Tendon-driven Flexible Manipulator

Project goals

This project aims to develop a flexible manipulator for transnasal/transoral surgery. Compared with existing surgical manipulators, the developed one should have better performance in workspace and dexterity, thus better facilitate the surgical operation.


A constrained tendon-driven serpentine manipulator (CTSM) is designed as shown in Figure 1. It includes an underactuated tendon-driven flexible section, a constraint and a set of tendons. The tendon-driven flexible section is similar to our previous wire-driven robot arm design. It comprises of several identical vertebras, and an elastic tube. Two successive vertebras form a joint and the joint rotation follows the elastic tube bending. Four tendons pass through all the vertebras. For each tendon, the two ends are attached to the distal vertebra and the motor respectively. These tendons are grouped to two pairs and are orthogonally arranged as shown in Figure. 1 (b). One tendon pair controls the bending about X axis and the other tendon pair controls the bending about Y axis. The manipulator bending is planar. The bending angle and bending direction are controlled by the motion of the four tendons. The constraint can be an elastic tube or rigid tube. The constraint translates along the tendon-driven flexible section. Vertebras in the range of the constraint are confined and vertebras out of the range of the constraint are free of rotation. Thus, the last constrained vertebra serves the base of the bending section.

Figure 1 3D design of the CTSM: (a) the assembled and explosion view of the CTSM; (b) the tendon configuration; (c) the cross section view of the joint.

The bending motion of the manipulator is shown in Figure 2: when the insertion of the constraint is 0, the CTSM bends by the tendons as a traditional TSM. By pushing the constraint forward the backbone is segmented to two parts: the proximal constrained section and the distal free bending section. Compared to the distal free bending section, the proximal constrained section is stiffer and the joints’ rotations are smaller. By pushing and pulling the constraint, the lengths of the two sections are controlled.

Fig. 2 Bending motion illustration: (a) the bending section is not constrained; (b) part of the bending section is constrained; (c) the whole bending section is constrained.


A prototype is built as shown in Figure 3. In the prototype, the flexible backbone has 27 vertebras. The vertebras are fabricated by 3D printing, and the material used is plastic. Each joint can rotate up to 7.25°.The total length of the flexible backbone is 104mm, and the diameter is 7.5mm. A silicon rubber tube serves the elastic tube. The outer diameter is 3 mm and inner diameter is 2 mm. Four steel wires with nylon coating are used to control the backbone bending. The diameter of the steel is 0.3 mm. The wires are arranged orthogonally, with opponent wires make a pair. Each wire pair is connected to a drum wheel. The rotation of the drum wheel is controlled by a servo motor. The diameter of the drum wheel is 50 mm. The wires are guided by a Teflon tube, whose outer diameter is 0.9 mm and inner diameter is 0.5 mm. The replaceable constraint is hold by a chuck, which is mounted on the linear actuator. The range of the linear actuator is 100 mm.


By changing the stiffness ratio between the flexible bending section and the overall stiffness λ, the workspace of the CTSM is as shown in Figure 4. In the simulation the length of the CTSM is 100 mm, and the number of vertebrae is 25.

Fig. 4 workspace comparison: (a) traditional TSM; (b) CTSM with elastic constraint; (c) CTSM with elastic constraint; (d) CTSM with rigid constraint.

When the CTSM with a rigid constraint is attached to a mobile base, the workspace and dexterity distribution are shown in Figure 5. For the tendon-driven serpentine manipulator (TSM), the dexterity is indexed as the kinematic flexibility. For a traditional TSM, the kinematic flexibility is 1 in most places; the maximum is 2. For the designed CTSM, the kinematic flexibility is enhances all over the workspace and the maximum is 15.


Figure 5 Comparison of the dexterity distribution over the workspace: (a) traditional TSM; (b) CTSM with λ=0.

People involved

Staff: Zheng Li
Visiting Students: Gui Fu, Zhengchu Tan, Jan Feiling
PIs: Hongliang Ren and Haoyong Yu

Experiment Videos

– Phantom tests

– CTSM Experiments in ex-vivo hearts and phantoms (2014/11/22)


1. Zheng Li, Haoyong Yu and Hongliang Ren, “A Novel Constrained Tendon-driven Serpentine Manipulator (CTSM)”, ICRA 2015 (under review)
2. Zheng Li, Haoyong Yu and Hongliang Ren, “A Novel Underactuated Wire-driven Flexible Robotic Arm with Controllable Bending Section Length”, ICRA 2014 Workshop on Advances in Flexible Robots for Surgical Interventions, Hong Kong, May 31-June 7, 2014
3. Zheng Li, Ruxu Du, Haoyong Yu and Hongliang Ren, “Statics Modeling of an Underactuated Wire-driven Flexible Robotic Arm”,IEEE BioRob 2014, Sao Pauo, Brazil, Aug12-15, 2014

Presentation at BIOROB2014

Presentation at ICRA 2014

Poster at ICRA 2014


Fig. 3 CTSM prototype.