CAE-Methods For Design of a Fast Digital Controlled Hydraulic Test Robot Manipulator

Copyright © IFAC Computer Aided Design in Control Systems. Swansea. UK. 1991

CAD/CAE-METHODS FOR DESIGN OF A FAST DIGITAL CONTROLLED HYDRAULIC TEST ROBOT MANIPULA TOR F. Conrad, L. F. Nielsen, P. H. SI:frensen, E. Trostmann, S. Trostmann and J. Zhou Control Engineering Institute. Technical University of Denmark. DK-2800 Lyngby. Denmark

Keywords: CADI CAE of hydraulic robots. robot contol, hydraulic robot, servoactuators, signalprocessors, digital control , adaptive control. Abstract The paper describes a very fast digitally controlled hydraulic test robot manipulator which has been developed and designed by the use of CAD/CAE-methods. The robot is implemented in the hydraulic laboratory at the Control Engineering Institute, the Technical University of Denmark. The purpose of the test robot manipulator is to carry out research activities within CAD/CAE in hydraulic control system design, digital control and adaptive control design and implementation. Furthermore the test robot is used for test and evaluation of off-line and on-line system identification methods and digital algorithms for modelling and adaptive control of multi variable hydraulic systems such as hydraulic robots, escavators og multi-axes test equipment. The paper describes and discusses the control and mechanical design problem and CAD/CAE-methods. The special software package KISMET and ROBCAD can be used in the design and 3D-animation of the robot manipulator for investigation of the robot and the workspace. The control design of the digital control system is done with the packages MATLAB and ACSL. The digital control computer is implemented with a fast AT&T-signalprocessor. A developed adaptive geometrical compensation control scheme is proposed for control of hydraulic robot manipulators.

INTRODUCTION

Other important research actIVIty areas are implementation aspects and evaluation of multivariable digital controllers for hydraulic systems such as fast robots, cranes, earth moving machines, excavators. multiaxes test equipment.

Robotics and fluid power control are important basic research and industrial areas within the general area of flexible automation and intelligent machine tools. In view of the need for faster robots, including mobile robots and hydraulic machines such as heavy load manipulators. cranes, earth-moving machines and vibration test facilities, a hydraulic test robot with two links digitally controlled by two linear hydraulic servo actuators has been developed and installed in the hydraulics laboratory of the Control Engineering Institute at the Technical University of Denmark. The digital control system is implemented on an AT&T signal processor.

The above research activities are carried out within the project MULTIDOS (MULTIvariable Digital Oilhydaulic Servo Systems) supported by the Danish Technical Research Council (STVF). initiated in 1985 by Conrad and Trostmann.

THE DESIGNED FAST HYDRAULIC TEST ROBOT MANIPULATOR

The use of digital controllers for the control of hydraulic systems is proliferating more and more to areas where the microprocessor is not normally present. It is for instance becoming increasingly interesting to use microprocessor control in earth moving machines. cranes, hydraulically powered lorries as well as hydraulic robots. The general problem in all these machines is the closed loop servo control of a multivariable system using digital control.

The multiam delign Qroblem

The robot design problem in MULTIDOS has been to design a very fast test robot manipulator with high performance linear hydraulic servo actuators and components with optimal properties in order to eliminate the unwanted effects of friction (stick-slip), backlash and dead bands. By using linear actuators the robot arms can be directly controlled without gears which are necessary in most robot manipulator systems involving rotational motors.

The purpose of the robot is to use it in research activities within CAD /CAE in multiaxes hydraulic control system design, hydraulic robot manipulator design, control system design, digital control, system identification, adaptive control and multivariable control.

