A DEVICE AND A METHOD FOR CONTROLLING THE MOTION OF A PNEUMATIC ACTUATOR

Abstract: A method for controlling the motion of a pneumatic actuator (3), comprising at least one elongated cylinder (6) provided with a piston (4), dividing the cylinder (6) into a first chamber (5a) and a second chamber (5b), an actuating pressure supply line (14) connectable to at least one of said first and second chambers (5a, 5b), a control means (22a, 22b) for controlling the motion of the piston (4) relatively to the cylinder (6), and at least one sensor means (23, 30) adapted to sense a piston feature related to the piston motion and co-operating with the control means (22a, 22b). According to the invention a control device is provided with deterministically moving designs for the piston (4), a mathematical model (105, 105') in said control means (22a, 22b) for providing a control design system for controlling in a closed loop system a momentary movement of said piston (4) for each moving design based on said at least one piston feature signal, and generating a control signal (102) from said control means (22a, 22b) to move said piston (4) in accordance with an instant portion of the actual moving design.

Description: A device and a method for controlling the motion of a pneumatic actuator TECHNICAL FIELD OF THE INVENTION The present invention relates to a method for controlling the motion of a pneumatic actuator, according to the preamble of claim 1, and an apparatus to perform the method according to the preamble of claim 6. An apparatus making such control is known from Design and testing of a servo controller for pneumatic cylinders, 1989, Journal of Process Mechanical Engineering, p 21-27. A jerky motion of the pneumatic device and an unstable motion of the robot arm is not desirable while the robot arm must perform a smooth movement in three dimensions. The repetition of a closed-loop system implemented in a control means is according to known technique not satisfactory and has an inadequate precision. Known control means comprising algorithms implicated for performing a motion of a pneumatic device with a closed-loop do not include any satisfactory mathematical model with determined parameters obtained from data collected in an experiment. SUMMARY OF THE INVENTION This has been solved by a method as initially defined, which is characterized by providing for each elongated cylinder deterministically moving designs i.e. way of movement of the piston, providing a mathematical model in said control means for providing a control design system which controls in a closed loop system a momentary movement of said piston for each moving design based on at least one signal from the sensor means regarding the piston feature, and generating a control signal from said control means to move said piston in accordance with an instant portion of the actual movin" desi-m. Hereby the pneumatic actuator can be controlled in a smooth, even and exact movement. Preferably, said design system in said control means comprises an algorithm with a system identification technique adapted for an automatic external influence from the mathematical model. Hereby the pneumatic actuator can be controlled in a smooth performance and the parameters of the control signal can be adjusted automatically in case of altered parameters during normal operations, or after an installation of a new robot device or changing said pneumatic components. Suitably, said piston feature signal fed to the control means, comprises a position signal indicating the actual position of said piston, providing said mathematical method estimating at least one parameter in co-operation with a closed one-loop system in said control means, providing said model based control design system in said control means including an algorithm with a system identification technique adapted for an external influence from said mathematical model, said operation signal and said position signal, and generating said control signal from said first control means for said piston to be moved in accordance with the instant portion of said design system. Hereby the pneumatic actuator can be controlled in a smooth, even and exact performance according to the position signal and the parameters of the control signal can be adjusted automatically in case of altered parameters during normal operations, or after an installation of a new robot device or changing the pneumatic components. Preferably, said piston feature signal comprises a pressure difference signal indicating the actual pressure difference between said first and second chambers, providing said mathematical method estimating at least one parameter in co-operation with a closed two-loop system in said control means; providing a model based control design system in said control means including an algorithm with a system identification technique adapted for an external influence from said mathematical model, said operation signal and said piston feature signal, and generating said control signal from said second control means for said piston to be moved in accordance with the instant portion of said design system. Hereby the pneumatic actuator can be controlled in a smooth, even and exact performance according to the pressure difference signal and the parameters of the control signal can be adjusted automatically in case of altered parameters during normal operations, or after an installation of a new robot device or changing the pneumatic components. Suitably, comparing said position signal with said pressure difference signal by said control means, which comprises a comparing algorithm for generating a control signal depending on the actual piston feature. Hereby the pneumatic actuator can be controlled by the external operation signals by either said first piston feature signal or