Feedforward and feedback dynamic trot gait control
for quadruped walking vehicle
Ryo Kurazume (kurazume@cvl.iis.u-tokyo.ac.jp)
The University of Tokyo, 4-6-1, Komaba, Meguro-ku, Tokyo 153-8505, JapanKan Yoneda (yoneda@mes.titech.ac.jp) and Shigeo Hirose(hirose@mes.titech.ac.jp)
Tokyo Institute of Technology, 2-12-1, Oo-okayama Meguro-ku, Tokyo 152-8552, Japan
September 1, 2001
Abstract. To realize dynamically stable walking for a quadruped walking robot, the combination of the trajectory planning of the body and leg position (feedforward control) and theadaptive control using sensory information (feedback control) is indispensable. In this paper,
we propose a new body trajectory, the 3D sway compensation trajectory, for a stable trot
gait; we show that this trajectory has a lower energy consumption than the conventional sway
trajectory that we have proposed. Then, for the adaptive attitude control method during the 2leg supporting phase, we consider four methods, that is, a) rotation of body along the diagonalline between supporting feet, b) translation of body along the perpendicular line between
supporting feet, c) vertical swing motion of recovering legs, and d) horizontal swing motionof recovering legs; we then describe how we verify the stabilization efficiency of each method
through computer simulation, stabilization experimentation, and experimenting in walking onrough terrain using the quadruped walking robot, TITAN-VIII.
Keywords: Quadruped robot, Trot gait, ZMP, Energy efficiency, Attitude control, Swing legs
control

1. Introduction
To increase the walking speed and the energy efficency of a quadruped walking vehicle, it is necessary to realize dynamically stable walking in cases
where the vehicle body is supported by fewer than 3 legs and is therefore
unstable.
In studying the trot (Yoneda and Hirose, 1995),(Hirose et al., 1989),(Yoneda
et al., 1996),(Yoneda et al., 1994), pace (Sano and Furusho, 1989), and bound(Raibert, 1986),(Furusho et al., 1994),(Furusho et al., 1995) gaits, which are
fundamental dynamically stable gaits, we have particularly noticed the trot
gait because of its close affinity to the crawl gait, which is one of the standard
statically stable gaits, and can be classified as a “safety gait” (Hirose andYoneda, 1993) that avoids complete tumbling by touching a swing leg toground. Consequently,in this paper we propose a generalized trot gait (Yoneda
and Hirose, 1995),(Hirose et al., 1989) which can smoothly shift from thecrawl gait to the trot gait in proportion to walking speed, and an intermit-­c 2002 Kluwer Academic Publishers. Printed in the Netherlands.

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2
VehicleInertia

GravityGroundZMP (Zero Moment Point)

Figure 1. Zero moment point(ZMP)
The sway compensation trajectory

ZMP
z (Vertical direction)x (Longitudinal direction)y (Lateral direction)

Figure 2. The sway compensation trajectorytent trot gait (Yoneda et al., 1996) which makes the diagonal legs swing andsupports legs simultaneously to reduce the dynamic effect of the recoveringswing legs on the body.
In addition we propose the sway compensation trajectory of the vehicle body (Yoneda and Hirose, 1995) which controls the position of a zero
moment point (ZMP); we describe how we verify the effectiveness of thistrajectory control to realize a dynamically stable walk by performing walking
experiments using the TITAN-IV and TITAN-VI. ZMP is the projection onto
the ground of the force acting on the body as shown in Fig.1. At this point,
no moment exists. Therefore, positioning this point on the diagonal line of
the supporting legs while the robot is using the trot gait, enables the robot tokeep walking stabley since there is no moment along this line which causes
the inclination of the robot body. The proposed sway compensation trajectoryuses lateral (along y axis) body motion to keep the ZMP on a diagonal line
between the support legs as shown in Fig.2.
The improvement of the energy efficiency for walking is one of the mostimportant topics in design and control of a mobile vehicle. Especially, formobile vehicles which have an energy source such as a battery, the improved
energy efficiency realizes not only the extension of the work hours but also

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3
the increase of transportable weight by the miniaturization of the battery andconcentrated utilization of limited resources for the desired task.
Kimura (Kimura et al., 1990) examined the energy efficiency of a quadrupedvehicle and showed the relation between the walking cycle and energy consumption for trot and pace gaits. However, a quadruped walking vehicle wasmodeled as an inverted pendulum and the dynamically stable gait which
always maintains the equilibrium state, such as the gait realized by sway
compensation trajectory, was not examined.
As design methodology, we also propos GDA (Gravitationally Decoupled
Actuation) (Hirose and Yoneda, 1993),(Hirose, 1984) which reduces energy
consumption of the walking vehicle mechanically.
Then, if the sway compensation trajectory is examined from the viewpointof energy efficiency, the energy consumption is expected to be larger than in
the case without specific controls of body trajectory, since acceleration anddeceleration of the body in the lateral direction along y axis has to be repeatedperiodically.
On the other hand, the position of ZMP is controllable not only by thelateral motion along the y axis but also by the longitudinal and vertical motion
along the x and z axes, respectively. And thus, by combining these motions,
it may be possible to improve the energy efficiency against the conventionalsway compensation trajectory which controls the position of ZMP only by
the lateral motion.
In this paper, we propose the 3D sway compensation trajectory of a vehicle
body which uses lateral, longitudinal, and vertical motion of the vehicle bodyto move the ZMP on the diagonal line of the support legs. It is verified thatthe proposed trajectory improves the energy efficiency through theoreticalanalysis and the dynamic simulation software, ADAMS.
Furthermore, we point out that the body attitude becomes unstable andoscillates due to the dynamic effect of the recovering motion of the swing
legs, when the generalized trot gait using the sway compensation trajectoryin which the duty factor is 0.5  0.75 (Yoneda et al., 1996) is applied. In
addition, though the 3D sway compensation trajectory can make dynamicallystable walking on flat ground possible, stable walking is impossible by using only gait control planned by the 3D sway compensation trajectory when
unknown roughness or inclination exists. Therefore, a feedback control system using mounted attitude sensors, gyroscope sensors, etc. which adaptively
corrects the body attitude is required.
For the crawl gait, which is one of the standard statically stable gaits, several quadruped walking vehicles using attitude sensors (Hirose et al., 1984)
or force sensors on the tip of the legs (Klein and Chung, 1987) to adapt toinclination of slope automatically have been developed. We also propose the“Sky-hook suspension control” (Yoneda et al., 1994) in which the specificimpedance relation in the deviation from desired position and attitude is es-