For economical reasons. it was decided as a first step to design a two-link test robot system with rigid arms digitally controlled by

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TECIINOVISIO!'l, CATIA KI S ~{ET, GRASP, ROBCAD

Animation and simulation

STRUDL

ProMatl a b (d esign)

Simulation and dynamic optimization

ACSL (Simulation)

Figure 1 A block digram for CAD/CAE design

two high performance linear hydraulic servo actuators. The test robot has been prepared for a later extension with additional axes. The main requirement for the design was that the test robot should be able to move a payload of maximum 50 kg path controlled as fast as possible in the specified work space within the limitation of the existing hydraulic power supply which is 841/min at 210 bars, The hydraulic actuators are equipped with hydrostatic bearings to eliminate friction (stick-slip) and with two-stage high frequency response servo valves. The digital controller should be designed and implemented with a very fast processor (signal processor) in order to obtain minimum computational time , A target is to study and identify the physical limitations of implementing a hydraulic digitally controlled robot system with respect to the mechanical subsystem, the hydraulic actuators, the digital controller circuits and the control algorithms itself.

end effector may be derived from the kinematic model. The geometric model express information of the physical dimensions of all the parts in the robot system, In CAD terms the geometric model is a series of solid models of all the parts in the robot system, The geometric model is useful for computer simulation

because together with the kinematic model it defines the information necessary to visualize the present spatial state of the robot and obtain an animation of the robot arm movements. The geometric model is an integrated part of the kinematic model.

KISMET GRASP ROBCAD

Inverse Kinematic Transformation

The applied design approach and CAQ,ICAE-methodJ In the design and implementation of the above described digitally controlled hydraulic test robot manipulator a system engineering approach has been followed as illustrated in Fig, I. Suitable CAD /CAE-methods have been applied for mechanical design, the actuator design and the controller design,

Dynamic and

Dynami c Model

STRUDL

Structural analyse

KISMET

Direct Kinematic

GRASP ROBCAD

Transformation

Kinematic, Geometric and structural design, In order to apply 3D-CAD/CAE methods in the design of the robot manipulator four models of the robot are needed (Trostmann et al 1990).

~melriC MO~

The kinematic model carries information of the connection and conceivable movements of the different links of the robot. The kinematic model of a robot arm system rule the spatial displace-

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ment of the robot links and thereby the position and orientation of the end effector. The kinematic model must include information such that the spatial displacement of the robot can be defined. This means that explicit mathematical equations (direct - and inverse

3 D CAO Systems

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3D Animation

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TECH:\O\' ISlO!'l CAnA

Figure 2 Models for CAD/CAE design

kinematic transformations) describing the coherence between the joint evaluated variables and the position and orientation of the

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KISMET GRASP (ROBCAD)

The dynamic model are defining a set of mathematical equations describing the dynamic behaviour of the robot arm. Dynamic behaviour in this context is defined as the relationship between position, velocity, acceleration and associated force and torque. The dynamic model is also an integrated part of the kinematic model. The structural model carries necessary information for computing elasticity, deformation, structural egenfrequencies etc. The structural model is also an integrated part of the kinematic model. A general configuration of algorithms and their interface to the above described models, is outlined in Fig. 2. Such a software system facilitates " Analysing and designing the robot geometry and the work space. " Computing the direct kinematics: Position of the robot joints in Cartesian coordinates from robot coordinates. "Computing the inverse kinematics: Position of the robot joints in robot coordinates from cartesian coordinates.

Figure 3 The implemented two-link test robot

" Computing velocities and accelerations of robot joints. more, the dynamic behaviour of the designed controUer, the designed actuators and the designed robot manipulator can be investigated by simulation using the simulation package ACSL

" Computation of the static behaviour: Forces and torques reacting on the robot links generated from gravity and load conditions.

(Advanced Computer Simulation Language).

" Computation of the dynamic behaviour : Forces and torques reacting on the robot links generated from

Design concem and mechanical ftructure In the MULTIDOS project (Conrad et ai, 1989) various mechanical structures for a rigid hydraulic robot controUed by two linear

dynamic load conditions. At our institute we are using several software packages for creating the different models and for facilitating a 3D simulation (animation). KISMET (Trostmann and Nielsen, 1989), ROPCAD (Nielsen 1989), and GRASP are robot off-line programming and simulation systems. Here the kinematic model of the robot manipulator is developed. All the simulation systems has some

actuators have been studied. This work has been continued. The aim has been to find promising design concepts and mechanical structures in order to solve the design problem described above.

facilities for creating geometric models, but these are limited.

