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Smart Actuators in Robotics
Copyright 0 IFAC Intelligent Assembly and Disassembly, Bled, Slovenia, 1998
SMART ACTUATORS IN ROBOTICS
Niko
Herakovi~
and Tomaf Perme
University ofLjubljana, Faculty of Mechanical Engineering, Laboratory for Handling, Assembly and Pneumatics
Abstract: A piezoelectric actuator is an example of alternative actuators, which are successfully implemented in many applications today. The main advantages of these actuators are their very high dynamics (up to 40 kHz), theoretically unlimited resolution (in the field ofnanometers), low consumption of electrical energy, high force and very compact construction. One very important advantage that piezoactuators can offer is the possibility of having the actuator, force sensor and position sensor contained in a onepiece unit. Such a piezoactuator is called a "smart actuator". The main problem with implementation of these actuators is related to very small oscillating movements caused by its expansion and contraction, which have to be transferred into continuous movement. In this paper, the characteristics of "smart actuators" are presented and their potential, as well as some ideas for their useful implementation and utilisation of the grippers and robots for assembly and disassembly, are discussed. Copyright© 1998lFAC
Keywords: smart piezoactuators, robots, robot arms, grippers, sensors
1. INTRODUCTION
actuators very complicated. Hydraulic drives are very appropriate and often used for robotic drives, but their cost is high.
In order for robots to become more efficient, they need to be continuously improved. In addition to economic and technical aspects, nowadays the ecology is an ever greater consideration in the development and implementation of new technologies and ideas.
These are the main reasons why actuation is one field where new technologies and ideas are constantly needed. Faster, smaller and more powerful are the demands which propel the world of actuation.
Power for robot motion is provided by various types of actuators, such as electric, hydraulic, pnewnatic, etc. Many robot manufacturers usually use electric motors of two types: stepper and direct current (DC) motors. Electric actuators are fast and accurate, but very weak or unpleasantly heavy because of their complexity.
One alternative idea in this world is a piezoelectric actuator. The piezo effect is well known for sensing, but more and more solutions have been developed so that this effect may also be used for actuation. In robot arms piezoactuators are already being very successfully used to compensate for the bending of arms (Brand, and Laux, 1990). It is generally known that supporting elements in mechanical systems (e.g. robot arms) possess some limited stiffness both dynamically and statically and, because of that, tend towards bending. As a consequence, inaccuracies appear in the positioning process. The stiffness could
Pneumatic actuators are mostly used for opening and closing of grippers. Despite the large quantity of energy per unit mass that gas can store, its compressibility makes servocontrol of pnewnatic
177
simply be increased with more stable construction of the robot arm, but this step is connected with increasing its own mass. Another way to increase a robot arm's stiffness without increasing its mass is to use active compensation for bending by using an adequate controller and piezoactuators, as shown in Fig. I.
of products are assembled along three perpendicular axes (up to 59% are assembled in one direction) ~e
requirements for robots used for assembly and disassembly are different than those used for welding or painting. A robot's main functional features are: numb~r of degrees of freedom (DOF); speed, e.g. velOCity of tool centre point (TCP); accuracy and repeatability; load capacity (payload); working space (working volume); and the possibility of including external sensors.
piezo actuators ./4
robot arm
i=='==~::=:I
The following requirements for an industrial robot are of major importance. In assembly applications ~ost .operations, such as placing of parts (40%), Insertion (30%) and fastening (24%), required simple movements that can be accomplished by robots with less than 6 DOF.
- ...............•
\\ CCD-array
controler
Load ca!'acity: In the case of assembly robots, acceleratIon and structural stiffness are more important design parameters than peak velocity or maximum load capacity, as minimising small motion times is generally a top priority (Seering, 1985).
Fig. I. Active compensation for bending of robot arms using piezoactuators For detecting the bending of a robot arm under a load, a laser measurement system is used. The aberration from the given position is detected with a C.CD sensor and actively compensated for by pIezoactuators and a PID controller. The use of piezoactuators in such compensative systems makes it possible to achieve higher positioning accuracy and shorter positioning times.