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4tablished, and the body motion is controlled by force control using force sensors on the tip of the support legs. However, this method fundamentally owes
the attitude stabilization control in statically stable walking gait in which 3 ormore legs are in contact with the ground, and thus, stabilization performancefor the two-leg supporting phase is not sufficient.
In contrast, Hiraki et al. (Hiraki et al., 1996) previously proposed a method
to produce the required moment for attitude recovery by rotating the bodyalong the diagonal line between the support legs. In this method, the two-leg
supporting phase is modeled as an inverted pendulum. But from the resultsof computer simulation examined later, the stabilization performance of thismethod is not always sufficient when the initial deviation of attitude is large.
In this paper, several attitude control methods, including body translation
along the direction perpendicular to the diagonal line between the support
legs, and the use of swing legs waving in the vertical and lateral direction
to suppress the oscillation of attitude, are proposed. Then, we describe: how
analytical models of each method are derived and optimum regulators are designed; and how the control performance of each method is compared throughcomputer simulation and damping control experimentation in the two-leg
supporting phase using the quadruped walking vehicle named TITAN-VIII(Arikawa and Hirose, 1996).
Furthermore, for the realization of the practical dynamically stable trot
gait for a quadruped walking vehicle, we believe that the combination of
feedforward control based on the sway compensation trajectory and feedback
control based on the adaptive attitude control is one possible effective control
method. The basic ideas underlying this feedforward and feedback dynamic
trot gait control are as follows: first, the state of the system is generouslytransferred adjacent to the unstable equilibrium point by off-line, feedforward
gait planning such as the sway compensation trajectory; then, the remaining
small deviation from the equilibrium point is adaptively compensated for bya simple, linear feedback control system. In this paper, we describe how wedeveloped the feedforward and feedback dynamic trot gait control systemthat combines the 3D sway compensation trajectory and the adaptive body
position and swing leg motion control and the walking experiment on roughterrain using TITAN-VIII that was carried out.

2. 3D sway compensation trajectory
2.1. C ONVENTIONAL SWAY COMPENSATION TRAJECTORYFirst, the formulation of the conventional sway compensation trajectory is
shown. Here, we consider a vehicle as a point mass at       . If theground is flat and the height of the body from the ground,  , is constant, thesubmit.tex; 8/01/2002; 13:34; p.4

5
position of ZMP       is given as






 















(1)



where,    . Next, the diagonal line of the support legs is defined as  





(2)

Then, in order for ZMP to stay on this line, the center of gravity has to satisfy        (3)Assuming that the vehicle moves along the x axis and acceleration of thecenter of gravity toward the moving direction is constant, the position of the
body along the x axis is expressed as   





(4)

By substituting this equation into Eq.(3), we get     

     (5)The solution of this differential equation,   , is given as the addition of thesolution of the next equation    that is,

 











(6)



(7)

and one particular solution that satisfies Eq.(5). Here, we assume that the general form of the particular solution is expressed by a polynominal expressionof time ; then the solution of Eq.(5) is derived as

 













 





(8)

From the boundary condition about the continuity of trajectory      ,    , each coefficient is determined as¾¾

 ¾ 







¾ 
 ¾     ¾    
 


¾ 
 ¾                  










(9)

(10)
(11)
(12)
(13)

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6


 



 






  ¾ 

¾ 








¾ 






¾ 





     (14)

Where, is a walking cycle. This solution is a trajectory of the center of gravity that always keeps the ZMP on the diagonal line of the support legs, andthis trajectory is defined as the (conventional) sway compensation trajectory.
2.2. E XPANSION FOR LONGITUDINAL MOTION
The conventional sway compensation trajectory mentioned above is expanded
to the form including a sway toward the longitudinal direction.
First, Eq.(3) is decomposed into two equations for the x and y directions,and each solution trajectory is assumed to be given as Eq.(8) and

 













 





(15)

By substituting the boundary condition about the continuity of trajectory, thefollowing equations with two parameters   and  are derived.


 



 

 



 



 



 





 

¾ 




   

 



  (16)  

(17)

 


   ¾      


¾ 
 ¾     ¾      


¾ 
 ¾                 




¾ 

  ¾   






¾ 
 ¾  



¾ 

(18)

(19)
(20)
(21)
(22)
(23)

Where,  is the body stroke in one walking cycle.

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72.3. E XPANSION FOR VERTICAL MOTION
Moreover, the above equations are expanded to the form including a sway inthe vertical direction.
Considering    , the solution is assumed to be given as





 











(24)

Where,  is an arbitrary constant. By substituting the boundary condition
about the continuity of trajectory, coefficients with a parameter  are derived
as


 



 



   






(25)

¾ 

   ¾





¾ 

(26)

Where,  is the body height at    .
We define these expansions of conventional sway compensation trajectory
toward longitudinal and vertical direction as “the 3D sway compensation trajectory”. As stated above, after the fundamental trajectory parameters  and  are determined, all 3D sway compensation trajectories that containlateral and longitudinal movement can be expressed with only two parameters and  , and if vertical movement is included, all trajectories are expressedwith three parameters,  ,  , and .
2.4. E NERGY EFFICIENCY OF THE 3D SWAY COMPENSATIONTRAJECTORY

The energy consumption of a walking vehicle is affected by many factors
such as mass of the body and legs, configuration of the degrees of freedom,
trajectory of body and legs, the negative power at each actuator, etc (Marhefka
and Orin, 1997),(Arikawa and Hirose, 1995). In this paper, however, the trajectory which minimizes the total of the external force applied dynamically,that is, the sum of squared acceleration through the entire trajectory as defined
below, is considered.



