecontact position on the fingertip, and α of the fingerprint ridgeatthat point crossing through a the relative orientation between the force sensor and the small neighborhood centered atthat pixel. Since fingerprint fingertip. ridges are not directed, the direction angle α is defined in the range of [0 180]. The simplest and most natural approach A. Contact position Relative to the Force Sensor for extracting local ridge orientation is based on computation Since the fingerprint sensor is rigidly attached to the force of gradients in the fingerprintimage. Asshown in Figure 10, sensor and aligned as described in Section II, thecontact the orientation around thecore pointin both the fingerprint area on the fingerprint sensor canbe used to compute where map and thearbitrary fingerprint canbe obtained. Their angle thecontact force is applied without assuming that it always difference α canbecomputed as the rotation angle. goes through the originor thecontact area covers the whole With the translation and rotation between the new finger- sensor surface. For many humangrasping studies, it is very print and fingerprint map, a point p" = (x",y") in an arbitrary importantto knowwhereexactly thecontactis on the force fingerprintimage canbe transformed to a point p = (x,y)in sensor. the fingerprint map with

1117 (Px,Py,Pz) thatis relative to the center of the force sensor and it is a variable during a grasping. The fingerprint sensor can also measure thecontact position (φ,θ) and orientation α relative to the fingertip. Now the force (Fx,Fy,Fz) and torque (Tx,Ty,Tz) canbeexpressed in the human finger coordinate system.

V. EXPERIMENT RESULTS AND VERIFICATION Three subjects have tested the new FFPO sensor. Subjects used their index fingers topress on the FFPO sensor system. The fingerprint sensor of the FFPO sensor collected the fingerprintimageat 10 fps and the force sensor measured Fig. 9. Thecore point automaticallydetected with acomplex-filter-based the force at 1000 Hz. With our software, it took roughly 30 approach. seconds to build the fingerprint map and then the sensor is ready to measure the force/torqueand thecontact features. Subjects wereasked to randomlyplace their fingertip on the fingerprint sensor and apply arbitrary force as theydesired. As expected, the force sensorreading is not affected by the fingerprint sensor after initial force sensor offset calibration. Thecurrent software can estimate thecontact position and orientation within one second for each fingerprint. The main computation time is consumed by thecore pointlocalization and it depends on the resolution of the images and the precision requirement. Totally 1500 fingerprintimages werecollected. Because the ground truth aboutthe positions of thecore points in the fingerprintimages were not known and it wouldbea tedious task to manually find thecore points and record Fig. 10. The local ridge orientations around thecore points. their positions in all the images, we randomly selected 100 fingerprintimages out of 1500 images to estimate the accuracy. Thecore points in those images were manually labeled and their x and y pixel positions were recorded − " " − " " − (p pcore) = R(α)(p pcore) + (pcore pcore), (5) and compared with the results of thecore pointlocalization approach we used. We found thattheaverageerror was and within6pixels and the fingerprint sensor has 500 dpi, so cos(α) sin(α) R(α) = (6) on average, theerror was within6/500 inches that was 0.3 −sin(α) cos(α) # $ mm. " where pcore and pcore are thecore points of the fingerprint Thereare several factors that could affectthe resolution map and thearbitrary fingerprint respectively; α is the of the fingerprint registration result. Among them, the most relative rotation angle between the fingerprint map and the important ones are the resolution of the fingerprint sensor arbitrary fingerprint. Now the center point of thearbitrary and the feature ofridges and valleys in the fingerprint which fingerprint canbe registered to the fingerprint map with are different between human subjects. The resolution of Equation 5. Then with Equation 3 and 4, thecontact property the fingerprint sensor directly affects the resolution of the canbe fully characterized. finger contact resolution. However, since thecomputation time of core pointlocalization approach is correlated with C. Contact Characterization the fingerprint resolution, it is a trade-off between the spatial With the force sensor and the fingerprint sensor, not only and temporal resolutions. the force and torque level but also thecontact position relative to the force sensor, thecontact position relative to VI. DISCUSSIONAND FUTURE WORK the fingertip, and the relative orientation between the force This paper proposed a novel fingertip force and position sensor and the fingertip canbe measured. A contact force measurement apparatus, FFPO sensor, that can measure between the human subject’s fingerpad and the objectin thecontact position and orientation on the force sensor the world canbecharacterized in both the human subject’s and the fingertip, as well as thecontact force. In many coordinate system and the world coordinate system. grasping or manipulation studies, with this device, not only Thecontact force (Fx,Fy,Fz) and torque (Tx,Ty,Tz) are not thecontact force but also thecontact spatial features on assumed tobealways applied on the center of the force sen- both the fingerpad and the sensor canbe obtained without sor. The fingerprint sensor measures the real contact position sacrificing any precision or dimension. It allows roboticists

