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Mechatronics-a concept with examples in active magnetic bearings
Mechatronics Vol. 2, No. 1, pp. 65-74, 1992 Printed in Great Britain
0957-4158/92 $5.00+0.00 © 1991 Pergamon Press plc
MECHATRONICS--A CONCEPT WITH EXAMPLES IN ACTIVE MAGNETIC BEARINGS
G.
SCHWEITZER
Institute of Robotics and Mechatronics Lab., ETH Zfirich, CH-8092 Ztirich, Switzerland
(Received 2 January 1991; accepted 20 March 1991) Abstract--What kind of knowledge is necessary in order to design a mechatronic product? What are the main issues, what are the required interconnections between the fields of mechanics, electronics, and computer science? Which means are available for research and education? A typical example of a mechatronic system will be presented: active magnetic bearings for a high-speed milling spindle, for applications in vacuum techniques, and in vibration control. Based on these examples from industry and research, a concept developed at the ETH (Eidgen6ssische Technische Hochschule) will be discussed for structuring the field of mechatronics, offering answers to the above questions.
1. WHAT IS MECHATRONICS?
Let us start with an example. In classical mechanics we ask: how does a body move, when a given force, for example the gravitational force, is acting on it? For answering that question, we write down the well known equations of motion and we solve them. This means we are dealing with a p r o b l e m of analysis. The question, however, can be formulated in an inverse way, too, i.e. what kind of force do you have to apply so that the body performs a desired motion? What, for example, does the braking force of an automobile have to be for negotiating a curve in a safe way during braking? This is a problem of synthesis, and in the case of the automobile the answer has led to the automatic ABS-system. For solving such synthesis tasks, we need, in addition to the mechanics, mainly system theory or control techniques. If we also want to realize a potential solution, we often also need electrical engineering, electronics, and computer science. The term 'mechatronics' for the combination of these fields probably came from Japan, and is quite widely used there. In fact this combination is not new at all: in aerospace and in aeronautics it has been successful for a long time. However, for some years now, this multidisciplinary field has grown rapidly, and has been developing a character of its own [1,2]. This development has been spurred by the availability of cheaper c o m p u t e r power, and it will be further supported by the availability of 'intelligent' power electronics. Therefore, we suggest a definition which emphasizes the new possibilities of this combination and its potential for 'intelligence' in a m o r e distinctive way: Mechatronics is an interdisciplinary field of engineering science, combining the 65
66
G. SCHWEITZER
classical areas of mechanical engineering, electrical engineering, and computer science. A typical mechatronic system picks up signals, processes them, and, as an output, generates forces and motions. The main issue is the extension and completion of mechanical systems by sensors and microcomputers, which leads to the characterization as 'information driven mechanical systems'. The fact that such a system detects changes in its environment by sensors, and, after suitably processing this information, reacts to them, makes it quite different from conventional machines (Fig. 1). Examples are spread over the mechanical engineering area and include robots, digitally controlled combustion
engines, machine tools with self-adaptive tools, contact-free magnetic bearings, and automated guided vehicles. Typical for such a product is the substantial amount of system's knowledge and software that goes into its design, construction and operation. It is quite justified to say that 'software is a machine element'. First, we will present a typical mechatronic example: active magnetic bearings for various applications. Finally, we will demonstrate briefly how this new field is structured at the E T H in research and in education.
2. PRINCIPLE OF THE ACTIVE MAGNETIC BEARING Magnetic bearings can support a rotor without any contact, and they can control its motion within the air gap in a well-defined way. The electromagnetic bearing forces depend on the measured state of motion of the rotor, and they are governed by a suitable control law. In addition to mechanical components, the bearing system contains electronic elements like sensors and power amplifiers, a controller realized, for example, by a microprocessor, as well as an increasing amount of software, which actually determines the 'intelligence' of the system. The increasing availability and handiness of these 'machine elements' makes the magnetic bearing more and more attractive for the solution of classical bearing problems in machine dynamics [3-5]. The functional principle of the electromagnetic suspension is based on a control loop as shown in Fig. 2. The control law can be adjusted within physical limitations to
Engineerin[~
_•MmeCh. echan.SystemI
t
Electrical I Engineering I
Sensors l Amphfiers I
Science [
L•Computer
!
Microprocessor I
Fig. 1. Mechatronic system: the system picks up signals from its environment, processes them 'intelligently', and reacts, for example, with forces or motions. Methods for connecting the various scientific areas come from the basic engineering sciences, system theory, control techniques and computer science.
Mechatronics--a concept with examples
67
the technical requirements, and it controls the dynamic stiffness of the suspension, and the damping and stability of the hovering state. A real rotor, of course, needs several of these suspension elements, which are connected by a multivariable control (Fig. 3). First, we will present some applications of the bearing principle and then the essential elements of this mechatronic product.
