Optimal digital controller design for a servo motor taking account of intersample behavior

A continuous-time plant with a discretized continuous-time controller does not yield stability if the sampling rate is lower than some certain level. Thus far, high-performance electronic control has made use of expensive hardware needed to implement discretized continuous-time controllers; low-cost hardware generally does not have a sufficiently high sampling rate. This technical note presents results comparing performance indexes with and without intersample behavior, and provides some answers to the question of how a low-specification device can control a plant effectively. We consider a machine simulating wafer handling robots, which is an electromechanical system driven by a direct drive motor, at a semiconductor factory. We illustrate the controller design for the robot with and without intersample behavior, and present simulations and experimental results using these controllers. Taking intersample behavior into account proves to be effective in improving control performance and makes it possible to choose a relatively long sampling period. Controller design in terms of the performance index with intersample behavior makes it possible to deal with situations in which a sufficiently short sampling period may not be employed, and freedom of controller design may be widened, especially regarding the choice of the sampling period.