The practically implemented robot manipulator

Instead a 3D CAD system, TECHNOVISION or CA TIA is used to create the geometric model. The communication of the geo-

Fig. 3 shows a picture of the implemented hydraulic robot manipulator. For economical reasons, the arms have been designed as welded steel parts build up by modifications of standard steel profiles. Particular attention has been focused on the bearing design to eliminate backlash and minimize friction

The design of the two arms should be shaped with respect to minimizing the moving masses of the arms without loss of the necessary stiffness of the arms and the base-unit fixed to the floor.

metric model from the CAD system to the off-line programming al'd simulation systems is realized through CAD"I, a neutral interface for CAD data exchange (Schlechtendahl, 1987). The 3D animation is conducted in KISMET, ROPCAD or GRASP. KISMET is the most suitable because it facilitates hidden surface removal, gouraud shading, interactive placement of viewpoint, etc. ROPCAD also includes these features but the CAD"I interface

(stick-slip). The base-unit is also built up as a rigid welded steel construction. It is bolted onto a heavy cast iron machine platform placed in the floor of the hydraulic laboratory.

to the CAD systems is not completed.

The two hydraulic servo actuators have been manufactured by the German company Mannesmann Rexroth GmbH. The hydraulic cylinders can be controUed by super high servo valves (MOOG,

Work is on-going to enlarge the robot model in the software system to include a dynamic model. The FEM software STRUDL is used to compute the structural behaviour of the robot manipulator.

Rexroth and others). The actuators are equipped with a hydraulic accumulator in the supply port and an other one in the return port to cope with the dynamic flow demands.

It is important to bear in mind that the work process is iterative and not sequential. It is very likely that the simulation (animation) reveals properties that cause a change of the relevant models.

The test robot is equipped with transducers for measuring: positions, velocities and accelerations of the moving piston shafts and the pressures in the hydraulic chambers. Further, the TCP (tool centre point) can be equipped with transducers.

Controller and actuator design. The 3D-robot manipulator model can then be used in the further robot design of the digital controUer and of the actuator design. Based on the given 3D-robot model a dynamic robot model is formulated for analysis and design using Pro-MA TLAB and its Control Tool Box. The robot dynamic model

As mentioned above, today the pressure and flow are limited by the existing hydraulic power station which can deliver a maximum flow rate of 84 I/min at a pressure up to 210 bars. Its is already

is most conveniently expressed as a state space model. Further-

planned to instaU a new more powerful supply unit in order to

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extend the available pressure and flow range. The robot manipulator system is described in detail in (Conrad et ai, 1990a).

case that the perimeter fence door is opened or if an emergency button is pressed. This is achieved by a solenoid operated overcentre valve on each of the cylinder.

DESIGN AND IMPLEMENTATION OF

In the current system the amplified transducer signals are sent to an analog-to-digital converter card (ADC/DAC) in an IBM PC-AT, where the transducer signals are converted to a 12-bit digital representation. These numbers are transferred via a fast data bus to a processor which calculates the control signal. This number is then transferred to the ADC/DAC and is supplied to the servo valve amplifiers.

THE CONTROL SYSTEM The arm is equipped with a measuring- and instrumentation system including devices for the measurement of various dynamic variables, devices for the supply of the valve control currents, and a security system for safety. There is the possibility for the measurement of piston position, which of course is essential for closed loop position control. This is achieved through LVDT position transducers integrated in the two cylinders. There is at the moment no possibility for measuring tool-centre-point position directly, neither is the system equipped with velocity transducers, but this will be implemented at a later stage. Furthermore, the cylinder chamber absolute pressures can be measured with four strain gauge based pressure transducers. Finally, the acceleration of each piston can be measured with two capacity type accelerometers. The characteristics of the transducer system is shown in table I.

In the microcomputer, the signals are processed in a Digital Signal Processor, which processes the control algorithm, and communicates with the PC via the PC-bus. The PC is used for data storage and further data processing and data display. The processor is a single chip AT&T WE DSP32 32-bit floating point signal processor with a cycle time of 160 nsec. It is specifically designed for performing floating point calculations at high speed. Its pipeline features allow for two 32 bit floating point numbers to be multiplied and added to a third in one instruction cycle (i.e. 160 nsec). This gives the processor a performance of 12.5 Mflops. The architecture of the DSP has the advantage over previous DSP's that it performs floating point operations and so the programmer does not have to worry about overflow and scaling problems.