3. MAGNETIC BEARING APPLICATIONS The various advantages of a magnetic suspension lead to applications of the bearings in three main areas: • vacuum and super clean room techniques (no contamination by the bearings); • machine tools (precision, high load, high speed); and • turbomachinery, centrifuges, vibration isolation and excitation (control and damping of vibrations, good dynamic properties). The requirements for magnetic bearings in these areas are different, especially as to the dynamic behaviour. Many properties can be influenced by a microprocessor Magnet~
Rotor
PowerAmplifier
Control
Sensor
Fig. 2. Functional principle: a sensor is measuring the deviation of the rotor from its reference position, a microprocessor acting as a controller derives a control signal from these measurements. The control signal drives a power amplifier, generating a control current through the actuator magnet in such a way that the magnetic forces keep the rotor in a hovering state.
Fig. 3. Electromagnetically suspended rotor: only the radial suspension in one plane is shown. The axial suspension can be realized in a quite similar way.
68
G. SCHWEITZER
controller. This possibility of influencing the characteristics by software allows the bearings to be made 'intelligent' (Table 1). The next two sections will show some actual examples.
3.1. Epitaxy centrifuge For the Max-Planck-Institute of Solid State Research in Stuttgart, G e r m a n y , we have built a special centrifuge, where solid state substrates can be covered in a well-defined way with very thin multi-layers [6]. To achieve this, a crucible rotates following a predefined speed program in a container under ultra-high-vacuum conditions (Fig. 4). There are intentions to upscale this liquid phase epitaxy process for generating semiconductors of the highest quality.
3.2. High-speed milling spindle In collaboration with a Swiss company, we built a magnetically supported milling spindle, which currently is launched as a prototype. The cutting power is about 35 kW, the rotation speed is up to 40,000 rpm, and the cutting speed for aluminium is up to 6000 m sec. This high-speed milling offers advantages with respect to the milling process and production costs. An extensive description is given in [7].
3.3. Actuator for generating test forces and for damping The design objective for the actuator shown in Fig. 6 was to generate high forces within a large frequency range that can be used to control rotor vibrations. The magnet has eight poles and can generate forces independently in two mutually orthogonal directions. This actuator is used firstly to generate test forces acting on a rotor for identification purposes and then to work as an active d a m p e r or as a bearing [8]. The rotor itself is 2.3 m long, 100 m m in diameter and supported in two oil bearings. These bearings are known to cause coupling effects between the lateral motions of the shaft. The overhung shaft carries disks with a mass of 93 kg. The actuator first exerts sequential test forces on the rotating rotor. The resulting shaft deviations are measured as test signals. They are used in an algorithm to compute control forces which are subsequently applied to the rotor by the same actuator, synchronously with the rotor speed, in order to compensate for the unbalance. This procedure repeats itself while the rotor slowly runs up to its operational speed. Thus Table 1. Technical objectives which can be achieved by suitably programming the microprocessor controller of the system Contact-free and stable support of the rotor High accuracy of positioning Damping of vibrations and stabilization of self-excited vibrations Reduction of vibration forces in the bearings Reduction of resonance amplitudes of an elastic rotor Passing through critical speeds Improvement of safety properties (redundancy, diagnostics. . . . ) Integration of the bearing into the operating system of the overall process
M e c h a t r o n i c s - - a c o n c e p t with e x a m p l e s
69
uum Gohiuse
-Motor
. Radial Lager
|er Sensor ;estall nsor
Fig. 4. Epitaxy centrifuge: the rotor with the crucible is suspended within the container with no contact. The magnetic bearings are completely outside the container and the bearing forces are acting through the container wall.
IPowerArnp'i"erl=~
I
Digital JControllerF~-~
Performance:
Rotor:
Rotational Speed: 40'000 RPM Driving Power: 30 kW Cutting Speed: 6'500 m/min
Length: 486 mm Weight: 16 kg Max. Diameter: 144 mm
Fig. 5. High-speed milling spindle with digitally controlled magnetic bearings.
70
G. SCHWEITZER
Fig. 6. Electromagnetic actuator ETH-190/50/1 consisting of the magnet, the power amplifiers, the ferromagnetic laminated sleeve on the rotor, and sensors. The main specifications are given in Table 2a, the frequency response is shown in Fig. 9. the algorithm determines the control forces necessary for minimizing the synchronous rotor vibrations, without a priori knowledge of the unbalance distribution or the hydrodynamic bearing coefficients. This kind of procedure for automatically adjusting the actuator during operation is of special interest, because generally the dynamic characteristics of the rotor in oil bearings cannot be predicted from theoretical models in a satisfactory way. This adaptive control is indicating a direction even more advanced, to 'learning' magnetic bearings. The first experiments with non-linear, learning control using a neural network have already shown very promising results [9].
