Measurement of hydrodynamic forces and moment acting on Crabster,

CR200 using model tests

Abstract – Crabster CR200 is a remotely operated vehicle In this paper, we introduced measurement of
which has six artificial and articulated legs driven by hydrodynamic forces and moment acting on CR200 using
BLDC motors. It is supposed to walk on the sea floor. As a a 1/4-length scaled model and a 6-axis Force/Torque
result, the robot has a very complicated appearance and sensor. We researched in hydrodynamic forces acting on
hydrodynamic forces and moment acting on the robot legs [3] but, research on hydrodynamic characteristic
experience a variety of changes according to configurable acting on hull body has not yet accomplished. The scaled
legs and body postural position. This paper describes the model has a 1/4 length ratio. Strut were also designed and
experiments to estimate lift force, drag force and pitching manufactured. The experiments were conducted a
moment acting on CR200 using a 1/4 length ratio scaled circulating water channel. In order to find out ground
model. A 6-axis Force/Torque sensor was adopted and effect by the floor, we conducted the experiments in two
installed in the middle of inner space of the model to ways. The first is that the model was installed on the floor
measure the forces and moment. The experiments were and the other is that the model was installed in the middle
conducted in a circulating water channel whose width and of water stream.
depth are 3.5m and 3.0m, respectively. To define
hydrodynamic characteristics effected by the floor, the 2. System configuration and setup
authors approached in two ways. The first is that CR200
was assumed to stand on the ground. The other is when the 2.1 Crabster, CR200 and scaled model
robot is placed in the middle of stream of water flow. Crabster has 2.56m length, 2.60m width and 1.32m
height. Its weights in air and water are 682kg and 188kg,
Keywords – Crabster CR200, Drag force, Lift force, respectively. Therefore, it is inefficient and high-cost to
Pitching moment, 1/4 length ratio scaled model use the real prototype one to measure hydrodynamic
characteristics due to its size and weight. We designed the
1/4 length ratio scaled model. Figure 1 shows the
1. Introduction
prototype and the model. The length ratio was decided
Crabster CR200 is a kind of remotely operated vehicles considering uniform flow area and flow speed of the
(ROVs.) It is connected to its remote control unit on the circulating water channel, frontal area of the robot and
mother ship via a tether cable. Major difference between desired Raynolds number. In result, the model has 640mm
CR200 and any conventional ROV is how to thrust. length, 647mm width and 327mm height. In the center of
CR200 uses six artificial legs driven by BLDC motors. the hollow body of the model, the 6-axis Force/Torque
Each leg has four degrees of freedom. Using these legs, (F/T) sensor was installed as shown in Fig. 2. The sensor is
CR200 is able to walk and control its body postural motion. OMEGA191 SI-3600-700 by ATI Industrial Automation.
It is supposed to inspect seabed floor surface and its Between the model and the sensor, an angle-adjustable
surroundings while keeping its position and attitude. structure was adopted.
Because there is no propeller, there is no disturbance to
environment such like sediment or corals by propeller
wash. Experiments for underwater artefact search using
this advantage were conducted. [1] Walking and
swimming algorithms for Crabster has been investigated
[2]. But, its complicated configuration including six
foldable legs and several protruded sensors on the back
causes streamlines which pass by the vehicle itself
complicated.
Tendencies of hydrodynamic forces and moments
caused by water flow which passed by the legs and body
are presumed to be different with any other typical ROVs.
Therefore, investigation for hydrodynamic forces and
moments acting on CR200 has been raised. Especially,
downward lift force, forward drag force and pitching Fig. 1. The 1/4 length ratio scaled model (Left) and
moment are more important because they are related to Prototype (Right)
stability, rollover, and slip of the robot directly.
 Fig. 2. F/T sensor installed on the model with an Fig. 4 Streamline-shaped strut
angle-adjustable structure

Fig. 3. Coordinate system

2.2 Coordinate and setup Fig. 5. Two modes: Gliding mode (Left) and Grounding
The coordinate system is shown in Fig. 3. Forward mode (Right)
direction (surge) and downward direction (heave) of the
robot are designated as x and z, respectively. Right
direction (sway) direction is y. The origin of the coordinate
was coincident with the center of the F/T sensor.

