Deployable Convex Generalized Cylindrical Surfaces Using Torsional Joints

The objective of this research is to present a model which can be used to create a deployable surface with a simple actuation that closely approximates a generalized cylindrical surface, such as the one shown in Fig. 1, using torsional joints which can be manufactured as 2D geometries with thickness. The ability to transition from a flat sheet to a predetermined shape can lead to advances in applications such as stowable reflectors incorporated in space mechanisms, tunable optical devices, morphing aerodynamic or hydrodynamic structures, and conforming components like circuit boards to curved shapes. This research includes and expands on work presented at the Mechanisms and Robotics Conference at ASME IDETC 2020 [1].

A generalized cylindrical surface is one of the three classes of curved developable surfaces. A developable surface can be formed from a planar surface through bending without any stretching or tearing [2]. A generalized cylinder is formed by translating a straight line, called the generator line, along a path in a plane perpendicular to the generator line. The path is called the directrix of the generalized cylinder. For example, a circular directrix would give rise to the common right circular cylinder. The method presented in this research enables the design of deployable, approximate generalized cylindrical surfaces where the directrix is a convex curve that does not self-intersect. Furthermore, the determination of the joint placement is investigated through optimization techniques to minimize error in the approximation of the surface and to include constraints on various parameters to ensure the functionality and manufacturability of the surface.

Part of the modeling presented in this research includes the characterization of the stiffness of a deployable surface. While this research focuses on using compliant torsional joints to introduce flexibility along straight lines in the surface (simulating the introduction of pin joints with a stiffness), there is a substantial body of work on modeling stiffness of structures and manipulators in more general configurations and with joints exhibiting more degrees-of-freedom. For example, work in characterizing the stiffness of manipulators and soft robotic arms has been approached through matrix structural analysis [3,4], finite element analysis [5,6], and the virtual joint method [7–9].

Surfaces which can be controlled or predictably changed offer advantages in situations where a change in occupied space or a dynamic response to an environment is desired [10]. Varying geometry of morphing elements can produce smooth curved surfaces [11], while other techniques achieve morphing through approximating curved surfaces with discrete morphing elements [12]. Morphing can also be used to provide locomotion as was accomplished with printing liquid crystal elastomer bilayers [13] or transition between stable states as was done with a compliant six-bar dwell mechanism [14].

Origami and origami-inspired mechanisms provide examples of surfaces where a change from one state, perhaps a tightly compacted state, to another state, a deployed state, is desired. Examples of these include bio-inspired wing structures [15], a diameter-changing origami wheel [16], and an origami-based heart stent [17]. Self-folding qualities can induce morphing with actuation methods such as thermally activated shape memory alloys [18]. Kirigami has also been leveraged to create morphing structures in response to temperature and light changes [19,20]. Origami and kirigami methods can enable simple manufacturing methods with complex morphing motions.

Morphing surfaces have also been used to enhance the performance through creating desirable geometries and profiles. Morphing wings and flight surfaces have been investigated to create structures which can be dynamically modified to achieve geometries which perform well under varying environments or conditions [21–25]. Deployable reflectors use the principle of morphing or changing surfaces to create the desired collection shape while still having the ability to stow compactly [26–30]. Optimization is commonly employed to assist in the determination of geometry for these types of morphing geometries [31,32].

This work specifically looks at generalized cylindrical surfaces, which are one of the four possible developable surfaces resulting from curved folds or creases in a surface [2]. The results of this work could possibly be incorporated into mechanisms and structures derived from curved-fold origami patterns to facilitate specific panel shapes or further increase the propensity of a panel to take a certain shape during deployment [33]. In comparison to the methods presented in the previous paragraph, the method presented here has characteristics of having an analytical model to create a geometry to approximate a deployed profile (as opposed to relying on finite element simulations or other numerical methods) and being simple to actuate through a single contractile element. Additionally, when torsion joints are used to form the flexible portions of the surface, the entire deployable surface can be made with planar-limited manufacturing techniques. The presentation of an optimization method to determine the placement of joint locations to minimize the error from approximating a continuous curved surface as a series of rigid links could be applied to several of the existing methods for creating morphing surfaces.

A model for creating generalized cylindrical deployable surfaces uses a system of rigid links joined by torsional springs that is actuated from a flat state by opposing tip loads to form a desired directrix in a deployed state. The desired directrix is used to calculate the required stiffness of the torsional springs between each rigid link.