4. ELEMENTS OF A MAGNETIC BEARING SYSTEM
Some essential elements of that mechatronic product are described briefly below. The mechanical element, the rotor, can be modelled in simple cases as a rigid body with five degrees of freedom which have to be controlled. In general, however, the rotor has to be seen as a flexible, vibrating structure. The natural modes of the milling spindle, for example, had been derived by a finite element model and verified by experimental modal analysis. The electronic actuator is the element within the control loop which transforms an electrical input into a mechanical output, the bearing force, and therefore this actuator, in a simple way is a magnetic bearing already (Figs 7 and 8). The character-
Mechatronics--a concept with examples iy
71
~Y I
Fig. 7. Radial magnetic bearing as input/output element, the control current through the coils is the input. istics of an actuator can be derived from geometric and electrical data, or they have to be determined experimentally. The specific load capacity, with respect to the cross-sectional area of the bearing, is about 30 N cm -2. Of course, we can only m a k e use of that potential of the actuator if the magnet, as a part of the complete system, is controlled suitably. The power amplifiers have to amplify the control signals and to feed the magnetic coils. The coils essentially represent an inductive load. With their low pass characteristics they are limiting the frequency response and thus the dynamics of the whole bearing system. We distinguish between D.C.-amplifiers and switched amplifiers. The
Fig. 8. Radial bearing for a flexible shaft.
72
G. SCHWEITZER
first ones are applied for small loads below 0.5 kVA (Table 2). The high power of an amplifier would be essential for achieving good dynamic properties (Fig. 9). The actual consumption of energy, however, is low, and at high speeds it is an order of magnitude less than with conventional bearings. The control requires information about the rotor motion, which usually will be determined by displacement sensors. The velocity signals are then derived in the computer. For the multivariable control which we need here, a complete state feedback is actually of limited value only. There we would need too many, not really exactly known, parameters and the system order increases too much, especially by taking into account an observer as well, making it hard to implement the controller on a microprocessor. We are using a decentralized structure, where the structure has been chosen reasonably from an engineering point of view. As a consequence of the simplified control structure, it is feasible now to realize a digital control with all its inherent possibilities [10]. Thus, the complete radial motion of an elastic rotor can be controlled with a control rate of 2.5-15 kHz, depending on the processor to be used (Motorola-68000, signal processor TMS 320 [7]). Some consideration has to be given to the design software. Packages like Matrix X or ACSL for the simulation of the complete nonlinear system have been quite valuable to us.
5. M E C H A T R O N I C S IN R E S E A R C H AND EDUCATION
Our work on these projects had pointed to gaps within the existing educational and research structure of our school. In order to do justice to the broad requirements of synthesis tasks in mechatronics, we decided among colleagues to cooperate more closely. We institutionalized the option to ask specialized researchers for their advice thus opening a source for expertise and synergy, without largely increasing the existing infrastructure. The school supported the foundation of a Mechatronics Group. Full and partial members of this group come from the Departments of Mechanical and Industrial Engineering, and Electrical Engineering: Profs Geering (Control), Guggenbtihl (Electronics), Hugel (El. Tech. Construction), KiJbler (Computer Vision), Mansour (Automation), Reichert (Electrical Drives) and Schweitzer (Robotics). Objectives of this group are to systematically support research and education in mechatronics. We initiate and coordinate research projects, for example on a high speed/high power electrical drive in magnetic bearings, or on a robot Table 2. Survey on actuator/amplifier combinations: (a) actuator for generating testforces and for damping shown in Fig. 6; (b) actuator for supporting a very flexible rotor and, (c) actuator for the radial bearing of a high-speed milling spindle
Max. bearing force Bearing diameter Bearing gap Switching freq. Voltage Power
(N) (mm) (mm) (kHz) (V) (kVA)
(a)
(b)
(c)
1300 190 1 50 310 1
140 58 1 D.C. +54 Perm 0.16
1800 96 0.35 max 100 310 2.4
Mechatronics--a concept with examples
73
F/N 1000
100
10 1 O0
1000
f/Hz
Fig. 9. Frequency response: the actuator/amplifier unit of Fig. 6, with the specifications of Table 2a, can, for example, generate a sinusoidal force of about 100 N at 400 Hz.
