Realization and swimming performance of the breaststroke by a swimming humanoid robot
© Nakashima and Kuwahara. 2016
Received: 8 January 2016
Accepted: 11 April 2016
Published: 26 April 2016
In order to clarify the mechanics of human swimming, a full-body swimming humanoid robot called “SWUMANOID” was developed as an experimental platform for research about human swimming. SWUMANOID had a detailed human body shape, created using three-dimensional scanning and printing equipment, and was developed as an experimental model substituting for human subjects. Not only the appearance but also the methodology to realize various swimming strokes was considered. In order to reproduce complicated swimming motions with high fidelity, 20 waterproof actuators were installed. The free swimming of the crawl stroke at a velocity of 0.24 m/s was realized in the previous study. However, it could not perform the breaststroke due to mechanical limitations. The objectives of this study were to realize the breaststroke for SWUMANOID by improving its lower limbs, and to investigate the swimming performance of the breaststroke experimentally. The lower body of SWUMANOID was fully redesigned, built, and connected to the upper body. The swimming motion of the breaststroke was created based on that of an actual swimmer. A free swimming experiment was conducted in a 25 m outdoor swimming pool. In addition, in order to discuss the experimental results in detail, the experiment was reproduced by the simulation. From the experiment, it was found that SWUMANOID could perform the breaststroke successfully. The swimming speed for the stroke cycle of 2.3 s was found to be 0.12 m/s. Since this swimming speed was considered low compared to that of the actual swimmer, the reason for the discrepancy was examined by simulation. From the simulation, it was found that one main reason for the low swimming speed was insufficient output power of the motors, especially for the knee and shoulder joints.
In spite of such a long history of human swimming, its mechanics still have not been fully clarified since it is an extremely complicated phenomenon, in which a complex human body moves unsteadily with many degrees-of-freedom (DOF) in the three-dimensional water flow. For example, the hand path in the crawl stroke depicts a distorted ellipse when viewed from the side in absolute space , showing that the hand does not push the water straight at a constant depth. Furthermore, the kinematics of the arm and hand during the underwater stroke is highly unsteady . From this viewpoint, many attempts were made recently to quantify the unsteady fluid forces acting on a swimmer while swimming. The first approach was an experiment involving a human subject [3–5]. However, this method had problems with insufficient repeatability, physical fatigue of the subject, and difficulty in installing sensors on the subject. For this reason, some researchers have conducted experiments using physical models such as robots instead of human subjects. A lot of measuring experiments using physical models have been conducted to date [6–9], but there was no full-body experimental platform which could consider interactions between the many segments involved in normal swimming motions. Therefore, an analysis using physical models had been performed on an isolated segment and misleading conclusions could have been developed. To solve such problems, a full-body swimming humanoid robot was developed for research about human swimming by Chung and Nakashima . The robot was named SWUMANOID. SWUMANOID had a detailed human-body shape, and was created using three-dimensional scanning and printing equipment since it was developed for the experimental model substituting for human subjects. The size of SWUMANOID was 1/2 scale of an actual swimmer. Not only the appearance but also the methodology to realize various swimming strokes was considered. In order to reproduce complicated swimming motions with high fidelity, 20 waterproofed actuators were installed. The free swimming of the crawl stroke at a velocity of 0.24 m/s was realized in the previous study .
Developing a swimming humanoid robot such as SWUMANOID is important for the following two reasons. First, it is expected that it will become an experimental platform for the research of human swimming. To date, many simulation studies about human swimming have been conducted [12–15]. Such simulation technique is very useful and powerful tool for analysis. However, simulation always needs validation and improvement by comparing with experimental results, since it is not an actual phenomenon after all. In order to conduct more accurate experiments for that purpose, more elaborate physical models, such as swimming humanoid robots, will be necessary. The second reason for developing a swimming humanoid robot is that it can be applied to robots for special tasks accompanying water environment, such as rescue robots in the sea and working robots around a pool in a nuclear plant. Developing a swimming humanoid robot and studying how it can swim will be useful for developing such robots in the future.
SWUMANOID had sufficient DOF to perform not only the crawl stroke, but also the back and butterfly strokes. However, it could not perform the swimming motion of the breaststroke due to the following three reasons: (1) the thigh joint only had one DOF, (2) the knee joint could not be as fully flexed as a human’s, (3) and the ankle joint could not be as fully dorsi-flexed as a human’s as well. These points for the lower limbs did not become problems when SWUMANOID performed the flutter kick for the crawl stroke, but did when it performed the breaststroke. Indeed, for the breaststroke, the thigh joints have to possess three DOF since the legs move in a complicated manner. The knee and ankle joints also have to be fully flexed and dorsi-flexed, respectively, for the recovery position of the legs.