Let L₁, L₂, …, L_N+1 be the lengths of N + 1 rigid links in the system and N is the number of joints between the links. Let k₁, k₂, …, k_N denote the equivalent stiffness of the torsion springs between the rigid links as shown in Fig. 2(a). The rigid-link system can be placed in the desired curved shape, represented by the function y_desired(x), where all of the endpoints of the rigid links lie upon the curve. The deployed link angles θ₀, θ₁, …, θ_N, the deployed heights y₁, y₂, …, y_N, and the distance between the tip loads r as shown in Fig. 2(b) can be determined from geometry. This information can be found using vectors that represent each of the links to calculate the angle between these vectors to obtain the deployed link angles and using y_desired(x) to find the deployed heights for each x corresponding to a joint location.

The lamina emergent torsion (LET) is a compliant joint that allows for a twisting motion [34–36]. A full LET joint consists of four torsion bars, a set of two parallel torsion bars (also called a torsional parallel joint [37]) in series with another set of two parallel torsion bars. LET joints can be chained together to form lines of torsion bars [33,38]. The lines of parallel torsion bars can act as the torsion springs in the model. An example of the geometry of a LET joint chain with two torsion bars on each ruling line is detailed in Fig. 4. The torsion bars of the LET joints are well suited to the model for deployable surfaces as the joints can be planar manufactured as part of a sheet, have a relatively stable axis of rotation, and have geometry which can be modified to change the stiffness of the joint in a predictable way using spring models [34,39]. For example, the length, width, or thickness of the torsion bars can be changed to specify a certain stiffness.

The second grouped term in the expression for J can be replaced by a fifth degree polynomial divided by another fifth degree polynomial for greater accuracy (see Ref. [40]) and this more accurate form of the equation was used in the implementation sections of this paper. These terms come from curve fits applied to a compensation function that seeks to resolve the exact series solution with a symmetric approximate solution for the torsional stiffness of a rectangular bar.

There is some flexibility in determining which geometric parameter(s) are allowed to vary to satisfy this equation. For example, all terms could be defined except for w and the equation can be solved numerically to find the width of the torsion bar.

For more complex joint types or considering degrees-of-freedom other than just twisting about a single axis, more in-depth stiffness modeling techniques could be used to obtain a relationship between the geometry, loading, and the stiffness [41].

Multidimensional optimization can be used to determine joint locations for a specified number of torsion joints, N, which will minimize the error in approximating the desired profile with a series of rigid links and allow for the introduction of additional constraints in the design problem.

This prevents the shortest distance from an arbitrary point on y_desired to y_poly from growing too large. If this constraint is not in place, the optimization process tends to aggregate all of the areal profile error to the location where the curvature of the desired function is the greatest, even if the distance from the desired curve to the actual deployed surface becomes large.

By imposing a limit on the maximum lineal profile error, the minimum areal profile error is increased, but distributed more evenly throughout the system. To gain an appropriate understanding of the range of this constraint, the optimization can be performed with a large value for the allowable lineal profile error such that the constraint is not binding. The value of the constraint can then be reduced until it reaches the other extreme value which will cause the optimization problem to have no feasible solution.

In the implementation to evaluate this constraint, the lineal profile error was found by locating the largest lineal profile error for each segment described by δ_i. This was accomplished with another optimization routine where a single free variable, the x coordinate on y_desired(x), was determined to maximize the objective function of the distance between the point (x, y_desired(x)) and the line segment (found using a form of the distance formula for the shortest length between a point and a line).

If desired, additional constraints can also be introduced such as the maximum angular rotation of a torsional spring and the minimum or maximum lengths of the rigid links. Once a mechanism, such as a torsion bar, is decided upon to serve as the torsion spring, further constraints can be imposed to limit stress or size of the mechanism.

To illustrate these additional constraints, consider a rigid-link system which uses N torsion springs, which corresponds to the number of design variables. Let us assume a geometry similar to the one shown in Fig. 4, where the number of torsion bars on each ruling line is two (m = 2) and the only geometric parameter we are allowing to change is the torsion bar widths w_i.

Scaling of the desired profile and constraints was used to increase the convergence rate for the optimization during implementation. For example, the desired profile and allowable lineal profile error were scaled by a factor such that the maximum x-axis length of the desired profile was one, though care was taken to use the full-scale geometry when calculating dimensions such as widths, stresses, or the constraints in Eq. (13). Several constraints, such as Eqs. (10), (12), and (14), were scaled by their allowable values.

