Twist drive

The Verdrillantrieb (engl. Twisted string actuator ) is a linearly acting actuator , which has a relatively high tensile force can produce.

Structure and principle

The structure of the twist drive is shown schematically in Fig. 1 . The cord is axially supported at the upper end and connected to a rotary motor . The other end of the cord is linearly supported and attached to a load. The dead weight of the load creates a force that acts downwards.

Fig. 1 : Principle structure of the cord drive, illustration based on: The Twisted String Actuation System: Modeling and Control: Fig. 1

The cord used consists of several individual cord fibers. If the motor performs a rotary movement, a force is applied against the load. This moves the load in the direction of the motor. The movement is guided by the linear bearing. This structure uses the principle of the cord gear, which converts a rotary movement of the motor into a linear movement of the load. For a better understanding of the cord drive, the principle is considered in the following, greatly simplified, in the work area ( ) of the cord. A cord with fibers is used. Influences such as friction, start-up and breakaway conditions, hyper-twisting and non-round cross-sections of the cord are simply neglected. The model described below was developed in The Twisted String Actuation System: Modeling and Control. ${\displaystyle 0\ll \theta \ll \theta _{\text{max}}}$ ${\displaystyle n}$

When twisting the cord, the individual cord fibers are subjected to a rotational movement at one end. At the same time, the linear bearing at the other end of the cord (the load side) prevents any rotational movement. As a result, the line fibers twist more and more on their own with every rotation θ on the motor side. A helix is ​​created and the effective length of the line is shortened. The development of the helix forms a triangle (see Fig. 2 ) which can be used to better illustrate the movement relationships. The length of the hypotenuse in this triangle is the length of the cord fibers. This length is constant, neglecting forces and material strains. At the apex of the triangle, the angle of rotation θ of the cord is zero due to the linear bearing. On the opposite side of the triangle , the angle of rotation of the cord corresponds to the angle of rotation of the drive. Twisting the cord further increases the length of the page . This shortens (assuming a constant fiber length and a constant cord diameter) the effective length of the cord , while the angle α at the apex of the triangle increases. ${\displaystyle p}$ ${\displaystyle L}$ ${\displaystyle \theta \cdot r}$ ${\displaystyle \theta \cdot r}$ ${\displaystyle p}$

Fig. 2 : Representation of the unwound cord as a helix. Illustration based on The Twisted String Actuation System: Modeling and Control: Fig. 5

Described mathematically this means:

(1)

${\displaystyle \tan(\alpha )={\frac {\theta \cdot r}{p}}}$,

(2)

${\displaystyle p={\sqrt {L^{2}-\theta ^{2}\cdot r^{2}}}}$.

In the next step, we also consider the forces acting, further neglecting the effects of friction and elongation.

The torque acts on the motor side of the cord , which acts as an axial force on the load via the twist angle α . In the case of path-controlled twisting, there is a balance between the resulting axial force in the cord and the load force in the moving state. When considering the number of line fibers , the force acting in a line fiber is distributed over this number. ${\displaystyle \tau _{L}}$ ${\displaystyle F_{z}}$ ${\displaystyle n}$ ${\displaystyle F_{i}}$

This results in the following relationships:

(3)

${\displaystyle F_{Last}=F_{z}=n\cdot F_{i}\cdot \cos(\alpha )=n\cdot {\frac {F_{\tau }}{n\cdot \sin(\alpha )}}\cdot \cos(\alpha )=n\cdot {\frac {\frac {\tau _{L}}{r}}{n\cdot \sin(\alpha )}}\cdot \cos(\alpha )}$,

and thus:

(4)

${\displaystyle F_{Last}=F_{z}={\frac {\tau _{L}}{r\cdot \tan(\alpha )}}}$.

It can be seen that the twist angle α creates a connection between the force ratio and the length ratio of the cord drive. Therefore, the gear ratio of the cord gear is a direct function of the twist angle. The result is a non-linear behavior of the transmission.

Next, consider the effects of stretching in the cord. When using cords with small diameters, a very high linear force can be generated with a very small torque. However, thinner cords have a lower elastic stiffness , so that they deform more. Under load, the initial length of the line fibers changes in the initial state (untwisted) depending on the load acting on each fiber . This can be described as follows: ${\displaystyle K}$ ${\displaystyle L}$ ${\displaystyle F_{i}}$

(5)

${\displaystyle L=F_{i}{\frac {L_{0}}{K}}+L_{0}}$.

