Autonomous Underwater Biorobots A Wireless System for Power Transfer

  A Wireless System for _{PHOTO COURTESY OF TAREQ ASSAF,} Power Transfer _{CESARE STEFANINI, AND PAOLO DARIO} Autonomous

Underwater Biorobots

  his article describes a new design for wireless power transfer in autonomous underwater robots. The aim is to propose a solution for battery charging by taking into account the morphological and dimensional constraints of robots requiring small and low-weight internal modules. An innovative design is presented for inductive power transfer suitable for a wide range of applications. The system is

  Tareq Assaf, Cesare

  conceptually equivalent to a transformer in which the core can be separated into two parts

  T Stefanini, and Paolo

  during operation, one for each coil. Inductive power transfer is selected to have a system to

  Dario

  easily and reliably charge different kinds of underwater robots. The secondary coil and its magnetic core are designed to be placed inside a bioinspired robot; the weight, dimensions, and power output for battery charging are optimized. The shape of the secondary magnetic core section is hollow to house the control electronics and sensors. The primary coil is the power inductor, which is placed in a docking unit outside the robot. Experimental results are also reported.

  Project LAMPETRA

  The framework for the development of the device described in this article is the project Life- Like Artifacts for Motor-Postural Experiments and Development of New Control Technologies _{Date of publication: 1 August 2013 Digital Object Identifier 10.1109/MRA.2012.2201577} Inspired by Rapid Animal locomotion (LAMPETRA), a three-year (2008–2010) collaborative

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  research project funded under the Seventh EU framework program, theme ICT-2007.8.3-FET proactive 3, “Bio-ICT Convergence,” aimed at consolidating multidisciplinary Bio-

  ICT research carried out by a recently established scientific community. It involves four European countries: Italy, Swe- den, France, and Switzerland. The objective of the project LAMPETRA is to develop lamprey and salamander bioin- spired artifacts as tools for neuroscientific studies on goal- early prototypes: a lamprey-like artifact by Scuola Superiore Sant’Anna—Pisa (SSSA) [1], [2] and a salamander-like artifact [3], [4] by the Biologically Inspired Robotics Group-Lausanne—Switzerland (EPFL). In the lamprey prototype (SSSA), both power supply and con- trol were wired. This article focuses on the adoption of on-board batteries to enhance free locomotion. It also addresses the on-board integration of control electronics and sensors. In this article, a wireless solution for battery charging is described, as a number of constraints are provided by the artifact. The robot is covered by a smooth thin waterproof skin layer, which is supposed to be deli- cately touched during a charging process carried out through a non- contact power transfer. In addition, issues such as body flexibility and space for internal electronics mean that there has to be a bulk miniatur- ization (axial and transversal) of the on-board power receiving module. The device introduced in this article is a transformer-based system that can transfer electric energy from a burrow-like docking station to the robot, for feeding through battery charging.

  State of the Art Docking and Power Transfer System

  Despite the wide range of theories, methodologies, and devices for wireless power transfer described in the literature, we focus on those solutions adopted in the field of oceanic exploration for autonomous underwater vehicles (AUVs). Shape, dimensions, and energy requests change case by case, and the working principles of the charging systems can be grouped into three classes: direct electric contact, electrome- chanic generators, and inductive transfer [5], [6].

  In the first type, energy is usually transferred to electric contacts powering an on-board isolated transformer. The contacts can be damaged by oxidation and mechanical stress or by other interactions with the environment, and they are also a critical in terms of waterproofing. The second system

  (electromechanic generators) can be applied to vehicles that use propellers and are less suitable for bioinspired robots that perform swimming by body deformation. The electric energy is transferred from the docking station to the robot by a rotating shaft. A motor in the docking station turns the shaft that is coupled to a generator or to an alternator in the AUV. Conceptually, the third system (inductive transfer) is a transformer where the core can be split into two parts. It energy from a primary coil, placed in the docking station, to a secondary coil, in the AUV. The power is transferred across the air/ water gap between the two coils. A transformer-based system was there- fore selected as the best solution for the given application.

