Research Article Electromechanical Characterization and ...tive, shape memory alloy (SMA) and...

Hindawi Publishing CorporationAdvances in Materials Science and EngineeringVolume 2013, Article ID 683041, 17 pageshttp://dx.doi.org/10.1155/2013/683041

Research ArticleElectromechanical Characterization and Locomotion Control ofIPMC BioMicroRobot

Martin J.-D. Otis

Applied Sciences Department, REPARTI Center, University of Quebec at Chicoutimi, Chicoutimi, QC, Canada G7H 2B1

Correspondence should be addressed to Martin J.-D. Otis; martin [email protected]

Received 22 June 2013; Accepted 6 September 2013

Academic Editor: Kean Aw

Copyright © 2013 Martin J.-D. Otis. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

This paper presents the electromechanical characterization of Nafion-Pt microlegs for the development of an insect-like hexapodBioMicroRobot (BMR). BMR microlegs are built using quasi-cylindrical Nafion-Pt ionomeric polymer-metal composite (IPMC),which has 2.5 degrees of freedom. The specific manufacturing process using a laser excimer for one leg in three-dimensionalconfigurations is discussed. Dynamic behavior and microleg characteristics have been measured in deionized water using a laservibrometer. The use of the laser vibrometer shows the linear characteristics between the duty cycle of square wave input anddisplacement rate of the actuator at multiple frequencies.This linearity is used to design a servo-system in order to reproduce insecttripod walking. As well, BMR current consumption is an important parameter evaluated for each leg. Current passing throughoutthe IPMCmembrane can result in water electrolysis. Four methods are explained for avoiding electrolysis.The hardware test benchfor measurements is presented. The purpose of this design is to control a BMR for biomedical goals such as implantation into ahuman body. Experimental results for the proposed propulsion system are conclusive for this type of bioinspired BMR.

1. Introduction

Microrobotic development in submillimeter size has reallyappeared with MEMS (Microelectromechanicals systems),using the MUMP process (Multiuser MEMS processes)and microfabrication process with excimer laser and LIGA(Lithographie, Galvanoformung, and Abformung) microfab-rication technology [1]. Several designers and researchersin micro- and nanorobotics hope their robots will per-form biomedical applications. Currently, these robots are inthe early stages of development, and the objectives to bereached are spread out over several decades [2]. Micro- andnanorobotics are an important area covering the knowledgeof science and applied science in robotics and microsystems.Natural law explains that the smaller a living organism is,the harder it is to kill it. BioMicroRobot is designed withthis simple rule. If the control of micro- and nanotechnologyis possible, it will be also possible to easily help or to killliving organisms (cancer, bacteria, etc.), as well as organic orinorganic substances (kidney stones).

Many microactuators are in development for insect-likemicrorobot legs, such as bimorph cantilever structures acting

as thermal actuators coupled or not with photonic band gap(PBG) materials, piezoelectric, electrostatic, magnetostric-tive, shape memory alloy (SMA) and electroactive polymer(EAP) such as ionic polymers to name a few [3]. Someapproaches are very interesting, but in biomedical appli-cations, greater problems involve material biocompatibility.This constraint limits the use of some inert materials, largelyused in human body like platinum, NiTi (Nitinol), somestainless steels, ceramic alumina, and some polymers. Mate-rials from a biological source are interesting for microroboticapplications. This material should work overall in robotmission time without degradation, corrosion, or wear in thechemical environment (enzyme, pH, and under lymphocyteaction). Of course, this material must not be toxic to the hostsystem [4].

BioMicroRobot (BMR) is amicrorobot used to help livingorganisms such as the human body in its metabolism and toresolve health problems like kidney stones. It can be used tounderstand, explore, change, or control biological systems.BMR with biosensors can replace medical instrumentationfor monitoring and measuring physiological events such asan in vivo glucose biosensor with hydrogel.

2 Advances in Materials Science and Engineering

BMR propulsion needs specific studies considering itsmany constraints. In this order of size and magnitude, itis necessary to study the context of its use and the BMRenvironment (aqueous medium, its conductivity and veloc-ity, ambient temperature, pressure, etc.). Manufacturing isalso an important challenge because of three dimensionalhandling of independent microparts. With the power supplybeing limited, it is necessary to reduce current consumptionwhile optimizingmechanical properties of the actuator (forceand velocity).

This paper covers all IPMC actuator parameters to beoptimized for use on a BMR. Based on the choice made forthe design of the microleg, it will be possible to manufactureit. The first part of the paper presents the chemical andmechanical manufacturing process of the microleg, whichis critical for good quality and electromechanical properties.A failure in the procedure and bad handling could damageactuators and decrease performance. Indeed, metal electrodedistribution particles added at themembrane surface are verysensitive to the chemical conditions of treatment, impreg-nation, and reduction. Second, for reducing some disad-vantaged electromechanicals properties, a detailed model iscarried out. Lastly, this paper recommends a central patterngenerator for locomotion control using the actuator model.Displacement rate according to pulse width modulation(PWM) duty cycle and the resonance frequency coupled witha fluid according to the length is presented. This relationshipwill be very important for the design of a bioinspired servo-system in order to control the BMR like an insect in two- orthree-dimensional environments.

1.1. Previous Work. Microrobot projects coming from Shuxi-ang Guo’s laboratory are particularly advanced in designingmicrorobot with IPMC actuator. Recently, they study theposition accuracy using 8 legged-locomotion using an IPMCmodel for onemotion direction (using two electrodes on eachsurface) [5]. Also, other studies relates IPMC position control[6, 7] and in-pipe microrobot [8]. However, these micro-robots are designed in order tomove along a plane and to floatin suspension without maneuverability in three-dimension.

This paper proposes a BMR similar to an insect with sixmicrolegs. In order to move the body in a three-dimensionalenvironment, each leg has tomove in at least three axes (threedegrees of freedom or 3 DOF). Since the robot is limitedin size, power consumption, and manufacturing complexity,this paper suggests to design a leg which has one axisof movement coupled with the others two. Therefore, thisconfiguration is considered as aminimum,which correspondto 2.5 degrees of freedom motion. This configuration allowsdriving the BMR as an insect tripod. However, with a lowerDOF, the use of the push-slip method is also a design optionsuch as piezoceramic actuation on a plane surface [9], but thispropulsion reduces the application to a rigid surface. HigherDOF is better when the propulsion surface is deformed andmany obstacles are present. To avoid obstacles, microlegsmust leave the surface, which explained the need for 2.5 DOFper microleg.

