NASA has developed a technique designed to prevent cross-coupling in systems where two or more linear electro-mechanical actuators (EMA) are rigidly connected and are in danger of becoming cross-coupled. In such systems where the linked EMAs are commanded to achieve two distinct goals, such as position and load control, control problems often arise — especially at higher load and linear velocity levels. Both position and load control become inaccurate and in certain situations, stability of the overall system may be compromised. The NASA-developed approach mitigates the problem and achieves both accurate position following and desired load levels between the two (or more) actuators.
The solution rests on the idea of using homogeneous control loops to achieve heterogeneous, simultaneous objectives controlling for both load and position values in this case. As a first step, the actuators are synchronized, moving in unison, so the load cell will not register any load. The proper load is then generated by calculating an offset, or virtual position, that one or both actuators can achieve in position tracking to generate the right load in the coupled system. For example, both actuators could be moved to position 50 with zero load. Then, while one is kept at 50, the other could be moved to 49 (for a tensile load) or 51 (for a compressive load). The key computation is to determine the correct virtual positions for each of the actuators that will result in both the desired actual position and the desired load. The primary benefit of the described approach is that it allows stable control of highly dynamic systems and not just static ones. Complicated position profiles can be executed while simultaneously maintaining accurate load profiles.