During the implementation of the optimization algorithm for parabolic profiles, it was noted that when the additional constraints for lineal profile error, stress, and manufacturing were not binding, the minimum areal profile error was achieved through equal δ_i spacing of the torsion springs. This can be shown mathematically as follows.

Equations (17), (18), and (19) give N + 1 equations which can be solved for the N + 1 angles of θ_i, where i = 0 to N when subjected to a force F. Equation (16) can then be used to find the distance between r between the ends of the deployable surface.

This system of equations is amenable to solving numerically with a single unknown and using substitutions. Let θ₀ be a single unknown and use Eq. (18) to calculate θ₁. The process can then be continued to sequentially calculate each θ_q for q = 2 to N using Eq. (19). These values can then be used in Eq. (17) as the single equation to satisfy.

This set of equations can have multiple valid solutions corresponding to stable and unstable equilibrium configurations of the deployable surface. The stable configurations correspond with minimums in the potential energy of the system. Kinematic chains with passive joints with stiffness can have multiple equilibrium positions as demonstrated in Ref. [8]. For the deployable surfaces in this research, an additional stable position is observed by reflecting a convex deployable surface about the line of action of the actuation force. Unstable equilibrium positions can be observed when links create a zig-zag shape, where some joint angles are positive and others are negative. Limiting the domain of the joint angles and providing an initial guess to the solver of a convex profile can help ensure that the system of equations is solved for a stable equilibrium position where a convex profile is formed. In-depth characterization of all equilibrium states is left to future work.

The methodology described above was used to create prototypes of a deployable airfoil surface and deployable parabolic reflector.

A deployable airfoil surface was designed using torsion bars as the torsion springs in the model with optimization to place the joint locations and ensure constraints for the lineal profile error and for manufacturing were met. The desired directrix was chosen to be the top surface of a Clark Y airfoil with a 10 in. (25.4 cm) chord length [43]. The optimization and constraint setup parameters are reported in Table 1.

The resulting joint positions after the optimization are shown by the thick black line in Fig. 6(a). The stiffnesses required for the desired profile and actuation force were calculated and the corresponding LET joint geometry determined. With the stiffness values for the joints, the shape throughout deployment was calculated as described in Sec. 2.3 and plotted in Fig. 6(a). The force-deployment shape results were used to plot the maximum height of the deployed shape against the deployment force as shown in Fig. 6(b). This plot shows how small amounts of shape change occur as the actuation force increases until a critical buckling force where large amounts of deformation occur rapidly. This nonlinear shift in stiffness has been investigated for kinematic chains containing joints with passive stiffness [8].

The resulting LET joint pattern was cut from a polypropylene sheet using an abrasive waterjet. The pattern is shown in the flat and deployed states in Fig. 7. The optimization results and experimental measurements taken on the prototype are summarized in Table 2. Basic photogrammetry was carried out using matlab to measure approximate lineal profile error in the deployed shape. The largest error occurred in the leading edge where some parasitic or unwanted motion occurred due to compression of the LET joints by the tip loads. A thin polyethylene terephthalate (PET) adhesive film 2.5 mils (0.064 mm) thick was then applied to the top layer of the pattern to decrease the parasitic motion while minimally affecting the torsion performance, similar to a technique investigated by Chen et al. [44]. This appeared to reduce some of the parasitic motion, yet increased the lineal profile error. The pattern with the PET covering is shown in Fig. 7(c). Applying a thin film with full bonded contact along a surface can impact the neutral axis of bending and torsional axes and thus change the modeling, and some options exist to minimize this impact. Specifically, a sandwich structure where the thin film is applied between two sheets of the same thickness can maintain the neutral axis of bending and torsional axes locations. Another option is to adhere the film to only the areas of the pattern that are not in torsion to help maintain the torsional behavior of the joints, as is further discussed in Ref. [44].

The force to deploy the pattern to the profile was measured using a small digital scale. Accurate, repeatable measurements were limited with the current measurement setup and more rigorous instrumentation is recommended for future work. There is some discrepancy between the designed tip load force and the measured force. This could be due to a number of factors including errors in the shear modulus value used, manufactured kerf width, and perhaps most importantly bending observed from compressive forces in the structure that accompanied the torsional deflection.

