EP0087414A1

EP0087414A1 - Method and means for determination of the position of a work arm

Info

Publication number: EP0087414A1
Application number: EP19810902573
Authority: EP
Inventors: Per Göran FAXÖ
Original assignee: Individual
Current assignee: Individual
Priority date: 1981-09-02
Filing date: 1981-09-02
Publication date: 1983-09-07
Also published as: WO1983000834A1

Abstract

Procédé de détermination de la position d'un robot ou d'un bras manipulateur ou autre de manière à ce que la détermination de la position soit totalement indépendante des déformations des matériaux et du jeu pouvant avoir lieu pour de grandes charges s'exerçant sur le bras. Ceci est effectué en détectant la position du bras robotisé ou manipulateur (1) par rapport à un bras indépendant non chargé (2), dont la position peut être déterminée de manière précise et qui est agencé en association au bras de travail (2). L'invention concerne également un bras manipulateur ou de robot conçu pour une telle détermination de position.Method for determining the position of a robot or a manipulator or other arm so that the determination of the position is completely independent of the deformations of the materials and of the play which can take place for large loads acting on the arms. This is done by detecting the position of the robotic or manipulator arm (1) relative to an unloaded independent arm (2), the position of which can be determined precisely and which is arranged in association with the working arm (2). The invention also relates to a manipulator or robot arm designed for such a position determination.

Description

Method and means for determination of the position of a work arm

The present invention relates to a new method of determining the position of a work arm, such as a robot or manipulator arm, with high accuracy independently of the load on the work arm. The invention also relates to a work arm, the position of which is determined in this manner. An industrial robot generally comprises a work arm which is movable in one or more degrees of freedom and which in the free end thereof is provided with a grab means or the like for performing a desired operation, e.g. picking, lifting/moving etc. The work arm may consist of several arm members which are pivotally or slidably connected to each other. The position of said grab means is monitored by suitable sensing means, which are arranged at the respective connecting joint. For example, the angle between mutually pivoting , arm members may be determined with an angle sensor arranged at the connecting joint. A location or position determination performed in such manner, however, disregards plays and material deflections in the robot arm itself. In case of longer robot arms and great handling weights the precision of the grab means will hereby be so reduced that operations requiring greater precision, e.g. various processing operations, are rendered difficult. Various approaches have been made to manage this problem.

For example, the U.S. patent application 4,119,212 describes a method of eliminating the effect of load deformations of a robot arm having a pivotal joint. In order to determine the location of the robot hand two linking elements of known lengths interconnected through a free-floating pivotal joint are used, the free ends of which elements are pivotally coupled to the robot arm hand and mounting end, respectively. By sensing the angle between the two linking elements and the position of the rear linking element, respectively, with suitable sensors the location of the work hand may be determined. Since the linking elements do not support any load the determination will be independent of any deformations of the very robot arm through great loads. This method of position determination has, however, several limitations and may in practice hardly be applied to pivoting in more than one plane.

According to the invention there is therefore suggested a new and relatively uncomplicated method of determining the position of a work arm with high accuracy totally independently of the load on the arm and, which can be used for most types of robots or manipulators - and also for other kinds of work arms - without any restriction as to the mobility of the arm and the number of joints. This is made possible by relating the position of the work arm to an ^^ rl xxYζ O PI unloaded position determining arm which is substantially independent of the work arm and arranged in connection with the work arm. By accurately determining the position of the unloaded arm and simultaneously determining the relative distance between the unloaded arm and the work arm near the free end portions of the two arms a correspondingly accurate position determination for the operative end or hand of the work arm is obtained. This position determination is thus totally independent of the plays and material deflections which may occur in the loaded work arm.

The method of the invention may advantageously be used in all kinds of loaded work arms, where it is necessary to determine the position of the operative end portion of the work arm with great precision. It is, however, particularly suitable for such work arms as robots or manipulators.

In a robot or manipulator the unloaded arm is arranged such that the work ar substantially follows the movements of the unloaded arm, or vice versa, said arms having separate drive systems. The unloaded arm may possibly be wholly or partially arranged outside τiie work arm, but it is preferably arranged within the same and suitably has the corresponding number of joints or pivotal sites as the work arm or the loaded arm.

