As a powder bed fusion additive manufacturing (AM) process, selective laser sintering (SLS) has seen widespread use with unique advantages in achievable part complexity and processable materials. However, greater applications of SLS AM remain hindered by insufficient assurance of the part qualities. A major barrier to such long-felt but not fully realized quality assurance rises from challenges in sensing and control mechatronics in presence of the multi-scale laser-material interactions. This paper presents the design of a full-scale, fully-accessible polymer SLS testbed with tailored mechatronics for in-situ monitoring and controls. We discuss approaches for controlling key process parameters and for measuring process signatures that are crucial for next-generation assurance of part accuracy and quality. We validated these designs in actual 3D printing of nylon powders among a tested list of low- to high-temperature polymers. Along this process, we (1) test a new scanning control to address a salient challenge in maintaining melting pool properties within each layer, and (2) reveal the characteristics of optic contamination as a process disturbance.