Velocity and acceleration: To reduce the cycle time of assembly operations, movements have to be carried out as quickly as possible. Acceleration affects gross motion time as well as cycle time. A general design guideline for robots is that most motions should be performed with the system at its velocity limit part of the time. Accuracy, resolution and repeatabi/ity: Robot accuracy is important for non-repetitive tasks. A typical accuracy standard for manipulators with precisely manufactured and measured kinematic elements and simple accurate models is up to 0.01 mm. More important for assembly and disassembly operations is repeatability of the manipulator. For very precise micropositioners it is 0.005 mm, but the typical industrial robot has a standard of 0.025 mm.
~ecently,
some experiments were made using plezoactuators for micro- and minirobots. The aim of this paper is to launch into the public eye these ideas along with some potential solutions to related problems in the field of assembly and disassembly.
2. ROBOTS FOR ASSEMBLY AND DISASSEMBLY
Actuators and Sensors: The main characteristic for actuators is power-to-size ratio. More recently, many actuators have included built-in position and velocity sensors, which are needed to measure variables within the robot and in the environment. Sensors are normally mounted on the motor shaft and not on the joint shaft.
The ideal robot - characterised by rigidity, minimal weight, as quick a response as possible, a maximal number of degrees of freedom, high speed, good repeatability and good accuracy - is unrealistic to build. Therefore, it is necessary to make a few concessions and to consider assembly's specific feat,ures. Several studies have stressed the following basiC features of most electromechanical assemblies (Delchambre, 1992):
Transmission: Power has to be transmitted from the actuator to the linkage. Typical transmission devices such as gears, tendons and linkage contain many disadvantages. The transmission elements can be eliminated by using a direct drive system, which has no backlash, low friction, low compliance, high reliability and a fast dynamic response, but is, however, more difficult to control.
of the parts to be moved weigh less than I kg the workspace is relatively small (projection of the volume required to assemble a product is 2 smaller than 400 cm )
178
energy transducer - piezo actuator
----------
I
I~-" I
I I
transducer I Cp completition '~;
'''' compensation of ___x_(t_) -.-loadinfluences ~_x_'(_t)_~electronic power amplifier and hysteresis actuator control signal
I I
TV F l 2
2
I
_____ 1
i(t)
nonlinear parameter
Fig. 2.
hysteresis model
u(t)
transducer output
The principle of a smart actuator
3. SMART ACTUATORS
The current i(t) and the voltage u(t) are measured at the electrical gate of the transducer and transferred to the digital signal processing unit. The signal processing unit calculates, on the basis of non-linear transducer and hysteresis models, the parameters of an inverse filter which idealises the input-output transfer behaviour of the actuator. Special attention should be paid to the fact that mechanical parameters, such as the expansion velocity v and the force F at the transducer output and at its completion, do not need to be known because they are calculated with a transducer model.
Piezoactuators consist, as do all actuators, of an electronic power amplifier and an electromechanical energy transducer, connected in a series. Many frequently used energy transducers can be described as three-gate or two-gate, within the framework of four-pole theory. This presentation of the phenomenon represents a basic tenet in the manufacture of so-called smart actuators, in which a signal processor calculates, with the aid of a suitable transducer model, output-stroke and input-current characteristics as well as force-voltage-transfer function, usable for sensing.
Another possibility of creating a kind of smart piezoactuator is shown in Fig. 3. This new class of devices stems from the combination of acting and sensing (in a monolithic device). The entire actuator is made from the piezoelectric ceramic material lead-
This type of smart actuator is based on transducer materials which possess sensor and actor characteristics simultaneously and represent a mechatronic subsystem. With such an actuator it is possible to exactly determine the output stroke and/or output force without the use of additional sensors for stroke and force. In this sense, piezoactuators are very appropriate for use as smart actuators.
ceramic body sensor
actuator
In a technical application, electric current and voltage have to be measured at the electrical gate of the transducer so the force and the positioning velocity at the transducer output gate may be determined. While in previous applications only the current (and sometimes also the voltage) has been wired to the electrical transducer input, a smart actuator has been used for all disposed control signals. Using this additional information, a microprocessor-controlled signal can turn a transducer into a sensor. The principle of a smart actuator (piezoelectric actuator) is shown in Fig. 2.
isolation material
Fig. 3. Smart piezoactuator - an actuator and sensor in one piece zirconate-titanate (PZT). This is one of the best materials to use when considering the energy density and conversion factors, based on ferroelectric ceramics which have a perovskite structure. With an adequate electrode structure, the main part of the
179
ceramic body functions as an actuator while a small fraction is used for sensing. This may result in an actuator with improved properties, e.g. linearity, if the sensor signal is fed back. With the use of suitable electronics it is also possible to measure the output force and the strain of the piezoactuator. Materials and devices which function in this manner are called 'smart'.
