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2007 IEEE International Conference on Robotics and Automation Roma, Italy, 10-14 April 2007

ThC7.2

Adaptive Trajectory Tracking Control of Skid-Steered Mobile Robots

Jingang Yi

Dept. of Mech. Eng. San Diego State University San Diego, CA 92182 jgyi@mail.sdsu.edu

Dezhen Song

Dept. of Computer Science Texas A&M University College Station, TX 77843 dzsong@cs.tamu.edu

Junjie Zhang

Dept. of Mech. Eng. Texas A&M University College Station, TX 77843 jjzhang@tamu.edu

Zane Goodwin

Dept. of Computer Science Texas A&M University College Station, TX 77843 zgoodwin@cs.tamu.edu

Abstract— Skid-steered mobile robots have been widely used for terrain exploration and navigation. In this paper, we present an adaptive trajectory control design for a skid-steered wheeled mobile robot. Kinematic and dynamic modeling of the robot is ?rst presented. A pseudo-static friction model is used to capture the interaction between the wheels and the ground. An adaptive control algorithm is designed to simultaneously estimate the wheel/ground contact friction information and control the mobile robot to follow a desired trajectory. A Lyapunovbased convergence analysis of the controller and the estimation of the friction model parameter is presented. Simulation and preliminary experimental results based on a four-wheel robot prototype are demonstrated for the effectiveness and ef?ciency of the proposed modeling and control scheme.

I. I NTRODUCTION Skid-steered mobile robots have been widely used in many applications, such as terrain navigation and exploration, waste management, defense, security, and household services. Figure 1 shows an example of a skid-steered four-wheel mobile robot. The absence of a steering system for a skidsteered mobile robot (vehicle) makes the robot mechanically robust and simple for terrain or outdoor environment navigation. Due to the varying tire/ground interactions and overconstrained contact, it is quite challenging to obtain accurate dynamic models and tracking control systems for such mobile robots. Although there is a great deal of research on dynamic modeling and tracking control of differential-driven mobile robots that are under the nonholonomic constraint of zero lateral velocity, such as unicycles or car-like robots (readers can refer to [1] and references therein), the counterpart research on skid-steered mobile robots is less frequently reported. Because of the similarity between skid-steering of tracked and wheeled vehicles, the method of modeling the track/ground interaction for tracked vehicles can be utilized for skid-steered wheeled robots. Song et al. [2] use the tracked vehicle models discussed in [3]. In [4], localization of a tracked vehicle based on kinematic models is presented. For skid-steered modeling of tracked vehicles, readers can refer to [5]–[7] for details. Because of the dif?culty in accurately capturing skid-steering, Anousaki and Kyriakopoulos [8] propose an experimental study to model the kinematic re-

Fig. 1.

A skid-steered four-wheel mobile robot.

lationship and demonstrate that a kinematic model for an ideal differential-driven wheeled robot cannot account for skid-steered robots. There is little work discussing the dynamic control of skidsteered mobile robots due to the lack of a good understanding of skid-steering and the complexity of the wheel/ground interactions. In [9], a dynamic model was presented for a skid-steered four-wheel robot, and a nonholonomic constraint between the robot’s lateral velocity and yaw rate is considered. A perfect knowledge of the wheel/ground contact was assumed. In [10], a simple Coulomb friction model is used to capture the wheel/ground interaction and a nonlinear feedback controller is designed to track the desired path. Ahmadi et al. [11] discuss tracked vehicle trajectory control and a linearized track-soil interaction model with known parameters is used for the controller design. There is some research on modeling the wheel/ground interaction for mobile robots. In [12], a comparison study is presented for the control performance of an omni-directional mobile robot with and without considering wheel slip. It was found that the signi?cance of slip increases when the wheel/ground friction coef?cient is larger. A tire/road friction model in automotive study was considered for the longitudinal friction force in [12]. For a detailed review of the tire/road friction model, readers can refer to [13]. Dynamic modeling of wheel/ground interaction is also presented in [14] for

1-4244-0602-1/07/$20.00 ?2007 IEEE.

