The bi-axial testing system (BATS) is one of the few testing systems in the world capable of producing horizontal high-speed movement under high compression force conditions. It is commonly used for testing seismic isolation units. The force and displacement outputs in both horizontal and vertical directions are crucial for analyzing the mechanical behavior of specimens. While specimen displacements can be accurately obtained using conventional measurement instruments, measuring specimen forces poses challenges in most testing systems today. Because of the immense testing force requirements of the system and considerations of space and cost, installing external load cells is difficult. Thus, the force data are typically derived from actuator records through measurements from their internal differential pressure transducers. However, this approach introduces errors because the system’s inertial force, system friction force, and specimen outputs are recorded together without distinguishing between these three forces, particularly in high-speed tests or when specimen force outputs are low.