In spite of the advantages that smart piezoactuators possess, there is a great disadvantage which makes using them for direct actuation of a robot arm joint impossible - a very small strain or extension of the piezoactuator. Normally, piezoactuators can achieve an extension of about 0.1 to 0.15% of their length. Such small extension of about 40 to 60 Jlm could perhaps be effectively used for direct actuation in micro robots, while in larger robots other solutions should be found to enlarge the extension of smart piezoactuators.
Because the dynamics, positioning accuracy and efficiency of the entire piezoactuator system are decisively influenced from the electronic power amplifier and from control electronics, close attention has to. be paid to choosing the right electronics (Herakovit, 1996).
Linear piezoelectric smart motor: Usually all joints of a robot arm, wrist and gripper are rotary or revolute joints. In Fig. 4 the principle is shown which illustrates how a linear piezo motor, using smart piezoactuators, could be used for the actuation of robot arm joints. This principle is very well known and already used in practice.
Basically, there are two ways of controlling the piezoactuator: voltage and charge control. Voltage control is simple to achieve but the strain of the piezoactuator, as a function of the driven voltage, shows considerable evidence of hysteresis and nonlinearities. Charge control, on one hand, requires more complex circuitry; on the other hand, it offers almost linear dependence between the output strain of the piezoactuator and the loading charge at the input.
smart piezo actuator\
4. SMART ACTUATORS IN ROBOTICS Its main advantages, such as very high dynamics (up to 40 kHz), theoretically unlimited resolution (in the field of nanometers), low consumption of electrical energy, high force, very compact construction and the possibility of acting as a smart actuator, make piezoactuators very suitable for the actuation of grippers and robot arm joints. Using an appropriate control and amplifier electronics, the position of the robot arm or the gripper, as well as the gripping and carrying force, would be able to be exactly determined without the use of additional sensors. At the same time, the arm joints and the gripper could be minimised and the cost reduced. The main question that here presents itself is: How can smart piezoactuators be used in robotics?
Fig. 4. Principle for the actuation of an arm joint with a linear piezo motor The linear piezo motor, usually called an inchworm or walking piezo motor, is driven by a few smart piezoactuators and is based on the walking principle (Fig. 5). y
•
X
!.._~
1 I
lA
4.1 Robot arm with joints and wrist
'\"~,i']~
cpA The motion of a robot arm, which consists of links and joints, differs from the motion of the human arm. Robotic joints can move through greater angles, but have fewer degrees of freedom. The elbow of an articulated robot can bend up or down, whereas a human can only bend an elbow in one direction with respect to the straight arm position. The greater range of motion available to joints in robot arms gives them greater flexibility than human arms, but this increased flexibility also increases the complexity of the systems used for the actuation ofjoints.
wpA
wpB
cpB
Fig. 5. Principle of walking Here wp are the walking piezoactuators. They expand and contract in the walking direction x. The clamping piezoactuators (cp) expand and contract in the y direction perpendicular to the walking direction. Altematingly, they clamp the A-side or the B-side of the motor to the frame. The output member is attached to the centre C.
180
frequency of the walking wp and clamping cp piezoactuators. The walking frequency should be kept far enough below the lowest natural frequency to maintain control of walking amplitude.
In Fig. 6 the sequence of action of the respective piezoactuators is shown. The time needed for one complete cycle is characterised as teIf clamp changing, which takes a certain amount of time (tch), happens during an interval of matching velocities of the piezoactuators wpA and wpB, constant velocity of the centre C will be achieved. As a consequence, the entire stroke h of the walking piezoactuators wp cannot be used. The effective stroke is
h' = Tlth
To adapt the characteristics of the piezoactuators to the target specifications of the linear motor, mechanical amplification can be used to enlarge the stroke of the walking piezoactuators. An example of a linear piezo motor with mechanical amplification of the walking piezoactuator stroke is shown in Fig. 7.
(I)
Rotary piezoelectric smart motor: The principle
where
which illustrates how a rotary piezo motor could be used for the actuation of robot arm joints is shown in Fig. 8. The rotary piezo motor, usually called an ultrasonic-walking-wave motor, can be mounted directly in the joint, and makes actuation of the arm joint very simple.