cooperating with a human operator, and we are responsible for a postgraduate course in'mechatronics. The Postgraduate Course in Mechatronics is offered in the Department of Mechanical and Industrial Engineering. It is of special interest to mechanical and electrical engineers, comp'uter scientists and technical physicists, for continued education of industrial staff engineers, and for accompanying courses during doctoral research. The complete course (Table 3) is scheduled for 1 year, and can be extended to accommodate various needs. The admission requirements for regular participation is a qualification equivalent to an E T H Master's Degree. The course offers a menu where Table 3. Lectures in the Mechatronics Postgraduate Course Winter 1989-1990 Computer Aided Kinematics and Kinetics of Mechanisms Computer Vision I Robust Control Optimal Filters Microprocessors I Applied Artificial Intelligence I Electrical Drive Systems I Robotics I Magnetic Bearings Microtechniques I Courses from other areas like Reliability, Electronics, Computer Science, Industrial Engineering
hr/week 4 4 3 3 4 4 4 3 3 2
Summer 1990 Computer Vision II Adaptive Control Realtime Data Processing Microprocessors II Applied Artificial Intelligence II Neural Networks Electrical Drive Systems II Robotics II Microtechniques II Electrotechn. Equipment in Road Vehicles Postgraduate Master's Thesis
4 3 4 4 4 3 4 3 2 2 150 hr
74
G. S C H W E I T Z E R
the selection has to b e d o n e a c c o r d i n g to p r e v i o u s e d u c a t i o n . A f t e r c o m p l e t i n g the c o u r s e successfully, the s t u d e n t will r e c e i v e a f o r m a l c o n f i r m a t i o n . S o m e of the courses are o f f e r e d as s h o r t courses, t o o , e s p e c i a l l y for industry. S e v e r a l s y m p o s i a on v a r i o u s topics o f the c o u r s e h a v e b e e n o r g a n i z e d at t h e E T H in the m e a n t i m e . T h e Undergraduate Curriculum f o r M e c h a n i c a l Engineers allows o u r s t u d e n t s to specialize to s o m e e x t e n t in M e c h a t r o n i c s , too. T h e y h a v e to c h o o s e two a r e a s f r o m a selection o f four, consisting o f e n e r g y , c o n s t r u c t i o n , p r o d u c t i o n a n d m e c h a t r o n i c s .
6. C O N C L U S I O N S B a s e d on e x a m p l e s f r o m t h e field o f m a g n e t i c b e a r i n g s , the r e q u i r e m e n t s a n d the p o t e n t i a l o f the n e w discipline o f m e c h a t r o n i c s h a v e b e e n o u t l i n e d . M a g n e t i c b e a r i n g s offer p r o p e r t i e s allowing classical b e a r i n g p r o b l e m s in m a c h i n e d y n a m i c s to b e t a c k l e d in a n e w way: no m e c h a n i c a l w e a r , no l u b r i c a t i o n , low m a i n t e n a n c e costs, a d a p t i v e d y n a m i c s . This is b a s e d o n the s y s t e m a t i c c o m b i n a t i o n of e l e m e n t s f r o m classical m e c h a n i c a l a n d e l e c t r i c a l e n g i n e e r i n g , a n d c o m p u t e r science in o r d e r to c o m e up with a new, ' i n t e l l i g e n t ' p r o d u c t . T h e m a g n e t i c b e a r i n g is a typical e x a m p l e o f such a p r o d u c t d e m o n s t r a t i n g t h e state o f the art of m e c h a t r o n i c s . T h e r e s e a r c h a n d e d u c a t i o n a l activities at the E T H are i n t e n d e d to intensively s u p p o r t this scientifically a n d e c o n o m i c a l l y p r o m i s i n g field. Acknowledgement--(An earlier version of) this paper was published in the Proceedings of the Conference on "Mechatronies", 1990. (The material) It is reproduced by permission of the Institution of Mechanical Engineers.
REFERENCES 1. Schweitzer G. and Mansour M. (eds), Dynamics of controlled mechanical systems. Proc. 1UTAM/IFAC Symposium, ETH Ztirich, 30 May-3 June 1988, Springer-Verlag, Berlin (1988). 2. Ishii T., Future trends in mechatronics. Keynote Address at the Internat. Conf. on Advanced Mechatronics (JSME), Tokyo, May (1989). 3. Habermann H. and Brunet M., The active magnetic bearing enables optimum control of machine vibrations. ASME Paper 85-GT-22, Gas Turbine Conf., Houston, March (1985). 4. Sehweitzer G. (ed.), Magnetic bearings. Proc. First Internat. Symposium, ETH Ztirich, 6-8, June 1988. Springer-Verlag, Berlin (1988). 5. Higuchi T. (ed.), Magnetic bearings. Proc. Second lnternat. Symp. on MB, Univ. Tokyo, Inst. of Industrial Science, 12-14 July (1990). 6. Bauser E. et al., Centrifuge for epitaxial growth of semiconductor multilayers. In Proc. IUTAM/IFAC Symposium (Edited by Schweitzer G. and Mansour M.). 7. Siegwart R. and Traxler A., M6glichkeiten und Grenzen schneller Aktuatoren am Beispiel einer magnetisch gelagerten Hochgeschwindigkeits-Fr~isspindel. VDI-Tagung Mechatronik - Kontrollierte Bewegungen im Fahrzeug- und Maschinenbau, Bad Homburg, VDI-Berichk 787, November (1989). 8. Burrows C. R., Sahinkaya N., Traxler A. and Schweitzer G., Design and application of a magnetic bearing for vibration control and stabilization of a flexible rotor. In Proc. IUTAM/1FAC Symposium (Edited by Schweitzer G. and Mansour M.). 9. Bleuler H., Diez D., Lauber G., Meyer U. and Zlatnik D., Nonlinear neural network control with application example. Proc. lnternat. Neural Network Conf., Paris, July (1990). 10. Larsonneur R., Design and control of active magnetic bearing systems for high speed rotation. Thesis ETH No. 8962, Ziirich (1990).
