CHR0075 Synopsis

8/8/2019 CHR0075 Synopsis

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Smooth Touch and Force Control toUnknown Environment without Force Sensor

for Industrial RobotNaoki Shimada, Kiyoshi Ohishi, Satoru Kumagai

Nagaoka University of Technology1603-1 Kamitomioka-cho,

Nagaoka, Niigata, 940-2188JAPAN Tel:+81-258-47-9525

Fax:+81-258-47-9500Email:[email protected]

Email:[email protected]:[email protected]

Toshimasa MiyazakiNagaoka National College of

Technology, 888 NishikatakaiNagaoka, Niigata, 940-8532JAPAN Tel:+81-258-32-6435

Fax:+81-258-34-9700Email:[email protected]

Abstract —This paper proposes a new smooth touch control tounknown environments for industrial robot. The touch controlis one of the useful methods for the smooth force control. Manyapplications of the manipulator should have a high stiffnessposition control method. However, the controller has no exibilityto the environments. Therefore, a method of high performanceforce control is an important factor for many applications. Forthis purpose, this paper proposes a new force control structurebased on I-PD force controller and disturbance observer. Theexperimental results in this paper conrm that the proposedmethod has ne validity of smooth touch control to unknownenvironments such as concrete, foamed styrol and cardboard boxby using the tested 3-DOF industrial manipulator.

I. INTRODUCTION

The motion control method for industrial robot is used inmuch application. For example, we have already proposedmany methods of high performance motion control[1][2].Some of these high performance position control method havebeen using in the industrial applications and the industrialproducts. Recently, it is important for robot manipulator to re-alize the high performance force control such as smooth touchcontrol to unknown environments for several applications suchas industrial eld, medical care and so on, in particular withregard to safety[3].

The force control has low stiffness in contrast with the po-sition control[4]. When the robot manipulator uses a positioncontrol and a force control, the hybrid control of position andforce is one of the solutions to the task [5].In this case, thecontrol system selects by the boundary conditions whetherposition control or force control is better or not. Generally, arobot manipulator has the touch process in the process of thecontact to environment. The robot manipulator generates theimpulsive force to the environment on condition of this touchprocess. The response of force control system has inuenceon the impulsive force.

This paper proposes a new smooth touch and force controlto unknown environment without force sensor for industrialrobot manipulator, which is a solution of this problem in thispaper. The proposed method is constructed by D-PD positioncontrol and I-PD force control. The transient phenomenonfrom D-PD position control to I-PD force control is carriedout smoothly. The integrator of I-PD force control systemis quickly initialized to the optimal value by using the dis-turbance observer and the manipulator dynamics calculationwithout force sensor, as shown in Fig.1. The proposed methodis treated as a new sensor-less force control system basedon the disturbance observer and the manipulator dynamics

calculation. The validity of proposed method is conrmedby the experimental results using the tested 3-DOF robotmanipulator. The environments of the experimental results usesa concrete block, a foamed styrol block and a cardboard box.

I I . R EACTION FORCE OBSERVER

A. Manipulator dynamics calculation

Manipulator Dynamics

calculationLθ

extτ

Lτdynaτ

+-

Link Gear Motor

K S

θM

θL

θS

Link Gear Motor

K S

θM

θL

θS Ic

L

mK T

1Jm s + Dm

-

+

1R g

K 1s +

1J

Ls + D

L-

-τL 1s

md

θS

θ

θ

S

+

1s

1R g

τ

State and DisturbanceObserver

ωm

Sθ

Lω

ωL

Gear & motor model

Fig. 1. Model of reaction force observer.