Moreover, the actuator must have sufficient dimensionsto support a microrobot of millimeter-length size. Along

these lines, previous paper was devoted to the manufacturingprocess of tubular and fiber IPMC [10, 11]. A clear explanationon the manufacturing process of the four electrode microlegsusing a laser is provided to allow actuation with severaldegrees of freedom. Starting with some samples, it will bepossible to measure the electromechanical characteristicsquantitatively.

1.2. BioMicroRobot Geometry and Design. The BMR micro-actuator consists of six (6) quasi-cylindrical Nafion-Pt ion-omeric polymer-metal composites (IPMC), and one tubularIPMC connected to a micropump enabling thrust vectorcontrol. IPMC has three (3) components: an ion-exchangemembrane (IEM) like the perfluorosulfonic acid polymer,some cations, and at least two electrodes. Using a 6-leggedlocomotion is generally used to improve the stability of themicrorobot during a displacement on a flat surface such asthe cockroach tripod walking. However, some applicationswhere more complex maneuvers in fluid such as obstacleavoidance and trajectory following require navigationin three dimensions enabling swimming capabilities. Inthis situation, it would be possible to use a thrust vectorcontrolled by a tubular IPMC. First, a micropump installedon the back of the robot pushes the ambient fluid in a tubularIPMC. The curved tube directs the fluid jet generating athrust (force) vector located at the IPMC anchored pointwhich push microrobot following Newton’s laws of motion.The use of a micropump is however necessary with the tube,which still represents a significant technological challenge.The characterization of this system (micropump, IPMC, andtubular fluid density) is not a part of this paper.

IEM consists of a cross-linked ionomer matrix withrelatively uniform distribution of ion-active sites (cluster net-work) throughout the structure. The effects of different typesof cations, electrode-membrane interfacial areas, and surfaceelectrode resistance on performance are well known [12–14].

Each IPMC component has an effect on the electrome-chanical characteristics. IPMCworks on two principles. First,the electroosmosis in the ionomermoves the cation andwaterto the cathode side by the electrostatic force when an electricfield is applied. The osmotic pressure inside the polymercreates a bending that produces a corresponding displace-ment [15]. The second principle is the pumping effect of thecations. It would be the dominant contribution in the caseof hydrophobic cations. It seems that the superposition effectworks with different cation types, such as the ammoniumform and alkali cations [16].

Our BMR design is presented in Figure 1. A vision systemis also currently being evaluated, which is designed like anechosounder. Since the leg design should move in 2.5 degreesof freedom, the microleg tip movement could be representedby Figure 2. The motion of the microleg is composed oftwo gait phases: swing phase (recovery phase) and stancephase (support the weight body). The anterior transitionpoint (transition from swing to stance) is called the anteriorextreme position (AEP), and the posterior transition point iscalled the posterior extreme position (PEP).

This electroactive polymer or IPMC actuator has manyadvantages over all others; it has a quick response to low

Advances in Materials Science and Engineering 3

(a) (b)

Figure 1: BioMicroRobot geometry using IPMC micropump and six IPMC microlegs.

0

0.5

1

1.5

2 00.2

0.40.6

0

0.5

Amplitude (m

m)

Swing

BMR body

Distance (mm)

Pivo

t (m

m)

AEPStance

PEP

−0.5

Figure 2: Ideal microleg tip movement in 2.5 DOF.

voltage, bends more than 100K times, generates distributivedeformation, has a low weight, can be downsized easily, andcan be used in water [17]. It seems to be safe in the humanbody [18] if no water electrolysis is produced. It can be drivenat low voltage, less than 2 volts. However, as described inFigure 3, we havemeasured a step transient response showinghigh instantaneous current consumption with Nafion-Pt andLithium counter ion.

Water electrolysis and current consumption are twomajor IPMC disadvantages for microrobot applications. Inpart of this paper, we will demonstrate how to avoid elec-trolysis of Nafion-Pt IPMC with Li+. Platinum dissolution isalso an inevitable consequence ofmetal use in the stimulationregime. Dissolved platinum is toxic in living organisms [19].

0 0.02 0.04 0.06

00.0020.0040.0060.008

0.01

Time (s)

Am

plitu

de (A

)

−0.06 −0.04 −0.02−0.01−0.008−0.006−0.004−0.002

Square wave 0-1VSquare wave 0–2VSquare wave 0–3V

Figure 3: Current response to different squares waves voltage input.

2. Materials and Methods

In previous studies, we have used the tubular Perma PureInc. TT-030 Nafion [10] for the micropump design. Actuatormovement is adjusted to orient the fluid direction. Fora microleg design, the tubular shape generates too smalldisplacement for this application. In another application,Nafion tube will be used to create an implantable inductivecoil for BMR telemetry and power supply. In fact, the use ofNafion N-117 (DuPont) fiber (quasi-cylindrical cross section)is better for the displacement rate and current consumption[11]. The size of this new microactuator is about 200 𝜇m× 200𝜇m. Its length L is about 4,000 𝜇m for the actuatorcharacterization. Expected final microleg length on the BMRbody is about 1,500 𝜇m.

2.1. IPMC Manufacturing. This method can be done usingany ionomer membrane. Ion exchange membrane (IEM) isa hydrophilic ionic polymer, which swells up about 10% inwater at room temperature. The membrane N-117 is cut into

4 Advances in Materials Science and Engineering3,

00

2,33

1,00

1,50

7,00

10

1,00

0,10

4,50

0,05 0,05

Delrin

Minimum

Maximum

6

7

8

Ø0,80

Figure 4: Six fiber holders for the manufacturing process.

fibers of 187 𝜇m with a sliding microtome LEICA SM 2500.The speed of microtome blade is set to 0.5mm/second withan angle of attack set at about −2 degrees. Next, the six fibersare attached to a homemade support as shown in Figure 4.

The ionomer is cleaned in an ultrasonic water bathand boiled in HCl and water for about 30 minutes each.The ionomer is chemically plated with platinum using theimpregnation and reduction (IR) process [20] as shown inFigure 5. Ion exchange (absorption in Figure 5(a)) and ionreduction (Figure 5(b)) are repeated up to two times to make3 𝜇m thick porous platinum layers (Figure 6). Li + cations areused on all IPMC to obtain faster responses.

