Feedforward augmented sliding mode motion control of antagonistic soft pneumatic actuators

Abstract

Soft pneumatic actuators provide many exciting properties, but controlling them without the use of bulky and expensive flow-control valves can be difficult and computationally expensive. We seek a solution to this problem by introducing an inexpensive and reliable muscle-like linear soft actuator used antagonistically to operate a rigid 1-DoF joint, resulting in a system that combines the advantages of rigid and soft robotics. Using this setup, we performed precise motion control using a sliding mode feedback controller as well as a sliding mode controller augmented by a feedforward term to modulate the state of solenoid valves that drive each actuator. We found that both controllers performed equivalently well in following a step function and in responding to a disturbance. The feedforward augmented controller performed significantly better when following dynamic trajectories over a range of frequencies and with the addition of an external force. The next step will be to modify our valve control scheme to allow for the determination of both the position and stiffness of the joint, better leveraging the advantages of soft pneumatic actuators.

A composite soft bending actuation module with integrated curvature sensing

“…We previously demonstrated that average pressure inside soft actuation chambers can be regulated through pulse width modulation (PWM) using valves [1]. The segment is driven by two actuation chambers.…”

“…To control the curvature of the bending actuator, we adapted an iterative sliding mode controller from [1], the final control law of which is given as:…”

“…constraint layer (neutral bending axis) for curvature measurement. For segment control, we previously presented an approach that used pulse width modulation (PWM) of miniature solenoid valves to regulate pressure inside chambers [1]. Work in this paper utilizes this controller with embedded sensing to control the curvature of a soft segment.…”

System-level challenges in pressure-operated soft robotics

“…17 Similarly, we developed extensile fiber reinforced soft actuators called reverse pneumatic artificial muscles (rPAMs) that relieve contractile stresses upon pressurization to apply antagonistic forces on underlying kinematic chains with large displacement ranges. 18,19 If bending deformation is required (for a snake-like robot 15 ), a semi-circular cross-sectional area is used as well as an inextensible (e.g. fabric, plastic, paper) thin constraint layer on the flat edge such that axial stresses are converted to bending deformations 20 as shown in Figure 1.…”

“…30 Compared to traditional electromechanical actuation, our soft actuators are low-bandwidth systems, which increases the burden on control to achieve precise positioning, although the inherent safety of soft robotic systems compensates for small positioning errors without resulting in large contact forces. To decouple potential sources of error due to sensing difficulties in a soft body, we studied motion control on sensorized bench-top rigid kinematic modules, including a simple revolute joint operated by two antagonistic fluidic soft muscles, 18 and more recently, a 2-DoF universal joint module operated by three fluidic soft muscles 31 as shown in Figure 5-A. For operation, these muscles articulating the module are connected to a pressure source and each controlled by a separate miniature fast-response solenoid valve.…”