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wheeled omni-directional robots. The longitudinal and lateral friction coef?cients are considered independently. Recently, Ray et al. [15] use the force-slip relationship from tire/road interaction to control the slip for the cooperative control of a group of skid-steered autonomous mobile robots. In this paper, a kinematic and dynamic model of a skid-steered four-wheel mobile robot is ?rst presented to characterize the skid-steering properties. A wheel/ground friction model is incorporated into the robot model for both the longitudinal and lateral friction forces. Based on these models, an adaptive trajectory control algorithm is utilized to asymptotically track the desired trajectory. The contribution of this paper is twofold. First, we propose a dynamic model for skid-steered four-wheel mobile robots. Most existing work only discusses the kinematic model due to the complex dynamics involved in the wheel/ground interactions. Second, we propose an adaptive tracking control mechanism that can estimate the wheel/ground interaction in real time. Such a control system design can enhance the tracking control performance of a skid-steered mobile robot. This paper is organized as follows. In section II, we discuss the kinematic and dynamic modeling of a four-wheel skidsteered mobile robot. A wheel/ground interaction model is also discussed in this section. Section III presents a trajectory control design for the skid-steered robot. Simulation and experimental results of a prototype robot are presented in section IV. Finally, we conclude the paper and discuss future research directions in section V. II. DYNAMIC M ODELS Figure 2 shows the kinematic schematic of a skid-steered robot. Without loss of generality, we consider the following assumptions. Assumption 1 Robot modeling assumptions. 1. The mass center of the robot is located at the geometric center of the body frame 1 . 2. There is point contact between the wheel and the ground. 3. The contact rolling resistance force is negligible 2 . 4. Each side’s two wheels rotate at the same speed. 5. The normal forces at the wheel/ground contact points are equally distributed among four wheels during motion. 6. The robot is running on a ?at ground surface and fourwheels are always in contact with the ground surface. Denote the wheel angular velocities ωi and the velocities of the wheel contact points as vi , i = 1, . . . , 4, for the leftfront, left-rear, right-front, and right-rear wheels, respectively. Assumption 1.4 implies ω1 = ω2 , ω3 = ω4 . The longitudinal and lateral forces at each wheel’s contact point are Fi and Pi , i = 1, . . . , 4, respectively. The velocity of the robot mass center is denoted as vG . We can de?ne a ?xed frame (X, Y ) and a robot body frame (x, y ) as shown in Fig. 2.

1 Similar results could be obtained if the mass center of the robot were located somewhere other than the robot’s geometric center. 2 Since we only consider wheel/ground point contact, the ground resistance force is negligible.

˙ ICR φ ICRl

θ2 θ1

Δv1

Trajectory Γ

θG

θ1

x P1 D

φ

y F1

P2 F2

vG S ?Δv3

F3

θ3

Δv2 G

Y

θ3 θ4

P3

P4 θ4 W O

Fig. 2. surface.

F4

ICRr L

X

?Δv4

A top-view schematic of a skid-steered mobile robot on a ?at

We also denote the longitudinal and lateral wheel bases as L and W , respectively. Because of Assumption 1.4, we denote the instantaneous center of rotation (ICR) of the left-side wheel contact points, right-side wheel contact points, and the robot body as ICRl , ICRr , and ICRG , respectively. It is known that ICRl , ICRr and ICRG lie on a line parallel to the y -axis [4], [7], ˙ ) be the longitudinal, lateral, and angular [16]. Let (x, ˙ y, ˙ φ velocity of the robot in body frame (x, y ). It is straightforward to calculate the relationship of the robot velocities and accelerations in both frames as follows. ˙ ¨ ˙ X X x ˙ x ¨?y ˙φ = RT (φ) = RT (φ) , (1) ¨ ˙ ˙ , y ˙ Y Y y ¨+x ˙φ where R(φ) = cos φ sin φ . ? sin φ cos φ