978-1-4244-6669-6/10/$26.00 ©2010 IEEE 36

The 11th IEEE International Workshop onAdvanced Motion ControlMarch 21-24, 2010, Nagaoka, Japan



+-

+-

-

State &Disturbance

observer +

-

Force Controller

+-

+-

Manipulator Dynamicscalculation

+

RgK T1

ref τref ω

PI Speed Controller

extτmω

1

1s

Gear Model

θM

θL

+

-

+-

K S

R g

1R

g

1s

1JLs +DL

1JMs +DM

K T+ - 1

ss

K fs2

3fsK

11 fsdfsp sK K +

dynaτ

1K

2K

3K

+

++

cmdI

s

1

Lw

sθˆ

+

Lτ

Lτ

θL

θ0

K en

D en

MenSS++++

Unknown Environment

WPfb

Wint

WFfb

Fig. 2. Block diagram of I-PD force control based on reaction force observer and manipulator dynamics calculation.

τ dyna = H (θL )ωL + D L ωL + b (θL , ωL ) + c (θL )+ f (θL ) (1)

The robot manipulator dynamics τ dyna is shown in (1)[6].where H (θL )ωL is the inertia force, D L ωL is the viscousfriction force, b(θL , ωL ) are the centrifugal force and thecoriolis force, c(θL ) is the gravity force and f (θL ) is thecoulomb friction force. The control block diagram is shown

in Fig.2, which has the two-inertia mechanical model of theexible joint between motor and link. θ0 is the position whenthe external force is inserted to the robot manipulator. Theexible joint is dened as the two-inertia model illustrated bya linear torsion spring between motor and link. Hence, it isdifcult to mount force sensor at load side. In this paper, loadposition θL is estimated by state observer, as shown in Fig.2.

B. Estimation of Reaction Force

In this paper, the reaction force is estimated by the stateand disturbance observer and the manipulator dynamics cal-culation. The reaction force observer is shown in Fig.1.The disturbance observer estimates the disturbance torque τ Lwhich contains the dynamics torque τ

dynawithout the nominal

driving torque τ dyna − n and the external torque τ ext of robotmanipulator, as shown in (2).

Moreover, the nominal driving torque τ dyna − n contains thenominal inertia J Ln and the coefcient of kinetic friction D L ,as shown in (3). Therefore, the external torque τ ext of robotmanipulator is estimated in (5) by using the state and distur-bance observer and the manipulator dynamics calculation.

τ L = τ dyna − τ dyna − n + τ ext (2)

τ dyna − n = J Ln θL + D L θL (3)

τ L = ( H (θL ) − J Ln )ωL + b(θL , ωL ) + c(θL ) (4)

+ f (θL ) + τ ext

τ ext = τ L − (H (θL ) − J Ln )ωL − b(θL , ωL )− c(θL ) − f (θL ) (5)

III . F ORCE CONTROL SYSTEM

A. Force controller The sensor-less force controller based on the reaction force

observer is shown in Fig.2. In Fig.3, since the environmentvelocity θen and the environment acceleration θen is treatedas zero, the transfer function of force controller becomes (6) oncondition that the bandwidth of PI speed controller is sufcientwider than that of force controller. K en is the stiffness of anunknown environment. From (6), the natural angular frequencyωn is obtained in (7).

F reac

F ref =

K f s 2 K en

(1 + K f sD 1 )s 2 + ( K fs 3 K en + K f sP 1 )s + K en K f s 2(6)

ωn = K en K fs 2

1 + K f sD 1(7)

ζ =K fs 3 K en + K fsP 1

2ωn(8)

In (7), the response of force controller is faster by increasingthe integral gain K fs 2 . The response speed and the bandwidthof force controller depend on K fs 2 and K en , on condition thatthe inequality K en K f s 2 >> K fsD 1 is satised , which is thegeneral condition. In ordinary cases, the condition is satised

37



because K en is so larger than K f sD 1 . Therefore, K f s 2 is theimportant constant to be considered when the force control isdesigned.

From (6), the damping factor ζ is also induced in (8).(8) indicate that the stability of force control system can bedetermined by the force feedback gain K fs 3 . On the otherhand, when the force feedback gain K f s 3 is too large, therobot manipulator often rebounds to the large opposite sidefrom the collision direction.