0957-4158/92 $5.00+0.00 © 1991 Pergamon Press plc
MECHATRONICS--A CONCEPT WITH EXAMPLES IN ACTIVE MAGNETIC BEARINGS
G.
SCHWEITZER
Institute of Robotics and Mechatronics Lab., ETH Zfirich, CH-8092 Ztirich, Switzerland
(Received 2 January 1991; accepted 20 March 1991) Abstract--What kind of knowledge is necessary in order to design a mechatronic product? What are the main issues, what are the required interconnections between the fields of mechanics, electronics, and computer science? Which means are available for research and education? A typical example of a mechatronic system will be presented: active magnetic bearings for a high-speed milling spindle, for applications in vacuum techniques, and in vibration control. Based on these examples from industry and research, a concept developed at the ETH (Eidgen6ssische Technische Hochschule) will be discussed for structuring the field of mechatronics, offering answers to the above questions.
1. WHAT IS MECHATRONICS?
Let us start with an example. In classical mechanics we ask: how does a body move, when a given force, for example the gravitational force, is acting on it? For answering that question, we write down the well known equations of motion and we solve them. This means we are dealing with a p r o b l e m of analysis. The question, however, can be formulated in an inverse way, too, i.e. what kind of force do you have to apply so that the body performs a desired motion? What, for example, does the braking force of an automobile have to be for negotiating a curve in a safe way during braking? This is a problem of synthesis, and in the case of the automobile the answer has led to the automatic ABS-system. For solving such synthesis tasks, we need, in addition to the mechanics, mainly system theory or control techniques. If we also want to realize a potential solution, we often also need electrical engineering, electronics, and computer science. The term 'mechatronics' for the combination of these fields probably came from Japan, and is quite widely used there. In fact this combination is not new at all: in aerospace and in aeronautics it has been successful for a long time. However, for some years now, this multidisciplinary field has grown rapidly, and has been developing a character of its own [1,2]. This development has been spurred by the availability of cheaper c o m p u t e r power, and it will be further supported by the availability of 'intelligent' power electronics. Therefore, we suggest a definition which emphasizes the new possibilities of this combination and its potential for 'intelligence' in a m o r e distinctive way: Mechatronics is an interdisciplinary field of engineering science, combining the 65
66
G. SCHWEITZER
classical areas of mechanical engineering, electrical engineering, and computer science. A typical mechatronic system picks up signals, processes them, and, as an output, generates forces and motions. The main issue is the extension and completion of mechanical systems by sensors and microcomputers, which leads to the characterization as 'information driven mechanical systems'. The fact that such a system detects changes in its environment by sensors, and, after suitably processing this information, reacts to them, makes it quite different from conventional machines (Fig. 1). Examples are spread over the mechanical engineering area and include robots, digitally controlled combustion
engines, machine tools with self-adaptive tools, contact-free magnetic bearings, and automated guided vehicles. Typical for such a product is the substantial amount of system's knowledge and software that goes into its design, construction and operation. It is quite justified to say that 'software is a machine element'. First, we will present a typical mechatronic example: active magnetic bearings for various applications. Finally, we will demonstrate briefly how this new field is structured at the E T H in research and in education.
2. PRINCIPLE OF THE ACTIVE MAGNETIC BEARING Magnetic bearings can support a rotor without any contact, and they can control its motion within the air gap in a well-defined way. The electromagnetic bearing forces depend on the measured state of motion of the rotor, and they are governed by a suitable control law. In addition to mechanical components, the bearing system contains electronic elements like sensors and power amplifiers, a controller realized, for example, by a microprocessor, as well as an increasing amount of software, which actually determines the 'intelligence' of the system. The increasing availability and handiness of these 'machine elements' makes the magnetic bearing more and more attractive for the solution of classical bearing problems in machine dynamics [3-5]. The functional principle of the electromagnetic suspension is based on a control loop as shown in Fig. 2. The control law can be adjusted within physical limitations to
Engineerin[~
_•MmeCh. echan.SystemI
t
Electrical I Engineering I
Sensors l Amphfiers I
Science [
L•Computer
!
Microprocessor I
Fig. 1. Mechatronic system: the system picks up signals from its environment, processes them 'intelligently', and reacts, for example, with forces or motions. Methods for connecting the various scientific areas come from the basic engineering sciences, system theory, control techniques and computer science.
Mechatronics--a concept with examples
67
the technical requirements, and it controls the dynamic stiffness of the suspension, and the damping and stability of the hovering state. A real rotor, of course, needs several of these suspension elements, which are connected by a multivariable control (Fig. 3). First, we will present some applications of the bearing principle and then the essential elements of this mechatronic product.