  Design Robot Overview

  The prototype of the lamprey-like artifact is supposed to be autono- mous, not only in terms of behavior but also concerning energy and mobility. To meet these require- ments, the robot is equipped with 12 vertebras, four active segments, six Lipo batteries, and a large number of sensors, such as vision, vestibular, and light sensors (Figure 1). Because of the critical dimensions of the robot, it was necessary to find solu- tions to different integration issues to include all the components on board. As can be seen in Figure 2, the robot can be divided into three main parts: the head, the segments, and the tail. The tail provides a significant contribution to the propulsion. It is a passive element that transmits the locomotion wave to the water with high efficiency. The segments are actuated by novel muscle-like actuators contained within a number of vertebras, which also host central pattern generator electron- ics and batteries. The vision and vestibular systems, the high- level control electronics, and the recharging device are all placed in the head. The head section represents the central computational unit. Placing all the above-mentioned compo- nents in the head was a big challenge considering the con- straints imposed by the project.

  Figure 1. The head segment. The vision, the central board, and the secondary winding for the wireless charge system are placed inside this module.

  Head (11 cm) Segments (55 cm) Tail (16 cm) Figure 2. The entire robot without the skin cover. The active vertebras and the batteries modules are visible in the segments section.

  Figure 5. Longitudinal section of the device shows the architecture of the transformer. The secondary magnetic core section is coupled with the primary, which is divided into six sections mounted around the secondary to maximize the coupling area.

  Equations and Dimensioning

  Primary Core Section Coupling Interface Secondary Area for Coil Windings Secondary Core Section Magnetic Flux

  Figure 4. Comparison between the designs of the secondary magnetic core section. The room available and the coupling in Case 2 are better than in Case 1.

  Robot Skin Case I

  Usable Room Case II

  Figure 3. LAMPETRA artifact. The first segment is the head- like section. This hosts the charging system and the control electronics. The total length of the robot is around 80 cm depending on the number of segments and on the tail length. Every segment implements magnetic muscle-like actuators [7] and local control electronics.

  Winding Control Electronics Segments

  Head Artifact Tail Secondary

  To design the secondary coil, we started with the sec- ondary coil core design. Equations (1) and (2) describe the core geometry in relation to the length and diameter, respectively. The magnetic flux flows from the primary coil to the secondary coil in the magnetic core, crossing the lit- tle air gap between the two (Figure 5). In the cross section

  Figure 6 shows dimensioning parameters of the cross section of the secondary core shape.

  Design constraints are the overall dimensions and mass, turns, magnetic flux, and desired power. After a parametric study reported in the following, we chose the optimum working point for our application in terms of maximum delivered power to weight ratio. The primary coil was dimensioned after the secondary, but in this case, dimen- sions were not a constraint because coupling geometry (interface with the secondary magnetic core section) and electromagnetic flux generation were the only two require- ments. Again, we chose the best compromise between power, dimensions, and number of turns.

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  The resulting longitudinal section envisaged for the device is shown in Figure 5. The secondary magnetic core section is separated from the primary section by a coupling interface, which is the air/water gap. The primary metal core section, consisting of six ele- ments, surrounds the secondary.

  On the other hand, Case 2 has more free room available for the electronics board, and the magnetic core is attached to the head shell, thus minimizing the gap with the external inductor.

  For the secondary coil magnetic core section, two dif- ferent designs were considered to reduce the bulk and the weight and to maximize the transferred power. Figure 4 presents the two possible approaches. The two repre- sented metal cross sections have the same cross area, but Case 1 has a uniform metal core, whereas Case 2 has a hollow shape. Case 1 is sometimes used for underwater vehicles [8], [9]. However, this solution does not fit our case because of the constraints related to bioinspiration. For example, with the increase in the air gap, the magnetic inductance would be reduced too much, and the elec- tronic components would have to be placed around this structure, which would result in a difficult design.

  The secondary coil was modeled first to fit the constraints imposed by the size and shape of the robot, which is eel around 50 mm, and the length of the robot depends on the number of segments (Figure 3). The charging system is supposed to be placed in the head segment with a length of 70 mm.

