I. Field of the Invention
This invention relates generally to control of machines and particularly to eliminating relative misalignment of a plurality of driving means spaced along the length of a moveable member, the member being propelled by the driving means to move linearly transverse to its length and the misalignment being measured parallel to the direction of motion.
II. Description of Related Art
An example of machine construction giving rise to misalignment of driving means of a machine member, referred to as skew, is illustrated in FIG. 1. Machine member 20 is moveable along rails 24 and 26 as indicated by the double ended arrow "X". Skew is defined as a difference of the positions of the driving means or driven points of machine member 20 as measured relative to a common reference and parallel to the axis of motion. For example, in FIG. 1 skew is represented by a difference in the values of X' and X". Movement of member 20 is imparted by, respectively, actuators 28 and 30 driving through transmissions or gear trains 32 and 34, respectively, at distal ends 21 and 22. It will be appreciated that machine constructions may include additional driving means spaced along the length of member 20. The driving means may include, for example, pinions engaging racks on the rails to propel member 20. Other driving devices may be included in the driving means for driving member 20 such as, for example, nuts rotated by actuators and engaging screws mounted parallel to the rails.
Member 20 and driving means 28 and 30 are arranged to propel member 20 linearly along the rails 24 and 26, the rails being substantially parallel to one another and substantially perpendicular to member 20. Skew may introduce binding of member 20 against rails 24 and 26 as well as poor engagement of the driving means. Skew can result in unsatisfactory response of actuators 24 and 26 to the control thereof and may result in excessive mechanical wear or damage to member 20, rails 24 and 26 or the driving means. Skew may arise as a result of forces acting on member 20 during periods when actuators 28 and 30 are not energized and therefore not active to maintain positions of ends 21 and 22. In light of the adverse consequences of skew, it is desirable to eliminate skew before initiation of extensive motion of member 20.
It is known from U.S. Pat. No. 4,045,660 to return a machine member to a desired location following power interruption using values of measured position determined upon power loss and upon power restoration. It is known from U.S. Pat. No. 4,484,287 to restore a moveable machine member to a desired position following power interruption by storing position information in a nonvolatile memory upon power loss for recall upon restoration of power. From U.S. Pat. No. 5,013,988 it is known to use presettable counters in conjunction with absolute encoders to create and update absolute position data, the data being updated so long as power is applied to the measuring system. From U.S. Pat. No. 4,629,955 it is known for control of motion of a machine member driven at distal ends to vary servomechanism gain of the driving means to eliminate skewing therebetween. The control of this patent provides only for reduction of skew attributable to differences in loading on the driving means during execution of motion. Servomechanism position control effected in accordance with this patent does not provide for detection, reduction or elimination of skew between the driving means upon application or restoration of power.
The controls of the aforesaid references addressing restoration of position after power interruption all rely on absolute position information which is immediately available upon application or restoration of power. However, in the event sufficient absolute position data is not available on application of power, it must be determined from position transducers. Sufficient position data will not exist if, during a period when position control is disabled, positional changes are not monitored and the range of position measurement transducers is less than the potential magnitude of positional change or the output of the position transducers while the machine member is stationary do not provide any indication of position whatsoever. Position transducers of the latter type include incremental or semi-absolute encoders, which are favored for providing high resolution position measurement over extended distances.