˙ , the lonGiven mass center velocity vG and yaw rate φ gitudinal velocities vix of the wheel/ground contact points are W ˙ W ˙ ˙? ˙+ v1x = v2x = x φ, v3x = v4x = x φ, (2) 2 2 where r is the wheel radius. Then we can de?ne the longitudinal wheel slips λi as rωi ? vix Δvix =? , i = 1, . . . , 4, (3) rωi rωi where Δvix = vix ? rωi . Note that λ1 = λ2 and λ3 = λ4 due to Assumption 1.4. It is also observed that under the above de?nition, λ ∈ [0, 1] if the wheel is under traction, and λ ∈ (?∞, 0] if the wheel is under braking, which is undesirable for uniformly modeling the wheel/ground friction under traction and braking cases. To avoid such a problem, using the same treatment as in [5], we restrict the magnitude of λ to a maximum magnitude of 1.0 for λ < 0 under braking. On the other hand, we denote the x-y coordinates for ICRl , ICRr , and ICRG as (xl , yl ), (xr , yr ), and (xG , yG ), λi =

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ThC7.2

respectively. We can ?nd that the x-coordinate S of the ICRs satis?es the following constraints [4], [16]. y ˙ S = xl = xr = xG = ? . ˙ φ (4) where function sgn(x) = 1 if x ≥ 0 and sgn(x) = ?1 if x < 0 and 0 if 0 ≤ λ < λm σ1 (λ) = 1?αλm 1?λm λm if λ ≥ λm , and σ2 (λ) = 1

?α ? 11 ?λm λm

We can also write the longitudinal skid velocities of the wheel/ground contact points as3 Δv1x = Δv2x = Δv3x = Δv4x = W 2 W yr + 2 yl ? ˙ φ, ˙ φ. (5)

if 0 ≤ λ < λm if λ ≥ λm .

(9)

Combining Eqs. (2), (5), and Δvix = vix ? rωi , we can obtain x ˙ ? rω1 x ˙ ? rω3 x ˙ yl = , yr = , yG = . (6) ˙ ˙ ˙ φ φ φ We consider the longitudinal friction forces Fi = Ni μi for the ith wheel, where μi is the friction coef?cient and Ni is the normal force. It has been widely considered that the friction coef?cient μ is a function of the longitudinal slip λ [7], [13], [17]. Figure 3(a) shows the μ-λ curve that is obtained by ?tting the experimental data [18]. Here, we consider a linear approximation of the μ-λ curve as shown in Fig. 3(b). For the traction case, the friction coef?cient μ can be approximated by the following functions. μ(λ) = Kλ Kλm ?

Kλm ?μs 1?λm (λ

Eq. (8) can still be used to calculate the magnitude of the friction coef?cients for the braking case while the longitudinal slip λ < 0 and μ < 0. The longitudinal friction force Fi and the lateral friction Pi are dependent on each other and their magnitudes form a friction force circle [16], [17], [19], namely, Fi = Fir cos θi , Pi = Fir sin θi , where Fir is the resultant maximum friction force and θi is the slip angle at the ith wheel (see Fig. 2). Noting that the longitudinal friction force Fi = Ni μi (λi ), then we can rewrite the lateral friction force Pi as (10) Pi = Fi tan θi , i = 1, . . . , 4, where slip angles θi can be calculated as the angles between the line formed by the wheel contact point and the instantaneous rotating center and the centerline of the wheel’s rotating axis (Fig. 2). Figure 4 shows the four combinations of friction forces for each side of wheels. The longitudinal forces Fi and the lateral forces Pi follow the relationship in Eq. (10). Denote the ICR coordinates as (x, y ) and for all cases shown in Fig. 4 we can rewrite Eq. (10) as follows.

