The digital injection of signals into machine feedback loops via incremental encoder interfaces

The paper outlines the methods available for the application of test inputs and the measurement of the response of computer numerical control (CNC) machines to changes in position set point. A new approach is described where a signal is injected digitally into the feedback loop of a CNC machine, and how this can be done using the compensation register of an incremental encoder interface board is considered. In principle, any signal can be added in this way, error correction, a test signal, etc. Indeed, several signals can be added 'simultaneously' into the various axes of the machine. However, the controller has built-in limits to the unpro-grammed moves that it will tolerate and will shut down the machine if these are violated. Hence, there is a limit in practice to the size of signal that can be injected, but the limit depends on the rate of change in the signal rather than its magnitude. Results are presented for a typical milling machine both for a step input and for system identification using pseudo-white-noise input. The method is very simple to apply on a machine having the necessary incremental encoder cards in the feedback loop, since no extra hardware is required