 ¾






 

  (27)

Here, only a regular walk    is considered for simplicity.
First, for the conventional sway compensation trajectory in which the body
is moved only in a longitudinal direction, the sum of squared acceleration

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8
through the entire trajectory, , is obtained by substituting   
Eq.(27) as







  







 ¾   




into




¾  

(28)

Next, the proposed 3D sway compensation trajectory including a swayin the longitudinal direction is considered. Since the parameter that can be
designed is   ,  that minimizes sum of squared acceleration through theentire trajectory is given by solving




as


 








¾  



(29)




¾  
¾    



(30)

And the minimum of the sum of squared acceleration  is derived as







  ¾   

 ¾   
¾     


(31)

By comparing Eqs.(28) and (31), the sum of the squared acceleration for the
3D sway compensation trajectory is smaller than the sum for the conventionalsway compensation trajectory except  ,    , or      .
Next, for the 3D sway compensation trajectory including a sway in thevertical direction, even though two parameters   and  should be designed,deriving an analytical solution that minimizes the sum of the squared acceleration is impossible. Therefore, in the following simulation, the minimum of
the sum of the squared acceleration is calculated repeatedly using the Newton
method.
Simulation results for               ,and    are shown in Table I. And Fig.3 shows the obtained body trajectories for the conventional sway compensation trajectory, the expansion in
the longitudinal direction, and the expansion in the longitudinal and verticaldirection, respectively. In Fig.3, the upper figure shows the trajectories projected on the ground (x-y plane), and the lower figure shows the trajectories
projected on the x-z plane from the lateral direction in which the vertical axis
is magnified 20 times.
From these figures, it is verified that the sum of the squared acceleration
can be reduced by swaying not only in the lateral direction but also in thelongitudinal and vertical directions. This means that it might be possible toreduce energy consumption for walking.

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9Table I. Minimum of squared acceleration
lateralonly

lateral andlongitudinallateral,longitudinal,
and vertical



0.623

0.334

0.331

 

0

0.155

0.153

 

0.623

0.179

0.175

 

0

0

0.003

½

0.2

0.107

0.107



0.02040.02040.0207Moreover, Fig.4 and Fig.5 show minimum of the sum of the squared acceleration for a change of  which is the intersection angle between the yaxis and diagonal line between the support legs, and the height of the body
 , respectively. These figures show that the smaller the angle , that is, thediagonal line between the support legs lies perpendicularly to the moving
direction, the more efficient the 3D sway compensation trajectory is.
Next, Fig.6 shows the minimum of the sum of the squared acceleration fora change of walking cycle . This figure shows that the effect of the 3D sway
compensation trajectory increases as the walking cycle becomes shorter, andthus, walking speed increases.
And now, if the walking velocity    is assumed to be constantin Eq.(31), we can derive that  is monotone increasing, and    when  and   ¾ ¾ when  . This means that for the purpose of
reducing the sum of the squared acceleration , the robot should walk with avery small walking cycle and a long slide.
Meanwhile, Kimura et al. (Kimura et al., 1990) proposed that there is a
specific walking cycle which gives a minimum energy consumption whena robot walks by trot gait with a constant velocity. They adopted a modelof an inverted pendulum as a modeling of the trot gait, and took an energy
consumption due to the swing motion of legs into consideration. Therefore,
in cases where the walking cycle is short, the energy consumption becomeslarge since swing legs have to be moved quickly. On the other hand, if thewalking cycle is long, the body is inclined steeply, and thus, a large amountof energy is required to recover from the body inclination.

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10
moving direction

0.1 ygfoot positionfoot position

diagonal line
0.05
Trajectories

-0.05

0.05

0.1

0.15

xg

with longitudinal motion
andwith vertical motion-0.05

lateral motion only
(conventional trajectory)-0.1

foot positions
moving direction
zgwith vertical motion

0.202

-0.05

0.201
0.2
0

0.05

0.1

xg
0.15

lateral motion only
(conventional trajectory)
andwith longitudinal motion

Figure 3. Trajectories of vehicle body8ρ6

lateral only

4
lateral and longitudinallateral, longitudinal, and vertical2
0
10Figure
4. Effect(      of20 

30



40
50
θ [deg.]

angle

60

between

70

80

the

support

legs

 )

The maximization of energy efficiency, which includes the energy consumption of the leg motion or the energy dissipation due to the impact andfriction between the feet and the ground, is very important and should be
discussed. However, the energy efficiency depends on the leg structure, thedesign of an actuator and gear system, and the regeneration ratio of negative
power produced at actuators. Therefore, as a more fundamental index, we

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11
4

ρ

3

lateral only
lateral and longitudinal

2lateral, longitudinal, and vertical

1
0
0

0.05

0.1

0.15

0.2

0.25

 



0.3

H [m]

Figure 5. Effect of height of the body (  







 )



 )