L ?x +x , P2 = ?F2 sgn(λ2 ) 2 . (11) y y Notice that y = 0 and F1 , F2 ≥ 0 is the magnitude of the longitudinal friction force in the above equations. For the four-wheel robot, we assume that the normal load at each wheel Ni = mg 4 is a constant and that the ground soil conditions are the same for the four wheels 4 . Due to the fact λ1 = λ2 , λ3 = λ4 , we can obtain F1 = F2 , F3 = F4 . Using the relationship given by Eq. (11), we can obtain

λ ∈ [0, λm ) ? λm ) λ ∈ [λm , 1],

(7)

where K is the friction stiffness coef?cient, λm is the longitudinal slip value which corresponds to the maximum wheel/ground friction coef?cient, and μs is the longitudinal wheel/ground sliding friction coef?cient. μ

Dry asphalt Loose gravel K

P1 = F1 sgn(λ1 )

L 2

1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

μp ?1 ?λm

Traction

μs 1

μ

λm

λ

Glare ice

0 0.1 0.2 0.3 0.4

P1 = F1 sgn(λ1 )

0.8 0.9 1

λ

0.5

0.6

0.7

Braking

L ? xl 2 + xl , P = ? F sgn( λ ) , 2 1 1 W yl ? 2 yl ? W 2 (12) L 2

(a)

(b)

P3 = F3 sgn(λ3 )

Fig. 3. (a) Relationship between the wheel/ground friction coef?cient μ and the longitudinal slip λ under various road conditions. (b) A linear approximation of the μ-λ relationship.

L ? xr + xr , P4 = ?F3 sgn(λ3 ) 2 . W ?(yr + 2 ) ?(yr + W 2 ) (13) L 2

Therefore, we can write the dynamic equations in the (x, y ) frame as follows. mx ¨ = 2 [sgn(λ1 )F1 + sgn(λ3 )F3 ] , ¨ = 2 [? sgn(λ1 )F1 + sgn(λ3 )F3 ] W ? IG φ 2 L (?P1 + P2 ? P3 + P4 ) , 2

4 This

We assume that the sliding friction coef?cient is a fraction of the peak friction coef?cient μp , i.e. μs = αμp = αKλm , where 0 ≤ α ≤ 1. With such a simpli?cation, we can rewrite Eq. (7) as μ(λ) = K [σ1 (λ) + σ2 (λ) sgn(λ)λ] ,

3 The

(14a) (14b)

my ¨ = P1 + P 2 + P 3 + P 4 ,

(8)

(14c)

ICRs are well-de?ned at a ?nite distance from the wheel/ground ˙ = 0 since y contact point for zero yaw rate φ ˙ = 0 in this case [4].

is a reasonable assumption since the robot size is relatively small.

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ThC7.2

Δv1

x

F1

x P1

˙>0 φ

x P1

˙>0 φ

Δv1

ICR (x, y ) ˙<0 φ

x

F1

P1 y Δv2

θ F1 θ 1 2

P1

ICR (x, y )

ICR (x, y )

y

θ1 θ2

F1

Δv2 P2

F2

(a) Fig. 4.

y

Δv1

θ1 θ2 F2

ICR (x, y )

y

θ1 θ2 F2

˙<0 φ

Δv1

P2 Δv2

(b)

P2

F2

(c)

P2 Δv2

(d)

Friction forces. (a) Braking while turning left, (b) traction while turning left, (c) braking while turning right, and (d) traction while turning right.

where m is the mass of the robot and IG is the mass moment of inertia of the robot about its mass center G. Using Eqs. (3), (5), (6), and (8), we can rewrite the traction/braking forces as F1 = F2 = mg Kσ1 (λ1 ) ? 4

as ¨ + c(q, q ˙ ) = E(q, q ˙ )v , Mq where v = [v1 v2 ]T and ? ? m 0 0 M = ?0 m 0? , 0 0 Ig ? 0 1 ˙ ??1 0 ˙ ) = mφ c(q, q 0 0 ˙ Wφ E1 = σr 2 y ˙

2 ˙ e2 = W x ˙+1 2 L σr 1 φ

(17)

mg K sgn(λ1 )σ2 (λ1 ) 4 mg Kσ1 (λ3 ) ? F3 = F4 = 4 mg K sgn(λ3 )σ2 (λ3 ) 4

x ˙?

