Position control system of the discontinuous feedback type

A control apparatus for positioning a dynamically controlled unit having at least a first axis of controlled movement, comprising in combination:

• actuator means operatively associated with said controlled unit and having discrete speed, forward and reverse drive modes for displacing said unit with respect to said first axis and having a stop mode;
• position sensing means operatively associated with said controlled unit for producing a signal (AP) representing the actual position of said unit with respect to said first axis;
• signal source means for producing a signal (DP) representing a desired position for said controlled unit with respect to said first axis;
• summing means connected to receive the AP and DP signals, said summing means producing an error signal (E);
• first and second signal processors connected between said summing means and said actuator means, said first signal processor comprising the subcombination of:
• (a) means for producing a predicted position signal (PP) in response to the AP and E signals and a constant factor related to a stopping time of said controlled unit that transpires when said actuator means undergoes a transition from one of said drive modes to said stop mode;
• (b) comparator means for comparing said signal PP and said signal DP, said comparator means having output means that has first, second and third discrete actuator drive signal conditions, said comparator means assuming said first signal condition at its output when said signal DP exceeds said signal PP by an amount greater than a predetermined deadband, and assuming said second signal condition when said signal DP is less than said signal PP by an amount greater than said deadband, and assuming said third signal condition when the difference between the signals DP and PP lies within said deadband;
• said second signal processor comprising the subcombination of:
• (c) signal averaging means for taking a time average of said error signal E over a predetermined interval T.sub.b, said averaging means being activated when said output of said comparator means of said first signal processor assumes said third signal condition, and said averaging means having an output at which a signal (AE) is produced representing a time average of said error signal at the end of said predetermined interval T.sub.b ;
• and,
• (d) function generator means having an input connected to the output of said averaging means and having output means that has first, second and third discrete actuator drive signal conditions corresponding to said first, second and third signal conditions of said comparator means of said first processor, said function generator means normally assuming said third actuator drive signal condition and being responsive to said averaging means so as to assume said first actuator drive signal condition when the average error signal AE is less than a predetermined negative threshold value and to maintain said first actuator drive signal condition for a time interval that varies as a function of the magnitude of said average error AE and so as to assume said second actuator drive signal condition when the average error signal AE is greater than a predetermined positive threshold level and to maintain said second signal condition for a time interval that varies as a function of the magnitude of said average error AE and then resuming third actuator drive signal condition;
• and,
• combining means for combining the actuator drive signal conditions of said first and second processors, said actuator means assuming said forward speed mode when either of said output means of said comparator means or said function generator means is in said first signal condition, and assuming said reverse speed mode when either of said output means of said comparator means or said function generator means is in its second signal condition, and assuming said stop mode when both said output means of said comparator means and said function generator means are in their first signal conditions.

A method of controlling the position of a dynamically controlled mechanism wherein a mechanism actuator is selectively operated in forward or reverse drive modes or a stopped mode by an actuator drive signal developed in response to signals representing desired position (DP), actual position (AP) and a position error (E) which is the difference between signals DP and AP, comprising the steps of:

• initially processing said DP, AP and E signals to effect a coarse positioning of said mechanism, the step of initially processing said signals comprising the substeps of:
• producing a predicted position signal (PP) that represents the actual position AP plus an additional factor representing a stopping distance of the mechanism when said actuator is caused to change from one of said drive modes, to said stop mode;
• and
• comparing the predicted position signal PP with the DP signal, and producing actuator drive signal that drives the actuator in one of its drive modes so long as the difference between the signals DP and PP is greater than a predetermined difference threshold, and producing an actuator drive signal that causes the actuator to assume its stop mode when the difference between the DP and PP signals is less than said threshold;
• and
• subsequently processing said signal E to effect an improvement of said coarse positioning, comprising the substeps of:
• receiving and taking an average of said signal E over a predetermined time to produce a discrete time average error signal (AE), and generating an actuator drive signal in response to said average error signal AE for causing said actuator to assume one of said drive modes, depending on the sense of the signal AE, for a discrete duration T.sub.c that is a predetermined function of the magnitude of the signal AE for removing an offset error in position that remains after said coarse positioning performed by the initial processing of said signals.