Tri-SedimentBot: An Underwater Sediment Sampling Robot

Jun Han Bae1, Jee Hwan Park2, Sangjun Lee1, and Byung-Cheol Min1

Abstract— In this paper, we present Tri-SedimentBot (TSB),an underwater sediment sampling robot that uses combinedrotation and linear motion to collect sediment. A main contribu-tion of this system is a novel design for the sampling system thatemploys a dual-motion system with feedback control in orderto maintain the stability of the TSB and the sediment collectionmethod. We demonstrate the performance of the TSB with a setof experiments using both open and closed-loop control systems;experiments show that the TSB with closed-loop control systemoutperforms the TSB with open-loop control system.

I. INTRODUCTION

Rivers are one of the most important natural resources toconserve. Rivers provide water, not only for drinking, butalso for recreational and commercial opportunities, such aspower generation, farming, and factory operation. However,rivers are prone to contamination due to hazardous chemicalsor sewer overflows from urban, industrial, and agriculturalsites. The US Environmental Protection Agency (EPA) re-ported that 55% of rivers and streams in the US are inpoor biological condition [1]. River monitoring is necessaryto supply clean water to humans, animals, and plants. Inaddition, river monitoring should be continuous over a longperiod of time to analyze the trends in river condition andquality [2]. In order to observe the condition and qualityof a river, a periodical water and sediment sampling isnecessary and a crucial measure. For a long period of time,water and sediment sampling methods have been generallybased on human samplers [3]. However, human samplingmethods involve potential safety risks. For instance, in thecase of a large river with rapid water flow, it could behazardous for samplers to approach the required samplingpoint. If the sampling point is deep, it is difficult to collectsamples manually. This process requires a heavier samplerand additional equipment, such as an electrical cable reel, aboat, and possibly additional manpower.

Recently, some research groups have developed au-tonomous systems capable of monitoring water conditions orsediment sampling in order to overcome human limitationsof preexisting monitoring systems. Autonomous water mon-itoring systems composed of mobile sensors for real-timemonitoring and data collection, such as flow rate, pollutants,and fish migration, have been introduced to the environmentmonitoring research [4][5]. One development in the fieldof the unmanned aerial vehicle (UAV) is a water-sampling

1Jun Han Bae, Sangjun Lee, and Byung-Cheol Min are withthe Department of Computer & Information Technology, Purdue Uni-versity, West Lafayette, IN 47907 USA bae21@purdue.edu;lee1424@purdue.edu; minb@purdue.edu

2Jee Hwan Park is with the School of Mechanical Engineering, PurdueUniversity, West Lafayette, IN 47907 USA park458@purdue.edu

(a) Submerging (b) Landing (c) Drilling

Fig. 1: Sampling procedure of the proposed underwater sediment samplingrobot, Tri-SedimentBot (TSB).

UAV [6][7]. In the case of a large scale water environment,the ocean for example, autonomous underwater vehicles(AUVs) have been developed since the 1990s [8]. Generally,AUVs are large and heavy (200-300kg) due to the requireddeep water operation [9]. However, relatively light weight(34kg) and human portable sediment sampling AUV havebeen introduced [10][11]. The underwater robot describedin this paper is intended for small-scale environments, suchas rivers, ponds, and streams. While both river water sam-pling and river sediment sampling are important monitoringmeasures, this paper focuses on sediment sampling. From adesign point of view, the sediment sampling mechanism ismore challenging than water sampling. Since the sedimentis at the bottom of the water, sediment sampling requiresmore components and more complex mechanism than watersampling.

As a first step, an objective of this paper is to developan underwater sampling robot, Tri-SedimentBot (TSB), thatcollects the sediment samples. TSB has a dual-motion systemto efficiently collect sediment. The dual motion combinesrotation and linear motion. Figure 1 depicts the stages inwhich the TSB collects sediment. A DC motor creates therotation motion of the auger drill to penetrate the sediment.A linear actuator generates the linear motion to provide thenormal force to the auger drill when the TSB is drilling thesediment. Once the TSB submerges, it free-falls under itsown weight until it reaches the bottom of the river. Afterlanding, it starts drilling the sediment in order to collect thesamples. This paper describes the robot building process,incorporates concepts of dual motion and feedback systems,and considers the robots stability and control issues. Alsoa set of experiments were conducted to demonstrate theperformance of the proposed system.