The objectives of this study were to realize the breaststroke for SWUMANOID by improving its lower limbs, and to investigate the swimming performance of the breaststroke experimentally. In this paper, the improvement for the breaststroke, creation of swimming motion, and experimental and simulation methods are explained. Next, the experimental results are shown and discussed. Finally, the obtained findings are summarized.
Improvement for the breaststroke
Electrical parts and other specifications
Specifications of improved SWUMANOID
(H) 925 mm (W) 270 mm (D) 119 mm
Actuators (dynamixel: Robotis Corp.)
RX28: 2.5 Nm (at 14.8 V)
MX28: 3.1 Nm (at 14.8 V)
MX28: 7.3 Nm (at 14.8 V)
Total: 24 DOFs
Arm: 2 arms × 6 DOF
Waist: 2 DOF
Leg: 2 legs × 3 DOF
CM700 (Robotis Corp.)
Li-Po 14.8 V 1550 mhA × 2
ZigBee module ZIG-110A
Parameters of body segments
Added weight [g]
Thigh × 2
Shank × 2
Foot × 2
Head and neck
Upper arm × 2
Elbow × 2
Forearm × 2
Breast × 2
Whole upper body
Whole lower body
Creation of swimming motion
Results and discussion
In addition, the trial of stroke cycle of 3.27 s was conducted. The average swimming speed was 0.11 m/s in this case. This value was only slightly lower than 0.12 m/s in the case of 2.3 s. Conversely, 0.12 m/s of 2.3 s was only slightly higher than 0.11 m/s of 3.27 s although the stroke cycle became 70 %. Ideally, swimming speed is inversely proportional to stroke cycle. Therefore, from the result of 3.27 s, the average swimming speed of 2.3 s was expected to be 0.11/0.7 = 0.15 m/s. Since 0.11 m/s of 2.3 s in the experiment was certainly lower than 0.15 m/s, the insufficient output power of the actuators were suspected from this result as well.
Examination by simulation
Simulation results of swimming speeds for four conditions
Swimming speed [m/s]
Increase amount [%]
The maximum joint torques of the shoulder and knee were calculated for condition 4 by the simulation. These were 0.84 and 2.37 Nm, respectively. Indeed, these were sufficiently smaller than 7.3 Nm in Table 1 for MX-64, which was used for the shoulder and knee joints. However, 7.3 Nm was the stall torque, which was the maximum torque without rotation. It suggests that there is a possibility to improve the actuator performance by selecting more appropriate reduction ratio for the motor gears.
From above results, it was found that the insufficient output powers of the motors were one of the main reasons for the low swimming speed. Therefore, it is expected that the swimming speed will increase largely if the motors have sufficient output powers to realize the target joint motions. However, the swimming speed of 0.210 m/s (condition 4) in Table 3 means the normalized stroke length of 0.522. It is still much lower than 1.358 of an actual swimmer. One reason for this discrepancy may be that the difference in the actual joint angles and measured ones. The joint angles in the experiment were measured by the internal function of the motors, and therefore mechanical errors such as backlash of the joints were not taken into account. Therefore, it was possible that the actual joint angles had some differences from the measured values. Another possible reason is that the target joint angles themselves were not sufficiently well-considered. Although they were determined based on the actual values of an actual swimmer, they had to be modified for SWUMANOID due to the limitation of the degrees-of-freedom as well as the range of motion of SWUMANOID. For example, in Fig. 12a, the fluid force acting on the hand and forearm still had a large negative peak at t * = 0.5–0.6. This means the recovery motion, in which the hand moves forward, still was not sufficiently good. If such problems in the swimming motion are all solved by modification, the swimming speed may increase more.
In the present study, the swimming humanoid robot SWUMANOID was improved to perform the breaststroke. The lower body was fully redesigned, built and connected to the upper body. The swimming motion of the breaststroke was created based on that of an actual swimmer. From the experiment, it was found that SWUMANOID could perform the swimming motion of breaststroke successfully. The swimming speed for the stroke cycle of 2.3 s was found to be 0.12 m/s. From the examination by simulation, it was found that one of the main reasons for the low swimming speed was insufficient output power of the motors, especially for the knee and shoulder joints.
As the future tasks, the actuation system for the knee and shoulder has to be improved by some methods, such as providing instantaneous large current to the motors for the power peak timings, redesign of the motors or introducing subsidiary active/passive actuators.
MN provided the basic ideas of the overall system, and KK designed the robots and the overall system. All of the experiments were performed by MN and KK. All authors joined the discussions for this research. All authors read and approved the final manuscript.
This work was supported by JSPS KAKENHI Grant Number 26282174.
The authors declare that they have no competing interests.
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