a second piston feature signal or both of said signals depending on which desirable operations the pneumatic actuator has to perform. Preferably, the control means is provided with determmistically moving designs; said control means comprises a mathematical model for providing a control design system for controlling in a closed loop system a momentary movement of said piston for each moving design based on said at least one piston feature signal; and said control means is adapted to move said piston in accordance with an instant portion of the actual moving design. Hereby said first piston feature signal can be fed to said model based control design system adapted in a one-loop control means. Suitably, said sensor means comprises a differential pressure transductor adapted to sense the pressure difference between the said first and second chambers and co- operating with said control means for controlling said piston relatively to said cylinder. Hereby both the first piston feature signal and the second piston feature signal can be fed to said model based control design system adapted in a two-loop control means. Preferably, a valve means is associated with said control means and connectable to said first and second chambers: and said valve means operates to control said piston relatively to said cylinder. Hereby the piston relatively to the cylinder can be controlled as the valve means controls the pressure into the chambers of the cylinder. Suitably, a first pressure supply line is connectable between said first chamber and said valve means, and a second pressure supply line is connectable between said second chamber and said valve means. Hereby the pneumatic pressure may have an effect on the piston relatively to the cylinder in at least one direction. Preferably, at least two pneumatic actuators are provided in an operating system, a co- ordinator means, common for the actuators, is associated with said control means in said control devices in the operating system in order to provide a coordinated operation of them. Hereby a robot arm can be controlled with a good controllability and in an efficient manner. Suitably, an apparatus for performing an animal related operation having said robot arm, wherein said robot arm is steered by at least two pneumatic actuators each comprising said control device for controlling the motion of said robot arm. Hereby is achieved a controllable motion of said robot arm. Preferably, said apparatus comprises a co-ordinator means for co-operating with said control device in each of the pneumatic actuators, and said co-ordinator means is adapted to control said control devices in a coordinated operation. Hereby the robot arm can be moved in a co-ordinated smooth, even and exact operation. Suitably, said robot arm is connected to a robot arm suspension means, which is furthermore hingedly connected to a support means, and said robot arm suspension means being arranged to said support means about an essential horizontal axis, and at least one of said control devices is arranged between said robot arm suspension means and said support means, for allowing said robot suspension means to perform a substantially pendulum movement about said essential horizontal axis (X-X). Hereby is achieved that the arm has good access to e.g. teats and that the risk is less for dirt, such as manure, to hamper the movement of mechanical parts, as the connection between the robot arm suspension means and the support means is arranged at a level substantially above said animal. Preferably, at least one of said control devices is arranged between said robot arm and said support means, for actively moving said robot arm in a substantially vertical plane. Hereby is achieved a controllable pendulum movement. Suitably, said robot arm is pivotally connected to said robot arm suspension means, for allowing said robot arm to perform a pivotal movement in relation to said robot arm suspension means. Hereby is achieved a movability of the robot arm in addition to said pendulum movement. Preferably, at least one of said pneumatic actuators is arranged between said robot arm and said robot arm suspension means, for actively moving said robot arm in a substantially sideward direction. Hereby is achieved a controllable pivotal movement. Suitably, said robot arm suspension means comprises a pivot means having a bar movable about a substantially horizontal axis, said bar being provided with a first connection member for said robot arm and a second connection member for said at least one of said pneumatic actuators, each of said first and second connection members being provided with a hinge movable about a substantially vertical axis. Hereby a joint for the pivotal movement is achieved. DRAWING SUMMARY The invention will now be described schematically with reference to the accompanying drawings, in which Figure 1 illustrates a control device according to a first embodiment of the invention, Figure 2 illustrates a control device according to a second embodiment of the invention, Figure 3 illustrates a co-ordinator means associated with three control devices for coordinating at least two of said control devices, Figure 4 illustrates an apparatus for performing an animal related operation arranged with the pneumatic actuators 3a, 3b, 3c. DETAILED DESCRIPTION OF THE INVENTION Referring to figure 1 a pneumatic device 2 comprises a pneumatic cylinder 6 provided with a piston 4 connected to a piston rod 8. A first pressure line 10 is connected to a first chamber 5 a of a cylinder 6 and to a servo valve 20. A second pressure line 12 is connected to a second chamber 5b of the cylinder 6 and to the servo valve 20. The servo valve 20 may be a 5/3 proportional flow control valve, and is controlled by a first control unit 22a controlling the valve 20. The valve 