3. MAGNETIC BEARING APPLICATIONS The various advantages of a magnetic suspension lead to applications of the bearings in three main areas: • vacuum and super clean room techniques (no contamination by the bearings); • machine tools (precision, high load, high speed); and • turbomachinery, centrifuges, vibration isolation and excitation (control and damping of vibrations, good dynamic properties). The requirements for magnetic bearings in these areas are different, especially as to the dynamic behaviour. Many properties can be influenced by a microprocessor Magnet~
Rotor
PowerAmplifier
Control
Sensor
Fig. 2. Functional principle: a sensor is measuring the deviation of the rotor from its reference position, a microprocessor acting as a controller derives a control signal from these measurements. The control signal drives a power amplifier, generating a control current through the actuator magnet in such a way that the magnetic forces keep the rotor in a hovering state.
Fig. 3. Electromagnetically suspended rotor: only the radial suspension in one plane is shown. The axial suspension can be realized in a quite similar way.
68
G. SCHWEITZER
controller. This possibility of influencing the characteristics by software allows the bearings to be made 'intelligent' (Table 1). The next two sections will show some actual examples.
3.1. Epitaxy centrifuge For the Max-Planck-Institute of Solid State Research in Stuttgart, G e r m a n y , we have built a special centrifuge, where solid state substrates can be covered in a well-defined way with very thin multi-layers [6]. To achieve this, a crucible rotates following a predefined speed program in a container under ultra-high-vacuum conditions (Fig. 4). There are intentions to upscale this liquid phase epitaxy process for generating semiconductors of the highest quality.
3.2. High-speed milling spindle In collaboration with a Swiss company, we built a magnetically supported milling spindle, which currently is launched as a prototype. The cutting power is about 35 kW, the rotation speed is up to 40,000 rpm, and the cutting speed for aluminium is up to 6000 m sec. This high-speed milling offers advantages with respect to the milling process and production costs. An extensive description is given in [7].
3.3. Actuator for generating test forces and for damping The design objective for the actuator shown in Fig. 6 was to generate high forces within a large frequency range that can be used to control rotor vibrations. The magnet has eight poles and can generate forces independently in two mutually orthogonal directions. This actuator is used firstly to generate test forces acting on a rotor for identification purposes and then to work as an active d a m p e r or as a bearing [8]. The rotor itself is 2.3 m long, 100 m m in diameter and supported in two oil bearings. These bearings are known to cause coupling effects between the lateral motions of the shaft. The overhung shaft carries disks with a mass of 93 kg. The actuator first exerts sequential test forces on the rotating rotor. The resulting shaft deviations are measured as test signals. They are used in an algorithm to compute control forces which are subsequently applied to the rotor by the same actuator, synchronously with the rotor speed, in order to compensate for the unbalance. This procedure repeats itself while the rotor slowly runs up to its operational speed. Thus Table 1. Technical objectives which can be achieved by suitably programming the microprocessor controller of the system Contact-free and stable support of the rotor High accuracy of positioning Damping of vibrations and stabilization of self-excited vibrations Reduction of vibration forces in the bearings Reduction of resonance amplitudes of an elastic rotor Passing through critical speeds Improvement of safety properties (redundancy, diagnostics. . . . ) Integration of the bearing into the operating system of the overall process
M e c h a t r o n i c s - - a c o n c e p t with e x a m p l e s
69
uum Gohiuse
-Motor
. Radial Lager
|er Sensor ;estall nsor
Fig. 4. Epitaxy centrifuge: the rotor with the crucible is suspended within the container with no contact. The magnetic bearings are completely outside the container and the bearing forces are acting through the container wall.
IPowerArnp'i"erl=~
I
Digital JControllerF~-~
Performance:
Rotor:
Rotational Speed: 40'000 RPM Driving Power: 30 kW Cutting Speed: 6'500 m/min
Length: 486 mm Weight: 16 kg Max. Diameter: 144 mm
Fig. 5. High-speed milling spindle with digitally controlled magnetic bearings.
70
G. SCHWEITZER
Fig. 6. Electromagnetic actuator ETH-190/50/1 consisting of the magnet, the power amplifiers, the ferromagnetic laminated sleeve on the rotor, and sensors. The main specifications are given in Table 2a, the frequency response is shown in Fig. 9. the algorithm determines the control forces necessary for minimizing the synchronous rotor vibrations, without a priori knowledge of the unbalance distribution or the hydrodynamic bearing coefficients. This kind of procedure for automatically adjusting the actuator during operation is of special interest, because generally the dynamic characteristics of the rotor in oil bearings cannot be predicted from theoretical models in a satisfactory way. This adaptive control is indicating a direction even more advanced, to 'learning' magnetic bearings. The first experiments with non-linear, learning control using a neural network have already shown very promising results [9].