1118 and neuroscientists to characterize humanhand locomotions [2]Ingvars Birznieks, Per Jenmalm, Antony W. Goodwin,,and Roland S. in more detail. Johansson. Encoding of direction of fingertip forces by human tactile afferents. Journal of Neuroscience, 21:8222–8237, 2001. In this paper, thecontact position relative to the sensor is [3] L.G. Brown. Image registration techniques. ACM Computing Surveys, obtained with the centroidof the fingerprint region, and the 24(4):326376, 1992. contact position and orientation relative to the fingertip is [4] A. A. Cole, P. Hsu, and S.S. Sastry. Dynamiccontrol of sliding by robot hands forregrasping. IEEE Trans Robotics and Automation, obtained with the registration from the fingerprint reading 8:42–52, 1992. to the prebuilt fingerprint map. Three subjects has used [5] M.R. Cutkosky. Ongrasp choice, grasp models, and the design of this FFPO sensor. They all reported thatthe device was hands for manufacturing tasks. 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Theeffect of keyboard to include more subjects tobetter characterize the precision keyswitch make force on applied force and finger flexor muscle activity. Ergonomics, 40(8):800808, 1997. and resolution of this new device. We will develop new [18] Bolle R.M., N.K. Ratha, and J.H. Connell. Image mosiacing forrolled experimental approaches tovalidate the sensor system with fingerprint construction. In ICPR, pages 1651–1653, 1998. actual known contact spatial features. [19] M. Santello and J. F. Soechting. Force synergies for multifingered grasping. Exp. Brain Res., 133:457–467, 2000. In addition, the fingerprint sensor also allow us to measure [20] J. K. Shim, M. L. Latash, and V. M. Zatsiorsky. Prehension syn- thecontact area which is correlated to thecontact force ergies:trial-to-trial variability and hierarchical organization of stable and the tissuecompliance feature, which is an important performance. Exp. Brain Res., 152:173–184, 2003. [21] J. K. Shim, M. L. Latash, and V. M. Zatsiorsky. Prehension synergies study itself[14]. Our current work doesn’tinclude the skin in three dimentions. J. Newrophysiol, 93:766–776, 2005. plasticity and consider the fingerprintimageas non-distorted [22] C.L. Taylor and R.J. Schwartz. Theanatomy and mechanics of the by assuming thatthecontact force is normal (orthogonalto humanhand. Artificial Limbs, 2:22–35, 1955. [23]Inc. Tekscan. www.tekscan.com. the fingerprint surface) and the user doesn’t apply traction or [24]Inc. Upek. www.upek.com/solutions/government/sensor.asp. torsion. However when we study varieties of fingertip forces [25] G. Westling and R.S. Johansson. Responses inglabrousskin including shearforce, this assumption may not hold for many mechanoreceptors during perecision grip in humans. Exp. Brain Res., 66:128–140, 1987. cases. In the future, we plan to include the skinplasticity in [26] Kurita Y, Ikeda A., Ueda J., and Ogasawara T. A fingerprint pointing consideration and use the deformation in the fingerprintto device utilizing the deformation of the fingertip during the incipient helpus to model tissue features and provide more detailed slip. IEEE Transactions on Robotics, 21:801– 811, 2005. information aboutthe fingerpad contact features in a grasping and manipulation. It wouldbeaveryusefultool for studies in robotics, haptics, and neuroscience. VII. ACKNOWLEDGMENTS This research wassupported by the Universityof South Florida Seed Grant.

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