Compliant robots

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Compliant Actuators

To define what a compliant actuator is, a definition of a non-compliant actuator —better known as a stiff actuator—is useful. A stiff actuator is a device, which is able to move to a specific position or to track a predefined trajectory. Once a position is reached, it will remain at that position, whatever the external forces exerted on the actuator (within the force limits of the device).

A compliant actuator on the other hand will allow deviations from its own equilibrium position, depending on the applied external force. The equilibrium position of a compliant actuator is defined as the position of the actuator where the actuator generates zero force or zero torque. This concept is specifically introduced for compliant actuators, since it does not exist for stiff actuators.

Some remarks must be made concerning terminology. Since compliance is the opposite of stiffness, both terms are used to describe the compliant or non-stiff behavior of an actuator. To describe an actuator with a variable stiffness, the term adjustable compliance can be used, but also variable compliance, adjustable stiffness, and controllable stiffness are used. These examples are given to show that there is not a standard terminology (yet) to describe these types of actuators. Also the term Variable Impedance Actuators is sometimes found. Here impedance can be both compliance as also the damping factor.

Examples of compliant actuators

Passive compliant actuators contain an elastic element, e.g. a spring which can store energy, which is not the case for actuators with active compliance, where the controller of a stiff actuator mimics the behavior of a spring [1]. The latter has the disadvantage that no energy can be stored in the actuation system and due to the limited bandwidth of the controller, no shocks can be absorbed. An advantage of active compliance is that the controller can make the compliance online adaptable. Online adaptability means that the compliance can be adapted during normal operation. A famous robot that uses active compliance for safety is the Kuka/dlr leightweight arm [7].

The most well know example of a passive compliant actuator is the original Series Elastic Actuator SEA [17]), which is a spring in series with a stiff actuator. The compliance of this actuator is fixed and is determined by the selection of the spring; thus, the physical compliance cannot be changed during operation. To obtain variable stiffness, the virtual stiffness of the actuator is adjusted by dynamically adjusting the equilibrium position of the spring.

To combine energy storage and adaptable compliance, an elastic element to store energy is needed, together with a way to adapt the compliance. A substantial number of designs have been developed. An overview is presented in 'Van Ham et al. [21], some classes are:

  • Antagonistic-controlled stiffness: two actuators with nonadaptable compliance and nonlinear force-displacement characteristics are coupled antagonistically, working against each other. By controlling both actuators and using nonlinear springs, the compliance and equilibrium position of this antagonistic setup can be set. To be able to vary the compliance, it is required that the spring characteristic of the two actuators is non-linear, while the resulting spring characteristic is linear. Examples in this section includes the use of pneumatic muscles [4], the VSA joint [18], AMASC [11],. . .
  • Structure Controlled Stiffness: Unlike the previous concept, structure control modulates the effective physical structure of a spring to achieve variations in stiffness. When using a beam as elastic element, the stiffness depends on material modulus, the moment of inertia, and the effective beam length (Mechanical Impedance Adjuster [14]). By adjusting one of these parameters during operation, the stiffness can be controlled. In the Jack Spring [9], the number of active coils in a helical spring is adjusted to vary the stiffness.
  • Mechanically Controlled Stiffness: Similar to structure control, mechanical control also adjusts the effective physical stiffness of the system. However, in this case, the full length of the spring is always in use. The variation is done by changing the pre-tension or the preload of the spring, as is the case in the MACCEPA [22] and the VS-Joint [27]. These actuators require only one compliant element. The complete actuator behaves as a torsion spring where the spring characteristics and the equilibrium position can be controlled independently during operation.
  • Variable Transmission Ratio: Also by changing the transmission ratio differ- ent stiffnesses can be obtained. In the versatile energy efficient (V2E2) actuator [19] an energy efficient actuator is achieved by a combination of an Infinite Vari- able Transmission (IVT) and an elastic element. Another approach to change the transmission ratio is by using a lever mech- anism. A lever has three prin- cipal points; the pivot: the point around which the lever can rotates, the spring point: the point at which springs are located and the force point: the point at which the force is applying to the lever. By changing the position of one of the three one obtains a variable stiffness which is independent controlled from the equilibrium position. Examples here are the AWAS-I [13], AWAS-II [12] and vsaUT [25].

Robotic applications of compliant actuators

Industrial robotics focuses on the interface between motor and loads, which is “as stiff as possible”. This rule of thumb arose because stiffness improves the precision, stability and bandwidth of position control. A welding robot for ex- ample needs stiff actuators. But a precise tracking of a trajectory is not always that stringent anymore. New requirements arise to let a robot walk or run and those can be found in compliant actuation: good shock tolerance, lower reflected inertia, more accurate and stable force control, less damage during inadvertent contact, and the potential for energy storage. For robots that need to interact with humans safety is of primary concern. The biggest danger presents when working in close proximity with robots is the potential for large impact loads resulting from the large effective inertia (or more generally effective impedance) of many robotic manipulators.

So compliant actuators can be found in walking and running robots for energy efficiency since the can exploit the natural dy- namics like is found in passive walkers like Denise [26], Nagoya walkers , Flame [8], Lucy [23]. . . or because they can store and release energy like Mowgli [15], BiMasc [10],. . . Also for prosthesis like ankle-foot prosthesis like from H. Herr [2], AMPfoot,... uses compliant elements to walk energy-efficient.

However, passive compliance can greatly improve robot safety in cases of im- pact between a human and a robot. It also has the ability to store energy (very similar to loading a spring). However, when this energy is suddenly released, it can result in high speed motions of the robot, and correspondingly in a high risk for humans in case of collision [20]. So the control must be adapted to this as is done e.g. in a proxy based sliding mode controller. Examples of robots including compliance for safety are DLR hand/arm system [6], Meka Robot, Probo [5], Ecce Robot [16],. . . . Also for rehabilitation purposes as exoskeletons compliant actuators are used for safety and also adaptability to be able to give assistance as needed as is for example the case in Lopes [24], Knexo [3],. . .


Bram Vanderborght (Vrije Universiteit Brussel) [1]


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