4. ELEMENTS OF A MAGNETIC BEARING SYSTEM
Some essential elements of that mechatronic product are described briefly below. The mechanical element, the rotor, can be modelled in simple cases as a rigid body with five degrees of freedom which have to be controlled. In general, however, the rotor has to be seen as a flexible, vibrating structure. The natural modes of the milling spindle, for example, had been derived by a finite element model and verified by experimental modal analysis. The electronic actuator is the element within the control loop which transforms an electrical input into a mechanical output, the bearing force, and therefore this actuator, in a simple way is a magnetic bearing already (Figs 7 and 8). The character-
Mechatronics--a concept with examples iy
71
~Y I
Fig. 7. Radial magnetic bearing as input/output element, the control current through the coils is the input. istics of an actuator can be derived from geometric and electrical data, or they have to be determined experimentally. The specific load capacity, with respect to the cross-sectional area of the bearing, is about 30 N cm -2. Of course, we can only m a k e use of that potential of the actuator if the magnet, as a part of the complete system, is controlled suitably. The power amplifiers have to amplify the control signals and to feed the magnetic coils. The coils essentially represent an inductive load. With their low pass characteristics they are limiting the frequency response and thus the dynamics of the whole bearing system. We distinguish between D.C.-amplifiers and switched amplifiers. The
Fig. 8. Radial bearing for a flexible shaft.
72
G. SCHWEITZER
first ones are applied for small loads below 0.5 kVA (Table 2). The high power of an amplifier would be essential for achieving good dynamic properties (Fig. 9). The actual consumption of energy, however, is low, and at high speeds it is an order of magnitude less than with conventional bearings. The control requires information about the rotor motion, which usually will be determined by displacement sensors. The velocity signals are then derived in the computer. For the multivariable control which we need here, a complete state feedback is actually of limited value only. There we would need too many, not really exactly known, parameters and the system order increases too much, especially by taking into account an observer as well, making it hard to implement the controller on a microprocessor. We are using a decentralized structure, where the structure has been chosen reasonably from an engineering point of view. As a consequence of the simplified control structure, it is feasible now to realize a digital control with all its inherent possibilities [10]. Thus, the complete radial motion of an elastic rotor can be controlled with a control rate of 2.5-15 kHz, depending on the processor to be used (Motorola-68000, signal processor TMS 320 [7]). Some consideration has to be given to the design software. Packages like Matrix X or ACSL for the simulation of the complete nonlinear system have been quite valuable to us.
5. M E C H A T R O N I C S IN R E S E A R C H AND EDUCATION
Our work on these projects had pointed to gaps within the existing educational and research structure of our school. In order to do justice to the broad requirements of synthesis tasks in mechatronics, we decided among colleagues to cooperate more closely. We institutionalized the option to ask specialized researchers for their advice thus opening a source for expertise and synergy, without largely increasing the existing infrastructure. The school supported the foundation of a Mechatronics Group. Full and partial members of this group come from the Departments of Mechanical and Industrial Engineering, and Electrical Engineering: Profs Geering (Control), Guggenbtihl (Electronics), Hugel (El. Tech. Construction), KiJbler (Computer Vision), Mansour (Automation), Reichert (Electrical Drives) and Schweitzer (Robotics). Objectives of this group are to systematically support research and education in mechatronics. We initiate and coordinate research projects, for example on a high speed/high power electrical drive in magnetic bearings, or on a robot Table 2. Survey on actuator/amplifier combinations: (a) actuator for generating testforces and for damping shown in Fig. 6; (b) actuator for supporting a very flexible rotor and, (c) actuator for the radial bearing of a high-speed milling spindle
Max. bearing force Bearing diameter Bearing gap Switching freq. Voltage Power
(N) (mm) (mm) (kHz) (V) (kVA)
(a)
(b)
(c)
1300 190 1 50 310 1
140 58 1 D.C. +54 Perm 0.16
1800 96 0.35 max 100 310 2.4
Mechatronics--a concept with examples
73
F/N 1000
100
10 1 O0
1000
f/Hz
Fig. 9. Frequency response: the actuator/amplifier unit of Fig. 6, with the specifications of Table 2a, can, for example, generate a sinusoidal force of about 100 N at 400 Hz.
