Motor Control Analysis and Digital Motion Control

This consultant was contracted to evaluate and redesign an ultra-precise motion control system used on a prototype printed circuit board drill and router. This system incorporated a novel approach to both X-Y motion and the drill head Z-motion. The designer had fitted the drill system with sensors capable of detecting positions on all three axes to within a fraction of a mill but the controller they developed was unable to maintain X-Y position accuracy and the Z-axis drill rate and depth control due to control loop response latency. The original system was comprised of a PC based controller and user-built digital and analog I/O modules.

Expert Analysis

We determined that there were several problems with the current data system approach including, excessive interrupt response latency from the PC, improper analog sampling techniques that would not allow accurate phase measurements of the position sensors, and slow digital I/O.

We developed a custom data system based on a high-performance digital signal processor (DSP) and a custom field programmable gate array (FPGA). The FPGA provided very fast and flexible digital I/O to the existing motor controllers and was field re-configurable so modification and improvements could be made without redesigning the control circuit. The DSP was used to offload interrupt s form the PC and do dedicated digital closed loop control of all three motors. The DSP would seek and maintain programmed positions while PC was used to read the PCB net list and handle mass storage and a graphical user interface.

The system, using the new hardware and software, was successfully demonstrated and subsequently purchased by a large PCB equipment manufacturer to be produced and sold worldwide.

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