  Model

  The inductive power transfer was chosen after considering the advantages and drawbacks of the systems in the literature.

  Low weight and low encumbrance are the most important within the robot. The main criterion in the design is the max- imization of the ratios, transferred power versus weight, and size. Efficiency is of secondary importance for our purposes and is just considered for heating issues. The novelty of the system is the hollow shape of the secondary magnetic core section in which the electronics can be hosted.

  A transformer-like system that can transfer energy from a dock- ing station to a robot is described here. The system is a kind of transformer with a split core: 1) the primary coil (the source) and 2) the secondary coil (the receiver). Constraints due to the bioinspired approach during the artifact development are on the basis of the design of the device.

  SEPTEMBER 2013 Device Overview

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  w W L (

• 2 A )

  h ^D

  (Figure 6), and ( ) c r , where r = b 2 / , are dependent vari- N = . (9) _{2 f}

  d

  ables for the wire thickness and diameter of the device, respectively. These dependencies enable the magnetic flux Resistance was estimated in to be maintained constant at each point of the magnetic _t

  4 D t

  N l

  core. We approximated these values to a constant size in

  R = . (10) _{r 2 f} l

  this prototype. ₂

  bd b + C r ( ) = , (1)

  2

  r _{f f} bd b + h = . (2) _{h h} D _{a a}

  The maximum number of turns ( ) N t was calculated using _{b b b} (3) by varying the inner diameter ( ) d and considering the fol- _{c c D d d} (r) (r) lowing values: external diameter ( ) D 50 mm, diameter of iron length ( ) strips ( ) b 1 mm ( r = b 2 / ), L of the device 60 mm, _f and copper wire diameter ( ) d 0.5 mm: _{2 L L t}

  L 2 2 a (( bd b ) / (

• 2 b d )) + + 2 .

  N = (3)

  $ _{2 2}

  d Figure 6. Secondary cylindrical magnetic core unit cross section. _{EMF ; sec} How the laminations were shaped for the secondary magnetic

  The voltage (

  V ) and inner resistance ( ) R are calcu- D the column core section is shown. The external diameter ( ), b the wings for coupling with the primary core ( ), a

  lated using (4) to estimate the output power ( Pow u ) estimated diameter ( ),

  L are given data. The wing diameter and length of the device ( )

  in (6) and the current transferred to the secondary coil:

  ( ( )) h d and the winding area ( ( )) c r are dependent variables ( r b / ). 2 d is the independent parameter.

  The inner diameter ( ) =

  1 D + + d 2 b _{t t t t t} The magnetic flux is .f

  R = N l =

  2 N , (4) $ $ $ _{f 2} S f _{EMF sec t r max 2 2} d V | = N

  2 f B ( bd b ), (5) + $ $ _{EMF | sec}

  Pow_u

  V

  1 Veff_load

  Pow u = ,

  $ (6)

  10

  2

  4 R _{t max}

  leff_load

  where is the resistivity, f is the frequency, B is the

  N_t/100 _t

  5

  maximum magnetic field for the given material, and l is

  Mass [hg]

  the turn length. Relevant characteristics, plotted in Fig-

  Pow_s

  ure 7, were evaluated to choose the dimension of the inner _d diameter. We chose the maximum power/mass ratio as 0.01 0.02 0.03 0.04 0.05 0.06 working point.

  The primary coil consists of a number of subcores (six)

  Figure 7. Output power, voltage, current, number of turns, mass, and

  that surround the secondary coil. The primary coil was

  power/mass ratio are plotted as a function of the inner diameter d,

  designed to ensure an adequate magnetic flux in the second-

  considering the external diameter, the axial length, and the diameter

  ary coil, by exploiting (1)–(6) and the following relations: of the wire (0.5 mm) as given. Arrow: the working point chosen, _{D {} whereas the related obtained values can be found in Table 1.