4

ρ

3
2
1
0
0

lateral only
lateral and longitudinallateral, longitudinal, and vertical1

2
T [sec.]Figure 6. Effect of walking cycle (    

3

4



 

adopted the sum of the squared acceleration during one walking cycle shownin Eq.(27) in this paper.
2.5. C OMPUTER SIMULATION USING THE DYNAMIC MOTIONSIMULATER , ADAMS
To verify the energy efficiency of the 3D sway compensation trajectory thatminimizes the sum of squared acceleration derived in Section 2.4, for theactual model with the same configuration of degrees of freedom as in a typical quadruped walking vehicle, the computer simulation using the dynamic
motion simulator, ADAMS  , was carried out. The simulation was executedon an Ultra sparc 30 Workstation of Sun Microsystems, Inc..
The configuration of degrees of freedom, weight, etc. of a quadrupedwalking vehicle model for computer simulation is the same as the TITANVIII (Arikawa and Hirose, 1996) which was developed in our laboratory.
Fig.7 and Fig.8 show the computer model and an example of simulation
results. The sum of power consumption at each joint and specific resistance (Gabrielli and Karman, 1950) are shown in Table II where the walking cycle½

Mechanical Dynamics, Inc., Ann Arbor, Michigan, USA

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12

Leg 2
Leg 1
Joint 2
Leg 3

Joint 1
Leg 4

Joint 3Figure 7. Simulation model for ADAMS

is 1 [s], walking speed is 0.2 [m/s] and duty factor is 0.5. Specific resistance is the index term which makes unified evaluation of energy consumption
possible even on a wall or horizontal surface.

     ! !  " #       !    (32)Specific resistance becomes smaller with higher transfer efficiency, and isminimized if the energy for the transfer is equal to the difference of potential
energy before and after the transfer. That is to say, the highest value is 0 on ahorizontal surface, and 1 on a vertical surface. The columns show the resultsof the conventional sway compensation trajectory, the trajectory including
lateral and longitudinal sway, the trajectory including lateral, longitudinal,
and vertical sway, and straight line with no sway control. The trajectories thatminimize the sum of squared acceleration derived in section 2.4 are used forthe 3D sway compensation trajectories. In the case in which the body moves
on a straight line with no sway control, however, the body attitude cannot be
maintained to be parallel to the ground, and the swing leg touches the groundunexpectedly.
From these results, by using the 3D sway compensation trajectory thatminimizes the sum of squared acceleration, specific resistance improves morethan for conventional sway compensation trajectory even for the same walking cycle and walking speed.
And Fig.9 shows power consumption at each joint of leg 2. In particular, power consumption at joint 3, which mainly produces the lateral sway
motion, becomes low.

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13

Figure 8. Simulation view of ADAMS
Table II. Comparison of energy consumption
lateralonly

lateral andlongitudinallateral,longitudinal,
and verticalstraight

Energy cons. [J]

142.0

114.6

108.6

96.8

Specific res.

2.35

1.90

1.80

1.60

Next, we compare the power consumption between the conventional sway
compensation trajectory and the 3D sway compensation trajectory for threemodels with different body lengths shown in Fig.10. In this simulation, thewalking cycle is 1 [s], walking speed is 0.2 [m/s] and duty factor is 0.5. The
size of the model shown in Fig.10(b) is the same as TITAN-VIII, and themodels in Fig.10(a) and (c) have half and double body length, respectively.
Table III shows the specific resistance and power consumption for thesemodels. It is verified that the effect of the 3D sway compensation trajectory
increases as the distance between the fore and hind legs shortens, and theangle of the diagonal line between the support legs in the moving direction
becomes perpendicular.

3. Adaptive attitude control methods
Even though the 3D sway compensation trajectory proposed above can realize
dynamically stable walking on flat ground, stable walking is impossible using

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1420Power [W]

Swinging

10

Supportinglateral only

0

-10 lateral, longitudinal,

lateral andlongitudinal

and vertical

-20

0

0.2

0.4
0.6
Time [sec.]

0.8

1.0

(a) Joint 1
Power [W]20
lateral, longitudinal,
lateral onlyand vertical

100lateral andlongitudinal
Supporting-10
Swinging

-20

0

0.2

0.4
0.6
Time [sec.]

0.8

1.0

(b) Joint 2
20
Power [W]

Swinging

10

Supportinglateral only0
lateral, longitudinal,
and vertical

-10
-20

0

0.2

0.4
0.6
Time [sec.]lateral andlongitudinal

0.8

1.0

(c) Joint 3
Figure 9. Power consumption of each joint of leg 2

only gait control planned with the 3D sway compensation trajectory when
unknown roughness or inclination exists. In addition, for example, when an
extended trot gait with the sway compensation trajectory is applied, body
attitude sometimes becomes unstable and oscillates due to the dynamic effectof the recovering motion of the swing legs, because the point mass is assumed
for the derivation of the sway compensation trajectory (Yoneda et al., 1996).
Therefore, a feedback control system that uses mounted attitude sensors,
gyroscope sensors, etc. which adaptively corrects body attitude is required. In
this section, we propose the attitude control method during 2-leg supporting
phase using the body translation and rotation, and leg swing motion.