2016 IEEE International Conference on Automation Science and Engineering (CASE)Fort Worth, TX, USA, August 21-24, 2016

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(a) Grab sampler (b) Corer sampler

Fig. 2: Examples of a sediment sampler.

II. RELATED WORKS

The purpose of collecting sediment samples is to analyzethe physical, chemical, biological, and toxicological condi-tions of water resources [12]. Moreover, it is important tochoose the appropriate sampling instrument and techniquedepending on the characteristics of the sediment and sam-pling location [12]. There are different types of samplingmethodologies. The grab sampler and corer sampler arecommon instruments for sediment sampling. Figure 2 (a)shows an example of the grab sampler. The grab samplersimply grabs the sediment by manual operation. It is a simpleand low-cost device, but it is practical for surface sedimentsampling in shallow water. It is not appropriate for increasedwater depth and it can be easily lost and contaminatedby the underwater conditions [13]. Figure 2 (b) shows anexample of the corer sampler. The main type of corer sampleris a cylindrical barrel that operates by the weight of thecorer. The corer penetrates the sediment by free-fall andthe sediment rushes into the barrel. To collect the sedimentsample, the corer sampler must be heavy enough to penetratethe sediment. However, it often drops some of the sedimentsample, so some corer samplers have a spring-loaded lid toclose the cylinder as soon as the sampling is complete. In themost extreme cases, scuba divers collect sediment samplesdirectly.

The methodologies mentioned above are based on humanactivity. As mentioned in section 1, sampling conditions canbe dangerous and even hazardous to humans and samplingassessments are lengthy due to various factors such asweather, expense, number of sampling locations and man-power. Thus, an effective sediment sampling device is neces-sary to improve river monitoring systems. Existing sedimentsampling AUVs are for large deep-scale environments; theAUVs themselves are large and heavy. The lightest sedimentsampling AUV is 34kg [10][11]. The TSB is lighter andsmaller than any existing sediment sampling robot. Givenits super lightweight and compact design, the TSB can beattached to various unmanned platforms, such as USV andUAV. This capability of the TSB can reduce the manufac-turing cost and achieve more frequent sediment sampling toconstruct more effective water monitoring systems.

Designing an optimized control system to maintain sta-bility is important consideration when developing the TSB.The control of underwater robots is challenging due to theuncertain disturbances and nonlinearity [14]. In this paper,

the only disturbance is the force of resistance acting on theauger drill when the TSB is drilling the sediment.

III. DESIGN & CONTROL SYSTEM

A. Robot System Design

The TSB's main components include a linear actuator, aDC motor, an auger drill, a syringe, motor housings, externalaluminum frames, and weights. The sampling system ofthe TSB is motivated by the typical underwater sedimentsampling and ground drilling methods. From the design andcontrol points of view, the corer sampler is better than thegrab sampler since it is simpler and has a wider usage range.The TSB adopted the corer samplers mechanism rather thanthat of the grab sampler. According to the design goal, itis insufficient to penetrate the sediment by its own weight.Therefore, the ground soil drilling method was reflected topenetrate the sediment in this paper. An auger drill is themost common drilling device with helical screw blades forground soil drilling. The TSB includes the cylindrical corerto collect the sediment and helical screw blades on the corerto penetrate the sediment surface.

1) Mechanical & electronic components: Figure 3 showsthe main components of the TSB. A linear actuator is locatedat the top of the system to implement the normal force to thedrill system, which combines a DC motor and a drill. TheDC motor is connected to the linear actuator and generatesthe rotational motion of an auger drill, which is hollow andcontains the syringe that collects the sediment. Two holesare drilled on the wall of both the auger drill and syringe.

Figure 4 shows the mechanism of the sediment samplingand the collected sediment in a syringe. The location of theholes depends on the desired quantity of sediment. The augerdrill and syringe assembly is an innovation of this mechanicalsystem since this assembly can collect the sediment samplewhile drilling. The core parts of the TSB are fixed to theexternal metal frame, which is an aluminum extrusion. Threesupport frames with weights are attached to main frames atthe bottom of the TSB to make a bottom-heavy system forstability [15][16].

