**Hang Yu \*, Jiulu Gong and Derong Chen**

School of Mechatronical Engineering, Beijing Institute of Technology, Beijing 100811, China; lujiugong@bit.edu.cn (J.G.); cdr@bit.edu.cn (D.C.)

Received: 16 July 2020; Accepted: 20 August 2020; Published: 1 September 2020

**Abstract:** Detecting small objects and objects with large scale variants are always challenging for deep learning based object detection approaches. Many efforts have been made to solve these problems such as adopting more effective network structures, image features, loss functions, etc. However, for both small objects detection and detecting objects with various scale in single image, the first thing should be solve is the matching mechanism between anchor boxes and ground-truths. In this paper, an approach based on multi-scale balanced sampling(MB-RPN) is proposed for the difficult matching of small objects and detecting multi-scale objects. According to the scale of the anchor boxes, different positive and negative sample IOU discriminate thresholds are adopted to improve the probability of matching the small object area with the anchor boxes so that more small object samples are included in the training process. Moreover, the balanced sampling method is proposed for the collected samples, the samples are further divided and uniform sampling to ensure the diversity of samples in training process. Several datasets are adopted to evaluate the MB-RPN, the experimental results show that compare with the similar approach, MB-RPN improves detection performances effectively.

**Keywords:** object detection; small object; multi-scale sampling; balanced sampling
