1. Introduction
Physical Human–Robot Interaction (pHRI), requires a technology to fabricate the robotic platform that is fundamentally different from industrial robots [
1]. Examples are surgical robots, prosthetic devices and exoskeletons. These platforms are supposed to physically interact with the soft tissues of the human body, where exerting forces outside the tissues structural limitations would lead to medical hazards. Industrial robots are designed for fast and accurate position control applications where everything about the environment is known and predictable. In pHRI, however, we need to take into account the uncertainties regarding the force interactions [
2]. An intrinsically soft robotic platform can provide a novel and safer option for biomedical applications [
3,
4].
The source of producing force into robotic platforms is actuators. Actuators have a critical role in the perceivable softness of the whole robotic platform. Recently, actuators with intrinsically soft bodies have been gaining interest among researchers and as a result different soft actuators have been proposed and fabricated. Pneumatic soft actuators [
5,
6,
7] were actually the first types of soft actuators that were introduced. In these actuators, air pressure inside a stretchable tube, usually made of Polydimethylsiloxane (PDMS) [
8], leads to inflate-deflate the tube and creates axial elongation-shrinkage. Therefore, this axial motion can create a desired amount of force. These actuators have high bandwidth, and output a relatively high amount of force and deflection. Looking at the whole picture, however, there has to be air pressure to be supplied by an air pump which is indeed bulky and rigid and also the actuator has to be tethered to the pump. This not only makes the entire robot’s body “not soft” but also very huge if the source of power is taken into account.
Another type of soft actuator is the shape memory alloy [
9,
10]. A shape-memory alloy (SMA) is deformed when it is cold; however, it regains its pre-deformed shape once it is heated. These actuators are indeed intrinsically soft and usually do not require a high amount of input power. However, they usually have very limited bandwidth, as it takes a long time for the alloy to cool down and retrieve its primary shape and compliance following electrical actuation.
Dielectric polymers [
11,
12,
13] are another type of soft actuators. These elastomers are in fact smart materials that generate large amount of strain when voltage is being applied to them which is converted into mechanical work. High elastic energy density while being lightweight are advantages of these actuators. However, the most limiting disadvantage is the extensively high amount of voltage (in order of 20 kV) that these actuators need to be activated.
Another type of recently developed soft actuators works based on liquid-gas transition [
14,
15]. In these thermo-active soft actuators a liquid with low boiling point is encapsulated inside a soft and stretchable tube usually made of PDMS. Generating heat by introducing electric current through a resistive element leads to increasing the temperature. The increased temperature changes the phase from liquid to gas and consequently large amount of volume changes. This large change in the volume inflates the soft tube and so motion and force are generated. The disadvantage of this actuator is, however, its very limited bandwidth and therefore, very slow response.
According to the Lorentz law a conductive element carrying electrical current in a magnetic field will experience a force acting perpendicular to both the magnetic field and direction of electric current. Electromagnetic working principle of traditional rigid actuators [
16,
17] has been implemented in another recent type of soft actuators. This type of soft actuators do not require high amount of input voltage as dielectric actuators do, and are fast in response and can develop considerably high amount of deflection. However, the challenge in these types of actuators is the relatively low amount of generated force. The reason is due to several factors: first of all, these actuators use a conductive liquid (usually Eutectic Gallium Indium or EGaIn) which have higher electrical resistance as compared to copper wires in traditional rigid actuators. Also, these actuators usually use flexible permanent magnets made by a mixture of PDMS and magnetic particles, which have lower magnetic strength as compared to rigid permanent magnets in traditional rigid actuators.
We have developed a novel Electromagnetic Soft Actuator (ESA) based on the working principle of solenoids with permanent magnets [
18,
19,
20]. In our ESA, electromagnetic field is created by applying electric current through a soft conductive coil. The coil is made of PDMS micro-pipe with diameter of around 0.1 mm filled by EGaIN. In our design, two coils are antagonistically embedded inside a PDMS body with a springy connection in between as it is shown in
Figure 1. Once electric current is being supplied the two coil can get magnetized based on the Lorentz law and attract each other. In order to intensify the electromagnetic field between the two coils, a flexible permanent magnet is placed inside the coils.
The permanent magnet is made by pouring a mixture of PDMS and magnetic particles into a 3D printed cylindrical-shaped mold, where the mixture in placed inside a strong external magnetic field while it is curing, to align the magnetic orientation of each particle. As a result, once the mixture is completely cured, the magnetic particles remain aligned even when the external magnetic field is abolished. The product is a cylindrical flexible material with magnetic properties.
This type of soft actuator is highly scalable where the scaling factor is determined by the available manufacturing technology. For example, the smallest PDMS micro-pipe available has a diameter around 0.1 mm, whereas using high-end 3D printers, PDMS can be printed as a coil-shape with embedded helical micro-channel inside with diameter in the order of micrometers. However, due to extremely high price of such 3D printers, this manufacturing technology is very expensive and only justifiable in mass production.
