Review of Actuators with Passive Adjustable Compliance ...

The variation of the compliance isbased on the variation of the lengthof the lever arms and uses only onepassive element, which isfundamentally different from theantagonistic setup or structuralcontrolledstiffness.Note that the VS-Joint and the MACCEPA actuator are ina way complementary, because, in the VS design, the spring isused in compression, whereas the latter uses the spring in tension.Both designs use the pretension or preload of the springto vary the stiffness, and both have one motor for the stiffnesssetting and one for the equilibrium position.Discussion and ComparisonComparing the different designs is not easy, because eachdesign might be optimized for a specific task or property.Table 1 gives an overview of some of the properties for the differentgroups of controllable stiffness actuators.These properties of each group are a generalization.Within each group, there are differences, and the describeddesigns might be optimized for a specific application. Otherdesign criteria to be taken into account are the complexity ofthe design, cost, speed of the stiffness variation, maximumtorque, and size of the complete system. In addition, therequired power source, e.g., in case of pneumatic muscles, canalso be a criterion.ConclusionsFour concepts to create adaptable, passive-compliant actuators,which can store energy and can be used for passiveapplications, are presented and examples are given. Thefirst concept, equilibrium-controlled stiffness, is based onadjusting the equilibrium position of springs. This conceptis simple to control but constantly requires energy to regulateits actuator position. The second concept, antagonistic-controlledstiffness, covers the compliant actuatorsbased on the antagonistic setup of two nonlinear springs.Again, in this system, extra work is needed to adjust thestiffness, because two nonlinear springs must be controlled.Table 1. An overview of some of the properties for the different groupsof controllable stiffness actuators.1) Minimum number of springs required(can be more to obtain symmetry)2) Can linear springs be used (related toavailability, linear spring are standard)3) Always using the full length of the springelement4) Preload/Pretension in equilibrium position(= forces acting on joint in eq. pos.)5) Completely stiff setting possible (limitedby the force of the spring elongation)6) Vary compliant setting possible (conceptual,can lead to large designs)7) Infinite bandwidth for shock absorbance(compliant behavior at high speed)8) Infinite bandwidth for chosen compliance(stiffness controllable at high speed)9) Independent control stiffness andequilibrium position10) Possibility to vary linearity of the stiffnesscurveEquilibrium-ControlledStiffnessAntagonistic-ControlledStiffnessStructure-ControlledStiffnessMechanicallyControlledStiffness1 2 1 1Yes No Yes YesYes Yes No YesNo Yes No YesNo No Yes NoYes No Yes YesYes Yes Yes YesNo Yes Yes YesNo No/Yes Yes YesYes No Yes YesRemark on 4: This can be either positive or negative. Preload/pretension is positive to reduce or even avoid play and backlash;on the other hand, it generate forces on the structure and the bearings, which can lead to a heavier structure or wear.Remark on 9: Some designs of antagonistic setups allow independent control, however this results in a relatively complexdesigns.Remark on 10: In this way, a progressive, degressive, or linear system behavior can be chosen. In equilibrium-controlled stiffness,the motor feedback can be used to vary the desired stiffness curve as long as the control commands are within the bandwidth ofthe motor. In a Jack Spring actuator, feedback can be used to add or subtract coils for a given deflection allowing the linearityresponse to vary.92 IEEE Robotics & Automation MagazineSEPTEMBER 2009