(2)
Fig. 8. Fig. 6.
Ultrasonic-walking-wave rotary motor
Sequence of action A piezoelectric smart rotary motor (Fig. 9) consists of: a piezoceramic ring a metallic stator a rotor consisting of a rub covering and a metal part a middle axle, fastening and housing
There are two major requirements for linear motors: very high walking or travelling velocity holding stiffness in order to meet the demand for accuracy
.2.5
Fig. 9. Fig. 7.
Linear piezo motor (Koster, 1994)
Rotary piezo motor (Jendritza, 1994)
The stator consists of a specially shaped brass part, which is glued together on one side with a piezoceramic ring, with a number of segment electrodes of the opposite polarisation and with
The walking velocity of a linear piezo motor depends on the extension magnitude and the working
181
sensor electrodes (Fig. 10). With adequate electronic control of the piezo motor, an elastic wave on the other side of the stator is generated. With the help of this wave and of a rub covering which pushes on the rotor, a rotary motion can be achieved.
piezoelectric grippers could be also used very effectively in quality control and inspection operations, because the gripper itself represents an intelligent actuator and sensor for position and force (or torque) simultaneously. Their advantage here is that they are small and compact and don't need to be equipped with additional sensors and electronics for use in control and inspection operations.
The main advantages of the rotary piezo motor are its high torque, small size, high holding torque, and ability to control the position without additional sensors.
5. CONCLUSION sensor electrodes
In this paper several ideas were given about new possibilities of actuation of robot arm joints. Piezoelectric smart actuators which have an actuator and a sensor in one body seem to be a possible alternative to the present actuators used in robotics. A great advantage that the new actuators offer is that they need almost no additional position and force sensors. In particular, a linear piezo motor could be very appropriate for actuation of grippers and of larger arm joints, because it can carry very high loads if adequately constructed.
Fig. 10. Principle of rotation
REFERENCES
4.2 Robot grippers Boothroyd, G., (1992). Assembly Automation and Product Design, Marcel Dekker, Inc., New York,1992. Brand, S., Laux, T., (1990). Einsatzmoglichkeiten und -bereiche von piezokeramischen Aktoren, VDINDE-Technologiezentrum Informationstechnik, Berlin, 1990. Deichambre, A., (1992). Computer-aided Assembly Planning, Chapman & Hall, London, 1992. Herakovic, N., (1996). Die Untersuchung der Nutzung des PiezoafJektes zur Ansteuerung fluidtechnischer Ventile, Verlag Mainz, Aachen, 1996. Jendritza, D. J., (1994). Einsatzbereiche und Designkriterien fUr Festkorperaktoren mit piezoelektrischen Keramiken und magnetostriktiven Seltenerdmetall-Eisen-verbindungen, Fachseminar, Magdeburg, 1994 Koster, M. P., (1994). A walking piezo motor, ACTUATOR 94, Bremen, 1994. McKerrow, P. J. (1991). Introduction to Robotics, Addison-Wesley, Singapore Seering, W. P. and Scheinman, V. (1985). Mechanical Design of an Industrial Robot. In: Handbook of Industrial Robotics (Nof, S. Y. (Ed», pp. 29-43. John Willey & Sons, New York.