The model was also used to design a deployable parabolic reflector made of Aluminum 7075-T6. The target deployed shape has a focal point 4 in. from the vertex. The flat, undeployed dimensions are 18.36 × 10.2 in. (46.6 × 25.9 cm). The parameters used to setup the optimization are shown in Table 3. The optimized joint locations are shown on the thick black line in Fig. 8(a). As was done with the airfoil from the previous section, the calculated stiffnesses of the joints were used with the procedure in Sec. 2.3 to trace the shape through deployment and beyond the desired deployment shape as shown by the red profiles in Fig. 8(a). The tip load force was plotted against the maximum deployed surface height in Fig. 8(b) with points shown corresponding to the profiles plotted in part (a) of the figure. The presence of a critical load that results in buckling-like behavior is present in the plot.

The optimized parabolic reflector was manufactured from the aluminum using an abrasive water jet and is shown in the flat and deployed states in Fig. 9. The same testing setup as for the airfoil surface was used to experimentally determine a force at the desired deployed profile and photogrammetry was used to investigate the error in the profile. The resulting values of the final geometry and from experimental testing are described in Table 4. The deployed profile and desired profile matched quite well with a small observed lineal profile error. The required tip load force to achieve the desired profile also matched with small error. Considerably less bending was seen in this prototype than for the airfoil surface and no covering was used.

From left to right, these terms are determined by the desired actuation force, the material used, and the desired profile curve, and on the right side of the equation all the factors in the term are functions of the geometry of the torsion bar pattern. By grouping these terms, we can see relationships such as if the geometry of a pattern is maintained (the right side of the equation), and changing the material will scale the actuation force as long as the stresses do not exceed the new material’s stress limit. Similarly, errors in estimating the value used for the shear modulus of a material should not affect the ability to achieve a desired profile shape as long as one is willing to adjust the actuation force to compensate for the modulus error.

Equation (20) implies that the number of torsion bars along a ruling line m can be changed to affect the magnitude of the tip load, yet m has no effect upon the stress, as m is not present and does not affect any of the terms in Eq. (21).

Minimizing the areal profile error in the model pushed all of the error between the desired and deployed directrix to the area of greatest curvature of the desired directrix. This was corrected by placing a constraint on the lineal profile error. The effect of minimizing the lineal profile error with a constraint on the areal profile error was also investigated with similar results, though the optimizer required more iterations to converge. Further investigation of definitions of profile error and suitable objective functions to minimize the error for the model present possible directions for future work.

As seen in the results from the physical prototypes, it appeared that bending accompanied the torsional deflection. Work can be done to incorporate bending effects into the analytical modeling for more accurate design estimates.

Both prototypes were deployed by moving two smooth barriers towards one another to create a compressive force. Tension elements, such as cables or strings, could be used to pull the two edges of the panel together to create an ultra-lightweight actuation method, though the effects of the point loads caused from the elements should be considered.

The model constructed using chains of torsion joints results in a deployable surface that can function as the final surface or as a structure underneath a flexible skin. Further studies need to be conducted to see how well the deployed surface would behave if used in an application with multiple loading conditions, such as with a distributed pressure load from an aerodynamic loading. Because aerodynamic loads fluctuate, it poses a challenge to design a single structure for multiple load cases where both the deployed structure’s stability and aerodynamic stability are considered. One possible path forward to address a more complex loading scenario would be to use a support structure underneath the deployed surface where hard stops align during deployment to resist compression loads on the surface of the structure.

A model has been presented as a way to create generalized cylindrical deployable surfaces with convex directrices conducive to being implemented with torsional compliant joints. The shape during the deployment process is also described. The model is demonstrated using the torsion bars of LET joints as torsion springs. A multidimensional optimization model was used to efficiently locate a given number of compliant joints along the directrix to minimize the error between the desired and actual directrix. Constraints were also enforced to ensure performance and a manufacturable geometry. The model was demonstrated through the physical creation of several prototypes in various materials. While the methodology shows potential for advancing the design of precise deployable surfaces, further investigation of the effects of bending accompanying the torsion and the effects of various loading conditions upon the deployed surfaces should be conducted. Actuation methods could also be incorporated into the material itself to create a compact package that can morph when a stimulus is applied.

This material is based on work supported by the National Science Foundation under NSF Grant 1663345 and NSF GRFP Grant 1247046. The authors acknowledge the University of Southern Indiana (USI) Foundation for supporting this research through a Science, Engineering, and Education Research Grant Award (SEERGA) and Justin Amos for his assistance with prototype fabrication. The authors would also like to acknowledge John Sinkovic for his insight into proving the optimal spacing for parabolic profiles where additional constraints were not binding.

There are no conflicts of interest.

The authors attest that all data for this study are included in the paper. Data provided by a third party are listed in Acknowledgement.