Dual loop control is often used to improve the performance of a motion control system. Although this appears more complex at first look, the overall system cost and complexity needed to reach the desired level of precision may often be significantly reduced. In the example system, mechanical stiffness was improved by a factor of approximately 100 by the use of a secondary feedback device.In the case of a single loop lead screw system, the feedback is usually located on the back of the motor. Each element in the system between the sensor and the load adds to the uncertainty. The motor mounted feedback is blind to torsion of the shaft with load. The coupler between the motor and the lead screw distorts under load, causing error between the motor shaft and the lead screw. The lead screw itself can also exhibit torsion under load, varying with both load and position of the nut with respect to the driven end of the screw. The lead screw itself typically has periodic error which can be very expensive to minimize, and the nut backlash minimization can add load as well as wear to the system. Finally, the thrust bearings’ accuracy also directly affect the system. The result is a system in which many elements can degrade the performance, and tightening the system can be expensive. As the feedback only senses the back shaft of the motor, much of what happens after that is unknown to the control system and thus not able to be corrected.

Adding a second feedback sensor (the optical track shown in insert) that measures close to the end effector of the system can significantly improve the performance while reducing the cost. Looking at the above system again, most of the error contribution is inside the secondary control loop; this error can be measured and thus corrected. With a good gain margin, the system error can be brought down nearly to error of the secondary feedback sensor. Rolled lead screws can replace ground lead screws, with their thread pitch variation, both cumulative and periodic, compensated for by the use of the additional feedback sensor. Differential thermal expansion coefficients of various elements may also be compensated.

For effective dual loop stiffness, a good system bandwidth is still needed. Elements of the system should be selected to minimize lost motion. The tighter system reduces the corrective action required of the control system just to hold a stationary position. Other periodic errors, such as lead screw pitch variation and thermal effects do not affect system stability and are well tolerated, but if significant, may require more control effort. The contribution of the errors within the loop are typically reduced by approximately the gain of the system, so a high gain system is desired.

The velocity feedback provides the phase stability for the system, as the position feedback is more than 180 degrees behind the phase of the supplied torque (180 degrees is the most optimistic). This velocity feedback is typically taken from as close to the motor to allow sensing and control of the energy being put into the system while lost motion or backlash is being corrected.

The IBEX lead screw system tested uses a 0.1-micron secondary feedback, direct driven by a QuickSilver hybrid servo with integral resolver – Mosolver – to provide 32,000 count resolution at the motor, corresponding to .25 micron per count. The closed loop system holds position within +/- 2 counts when adding a 20 Newton (5lb) load. The motor corrects the compliance in the system under this load by moving approximately +/-225 counts (2.5 degrees) when the load is applied. With the same system operating single loop, the motor will hold steady within +/-2 counts as a 20 Newton (5 pound) load is applied in either direction, while the linear scale shows a movement of approximately +/- 120 counts (12 microns). Thus, a properly implemented dual loop system excels not only in compensating lead screw tolerances, but also in significantly stiffening the resulting system under load.

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