Shape 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 technologies 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 techniques 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 feedforward 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 measurements 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.
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