20 operates to control the movement of the piston 4 inside the cylinder 6. The servo valve 20 is connected to a pressure source 16 by a supply line 14 and is controlled by the unit 22a to feed the pressure of the source 16 to one or the other of the chambers 5a and 5b. A position sensor 23 is adapted to the cylinder 6 for sensing the position of the piston 4. The position sensor 23 provides a position signal 103 and is connected to the first control unit 22a, which furthermore is connected to the servo valve 20 and to a co- ordinator unit 100. An operation signal 101 is fed to the control unit 22a from the coordinator unit 100. The co-ordinator unit 100 is arranged to co-ordinate for example three pneumatic actuators 3 a, 3 b, 3 c mounted to a robot arm 38 of a milking robot. The robot arm will be further described bellow in connection with fig. 4. The output of the sensor 23 is fed to the control unit 22a, which for example can be an ordinary kind of computer, with an one-loop implication and which controls the operation of the pneumatic actuator 3. It is possible to have the position sensor 23 to directly indicate the speed of the piston 4, i.e. the position per unit of time. However, it is also possible to have the position as an output and provide the speed indication by for example the sampling rate of an analog-to-digital converter (not shown) converting the analog reading of the sensor 23 into digital values fed to the unit 22a. The control unit 22a comprises a mathematical model 105 with determined parameters obtained from data collected experimentally. This mathematical model is automatically activated by the algorithm arranged with a system identification technique. An operation signal 101 is fed to the control unit 22a from a co-ordinator unit 100, which will be described further bellow. The control unit 22a generates a control signal 102 to the servo valve 20 for controlling the actuator 3. It is to be noted that the pneumatic device shown in fig.1 is intended to be inserted as an element in a steering system, and that the algorithm therefore is dependent on the design of that system and therefore must be determmistically obtained. The position sensor 23 may be a linear potentiometer inside or outside the cylinder 6 or a linear inductive contactless sensor. Also may at least one optical incremental encoder, rotary potentiometer or rotary incremental or absolute encoder, adapted on a robot arm. be used, which will be further described bellow in connection with fig. 4. Figure 2 shows an example of a second embodiment of the invention, in which parts denoted with a reference sign correspond to parts of the first embodiment with the same reference sign. The protection device according to the second embodiment thus includes a differential pressure transductor 30, which is provided in a first connection line 31 between the two chambers 5 a and 5b, and in practice connects the first pressure supply line 10 and the second supply pressure line 12. Furthermore the second embodiment includes a control unit 22b, which is connected to the servo valve 20, to the position sensor 23 and to the differential pressure transductor 30. The transductor 30 is adapted between the first and second chambers 5 a. 5b of the cylinder 6 for sensing the pressure difference between the chambers. The transductor 30 provides a pressure difference signal 104 to the control unit 22 b. The outputs of the transductor 30 and the sensor 23 are fed to the control unit 22b, which for example can be an ordinary kind of computer, with a two-loop implication and which controls the operation of the pneumatic actuator 3. The control unit 22b comprises a mathematical model 105' with determined parameters obtained from data collected in an experiment. The mathematical model 105' is more complicated than the model 105 in the first embodiment, since it is dependent by two detected signals, the instant position of the piston and the pressure difference between the chambers 5a and 5b and gives a more defined movement indication for the control of the piston movement. Said operation signal 101 is fed to the control unit 22b from said co-ordinator unit 100 and a signal is of course fed from the co-ordinator unit 100 to the control unit 22b. The co-ordinator unit 100 operates to steer at least two pneumatic actuators in a coordinated smooth motion. This will further be described in fig.3 Said mathematical model 105' is automatically activated by the algorithm arranged with a system identification technique. The control unit 22b generates a control signal 102 to the servo valve 20 for steering the piston motion in said cylinder 6. Figure 3 shows an example of an embodiment wherein the co-ordinator unit 100 is connected to a first, a second and a third control device 2a, 2b, 2c. The co-ordinator unit 100 can for example be an ordinary kind of computer. The co-ordinator means provides operation signals 101a. 101b, 101c to the control devices 2a, 2b, 2c and operates to co-ordinate the control devices 2a, 2b, 2c for controlling for example a robot arm 38, to make a smooth, exact and even movement along an indicated path. This means that the movement control must be able to provide different speeds for the pistons in the individual pneumatic devices in dependence of the position of the arm along its movement along a path. Therefore, the mathematical models are individual for each pneumatic system. As shown in figure 4, which illustrates an embodiment of a milking machine in which the control devices described above is included, a robot arm 38 is connected to a robot arm suspension means 40. The robot arm suspension means 40 is hingedly connected to a support means 42 and is arranged to said support means 42 about a horizontal axis (X-X). Said robot arm (38) is steered by said actuators (3) comprising said control device (2) The support means 42 is connected to a railing 50 of an animal stall 60. The robot arm 38 is pivotally connected to the robot arm suspension means 40. A first pneumatic actuator 3 a is arranged between the robot arm suspension means 40 and the support means 42. A second pneumatic actuator 3b is arranged between the robot arm 38 and said support means 42. A third pneumatic actuator 3 c is arranged between the robot arm 38 and the robot arm suspension means 40. Furthermore, the robot arm 38 is connected to the suspension means 40 via a pivot means 44, including a tubular member 47. movable about a pair of hinge members 46a, 46b, the tubular member 47 forming a substantially horizontal axis A-A. and furthermore via said third pneumatic actuator 3c connected to the tubular member 47 of the pivot means 44. The first, second and third pneumatic actuator 3 a, 3b, 3c could be connected to a common control unit (not shown) or separately to its own special control units 22a, 22b (shown in respectively fig.l and 2). The control units 22a, 22b furthermore can be connected to a co-ordinator unit 100 (not shown), which, for example, may be an ordinary kind of computer. At least one optical incremental or absolute encoder, rotary potentiometer or rotary incremental encoder, adapted on a robot arm. may be used (not shown) separately or in combination with the position sensor. OPERATION The control device according to the invention can be arranged for controlling many different functions. It is an advantage to combine a position sensor means 23 with a differential pressure transductor 30 as the transductor 30 can give the robot arm a sensitivity for detecting obstacle or if, for example, the animal comes in a wrong position. The force applied to the robot arm is controllable. For example, for teat cleaning, the cleaner can be pushed towards the udder of the animal with a controllable force. An animal to be milked, such as a cow, has normally different length on the teats. Obviously, the differential pressure transductor 30, as illustrated in figure 2, may be used as an alternative to or in combination with the position sensor 23. For example, when a teat cup (not shown) connected to the robot arm 38 is moved towards a teat of the animal, the teat cup is attached onto the teat in response to said differential pressure transductor 30 for permitting a safe motion towards the teat. Consequently this function is comfortable for the animal and the teat cup do not press on the udder of a animal if the teat to be milked is shorter than a normal size of the teat. Furthermore, after the installation of a new robot, or changing pneumatic components the controller shall be adjusted according to the new installed robot system. By means of the automatic parameter tuning of the controller, the parameters for controlling the control device according to the invention, can be adjusted automatically. This automatic parameter tuning operates to adjust the control signal, even if changes of the position features of the piston arises caused by wear and tear in valves or cylinders, altered data caused by friction, change of temperature e.t.c. By means of the algorithm implemented in the control means according to the invention, the pneumatic actuators 3 can be controlled in a smooth motion and have a good precision. Thus the algorithm implies that the robot arm can be moved in a smooth motion.

Claims: CLAIMS 1. A method for controlling the motion of a pneumatic actuator (3), comprising at least one elongated cylinder (6) provided with a piston (4), dividing the cylinder (6) into a first chamber (5a) and a second chamber (5b), an actuating pressure supply line (14) connectable to at least one of the first and second chambers (5a, 5b), a control means (22a, 22b) for controlling a motion of the piston (4) relatively to the cylinder (6), and at least one sensor means (23, 30) adapted to sense a piston feature related to the piston motion and co-operating with the control means (22a, 22b), characterized by providing for each elongated cylinder (6), deterministically moving designs i.e. way of movement of the piston (4); providing a mathematical model (105, 105') in said control means (22a, 22b) for providing a control design system which controls in a closed loop system a momentary movement of said piston (4) for each moving design based on at least one signal (103, 104) from the sensor means (23,30) regarding the piston feature; and generating a control signal (102) from said control means (22a, 22b) to move said piston (4) in accordance with an instant portion of the actual moving design. 2. A method according to claim 1, wherein said design system in said control means (22a, 22b) comprises an algorithm with a system identification technique adapted for an automatic external influence from the mathematical model (105, 105'). 3. A method according to claim 1, wherein said piston feature signal fed to the control means (22a), comprises a position signal (103) indicating the actual position of said piston (4); providing said mathematical method (105) estimating at least one parameter in co-operation with a closed one-loop system in said control means (22a); providing said model based control design system in said control means (22a) including an algorithm with a system identification technique adapted for an external influence from said mathematical model (105), said operation signal (101) and said position signal (103); and generating said control signal (102) from said first control means (22a) for said piston (4) to be moved in accordance with the instant portion of said design system. 