Investigations on Lightweight Industrial Robots

Copyright © lFAC Robot Control, Vienna, Austria, 1991

MISCELLANEOUS

INVESTIGATIONS ON LIGHTWEIGHT INDUSTRIAL ROBOTS I.D. <;aushi Mechanical Constructions, University o/Tirana, Albania and Institute/or Handling Devices and Robotics, Technical University o/Vienna, Austria

Abstract. In order to achieve higher productivity the industrial robots must move faster and accurately, what leads to the necessity of their weight reduction. This paper documents simulation investigations by reducing the inertia parameters of an existing industrial robot, IGM Limat RT-280. The compute model of the energo-mechanical system includes all six moving links, the payload with all its inertia parameters, inertia and gyroscopic effects of the driving rotors, electromagnetic control effects of the driving units, which will all be important also for future design of appropriate controllers for lightweight robots. Simulations are performed on a PC using a software for dynamic analysis of multibody systems (AUTOLEV) . Analyzing kinetic and potential energy of the links, it is shown which inertia parameters have to be reduced in order to minimize energy consumption, as a representative index of lightweight design . Keywords. Industrial robots; modeling; computer simulation; optimization; lightweight industrial robots .

deformations within the adjustable range of an adaptive control. The joint arm was designed as a mixed form of construction of integrated framework (with pressure and draw bars) and plate connections. Weight reduction of robots can also be accomplished by using more powerful motors or realizing the idea of coupled drive (Hirose and Sato, 1989). By means of lightweight materials, such as carbon fibre composites, it is also possible to reduce robot weight enormously k e eping stiffness relatively high.

INTRODUCTION The demand for higher industrial productivity requires continuously further optimizations of conventional robots . In order to achieve higher working velocities, lower weight-to-payload ratio, less energy consumption etc. for the industrial robots, there are being carried out several multidisciplinous scientific works aiming at designing lightweight robot . These researches are being concentrated on the three following directions : (1) mechanical construction and component improvements, (2) modeling and simulations, and (3) appropriate control design.

Recently (Maschinenwelt Elektrotechnik, 1991) a novel industrial robot with a complete framework construction (instead of bending links) has been developed, which achieves a weight-to-payload ratio of 1. 25/1.

The first step in perfecting the mechanical system of industrial robots should consist in improving their technical level. Efforts in order to obtain high positioning accuracy and repeatability has brought about massive structures of industrial robots having a very low degree of material utilization and which cannot move at the required speeds. Through a systematic optimization of a robot arm structure, Feyerabend (1988) achieves practically a weight reduction by 58%±5% of the supporting structure of one bending link, using the same material and having dynamic

Analytical dynamic models of light robots have been developed from systems of rigid links with ' flexible' joints (e.g., Thomas, Yuan-Chou and Tesar, 1985) to systems of rigid bodies with one flexible link (Hennessey, Priebe, Huang and Grommes, 1987) or with two flexible links (Tsujisawa and Book, 1989). However, they represent experimental laboratory prototypes and do not include all six degrees

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rotors, whose whole effects can be taken into accoun t th e reby. (Desoyer , Kopacek and Troch, 1985 ; Hittmair, 1 988 ). As to dynamic fricti on effec t s, the following expression r e lat ed to th e friction moments is us e d :

of freedom of an indu stria l robot. Thr oug h simulation of dynamics based on an appropriate robot model, in vest igation s upon influenc e and optim iz at i on of mechanical system parameters and/or design of a satisfactory contro ll er are carried out.

MFI 1

Considerable progress has been made in the fields of robot control design by developing adaptive control systems capable of compensating vibrations of flexible manipulators, such as payload-adaptive control (Menq and Chen, 19 88) o r feedforward plus feedback c ontro l (Oosting and Dickerson, 19 88 ) .