2.2. Excimer Laser Process. The electrodes on the tube andfiber are cut into four sections using an excimer laser process.The laser beam removes the plated surface in order to formfour sections around the IPMC. These four sections drivethe IPMC in 2.5 degrees of freedom. Figure 7 shows themicropump propulsion using tubular IPMC while Figure 8represents one microleg. Both Figures are resulting from the

laser process. This configuration around the IPMC shouldallow eight directions of motion labeled as S

1to S8. This

technical process is also used to build up the BMR body andinduction coil for the power supply.

2.3. Hardware Test Bench. In order to obtain the dynamicbehavior ofmicrolegs under electrical solicitation, a scanninglaser vibrometer is used to measure the tip (free end) speedof displacement as a function of both frequency and theelectrical current induced. The effect of microleg length onthe first vibration mode is also inspected in a fluid.

The beam formed by the actuator is clamped at oneend and is acting freely in 18MΩ deionized water at roomtemperature (Figure 9). The laser beam goes through anoptical window and reaches the microleg perpendicular toits neutral plane. Refraction of light has not been taken intoaccount in the related measurements.

The applied voltage is a periodic PWM signal pro-grammed as an arbitrary waveform generator-like modelHP33120A. The waveform consists of a 1024-point signalwhere half the points form a positive PWM and the otherhalf a negative one. Instantaneous speed was measured at thetip of the actuator and stored for all duty cycles used in thePWM signals of the waveform.Thewaveform has a 3V peak-to-peak and is applied at 10 and 20Hz.

The current is measured using two types of instruments.The first type is a current probe thatmeasures the electromag-netic field around the cable (Tektronix P6042). The secondtype is a transimpedance amplifier (with gain set at 100 usingthe resistor) placed in serial with H-Bridge MOSFET andIPMC. The isolation circuit using H-bridge MOSFET forthe stimulus circuit is placed to drive opposite electrodes onIPMCmicrolegs.This circuit is used with themicrocontrollerparallel output port to drive all six microlegs and one vectorcontrol propulsion (this represents a total of 28 electrodes).

These measurements were done far away from the firstresonance of the mechanical system: a modal analysis ofthe microleg using a randomly applied voltage and scan-ning capability of the vibrometer revealed an initial naturalfrequency of 180Hz. This frequency takes into account thecoupling between the fluid and the structure.

3. Microleg Model

Our previous paper demonstrates three-dimensional micro-leg movements with mathematical equations and simulationfor the actual electrode configuration [10, 11]. Two voltageamplitudes were simulated. This section presents all resultsfor the frequency response and includes theoretical explana-tions and an analysis of each result.

3.1. Analog Electrical Stimulus. Microlegs will be driven by avoltage-controlledCMOS technology. It is important to knowthese stimulation effects on the current consumption at anyfrequency.

The current transient step response shows amajor currentpeak that decreases rapidly (Figure 3). A negative peak is alsoproduced when the square wave stimulus return to zero.This


IEM

H+H+

[Pt(NH3)4]2+

(a)

NaBH4

Pt++

Pt++Pt++

B(OH)−4

(b)

M+ M+ M+ M+

Na+Na+

Na+

(c)

Figure 5: Chemical process of Nafion plating with platinum [20].

Epoxy

Platinum

IEM

Figure 6: IPMC cross section.

S1

S2

S3

S4

S5

S6

S7

S8

III

III IV

SEM view

PlatinumNafion

Figure 7: Tubular IPMC for the micropump propulsion.

phenomenon is explained by a serial resistance-capacitance(2𝑅𝑠+ 𝐶dl) network introduced via the electrode-electrolyte

interface (Figure 10).Capacitance is due to double-layer capacitance 𝐶dl. If the

voltage is increased, 𝐶dl increases until the electron transferoccurs at the electrode/solution interface. The continuouscurrent remains in steady state, which shows a nonlinearresistive element 𝑅

𝐹through the IPMC parallel to RC net-

work [21]. This reaction was analyzed previously for differentIPMC and cations types [22]. Normally, electroneutralityis maintained across the interphase, but it may be leaky,allowing some faradic current to flow across it, explained

S1

S2S3

S4

S5

S6

S7

S8

III

III IV

(a) (b)

Figure 8: Square section of the microleg.

by faradic resistance 𝑅𝐹. 𝑅𝐹may behave as simple ohmic

resistance, but it also can be a hyperbolic or an exponentialfunction of potential, depending on the range of overpoten-tial considered. 𝐶dl also is a complex function of potential,although it may not vary as much as 𝑅

𝐹[23].

Analysis of microleg electromechanical characteristicsdoes not need to include an analysis of Warburg impedanceand absorbing pseudocapacitance due to the limitation ofmass transport by the diffusion and transfer process of charge.However, it is important to note that Warburg impedanceadds a constant phase of −90∘ between current and voltage,which may influence transient or frequency analysis as seenin Figure 11(b). Only 𝐶dl is treated as pure capacitance, in thephysical sense.

Electrode surface resistance, 𝑅𝑆𝑆, can be represented and

explained by low conductivity, insufficient electrode thick-ness, microscopic cracks and the heterogeneous depositionof metal particles. The cyclic deformations in traction andcontraction during cyclic movement of the IPMC can causefatigue and an increase in this resistance [24, 25].Thenetwork𝑅𝐹//(2𝑅𝑆+ 𝐶dl) is an approximation of the proposed model.

In steady state, two regions have been defined for themicroleg: the linear zone below the decomposition voltage(sum of metal overpotential used as the electrodes and stan-dard equilibrium potential or standard electrode potential)and nonlinear exponential zone above decomposition voltage(Figure 12).

The frequency response shows interesting particularity.Measurements are taken using the sinusoid 2Vpeak-to-peak.


Computer PWM generator H-bridgeMOSFET

Transimpedanceamplifier

IPMC

RO/DIwater

Opticalwindow

Scanning laservibrometer

Laser beam+ −

Figure 9: Experimental setup for microleg speed measurement.

2h

x

z

Rs

Rs

RF

Zw

nOutside

PtInside

PtIEM

Rss Rss

RssRss

Cdl

Figure 10: IPMC electrical model [24].