  ( ) Ni = . (7) Table 1. Working values obtained from the model. The general law governing the relation between the

  Parameters Values (IU)

  number of turns ( N D ), the current ( ), i and the magnetic

  N t , number of turns 560

  flux are represented by (7). The reluctance ( ) calcu-

  Veff load in voltage 7.63 (V)

  lated by using (8) is an intrinsic value of the magnetic

  R, inner resistor 7.07 (X)

  core, depending on its shape. Air reluctance is calculated

  Ieff load, current 1.08 (A)

  by approximating the air gap size:

  8.23 (W) Pow u , output power * _tot M, mass * 0.22 (kg) 2 air/water sec pr .

  (8) = + +

  Pow s , power/mass ratio * 36 (W/kg) _D Marked (*) parameters are not dependent on the wire diameter

  N is calculated by using (9) considering the free space (d ). Other parameters are strictly related to d . The wire diameter _{f f}

  around the magnetic core where W is the width of wind- _w was chosen considering the current and voltage desired for the ing, L is the length, and A are “wing” dimensions (refer application. to a in Figure 6):

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  29 We used a Delrin core to assemble the metal laminations in the cylindrical-shaped ending (Figure 10). Epoxy material was used to create a rigid structure and to provide electronic insu- lation. The magnetic core received further electric insulation by means of insulating lacquer. The module was then wound in copper wire. More than 500 windings were utilized. Weight, resistance, and dimensions were checked after fabrication.

  The primary unit was developed by following similar design criteria to those adopted for the secondary side, particularly with regard to the maximum magnetic induction available in the material and target voltage. The module is composed of six radial coils (Figure 11), generating a magnetic flux, and poles are located with a small radial gap at the ends of the sec-

  Figure 8. The final and intermediate shape of the iron wire.

  ondary module. Epoxy material was used to link iron pieces in the six cores. Coils were made by using a wire with a diam- eter of 0.8 mm, which gives an impedance for the six coils in series that matches the voltage given by a commercial transformer, as detailed in the following.

  After preliminary tests on overall functionality, the mod- ule was embedded in polyurethane (PU) to strengthen the structure, increase the heat dissipation, and make the system waterproof. The device will also be coated with a special lac- quer paint to obtain a smooth surface and to ensure complete isolation when underwater.

  Experiments

  Once the final prototype was ready (Figure 12), experi- ments were performed to verify compliance with the model and to characterize the device. Particular attention was paid to the testing of the output power supplied by the secondary coil. Table 2 summarizes the main charac- teristics of the system.

  Figure 9. Iron wire modeling. About 200 laminations were

  The complete device is powered by an external worksta-

  crafted for the entire device. A simple but stable setup was built

to ensure constant dimensions for all the laminations. tion and works at 48 V and 50 Hz. The most important value

  to investigate is the maximum output power of the second-

  Materials and Methods Secondary Module

  As already mentioned, the secondary coil dimensions are approximately 50 mm in diameter and 60 mm !2 mm in length. Off-the-shelf materials were chosen as building elements. _● Materials: _● iron wire (1.2 mm diameter) _● Delrin _● copper wire (0.5 mm diameter) epoxy material.

  The iron wire was shaped into curved laminations (Figures 8 and 9) in accordance with the transformer theory. In fact, using a low-frequency magnetic field (50 Hz), hysteresis losses and eddy currents can influence the device by generat-

  Figure 10. The secondary coil is mounted using the laminations,

  ing side effects in the magnetic core. To reduce these effects,

  and a Delrin core is used as a building frame for the metal. First,

  which may decrease the efficiency of the device, the magnetic

  the metal is bound in a solid structure using Epoxy material and then wound in copper wire.

  core needs to be built with lamination-like elements.

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  Figure 12. Final result before coating.

  Figure 11. The secondary coil was used to mount the six different primary subcores at the correct distance. The secondary coil needs to be free to get in and out of the primary magnetic coil.

  1.5 *Waterproof.

  2

  83 Air/water gap max (mm) —

  68 ^Q 115 ^# 68 178 ^# 173 ^#

  50 ^#

  (mm) ^Q

  10 Electrical Isolation Yes Yes Yes* Dimensions

  10

  6.5

  Data Secondary Primary Primary + PU Mass (kg) 0.206 1.805 3.702 Resistor (X)

  Table 2. Summary of working values obtained from the model.

  Stop Figure 13. Output voltage amplitude is approximately 14 V.