cooperating with a human operator, and we are responsible for a postgraduate course in'mechatronics. The Postgraduate Course in Mechatronics is offered in the Department of Mechanical and Industrial Engineering. It is of special interest to mechanical and electrical engineers, comp'uter scientists and technical physicists, for continued education of industrial staff engineers, and for accompanying courses during doctoral research. The complete course (Table 3) is scheduled for 1 year, and can be extended to accommodate various needs. The admission requirements for regular participation is a qualification equivalent to an E T H Master's Degree. The course offers a menu where Table 3. Lectures in the Mechatronics Postgraduate Course Winter 1989-1990 Computer Aided Kinematics and Kinetics of Mechanisms Computer Vision I Robust Control Optimal Filters Microprocessors I Applied Artificial Intelligence I Electrical Drive Systems I Robotics I Magnetic Bearings Microtechniques I Courses from other areas like Reliability, Electronics, Computer Science, Industrial Engineering
hr/week 4 4 3 3 4 4 4 3 3 2
Summer 1990 Computer Vision II Adaptive Control Realtime Data Processing Microprocessors II Applied Artificial Intelligence II Neural Networks Electrical Drive Systems II Robotics II Microtechniques II Electrotechn. Equipment in Road Vehicles Postgraduate Master's Thesis
4 3 4 4 4 3 4 3 2 2 150 hr
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G. S C H W E I T Z E R
the selection has to b e d o n e a c c o r d i n g to p r e v i o u s e d u c a t i o n . A f t e r c o m p l e t i n g the c o u r s e successfully, the s t u d e n t will r e c e i v e a f o r m a l c o n f i r m a t i o n . S o m e of the courses are o f f e r e d as s h o r t courses, t o o , e s p e c i a l l y for industry. S e v e r a l s y m p o s i a on v a r i o u s topics o f the c o u r s e h a v e b e e n o r g a n i z e d at t h e E T H in the m e a n t i m e . T h e Undergraduate Curriculum f o r M e c h a n i c a l Engineers allows o u r s t u d e n t s to specialize to s o m e e x t e n t in M e c h a t r o n i c s , too. T h e y h a v e to c h o o s e two a r e a s f r o m a selection o f four, consisting o f e n e r g y , c o n s t r u c t i o n , p r o d u c t i o n a n d m e c h a t r o n i c s .
6. C O N C L U S I O N S B a s e d on e x a m p l e s f r o m t h e field o f m a g n e t i c b e a r i n g s , the r e q u i r e m e n t s a n d the p o t e n t i a l o f the n e w discipline o f m e c h a t r o n i c s h a v e b e e n o u t l i n e d . M a g n e t i c b e a r i n g s offer p r o p e r t i e s allowing classical b e a r i n g p r o b l e m s in m a c h i n e d y n a m i c s to b e t a c k l e d in a n e w way: no m e c h a n i c a l w e a r , no l u b r i c a t i o n , low m a i n t e n a n c e costs, a d a p t i v e d y n a m i c s . This is b a s e d o n the s y s t e m a t i c c o m b i n a t i o n of e l e m e n t s f r o m classical m e c h a n i c a l a n d e l e c t r i c a l e n g i n e e r i n g , a n d c o m p u t e r science in o r d e r to c o m e up with a new, ' i n t e l l i g e n t ' p r o d u c t . T h e m a g n e t i c b e a r i n g is a typical e x a m p l e o f such a p r o d u c t d e m o n s t r a t i n g t h e state o f the art of m e c h a t r o n i c s . T h e r e s e a r c h a n d e d u c a t i o n a l activities at the E T H are i n t e n d e d to intensively s u p p o r t this scientifically a n d e c o n o m i c a l l y p r o m i s i n g field. Acknowledgement--(An earlier version of) this paper was published in the Proceedings of the Conference on "Mechatronies", 1990. (The material) It is reproduced by permission of the Institution of Mechanical Engineers.
REFERENCES 1. Schweitzer G. and Mansour M. (eds), Dynamics of controlled mechanical systems. Proc. 1UTAM/IFAC Symposium, ETH Ztirich, 30 May-3 June 1988, Springer-Verlag, Berlin (1988). 2. Ishii T., Future trends in mechatronics. Keynote Address at the Internat. Conf. on Advanced Mechatronics (JSME), Tokyo, May (1989). 3. Habermann H. and Brunet M., The active magnetic bearing enables optimum control of machine vibrations. ASME Paper 85-GT-22, Gas Turbine Conf., Houston, March (1985). 4. Sehweitzer G. (ed.), Magnetic bearings. Proc. First Internat. Symposium, ETH Ztirich, 6-8, June 1988. Springer-Verlag, Berlin (1988). 5. Higuchi T. (ed.), Magnetic bearings. Proc. Second lnternat. Symp. on MB, Univ. Tokyo, Inst. of Industrial Science, 12-14 July (1990). 6. Bauser E. et al., Centrifuge for epitaxial growth of semiconductor multilayers. In Proc. IUTAM/IFAC Symposium (Edited by Schweitzer G. and Mansour M.). 7. Siegwart R. and Traxler A., M6glichkeiten und Grenzen schneller Aktuatoren am Beispiel einer magnetisch gelagerten Hochgeschwindigkeits-Fr~isspindel. VDI-Tagung Mechatronik - Kontrollierte Bewegungen im Fahrzeug- und Maschinenbau, Bad Homburg, VDI-Berichk 787, November (1989). 8. Burrows C. R., Sahinkaya N., Traxler A. and Schweitzer G., Design and application of a magnetic bearing for vibration control and stabilization of a flexible rotor. In Proc. IUTAM/1FAC Symposium (Edited by Schweitzer G. and Mansour M.). 9. Bleuler H., Diez D., Lauber G., Meyer U. and Zlatnik D., Nonlinear neural network control with application example. Proc. lnternat. Neural Network Conf., Paris, July (1990). 10. Larsonneur R., Design and control of active magnetic bearing systems for high speed rotation. Thesis ETH No. 8962, Ziirich (1990).