Fall-related injury is a significant health problem on a global scale and is expected to grow with the aging population. Laboratory-based perturbation systems have the capability of simulating various modes of fall-inducing perturbations in a repeatable way. These systems enable fundamental research on human gait and balance and facilitate the development of devices to assist human balance. We present a robotic, rope-driven system capable of rendering bumps and force-fields at a person's pelvis in any direction in the transverse plane with forces up to 200 N, and a 90% rise time of as little as 44 ms, which is faster than a human's ability to sense and respond to the force. These capabilities enable experiments that require stabilizing or destabilizing subjects as they stand or walk on a treadmill. To facilitate use by researchers from all backgrounds, we designed both a configuration with simpler open-loop force control, and another with higher-performance, closed-loop force control. Both configurations are modular, and the open-loop system is made entirely from 3D-printed and catalog components. The design files and assembly instructions for both are freely available in an online repository.