W ˙ φ u1 ? 1 , 2 W ˙ φ u2 ? 1 , (15) 2

1 RT E1 mgK , e2 4 ? ? ? P 0 T σ 1 2R ˙ ? mgK ? 0 ?, 0? q 4 ?W σΔ 0 ˙) = E(q, q ?2x ˙ ?2y ˙ 1+ 1 2 σr 1

˙

x ˙+

, .

1 1 where u1 = rω and u2 = rω . De?ning the control input 1 3 variables v1 = σ2 (λ1 )u1 ? σ2 (λ3 )u2 , v2 = σ2 (λ1 )u1 + σ2 (λ3 )u2 and using the friction force model (15) and the relationship for λi , Eqs. (14a)-(14c) become

2 2 1+ 1 ?φ 2 W +L 2 σr 2

1 P 1 ˙ 1 σ + W φv1 ? xv ˙ 2 , (16a) 2 4 2 1 1 1 ˙ v 2 1 + σ r 1 + σ r 2 v1 , (16b) y ¨ = ? gK y 2 2 2 L2 ˙ ¨ = mg K ?W σΔ + W x φ ˙+ φσr1 v1 ? 4IG 2 1 1 ˙ 2 , (16c) W 2 + L2 1 + σr2 φv 2 2 x ¨ = gK where σr 1 = σr 2 ˙ ) σ1 (λ3 ) sgn(λ3 φ ˙) σ1 (λ1 ) sgn(λ1 φ ? , λ1 σ2 (λ1 ) λ3 σ2 (λ3 ) ˙ ) σ1 (λ3 ) sgn(λ3 φ ˙) σ1 (λ1 ) sgn(λ1 φ + , = λ1 σ2 (λ1 ) λ3 σ2 (λ3 )

We have to incorporate the nonholonomic constraint (4) into the above dynamics. Using Eq. (1), we can rewrite Eq. (4) as ˙ = 0, A(q)q (18) where A(q) = [? sin φ cos φ S ]. Following a similar derivation in [9], we can ?nd the following reduced statespace model 5 ˙ = G(q)η , q (19a) T ?1 T ˙ ˙ η = (G (q)MG(q)) G (q)(Ev ? MG(q)η ? c),(19b) where η = [η1 η2 ]T = [x ˙ y ˙ ]T is the pseudo-velocity and matrix G(q) has its columns in the null space of A(q) 6 . ? ? cos φ ? sin φ RT 1 G(q) = ? sin φ cos φ ? = . , g1 = 0 ? S g1 1 0 ?S We can simplify Eq. (19b) as follows. ˙ = η mgK Mη 4

T E1 + g1 e2 v +

and σP = σ1 P + σ2 P , σΔ = σ1Δ + σ2Δ , σ1 P = σ1 (λ1 ) sgn(λ1 ) + σ1 (λ3 ) sgn(λ3 ), σ1Δ = σ1 (λ1 ) sgn(λ1 ) ? σ1 (λ3 ) sgn(λ3 ), σ2 P = σ2 (λ1 ) + σ2 (λ3 ), and σ2Δ = σ2 (λ1 ) ? σ2 (λ3 ). De?ning the generalized coordinates q = [X Y φ]T and using Eq. (1), we can rewrite Eqs. (16) into the XY -frame

2σP

W σΔ S

,

(20)

5 We drop the variable dependency for those variables that have been previously de?ned. 6 Here we can enforce S = 0 to de?ne the matrices well. It is also observed 1 in [4] that S is ?nite such that S = 0.