The relative position determination between the loaded and unloaded arms may be made in various ways. Suitably, it is effected by appropriate means arranged in the respective free end portions of the arms which permit an accurate determination of the distance between defined parts of said end portions in the necessary directions. Such devices or systems for sensing small position changes between parts which are movable relative to each other are known per se and may, e.g., be based upon measuring units or sensors of the contact-free as well as non-contactfree type. As examples inductive sensors, magnetic sensors, such as, e.g. field-plates (Hall generators), pneumatic sensors, mechanical sensors, such as strain gauges, hydraulic balancing valves, optical sensors, and the like may be mentioned. The position sensing may be arranged such that either the loaded arm o the unloaded arm is the measuring arm, i.e. that the loaded arm senses its position in relation to the unloaded arm, or vice versa. A resulting differentia signal may then be utilized to move one of the two arms, such that a predetermined mutual blancing position of the arms is reset. By continuously relating the position of the loaded arm to the position of the unloaded arm, a mentioned above, a position determination of the work arm or the loaded arm i obtained which is not affected by material deflection or plays. The adaptation of the movements of the two arm systems in relation t each other may be effected with known control circuits for the drive means o the two arm systems, which drive means, for example, may be electric o hydraulic. The loaded as well as the unloaded arm may function as the drivin arm, i.e. the arm which is directly actuated to perform the desired movements and to which - in case of a robot arm - the program unit is connected. In a robo or manipulator according to the invention there are thus two separate control o servo-systems, which suitably are stabilized in relation to each other in per s known manner. In the following the invention is described with regard to a particula embodiment with reference to the accompanying drawings, wherein

Figure 1 is a diagrammatical side-view elevation of a robot ar according to the invention,

Figure 2 is a diagrammatical perspective view of the position deter mining part of the device shown in Fig. 1,

Figure 3 is a principal diagram of a servo-system for the slave-unit o the device shown in Fig. 1, and

Figure > is a principal diagram of a servo-system for the master-unit o the device shown in Fig. 1. The robot shown in Fig. 1, which is designed to be mounted to a floor, wall or the like, comprises an outer robot arm 1 and a substantially completel self-supporting inner robot arm or master robot 2 therein. The outer robot arm 1 constitutes the loadable work arm of the robot and is therefore dimensioned with regard thereto, while the inner robot arm 2 only is intended to be a "measuring arm" which is not loaded and thereby may have considerably thinner dimensions.

The outer robot arm 1 is composed of three arm parts, viz. a base part 3, an intermediate part k and an outer part 5, at the end of which a wrist 6 is mounted which optionally may be pivotal in several degrees of freedom. . The wrist 6 is provided with suitable means for the work moments to be performed by the robot, e.g., a grab means. The base part 3 is pivotally attached to a bottom plate

7 such that it may be rotated about its longitudinal axis, i.e. in the plane of the plate 7. The intermediate part 4 is at one end thereof pivotally mounted to the base part 3 for rotation about .an axis of rotation A. In its opposite end the intermediate part is pivotally connected in the same way to the end of the outer part 5 opposite to the wrist 6 to permit rotation about an axis of rotation

B. The robot wrist 6 shown in the Figure may thus be caused to occupy a desired position by rotation of the arm parts .and 5 about the axes A and B, respectively, and by rotation of the robot arm about its "longitudinal axis" via__ the base part 3. The desired pivotal movements are effected by means of suitable, e.g. hydraulic or electric, actuators arranged in suitable manner at the respective pivotal joints. Said actuators are controlled by a first servo-system, as will be described further below. The inner unloaded robot arm or master robot 2 (which for the sake of clearness is in drawn in dashed lines in the Figure) is, in similar manner to the outer robot 1, in the shown case composed of three pivotally connected arm members, viz. a base part 8, an intermediate part 9 and an outer part 10. The base part 8 is, in the same way as the base part 3 of the outer robot 1, pivotally mounted about its longitudinal axis in a mounting portion 7a (possibly a part of the mounting plate 7) which in turn is attached to the floor, the wall etc., preferably totally independent of the attachment of the outer robot 1. At its other end the base part 8 is pivotally connected to the intermediate part 8 through a pivotal joint . C, and the intermediate part 9 is at its opposite en pivotally connected to the outer part 10 through a pivotal joint D. The axes of rotation A and B of the outer robot may possibly pass through centre holes i the joints 11a and lib, respectively. The inner robot arm 2 is provided wit suitable drive means, e.g. hydraulic or electric ones, to accomplish the desire rotation at the above mentioned pivotal joints. Further, sensors 11a, lib and lie e.g. potentiometers, are arranged at the pivotal joints for sensing the relativ angular position of the arm parts. The control of said drive means is effected b means of a second servo-system, as will described further below. The free en 10a of the outer arm part 10 of the master robot 2 is in suitable manne surrounded by three pairs of sensors 12a - 12c arranged inside the arm part 5 o the outer robot 1. In the shown case the end portion 10a together with th sensors 12a - c form a contact-free three-axled balancing device. Thus, th sensor pair 12a is arranged to sense and balance the rotation about the abov mentioned joint D, while the sensor pairs 12b and 12c in corresponding manne sense and balance the rotation about the joint A and about the longitudinal axi of the base part 8, respectively. An embodiment of the above mentione balancing device is shown in Fig. 2. The latter comprises an armature 13 of low remanence iron, which is arranged to be mounted at the free end of the arm par 10 of the inner robot 2, e.g. through bores 14 in a fore mounting portion 15 of th armature 13. Around the proper armature portion 16 the above mentioned senso pairs 12a - 12c are arranged in the form of six horseshoe-formed permanen magnets, at one branch end of which field plates 17a-c of Hall generator type ar arranged, i.e. thin plates .based upon semi-conductor material the output voltag of which in a magnetic field are dependent on the flux density. The horsesho magnets 12a-12c are, as mentioned above, attached to the inner side of the en portion of the outer robot 1 and spaced somewhat, e.g. up to some centimeter to the armature portion 16. For reasons that will appear below the length of th inner robot 2 should be. adjusted such that the outer robot 1 cannot b straightened out completely.

