Patent ID: 8215673

Claim:
An apparatus for compensating for drift in a perceived position of a jack leg, the apparatus comprising: a jack including a jack base and a jack leg supported for extension and retraction relative to the jack base along a stroke having retraction and extension limits, the jack also including a jack drive operably connected to the jack leg and configured to drive the jack leg along its stroke; a controller connected to the jack drive and configured to command the jack drive to extend and retract the jack leg, and a position change sensor connected to the controller and positioned and configured to sense changes in jack leg position along the stroke and to provide corresponding jack leg position change signals to the controller, the controller being configured to track jack leg position by updating a perceived present leg position value in response to the position change signals; the controller being further configured to bias any leg position perception errors in a direction away from the retraction limit by occasionally applying a compensation factor to the perceived present leg position value, and to reset the perceived present leg position value whenever the jack leg reaches the retraction limit.