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ThC7.2

where Mη = where r = ?γ ? f (z, ) ? ξ s and ξ > 0 is a constant. Combining the above control design with Eq. (24) and taking the derivative of the Lyapunov function V , we have ? 1 ?? θ ˙ ˙ = sT s ˙+ θ θ = ?ξ sT s ? V ρθ θ ˙ ? θ + sT r . ρ

1 m

0

1 IG m+ S 2

0

.

III. C ONTROL S YSTEM D ESIGN Given the dynamic model of the skid-steered mobile robot (19a) and (20), we can design a dynamic feedback linearization based controller system [9]. We can ?rst use the following input transformation for Eq. (20).

T e2 v = E1 + g1 ?1

? as We can design the adaptation law for the estimate θ ˙ ? θ = ?ρsT r. (26)

4 2σP 1 M? η τ ? W σΔ mg S

,

(21)

where τ = [τ1 τ2 ]T is the new control input. Under such a transformation, Eq. (20) becomes η ˙ 1 = Kτ1 η ˙ 2 = Kτ2 . Consider the new output function z(t) as the coordinates (in a ?xed frame) of the ICR projection point on the x-axis (point D in Fig. 2). X + S cos φ z(t) = . (22) Y + S sin φ ˙ = w1 (dynamic extension), τ2 = w2 , and De?ning τ1 = ζ , ζ T w = [w1 w2 ] , then we obtain ... z = K α(q, η )w + K β (q, η ) + γ (q, η ), (23) where α(q, η ) =

η1 2ζη2 cos φ S sin φ S sin φ , β (q, η ) = , η1 2ζη2 sin φ ? S cos φ ? S cos φ

2 2

Then the stability of the adaptive control system follows ˙ = ?ξ sT s ≤ 0 and Barbalat’s Lemma [20]. Note from V ? to its true that the convergence of the estimated parameter θ value depends on the persistent excitation conditions for the adaptation law by Eq. (26). IV. S IMULATION AND E XPERIMENTAL R ESULTS In this section, we present some simulation and experimental results based on the skid-steered four-wheel robot platform shown in Fig. 1. For this robot, we have: W = 0.43 m, L = 0.28 m, r = 0.08 m, m = 5 kg, and Ig = 0.45 kgm2 . Fig. 5 shows one test which compares the experiments with the simulation results. The robot was run under constant angular velocities ω1 = ω2 = 60 rpm and ω3 = ω4 = 120 rpm on a concrete surface. Fig. 5(a) shows the PID-controlled angular wheel speeds and Fig. 5(b) shows the real trajectory of the robot’s center with a simulated circular trajectory under the same wheel speeds. Considering the mechanical alignment and geometric variations on the real system, the actual robot trajectory ?ts well with the simulated trajectory. A comparison of the results clearly shows that the dynamic model can predict the robot dynamics.

η1 η2 1 η2 . Let zd (t) and γ (q, η ) = ? ηS ? S 2 cos φ 2 sin φ denote the desired trajectory of the mobile robot. Then we can de?ne the tracking error = z(t) ? zd (t) and the sliding 2 d +a = ¨ + k1 ˙ + k2 , where k1 = 2a, surface s as s = dt 1 . From the friction model k2 = a2 , and a > 0. De?ne θ = K discussed in the previous section, we know that 0 < θ < ∞. Taking the time derivative of s, we have ... ... ˙ = z ? z d + k1 ¨ + k2 ˙ . s

T

140

1.5

120

Experiment Simulation

1

100

ω (rpm)

80

Y (m)

Wheel 1 Wheel 2 Wheel 3 Wheel 4

0.5

0

60

?0.5

40

?1

20

?1.5

0

0

2

4

6

Time (sec.)

