High-Speed Scooping Manipulation
Using Controlled Compliance
Ka Hei Mak and Jungwon Seo
The Hong Kong University of Science and Technology (HKUST)
Clear Water Bay, Hong Kong
Email: khmakac@connect.ust.hk, junseo@ust.hk

t=0s
0.28 sFinger pf
1.73 sz

Thumb pt
Objectx0.61 s+
Virtualspring

surfacepdf

1.17 s1.62 spdt

Thumb ptψFinger pf
1.46 sFTψ1.38 s
pdf

pdt

G(c)
(a)
(b)
Fig. 1. (a) Our high-speed scooping performed by a direct-drive gripper, for picking a domino block off from a hard surface. The arm can practically slamthe gripper onto the surface while the object is being scooped. (b) Two-step procedure for scooping through stiffness control. All the bodies (finger, thumb,object, and surface) are assumed to be rigid. (c) Scoopability analysis following [2]. Due to the rigid contact constraints shown as the contact normals at thepoints F (finger-object), T (thumb-object), and G (ground-object), the object is kinematically unable to escape through the gap between F and G. In otherwords, the object at the configuration is only allowed to instantaneously rotate about a point in the region shaded red, counterclockwise (encoded by the “+”sign). Scoopability is valid as long as the triangular region shaded red does not vanish.

Abstract—The presented study addresses how to perform themanipulation of scooping over a wide range of objects at highspeed, termed as high-speed scooping (Fig. 1(a)). This is a
departure from existing solutions to robotic scooping throughunderactuation, with a limited ability to actively adapt to various
situations, or motion control, with dependence on accurate
geometric information. Critical to high-speed scooping are the capabilities for rapidly responding to contact interactions between
the object, robot, and environment under errors and uncertainties
regarding the geometric and physical properties of the bodies thatphysically interact.

I. H IGH -S PEED S COOPING : P RINCIPLESOne viable approach to address the challenges of high-speed
scooping is to have the robot emulates the mechanical systemthat interacts with its environment as desired, according to
the formalism of impedance control [3]. Specifically, here therobot is abstracted as a finger-thumb pair (see the point finger
and the flat-faced rigid thumb in Fig. 1(b)) and each digit,
assumed to move freely, is controlled to render a mass-springdamper system:mf p̈f + bf ṗf + kf (pf − pdf ) = ff

(1)

mt p̈t + bt ṗt + kt (pt − pdt ) = ft

(2)