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15(a) Short body length(b) Middle body length(c) Long body length

Figure 10. Three simulation models with different body length
Table III. Specific resistance for each body length(a) lateral only(b) 3D sway(b)/(a)Short

3.90 (236.0[J])1.90 (114.5[J])48.7 [%]

Middle2.35 (142.0[J])1.80 (108.6[J])76.6 [%]

Long

1.81 (109.4[J])1.75 (105.4[J])96.7 [%]

To produce the required moment for attitude recovery in the two-leg supporting phase, the method using rotation of the body along the diagonal line
between the support legs as shown in as shown in Fig.11(a) has been proposed so far (Hiraki et al., 1996). The required moment for attitude recovery,
however, can be produced by the translation of body position as shown in
Fig.11(b). Furthermore, although the recovery motion of swing legs has been
considered as disturbance that induces the oscillation of body, so far (Yoneda
et al., 1996), by controlling the recovery path appropriately, swing legs canbe used for the oscillation control of body attitude.
From the above discussion, four stabilization control methods of body
attitude as shown in Fig.11 are considered in this paper.
(a) Rotation of the body along the diagonal line between the support legs.
(b) Translation of the body along the direction perpendicular to the diagonalline between the support legs.
(c) Vertical motion of swing legs during recovery.

submit.tex; 8/01/2002; 13:34; p.15

16(a) Rotation of body(b) Translation of body(c) Vertical motion of swing legs(d) Horizontal motion of swing legs

Figure 11. Four attitude control methods

Link 3

φ1 φ
2Link 2

φ1

x

Joint 2
Link 3

Joint 1

Joint 2
Joint 1

Link 1(a) Rotation of body
φ1
φ2(b) Translation of bodyφ1

Joint 2

Joint 2

φ2
Joint 1
φ2(c) Vertical motion of swing legs

φ2

Joint 1(d) Horizontal motion of swing legs

Figure 12. Analysis models(d) Horizontal motion of the swing legs during recovery.
First, each method is simplified as a three-link model corresponding to asupport leg, a body, and a swing leg as shown in Fig.12. Appendix A shows
motion equations of each model and derived optimum regulators designed torepress the oscillation of the body.
Fig.13 shows examples of computer simulation when the initial conditions
are $   ,$   , and    

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1740
φ 1 [deg.]

20
0

-20
0

2

4
6
Time [sec.]

8

-40

10

80

20

60

10

x [mm]

φ 2 [deg.]

φ 1 [deg.]

10
8
6
4
2
0
-2

40
20

8

10

2

4
6
Time [sec.]

8

10

-20
2

4
6
Time [sec.]

8

0

10

(b) Translation of body
10
8
6
4
2
0
-2

φ 1 [deg.]

φ 1 [deg.]

0

2

4
6
Time [sec.]

8

10

0

2

80

80

60

60

φ 2 [deg.]

φ 2 [deg.]

4
6Time [sec.]

0(a) Rotation of body
10
8
6
4
2
0
-2

2

-10

0
0

0

40
20
0

4
6
Time [sec.]

8

10

40
20
0

0

2

4
6
Time [sec.]

810(c) Vertical motion of swing legs

0

2

4
6
Time [sec.]

810(d) Horizontal motion of swing legs

Figure 13. Simulation results

From Fig.13(a), the body has to be inclined up to 40 degrees toward the inclination direction to recover the inclination of the support leg for the method
using the rotation of body. Fig.13(b) shows that maximum body movement
to recover the inclination of the support leg is 0.2 [m] for the method using
the translation of body. Though this body motion can be executed by TITANVIII, the inclination of the support leg becomes vibrational. Next, in Fig.13(c)and (d), the maximum angles of the swing legs become 80 [deg.] and 70[deg.], respectively. Thus, it is difficult to stabilize the body attitude using
only the effect of vertical and horizontal motion of the swing legs since therealization of such a wide movable angle is difficult.

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18

Attitude sensor

Gyrosensors

Figure 14. 4-legged walking robot, TITAN-VIII

φ1 [deg.]

15
10
5
0
-5
-10
-15
0

2

4
6
Time [sec.]

8

10

Figure 15. Experimental result (no sensor)

4. Dynamically stable walking experiment with TITAN-VIII
4.1. DAMPING CONTROL IN THE TWO - LEG SUPPORTING PHASE
The damping control experiment using four attitude stabilization control methods examined in Section 3 was carried out using the quadruped walking robot
named TITAN-VIII.
As shown in Fig.14, this robot is equipped with a computer board (Pentium 200MHz, Japan Data System), AD/DA boards, Ethernet card, silicondisk, 3-axes attitude sensor, (Maxcube, Japan Aviation Electronics), and two
gyrosensors (Gyrostar, Murata).
In the experiment, the body of TITAN-VIII that stood with two support
legs was tilted by an external force applied to the body (pushing by hand),and the body attitude returned to the stable state by each control method wasmeasured.
The ankle of TITAN-VIII is restricted mechanically to let the sole beparallel to the body. Thus, as shown in Fig.15, the body attitude returns toa stable state without attitude control if the inclination angle is smaller thanabout 8 [deg.], since the moment for recovery of body inclination is producedby the soles.

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19

34[deg.](a) Recovering path is perpendicular (b) Recovering path is inclined
to groundwith respect to ground.

Figure 16. Recovering path of swing legs

For the purpose of increasing the dynamic effect of the swing legs to thebody, the swing legs are stretched laterally in the middle of the return path asshown in Figs.16(b), and therefore, the inertia of the legs is increased.
Inclination angle of the support leg along the diagonal line between the
support legs, $ , and control variables $  and  for each control method
are shown in Figs.17. Here, $  in Fig.17(a) is the body rotation angle,  in
Fig.17(b) is the body displacement, and $  in Fig.17(c) and (d) is the angleof joint 2 in Fig.12 converted from the amount of swing.
Fig.18 shows the damping control experiment using the vertical motionof the swing legs, and where the swing legs are swung up as the recovery ofbody attitude. From the experimental results for the method using the rotationof body shown in Fig.17(a), the swing leg of the leaning side contacted theground because the body was tilted more and more in the tilted direction,and large body vibration was generated by the reaction force. Next, for the
method using the translation of the body shown in Fig.17(a), though the
body attitude was vibrational, it was finally converged to horizontal. Fromthe comparison of Fig.17(c) and Fig.15 which are the results for the control
method using vertical motion of the swing legs and the results without attitudecontrol, performance of convergence was clearly improved by the dynamic
effect of swinging. However, the swing width of the swing leg is limited in
the joint movable area, and thus, recovery from a large tilt angle to horizontal
is difficult using only this method. On the other hand, Fig.17(d) using thehorizontal motion of the swing legs shows the convergence performance ishardly improved since displacement of actual links is smaller than the caseusing vertical motion of the swing legs, and the dynamic effect is small.
These results suggest that the attitude stabilization performance might be
the highest by combining the vertical motion of the swing legs and translationof the body.
Then, Fig.19 shows the experimental result when these two methods are
simply superimposed. In comparison with Fig.19, Fig.15 and Fig.17(b), bothconvergence performance and stability are improved.