The current amplitude response has a gain of −10 dB/decadebetween 5 and 100Hz and cut-off frequency near 45Hz.As well, linear characteristics are associated with the phasefrequency response.

This characteristic shows that current is out of phase fromvoltages between −100∘ and −180∘. It can be explained bythe presence of absorbing pseudocapacitance and Warburgimpedance.

3.2. Mechanical Displacement Analysis with PWM. Analysiscarried out until now does not make it possible to controlthe IPMC bending angle using CMOS technology. RISCmicrocontroller architecture does not support analog outputat the 8-bit parallel port [26]. Adding a digital-to-analogconverter (D/A) is a cumbersome solution for MEMS and/ormicroproduction. Although the IPMCmicroleg can be mod-eled using a R//RC network, the use of PWMmodulation canadjust the quadratic mean of the electric tension 𝑉RMS. Next,the bending angle can be controlled if a relationship existsbetween the IPMC bending angle and PWM duty cycle.

However, the first step is to check whether a certaindependence or correlation exists between the duration ofPWM impulse and microleg displacement. The scanning

laser vibrometer cannot directly measure displacement,because it can only analyze dynamic structure. Therefore, itis necessary to study the dynamic behavior in a cantileveredbeam configuration, that is, microleg tip displacement ratefunction of the PWM duty cycle. The waveform generatorprovided with scanning laser vibrometer requires program-ming the desired signal.

For the experiments, the IPMC is supplied periodicallyon each one of its electrodes. This periodic signal vibratesthe structure within two degrees of freedom. This impliesthe periodic generation of the standardized signal. Param-eter adjustment (frequency of the signal and the voltageamplitude) on the standardized signal makes it possible toobtain excitation signals used for the experiments.The signalis adjusted with a value of a 6-volt peak-to-peak for twofrequencies: 10 and 20Hz.

Figure 13 shows the tip microleg displacement rate in acantilevered beam configuration for 10Hz and 20Hz for theodd PWM duty cycle. Figure 14 shows a mean maximumspeed for each PWM duty cycle over the 30 measurementstaken from data as found in Figure 13. Experimental resultsmay be expressed as linear characteristics. The next analysisshows the degree of correlation between the mean maximumspeed movement and PWM duty cycle.

The correlation coefficient 𝑟 calculation provides anestimate of the correlation degree between two randomvariables of even population. The value obtained for 𝑟 mustbe checked to ensure that it does not differ significantlyfrom 0, which probably indicates a linear absence of thecorrelation between the variables observed. A significancethreshold of 0.05 and a population of ten (10) samples forthe mean maximum are used for the analysis. The variationof vibration function of frequency is distributed accordingto Student Law (bilateral case) with 8 degrees of freedom.Since the value of 𝑟2 is high (0.982 and 0.986), as shown inFigure 14, the straight line adjustment at the experimentalpoints is a good quality. It is obvious that this study cannotshow cause-to-effect between the two variables. However,predictable actuatormechanical and physical behaviorsmakeit possible to validate that there is a cause-to-effect between


0 0.5 1 1.5 2 2.5 3−66

−64

−62

−60

−58

−56

−54

−52

−50

−48

−46|H

(s)|

(dB)

log10(f) (Hz)

(a)

0 0.5 1 1.5 2 2.5 390

100

110

120

130

140

150

160

170

180

190

Phas

e (de

g)

log10(f) (Hz)

Phase = 29.098∗log10(f) + 97.597

(b)Figure 11: Current amplitude (a) and phase (b) response ratio of voltage amplitude function of frequency.

0 0.2 0.4 0.6 0.8 1 1.20

1

2

3

4

5

6

7

8

Voltage (V)

Curr

ent (

uA)

r = 0.99466

I = 6.3214V + −0.05

(a)

1 1.5 2 2.5 3 3.5 40

2000

4000

6000

8000

10000

12000

14000

16000

18000

Voltage (V)

Curr

ent (

uA)

I = 25.919 + 25.377∗exp(1.6079∗V)

(b)

Figure 12: Steady state current function of voltage in (a) linear and (b) nonlinear regions.

the two independent variables: the variations of one variableinvolve the variations of the other without an unspecifiedattribution of an external common cause variation.

3.3. Frequency Analysis Using FFT. Starting from theseresults, it is possible to investigate more IPMC properties inthe frequency domain. A structure of this type has particularmodes of resonance that can be used in the propulsion of therobot as a strategy for reaching optimal motion. It was shownthat IPMC dimensions vary its natural frequency: the naturalfrequency decreases when its length increases in a similar wayto a cantilever beam [27].

The distributed systems have an infinite number ofdegrees of freedom and natural frequencies. Each naturalfrequency has a single mode, which is known to be like anormal function. A transitory vibration forced or in a steadyoperation will generally excite several or all the frequencies

and modes in combination.The clear response in a point canbe expressed in their terms according to the superpositionprinciple. The first mode is that associated with the weakestfrequency [28]. The use of the first vibration mode will beprobably advantageous for moving the robot at its optimalspeed in terms of current consumption versus velocity. Thus,it is necessary to theoretically and experimentally analyze theIPMC first modes in the cantilever beam configuration.

In the case of the IPMC microleg, free end tip massis negligible. Next, the resolution of theoretical equationprovides all the proper frequency 𝑓

𝑛using [29]

𝑓𝑛=1

2𝜋

𝑘2𝑛

𝐿2√𝐸𝐼

𝜌𝑆, 𝑆 = 2𝑙ℎ, (1)

where 𝐿 is the IPMC length, 𝑙ℎ product is the IPMC crosssection (0.2mm × 0.2mm), 𝐼 is the inertia, and 𝐸 the Young


4 6 8 10 12 14 16

0

0.005

0.01

0.015

0.02

0.025

10%

30%

50%

70%

90%

Mic

role

g tip

spee

d

10 %30 %50 %

70 %90 %

Duty cycle (%)

−0.005

×0.0019531 s

10Hz PWM

(a)

3 4 5 6 7 8 9 10

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

10%

30%

50%

70%

90%

Mic

role

g tip

spee

d

10 %30 %50 %

70 %90 %

Duty cycle (%)

−0.005

×0.0019531 s

20Hz PWM

(b)

Figure 13: Tip microleg displacement rate in function of multiple PWM duty cycle at PWM period of (a) 10Hz and (b) 20Hz.