(Ai+Bil
(1)

reduced fri ction moment of the ith joint, motor and gear unit

4>;

angular velocity o f th e i th link relating to the (i_1)th l ink

The v alues of the coefficients A;, B;, C; and D; (i=l to 6) have been evaluated thr oug h measurements (Zhao, 1988) . The dri v ing electric DC mo tor s are modeled each as a first order lag unit with subsequent integrator (Desoyer, Kopacek and Troch, 1985). Th e y are included into the model through the foll o wing motor equations:

This pap e r documents simulation investigations on an existing industrial robot, IGM Limat RT-28 0. Th e mode l of the energo -mechanical system i nc lud es a ll s ix degrees of freedom of th e jOint links, the whole effects produced by the driving rotors, mechanical friction, payload along with its inertia moments, electromagne tic co ntrol effects of the driving units , which all would b e important also for fut ure design of appro pria t e controllers f o r lightweight r obots. Simulations are performed on a standa rd PC using a commercially available so ftw are. Ana lyzing kin e tic and potential energy of the links it is shown which inert ia parameters have to be reduced in order to minimize energy consumption, as a repr esen t at ive ind ex of lightweight design.

(2, a)

U. K, U, T,

armature vo lt age amplifying factor input voltage mechanical time constant

armature current arma ture resistance armature inductance t orq ue sensitivity rotor speed

MODEL I NG As specimen for the simulation investigations an exi s ting industrial robot (IGM Limat RT-28 0) with six revolving rigid links is used, which is destined f or welding. Modeling and symbolic generation of the dynamic equations are made with the software packag e for dynamic analysis of multibody systems AU TOL EV (Schaech t e r, Levinson and Kane, 1 988) based on th e Kane's meth od (Kane and Levinson , 19 83 ).

1, ... ,6

(2, c)

motor torqu e The six nominal voltage va lues of the DC motors are used as input quantities for th e mode l.

In most studies of robot dynamics , a reduced order model from the point of view o f the basic degrees of freedom is used. Since f o r the dynamics of lightweigh t robots, which have to move fast and ac cu rately, each elemen t could have a considerable influence, thi s study aims at creating a more complete model. This modeling implicates six local coordinate fram es for the links and an additional coordlnate system for the payload, in orde r to associate the inerti a moments t o the refl ec tion of the payload mass. Six othe r coordinate fra mes are related to the driving

ANALYZING SIMULATI ON RESULTS

Influence of the p a yl oa d Each program d ea ling with dynamics of robots would hav e to consider inertia parameters of the payload. In high-speed lightweight robotic systems, the influence of the payload on the dynamics of the system can be compensated through payloadadaptive control (Menq and Chen, 1988) . In o rd er to evaluate the influence of the

538

payload on the dynamics of the Limat RTrobot, it is fixed on the gripper a cylindrical aluminum body with 7.5 kg, the maximal value allowed. The influence of the payload on the motion of the end-point of the robot is to be seen in Fig.1, which shows the three projection planes for that motion. Inertia moments of payloads in the form of concentrated bodies, such as full cylindrical ones, have little influence on the motion of the end-point (see Fig.1, the corresponding lines lie nearly one upon another). 1.5

IAOTION OF THE END-POINT

N

0.5

2: w

IAOTION OF THE ENO-POINT

0.5

I

0

)-

~ I"~

,-

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0.5

0 Y

0.5

The influence of the rotors and the payload of a robot on the drive torque of each motor (see Fig.3) indicates how important is their including into a complete model with 6 DOF for the design of a satisfactory controller. The payload produces its greatest effect on the drive torque of the 5 th motor, on which the rotors have little influence, while the rotors produce their greatest effect on the drive torque of the 6~ motor, for which the payload effect is small.

-0.5

[m]

complete model without rotor effects with payload with payload as point mass

DRM: TORQUE: 13, T4

N

....

1.5

0.5

100

TIme t [sl Fig. 2. Motion energy of the links, rotors and payload.

