Beam focus control system for optical record carrier scanning apparatus

Abstract

A focus control system for optical record carrier scanning apparatus includes a mechanical actuator (13) for moving the focus of the scanning beam relative to a recording layer of the record carrier, a focus-error detection circuit (15, 16, 17) for generating a focus-error signal (FE), and a control circuit (26) for generating a first control signal (AS') for the actuator (13) which is proportional to the focus-error signal (FE). In order to ensure that the actuator is within its generating range when it locks-in to the focus-error signal, the system also includes a lock-in circuit (20) having a signal generator (21) and an integrator (23) for generating an additional control signal (AS') for the actuator (13) prior to lock-in. A lock-in detection circuit (25) generates a lock-in signal (IN FOCUS) when the focus error is in the operation range, which causes a switch (22) to change-over the input to integrator (23) to the focus-error signal (FE) and also enables the control circuit (26) to provide the first control signal (AS'). Thus, after lock-in the integrator (23) and the control circuit (26) constitute a controller for controlling actuator (13) in accordance with both the focus-error signal and the integral thereof. This compensates for low-frequency disturbances and renders the control system more immune to variations in the parameters affecting correct beam focus position.