Industrial robot grippers are generally single degree of freedom systems operated in an open control loop, or are else designed to a custom shape for a certain workpiece. A power for grip motion is provided by various types of actuators, such as electrical, hydraulic, pneumatic, etc. Recently, many actuators have included built-in position and velocity sensors. With classical grippers, problems can occur when the object to be grasped is not placed midway between the fmgers; it is then possible that excessive force could be applied to the object. Because of this, the grasping force must be measured with an additional sensor located in the gripper's jaws. There are also additional sensors used, such as an ultrasonic sensor in the palm for detection of the object before it is grasped. Because of all these conditions and requirements, classical grippers are very expensive, complex and huge. The linear piezo motor with smart piezoactuators described could be especially suitable for implementation as an actuator in the gripper. Using an appropriate control along with amplifier electronics, the position of the gripper as well as the gripping force could be exactly determined without using additional sensors. At the same time, the gripper could be minimised and the cost reduced. Another very important characteristic of piezoelectric grippers is the so-called fail-safe function. In the event of a sudden power outage, they are able to maintain their position and provide a constant and secure grip on the handled object. The new
182
SMART ACTUATORS IN ROBOTICS
Niko
Herakovi~
and Tomaf Perme
University ofLjubljana, Faculty of Mechanical Engineering, Laboratory for Handling, Assembly and Pneumatics
Abstract: A piezoelectric actuator is an example of alternative actuators, which are successfully implemented in many applications today. The main advantages of these actuators are their very high dynamics (up to 40 kHz), theoretically unlimited resolution (in the field ofnanometers), low consumption of electrical energy, high force and very compact construction. One very important advantage that piezoactuators can offer is the possibility of having the actuator, force sensor and position sensor contained in a onepiece unit. Such a piezoactuator is called a "smart actuator". The main problem with implementation of these actuators is related to very small oscillating movements caused by its expansion and contraction, which have to be transferred into continuous movement. In this paper, the characteristics of "smart actuators" are presented and their potential, as well as some ideas for their useful implementation and utilisation of the grippers and robots for assembly and disassembly, are discussed. Copyright© 1998lFAC
Keywords: smart piezoactuators, robots, robot arms, grippers, sensors
1. INTRODUCTION
actuators very complicated. Hydraulic drives are very appropriate and often used for robotic drives, but their cost is high.
In order for robots to become more efficient, they need to be continuously improved. In addition to economic and technical aspects, nowadays the ecology is an ever greater consideration in the development and implementation of new technologies and ideas.
These are the main reasons why actuation is one field where new technologies and ideas are constantly needed. Faster, smaller and more powerful are the demands which propel the world of actuation.
Power for robot motion is provided by various types of actuators, such as electric, hydraulic, pnewnatic, etc. Many robot manufacturers usually use electric motors of two types: stepper and direct current (DC) motors. Electric actuators are fast and accurate, but very weak or unpleasantly heavy because of their complexity.
One alternative idea in this world is a piezoelectric actuator. The piezo effect is well known for sensing, but more and more solutions have been developed so that this effect may also be used for actuation. In robot arms piezoactuators are already being very successfully used to compensate for the bending of arms (Brand, and Laux, 1990). It is generally known that supporting elements in mechanical systems (e.g. robot arms) possess some limited stiffness both dynamically and statically and, because of that, tend towards bending. As a consequence, inaccuracies appear in the positioning process. The stiffness could
Pneumatic actuators are mostly used for opening and closing of grippers. Despite the large quantity of energy per unit mass that gas can store, its compressibility makes servocontrol of pnewnatic
177
simply be increased with more stable construction of the robot arm, but this step is connected with increasing its own mass. Another way to increase a robot arm's stiffness without increasing its mass is to use active compensation for bending by using an adequate controller and piezoactuators, as shown in Fig. I.
of products are assembled along three perpendicular axes (up to 59% are assembled in one direction) ~e
requirements for robots used for assembly and disassembly are different than those used for welding or painting. A robot's main functional features are: numb~r of degrees of freedom (DOF); speed, e.g. velOCity of tool centre point (TCP); accuracy and repeatability; load capacity (payload); working space (working volume); and the possibility of including external sensors.
piezo actuators ./4
robot arm
i=='==~::=:I
The following requirements for an industrial robot are of major importance. In assembly applications ~ost .operations, such as placing of parts (40%), Insertion (30%) and fastening (24%), required simple movements that can be accomplished by robots with less than 6 DOF.
- ...............•
\\ CCD-array
controler
Load ca!'acity: In the case of assembly robots, acceleratIon and structural stiffness are more important design parameters than peak velocity or maximum load capacity, as minimising small motion times is generally a top priority (Seering, 1985).
Fig. I. Active compensation for bending of robot arms using piezoactuators For detecting the bending of a robot arm under a load, a laser measurement system is used. The aberration from the given position is detected with a C.CD sensor and actively compensated for by pIezoactuators and a PID controller. The use of piezoactuators in such compensative systems makes it possible to achieve higher positioning accuracy and shorter positioning times.
Velocity and acceleration: To reduce the cycle time of assembly operations, movements have to be carried out as quickly as possible. Acceleration affects gross motion time as well as cycle time. A general design guideline for robots is that most motions should be performed with the system at its velocity limit part of the time. Accuracy, resolution and repeatabi/ity: Robot accuracy is important for non-repetitive tasks. A typical accuracy standard for manipulators with precisely manufactured and measured kinematic elements and simple accurate models is up to 0.01 mm. More important for assembly and disassembly operations is repeatability of the manipulator. For very precise micropositioners it is 0.005 mm, but the typical industrial robot has a standard of 0.025 mm.
