# Scattering or Pushing for Object Singulation in Cluttered Environment: Case Study with Soma Cube
## 1 Introduction  
Picking up objects among clutter seems easy for humans because the human brain instantly and intuitively determines different ways to complete the task. However, even simple tasks are more complicated for robots. Assume that many objects are cluttered in the environment. Using non-prehensile manipulation, there are two ways to manipulate objects: quasi-static and dynamic manipulation methods. In case of quasi-static manipulation (i.e., pushing for object singulation), there is always a contact between the object and the manipulator. Usually, the quasi-static manipulation consumes time and as a result we need multiple pushing to get the desired singulation between objects. Pushing approach to grasp ungraspable objects inspired by human strategies has been surveyed [1]. Identification of smooth surfaces using vision based algorithms has been proposed for singulation of objects [2]. Various pattern recognition and image segmentation techniques have been proposedfor singulation of object in the cluttered environment [3]. Pushing objects at their center of mass in the target direction to isolate the object in the cluttered environment is proposed as an alternative approach for manipulation [4]. [PERSON] proposed push planners for various tasks related to non-prehensile rearrangements [5]. Physics engine based robotic pushing has been proposed for different scenarios. They utilized object tuning and modelling for the accurate prediction of the behaviors of the object [6]. Reinforcement learning based on the lateral pushing has been used as a technique for singulation of the object in clutter. Robust object grasping in clutter was presented [7]. Studies have been conducted to predict the rigid behavior of the object when manipulated [8; 9; 10]. Similarly, self-supervised learning methods have been proposed for grasping objects [11; 12]. Few studies used pushing to determine kinematic model of articulated objects and they used the assumption of known segmentation of objects [13; 14].  
On the other hand, in the dynamic manipulation (i.e., scattering for object singulation), the object keeps moving even after losing the contact with the manipulator. Collision-based scattering is one example of dynamic manipulation to achieve the desired target in short time. [PERSON] et al. [15] proposed a scattering based singulation technique to isolate the objects cluttered in the environment. Using the virtual world simulator based on the dynamic model, they estimated physical properties of all objects in the environment. Based on the desired input from the virtual world, real world scattering could be performed. Approaches related to impulse between robot and the environment have been studied [16; 17]. However, dynamic and quasi static manipulations alone cannot achieve the desired object singulation for every possible case. In some cases, combination of dynamic and quasi-static manipulation would be beneficial for effective object singulation. Therefore, this paper proposes a hybrid approach combining dynamic and quasi-static manipulation. This paper is organized as follows. Section 2 explains the methodology of the proposed approach. Modeling and identification of impact are introduced in Section 3. Parameter estimation and dynamics-based simulator are presented in Section 4. Section 5 explains the detail related to dynamic, quasi-static manipulation, and hybrid approach for singulation. Comparison between hybrid approach and pushing only approach is discussed in Section 6. Finally, discussions in 7 and conclusions are drawn in Section 8, respectively.