The fundamental construction of the above mentioned servo-system fo the outer robot or slave robot 1 appears from Fig. 3, showing a partial servo system for one of the sensor pairs 12a-c corresponding to movement of the oute robot 1 in one of the sensing directions. The two other partial servo- systems fo movement of the robot arm 1 in the other sensing directions are, of course constructed in identical manner.

The shown partial servo-system comprises a per se conventiona measuring bridge IS having two field plates (Hall generators) 17 connected to constant current source 19. The field plates 17 are each connected to an input o an OP-amplifier 20. The output of the latter is connected to a second OP amplifier stage 21, which in turn is connected to a servo-control means 22 fo actuation of the drive means for rotation at the intended pivotal joint. If, fo example, the drive means in question is a hydraulic motor the servo-control mean 22 is a servo-valve. Further, a velocity sensor 24 (e.g. a tachometer) sensing th relative velocity of movement of the arm parts is connected to the input o amplifier. 22 for introducing a negative compensating or stabilizing signal. It i seen from Fig. 1 that, e.g., the sensor pair 12b is affected differently o rotation of the joint C depending on the rotational position of the joint D. I order that the signal from the measuring bridge 18 should be essentiall independent of the degree of extension of the work arm, a compensating signa controlling the amplification of the amplifier 21 is fed in suitable manner from central program unit 25 for the robot via a digital/analogue-converter 26a.

A corresponding principal diagram for the servo-system of the maste robot 2 is shown in Fig. 4. The above mentioned central program unit 25, whic contains the desired control program for the master robot 2, is connected to servo-amplifier 27 via a digital/analogue-converter 26b. The latter is in tur connected to servo-control means 28, which, e.g., are servo-valves when the drive means for effecting a desired rotation at the pivotal joints of the master robot consist of hydraulic motors. The hydraulics and mechanics actuated by the servo-valve 28 are represented by a block 29. The sensors at the three pivotal joints (11a, l ib, l ie) mentioned in connection with Fig. 1 are represented by an inductosyn measuring scale 30. The latter is for sensing connected to a phase/pulse unit 31 via an amplifier 32 and fed back to the phase/pulse-unit via

OMPI an amplifier 33 and a band-pass filter 34. The phase/pulse-convertor 31 is further connected to an Up/Down-counter 35, which in turn is connected to the central program unit 25 for reading. The reading is done periodically by means of a system clock 36. The industrial robot shown in the Figures operates in the following way.

The totally unloaded master robot 2 is, as mentioned above, controlled in a predetermined manner through the central program unit 25 (Fig. 4). The digital signals from the program unit 25 are converted in the D/A-converter 26b, amplified in the servo-amplifier 27 and fed to the servo-valve 28, which gives the desired actuation of the hydraulic motors at the pivotal joint or joints in question (C, D or the "rotational joint"). The angular position of the sensors 1 la,

- lib, lie arranged at the pivotal joints of the master robot 2 are indicated by the inductosyn measuring scale 30. The latter is sensed by the phase/pulse-unit '3i by feeding with a reference signal (e.g. 1-5 kHz), the angular value of the inductosyn measuring scale 30 determining the phase of the feedback signal. The phase value is converted into pulses which are counted in the counter 35. The program unit 25 periodically reads the counter 35 on signal from the clock 36 (e.g. 100 Hz), processes the read value and feeds the necessary control signals to the servo-amplifier 27 and the D/A-converter 26. . Any change of the relative position of the end portions of the outer arm