0 20 40 60 80 1000

0.005

0.01

0.015

0.02

0.025

0.03

0.035

Duty cycle (%)

Disp

lace

men

t rat

e (m

/s)

r = 0.99102

r = 0.99312

Curve fitting 10Hz10Hz

Curve fitting 20Hz20Hz

Figure 14: Mean of maximum tip speed as a function of PWM dutycycle.

modulus. 𝑘𝑛comes from the resolution of characteristic

equation:

1 + cos (𝑘𝑛) cosh (𝑘

𝑛) = 0. (2)

However, this equation is not valid within the waterenvironment. It is necessary to consider the behavior ofa fluid-structure coupled system. In general, it should bechecked whether the fluid is incompressible. In this case, theeffect of fluid is then purely inertial. Using the representationof added mass makes, it possible to find a simple formula to

evaluate the natural frequency of the coupled system. Whenthe structure vibrates in water, it induces water acceleration,producing an additional force on the structure, in additionto the force of trail fluidic-dynamics. This added force can besuitably modeled as the product of hypothetical water massand structure acceleration. This implies that the structurein water vibrates at a natural frequency weaker than thefrequency in a vacuum. By replacing inertial 𝐼 of a circle ofradius 𝑎 and its surface 𝑆 in (1) of the eigen frequencies 𝑓

𝑛,

the result is given by

𝐼 =𝜋(2𝑎)

4

64=𝜋(𝑎)4

4,

𝑆 = 𝜋𝑎2

,

𝑓𝑛=1

2𝜋

𝑎𝑘2𝑛

2𝐿2√𝐸

𝜌.

(3)

The equation describing a vibrating beam in water shouldcomprise both the structure density of the beam described in(1) and that of water, which corresponds to a coupled systemin density. The coefficient of added mass 𝐶

𝑚defined as the

ratio of the addedmass to that of watermoved by the cylindermakes it possible to couple the structure withwater accordingto

𝑓𝑛=1

2𝜋

𝑎𝑘2𝑛

2𝐿2√𝐸

(𝜌𝑐+ 𝐶𝑚𝜌𝑤). (4)

A simulation of (3) and (4) is shown in Figure 15. Toprove this theory, modal analysis was carried out with the


0 200 400 600 800 1000 12001.5

2

2.5

3

3.5

4

4.5

5

First mode frequency (Hz)

Mic

role

g le

ngth

(m)

Coupled with fluid (water), circular baseIn air

×10−3

Figure 15: Simulation of the microlegs first mode frequencyfunctions of its length.

scanning laser vibrometer using a pseudorandom excitationvoltage (Gaussian white noise) of 6V peak-to-peak. Thisanalysis makes it possible to find the first mode vibratingfrequency 𝐹

𝑟1at about 180Hz for a 4-millimeter microleg

length (Figure 16). This frequency takes into account thecoupling between heavy fluid (deionized water) and thestructure. A digital simulation is used to find a frequencyclose to that found with the laser. Moreover, it is interestingto compare the tension/current cut-off frequency (about45Hz from Figure 11(a)) with the maximum frequency 𝐹MAXat about 50Hz. Note that frequency response fluctuatesaccording to the applied voltage and according to resistance𝑅𝐹and capacitance 𝐶dl values.

4. Water Electrolysis

Many other IEMmembranes can be used for this experiment,such as Flemion or Aciplex. Flemion-Au IPMC, with its den-drite inside ionomer, is known to have a higher ion exchangecapacity, higher surface electrical conductivity, higher hydra-tion capacity, and higher longitudinal rigidity than Nafion-Pt. These conclusions provide Flemion-Au with a bettercurvature angle for the same applied voltage without relax-ation [30]. Gold can be also envisaged given its advantages:stable in acid, ductile, good electrical conductor, and lessreactive in electrochemical reactions [31, 32]. Gold has also anoverpotential higher than Pt. It allows using higher voltagesto produce bending curvature without water electrolysis.

This section presents different ways to avoid water elec-trolysis. Currently, cation type and electrode metal havedemonstrated some useful characteristics for avoiding back-relaxation when stimulus is applied and for avoiding waterelectrolysis. First, an overview of cation and electrode charac-teristics is presented. Second, a new way to avoid electrolysis

0 100 200 300 400 5000

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

Frequency (Hz)

Am

plitu

de

FFT of microleg bending characteristics

Tip displacementTip displacement simulation

×10−5

Fmax

First mode Fr1

Figure 16: Modal analysis with laser vibrometer in frequency withFFT.

is introduced with signal waveform and frequency applied asthe stimulus.

4.1. Cation Type. Cations can be divided into three classes:𝐴, 𝐵, and 𝐶. These classes are categorized by water ionicconductivity and water state inside IEM. The 𝐴 classcontains small hydrophilic alkali and some alkaline-earthmetal (Li+, Cu2+, Na+, K+, Rb+, and Cs+). The 𝐵 class isformed by hydrophobic alkyl ammonium ions ((CH

3)4N+,

(C2H5)4N+, NH

4−𝑚(CH3)𝑚

+, and NH4−𝑚(C2H5)𝑚

+). Lastly,class 𝐶 includes the large hydrophobic cations, such as TBA(tetrabutylammonium+) and TPrA (tetrapropylammonium)[33].

Water displacement volume inside polymers causes con-tractions and dilations, which involves asymmetrical pressuredistribution.Thus, internal stresses produce temporary poly-mer deformation. For small cations, like Li+, the displace-ment response is fast. However, if tension is maintained, thepolymer will not keep its shape with a Nafion-Pt composite[22]. From another perspective, for bulkier cations, like tetra-n-butylammonium+ (TBA), the response is slower, but therewill be a little relaxing. The relaxation time depends on thewater leakage in the polymer interstices.

Relaxation probably occurs since the cations transportmore water molecules than there should be in an equilibriumstate. Relaxation will finish when equilibrium has beenreached between the osmotic water pressure (dilution of theions types) induced by electrical voltage, interfacial polymer-solvent, and polymer internal elastic energy (limitation ofpolymer chain elongation) [34]. No further relaxation isobserved with TBA cation type. It can be caused by thepossibility that the cations block the channels and preventwater leakage or by the possibility that the concentration ofcations requires more water than it moved. No relaxation


occurs with Flemion-Au IPMC for all cation types. Instead,the displacement rate drops without moving back [22].