In order to conrm the validity of I-PD force controller, thispaper carries out the experiment by using the tested three jointrobot manipulator as shown in Fig.3. The parameters of testedI-PD force control is shown in Table I, and the parameters andthe conditions of tested touch and force control are shown inTable II.

In this experimental condition, as the end-effecter of testedrobot manipulator is close to the target environment as shownin Fig.3, it is able to carry out the smooth touch and forcecontrol. Fig.4 points out that the tested I-PD force controlleris the stable response to the concrete block. Hence, Fig.4 also

points out that I-PD force controller is suited to the forcecontrol to the high stiffness environments such as the concreteblock.

TABLE IPARAMETERS OF EXPERIMENTAL TESTED I-PD FORCE CONTROL .

Description Symbol ParameterPosition feedback gain(P) K f sP 1 0.5Position feedback gain(D) K f sD 1 0.005

Force integration gain K fs 2 0.08Force feedback gain K fs 3 0.05

Torque reference[Nm] F ref -20

TABLE IIPARAMETERS OF TESTED TOUCH AND FORCE CONTROL .

Description Symbol ParameterPosition feedback gain(P) K fsP 1 0.5Position feedback gain(D) K f sD 1 0.005

Force integration gain K fs 2 0.08Force feedback gain K fs 3 0.05

Torque reference[Nm] F ref -20Threshold for switch[Nm] F th -15

Contact speed of arm[rad/s] ω L -0.25

B. Hybrid control system

Many applications for industrial robots need to have boththe exact position control and the stable force control. Theproposed hybrid control system is constructed as shown inFig.5 which is composed of I-PD force controller and D-PDposition controller. D-PD position controller is constructed asshown in Fig.6, which is one of high performance positioncontrol method for industrial robots [1][2]. The control modeof proposed hybrid control system is selected by the thresholdof external torque τ ext which is the estimated value of thereaction force observer as shown in Fig.1.

In order to conrm the validity of the proposed hybridcontrol system, this paper carries out the experiments of touchand force control based on the proposed hybrid control systemby using the tested three-joint robot manipulator.

3-DOF industrial robot Environment

(concrete or styrofoam block)

reacFreac

τ

Fig. 3. Overview of experimental method.

0 1 2 3 4 5 6 7 8 9 10-0.55

-0.545

-0.54

-0.535

-0.53

0 1 2 3 4 5 6 7 8 9 10-0.04

-0.02

0

0.02

0.04

0 1 2 3 4 5 6 7 8 9 10-40

-30

-20

-10

0

10

Time[sec]

A r m

p o s i t i o n

[ r a d ]

A r m

s p e e d [ r a d / s ]

R e a c t i o n

t o r q u e [ N m

]

(a)Arm position

(b)Arm velocity

(c)External torque

Fig. 4. Experimental result of I-PD force controller.

ref θ

Two Inertia

Resonant

Gear model

PI SpeedController&

State feedback

State &Disturbanceobserver

I-PD ForceController

D-PD PositionController


ref τ

ref I

mω

Lθ^

LωSθdynaτ

dynaτ^+-extτ

mω

ref ω

Fig. 5. Block diagram of the proposed hybrid controller.

State &Disturbance

observer +

-

+-


+

R gK T1

PI Speed Controller mω

1

1s

Gear Model

θm

θL

+

-

+

-K S

R g

1

R g

1s

1JLs +DL

1JMs +DM

K T+ -

1s

dynaτ

1K

2K

3K

+

++

cmdI

s

1

Lw

sθˆ

-

Lτ

Lτ

θL

Mref θ Kpp++

-

Kpd

Fpd M re f ω

D-PD position controller

extτ^

Fig. 6. D-PD position control.