5 of the slave robot 1 and the outer arm 10 of the master robot 2 is sensed by one or more of the measuring bridges 18 (Fig. 3) in that the armature 16 is displaced in relation to the horseshoe magnets 12 (Fig. 2), the magnetic field of the field plates 17 in question being changed. The resulting differential signal is amplified in the OP-ampiifier 20 and led to another amplifier 21 together with the stabilizing signal from the velocity sensor 24. As mentioned above the amplification of the amplifier 21 is controlled in dependence of the degree of extension of the work arm through a compensating signal from the program unit 25 via the D/A-converter 26a. The output signal from the amplifier 21 passes to the servo-valve 22. The latter therr actuates the hydraulic motor or motors in question at the pivotal joints of the slave robot 1, such that the balance is restored in the measuring unit 18. The differential signal obtained from the measuring unit 18, which signal is essentially proportional to the movement of the master robot 2, may be processed by a separate microprocessor unit for maximum mutual stability of the two servo-systems to be obtained. Any movement of the inner robot arm or master robot 2 thus gives a corresponding movement of the outer robot or work robot 1. Since the free end 10a of the master robot (in the shown case the armature portion 16) in the balancing position always occupies a predetermined, accurately defined position o equilibrium at the end portion of the work arm 1, also the position of the fre end or wrist 6 of the work arm 1 will be determined with great accuracy, an totally independent of plays and material deflections or deformations of the wor arm. As mentioned above, this is of great importance where higher loads ar concerned, e.g. up to a ton or more, and high precision is required, such as direc processing operations. With the robot of the invention it is, for example, possibl despite such loads to achieve a precision for the wrist 6 of 0.05 mm or better fo an extended arm length of about 4 meters. The invention is, of course, not restricted to the embodiment speciall described above and shown in the drawings, but many modifications and change may be made without departing from the inventive concept. For example, th devices for the relative position determination of the loaded and the unloade arms may be designed in various ways using various types of sensors. Further, th control systems for the outer and inner robots may be arranged in other ways. O course, also the numbers of degrees of freedom of the work arm and the unloade arm as well as the constructive design thereof may be varied.

OMPI

Claims

1. A method of determining the position of a loadable work arm, such as a robot or manipulator arm, characterized by sensing the position of the work arm (1) in relation to an unloaded arm (2) which is independent relative to the movable parts of said work arm and whose position may be accurately determined, which unloaded arm is arranged in connection with the work arm (1).

2. A method of claim I, characterized in that one of the unloaded arm (2) and the work arm (I) controls the movement of the other, such that the two arms (1, 2) substantially accompany each other when the controlling arm is moved.

3. A method of claim 1 or 2, characterized in that the position determination is related to a predetermined balance position of the work arm (1) and the unloaded arm (2), and that an obtained deviation is utilized to move the work arm such that said balance position is restored.

4. A method of claim 2 or 3, characterized in that the unloaded arm (2) totally or partially is arranged within the work arm (1).

5. A method of any one of claims 1 - 4, characterized by^' sensing the relative position of the work arm (1) to the unloaded arm (2) at or near the free end portions of the arms.

6. A robot or manipulator device comprising a movable, loadable work arm (1) having at least one pivotal joint (A, B), characterized in that it for the determination of the position of the work arm (1) comprises a relative to the movable parts of the work arm (1) independent, unloaded arm (2) of substantially corresponding mobility and provided with means (lla-c) for accurate position determination, which unloaded arm is arranged in connection with the work arm (1), said unloaded arm (2) at or near its free end portion (10a) comprising means (13) arranged to cooperate with means (12a-c) in an adjacent part of the work arm (1) to permit determination of the relative position of the free end portion of the unloaded arm (2) to the work arm (1), and one of the work arm (1) and the unloaded arm (2) being arranged to control the movement of the other such that the two arms substantially accompany each other when the controlling arm is moved.

7. A device of claim 6, characterized in that the mutual control of the movements of the work arm (1 and the unloaded arm (2) is arranged such that a predetermined balance positio of the free end portion of the unloaded arm (2) and adjacent parts of the arm (1 is sought.

8. A device of claim 7, characterized in that the unloaded arm (2) totally or. partially is arranged withi the work arm (1).

9. A device of any one of claims 6 - S, characterized in that the unloaded arm (2) is arranged to control the movemen of the work arm (1).

10. A device of any one of claims 6 - 9, characterized in that the means (12a-c; 13) for determining the relative positio of the work arm (1) to the unloaded arm (2) comprise means for static or dynami measurement of changes of a generated magnetic field.

OMPI