A new global model to characterise the dynamics of a pneumatic proportional-pressure valve for a biomechatronic application

Abstract

Traditionally, the pneumatic proportional-pressure valve dynamics has been characterised from its output pressure and input reference voltage relationship. However, in our application about the use of a pneumatic artificial muscle as actuator of a gait rehabilitation exoskeleton, where an adequate valve output pressure control is required to simultaneously attain a smooth joint movement and an efficient interaction force with the patient, it is also important to evaluate the effect on valve output pressure of the input airflow rate disturbance caused by downstream load variations during this actuator operation. In this paper we present a new global linear model for such application, considering the concurrent effect on valve output of both voltage and airflow rate inputs. From data acquired through an experimental setup and for reference inputs in the frequency range of human gait, two transfer functions describing the relationships between valve output pressure and their respective inputs were first obtained. Second, the individual models were combined in a global one, according to the superposition principle. Third, the resultant model was analysed in the time and frequency domains and finally, validated (MSE = 0.0499) and discussed, in the search for clues to design a suitable actuation system for this type of robotic device.