3. Experiments
3.1 Apparatus setup
The circulating water channel has 3.5m width and 3.0m
water depth. To install the model in the circulating water
channel, strut was designed. It has streamlined shape as
shown in Fig. 4. There were two kinds of modes to find out
ground effect by the floor. The first mode is to install the
model very close to the floor but without contact. The
second mode is that the model is located in the middle of
the water stream. Therefore, we called the two modes as
“grounding mode” and “gliding mode,” respectively. It
was assumed that thickness of boundary layer is about Fig. 6. Experiments in the circulating water channel –
0.25m from the floor and the wall of the channel. Figure 5 (Count-clockwise from left-upper) Grounding
shows the installation concept of two modes. The left and mode with positive pitch, Grounding mode with
right parts are gliding mode and grounding mode, negative pitch and gliding mode
respectively. In the case of grounding mode, partial
volume of the model was presumed to be immerged into considering strength and stiffness of the model and strut,
the boundary layer. maximum speed was limited to 3.0kts. In the case of pitch
angle of the model, seven cases from -10º to 10º with 2º
3.2 Experiments and signal processing gap were adopted. Negative pitch angle means that the
The experiments were conducted in the circulating robot’s head is raised up. Positive pitch angle means, on
water channel. Flow speed is able to be accelerated to 7kts. the other hand, that the robot’s head is bowed down.
Eight cases of speeds were adopted. Flow speed was Figure 6 shows two mode and different pitch angles. Joint
accelerated from 0.4kts to 2.8kts with 0.4kts gap. And angles of the model’s legs are adjustable for more realistic
configuration descriptions.
 After the water flow speed reached at the desired speed 4. Conclusion
and became stable, we acquired data of the F/T sensor for
six minutes or more. And then, we accelerated or Experiments to measure hydrodynamic forces and
deccelerated the speed to the next desired speed. Sampling moment acting on the 1/4 length ratio scaled model of
rate and samples to read were 100Hz and ten, respectively. CR200 were conducted in the circulating water channel.
Raw data were filtered by Butterworth filter. And then, Less drag force acted on the model when the model was on
data of the last three minutes were collected and averaged. the floor very closer. Therefore, to keep CR200 from
rollover or slip, it needs to prostrate itself closely on the
3.3 Results sea floor with bowing its head down properly to increase
downward lift force and pitching moment negatively.
Drag force (Fx), Downward lift force (Fz) and pitching
moment (My) were focused. Figure 7 and 8 show variation Acknowledgement
of the drag force (Fx) with respect to pitch (θ) and water
speed (kt) at grounding mode and gliding mode, This research was supported by the Korea Research
respectively. As expected easily, the drag force increases Institute of Ships & Ocean Engineering (KRISO) for “The
as water speed increases. The pitch did not have any development of a multi-legged walking and flying subsea
significant effect. The model experienced less drag force robot” (PES2250, PES2400)
at the ground mode. In the case of lift force (Fz), effect of
pitch angle becomes serious as shown in Fig. 9 for
grounding mode and Fig. 10 for gliding mode. When the
pitch angle was 4o, the model experienced the maximum
downward lift force at the two modes. More investigation
is required to explain dramatic changes of Fz between
1.5kts and 2.0kts in Fig. 9(b). Figure 11 and 12 show
results of pitching moment (My) at grounding mode and
gliding mode, respectively. Only when CR200 bowed its
head down to 10o, the pitching moment increases
negatively as the water speed increases.

Fig. 7 Drag force at grounding mode – (a) [Upper] with Fig. 8 Drag force at gliding mode – (a) [Upper] with
respect to pitch, (b) [Lower] with respect to respect to pitch, (b) [Lower] with respect to
water speed water speed
Fig. 9 Downward lift force at grounding mode – (a) Fig. 11 Pitching moment at grounding mode – (a)
[Upper] with respect to pitch, (b) [Lower] with [Upper] with respect to pitch, (b) [Lower] with
respect to water speed respect to water speed

Fig. 10 Downward lift force at gliding mode – (a) Fig. 12 Pitching moment at gliding mode – (a) [Upper]
[Upper] with respect to pitch, (b) [Lower] with with respect to pitch, (b) [Lower] with respect to
respect to water speed water speed
 References
[1] B.H. Jun, P.M. Lee and Y. Jung, “Experience on
Underwater Artefact Search Using Underwater
Walking Robot Crabster CR200,” Proceedings of
OCEANS’15 MTS/IEEE OES Washington D.C, 2015
[2] S. Yoo, H. Shim, B.H. Jun, J.Y. Park and P.M. Lee,
“Design of Walking and Swimming Algorithms for a
Multi-legged Underwater Robot Crabster CR200,”
Marine Technology Society (MTS) Journal, Vol. 50,
No. 5, pp. 74-87, 2016
[3] B.H. Jun, H. Shim and P.M. Lee, “An Approximation
of Generalized Torques by the Hydrodynamic forces
acting on Legs of Underwater Walking Robot,”
International Journal of Ocean System Engineering,
Vol. 1, No. 4, pp. 221-228, 2011