If ammonium size increases, IPMCdisplacement rate willgo down, because of the slower molecule mobility inside theionomer. However, the IPMC curving angle will be morepronounced, and relaxation by water leakage will decreasegradually. Moreover, ammonium’s slower mobility eliminateselectrochemical reactions like water electrolysis [35].

4.2. Electrode Characteristics. Electrode polarization, by elec-trical current, should alter half-cell potential. Half-cell vari-ation (difference between observed half-cell potential andequilibrium zero current half-cell potential) is known as over-potential. The element chosen for electrode composition isdirectly associated with IPMC performance such as ductilityand electrical conductivity. Electrodes must be also stablein electrochemical reactions like noble metals. Of course,overpotential should be very large for a higher decompositionpotential. This section explains the effects of overpotential.

Electrodes made with noble metal, such as platinumand gold, have a behavior similar to perfectly polarizableelectrodes. Both platinum and palladium have extremelysmall overpotentials for hydrogen evolution. Gold has asignificantly larger overpotential. As well, gold does notappreciably absorb hydrogen, and this factor together withits larger overpotential for hydrogen evolution makes goldthe metal of choice [21]. Such a relatively inert electrode isdifficult to oxidize and dissolve. The best reaction for IPMCis characterized by a displacement current (no actual chargecrosses the electrode-electrolyte interface) and the electrodebehaves as though itwere a capacitor. In this case, themajorityof the overpotential is a result of concentration overpotential(ion concentration variation at the interface) [36].

Frequency and waveform have a direct influence on ionmobility inside IEM. The next section explains frequencyand waveform influence on the displacement current at theinterface to approach IPMC behavior as a capacitor withouta resistive element.

4.3. Frequency andWaveform. Visual observations of micro-leg displacement show that the movement is greater in thenonlinear region. This result is not interesting for manybiomedical applications. However, it is possible to drive themicroleg with a charge-balanced waveform.

Control of the charge density and charge balance areessential for avoiding electrolysis and to provide a chemicallyreversible reaction (or safe stimulation). Waveforms can besymmetrical or asymmetrical. Symmetrical refers to biphasicstimuli in which the magnitude of the current density in thefirst pulse of the biphasic pair is the same as in the secondpulse. The use of biphasic stimulus waveforms with zero netcharge flow does not guarantee that toxic electrochemicalreaction products will not be produced [37].

Considering the IPMC electrical model, a minimumfrequency is necessary for eliminating water electrolysis. Theelectrochemical technique of cyclic voltammetry can delin-eate an operational potential window between hydrogen andoxygen evolution.This technique is applied to findminimum

frequency of symmetrical stimuli because water electrolysisdecreases thermodynamic efficiency and consumes power.

Figure 17 shows three hysteresis curves for the frequency10, 30, and 180Hz. Signal amplitude applied to system is 5Vpeak-to-peak in order to produce water electrolysis. To readcurrent amplitude, divide Δ𝑌, on this figure, by 100, whichis the transimpedance amplifier gain. Water electrolysis isshown by concave form at two hysteresis extremity. It is clearthat increasing frequency decreases power consumption andeliminates water electrolysis.

A study could be carried out to characterize themechani-cal reactions during stimulationwith a current source insteadof a voltage source. The use of a current source would makeit possible to control the quantity of charge flow injected byvarying amplitude and/or the width of the pulse phase withany IPMC impedance. Maximum charge density being ableto be injected is according to the surface of the electrodes.

5. Locomotion Model and Control

Avery special designwill be developed and explained to allowBMR to move in all directions within the constraints of theactuators and the eight-bit parallel port of themicrocontroller[26]. In this vein, the first part of this section is devoted to thestudy of the dynamics of an insect gait. This study highlightsthe main parameters to be considered in order to operatethe microleg. Among other things, the types of locomotionare covered. The type of locomotion is important because itdepends on the speed of the insect and hardware constraintsof the microcontroller. The analysis of the movement of theleg and themovement of an insectwith six legs, like ants, leadsthe design of the control system. It will also be necessary tounderstand the functioning of the physiognomic locomotionin the central nervous system for a robust adaptive algorithm.

5.1. Insect Locomotion. At this time,manyworks have studiedthe decentralized and central control systems involved inlocomotion.The combination of behavioral and electrophysi-ological experimental data shows that locomotion is the resultof both central and decentralized mechanisms [38]. Thisimplied that each leg has an autonomous sensitive elementcontrolled independently.

The RISCmicrocontroller architecture used in this appli-cation has only one eight-bit parallel port [26]. This stronginternal constraint only lets us use the central control. In fact,decentralized locomotion control would require a 28-outputbits on one parallel port (6 legs × 4 electrodes per leg, plus themicropump using thrust vectoring control) from the micro-controller to drive each BMR leg independently withoutsensitive input and reflex. Next, this paper is based essentiallyon central systems. With this bus width, a simple neuralnetwork architecture called Walknet, a biologically inspirednetwork for controlling six-legged walking, can be used froma cellular nonlinear approach [39, 40]. The central commanddeveloped here coordinates BMR movements with a higherstereotypy and less adaptivity. A feedback loop acts as a reflexto adjust the leg displacement. This reflex can move one legto another position to avoid or to climb over an obstacle.


0.03A

5.0V

(a)

0.01A

5.0V

(b)

0.01A

5.0V

(c)

Figure 17: Hysteresis of current response for (a) 10Hz, (b) 30Hz, and (c) 180Hz sinusoid voltage for the same voltage amplitude applied.

L1R2L3

R1L2R3

R1

R1

BMR

Direction

AEP

PEP

PEPAEP

PS RS

Time scaleCritical point

Tarsuspositionrelativeto body

StanceSwing

Figure 18:The cyclic movement of a leg and the tripod (fastest gait)walking stepping pattern.

5.2. Hexapod Insect Walking. The cyclic movement of awalking leg consists of two parts (shown in Figure 18), thepower stroke (PS, also stance or support phase that propelsthe body) and the return stroke (RS, also swing or recoveryphase).