The instrumentation system incorporates a set of servo valve amplifiers providing the current control input to the servo valves with additional current limiting functions and dither application. These are standard manufacturer supplied current amplifiers.

The DSP-chip is implemented onto a custom-made extension card which fits into the slots of a standard PC-AT.

For safety reasons the arm is surrounded by a fence connected to the security system which automatically shuts down oil flow in the

We are currently investigating a new and improved concept for the instrumentation- and control system, which from an industrial applications point of view is more reliable, versatile, flexible and electronically not prone to noise problems. In this new layout, in stead of having all digital hardware on extension cards inside the microcomputer, the analog as well as digital control hardware for

Table I. The Measuring System for the TIJD Test Robot. Variable

Transducer

Range

Linearity

Position

Messotron

0-650 mm

::to.5 %

Accelera tion

Zetra

0-150 g

1.0 %

Pressure

HBMPK4A

0-650 bar

0.5 %

each axis is placed in an separate 19" rack. In this way the hardware can be altered to accommodate any number of servo axis. Further, the cables carrying analog signals are kept to a minimum in length, thereby reducing the noise pick up to a minimum. Finally the

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L:::::===~--@]Figure 4 New and improved concept for an instrumentation and control system.

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hardware can be placed in a stand-alone cupboard designed and ruggidized for specific industrial requirements. The concept is shown in Fig. 4, which indicates that the microcomputer is only used for program up- and down-load and for data collection via a serial link of some kind. This modular concept can be extended and altered to fits a variety of different control applications requiring powerful control capabilities.

Feedforward Compensation

THE DYNAMIC MODEL OF THE ROBOT MANIPULATOR

q

Manipulator

Figure 5 An adaptive geometrical compensation scheme

The robot arm dynamics can be described by a Lagrange-Euler equation of motion D(q)i:j+H(q,q)+G(q)=T

Robot

An adaptive geometrical compensation scheme was proposed by

(Zhou et ai, 1990) for the hydraulic test robot. This control scheme consists of feedforward geometric compensation, computed from the desired robot joint velocity, and an adaptive feedback controller as seen in Fig. 5. Feedforward compensation involves the calculation of a control signal on the basis of the desired joint position, velocity and acceleration. Since a calculation of the complete dynamic equations of the system is too time consuming, a simplified compensation scheme was proposed.

( 1)

where q, q, and i:j are vectors signifying the joint positions, velocities, and accelerations, respectively, D (q) is an acceleration-related matrix, H (q, q) is a Coriolis and centrifugal force vector, G (q) is a gravitational force vector, and T is an applied torque vector generated by the actuators. The actuator equations consist of a servovalve dynamic equation, a cylinder continuity equation, a static valve equation, and a piston force equation. Since the two actuators, one for each link, are equal, it is assumed that they have the same static and dynamic characteristic.

For each link the total valve flow can be divided into two parts the displacement flow and the compressibility and leakage flow (see Fig. 6). In the case of the test robot it turn out that the dominant flow is the displacement flow, which is a function of piston velocity only. Therefore, for gross feedforward compensation the control signal may be computed from the desired velocity, neglecting pressure dependence on valve flow and piston motion. This leads to a feedforward compensation scheme which is a purely geometric relation between flow and velocity, hence the name geometric compensation:

The servovalve has a bandwidth of approximately 190 Hz. with a static flow equation of the usual non-linear square root frame. The force equations for two cylinders displays major non-linear behaviour in particular the line-of-action of the cylinder force varies considerably giving rise to a non-linear relation between cylinder force and large action on the link.

u d (/)=

AN ADAPTIVE GEOMETRICAL COMPENSATION SCHEME

[U1J [k u~

=

X1(/)J

l . ' A, ' k 2 •• A,' x~(/)

(2)

where k '. (i = 1, 2) are constant gains specified by the program and

x ~ (i = 1, 2) are the desired piston velocities. The computations

In the controller design extensive use of CAE and CADCS has been made. The controller design is largely done in the computational package MATIAB, which has established itself as a standard tool in the analysis of control systems. For further evaluation of system dynamic performance computer simulation has been undertaken using ACSL.

necessary for calculating the above control signals are three multiplications for each joint only. To secure sufficient dynamic response and for stationary error compensation an adaptive feedback controller is implemented. The controller is an independent joint controller for each link separately. The mathematical operations for computing the adaptive feedback control can be performed in about 0.5 m sec ..

