Drains are an integral part of every modern city, where drainage systems are entirely subsurface in most countries. Statistics from Asia, Europe, United States show that major cities contain 4000 to 7000 km of drainage lines. The primary purpose of these surface and subsurface sewage systems is to remove excess water in a safe and timely manner, which plays a vital role in controlling water-related diseases or water-borne diseases. Drainage systems have its disadvantages, where these systems give problems to mosquito-borne diseases, clogging, internal damages due to ageing, excessive traffic which causes contamination of groundwater or overflow. To control these problems, serious inspection, monitoring and maintenance of drainage systems is required. At present, this task is labor-intensive as shown in Figure 1
a that adds more difficulties in subsurface sewer lines like inaccessible areas with poor lighting, ventilation and safety concerns associated with insect bites. The cross-section of the drainage system with the approximate symmetric design shown in Figure 1
b are widely found in Singapore [1
]. The width, W
, of these drainages typically range from 1.1 to 1.8 m, w
from 0.2–0.8 m and the height h
between 0.3–1.1 m. Thus, there is a requirement to design the robot to traverse inside this type of drainage systems.
The specially designed mechanism with suited locomotion as per the internal geometry of the drainage system is essential. Classification of the inspection robots can be done on the basis of locomotion as tracked, wheeled and legged. PIRAT [2
] is a tracked small robot designed for the quantitative assessment of sewer systems surveyed in real time. The development of autonomous body for inspection of liquid filled pipes “pipe rover/pearl rover” has six-legged propulsion [3
]. In another work, an autonomous sewer cleaning robot was published that cleans underwater sewers [4
]. KARO is a wheeled tethered robot for smart sensor-based sewer inspection equipped with intelligent multi-sensors [5
]. KANTARO is a wheeled platform and uses a special mechanism called “naSIR Mechanism” to access straight and even bends pipelines without intelligence of sensors or controllers [6
]. KURT is a six-wheeled vehicle that can fit in 600 mm diameter pipelines [7
] and MAKRO a worm-shaped wheel, multi-segmented and autonomous bodies for navigation in drain systems [8
]. The wheeled robot with fixed morphology finds application in climbing of ropes for inspection task as in [9
]. Even though a bunch of studies in the literature validates for monitoring or inspection of sewer systems, they mostly suffer from performance issues like modularity and adapting its height as per the geometry of drains that diminish their full potential. One major factor that results in the performance degradation associated with inspection robots design is their fixed morphology. We have proposed the model of quadruped robot for drainage systems that are mainly constructed to carry excess water to reservoirs, unlike the sewage pipes that are used to dispose of solid wastes and water. Tarantula has four-wheel drive and steering locomotion. The drain inspection task can include the identification of the potential mosquito inhabitants and locations that are prone to mosquito-borne diseases as presented in [11
] using the images grabbed from the camera mounted on Tarantula in the near future.
Quadruped robots are gaining increased attention among robotics researchers across a wide range of applications with its unique morphology to carry out various kinds of field work. These quadruped robots bring with them the unique advantage of efficiency. Several developments have been made after pioneering research on quadruped robot from MIT [12
] and Tokyo University. Since then, a large number of quadruped robots have been developed, such as BISAM [13
], which has reptile-like walking and stabilizes itself using a flexible spine. In another work, WARP1 [14
] presents a standing posture controller for walking robots, which was successfully tested in simulations and experiments. The pioneering work of Hirose and Fukushima robotics laboratory mainly focused on legged robots for about 40 years. Typical quadruped robots born from this laboratory is TITAN series [15
] that is the development of a sprawling-type quadruped robot and capable of high velocities and energy efficient walking. Popular among these is TITAN VIII [17
]. An introduction to several quadruped robots along with its locomotion and control techniques were presented in [18
]. The large dimension quadruped robot equipped with drilling equipment and capable of walking on different terrains by incorporating impedance control for the foot-ground contact was reported in [19
]. These quadruped robots were mainly used in the fields like mine detection, walking uneven terrain, etc., but to access the drainage system with varying heights and cross-section, the robot should be designed accordingly to have the ability to reconfigure its morphology. In Table 1
, a comparison was made among the existing drainage and sewer inspection and cleaning robots. We have used the word quadruped with Tarantula
since the kinematics of wheel is coupled with the kinematics of leg. Note that it is not used here in the context of walking, trotting, etc., capabilities of robots.