~ecently,
some experiments were made using plezoactuators for micro- and minirobots. The aim of this paper is to launch into the public eye these ideas along with some potential solutions to related problems in the field of assembly and disassembly.
2. ROBOTS FOR ASSEMBLY AND DISASSEMBLY
Actuators and Sensors: The main characteristic for actuators is power-to-size ratio. More recently, many actuators have included built-in position and velocity sensors, which are needed to measure variables within the robot and in the environment. Sensors are normally mounted on the motor shaft and not on the joint shaft.
The ideal robot - characterised by rigidity, minimal weight, as quick a response as possible, a maximal number of degrees of freedom, high speed, good repeatability and good accuracy - is unrealistic to build. Therefore, it is necessary to make a few concessions and to consider assembly's specific feat,ures. Several studies have stressed the following basiC features of most electromechanical assemblies (Delchambre, 1992):
Transmission: Power has to be transmitted from the actuator to the linkage. Typical transmission devices such as gears, tendons and linkage contain many disadvantages. The transmission elements can be eliminated by using a direct drive system, which has no backlash, low friction, low compliance, high reliability and a fast dynamic response, but is, however, more difficult to control.
of the parts to be moved weigh less than I kg the workspace is relatively small (projection of the volume required to assemble a product is 2 smaller than 400 cm )
178
energy transducer - piezo actuator
----------
I
I~-" I
I I
transducer I Cp completition '~;
'''' compensation of ___x_(t_) -.-loadinfluences ~_x_'(_t)_~electronic power amplifier and hysteresis actuator control signal
I I
TV F l 2
2
I
_____ 1
i(t)
nonlinear parameter
Fig. 2.
hysteresis model
u(t)
transducer output
The principle of a smart actuator
3. SMART ACTUATORS
The current i(t) and the voltage u(t) are measured at the electrical gate of the transducer and transferred to the digital signal processing unit. The signal processing unit calculates, on the basis of non-linear transducer and hysteresis models, the parameters of an inverse filter which idealises the input-output transfer behaviour of the actuator. Special attention should be paid to the fact that mechanical parameters, such as the expansion velocity v and the force F at the transducer output and at its completion, do not need to be known because they are calculated with a transducer model.
Piezoactuators consist, as do all actuators, of an electronic power amplifier and an electromechanical energy transducer, connected in a series. Many frequently used energy transducers can be described as three-gate or two-gate, within the framework of four-pole theory. This presentation of the phenomenon represents a basic tenet in the manufacture of so-called smart actuators, in which a signal processor calculates, with the aid of a suitable transducer model, output-stroke and input-current characteristics as well as force-voltage-transfer function, usable for sensing.
Another possibility of creating a kind of smart piezoactuator is shown in Fig. 3. This new class of devices stems from the combination of acting and sensing (in a monolithic device). The entire actuator is made from the piezoelectric ceramic material lead-
This type of smart actuator is based on transducer materials which possess sensor and actor characteristics simultaneously and represent a mechatronic subsystem. With such an actuator it is possible to exactly determine the output stroke and/or output force without the use of additional sensors for stroke and force. In this sense, piezoactuators are very appropriate for use as smart actuators.
ceramic body sensor
actuator
In a technical application, electric current and voltage have to be measured at the electrical gate of the transducer so the force and the positioning velocity at the transducer output gate may be determined. While in previous applications only the current (and sometimes also the voltage) has been wired to the electrical transducer input, a smart actuator has been used for all disposed control signals. Using this additional information, a microprocessor-controlled signal can turn a transducer into a sensor. The principle of a smart actuator (piezoelectric actuator) is shown in Fig. 2.
isolation material
Fig. 3. Smart piezoactuator - an actuator and sensor in one piece zirconate-titanate (PZT). This is one of the best materials to use when considering the energy density and conversion factors, based on ferroelectric ceramics which have a perovskite structure. With an adequate electrode structure, the main part of the
179
ceramic body functions as an actuator while a small fraction is used for sensing. This may result in an actuator with improved properties, e.g. linearity, if the sensor signal is fed back. With the use of suitable electronics it is also possible to measure the output force and the strain of the piezoactuator. Materials and devices which function in this manner are called 'smart'.