The anterior transition point (transition from swing tostance) is called the anterior extreme position (AEP), and

Group AGroup B

L1

L2

L3

R1

R2

R3

H

Front

D

W

HindIpsilateral legs RiIpsilateral legs Li

Prothoracic

Mesothoracic

Metathoracic

d1

d2

d3

Figure 19: Top view of microrobot. Two legs groups (A, B) must beused. The tripod of support is represented by the dash triangle.

the posterior transition point is called the posterior extremeposition (PEP). The most critical point to maintain therobot’s stability is the transition from PS to RS [41]. Next, aninteresting location for the feedback sensor could be used todetect PEP movement.

Two conditions must be met to maintain the robot’sstability during all locomotion phases. The center of gravityof the body must lie within the triangle of support as shownin Figure 19. As well, three feetsmust always be on the groundsimultaneously.The body is supported alternatively, for equal


L3 L2 L1

R3 R2 R1

StanceSwing

(a)

L1R2L3R1L2R3

StanceSwing

Autowave

(b)

L1L2L3R1R2R3

StanceSwing

Autowave

(c)

Figure 20: (a) Checkerboard Turing pattern corresponding to tripod gait locomotion. (b) Insect fast-gait pattern. (c) Insect swimmingpattern.

Inputs

Cell11 Cell12

Outputs

(a)

Inputs

CPG

II, 11 I, 11 I, 12 II, 12

Group B Group AExcitationInhibition

(b)

Figure 21: (a) Reciprocal inhibition network consisting of two similar neurons (or two populations of neurons). (b) CPG CNN design withreciprocal inhibition network.

periods of time, by one of the two groups of legs (tripod gait):A (L1R2L3) or B (R

1L2R3).

The stabilitymargin is defined as the shortest distance (𝑑1,

𝑑2, and 𝑑

3) from the center ofmass to the edge of the tripod of

support. Many insects closely conform to this pattern whenmoving quite rapidly [42, 43]. Twomain strategies are knownto allow the insect to turn: increasing step frequency or steplength of legs on one side of the body versus the other [44].

5.3. Central-Pattern Generator Paradigm. The central ner-vous system (CNS) must produce specific patterns of motorneuron impulses during coordinate locomotion or move-ment. The central hypothesis purports that rhythmic move-ments or basic motor programs in walking are driven bythe central-pattern generator (CPG) located in the CNS. The

CPG includes subnetworks of command neurons (CNs) andlocal pattern generating neurons (LPGNs). The neurons ofLPGN are called motoneurons (flexors and extensors neu-rons) [45, 46]. The impulse in nerve membrane is modeledusing the nonlinear reaction-diffusion partial differentialequation (RD-PDE), which can be represented by autowaveswith cellular neural networks (CNN). Its capacity to generateplateau potentials or oscillations is a key issue for rhythmgeneration, but also for driving the transition among varioustypes of locomotion like walking, running, and swimming[47].

Taking into consideration the biological aspects of loco-motion, reaction-diffusion CNN (RD-CNN) is made up oftwo parts: one is devoted to generating autowaves (Figures20(b) and 20(c)) and the other is responsible for specific


9

8 1

Address

MemoryInitial

conditionPartial

product

Accumulator

SWB2

1Add/sub.

Shifter

State

dij

S0: add1: sub.

Limiter

Updateregisterykl,m(n) yij,m(n)m-th bits of m-th bits ofneighbor

outputoutput

output

xij(n + 1)

yij(n + 1)

Piecewise linear

∑Bv+1

(a)

Feedback 1 4 72 5 83 6 9

y1–3,m

y4–6,m

y7–9,m

Mem

ory,

23w

ords

Mem

ory,

23w

ords

Mem

ory,

23w

ords

dij

Aij:

dij,l dij,c dij,r

matrix 3×3

(b)

Figure 22: (a) Block diagram of DTCNN cell. (b) Memory partitioning scheme.

IPMC

IIIIII

IV

E1

E2

E3

E4

Li ,Ri

NafionElectrode

S1

S2S3

S4

S5S6

S7

S8

(a)

L1

L2

L3

R1

R2

R3

RE1

RE2

RE3

RE4

LE1

LE2

LE3

LE4

BMR

(b)

Figure 23: (a) Leg cross section with four electrodes in Cartesiancoordinates. The states 𝑆

𝑖generate bending moments and the

associated direction movement. (b) BMR body hardware.

types of locomotion obtainedwith a stable checkerboard Tur-ing pattern configuration (Figure 20(a)) [48]. Multitemplateapproach (MTA) is an extension of the model used in RD-CNN-based hexapod robot. CPG implementation is based

on the principle that the same group of neural cells can bereorganized by changing the synaptic connection [49].

For implementation purposes, CNN can be classified ascontinuous-time (CT) [50] and discrete-time (DT) models[51]. CNN is composed of basic nonlinear dynamic circuitsknown as a cell, where each unit is connected only as aset of adjacent cell neighbors to form an array 𝑀 × 𝑁. Inthis way, the 3 × 3 checkerboard Turing pattern needs a 1-neighborhood (Figure 22(b)). The CNN structure can formin any dimensions, but, in our application, we need only two-dimensional arrays to reproduce autowaves for the tripodlocomotion pattern.

An analogically programmable CNN in CMOS technol-ogy is reported with MOS transistor. This implementation isbased on operational transconductance differential amplifier(OTA) stage. The cell resistor is approximated by the MOStransistor and its current-voltage relationship is nonlinear[52]. Moreover, digital architecture for the discrete-timeCNN based on the combination of bit-serial computation ofdistributed arithmetic (DA) is presented. The system usingDA has some advantages over all others: small memory size,shared memory contents, and reduced bus width. It can beimplemented on an FPGA chip for reconfigurability, and adigital CMOS VLSI chip for high integration. The digitalapproach has the advantage of simplicity and expandability


Reflex width control 𝛼

LEGregister

F

CPG

8

DT-CNNCMOS p0.18

GAGBReset

Moore

CLK

Reflexsensor

I

I

II

II

III

III

IV

IV

𝛼

Outputcontrol

Outputcontrol

Electrode’s leg

Leftlegs

Leftlegs

L R

L1L2L3

R1R2R3

Figure 24: Hexapod walking system.

compared with the analog approaches, but the disadvantagesof hardware complexity and bus width [53].

6. CMOS DT-CNN Implementation

The whole system has been designed on BMR as follows[54]. The proposed design is a simplified version of theCPG (Figure 21). This design corresponds to the fact thatin most CPGs, the majority of interconnections betweenneurons are inhibitory [46]. The output saturation of theactivation function corresponds to the maximum IPMCactuation (bending angle).