CH1 RMS

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  CH2 Off Freq.

  27.8 V CH1 None CH1 5.00 V CH1/4.80 V

  1 CH1 Peak–Peak

  9.03 V CH1 None

  Tek M 10.0 ms M Pos: 0.000s Measure

  The primary coil can theoretically work up to a standard volt- age of 220 V at 50 Hz; however, it can work at a lower voltage and was developed to operate at 48 V. By reducing the voltage: 1) the device is less dangerous in a water environment, 2) the energy required is lower, and 3) the heat dissipation decreases. The work frequency is also an important parameter. Although an increase in frequency improves the performance of the device, the design proposed here works at 50 Hz, considering the international standard frequency (50–60 Hz); the device can thus be plugged in easily everywhere. These design choices

  Discussion

  The efficiency, as reported on Table 3, is roughly 16% and is close to the estimated value. The low efficiency of the device is due to the design in terms of operating frequency and also the handmade lamination and windings. Efficiency can be increased by tuning the frequency and by improving the magnetic core. On the other hand, the aim of this device is not to be highly efficient, above all to be suitable for bioin- spired purposes in water environment and to be safe. From this point of view, the device works as expected and is fully compatible with the electronics and all other robot compo- nents. Finally, the energy requirements of the robot are very low (power consumption is less than 5 W, and battery capacity on board is only 20 Wh) and therefore energy before conversion is also very limited.

  ; therefore, a negative trim of 75 g is present and must be negotiated by exploiting small additional adjacent empty spaces.

  3

  The results of the tests are summarized in Table 3. The “Real Device” column shows the experimental data obtained with the device, and the “Model” column shows the data on the theoretical dimensions retrieved using the model. The real values are very close to the parameters obtained with the model. Finally buoyancy, as given by the Archimedes princi- ple, was estimated: the mass of the secondary device is 200 g, and its overall volume is 145 cm

  ary coil. The output power is maximum when load imped- ance equals the conjugate inner one according to the maxi- mum power transfer theorem. An oscilloscope (Figure 13) was used to measure the voltage at the ends of a load: this is a series of resistors and capacitors whose resulting impedance is close to the conjugate of inner resistance and inductance.

  Scuola Superiore Sant’Anna, Pontedera 56025, Italy. E-mail: c.stefanini@sssup.it.

  1.3

  Paolo Dario,

  Scuola Superiore Sant’Anna, Pontedera 56025, Italy. E-mail: paolo.dario@sssup.it.

  Table 3. Test results.

  Data Model Real Device Output power (W)

  12.2

  12.5 Efficiency (%)

  18 _{+ 16} Current (A)

  1.4 Voltage (V)

  Cesare Stefanini,

  9

  9.5 Resistor (X)

  6.4

  6.5 Capacitor ( _n

  F) —

  67 Phase correction (°) — _{+ 151} Dimensions (mm) ^Q 52 # 68 ^Q 52 # 68 Metal core diameter (mm) ^Q

1.3 Q

  1.3 Turns 530 _{+ 530}

  Scuola Superiore Sant’Anna, Pontedera 56025, Italy. E-mail: t.assaf@sssup.it.

  32

  During the tests, the device did not cause any damage to the electronics when the magnetic field was applied. The device will be shortly integrated into the whole system.

  SEPTEMBER 2013

  are strictly related to the low efficiency of the device compared with the traditional transformers that can achieve a very high efficiency up to 95%. The air/water gap, the low working fre- quency, and the manufacturing process reduce the efficiency to 16%. This value is acceptable for our application in which external power is not an issue. Furthermore, the concept and the design are still valid, although applications would require higher performance and efficiency.

  Another important feature is the hollow shape of the secondary core section. Using this design, electronics can be placed into the secondary coil magnetic core, thus limit- ing any wasted space. This design also reduces the weight, which is an important parameter in underwater applica- tions when a neutral trim is required.

  The secondary coil final mass is 200 g. Considering the total length (and volume) of the artifact, this low weight enables the buoyancy of the robot to be designed, thus not having to take into account the weight of the secondary coil.