In spite of the advantages that smart piezoactuators possess, there is a great disadvantage which makes using them for direct actuation of a robot arm joint impossible - a very small strain or extension of the piezoactuator. Normally, piezoactuators can achieve an extension of about 0.1 to 0.15% of their length. Such small extension of about 40 to 60 Jlm could perhaps be effectively used for direct actuation in micro robots, while in larger robots other solutions should be found to enlarge the extension of smart piezoactuators.
Because the dynamics, positioning accuracy and efficiency of the entire piezoactuator system are decisively influenced from the electronic power amplifier and from control electronics, close attention has to. be paid to choosing the right electronics (Herakovit, 1996).
Linear piezoelectric smart motor: Usually all joints of a robot arm, wrist and gripper are rotary or revolute joints. In Fig. 4 the principle is shown which illustrates how a linear piezo motor, using smart piezoactuators, could be used for the actuation of robot arm joints. This principle is very well known and already used in practice.
Basically, there are two ways of controlling the piezoactuator: voltage and charge control. Voltage control is simple to achieve but the strain of the piezoactuator, as a function of the driven voltage, shows considerable evidence of hysteresis and nonlinearities. Charge control, on one hand, requires more complex circuitry; on the other hand, it offers almost linear dependence between the output strain of the piezoactuator and the loading charge at the input.
smart piezo actuator\
4. SMART ACTUATORS IN ROBOTICS Its main advantages, such as very high dynamics (up to 40 kHz), theoretically unlimited resolution (in the field of nanometers), low consumption of electrical energy, high force, very compact construction and the possibility of acting as a smart actuator, make piezoactuators very suitable for the actuation of grippers and robot arm joints. Using an appropriate control and amplifier electronics, the position of the robot arm or the gripper, as well as the gripping and carrying force, would be able to be exactly determined without the use of additional sensors. At the same time, the arm joints and the gripper could be minimised and the cost reduced. The main question that here presents itself is: How can smart piezoactuators be used in robotics?
Fig. 4. Principle for the actuation of an arm joint with a linear piezo motor The linear piezo motor, usually called an inchworm or walking piezo motor, is driven by a few smart piezoactuators and is based on the walking principle (Fig. 5). y
•
X
!.._~
1 I
lA
4.1 Robot arm with joints and wrist
'\"~,i']~
cpA The motion of a robot arm, which consists of links and joints, differs from the motion of the human arm. Robotic joints can move through greater angles, but have fewer degrees of freedom. The elbow of an articulated robot can bend up or down, whereas a human can only bend an elbow in one direction with respect to the straight arm position. The greater range of motion available to joints in robot arms gives them greater flexibility than human arms, but this increased flexibility also increases the complexity of the systems used for the actuation ofjoints.
wpA
wpB
cpB
Fig. 5. Principle of walking Here wp are the walking piezoactuators. They expand and contract in the walking direction x. The clamping piezoactuators (cp) expand and contract in the y direction perpendicular to the walking direction. Altematingly, they clamp the A-side or the B-side of the motor to the frame. The output member is attached to the centre C.
180
frequency of the walking wp and clamping cp piezoactuators. The walking frequency should be kept far enough below the lowest natural frequency to maintain control of walking amplitude.
In Fig. 6 the sequence of action of the respective piezoactuators is shown. The time needed for one complete cycle is characterised as teIf clamp changing, which takes a certain amount of time (tch), happens during an interval of matching velocities of the piezoactuators wpA and wpB, constant velocity of the centre C will be achieved. As a consequence, the entire stroke h of the walking piezoactuators wp cannot be used. The effective stroke is
h' = Tlth
To adapt the characteristics of the piezoactuators to the target specifications of the linear motor, mechanical amplification can be used to enlarge the stroke of the walking piezoactuators. An example of a linear piezo motor with mechanical amplification of the walking piezoactuator stroke is shown in Fig. 7.
(I)
Rotary piezoelectric smart motor: The principle
where
which illustrates how a rotary piezo motor could be used for the actuation of robot arm joints is shown in Fig. 8. The rotary piezo motor, usually called an ultrasonic-walking-wave motor, can be mounted directly in the joint, and makes actuation of the arm joint very simple.
