Variable impedance actuators: A review

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Abstract

Variable Impedance Actuators (VIA) have received increasing attention in recent years as many novel applications involving interactions with an unknown and dynamic environment including humans require actuators with dynamics that are not well-achieved by classical stiff actuators. This paper presents an overview of the different VIAs developed and proposes a classification based on the principles through which the variable stiffness and damping are achieved. The main classes are active impedance by control, inherent compliance and damping actuators, inertial actuators, and combinations of them, which are then further divided into subclasses. This classification allows for designers of new devices to orientate and take inspiration and users of VIA’s to be guided in the design and implementation process for their targeted application.

Section snippets

What is a Variable Impedance Actuator?

To define what a Variable Impedance Actuator (VIA) is, it is useful to start by defining a non-VIA (traditional stiff) actuator. A stiff actuator is a device, able to move to a specific position or track a predefined trajectory. Once a position is reached, the actuator will hold this position, (ideally) whatever the external forces (within the force limits of the device). It is a position source, i.e. a system with a very high (ideally infinite) mechanical impedance. This behavior is obtained

Active impedance by control

Active impedance by control is when an actuator mimics the impedance behavior using software control  [12]. Based on the measured output state, a correction is calculated by the controller and set by the (stiff) actuator. This type of VIA has an actuator, sensor and controller that are fast enough for the application, but no energy can be stored and due to the limited bandwidth of the controller no shock can be absorbed (e.g. hitting with a bat will not be handled by the system with the desired

Inherent compliance

In contrast to active impedance by control, passive compliance contains a passive or intrinsic compliant element. This category can be sub-divided into mechanisms where the compliant element cannot change its stiffness (fixed compliance) with the variable impedance created by software control, and adaptable compliance systems where the stiffness is controlled by mechanical reconfiguration. The advantage here is that the very high (virtually infinite) bandwidth for the passive compliance can

Inherent damping

Inherently compliant actuators do have drawbacks: the mechanical resonance is decreased, compromising achievable bandwidth  [78], [79], [80], [81]. The introduction of two complex conjugate poles also creates a sharp increase in phase lag, decreasing the stability margin when controlling the joint for link quantities and making the control difficult on the motor side due to the introduction of an anti-resonance at the same frequency. This creates problems particularly in the position/velocity

Combinations of inherent compliance and damping

Some devices combine a variable damping actuator with an elastic element. To give an overview of the different possibilities consider a system described by: (1) one motor with an output shaft, (2) one containment frame, (3) one elastic connection between the motor and the shaft, and (4) one source of damping action. Since the list of system topologies grows exponentially with the number of elements considered, we limit our analysis to systems composed of those four elements only. This gives

Inertial actuators

Since impedance is defined as the differential operator relating the time course of reaction force F(t) to the time course of position P(t), the impedance of inertia M is (in the Laplace domain for simplicity): I(s)=F(s)P(s)=1Ms2,

So a mass can also be used as a storage of kinetic energy apart from a spring and damper. For example, in a hammer kinetic energy can be accumulated to drive a nail into a piece of wood  [104]. A spinning flywheel is employed to act as a gyroscope to stabilize the

Conclusion

Variable Impedance Actuators are under investigation to achieve safe, energy-efficient, and highly dynamic motion for powering the next generation of robots which have to collaborate with humans and interact with an unknown environment. The advances in VIA technology will pave the way towards new application fields, such as industrial co-workers, household robots, advanced prostheses and rehabilitation devices, and autonomous robots for exploration of space and hostile environments.

This paper

Acknowledgments

This work has been funded by the European Commissions 7th Framework Program as part of the project VIACTORS under grant no. 231554. From the second name, the authors are in alphabetical order.

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