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Fig. 3: Main TSB components: 1) Linear actuator, 2) 3D printed motorhousing, 3) DC motor, 4) Auger drill, 5) Syringe, and 6) Weight.

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(a) (b)

Fig. 4: (a) Sediment sampling mechanism and (b) collected sample.

To control the linear actuator and DC motor, the TSBis connected to a system composed of a microprocessor, alaptop, a motor driver, and a power supply. Two signal linesfrom the linear actuator are connected to the microprocessorto transmit and receive its position value. The DC motor isconnected to the motor driver and the microprocessor forpulse-width modulation (PWM) control.

2) Sensors: Two different types of sensors are installedon the TSB. First, an accelerometer is installed at the top ofthe TSB and connected to the microprocessor to measure rolland pitch angles. The difference between the current angleand the previous angle determines the stability of the system.Second, three limit switches are attached to the bottom of45 degree frames, which are connected to the main externalframes. The limit switch detects whether or not the TSBsuccessfully lands at the bottom of the water.

B. Stability

The stability of an underwater robot is important tomaintain the robot’s pose. In case of the TSB, the attitude(pitch and roll) plays an important role in maintaining thevertical motion. A horizontal motion (yaw and x, y directionmotion) of the robot is less significant because it has asymmetric design and the stability is only considered afterthe TSB lands. Stability is the tendency to return back to theoriginal position whenever an object is unintentionally tippedor flipped over by disturbances, such as shifting payloads,water currents, and waves [17]. For TSB, a resistance forceby the drilling motion can be added to the list of the potential

CB

CG

Fig. 5: Center of buoyancy (CB) and center of gravity (CG) of the TSB.

disturbances acting on the system. A completely submergedrobot is stable when its center of buoyancy (CB) is aboveits center of gravity (CG). The CB is the center of gravityof the water displaced by a submerged object. The CG isthe point that is equivalent to the total weight of the objectconcentrated at one point. If the object has a perfectly evenlydistributed mass, the CB and CG will be equal. However, theCB and CG of the TSB are not in same location, due to itsnon-uniform distributed mass.

Figure 5 shows the CB and CG of the TSB; its CG islower than CB. The CB and CG were calculated from theCAD program by inserting the material properties of eachcomponent. The CB is located at 216.39mm and the CG isat 194.42mm above the bottom. According to the positionsof the CB and CG, the TSB is a stable, bottom-heavy system.To increase the stability of the robot, the distance betweenthe CB and the CG needs to increase. The CB and CG aredepend on the magnitude of the robot's weight and buoyantforce, which is generally determined by the size and thematerial of the robot. Therefore, finding an optimized sizeand weight is crucial to increase the stability of the system.

A resistance force acting on the system is the onlydisturbance considered in this phase. Therefore, there is nodisturbance before the TSB starts the drilling motion for thesediment sampling after it lands on the bottom of the river.Once the sampling process starts, the stability of the TSBchanges from a stable condition to an unstable condition dueto the resistance force of the drilling motion. Thus, designingan optimized stability control system is necessary to validateand improve the TSB's performance.

C. Control System

When the TSB dives into the water, it sinks until it reachesto the bottom of the river. All of the limit sensors at thebottom of robot's support frames indicate that the rangebetween each sensor and the ground is zero, which showswhether or not the robot is successfully lands on the riverbottom. The TSB begins collecting a sediment sample byoperating the linear actuator and DC motor. The DC motorrotates clockwise to rotate the auger drill and the linearmotor simultaneously extends the auger drill until it reachesits maximum position. Once the drill arrives at the desired

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Fig. 6: (a) Position difference between the default and sampling conditions(b) Angle difference between the default and sampling conditions.

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Fig. 7: Control system diagram.

position, it rotates for a certain time (e.g. 5-10 seconds) tocollect the sediment sampling with the proposed mechanism.Once sampling is complete, the robot retrieves the sampleby the reverse motion of the linear actuator. These steps area single cycle of the sampling process. However, repulsiveforces acting on the TSB due to its light weight and rotationalso occur. Therefore, proper control of the sampling motionshould be taken into account.