The CPG CNN structure shown in Figure 21 is builtfor the insect tripod gait. Each population of neurons isconnected to a group of legs (A or B). Thus, the first onecontrols the first tripod (L

1R2L3) and the second controls the

other tripod (R1L2R3). This pair of mutually inhibited cells,

with a suitable choice of the synaptic weights, is characterizedby an antiphase synchronization of the activities of thetwo cells. The cells are implemented on FPGA using DA(Figure 22) with the architecture proposed in [54].

Eachmemory contains only the partial products for threecoefficients. This configuration reduces storage by 95% in thedirect implementation.The accumulator in DA performs addand shift operations.

7. Control System

The BMR body is the support for the current microlegs. Theleg configuration on the body is an important considerationfor driving the robot in any direction (Figure 23). In partic-ular, the electrical configuration of the connections shownin Figure 23(b) of the legs is designed so as to produce awalking pattern. Experimental results show that the half-period of stance does not need any power consumption,while the other legs are in swing period (maximal currentconsumption). IPMC elasticity propels the BMR body. The

Bits

F

F

RLRL RL

Figure 25: BMR possible direction control by PWMmodulation.

bending angle decreases or increases the stance, and theswing time produces the BMR turn.

DT-CNN architecture for CPG has been included in theprevious BMR control system to improve adaptability to thisenvironment.TheDT-CNNmodule controls themoore finitestate machine (FSM) with GA and GB bits, the two tripodgroups. DT-CNN has two inputs and five outputs. FSM isdesigned with eight states, and the output is controlled by thePWM as depicted in Figure 24. The MSB bits, R (right) andL (left), control the PWMwidth, while the ten bits, includingLSB 𝛼 bits, are proportional to the bending angle of the fiberIPMC actuator.

The BMRwill move forward when R and L are set to zero.PWM width will be the same for the left and right legs asshown in Figure 25. In addition, it can turn the same way asan insect when the width of the PWM is changed for the left


41.6ms

0.002A

0.2V

Current

LE1LE2LE3LE4

RelaxationPWM period

Figure 26: Applied PWM for LE1 to LE4 and current consumptionfor LE1 and LE3.

or right legs. Only three bits, stored in the microcontrollerLEG register, are used to control the direction of the robot.Only these bits can be modified by user algorithms.

The overall system is coded as a driver in VHDL and istested on FPGA. Currently, the PWM period is fixed in thePWMmodule. Experimental results show that a frequency of763Hz and 24Hz for PWM and FSM, respectively, can driveone BMR leg. A half-period of stance is calculated at about0.042 seconds (maximum bending to relaxation position).Figure 26 shows the FSM signal from the FPGA and currentconsumption with timing parameters. In future work, themodule will be included in the microcontroller.

8. Conclusion

Microleg fabrication is an important challenge. It requiresmicroscopic handling and specific instrumentation. Also, afailure in the procedure and a bad handling could damage theIPMC surface. Indeed, the distribution of the metal particlesadded on the IEM is very sensitive to the conditions of treat-ment, impregnation, and reduction. Microleg dimensions,according to the new manufacturing process worked out inthis paper, make it possible to carry out a millimeter-sizemicrorobot. Furthermore, the configuration of the electrodesconfers 2,5 degrees of freedom. Thus, a hexapod robot willhave 15 degrees of freedom unlike insects, which have at least18 degrees of freedom. Using amicropumpwith a trust vectorcontrol improves maneuverability of the BMR.

Thereafter, several essential IPMC physical propertieswere given to drive 15 DOF hexapod BioMicroRobots. Tomeasure these characteristics adequately, a very particularassembly was carried out. Inter alia, it was possible to show,without any doubt and using a laser vibrometer, that there is alinear relationship between the IPMC displacement rate andPWMreport/ratio of cycle (duty cycle). It was also possible tomeasure the resonance frequency coupled with a heavy fluid.The minimum frequency for eliminating water electrolysiswhen the voltage is higher than the decomposition potentialwas also studied. The four methods for reducing waterelectrolysis explained in this paper are as follows:

(1) Using alkyl ammonium like TBA allow decreasingelectrolysis and the relaxation effect with the Nafionmembrane;

(2) keeping frequency higher than 30Hz, the limit ofmolecule diffusion through the electrode membrane;

(3) obtaining a high overpotential with better electrodesor keeping applied voltage lower than the decompo-sition potential;

(4) using bipolar current source to control the chargedensity and charge balance injected inside IPMC. Itmay be done by active and passive waveforms for thetwo phases of asymmetrical biphasic stimuli.

The evidence of linearity between the PWMdisplacementrate and the PWM duty cycle is a relationship, which will bevery important for controlling the BMR in all directions likean insect. Moreover, the electromechanical characteristicsprovided in this paper will allow developing a controlmethodfor each microleg. In effect, power consumption and legdegree of freedom numbers are two of the many restrictivecharacteristics to be observed at the time of system design forautonomous robots.

Obviously, other interesting applications can benefit fromthe process, which will be developed. It is possible to envisageactive electrodes for the mechanical stimulation in humanimplant prostheses. Indeed, these electrodes could stimulatethe sensitive receivers not only electrically, but also mechan-ically.

Lastly, a new artificial locomotion servo-system withdiscrete time cellular neural networks (DT-CNN) has beendeveloped for a bioinspired hexapod BMR. Using a statemachine and PWM modulation to drive the actuators, therobot can move in any direction similarly to an insect. Anoverview of the robot central pattern generator (CPG) inaccordance with the insect displacement principle has beendemonstrated. MTA is used to perform a particular loco-motion type with autowave generation. The control systemcan be structured as a digital control system done by CNNsgenerating the tripod locomotion pattern. Leg displacementis controlled by CPG as a function of the reflex sensor and theuser algorithm in the microcontroller.

Acknowledgments

This work was partly supported by Groupe d’acoustique etvibrations de l’Universite de Sherbrooke (GAUS) and InstitutdesMateriaux et des Systemes Intelligents (IMSI).The authorwould like to thank Pierre Magny and Irene Levesque fortheir advice and suggestions for taking SEM images of thecomposite. As well, The author would like to thank MagellaTremblay and Patrice Masson for the use of their laboratoryinstrumentations.

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