4. A method according to anyone of the preceding claims, wherein said piston feature signal comprises a pressure difference signal (104) indicating the actual pressure difference between said first and second chambers (5a, 5b); providing said mathematical method (105) estimating at least one parameter in co-operation with a closed two-loop system in said control means; providing a model based control design system in said control means (22b) including an algorithm with a system identification technique adapted for an external influence from said mathematical model (105), said operation signal (101) and said piston feature signal (103, 104); and generating said control signal (102) from said second control means (22b) for said piston (4) to be moved in accordance with the instant portion of said design system. 5. A method according to claim 4, wherein comparing said position signal (103) with said pressure difference signal (104) by said control means (22b), which comprises a comparing algorithm for generating a control signal depending on the actual piston feature. 6. A control device for controlling the motion of a pneumatic actuator (3), comprising at least one elongated cylinder (6) provided with a piston (4), dividing the cylinder (6) into a first chamber (5a) and a second chamber (5b), an actuating pressure supply line (14) connectable to at least one of said first and second chambers (5a, 5b), a control means (22a, 22b) for controlling the motion of the piston (4) relatively to the cylinder (6), and at least one sensor means (23, 30) adapted to sense a piston feature related to the piston motion and co-operating with the control means (22a, 22b), characterized in that said piston (4) is controlled by deterministically moving designs; said control means (22a, 22b) comprises a mathematical model (105, 105') for providing a control design system for controlling in a closed loop system a momentary movement of said piston (4) for each moving design based on said at least one piston feature signal; and said control means (22a, 22b) is adapted to move said piston (4) in accordance with an instant portion of the actual moving design. 7. A device according to claim 6, wherein said sensor means comprises a differential pressure transductor (30) adapted to sense the pressure difference between the said first and second chambers (5a, 5b) and co-operating with said control means (22b) for controlling said piston (4) relatively to said cylinder (6). 8. A device according to claims 6-7, wherein a valve means (20) is associated with said control means (22a, 22b) and connectable to said first and second chambers (5a, 5b); and said valve means (20) operates to control said piston (4) relatively to said cylinder (6). 9. A device according to claims 6-8, wherein a first pressure supply line (10) is connectable between said first chamber (5a) and said valve means (20); and a second pressure supply line (12) is connectable between said second chamber (5b) and said valve means (20). 10. A device according to claims 6-9, wherein at least two pneumatic actuators (3) are provided in an operating system; a co-ordinator means ( 100), common for the actuators, is associated with said control means (22a, 22c) in said control devices (2) in the operating system in order to provide a coordinated operation of them. 1 1. An apparatus for performing an animal related operation having a robot arm (38), wherein said robot arm (38) is steered by at least two pneumatic actuators (3) each comprising said control device (2a, 2b, 2c) according to claim 6 for controlling the motion of said robot arm (38). 12. An apparatus according to claim 1 1, wherein said apparatus comprises a coordinator means (100) for co-operating with said control device (2a, 2b, 2c) in each of the pneumatic actuators; and said co-ordinator means (100) is adapted to control said control devices (2a, 2b, 2c) in a coordinated operation. 13. An apparatus according to claim 1 1, wherein said robot arm (38) is connected to a robot arm suspension means (40), which is furthermore hingedly connected to a support means (42), and said robot arm suspension means (40) being arranged to said support means (42) about an essential horizontal axis (X-X), and at least one of said control devices (2a) is arranged between said robot arm suspension means (40) and said support means (42), for allowing said robot suspension means (40) to perform a substantially pendulum movement about said essential horizontal axis (X- X). 14. An apparatus, according to claim 1 1 or 12, wherein at least one of said control devices (2b) is arranged between said robot arm (38) and said support means (42), for actively moving said robot arm (38) in a substantially vertical plane. 15. An apparatus, according to claims 13 or 14, wherein said robot arm (38) is pivotally connected to said robot arm suspension means (40), for allowing said robot arm (38) to perform a pivotal movement in relation to said robot arm suspension means (40). 16. An apparatus, according to claim 15, wherein at least one of said control devices (2c) is arranged between said robot arm (38) and said robot arm suspension means (40), for actively moving said robot arm (38) in a substantially sideward direction. 17. An apparatus, according to claim 16, wherein said robot arm suspension means (40) comprises a pivot means (44) having a bar movable about a substantially horizontal axis (A-A), said bar being provided with a first connection member (45) for said robot arm (38) and a second connection member (46) for said at least one of said control devices (2c), each of said first and second connection members (45, 46) being provided with a hinge movable about a substantially vertical axis.

Inventors: LIAO BOHAO, ROSANDER STEFAN, LIAO, BOHAO, ROSANDER, STEFAN
Applicants: ALFA LAVAL AGRI AB, LIAO BOHAO, ROSANDER STEFAN, LIAO, BOHAO, ROSANDER, STEFAN
Application year: 1999
Main classification: B25J9
IPC codes: A01J5, B25J9, G05B19, G05D16