Figure 6 (a) depicts a position (extended length) of thelinear motor with respect to time. The position, L, will belinearly proportional to the time, t, if there is no disturbance(e.g. repulsive force). The proportionality can be set as yr, areference or desired output as shown in Figure 6 (a). Sinceyr is a function of time, it can be derived as follows,

yr(t) = L(t) · cos θ(t) (1)

where yr is a reference output of the drilled length fromthe ground and θ is a tilted angle of the robot body thatmay be caused by a disturbance, ud. Therefore, if there is nodisturbance (when θ = 0), Eq. (1) becomes,

yr(t) = L(t). (2)

However, as the disturbance occurs, the extended lengthof the linear motor is not the same as the drilled length.The relation is no longer linear, as shown in Figure 6 (b).Similarly, an actual output of the drilled length from theground, y, can be derived as follows,

y(t) = L(t) · cos θ(t) (3)

where θ = ud. The difference between these lengths, yr andy, is called an error, e as described in Eq. (4). As the tiltedangle, θ increases, the error e increases and thus the systembecomes unstable,

e(t) = yr(t)− y(t) = L(t)(1− cos θ(t)). (4)

In order to maintain the stability of the system andlower the error, P (proportional) control is used. As shownin Figure 7, tilt angle θ, which is measured by a sensor(e.g. accelerometer), produces the error. The error is thenmultiplied by a constant gain, kp, and it is used as an inputof the system, u, as follows,

u(t) =

{yr(t) if θ(t) ≤ c

yr(t)− kp · e(t) else(5)

where c is a threshold to determine a control execution. Theangle value, c, should be small enough (e.g. smaller than 5◦).A condition input shown in Eq. (5) is designed to cope witha relatively slow reaction by the linear motor compared tothe fast sensor measurement.

IV. EXPERIMENT

In this section, the experimental results verify the per-formance of the TSB by comparing open-loop and closed-loop control systems. Figure 8 shows the experiment setting.A plastic trashcan is used for a water tank to simulate theaquatic environment. About 17cm of sand is at the bottomand the trashcan filled with water. A power supply andelectronic control boards (e.g. a microprocessor and motordrivers) are located next to the test stand. The TSB isconnected to the electronic control boards and the powersupply with a waterproof electrical wire. The TSB wasmanually dropped into the water to begin the experiment.

Three different input conditions were applied to the linearactuator to verify the stability of the TSB. Input conditionswere 9V, 10.5V, and 12V. A different voltage input changesthe performance of the linear actuator, which is the normalforce. The purpose of the various input conditions of thelinear actuator was to examine whether or not the magnitudeof the drilling force effects to the stability of the TSB. Thedrilling force of the TSB contains two difference forces,which are the normal force of the linear actuator and thetorque of the DC motor. The input condition of the DC motorwas fixed as 12V and a maximum 10 rpm because the rpmrange was too narrow to collect meaningful data. Therefore,the normal force of the linear actuator was the only variablethat changes the drilling force.

The input conditions of the control system was based onEq. (5), which is the proposed control system and can beexpressed and simplified as Eq. (6). This shows that as theTSB tilts more than 3 degrees, the input to the system willbe set to return to the initial position, L0. Otherwise, theTSB will continuously extend until it reaches at the desiredposition as follows,

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Fig. 8: Experiment setting: 1) Power supply, 2) Electronic system, 3) TSB,4) Test stand, and 5) Trash can.

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u(t) =

{yr(t) if θ ≤ 3◦

L0 else.(6)

The following values define the reference output positionof the linear motor, yr in Eq. (2) for the experiment: Lf

= 100mm, L0 = 0mm, tf = 90 sec, and t0=0 sec, whichforms the following slope equation, (Lf−L0)

(tf−t0), where Lf is the

final position of the linear motor, L0 is the initial positionof the linear motor, tf is the time at the final position, andt0 is the time at the initial position. The angle value, c, waslimited to 3 degrees for the experiment based on the trial anderror demonstrations. The return condition of the TSB wasthe initial position of the linear actuator, L0, instead of theproportional control variable, yr(t) - kp·e(t). If this variableis applied as the return value, it should be continuouslychanged by the time step, proportional value, kp, and thereal-time error. To maintain stability, the upward motion ofthe drill should occur once the system detects that the anglevalue is greater than 3 degrees. However, if the return valuecalculation from the system delays the upward motion of thedrill to maintain stability, the TSB might collapse. Therefore,to remove the calculation delay, the return value was set atthe constant value, L0.