In addition, when twisting, the force in the direction of the fibers changes, so that when twisting itself, the total length of the fibers changes. This in turn has an influence on the effective length of the cord. ${\displaystyle L}$ ${\displaystyle p}$

The relationship is shown below:

(6)

${\displaystyle p={\sqrt {L_{0}\left(1+{\frac {F_{i}}{K}}\right)^{2}-\theta ^{2}r^{2}}}}$.

Since the force in turn depends on, the solution of this equation is not easy to achieve analytically. As a simplification, an infinitely high rigidity has always been assumed in previous scientific investigations . This assumption does not apply in reality, so that in all investigations a deviation between the theoretically calculated curves and the experimentally determined curves can always be seen (see The Twisted String Actuation System: Modeling and Control). ${\displaystyle F_{i}}$ ${\displaystyle p}$ ${\displaystyle K\rightarrow \infty }$

use

Table 1 shows the characteristics of the twist drive. The parameters for calculating the values ​​of the TSA are taken from Investigation of twisted string actuation with a programmable mechanical load test stand. The value for the efficiency of the system is taken from Twisted string actuation systems: A study of the mathematical model and a comparison of twisted strings. Cords made of UHMWPE were used in both experiments.

As a comparison, the properties for a human muscle are given according to The selection of mechanical actuators based on performance indices.

Table 1: Comparison of the properties of the twist drive with those of the human muscle. Source: The use of the twist drive in orthoses.

property	unit	Twist drive	Human muscle after
maximum elongation	%	28.6	30 to 70
maximum tension	MPa	285	0.1 to 0.4
maximum frequency	Hz	0.5	5 to 5000
Efficiency	%	90	20 to 25
Power density	W / kg	40732724	500

It should be noted that these results relate to a specific experiment. When using cords with different radii or elongations at break, different results are to be expected. The issue of wear also plays a major role in the properties of the drive, as it means that the load capacity of the cord varies.

In summary, it can be said that the twist drive is a comparatively slow drive which, with a maximum elongation of 28%, is very limited in its field of application. Due to its non-linear behavior, it is very difficult to control the drive. In addition, the drive can only apply tensile forces, so that complementary movements cannot be implemented with one drive alone. However, it can generate high voltages and has a high power density. Another advantage of the drive is its high flexibility, so that it can be used for applications that are difficult to implement with conventional drives. The drive has a simple structure and can therefore be manufactured easily. The comparison of the TSA with a human muscle was chosen because many possible applications are in the area of ​​human movement.

The twist drive has achieved one of the greatest successes to date when used as an actuator for a so-called robot hand. As part of the DEXMART project, a robot hand was developed that could perform sensitive tactile tasks. The basic concept of the shadow hand was adopted as the basic frame ; the drives were replaced by the twist drive. The twist drive is the subject of research at several universities, since the control is complex and the mechanical properties of the drive are difficult to control. In addition, most areas of application have not been fully researched, such as how human movement can be sustainably supported with actuators. Some research on the twist drive is listed below.

• University of Utah: Torsion of Rope-Connected Hoops Leads to Light Weight Prosthetic Actuator, Sept. 1973.
• Kremer, S .: Twisted Cord Actuator. U.S. Patent US4,843,921A , July 4, 1989.
• McGill University: Center for intelligent machines: Department of mechanical engineering: Experimental Validation of Compliance models for LADD transmission kinematics, 1995.
• Soham, M .: Twisting Wire Actuator. US Patent US7477965B2 , Jan 13, 2019.
• Tokai University: Complex and Flexible Robot Motions by Strand-Muscle Actuators, 2007.
• Uni of Kitakyushu Information and Media Engineering: A Five Fingered Robotic Hand Prototype by using Twist Drive, 2010.
• Saarland University: Laboratory of Actuation Technology
• Secondi Università di Napoli: Dipartimento Di Ingegneria
• Università di Bologna: Department of Electronics, Computer Science ans Systems
• Korea University of Technology and Education: Biorobotics Laboratory
• KAIST Korea: Mechatronics, Systems and Control : Development of anthropomorphic robot hand with dual-mode twisting actuation and electromagnetic joint locking mechanism, 2013.

• Cleveland State / West Virginia University: Design and Fabrication of an Assistive Device for Arm Rehabilitation Using Twisted String System, 2013.
• Technical University of Darmstadt: Institute of Electromechanical Design
• Oak Ridge Associated Uni. & SGT Inc. Intelligent Robotics Group: Impedance controlled twisted string actuators for tensegrity robots, 2014.
• University. Okayama: Computer Science & Uni Ritsumeikan Robotics: Robotic joint design by agonist and antagonist arrangement with twisting small diameter round belts, 2015.
• University Coimbra: Institute of Systems and Robotics : The uc softhand: Light weight adaptive bionic hand with a compact twisted string actuation system, 2016.

