The knit textile bending actuators documented here consist of two layers of fabric sewn around a balloon. This section describes the primitive motions of a textile actuator, and discusses some of the design considerations involved in making your own actuator. Textile actuators can also be designed to produce multiple motions by adding additional layers and balloons. The adding a motion section describes this in more detail. 

As described in the title page, textile actuators produce motion when a balloon constrained by fabric is inflated. The air pressure from the balloon transmits its strain directly into all surfaces in contact with the balloon. In order to control and utilize this strain, the right materials need to be selected.

An extending actuator can be made by putting a one-way stretch fabric on top of another piece of one-way stretch fabric. See the motion primitives videos below to see an extending actuator in action.

In order to create a bending motion, a significant difference in axial stretch properties between the top and bottom layers is necessary. These materials are described in more detail in the fabric section. An woven fabric is shown below to the left and the same fabric has tension applied to the right. Notice that this fabric does not stretch.

Another way to utilize one-way stretch fabrics is to cut it at an angle. For example, cutting a fabric layer on the bias at an angle of 45° allows the stretchiest direction to twist the actuator when pressurized. See a twisting actuator below in the motion primitives videos.

Motion primitives:

A key feature of textile actuators is the low cost and ease of fabrication. Using inexpensive tools, such as a sewing machine and an impulse sealer, functional actuators can be realized. The fabrication process is absent of complex 3D molding and casting processes that are required to build elastomeric fiber-reinforced actuators. Two simple, but separate fabrication processes are needed to make a fabric pocket and a balloon. The following sections detail how to make each component and assemble the two once they are made.

The two most important aspects of a textile actuator's performance are its range of motion and its torque. It is important to characterize these two aspects of an actuator to get an idea of how much force is being applied for the specific application. When an actuator is inflated, the energy from the air pressure will move the actuator until it is either fully actuated or stopped in its path due to an external constraint. If an actuator is constrained from completing its full range of motion, the actuator will apply torque to the constraint.

Torque can be measured by constraining one section of the actuator to a static cradle and the other section to an arm connected to an assembly that can rotate freely until it is pinned to a static angle for each measurement. A 6 axis force/torque sensor is sandwiched between the arm and the rotating base to measure torque. In this case, both flexion and extension torque in a dual-acting actuator were measured at 25 psi. Measurements were taken at angles ranging from 0 to 120 degrees of rotation; the resultant plot can be seen below, right.

Notice that the flexion torque decreases with an increase in angle while extension torque increases with an increase in angle. This means that the maximum flexion torque is achieved when the actuator is straight (lowest angle) and the maximum extension torque is achieved when the actuator is bent to its highest angle.

Once the actuator is characterized with the method described above, the torque can be determined using an image and a known pressure in the actuator. This can be done by photographing the actuator at various pressures and using image processing techniques to track the angle of the actuator tip relative to the base.

This testing method allows the actuator to have unconstrained motion during inflation. By using this technique of characterizing and visually measuring torque, an actuator's performance can be better understood and tailor made to a specific application. Note that there is often a trade-off between torque and range of motion in textile actuators. Based on the application, it is important to design an actuator to not only produce the desired motion, but also produce desirable torque at intermediate and final states.

This case study describes the design of a soft robotic device for grasping assistance and is based on work carried out by Cappello et al. (2018).

Clinical Need

As discussed in the PneuNets glove case study, loss of grasping ability is a significant problem which negatively impacts quality of life in patients. The aim of the project described here is to provide a device that can be worn comfortably all day and assist the user in activities of daily living. 

Solution

The proposed solution is a glove that consists of 5 dual-action textile actuators to assist in closing and opening the hand. The resulting device weighs less than 100g and can be worn for long periods of time without bothering the user. Each actuator has two tubes, one for the flexion balloon and one for the extension balloon. These tubes run to a control box that has nearly the same components as the fluidic control board reconfigured compactly into a portable box.

Shown below is a clinical trial where EMG sensing electrodes were placed on the muscles that close (flexor digitorium superficialis) and open (extensor digitorium communis) the hand. The subject goes through the same motions with and without the glove on. The following figure displays muscle activity during these exercises. Notice that there is a significant decrease in muscle activity to produce the same closing and opening motions when the subject is wearing the glove as opposed to when the subject is not wearing the glove.