Evaluation of three force-position hybrid control methods for a robot-based biological joint-testing system

Area of Science:

• Robotics in biomechanics
• Orthopedic biomechanics
• Control systems engineering

Background:

• Robot-based joint-testing systems (RJTS) enable unconstrained laxity tests, measuring joint stiffness.
• Previous force-position hybrid (FPH) control methods focused on anterior/posterior knee laxity, with limited evaluation for valgus/varus movements.
• The applicability of FPH control for diverse degrees of freedom (DOF) in knee laxity testing requires further investigation.

Purpose of the Study:

• Develop a 6-DOF RJTS using an industrial robot.
• Propose novel force-position hybrid control methods: force-position alternate control (FPA) and force-position hybrid control with force-moment control (FPHFM).
• Evaluate and compare the performance of FPH, FPA, and FPHFM for anterior/posterior and valgus/varus knee laxity tests.

Main Methods:

• An industrial 6-DOF robot equipped with a 6-component load cell was utilized for the RJTS.
• FPH, FPA, and FPHFM control methods were implemented and evaluated for unconstrained anterior/posterior and valgus/varus laxity tests.
• Performance metrics included control iterations, total time, and constraining forces/moments, compared against traditional constrained tests (CT).

Main Results:

• FPHFM, FPA, and FPH significantly reduced constraining forces and moments compared to CT for both anterior/posterior and valgus/varus tests.
• FPHFM demonstrated the lowest root-mean-squared constraining forces (<2.3 N) and moments (<0.14 Nm) across flexion angles.
• FPHFM and FPA achieved greater load reduction than FPH but required more control iterations and increased total testing time.

Conclusions:

• Force-position hybrid control with force-moment control (FPHFM) is the optimal method for unconstrained knee laxity testing when increased test duration is acceptable.
• The study provides valuable insights for selecting appropriate force-position hybrid control strategies for RJTS in clinical settings.
• FPHFM offers enhanced safety and accuracy in robotic knee laxity assessments.