See also

• Rubber motor

literature

SUZUKI, Masakazu. Complex and flexible robot motions by strand-muscle actuators. In: Climbing and walking robots: Towards new applications . InTech, 2007.

Individual proof

1. ↑ ^{a b c d e f} WÜRTZ, Thomas, et al. The twisted string actuation system: Modeling and control. In: 2010 IEEE / ASME International Conference on Advanced Intelligent Mechatronics . IEEE, 2010. pp. 1215-1220.
2. ↑ MAY, Chris, et al. Investigation of twisted string actuation with a programmable mechanical load test stand. In: Innovative Small Drives and Micro-Motor Systems; 9th GMM / ETG Symposium . VDE, 2013. pp. 1–6.
3. ↑ GAPONOV, Igor; POPOV, Dmitry; RYU, Jee-Hwan. Twisted string actuation systems: A study of the mathematical model and a comparison of twisted strings. IEEE / ASME Transactions on Mechatronics , 2014, 19th vol., No. 4, pp. 1331–1342.
4. ↑ HUBER, JE; STAIN, NA; ASHBY, MF The selection of mechanical actuators based on performance indices. Proceedings of the Royal Society of London. Series A: Mathematical, physical and engineering sciences , 1997, Volume 453, No. 1965, pp. 2185-2205.
5. ↑ Ehm, Benjamin: Seminar paper: The application of the twist drive in orthoses . Saarland University May 2016.
6. ↑ SICILIANO, Bruno (ed.). Advanced bimanual manipulation: Results from the dexmart project . Springer Science & Business Media, 2012.
7. ↑ JACOBSEN, SC; JERRARD, RB Torsion of Rope-Connected Hoops Leads to Light Weight Prosthetic Actuator. In: 26th ACEMB . 1973.
8. ↑ Kremer, Stephen R., 1989, Twisted cord actuator, Application April 18, 1988 U.S. Patent US4834921A. 4th July 1989
9. ↑ MENNITTO, G .; BUEHLER, Martin. Experimental validation of compliance models for LADD transmission kinematics. In: Proceedings 1995 IEEE / RSJ International Conference on Intelligent Robots and Systems. Human Robot Interaction and Cooperative Robots . IEEE, 1995. pp. 385-390.
10. ^ Soham, Moshe, 2009, Twisting wire actuator, registration: October 24, 2004 U.S. Patent US7477965B2, January 13, 2009
11. ↑ SUZUKI, Masakazu. Complex and flexible robot motions by strand-muscle actuators. In: Climbing and walking robots: Towards new applications . InTech, 2007.
12. ↑ GODLER, Ivan; SONODA, Takashi. A five fingered robotic hand prototype by using twist drive. In: ISR 2010 (41st International Symposium on Robotics) and ROBOTIK 2010 (6th German Conference on Robotics) . VDE, 2010. pp. 1-6.
13. ↑ SHIN, Young June, et al. Development of anthropomorphic robot hand with dual-mode twisting actuation and electromagnetic joint locking mechanism. In: 2013 IEEE International Conference on Robotics and Automation . IEEE, 2013. pp. 2759-2764.
14. ↑ SHISHEIE, Reza, et al. Design and fabrication of an assistive device for arm rehabilitation using twisted string system. In: 2013 IEEE International Conference on Automation Science and Engineering (CASE) . IEEE, 2013. pp. 255-260.
15. ^ PARK, In-Won; SUNSPIRAL, Vytas. Impedance controlled twisted string actuators for tensegrity robots. In: 2014 14th International Conference on Control, Automation and Systems (ICCAS 2014) . IEEE, 2014. pp. 1331-1338.
16. ↑ INOUE, Takahiro, et al. Robotic joint design by agonist and antagonist arrangement with twisting small-diameter round-belts. In: 2015 IEEE / RSJ International Conference on Intelligent Robots and Systems (IROS) . IEEE, 2015. pp. 1751-1756.
17. ↑ TAVAKOLI, Mahmoud; BATISTA, Rafael; SGRIGNA, Lucio. The UC soft hand: light weight adaptive bionic hand with a compact twisted string actuation system. In: Actuators . Multidisciplinary Digital Publishing Institute, 2016. p. 1.
18. ↑ SUZUKI, Masakazu. Complex and flexible robot motions by strand-muscle actuators. In: Climbing and walking robots: Towards new applications . InTech, 2007.