A. Open-loop System

This experiment shows the performance of the TSB with-out a feedback control. The experiment was conducted withthree different linear actuator conditions (9V, 10.5V, and 12Vpower inputs). The speed of the DC motor was 10 rpm. Thered plot of the distance of linear motor chart (1st chart fromeach set of charts) from Figure 9 (a), (b), and (c) shows thetransient position of the linear actuator within the open-loopsystem. The open-loop system chart (2nd chart from each setof charts) from Figure 9 (a), (b), and (c) shows the roll andpitch angle of the TSB within the open-loop system. Theseresults indicate that the TSB is unstable during the first 30to 40 seconds.

B. Closed-loop System

This experiment shows the performance of the TSB witha feedback control. The experiment was also conducted withthree different conditions (9V, 10.5V, and 12V power inputs)of the linear actuator. The rpm of the DC motor is fixed atthis time. The blue plot of the distance of the linear motorchart (1st chart from each set of charts) from Figure 9 (a),(b), and (c) shows the transient position of the linear actuatorwithin the closed-loop system. The closed-loop system chart(3rd chart from each set of charts) from the Figure 9 (a), (b),and (c) shows the roll and pitch angle of the TSB within theclosed-loop system. These results indicate that the stabilityof the TSB is more stable than the open-loop system.

C. Results

All Figure 9 charts suggest that the TSB with a closed-loop system control is more stable than the open-loop system.The attitude (roll & pitch) angle difference is greater withthe open-looped system than the closed-loop system. This

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Fig. 9: Result charts with varied voltage inputs to the linear actuator thatshow comparisons of the performance of the TSB between the closed-loopcontrol and the open-loop control.

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Fig. 10: Weight of the sampled sediment from each system.

means that the TSB lacking a feedback control tilted morethan the TSB with feedback control. The distance of thelinear actuator chart shows that the linear actuator arrivedat the final position (fully extended) earlier within the open-loop system than the closed-loop system. However, this chartdoesn't indicate the depth of the drill, since the position ofthe linear actuator originates from the linear actuator itself.For example, in 9V conditions (Figure 9 (a)), the 1st chartshows that the linear actuator takes around 14 to 15 secondsto reach the final position in the open-loop system (redplot) and around 25 seconds in the closed-loop system (blueplot). From the attitude chart of the open-loop system (2nd

chart), the angle rapidly increases at the beginning (when thelinear actuator starts to extend and the drill starts to rotate)and decreases once the linear actuator is fully extended.The attitude remains constant for 30 seconds after the TSBstarts the sampling process. This indicates that even thoughthe linear actuator is fully extended, the TSB is tilted dueto the resistance force; therefore, the drilled depth is notequal to the extension length of the linear actuator and thedrill takes longer than the linear actuator to reach to thedesired position. From the 3rd chart, the attitude chart of theclosed-loop system, plots fluctuate when the linear actuatoris stretching and the drill is rotating; plots stabilize after 25seconds passes. This indicates that the linear actuator anddrill arrive at the desired position practically simultaneously.

Figure 10 shows the weight of the averaged sampledsediment from the 3 trials, each with different systems. In9V conditions, 24g sediment was sampled with the open-loop system and 52g sediment was sampled with the closed-loop system. In 10.5V conditions, 13.67g sediment wassampled with the open-loop system and 56.67g sedimentwas sampled with the closed-loop system. In 12V conditions,24.67g sediment was sampled with the open-loop system and52.67g sediment was sampled with the closed-loop system.The different voltage input of the linear actuator didnt affectthe weight of the sampled sediment. However, the weightof the sampled sediment increased with feedback control(closed-loop control).

V. CONCLUSIONS

A sediment sampling robot (called as TSB in this paper)was developed as an initial step in the development of

continuous and real-time autonomous water and sedimentmonitoring systems for large rivers. The performance of theproposed control system that operates the linear actuatorand DC motor to generate the drilling motion was validatedby comparing experiments of the TSB with two differentsystems. The TSB with a feedback control system showedthe better and more stable performance than the TSB withoutit.

An improved feedback control system with DC motorcontrol as well as a field experiment in the actual riverenvironment remain as future work that would improve andvalidate the performance of the robot.

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