Eq. (1) represents the mass-spring-damper dynamics on target
for the finger (the subscript ‘f’); Eq. (2) is for the thumb(the subscript ’t’). m, b, k, and f denote the mass, damping
constant, spring constant, and external force. p = [x z]T and
pd = [xd z d ]T represent the actual and the desired position,respectively. pd is also interpreted as the rest position of thevirtual spring in the dynamics model.
We then consider how to move the set points of thevirtual springs, pd , in order to achieve desired contact interactions. Fig. 1(b) depicts our two-step procedure. First (top ofFig. 1(b)), pdf and pdt are set to be located below the support
surface. This will bring the finger (thumb) into contact withthe object (surface). Second (bottom of Fig. 1(b)), pdt is set
such that the thumb’s tip coincides with pdf , which is kept
fixed, after the gripper makes contact with the environment.
This will cause the gripper to close and pinch the object.
The suggested strategy poses a question: how it is possible
for the thumb to initially get through between the surface andthe object. We hypothesize that if the thumb is harder thanthe object and approaches to the object with a sufficiently
small angle of attack ψ (Fig. 1(b)), the initial penetration isachievable. This point was also verified in our recent study[2]. Another issue to consider is how to capture the object
into a stable final pinch grasp. We place the finger pf on theobject such that the object is scoopable [2] (Fig. 1(c)); thenthe object cannot escape through the gap between the finger
and the ground, if the finger and thumb are fixed, and thus theUR10 Arm(a)Our overall hardware setting is shown in Fig. 2(a). A UR10arm carries our two-fingered custom-built gripper, inspired by[1], with four degrees-of-freedom (DOF) and direct-drive actuation. Each two-DOF finger is constructed from a symmetricfive-bar linkage actuated by two T-Motor GB54-2 motors withno gear train. Using the direct-drive gripper, we implement thestiffness control task in Eq. (1-2) as proportional-derivative(PD) motion control.
The timeline of the implemented process of our high-speed
scooping is instanced in Fig. 2(b). The details are as follows:
1) Approach (before time t = 1.12 s): The gripper ispositioned above the object and then slammed on thesurface (height unknown) as fast as around 0.45 m/s.
2) Interact with Environment (t = 1.12 s): The collision
between the gripper and the object/surface is detected byproprioception: measuring the displacement of the digit
linkages using the encoders of the motors. Meanwhile,
the angle of attack ψ at the thumb is sensed, also by theproprioception, and adjusted as needed. The arm moving
downward to slam the gripper is then accelerated upward
to stop it from pressing the surface too hard.
3) Interact with Object (t = 1.17 s): At the instant of thecollision detection, the gripper is commanded to closeby moving the thumb’s tip towards the fingertip withlarger stiffness values (see the plots in Fig. 2(b)). As
the gripper closes, the finger keeps pushing down on
the object and the thumb slides on the surface towards
the finger and finally penetrates under the object.
4) Scoop (t = 1.26 s): The object is forced to rotate about
the contact point with the ground surface and then enters
the workspace of the gripper.
5) Pinch grasp (t = 1.38 s): Finally, a pinch grasp isobtained. The stiffness values are further increased tosecure the object. The gripper itself is shown to move
upward by the arm with a positive z-speed.
Note that the duration of the dynamic interaction is around
0.3 s and the arm does not need to be completely stopped fora finite amount of time.
We applied the operation to a range of picking scenarioswith various objects1 ; see Fig. 2(c). The results show that ourhigh-speed scooping, with success rate 87/90, can be more
effective in picking thin objects than other approaches suchas ad hoc direct pinch grasping, with success rate 41/90, andapplicable to picking fragile objects too (see the picking ofthe cracker in the figure).
1 [Online video]: https://drive.google.com/file/d/
1qP3fuFKnlvbISacJgcBIA0HoCvugLJ-I/view?usp=sharing

Odrivemotor driver

Task objects
Surface(b)Motor joint
angles (degree)

II. H IGH -S PEED S COOPING : E XPERIMENTS

Direct-drive gripper

MotorP-gain

object will comply with the gripper’s closing motion.
Our approach for high-speed scooping offers clear advantages. No exact geometric information is necessary becauseof the compliant dynamics and the robustness of scoopability(Fig. 1(c)). Compared to rigid motion control or underactuatedcontrol, the contact forces between the bodies interacting canbe actively adjusted.

Time (s)FingerThumbxCommanded

ψ

zx

t=0s

1.12 s1.17 s1.26 s1.38 s1.50 s(c)Thin plastic card

CrackerFig. 2. (a) Experiment setting. (b) Timeline of high-speed scooping withthe time plots of the gripper’s parameters and a corresponding sequence ofphotographs. (c) Time-lapse sequences of picking a thin plastic card and acracker with high-speed scooping, shown to be more opaque with time.

R EFERENCES[1] Ankit Bhatia, Aaron M Johnson, and Matthew T Mason.
Direct drive hands: Force-motion transparency in gripper
design. In Robotics: science and systems, 2019.
[2] Tierui He, Shoaib Aslam, Zhekai Tong, and Jungwon Seo.
Scooping manipulation via motion control with a twofingered gripper and its application to bin picking. IEEE
Robotics and Automation Letters, 6(4):6394–6401, 2021.
[3] Neville Hogan. Impedance Control: An Approach to Manipulation: Part I—Theory. Journal of Dynamic Systems,Measurement, and Control, 107(1):1–7, 1985.