Dynamic Path

Geometri c Path

Figure 6 Feedforward compensation design

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position error is less than 5% over the whole range from minimum to maximum piston position. The desired motion is a simultaneous motion of both links with piecewise constant acceleration. It is seen that the control signal in this motion saturates, but still the controller manages good dynamic performance.

and a c t ual m o ti o n o f jo i nt 2

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CONCLUSIONS

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As an important result of the MULTIDOS project a fast two-link hydraulic digitally controlled test robot manipulator with a maximum payload of 50 kg and a maximum speed of about 3.5 m/s in the tool point has been developed and implemented.

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Ti me [se c.) Co ntr ol sig nals f or li n k 2

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Our experience shows that the application of the described CAD/ CAE methods are key tools in the mechanical design and the controller design.

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The robot is used for further design and evaluation of hydraulic actuators, multivariable and adaptive control schemes and for the implementation of digital algorithms on a digital signal processor.

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REFERENCES -4 0

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Conrad, F., P. H. S0rensen, E . Trostmann,J. Zhou (1990a) . Design and Digital Control of a Fast Hydraulic Test Robot Manipulator. Proc. of the 9th BHRA International Conference on Fluid Power, Cambridge, England, 25-27 April.

Time [sec. ) Des ired and ac t ual mo ti on o f jo in t 1

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Conrad, F., P. H. S0rensen, E. Trostmann, 1. Zhou (1990b). Signal Processors for the Control ofa Multivariable Hydraulic Robot System

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Proc. of the 3th Bath International Fluid Power Workshop, Bath, England, 13-15 September.

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Nielsen, L. F. (1989). A Survey over Simulation and Off-line Pro-

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gramming Systems - ROBCAD etc., the Control Engineering Institute, Technical University of Denmark, Report No. S89.20.

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Schlechtendahl, E. G. (1987). Specifications of a CAD"I Neutral File CAD Geometry, Ver.3.2. 2nd Ed., Springer Verlag.

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Tim e [sec. )

10

Trostmann, E., F. Conrad, S., Trostmann, and L. F. Nielsen (1990).

Control sig n a ls for link 1

Implem entation of an Off-line Programming System Based on the .;

IRDATA Neutral Interface. Proc. of ISIR '90, Copenhagen, October.

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Trostmann, S. and L. F. Nielsen (1989). Introduction to KISMET the Control Engineering Institute, Technical University of Denmark, Report No. S89.85.

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Trostmann, S., E. Trostmann, L. F. Nielsen and T. G. Clausen (1990). Intelligent Sensor-based Real-time Control of Machines (in Danish). Proceeding from Seminar on Integrated Production Systems - IPS, Kal0, Denmark, 15-17 August 1990.

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Ti m e [sec. ]

1

2 3

Adaptive Con trol , Nominal Co ntrol, Actual Con trol

Zhou, J. (1989). Adaptive Control and Applications to Hydraulic Robot Manipulators._Ph .D.-thesis. Control Engineering Institute, Technical University of Denmark, Lyngby.

Actual Trajectory Desired Trajectory

Zhou, 1., F. Conrad, and P. H. S0rensen (1990). An Adaptive Geometric Compensation Control Scheme for Hydraulic Manipu Iators._Proc. OfIEEE International Workshop on INTELLIGENT

Figure 7 Simulation results

Fig. 7 shows simulation results for applying the proposed adaptive geometrical compensation scheme to the two-link test robot . The results indicate that the geometric compensation technique together with the adaptive controller gives an overall controller with good static and dynamic performance. The maximum angular

MOTION CONTROL, 20-22 August, Bogazici University, Istanbul, Turkey.

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