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20

6

15
10
5
0
-5
-10
-15
0

2

4
6
Time [sec.]

8

10

6

15
10
5
0
-5
-10
-15
0

2

4
6
Time [sec.]

8

10

10

15
10
5
0
-5
-10
-15
0

2

4
6
Time [sec.]

8

10

10

15
10
5
0
-5
-10
-15
0

2

4
6
Time [sec.]

8

10

10

φ 1 [deg.]

φ 1 [deg.]

20

0

-10
-20
0

2

3
4
Time [sec.]

5x [mm]

φ 2 [deg.]

6
4
2
0
-2
-4
-6

1

0

1

2

3
4
Time [sec.]5

φ 1 [deg.](b) Translation of bodyφ 1 [deg.](a) Rotation of body
15
10
5
0
-5
-10
-15
0

2

4
6
Time [sec.]

85

φ 2 [deg.]

φ 2 [deg.]

10

0

-5

-10

0

2

4
6
Time [sec.]

8(c) Vertical motion of swing legs(d) Horizontal motion of swing legs

Figure 17. Experimental results of attitude control for slow trot gait

In general, the crawl gait, which is one of the statically stable walking gaits
for a quadruped walking vehicle, makes it possible to walk maintaining static
stability of the body, since three or more legs always have contact with theground. However, in order to keep the projected point of the center of gravity
within the support leg polygon, and also to guarantee the convergence to theregular gait, the possible area for the swing leg to contact the ground is limited
compared with the movable area of leg. On the other hand, the possible areaof the contact point for the trot gait can be taken wider than the crawl gait,since two diagonal legs turn into swing legs simultaneously. Therefore, if astable trot gait can be realized using the proposed stabilization control even at
low walking speed, the more adaptive walking with a large contact area will
become possible.

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Time

21

Figure 18. Experiment of attitude control using swing legs

4.2. ATTITUDE CONTROL EXPERIMENT BY GENERALIZED TROT GAIT
AND STEP CLIMBING EXPERIMENTThe attitude stabilization method which employs feedback control using vertical motion of the swing legs and translation of body, and feed forward
control using the 3D sway compensation trajectory proposed in Section 2
was applied to TITAN-VIII, and the stabilization performance in dynamicallystable walking by a generalized trot gait was verified.
As stated above, body attitude sometimes becomes vibrational when the
duty factor is 0.5  0.75 for the generalized trot gait with the sway compensation trajectory by the dynamic effect of recovering motion of swing
legs.
The body attitude of TITAN-VIII using the proposed body attitude stabilization control was then examined. Fig.20 shows a comparison with andwithout the attitude control, and Fig.21 shows the measured inclination angleof the support legs around the diagonal line between the support legs. In thissubmit.tex; 8/01/2002; 13:34; p.21

22
20
10
x [mm]

φ 1 [deg.]

15
10
5
0
-5
-10
-15
0

0

-10
2

4
6
Time [sec.]

8

10

4

8

10

-20
0

2

4
6
Time [sec.]

810

φ 2 [deg.]

10
5
0

-5

-10

0

2

6

Time [sec.]

Figure 19. Experimental result (Combination of translation of body and vertical motion ofswing legs)

experiment, the duty factor is 0.65, walking cycle is 3[s] and walking speed
is 0.65[m/s].
From Fig.20, the inclination angle without attitude control was greatly
collapsed as shown in the 3rd and 4th photos from the top. However, by
the proposed attitude control, the right and left legs turned into swing legs
equally. Accordingly, dynamically stable walking using a generalized trot
gait with a duty factor of 0.65 can be realized by proposed body attitude
stabilization control.
In addition, the return path of the swing legs is also shown in Fig.22. The
horizontal axis is the walking direction, and the vertical axis is the verticaldirection. By using swing leg control only in the region where the heightof the swing leg from the ground is larger than a specific height (5 [cm]),the body vibration caused by the contact of the swing leg to the ground isprevented.
Next, we examined the attitude stabilization performance when TITANVIII walks dynamically in an environment where an unknown and leaning
step exists.
Fig.23 is a series of photos of the experiment, and Fig.24 shows the inclination angle of the support leg around the diagonal line between the support
legs. In this experiment, the duty factor is 0.5, walking cycle is 10[s], andwalking velocity is 0.02 [m/s]. The photos on the left in Fig.23 show theresults with the proposed attitude stabilization control, and the photos on theright show the results without attitude stabilization control.
From Fig.24, dynamically stable walking with small attitude fluctuation
can be performed by the 3D sway compensation trajectory on a flat surface
in 0  12[s], On the other hand, one of the right side legs runs on the stepin 1225[s]. The results without attitude control show that the body inclined

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23(a) attitude control(b) no sensor

Figure 20. Dynamically stable walking experiment (Duty factor is 0.65)

so much and the swing leg contacted the ground unexpectedly as shown in
the fourth photo of Fig.23. However, when the proposed attitude stabilization
control was applied, dynamically stable walking without contact of the swing
legs to the ground was realized and the effectiveness of the proposed attitude
stabilization control was confirmed.

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Inclination [deg.]

24
15
10
5
0
-5
-10
-15
0

with attitude control

2

no sensor
46Time [sec.]