  Finally, the expected self-docking feature has been observed. The secondary coil is attracted into the primary coil with a delicate force of about 0.5 N. This function is not mandatory for our application, where we have locomotion capability; however, it is still useful as additional self-guid- ance while docking.

  Conclusion

  We have successfully developed a low-weight system for bat- tery charging by proposing a new design for wireless power transfer devices. Our design is useful in situations when there are constraints regarding: small dimensions, a cylindri- cally shaped robot, low weight, and high electrical insulation. Another advantage is that only one primary coil is needed to charge different robots equipped with individual secondary coils. Robot docking is critical and our device provides a neat solution with self-guidance. All these features highlight that our device would work well in AUV applications.

  Acknowledgment

  Tareq Assaf,

  This work is primarily being carried out under the Euro- pean Project LAMPETRA (Life-like Artifacts for Motor- Postural Experiments and Development of new Control Technologies inspired by Rapid Animal locomotion) Proj- ect Reference: 216100 through the Seventh Frame Program research area: ICT-2007.8.3-FET “Bio-ICT Convergence.” It also comes under the European Project ANGELS (ANGuil- 231845 through the Seventh Frame Program, research area:

  ICT-2007.8.5-FET “Embodied intelligence.”

  References [1] C. Stefanini, S. Orofino, L. Manfredi, S. Mintchev, S. Marrazza, T. Assaf, L. Capantini, E. Sinibaldi, S. Grillner, P. Wallen, and P. Dario, “A novel auton- omous, bioinspired swimming robot developed by neuroscientists and bioen- gineers,” Bioinspiration Biomimetics, vol. 7, no. 2, p. 025001, 2012.

  [2] S. Grillner, A. Kozlov, P. Dario, C. Stefanini, A. Menciassi, A. Lansner, and J. H. Kotaleski. (2007). Modeling a vertebrate motor system: Pattern genera- tion, steering and control of body orientation. Prog. Brain Res. [Online]. 165(14), pp. 221–234, PMID: 17925249. Available: http://www.ncbi.nlm.nih. gov/pubmed/17925249 [3] A. J. Ijspeert, J. Hallam, and D. Willshaw. (1999, Mar.). Evolving swimming controllers for a simulated lamprey with inspiration from neurobiology.

  Adapt. Behav. [Online]. 7(2), pp. 151–172. Available: http://adb.sagepub.com/ cgi/content/abstract/7/2/151 [4] A. Crespi, A. Badertscher, A. Guignard, and A. J. Ijspeert, “AmphiBot I: An amphibious snake-like robot,” Robot. Auton. Syst., vol. 50, no. 4, pp. 163–175, 2005.

  [5] R. Stokey, M. Purcell, N. Forrester, T. Austin, R. Goldsborough, B. Allen, and C. von Alt. (1997). A docking system for REMUS, an autonomous under- water vehicle. presented at OCEANS MTS/IEEE Conf. Proc. 2, pp. 1132–1136. [Online]. Available: 10.1109/OCEANS.1997.624151 [6] R. Coulson, J. Lambiotte, G. Grenon, T. Pantelakis, J. Curran, and A. An. (2004). Development of a modular docking sub-system for 12" class autonomous underwater vehicles. presented at OCEANS MTTS/IEEE TECHNO-OCEAN. 3, pp. 1745–1749. [Online]. Available: 10.1109/OCEANS.2004.1406388 [7] C. Stefanini, S. Mintchev, and P. Dario, “Permanent magnet actuator for adaptive actuation,” Patent WO 0 15 997, Feb. 11, 2010. [8] B. Heeres, D. Novotny, D. Divan, and R. Lorenz. (1994). Contactless under- water power delivery. presented at IEEE Power Electronics Specialists Conf.

  PESC Rec. 25th Annu. 1, pp. 418–423. [Online]. Available: 10.1109/ PESC.1994.349700 [9] A. Bradley, M. Feezor, H. Singh, and F. Y. Sorrell. (2001). Power systems for autonomous underwater vehicles. IEEE J. Ocean. Eng. [Online]. 26(4), pp. 526– 538. Available: 10.1109/48.972089

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