No abstract

This is a postprint version of the following published document: Villoslada, A.; Escudero, N.; Martín, F.; Flores, A.; Rivera, C.; Collado, M.; Moreno, L. (2015). Position control of a shape memory alloy actuator using a four-term bilinear PID controller. Sensors and Actuators AbstractShape memory alloy (SMA) actuators have a number of appealing features, such as their low weight or their high force-to-weight ratio, that make them a potential alternative to traditional actuation tech-nologies in fields such as space applications, surgical devices or wearable robotics. In this paper, a type of bilinear controller consisting of a conventional PID controller cascaded with a bilinear compensator, known as BPID, is proposed. Bilinear controllers are a subset of nonlinear controllers, which is why the BPID may be a promising alternative to control the position of a SMA actuator. Nonlinear control tech-niques are commonly applied to control SMA actuators, because of their nonlinear behavior caused by thermal hysteresis. The BPID controller is simpler and easier to implement than other nonlinear control strategies, which makes it a very appealing candidate to control SMA actuators. The performance of the BPID controller has been compared with other two controllers, a conventional PID and a commuted feed-forward PIPD, controlling a real SMA actuator. To this end, a set of five tests has been defined, in which the controlled actuator must follow a series of position references. From these tests, the position and error of the actuator have been plotted, and a series of metrics has been computed to have quantitative mea-surements of the performance of the three controllers. It is shown that, in most of the experiments, the BPID has a better performance than the other two tested controllers, especially tracking step references. However, the power consumption is slightly higher when the actuator is controlled with this strategy, although the difference is minimal. Also, the BPID imposes greater energy variations to the SMA actuator, which might affect its service life. Overall, the BPID controller has proved to be a viable alternative to control SMA actuators.

Human exploration of the Solar System is one of the most challenging objectives included in the space programs of the most important space agencies in the world. Since the Apollo program, and especially with the construction and operation of the International Space Station, extravehicular activities (EVA) have become an important part of space exploration. This article presents a soft hand exoskeleton designed to address one of the problems that astronauts face during spacewalks: hand fatigue caused by the pressurized EVA gloves. This device will reduce the stiffness of the spacesuit glove by counteracting the force exerted by the pressurized glove. To this end, the system makes use of a set of six flexible actuators, which use a shape memory alloy (SMA) wire as the actuating element. SMAs have been chosen because some of their features, such as low volume and high force-to-weight ratio, make them a suitable choice taking into account the constraints imposed by the use of the device in a spacesuit. Besides describing the different mechanical and electronic subsystems that compose the exoskeleton, this article presents a preliminary assessment of the device; several tests to characterize its nominal operation have been carried out, as well as position and force control tests to study its controllability and evaluate its suitability as a force assistive device.

This paper presents a novel Shape Memory Alloy (SMA) linear actuator and its application driving the joints of an anthropomorphic robotic hand. Its main feature is that it is a flexible SMA actuator, it is designed so that it can be bent. The possibility of bending the actuator while preserving its capacity to produce a linear movement allows to use very long SMA wires, able to yield a great linear displacement, in a confined space. This feature provides better integration and adaptability than other existing high-displacement SMA actuators. The mechanical design of the presented SMA flexible actuator is based on the Bowden cable transmission system, using a multilayer sheath with a PTFE inner sheath to reduce friction losses and a stainless steel outer sheath that functions as a heat sink to reduce the cooling time of the SMA element. To test the SMA actuator with a real device, a robotic hand has been built using low cost rapid prototyping techniques from an open source design. The designed flexible SMA actuator has a great potential to be used in humanoid robots or robotic prostheses, due to its adaptability, output force, low weight and silent operation.