Scaling factors for several actuation technologies, such as electromagnetic, piezoelectric and electrostatics have been studied by other researchers [
21,
22]. It has been shown that for electromagnetic actuators the force is proportional to square of the length. This means that by decreasing the size of the actuator, the generated force will be reduced. However, interestingly we found that by scaling down the ESA’s size, its force to volume ratio increases. This was confirmed through experiments on different sizes of ESAs [
20]. This behaviour was also analytically proven using the geometry of the actuator and electromagnetic equations based on the Lorentz Law [
23]. The limitation on scaling down the size of ESA depends on available technology. Nano-Scribe 3D printer can create micro-scale size micro-channels but the cost of the 3D printer is about half of a million dollars.
This property of ESAs suggests that by reducing the size of ESAs and attaching them in series and parallel as a network, the output force can be enhanced compared to a single ESA with the same size of the networked ESAs. Due to their light weight, their fast response, and being able to be operated with voltage range around 10 V to 80 V, with increased output force, ESAs can reproduce a soft actuation technology suitable for pHRI, especially for rehabilitation and support of patients with mobility impairments.
In order to examine the capability of a networked ESAs to be used as drive train for a rehabilitation or force augmentation device, we consider a case of an active elbow brace and use of optimization method to find the optimal structure of the network of ESAs.
3. Results
Regarding the optimization of the soft permanent magnet as mentioned earlier we prepared five flexible permanent magnet samples each with different mixing ratios (between 8% to 28% with incremental mixing percentage of 4% of magnetic particles). For each sample, the flux charge density was measured by Magnetic Field Instrument (MFI), a device used to evaluate the magnetic field or flux produced around permanent magnets, solenoids, and electrical devices. The flux density for each mixing ratio was presented in an interval domain as the mixing ratio could not be precisely adjusted. The result is shown in
Table 2.
Result of the optimization problem for a single ESA has been presented in
Table 3.
Concerning the optimization of the spatial network of actuators as illustrated in the previous section, the comparison is done among the magnetic field of the actuator’s coil with various cross-section profiles (triangle, square, pentagon, hexagon, octagon, and circle) and the results is presented in
Table 4.
To compare the efficiency of the profiles, the power consumption ratio needs to be taken into account. Considering same electric current, the only parameter affects the power consumption would the wire length. For the same cross section areas the perimeter ratio of various section profiles are listed in
Table 4. Therefore, the calculated force ratios need to be normalized with perimeter ratios.
Table 5 compares the produced force over perimeter ratio (
) for aforementioned profiles. Another factor contributes to the final output of the network is packing density. Packing density is defined as the ratio of the cross-section taken up by the coils’ wire section profiles to the available space. The maximum amount of packing density is for triangular, square, and hexagonal configuration which is one, means that the available space is absolutely filled with the coils. The remaining section profiles, i.e., pentagonal, octagonal and circular ones do not completely cover the available space and have less than 1 packing density. Packing densities for all section profiles are listed in
Table 5.
Comparing the right column of
Table 5 indicates that the hexagonal section profile is the most efficient one in terms of magnetic field and force output considering the power consumption and circular profile has the second rank.
4. Discussion
In this paper, the optimal design for a network of novel ESA was presented. The novel electromagnetic soft actuator operates based on the working principle of solenoids, that consists of two antagonistically located coils made of flexible wires and share a flexible permanent magnetic core.
It was shown that by reducing the size of these ESAs, the force to volume size ratio increases, which suggest a network of miniaturized ESAs would achieve higher amount of output force compare to a single ESA with the same size of the whole network. In this work this was numerically tested for a case study of an active elbow brace.
The goal was to achieve optimal design of a single ESA and consequently optimal design of a networked ESAs to achieve the maximum output torque.
The result showed that having a network of ESAs as drive train for an active brace, we can satisfy the performance parameters, for supporting the elbow joint of a patient with decreased muscle performance and mobility.
This suggests that with the available manufacturing process discussed in this paper, the actuation technology based on electromagnetic soft actuators can be used as drive trains in robotic prosthesis and robotic exoskeletons, to support patients with decreased muscle function at their affected joints. Our future endeavors are focused in enhancing further the produced torque by the ESAs, in order to be utilized for robotic prosthetics and exoskeletons in patients with complete loss of muscle function.
This actuation technology is uniquely suitable in rehabilitation and/or force augmentation applications for those mobility impaired patients that have not completely lost the ability to move their affected joints and would need some extra help to recover or be able to perform their daily tasks. Considering the huge population of these types of mobility impaired patients (e.g., stroke patients, peripheral arterial disease, traumatic injuries, neuropathies, senescence and frailty) electromagnetic soft actuators provide novel potential solutions, for wearable and next-to-skin type of assistive technologies, at low production cost, safe, portable and yet sufficiently powerful with low power requirement and high bandwidth.
As another case study, and considering the unique scale ability of the proposed actuation method, we will also consider a prosthetic finger, where the network of soft actuators can be linked to a tendon mechanism to bi-directionally move a finger. The return motion can then be done through a pre-loaded spring.
Planning for the future, we will manufacture the active brace powered with the network of ESAs and test its potentials in rehabilitation trials, such as those for elbow stiffness or iso-kinetic motion for elbow spasticity and other motor dysfunctions from various medical pathologies.