Variable impedance actuators: A review
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 to the time course of position , the impedance of inertia is (in the Laplace domain for simplicity):
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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