8

10

Figure 21. Body inclination (Duty factor is 0.65)
with vertical motion of swing leg

z [m]

0.1

00

0.1
x [m]

0.2

Figure 22. Recovering path of swing leg

5. Conclusion
In this paper, the 3D sway compensation trajectory, that is, an expansion of
the conventional sway compensation trajectory with lateral motion towardthe 3D trajectory including longitudinal and vertical motion, is proposed.
The 3D sway compensation trajectory enables keeping ZMP on the diagonalline of the support legs more efficiently with less energy consumption. Then,
it is verified that the proposed trajectory improves the specific resistancethrough theoretical analysis and the dynamic simulation software, ADAMS.
The effect of the proposed trajectory increases as the vehicle walks faster,and even for the same walking speed, the proposed trajectory is especiallyeffective when the movable range of the body is mechanically restricted since
the swing length becomes smaller than when using the conventional sway
compensation trajectory.
As stated above, the energy consumption of a walking vehicle is affectedby many factors. The trajectory proposed in Section 2.4 that minimizes thesum of the squared acceleration, does not always consist of the optimum
trajectory that minimizes the actual energy consumption for walking. For
example, if the leg of a walking vehicle shown in Fig.7 lies on a straight line,
the required joint torque to support the force toward this direction is very

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25(a) attitude control(b) no sensor

Figure 23. Dynamically stable walking experiment on rough terrain

small and walking with little energy consumption becomes possible using
this straight form continuously.
On the other hand, for the proposed 3D sway compensation trajectory,
all trajectories can be described with two or three parameters. Therefore, it
might be possible to obtain the optimum trajectory that minimizes the energy
consumption for an actual walking vehicle with various joint configuration,
by repeat calculation with ADAMS, and this will be examined in the future.
Moreover, even though the 3D sway compensation trajectory proposed inthis paper can realize dynamically stable walking on flat ground, stable walking is impossible using gait control planned with the 3D sway compensation
trajectory only when unknown roughness or inclination exists. In addition, forexample, when an extended trot gait with the sway compensation trajectory
is applied, body attitude sometimes becomes unstable and oscillates due tothe dynamic effect of the recovering motion of the swing legs, because thesubmit.tex; 8/01/2002; 13:34; p.25

26
4Inclination [deg.]

0-4
3D sway compensation
trajectory
and attitude control-8-12

3D sway compensation
trajectory only

-16
0

5

10

15
20
Time [sec.]

25

30

35

Figure 24. Body inclination of robot walking dynamically on rough terrain

point mass is assumed for the derivation of the sway compensation trajectory(Yoneda et al., 1996).
For these problems, in this paper, several attitude control methods including body translation along the direction perpendicular to the diagonal line
between the support legs and the use of swing legs waving in the vertical andlateral direction to suppress the oscillation of attitude, are proposed to realize
dynamically stable walking for the quadruped walking vehicle.
Then, the analytical model of each method was derived and the optimum
regulator to stabilize the attitude oscillation was designed. The control performance of each method was compared through computer simulation anddamping control experiment using the quadruped walking vehicle, TITANVIII, standing with two support legs.
Furthermore, a dynamically stable walking experiment on rough terrain
using the generalized trot gait employing the 3D sway compensation trajectory and the proposed attitude control method was carried out, and the effectiveness of the proposed attitude control method that enables dynamicallystable walking on unknown and rough terrain was confirmed.
So far, we have considered that a light weight leg is a requisite for thedevelopment of a practical walking vehicle and created them by the optimization of actuator configuration using, for example, wire and pulley, theuse of light and rigid materials, and optimum mechanical design with highstiffness-weight ratio. However, experimental results show that even the lightweight leg such as the leg of TITAN-VIII is effective for vibration control ofthe body depending on the moving direction.
The basic idea underlying the attitude stabilization control method proposed in this paper is as follows: First, the state of the system is generouslytransferred adjacent to the unstable equilibrium point by off-line gait planning
using the 3D sway compensation trajectory. Then, the little deviation from theequilibrium point is adaptively compensated for by a simple linear feedback

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27
control system which utilizes the translation of the body and swinging of theswing legs.
Results of the experiment demonstrated that this control methodology wasvery effective for practical use and could make dynamically stable walking
of walking vehicle possible.

Appendix
A. Motion equations for three link models and design of optimum
regulators
A.1. V IBRATION CONTROL USING ROTATION OF BODY ALONG THE
DIAGONAL LINE BETWEEN SUPPORT LEGSIn Fig.12(a), we assume that each link length, mass, and inertial are "    %   , respectively. The motion equation of this model is derived as&  ½ ½ ¾  ¿ $
% ½  ¾  ¿ "

(33)&  %%  ¾ ¿  $%%  ¾ ¿ (34)$  $ 

By substituting the parameters of TITAN-VIII into the above equations, andif the maximum torque is 10 [Nm], the optimum regulator is designed as&  $

$

$

$

(35)

A.2. V IBRATION CONTROL USING TRANSLATION OF BODY ALONG THE
DIRECTION PERPENDICULAR TO THE DIAGONAL LINE BETWEENTHE SUPPORT LEGS

The motion equation of the model shown in Fig.12(b) is derived as$  ½ $    "'% %   "

¾

  

%  
 $   %%   "



%

%

"

"
 
%  "




"

(36)



'

(37)

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28
By substituting the parameters of TITAN-VIII into the above equations, andif the maximum force is 100 [N], the optimum regulator is designed as'  $$   

(38)

A.3. V IBRATION CONTROL USING VERTICAL MOTION OF THE SWINGLEGS DURING RECOVERYThe motion equation of the model shown in Fig.12(c) is derived as

$ 
$ 



&  ½ ½ ¾  ¿ $%&  %

"