38



0 1 2 3 4 5 6 7 8 9 10-0.8

-0.6

-0.4

-0.2

0

0 1 2 3 4 5 6 7 8 9 10-0.3

-0.2

-0.1

0

0 1 2 3 4 5 6 7 8 9 10-80

-60

-40

-20

0

20 Force reference

Force response

L o a d p o s i t i o n [ r a d ]

L o a

d s p e e d [ r a d / s ]

R e a c t i o n

t o r q u e [ N m

]

Time[sec]Impulsive force

(a)Load position

(b)Load velocity

(c)Reaction torque

Position control Force control

Fig. 7. Experiment results of touch and force control of proposed hybrid con-trol system for concrete block. : (a)load position (b)load velocity (c)externalTorque

3 3.5 4 4.5 5 5.5 60.6

0.59

0.58

0.57

0.56

0.55

0.54

0.53

0.52

0.51

Time[sec]

A r m

p o s i t i o n

[ r a d ]

Fig. 8. Enlarged picture of load position response in Fig.7.

The experimental results from Fig.7 to Fig.10 point outthat the proposed hybrid control system realizes the stabletouch and force control for some unknown environmentssuch as concrete block, foamed styrol block and cardboardbox without force sensor. However, the response of forcecontrol is slow. In Fig.7, after the tested robot manipulatorcontacts to the concrete block, the force response of forcecontrol is zero for about 3[sec]. Its force response is stabilizedat 7[sec]. Moreover, the tested robot manipulator moves tothe opposite side from its touch direction and has the largeimpulsive torque such as the maximum value 70[Nm ] to thetarget environments. Similarly, in Fig.9 and Fig.10, the forceresponses of proposed hybrid control system have the slowresponse and the large impulsive torque for foamed styrolblock and cardboard box after the touch process.

0 1 2 3 4 5 6 7 8 9 10-0.6

-0.4

-0.2

0

0.2

0 1 2 3 4 5 6 7 8 9 10-0.3

-0.2

-0.1

0

0.1

0 1 2 3 4 5 6 7 8 9 10-80

-60

-40

-20

0

20

Time[sec]


L o a d s p e e d [ r a d / s ]

R e a c t i o n

t o r q u e [ N m

]

Force reference

Force response

Impulsive force

(a)Load position

(b)Load velocity

(c)Reaction torque


Fig. 9. Experiment results of touch and force control of proposed hybridcontrol system for foamed styrol block. : (a)load position (b)load velocity(c)external torque

0 1 2 3 4 5 6 7 8 9 10-0.4

-0.3

-0.2

-0.1

0

0 1 2 3 4 5 6 7 8 9 10-0.3

-0.2

-0.1

0

0.1

0 1 2 3 4 5 6 7 8 9 10

-40

-20

0

Time[sec]



R e a c t i o n

t o r q u e [ N m

]

Force reference

Force response

Impulsive force

(a)Load position

(b)Load velocity

(c)Reaction torque


Fig. 10. Experiment results of touch and force control of proposedhybrid control system for cardboard box. : (a)load position (b)load velocity(c)external torque

IV. S MOOTH TOUCH AND F ORCE C ONTROL BASED ON

PROPOSED H YBRID C ONTROL

A. Initial value compensation method of integrator

In order to suppress the large impulsive force and the delaytime of force control, this paper proposes a new compensationmethod of touch and force control based on the proposedhybrid control system.

At rst, this paper proposes a new initial value compen-sation method of integrator of I-PD force controller afterthe touch process. When the robot manipulator has the largeimpulsive force after the touch process, the integrator storesthe large error between the force reference and the forcefeedback signal. As the result, the impulsive velocity referenceto the opposite side is generated in I-PD force controller.Moreover, when the integrator of I-PD force controller stores

39



+

-+-

-

+-

0θ

ref τ

s

K fs2

3fsK

11 fsdfsp sK K +

extˆmθ

ref wintW

FfbW

PfbW

τ

Fig. 11. Initial values of I-PD force controller.

3.3 3.35 3.4 3.45 3.5 3.55−0.4

−0.2

0

0.2

0.4

0.6

0.8

1

V e l o c i t y r e f e r e n c e [ r a d / s ]

Brake


3.3 3.35 3.4 3.45 3.5 3.55−35

−30

−25

−20

−15

−10

−5

0

R e a c t i o n

t o r q u e [ N m

]Threshold(-15[Nm])

Point of switching to force control,and initializing integrator.

