The control system according to the present invention may be used in general for controlling the operation of a machine. In the following, the invention will be exemplified by reference to a steering system.
Ships and vehicles, and in particular off-highway machinery such as wheel loaders, excavators, dozers, articulated vehicles, tractors, harvesters and similar heavy duty machines often operate with hydraulic, electro-hydraulic, and electric steering systems. Typically, the steering system receives an operator input to indicate a desired angular movement of the ship or vehicle. The input could be provided via input means such as a joystick, a steering wheel, or the input could be derived from a GPS system or similar system for positioning and tracking. The input is converted into a signal for an actuator, e.g. a hydraulic cylinder, which moves a steered element, e.g. a wheel of a vehicle or a rudder of a ship. Steering systems are disclosed e.g. in U.S. Pat. No. 6,668,967 and in EP0856453.
In connection with so called “steer by wire” (in the following SBW) applications, a position feed back signal and a closed loop control system are normally applied to increase the accuracy and reliability. In such systems, a sensor is fixed to the steered element and feeds back e.g. an angular position of the element. Since faults in the sensor may result in malfunction of the steering system and in worst case in wrong movement of the steered element, the sensor is a critical component.
To increase safety, the sensing task may either be handled by several sensors in parallel or it may be handled by very high quality sensors or sensors with an internal fault diagnosis feature, and evidently, the sensing task is expensive.
U.S. Pat. No. 5,275,251 discloses a hydraulic steering in which a steering angle error is compensated. In this case, a wheel position and a steering wheel position are compared, and a correction signal is utilized for correcting the wheel position relative to the steering wheel position in case the steering wheel is rotated. The compensation of the steering angle error is effected at every steering handwheel speed and at any steering angle error. Since correction of errors occur at any speed and at any error, system reliability is very important, and there is a risk that the driver of the vehicle feels overruled.