8 10 12

14

16

18

20

?4

?3.5

?3

?2.5

X (m)

?2

?1.5

?1

?0.5

0

Using the dynamics given by Eq. (23), the above equation becomes 1 1 ˙ = αw + β + γ + f (z, ), s (24) θ θ ... ?= θ?θ ?, where where f (z, ) = ? z d + k1 ¨ + k2 ˙ . Denote θ ? θ is an estimate of the true parameter θ. We consider the following Lyapunov function candidate V = 1 ?2 1 T s s+ θ , 2 2θρ

(a)

(b)

Fig. 5. Experimental results for a circular motion with ω1 = 60 rpm and ω3 = 120 rpm. (a) Angular wheel speed, and (b) experimental and simulated trajectory.

where ρ > 0 is the adaptation gain. Let the control input w be ?α?1 ?γ ? f (z, ) ? ξ s ? 1 β , (25) w=θ ? θ

In the simulation studying trajectory tracking control performance, the robot is designed to track a circle given by t t m and yd (t) = 6 cos 10 m. The location xd (t) = 6 sin 10 of the ICR of the robot is chosen as S = 0.167 m. For simplicity, the slope of the wheel/ground friction coef?cient μ-λ curve is kept constant at K = 5 during the entire maneuver. The other parameters in the μ-λ curve are α = 0.8 and λm = 0.2. The controller parameters are chosen as follows: k1 = 2, k2 = 1, ξ = 0.1, and ρ = 1. The

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robot starts at location (0, 3) with yaw angle φ(0) = 0 and horizontal velocity x ˙ = 0.3 m/s. The initial wheel velocities are ω1 (0) = ω2 (0) = 20π rpm and the initial condition for ?(0) = 1. Figure 6(a) shows the the estimated parameter is θ robot trajectory, and Fig. 6(b) shows the tracking error in the ?xed frame. It can be clearly seen from these plots that the robot trajectory quickly converges to the desired trajectory. Figure 7(a) shows the robot velocities in the body-?xed frame. It can be seen that since S = 0, the lateral velocity of the robot is non-zero, i.e. y ˙ = 0, and the robot is indeed ? is shown skidding on the ground. The estimated parameter θ ? in Fig. 7(b). Although the estimated parameter θ converges, 1 = 0.2. it does not converge to the true value θ = K

6

to its true value depends on the richness of the adaptation signals. Some experimental and simulation results were presented to demonstrate the effectiveness and ef?ciency of the proposed modeling and control scheme. In the future, we will report the experimental testing results of the proposed control mechanism on various road conditions. R EFERENCES

