The sketches are presented to illustrate general concepts of common position feedback control schemes and should be viewed as a food for thought exercise. In most applications, work is being performed on or near the slide's mounting surface, either by moving a target on the slide with respect to a fixed tool or by moving the tool on a slide with respect to a fixed target (something moving & something stationary). In either case, motion control should be near the slide. Cost vs. precision generally dictates the feedback method used for positioning systems.

The top figure illustrates an open-loop stepper-motor driven linear stage (not offered by Anorad). With this approach, a control system sends a string of pulses to the motor which causes it to rotate. For example, 10,000 pulses to a 2,000 pulses/rev motor will cause it to rotate 5 times. Directly coupled to a 5-pitch lead screw, 10,000 pulses will cause the slide to move precisely 1.0 inch (maybe). Precisely 1.0 inch will only happen if the stepper motor rotates exactly 1,800° and the lead of the screw is "exactly 0.200" and there is no compliance (zero backlash) in the nut and the coupling is stiff and there is no lost motion caused by stiction in the bearing systems. All of these If, ands, or buts can be minimized by using very high priced precision components. Fortunately, there are less costly, alternative approaches to consider.

The second figure replaces the stepper motor with a servo motor/rotary encoder combination. Now the rotation precision can be sensed and corrections made if necessary. All of the other "and" parameters listed above are still present though.

The third figure replaces the motor-mounted rotary encoder with a linear encoder scale and a slide-mounted reader head. Though this bypasses all the potential error causing parameters mentioned above, position control is only good to the point of reader head and scale interfacing. Work is typically not performed at the encoder but somewhere above the slide's surface. As shown in an earlier section, angular errors could adversely affect precision at the point of work.

The fourth figure illustrates a method for bypassing error potentials all the way to the point of work. In this illustration, a laser provides position feedback where the work is being performed (at point A).

Fair Precision at point (A). Precision at Slide is questionable (due to stalling, backlash, etc.).

Good precision at encoder (A). Questionable precision at B (coupling, lead, & backlash errors).

Good precision at slide (A). Bypasses coupling, lead, and backlash errors.

Excellent precision at point of work (A). Bypasses coupling, lead, backlash, and pitch errors.

A direct-drive linear servo motor stage provides superior servo stiffness with action, reaction & sense all where it's needed - near the point of work.