An interesting hybrid mode of locomotion robot named PAW used both the wheels and legs to achieve gaits, such as bounding, galloping and jumping, was reported in [21
]. In [21
], the four legs were having only a single degree of freedom which was used to incline the body and the formulation was presented for inclined turning and the wheel at the distal end to provide the locomotion. Tarantula has four degrees of freedom (DOF) in each leg to provide the change in the height of the body, contact with the inclined surface and for independent steering action. The contribution of this work is the designed mechanism, formulation for the coupled kinematics of legs and wheels along with the identification of the kinematic parameters of each leg.
The mechanical structure and the mechanisms are designed and assembled in CAD. The kinematics of legs is coupled with the wheel steering kinematics for the designed mobile robot Tarantula. The accuracy of these geometric parameters is critical for the control and steering. Hence, it becomes essential to identify the kinematic parameters of the legs after the assembly of the robot. Kinematic identification is a well established area that uses a geometric approach [22
] or the optimization based technique [23
] to estimate the kinematic parameters. Kinematic calibration of the legged mobile robot is presented in [24
] and used the optimization based approach that requires the knowledge of its nominal or theoretical kinematic parameters for its initial guess to find the calibrated parameters and consequently improves the positional accuracy. We have used the geometric approach that needs no prior information of geometric parameters and used the circle point method formulation as presented in [25
] to identify the widely used kinematic representation defined by Denavit and Hartenberg [26
]. However, Ref. [25
] did not account for the robots with prismatic joints. In this work, we have extended the approach proposed in [25
] for the prismatic joints as well and demonstrated it with the kinematic identification of each of the four legs of the assembled quadruped robot.
Traditional strategies to recognize kinematic parameters of a robot includes taking the robot to a controlled situation to take pose estimations utilizing a coordinate measurement machine (CMM) [27
] or laser tracker [28
]. In this work, we have proposed the use of the monocular camera with the AruCo markers to demonstrate it for the identification of Tarantula. Unlike the visual localization which is done using a single marker reported in [29
], we have used ArUco markers map (AMM) that resulted in the improved measurement accuracy. The measurement performance of this approach is compared using the standard industrial robot KUKA KR6 R900 robot (KUKA, Augsburg, Germany) [30
]. Being cognizant of the above facts, we set the following objectives:
Design of the robotic platform that can change its height and is holonomic,
Formulation for kinematics of the wheeled locomotion coupled with the leg kinematics,
Identification of kinematic parameters after the assembly of the robot, using monocular vision and ArUco markers,
Trajectory tracking of the robot using the same set-up of monocular vision and ArUco markers.
This paper is divided into five sections. Section 2
lists the design requirements and the mechanical layout, i.e., system architecture of the Tarantula
is discussed in detail. Section 3
introduces the workspace analysis of the Tarantula along with the kinematics of wheeled locomotion coupled with the leg kinematics. Experiments for identification of the kinematic parameters of the assembled Tarantula
along with the trajectory tracking in Section 4
. Finally, Section 5
concludes the paper.
5. Conclusions and Future Works
In this paper, Tarantula’s design, modeling, and its kinematic parameters identification are presented. The robot was designed for the specific geometry of the drains. The designed manipulator is modular and can adjust its height as per the environment. The simultaneous abduction or adduction of the two legs in a frontal plane was provided using the chain sprocket mechanisms connected using bevel gears arrangement with a single motor. A specially designed bi-directional suspension mechanism was fixed inside the trunk as a shock absorber. The limitation of the chain sprocket is that it imparts the rotational play because of slacking.
To the best of the author’s knowledge, this is the first attempt to estimate the kinematic parameters of the assembled robot with four leg using monocular camera mounted on the assembled Tarantula. This method also identified the effective dimension of the trunk where the proximal revolute joints were connected. The limitations of the monocular vision and fiducial markers to obtain the pose is that the camera must view the markers in the generated map. We also performed the experiments to evaluate the trajectory tracked by Tarantula using a similar set-up. Overall, the contributions of this paper are listed below:
Design of the modular robot Tarantula with the ability to reconfigure its height, mainly for inspecting the drains,
A mechanism designed for the simultaneous abduction/adduction of legs,
A methodology to identify the kinematic parameters using ArUco markers and monocular vision for the assembled mobile robot with legs. First, using the calibrated camera, poses of ArUco markers are reconstructed in 3D space. Second, by moving each joint and capturing images, a set of the tracked pose is determined. Then, the DH parameters were evaluated.
The domains set for future research work will mainly focus on: design optimization of the legs to reduce the total weight. Since the design is modular, the newly designed legs can be easily attached to the existing trunk. The second focus will be to mount the Light Detection and Ranging (LiDAR) sensor near the trunk opening of to map the drains.