(2)
Fig. 8. Fig. 6.
Ultrasonic-walking-wave rotary motor
Sequence of action A piezoelectric smart rotary motor (Fig. 9) consists of: a piezoceramic ring a metallic stator a rotor consisting of a rub covering and a metal part a middle axle, fastening and housing
There are two major requirements for linear motors: very high walking or travelling velocity holding stiffness in order to meet the demand for accuracy
.2.5
Fig. 9. Fig. 7.
Linear piezo motor (Koster, 1994)
Rotary piezo motor (Jendritza, 1994)
The stator consists of a specially shaped brass part, which is glued together on one side with a piezoceramic ring, with a number of segment electrodes of the opposite polarisation and with
The walking velocity of a linear piezo motor depends on the extension magnitude and the working
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sensor electrodes (Fig. 10). With adequate electronic control of the piezo motor, an elastic wave on the other side of the stator is generated. With the help of this wave and of a rub covering which pushes on the rotor, a rotary motion can be achieved.
piezoelectric grippers could be also used very effectively in quality control and inspection operations, because the gripper itself represents an intelligent actuator and sensor for position and force (or torque) simultaneously. Their advantage here is that they are small and compact and don't need to be equipped with additional sensors and electronics for use in control and inspection operations.
The main advantages of the rotary piezo motor are its high torque, small size, high holding torque, and ability to control the position without additional sensors.
5. CONCLUSION sensor electrodes
In this paper several ideas were given about new possibilities of actuation of robot arm joints. Piezoelectric smart actuators which have an actuator and a sensor in one body seem to be a possible alternative to the present actuators used in robotics. A great advantage that the new actuators offer is that they need almost no additional position and force sensors. In particular, a linear piezo motor could be very appropriate for actuation of grippers and of larger arm joints, because it can carry very high loads if adequately constructed.
Fig. 10. Principle of rotation
REFERENCES
4.2 Robot grippers Boothroyd, G., (1992). Assembly Automation and Product Design, Marcel Dekker, Inc., New York,1992. Brand, S., Laux, T., (1990). Einsatzmoglichkeiten und -bereiche von piezokeramischen Aktoren, VDINDE-Technologiezentrum Informationstechnik, Berlin, 1990. Deichambre, A., (1992). Computer-aided Assembly Planning, Chapman & Hall, London, 1992. Herakovic, N., (1996). Die Untersuchung der Nutzung des PiezoafJektes zur Ansteuerung fluidtechnischer Ventile, Verlag Mainz, Aachen, 1996. Jendritza, D. J., (1994). Einsatzbereiche und Designkriterien fUr Festkorperaktoren mit piezoelektrischen Keramiken und magnetostriktiven Seltenerdmetall-Eisen-verbindungen, Fachseminar, Magdeburg, 1994 Koster, M. P., (1994). A walking piezo motor, ACTUATOR 94, Bremen, 1994. McKerrow, P. J. (1991). Introduction to Robotics, Addison-Wesley, Singapore Seering, W. P. and Scheinman, V. (1985). Mechanical Design of an Industrial Robot. In: Handbook of Industrial Robotics (Nof, S. Y. (Ed», pp. 29-43. John Willey & Sons, New York.
Industrial robot grippers are generally single degree of freedom systems operated in an open control loop, or are else designed to a custom shape for a certain workpiece. A power for grip motion is provided by various types of actuators, such as electrical, hydraulic, pneumatic, etc. Recently, many actuators have included built-in position and velocity sensors. With classical grippers, problems can occur when the object to be grasped is not placed midway between the fmgers; it is then possible that excessive force could be applied to the object. Because of this, the grasping force must be measured with an additional sensor located in the gripper's jaws. There are also additional sensors used, such as an ultrasonic sensor in the palm for detection of the object before it is grasped. Because of all these conditions and requirements, classical grippers are very expensive, complex and huge. The linear piezo motor with smart piezoactuators described could be especially suitable for implementation as an actuator in the gripper. Using an appropriate control along with amplifier electronics, the position of the gripper as well as the gripping force could be exactly determined without using additional sensors. At the same time, the gripper could be minimised and the cost reduced. Another very important characteristic of piezoelectric grippers is the so-called fail-safe function. In the event of a sudden power outage, they are able to maintain their position and provide a constant and secure grip on the handled object. The new
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