(a)Reaciton torque

(b)Velocity reference

Fig. 12. Imege of brake input for smooth touch process.

the large error, the force response of force control has thedelay time.

In order to overcome these problems, when the output W int

of integrator is set to zero at the moment of its touch process,

the velocity reference ωref is shown in (9).W F f b = K f s 3 × F cmax

W P f b = K f sP 1 × (θc − θ0 )ωref = − (W F f b + W P f b ) (9)

where, F cmax is the maximum value of impact force whichis also the estimated value of the reaction force observeras shown in Fig.1. θc is the position at the moment of itstouch process. Therefore, the initial value of integrator of I-PDforce controller is shown in (10). The proposed initial valuecompensation method obtains the output W int of integrator byusing (10).

ωref = W int − (W F f b + W P fb ) = 0W int = W F fb + W P fb (10)

B. Brake input for smooth touch process

Next, this paper proposes a new additional input referencefor smooth touch process to the output of I-PD force con-troller after the touch process. The proposed additional inputreference calls ”Brake Input”. When the manipulator touchesto an environment, the proposed method inserts the additional

0 1 2 3 4 5 6 7 8 9 10-0.6

-0.4

-0.2

0

0.2

0 1 2 3 4 5 6 7 8 9 10-0.3

-0.2

-0.1

0

0.1

0 1 2 3 4 5 6 7 8 9 10-80

-60

-40

-20

0

20

Time[sec]


L o a d s p e e d [ r a d

/ s ]

R e a c t i o n

t o r q u e [ N m

]

Force response

Force reference

(a)Load position

(b)Load velocity

(c)Reaction torque


Fig. 13. Experiment results of proposed smooth touch and force control forconcrete block. : (a)load position (b)load velocity (c)external torque

2.5 3 3.5 4 4.5 5 5.5 6

-0.56

-0.55

-0.54

-0.53

-0.52

-0.51

-0.5

-0.49

-0.48

-0.47

3 3.1 3.2 3.3 3.4 3.5 3.6 3.7

-0.55

-0.548

-0.546

-0.544

-0.542

-0.54

-0.538

A r m

p o s i t i o n

[ r a d ]

Time[sec]

Time[sec]

A r m

p o s i t i o n [ r a d ]

Fig. 14. Enlarged picture of load position response in Fig.13.

velocity reference ”Brake Input” to the output of I-PD forcecontroller in order to reduce an impulsive force as shownFig.12. An impulsive force is reduced by ”Brake Input” whichis the opposite velocity reference − ωF B . ωF B is the actualvelocity feedback signal.

C. Experimental results

In this paper, the validity of the proposed method is

conrmed by the experimental results using the tested 3-DOF manipulator. The experimental results are shown in fromFig.13 to Fig.16.

The experimental results of concrete block are shown inFig.13. These results point out that the robot manipulatorkeeps the smooth contact to the concrete block after the touchprocess. The force response of Fig.13 is faster than that of inFig.7.

The experimental results of foamed styrol block is shownin Fig.15, and the experimental results of cardboard box is

40



0 1 2 3 4 5 6 7 8 9 10-0.4

-0.3

-0.2

-0.1

0

0 1 2 3 4 5 6 7 8 9 10-0.3

-0.2

-0.1

0

0.1

0 1 2 3 4 5 6 7 8 9 10-60

-40

-20

0

20

Time[sec]


L o a d s p e e d [ r a d

/ s ]

R e a c t i o n

t o r q u e [ N m

]

Force reference

Force response

(a)Load position

(b)Load velocity

(c)Reaction torque


Fig. 15. Experiment results of proposed smooth touch and force control forstyroform block. :(a)load position (b)load velocity (c)external torque

0 1 2 3 4 5 6 7 8 9 10-0.6

-0.4

-0.2

0

0.2

0 1 2 3 4 5 6 7 8 9 10-0.3

-0.2

-0.1

0

0.1

0 1 2 3 4 5 6 7 8 9 10-40

-30

-20

-10

0

10

Time[sec]