0 X [m] IAOTION OF THE END-POINT

complete model without rotor effects with payload with payload as point mass

200

0 0

0.5

1.5

0.5

ENERGY: UNKS,ROTORS,LOAO

links

I

I N

1.5

300

;::

x [m] Fig. 1. Motion of the robot end-point.

o Influence of driving rotors

0.5

0.5

TIme t [s]

TIm. t [s]

complete model wi thou t rotor effects with payload wi th payload as point mass

Internal rotating driving components-like rotors or gears, have considerable effects on the dynamics of robots. In order to develop highly accurate and fast control algorithms for lightweight robots, it is necessary to include into the model all inertia and gyroscopic effects of the internal rotating parts. This can be performed relatively simply by means of a program for multibody systems. The Limat RT-robot has six rotors: one located in the foundation, two in the first link, and three others in the fourth link. They are included in the whole model through an especial instruction ('axis') in AUTOLEV and the corresponding constraint equations, relating their rotating speeds to those of the links. The influence of the rotors on the positioning accuracy of the robot for the same input motor voltages is to be seen in Fig.1. Although the rotors have small inertia parameters, their kinetic energy is about 31 per cent of that of the links (see Fig.2). This considerable portion is due to the high rotation speeds of the rotors (high gear ratios).

TIme t [s]

Fig. 3. Drive torques of the six motors. Simulation with rotors fictitiously fixed to an inertial reference frame has shown that the kinetic energy of the rotors is essentially produced by the rotation around their own axes (inertia effects) and to a lesser extent by the coupling of these rotations with those of the links (gyroscopic effects).

Analyzing energy of motion For a given motion, the robot needs energy in order to achieve a certain velocity of the links, rotors and payload (kinetic

539

energy), to change their position (potential energy) and to overcome friction (dissipation energy) . By means of the developed program for the dynamic simulation of the studied robot Limat RT the curves of the kinetic and potential energy of each link are obtained (see Fig.4).

ANGULAR ROTATIONS: ql. q2

...C'

0.5

N

ANGULAR ROTATIONS: q3. q4

1.5

C'

,.; 0.5 er

KINETlC ENERGY

POTENTIAL ENERGY

/'----····"f 40

20

o

:(

... ) ..

//,

5

:',' ..... .

o

0.5 Time t

2 '"

/

O~~----------~

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ANGULAR ROTATIONS: qS. q6

complete model -30% mass of all links -40% mass of 2.,3.,4.link

Co W

/i;~;~··<:~~::~c.:i-- IOO L - - - - - - - - - '

o~----------~~

o

0.5 Time t [5}

[5}

0.5

o

0.5 Time t [5}

Time t [s}

O~~----------~

o

0.5

Time t [5} Fig. 5. Angular rotations of the links.

In the developed simulations program, the electric energy consumption (see Fig.?) is calculated in the case of driving and braking through electric motors, and holding in standstill through mechanical brakes:

Time t [5}

Fig. 4. Kinetic and potential energy of the links.

Pi (t): (

VOi~O

UA (t) IA (t) i 0 j

(3 )

Voi:O

i:1,,,.,6

The largest part of the motion energy is consumed by the second, third and fourth link. This fact leads to the conclusion that the mass of these links have to be reduced to a higher extent in order to minimize the energy consumption.

i

I

A;

(t)

0;

W;

The process of the weight reduction of a convectional industrial robot can be optimized according to certain criteria, as for example minimal energy consumption. For the robot in study it is supposed that, using special construction design as integrated framework and/or lightweight materials etc., the mass of all links is reduced by 30% in the first series of investigations on weight reduction. As it turned out from the analysis, of the energy of motion, the 2nd, 3,d a nd 4th link need the most energy, in second case a mass reduction by 40% of these three links only is supposed.

(4 )

power requi remen ts of the i th motor arma ture vol tage of the i th motor armature current of the ith motor rotation speed of the i th rotor energy consumption of the i th motor

UA; (t)

Reduction of inertia parameters

1, ... ,6

1 st,2nd,3I"d MOTOR

0.5 Tin"'le t

POWER :

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4th . ~th.6th

MOTOR

300~

200

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5 ".

0

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o

Since the weight reduction influences the motion of the links (see Fig.S), in order to compare the electric energy consumption among different cases (see Fig.6 and Fig.?), this energy is divided through the distance of the robot end-point covered respectively.

j 0.5

complete model -30% mass of all links -40% mass of 2.,3.,4. link Fig. 6. Power requirements: motors 1 to 6.