¾

 ¿
¾

 ¿

%

¿ ¾¾

½  ¾  ¿ % " ¿  $ $¿ ¾ ¿(39) $

(40)

By substituting the parameters of TITAN-VIII into the above equations, andif the maximum torque is 10 [Nm], the optimum regulator is designed as&  $

$

$

$

(41)

A.4. V IBRATION CONTROL USING HORIZONTAL MOTION OF THE SWINGLEGS DURING RECOVERYThe motion equation of the model shown in Fig.12(d) is derived as

$ 



&  ½ ½ ¾  ¿ $
%

$ 



½  ¾  ¿ %&    
¿

%

%

%


 ¿"$

¿ ¾¾$  " ¿  $ ¿   ""  $$ ¿   " ¿  $
¾

(42)

(43)

By substituting the parameters of TITAN-VIII into the above equations, andif the maximum torque is 10 [Nm], the optimum regulator is designed as&   $

 $  $  $(44)

References
S. Hirose and K. Yoneda, Toward the Development of Practical Quadrupe Walking Vehicles,
J. of Robotics and Mechatronics, vol.5, No.6, pp.498-504.

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29
K. Yoneda and S. Hirose, Dynamic and static fusion gait of a quadruped walking vehicle on awinding path, Advanced Robotics, pp.125-136.
S. Hirose, K. Yoneda, R. Furuya, and T. Takagi, Dynamic and static fusion gait of a quadrupedwalking vehicle, In Proc. of IEEE/RSJ Int. Conf. on Intelligent Robots and Systems ’89,
pp.199-204.
K. Yoneda, H. Iiyama, S Hirose, Intermittent Trot Gait of a Quadruped Walking Machine
Dynamic Stability Control of an Omnidirectional Walk, In Proc. Int. Conf. on Robotics
and Automation, pp.3002-3007.
S. Hirose, A Study of Design and Control of a Quadruped Walking Vehicle, In Int. J. Robotics
Research, Vol.3, No.2, pp.113-133.
G. Gabrielli and I. von Karman, What price speed?, Mechanical Engineering, Vol.72, No.10,
pp.775-781.
K. Yoneda, H. Iiyama, and S. Hirose, Sky-hook suspention control of a quadruped walking
vehicle, In Proc. Int. Conf. on Robotics and Automation, pp.999-1004.
H.Kimura, I.Shimoyama and H.Miura, Dynamics in the dynamic walk of a quadruped robot,
RSJ. Advanced Robotics, vol.4, no.3, pp.283-301.
D. W. Marhefka and D. E. Orin, Gait Planning for Energy Efficiency in Walking Machines, In
Proc. Int. Conf. on Robotics and Automation, pp.474-480.
K. Arikawa and S. Hirose, Study of Walking Robot for 3 Dimensional Terrain, In Proc. ofIEEE/RSJ Int. Conf. on Intelligent Robots and Systems ’95, pp.703-708.
K. Arikawa and S. Hirose, Development of Quadruped Walking Robot TITAN-VIII, In Proc.
of IEEE/RSJ Int. Conf. on Intelligent Robots and Systems ’96, pp.208-214.
A. Sano and J. Furusho, Dynamically Stable Quadruped Locomotion (A Pace Gait in The
COLT-3), In Proc. of the Int. Symp. on Industrial Robots, pp.253-260.
M. H. Raibert, Legged Robots That Balance, MIT Press.
J. Furusho, A. Sano, M. Sakaguchi, and K. Honda, Bounce Gait Control of a Quadruped
Robot, In Proc. of the Second International Conference on Motion and Vibration Control,Vol. 1, pp.198-203.
J. Furusho, A. Sano, M. Sakaguchi and E. Koizumi : Realization of Bounce Gait in a
Quadruped Robot with Articlar-Joint-Type Legs, In Proc. Int. Conf. on Robotics andAutomation, pp.697-702.
M. Hiraki, T. Emura, Y. Senta, and S. Okada, Trotting Gait of a Quadruped Robot Based on
Reaction Wheel Model (in Japanese), In Proc. of 14th Conference of the Robotics Societyof Japan, pp.967-968.
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Robotics and Automation, vol.2, no.2, pp.61-69.
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Vehicle, Robotics Research (1st Int. Symp.), The MIT Press, pp.253-277.
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30
Authors’ Vitae
Ryo Kurazume
is a researcher in Institute of Industrial Science at the University of Tokyo. He
received his Ph.D. degree from the Department of Mechanical Engineering
Science, Tokyo Institute of Technology in 1998. His M.E. and B.E. werefrom the Department of Mechanical Engineering Science, Tokyo Institute ofTechnology in 1991 and 1989, respectively. His research interests include
multiple mobile robots, computer vision, and walking robots.
Kan Yoneda
is an associate professor in the Department of Mechanical and Aerospace Engineering at Tokyo Institute of Technology. He received his Ph.D. degree from
Department of Mechanical Engineering Science, Tokyo Institute of Technology in 1992. His M.S. and B.S. were from Department of Physics, Tokyo
Institute of Technology in 1987 and 1985, respectively. His research interest
includes mechanical design and control of biped, quadruped, and hexapod
walking robot.
Shigeo Hirose
was born in Tokyo, Japan in 1947. He is a Professor in the Department of
Mechano-Aerospace Engineering at the Tokyo Institute of Technology. His
research interests are in mechanisms, sensors and control of novel robotic systems. He was awarded more than twenty academic prizes including the Best
Conference Paper Award in 1995 and the first Pioneer in Robotics and Automation Award in 1999 both from IEEE Robotics and Automation Society.
He has published several books, including ”Robotics” (Shokabo Publishing
Co. Ltd., 1987, in Japanese) and ”Biologically Inspired Robots” (Oxford
University Press, 1993).

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