L o a d p o s i t i o n

[ r a d ]


R e a c t i o n

t o r q u e [ N m

]

Force reference

Force response

(a)Load position

(b)Load velocity

(c)Reaction torque


Fig. 16. Experiment results of proposed smooth touch and force control forcardboard box. : (a)load position (b)load velocity (c)external torque

shown in Fig.16. These results point out that the manipulatoris keeping the smooth touch process and ne force controlquickly. Moreover, the impulsive forces at the touch processin Fig.15 and Fig.16 are lower than the result as shown in Fig.9and Fig.10. Especially, the force response result of cardboardbox is smooth and fast as shown in Fig.17. Therefore, theseexperimental results conrm that the proposed method realizes

the ne force control to unknown environment.

V. C ONCLUSION

This paper proposed a new smooth touch and force controlto unknown environment without force sensor for industrialrobot, which is based on the hybrid control system of D-PDposition control and I-PD force control by using the reactionforce observer. In the proposed method, the transient processfrom D-PD position control to I-PD force control is smoothlycarried out by using the proposed initial value compensation

Fig. 17. View of the exprimental result to cardboard box.

method of the integrator of I-PD force control.

The proposed method realizes the smooth touch and forcecontrol for the several target environments such as concreteblock, foamed styrol block and cardboard box, because it neednot to have any information of environments. The validityof proposed method is conrmed by the experimental resultsusing the tested 3-DOF manipulator. In these experimentalresults, the force response is obviously improved by theproposed method.

R EFERENCES

[1] T. Miyazaki, S. Otaki, S. Tungpataratanawong, and K. Ohishi, ”HighSpeed Motion Control Method of Industrial Robot Based on DynamicTorque Compensation and Two-Degrees-of-Freedom Control System”,IEEJ Trans. Ind. Applicat.,vol. 123-D, no. 5, 2003, pp. 525-532.

[2] S. Tungpataratanawong, K. Ohishi, and T. Miyazaki, ”Robust MotionControl of Industrial Robot Manipulator Based on Feedforward TorqueCompensation and D-PD Control”, in Proc. 6th Japan-France and 4thAsia-Europe Mechatronics Congress, Japan, 2003, pp. 432-437.

[3] A. Bicchi and G. Tonietti, ”Fast and Soft-arm Tactics”, IEEE. Robot.Automat. Mag., Vol.11, 2004, pp. 22-33.

[4] M. T. Mason, ”Compliance and Force Control for Computer ControlledManipulators”, IEEE Trans. on Syst., Man, and Cybern., vol. SMC-11,1981, pp. 418-432.

[5] K. Ohishi, M. Miyazaki, and M. Fujita, ”Hybrid Position and Force Con-trol without Force Sensor”, RSJ journal, vol.11(3),1993,pp.166-174.(inJapanese)

[6] M. C. Good, L. M. Sweet, and K. L. Strobel, ” Dynamic Models forControl System Design of Integrated Robot and Drive Systems”, ASMEJ. Dynam.Syst., Meas., Contr., Vol. 107, 1985, pp. 53-59.

[7] L. Sciavicco and B. Siciliano, ”Modeling and Control of Robot Manipu-lators”, McGraw-Hill, New York: 1996.

[8] S. Tungpataratanawong, K. Ohishi, and T. Miyazaki, ”Force sensor-lessimpedance control for elastic joint of industrial robot manipulator”, Proc.2005 International Power Electronics Conference, IPEC2005, Japan, No.S39-3, 2005, pp. 1-6.

[9] S. Manabe, ”Coefcient Diagram Method”, 14th IFAC Symp.Automat.Contr. in Aerosp., Korea,1998, pp. 322-327.

[10] N. Shimada, T. Miyazaki, K. Ohishi, ”Force control without force sensorby I-PD force controller using disturbance observer”, Proc. 27th Annualconference of the robotics society of Japan, Japan, No. 2M2-03, 2009.

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CHR0075 Synopsis