Research Papers

Graphical Facial Expression Analysis and Design Method: An Approach to Determine Humanoid Skin Deformation

[+] Author and Article Information
Yonas Tadesse1 n2

Center for Energy Harvesting Materials and Systems (CEHMS), Bio-Inspired Materials and Devices Laboratory (BMDL), Center for Intelligent Material Systems and Structure (CIMSS), Department of Mechanical Engineering,  Virginia Tech, Blacksburg, VA 24061yonas@vt.edu;

Shashank Priya

Center for Energy Harvesting Materials and Systems (CEHMS), Bio-Inspired Materials and Devices Laboratory (BMDL), Center for Intelligent Material Systems and Structure (CIMSS), Department of Mechanical Engineering,  Virginia Tech, Blacksburg, VA 24061spriya@vt.edu


Present address: Mechanical Engineering Department, University of Texas at Dallas 800 West Campbell Rd., Richardson, TX 75080.


Corresponding author.

J. Mechanisms Robotics 4(2), 021010 (Apr 25, 2012) (16 pages) doi:10.1115/1.4006519 History: Received October 10, 2010; Revised February 23, 2012; Published April 25, 2012; Online April 25, 2012

The architecture of human face is complex consisting of 268 voluntary muscles that perform coordinated action to create real-time facial expression. In order to replicate facial expression on humanoid face by utilizing discrete actuators, the first and foremost step is the identification of a pair of origin and sinking points (SPs). In this paper, we address this issue and present a graphical analysis technique that could be used to design expressive robotic faces. The underlying criterion in the design of faces being deformation of a soft elastomeric skin through tension in anchoring wires attached on one end to the skin through the sinking point and the other end to the actuator. The paper also addresses the singularity problem of facial control points and important phenomena such as slacking of actuators. Experimental characterization on a prototype humanoid face was performed to validate the model and demonstrate the applicability on a generic platform.

Copyright © 2012 by American Society of Mechanical Engineers
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Figure 11

Various facial emotions from the prototype face developed using shape memory alloy actuators. The size of the skull is 140 × 90 × 110 mm3 . (a) AUs on the skull, (b) normal face, (c) AU2L (risorius), left cheek stretched (smirk), (d) AU3 (philtrum), upper lip up (sneer action), (e) AU8 (procerus), both brow raised (concerned), (f) AU10 (lateral pterygoid + digastric), mandible pulled down (mouth opened), (g) AU9 (outer frontalis), right eye brow lift up (mischievous/ nervous), (h) AU10 + AU2L, left check and mandible opened (“eh”), (i) AU8 stretched (worried), (j) AU11 + AU8, surprised and angry, (k) AU10 shocked, (l) AU3 + AU11 “uhh.” The expressions (i)–(l) are when the upper head is stretched (AU9 and AU7 stretched).

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Figure 1

Baby humanoid face: (a) architecture of skin–actuator-structure interconnection, (b) back-side view after assembly, and (c) front view after assembly

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Figure 2

Illustration of GFEAD method using contractile actuators and supporting skull structure. Steps (a)–(j) demonstrate the method for case I. (a) Two views of the skull showing an anchor point and two sinking points. (For convenience in analysis and to focus on the left cheek, the usually considered side view of the skull was selected to be the front view.). (b) Schematic diagram of the skull boundary and contractile actuators (OA = SMA1 and OB = SMA2 ). The respective distances of anchor point, sinking point, and terminating points from reference line are also shown. (c) Step I: Construction of auxiliary plane [A1] that provides the true length of OA. A reference line (B-1) is drawn parallel to OF AF and distances (g, h, and i) are mapped from the bottom plane [B]. (d) Step II: Construction of auxiliary plane [A2] to find the edge view of plane O1 A1 B1 and introducing auxiliary plane [A3]. (e) Step III: Mapping the positions of point A3 , B3 , and O3 on plane [A3]. The exact shape of the plane A3 B3 O3 that holds the SMA actuators can be obtained on plane [A3]. (f) Step IV: Construction of circles on auxiliary plane [A3] to find the intersection points when one of the SMA2 actuator (O3 B3 ) deforms 4% and the other SMA1 (O3 A3 ) remains undeformed. (g) Step V: The position of the anchor point before actuation (O3 ), after actuation (O3a ), and the apparent deformation (O3a O3 ) observed in auxiliary plane [A3]. (h) Step VI: Projecting back O3a to the auxiliary plane [A2] to obtain O2a . (i) Step VII: Propagating the position of the anchor point O2a to auxiliary plane [A1] and principal plane [F]. The positions are shown with black triangular marks and named as O1a and OFa . (j) Step VII: Connecting the actuated position of the anchor point OFa and the sinking points (AF and BF ) in the frontal plane.

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Figure 3

Facial expression analysis and design in case II. (a) Schematic diagram of the desired direction of movement, anchor point, and boundary of the skull in two views. (b) Step I: Picking up any point on the directional line in the two principal planes (PF and PB ). (c) Step II: An orthographic view showing the true length of deformation line (OP) in auxiliary plane [A1]. (d) Step II: Orthographic view that shows the point view of the facial deformation vector in auxiliary plane [A2] (e) Step III: Construction of an edge view of a plane that holds the sinking and the anchor points with an angle γ from reference line 1–2 in auxiliary plane [A2]. (f) Step IV: Construction of a parallel reference line 2–3 to the edge view line in order to obtain the true shape of the plane that holds the sinking points and the anchor point in plane [A3] (g) Step V: Construction of arcs with radius R1 and R2 centered at O3 and Q3 , respectively, in auxiliary plane [A3] where R1 –R2 corresponds to the displacement of the SMA actuator. Alternative circles are also drawn with radius R3 and R4 keeping the R3 –R4 the same as the displacement of the SMA actuator. (h) Step VI: The candidate sinking points are the intersection of the arcs drawn in step V and marked as U3 , S3 , and T3 in auxiliary plane [A3]. (i) Step VI: Projecting back the candidate sinking points to the edge view line in auxiliary plane [A2]. (j) Step VII: Projecting back the sinking points to auxiliary plane [A1] by mapping equal distance i, j, and k from plane [A3] to plane [A1]. (k) Step VII: Projecting the sinking points to the bottom principal plane [B] by transferring equal distance (l, m, n) from plane [A2].

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Figure 4

(a) Analysis of facial expression on a prototype skull focusing on the right cheek movement. (b) Inset view of the front skull indicating three cases of positions and directions of control points (pink, maroon and green hollow circles).

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Figure 5

Illustration of the slacking phenomena

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Figure 6

Three different facial deformations on a prototype face, (a) and (b) upper SMA activated by 3.5 mm, (c) and (d) action unit AU2 activated resulting in 5 mm deformation, (e) and (f) both upper and lower SMA activated by 8 mm

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Figure 9

(a) GFEAD considering skin elasticity and number of actuators, (b) Zoomed view of the frontal plane considering skin properties

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Figure 10

(a) Arrangement of action unit and sinking points on a curved plane. (b) Discretization of action unit and sinking points on a curved plane.

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Figure 7

(a) Thickness-map of the baby face and left portion of the skin–skull assembly and contact between the skull and the skin, (b) schematic diagram of the inset of skin around the cheek, (c) friction contact only on the periphery, (d) supporting skull is partially touching the skin, and (e) supporting skull and skin contacts on the entire surface

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Figure 8

Effect of number of actuators and skin properties on the movement of control points




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