Abstract: To address dynamic trajectory accuracy requirements in ultra-precision machining of complex optical surfaces, a direct grating position signal acquisition method is developed using a power PMAC (programmable multi-axis controller). real-time recording of X/Z-axis grating coordinates is implemented through the NC system's native data interface, enabling on-machine evaluation of circular trajectory deviations. Compared with conventional ball-bar tests or laser interferometry, this approach eliminates the need for external sensors and avoids on-machine hardware installations. Experimental results demonstrate that during circular interpolation at a feed rate of 100 mm/min, the total trajectory deviation is quantified 3σ as ±5.5 nm (99.1% confidence), with X-axis and Z-axis deviations 3σ of ±4.1 nm (98.7% confidence) and ±3.7 nm (98.2% confidence) respectively. This method provides an efficient, low-cost detection solution for optimizing freeform surface machining processes, investigating dynamic error mechanisms, and implementing real-time compensation in ultra-precision machining, while offering performance visualization for cross-machine comparison and supplying foundational data for digital twin implementation.