[1] G. Oriolo, A. De Luca, and M. Vendittelli, “WMR Control Via Dynamic Feedback Linearization: Design, Implementation, and Experimental Validation,” IEEE Trans. Contr. Syst. Technol., vol. 10, no. 6, pp. 835–852, 2002. [2] Z. Song, Y. Zweiri, and L. Seneviratne, “Nonlinear Observer for Slip Estimation of Skid-Steering Vehicles,” in Proc. IEEE Int. Conf. Robotics Automation, Orlando, FL, 2006, pp. 1499–1504. [3] A. Le, D. Rye, and H. Durrant-Whyte, “Estimation of Track-Soil Interactions for Autonomous Tracked Vehicles,” in Proc. IEEE Int. Conf. Robotics Automation, Albuquerque, NM, 1997, pp. 1388–1393. [4] J. Martinez, A. Mandow, J. Morales, S. Pedraza, and A. Garcia-Cerezo, “Approximating Kinematics for Tracked Mobile Robots,” Int. J. Robot. Res., vol. 24, no. 10, pp. 867–878, 2005. [5] M. Kitano and M. Kuma, “An Analysis of Horizontal Plane Motion of Tracked Vehicles,” J. Terramechanics, vol. 14, no. 4, pp. 211–225, 1977. [6] J. Wong, Theory of Ground Vehicles, 3rd ed. Hoboken, NJ: John Wiley & Sons, Inc., 2001. [7] J. Wong and C. Chiang, “A General Theory for Skid Steering of Tracked Vehicles on Firm Ground,” Proc. Inst. Mech. Eng., Part D: J. Auto. Eng., vol. 215, pp. 343–355, 2001. [8] G. Anousaki and K. Kyriakopoulos, “A Dead-Reckoning Scheme for Skid-Steered Vehicles in Outdoor Environments,” in Proc. IEEE Int. Conf. Robotics Automation, New Orleans, LA, 2004, pp. 580–585. [9] L. Caracciolo, A. De Luca, and S. Iannitti, “Trajectory Tracking Control of a Four-Wheel Differentially Driven Mobile Robot,” in Proc. IEEE Int. Conf. Robotics Automation, Detroit, MI, 1999, pp. 2632– 2638. [10] P. Petrov, J. de Lafontaine, P. Bigras, and M. Tetreault, “Lateral Control a Skid-Steering Mining Vehicle,” in Proc. IEEE/RSJ Int. Conf. Intel. Robots and Systems, Takamatsu, Japan, 2000, pp. 1804–1809. [11] M. Ahmadi, V. Polotski, and R. Hurteau, “Path Tracking Control of Tracked Vehicles,” in Proc. IEEE Int. Conf. Robotics Automation, San Francisco, CA, 2000, pp. 2938–2943. [12] R. Balakrishna and A. Ghosal, “Modeling of Slip for Wheeled Mobile Robots,” IEEE Trans. Robot. Automat., vol. 11, no. 1, pp. 126–132, 1995. [13] J. Yi, “A Fault Tolerant Longitudinal Control and Tire/Road Friction Estimation System for Automated Highway Systems (AHS),” Ph.D. dissertation, Dept. Mech. Eng., Univ. Calif., Berkeley, 2002. [14] R. L. Williams, II, B. E. Carter, P. Gallina, and G. Rosati, “Dynamic Model with Slip for Wheeled Omnidirectional Robots,” IEEE Trans. Robot. Automat., vol. 18, no. 3, pp. 285–293, 2002. [15] L. Ray, D. Brande, J. Murphy, and J. Joslin, “Cooperative Control of Autonomous Mobile Robots in Unknown Terrain,” in Proc. ASME Int. Mech. Eng. Cong. Expo., Chicago, IL, 2006, IMECE2006-13435. [16] K. Weiss, “Skid-Steering,” Auto. Eng., pp. 22–25, 1971. [17] U. Kiencke and L. Nielsen, Automotive Control Systems. Berlin, Germany: Springer-Verlag, 2000. [18] J. Yi, L. Alvarez, and R. Horowitz, “Adaptive Emergency Brake Control with Underestimation of Friction Coef?cient,” IEEE Trans. Contr. Syst. Technol., vol. 10, no. 3, pp. 381–392, 2002. [19] X. Claeys, J. Yi, R. Horowitz, C. Canudas de Wit, and L. Richard, “A New 3D Tire/road Friction Model for Vehicle Simulation,” in Proc. ASME Int. Mech. Eng. Cong. Expo., New York, NY, 2001, IMECE2001/DSC-24528. [20] H. Khalil, Nonlinear Systems, 2nd ed. NJ: Prentice Hall, 1996.

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V. C ONCLUSION We presented an adaptive trajectory control design of a skid-steered wheeled mobile robot. An approximation of the wheel/ground friction forces was used to capture the dynamic relationships of the robot. The relationship between the longitudinal and lateral friction forces at each wheel was obtained through the rigid body kinematics of the robot frame and wheels on each side. An adaptive control algorithm was designed to simultaneously estimate the wheel/ground contact friction information and control the mobile robot to follow a desired trajectory. The stability of the adaptive controller was guaranteed by a Lyapunov stability analysis. However, the convergence of the estimated friction parameter

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