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WORK: 1st.2nd,3rd MOTOR ~

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REFERENCES

300

Desoyer, K" p, Kopacek , and I, Troch (1985), Industrieroboter und Handhabungsgerate: Aufbau, Einsatz , Dynamik, Modellbildung und Regelung, R, Oldenbourg Verlag, Munchen-Wien, Feyerabend, F, (1988), Systematic optimization of a robot arm structure, The Industrial Robot, 15(4), 219-222, Hennessey, M,P " J,A, Priebe, P,C, Huang, and R,J, Grommes (1987), Design of a lightweight robotiC arm and controller, IEEE International Conference on Robotics and Automation, Vol, 3, pp, 779-785 , Hirose, S,' and M, Sato (1989) , Coupled drive of the multi-DOF robot, IEEE International Conference on Robotics and Automation, Vol , 3, pp, 1610-1616, Hittmair, R, (1988), Dynamische Modellbildung eines mehrgliedrigen Industrieroboters und Untersuchung der Einflusse verschiedener Modellparameter mittels digitaler Simulation, Dr,tech, Thesis, TU Wien, Kane, T,R" and D,A, Levinson (1983) , The use of Kane's dynamical equations in robotics, The International Journal of Robotics Research, Vol, 2, No, 3, Maschinenwelt Elektrotechnik (1991), Ein neuartiger Arbeitsrobot e r fur das Schleifen und Polieren von Metallwerkstucken mit gro0en Angriffskraften, Maschinenwelt Elektrotechnik, Heft 2, 42-43, Menq, C,H" and J,S , Chen (1988), Dynamic modeling and payload-adaptive control of a flexible manipulator , IEEE International Conference on RobotiCS and Automation, Vol, 1, pp , 488-493, Oosting, K" and S,L, Dickerson (1988), Simulation of a high-speed lightweight arm, IEEE International Conference on Robotics and Automation, Vo!. 1, pp, 494-496, Schaechter, D,B" D,A, Le v inson, and T,R, Kane (1988), Autolev user's manual, OnLine Dynamics, Inc" Sunnyvale, Thomas, M" H,C, Yuan-Chou, and D, Tesar (1985), Optimal actuator stiffness distribution for robotiC manipulators based on local dyna miC criteria, IEEE International Conference on Robotics and Automation, pp, 275-281, Tsujisawa, T" and W, J , Book (1989), A reduced order model d e ri v ation for leightweight arms with a parallel mechanism, IEEE International Conference on Robotics and Aut o mation, Vol , 3, pp , 728-735 , Zhao, G, (1989), Regelung eines Industrieroboters unter Berucksichtigung der Reibung s einflusse , Dr,tech , Thesis, TU Wien,

,.,

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0,5 TIme t [,]

TIme t [,] l000r-____T~O~~~L~W~OR~K~___,

c 0 m pIe t e model -30% mass of all links -40 % mass of 2,,3,,4,link

TIme t [,]

Fig, 7, Work of motors 1 to 6,

Through the weight reduction by 30% of all links of the welding robot Limat RT-280, 22,64% of its moving energy consumption can be saved, Reducing the 2 nd , 3,d and 4th link by 40%, makes possible a saving of energy by 22,08%, In general, the mass reduction of the relatively massive links with the largest part of the motion energy should at first be preferable, i,e , design according to the anthropomorphic construction principle,

FINAL OBSERVATIONS AND CONCLUSIONS Industrial robots have to be mechanically improved through weight reduction and optimized mass distribution according to criteria of minimal energy consumption in order to achieve an anthropomorphic similarity , This article shows which effect different cases of weight reduction of the links for a welding industrial robot would produce on the energy consumption , The selection of the links whose weight is to be reduced more than the one of the others is oriented towards analysiS of the energy of motion (kinetic and potential) for each link, This analysis is done through dynamic simulations on a developed compute model, which includes all six links with their degrees of freedom, friction effects, payload along with its inertia moments, inertia and gyroscopic effects of rotors and electromagnetic control effects of the driving units, The investigation simulation results are documented grapnlcally, They show that it is important for the design of controllers for lightweight industrial robots to include all the above mentioned effects into the model of the robot system,

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