| 1. |
- Axelsson, Patrik, 1985-, et al.
(författare)
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H∞-Controller Design Methods Applied to One Joint of a Flexible Industrial Manipulator
- 2013
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Control of a flexible joint of an industrial manipulator using H∞ design methods is presented. The considered design methods are i) mixed-H∞ design, and ii) H∞ loop shaping design. Two different controller configurations are examined: one uses only the actuator position, while the other uses the actuator position and the acceleration of end-effector. The four resulting controllers are compared to a standard PID controller where only the actuator position is measured. The choices of the weighting functions are discussed in details. For the loop shaping design method, the acceleration measurement is required to improve the performance compared to the PID controller. For the mixed-H∞ method it is enough to have only the actuator position to get an improved performance. Model order reduction of the controllers is briefly discussed, which is important for implementation of the controllers in the robot control system.
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| 2. |
- Axelsson, Patrik, 1985-, et al.
(författare)
-
H∞-Controller Design Methods Applied to One Joint of a Flexible Industrial Manipulator
- 2014
-
Ingår i: Proceedings of the 19th IFAC World Congress, 2014. - : International Federation of Automatic Control. - 9783902823625 ; , s. 210-216
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Konferensbidrag (refereegranskat)abstract
- Control of a flexible joint of an industrial manipulator using H∞ design methods is presented. The considered design methods are i) mixed-H∞ design, and ii) H∞ loop shaping design. Two different controller configurations are examined: one uses only the actuator position, while the other uses the actuator position and the acceleration of end-effector. The four resulting controllers are compared to a standard PID controller where only the actuator position is measured. The choices of the weighting functions are discussed in details. For the loop shaping design method, the acceleration measurement is required to improve the performance compared to the PID controller. For the mixed-H∞ method it is enough to have only the actuator position to get an improved performance. Model order reduction of the controllers is briefly discussed, which is important for implementation of the controllers in the robot control system.
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| 3. |
- Axelsson, Patrik, 1985-, et al.
(författare)
-
H∞ Synthesis Method for Control of Non-linear Flexible Joint Models
- 2013
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- An H∞ synthesis method for control of a flexible joint, with non-linear spring characteristic, is proposed. The first step of the synthesis method is to extend the joint model with an uncertainty description of the stiffness parameter. In the second step, a non-linear optimisation problem, based on nominal performance and robust stability requirements, has to be solved. Using the Lyapunov shaping paradigm and a change of variables, the non-linear optimisation problem can be rewritten as a convex, yet conservative, LMI problem. The method is motivated by the assumption that the joint operates in a specific stiffness region of the non-linear spring most of the time, hence the performance requirements are only valid in that region. However, the controller must stabilise the system in all stiffness regions. The method is validated in simulations on a non-linear flexible joint model originating from an industrial robot.
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| 4. |
- Axelsson, Patrik, 1985-, et al.
(författare)
-
H∞ Synthesis Method for Control of Non-linear Flexible Joint Models
- 2014
-
Ingår i: Proceedings of the 19th IFAC World Congress, 2014. - : International Federation of Automatic Control. - 9783902823625 ; , s. 8372-8377
-
Konferensbidrag (refereegranskat)abstract
- An H∞ synthesis method for control of a flexible joint, with non-linear spring characteristic, is proposed. The first step of the synthesis method is to extend the joint model with an uncertainty description of the stiffness parameter. In the second step, a non-linear optimisation problem, based on nominal performance and robust stability requirements, has to be solved. Using the Lyapunov shaping paradigm and a change of variables, the non-linear optimisation problem can be rewritten as a convex, yet conservative, LMI problem. The method is motivated by the assumption that the joint operates in a specific stiffness region of the non-linear spring most of the time, hence the performance requirements are only valid in that region. However, the controller must stabilise the system in all stiffness regions. The method is validated in simulations on a non-linear flexible joint model originating from an industrial robot.
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| 5. |
- Axelsson, Patrik, 1985-
(författare)
-
On Sensor Fusion Applied to Industrial Manipulators
- 2011
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- One of the main tasks for an industrial robot is to move the end-effector in a predefined path with a specified velocity and acceleration. Different applications have different requirements of the performance. For some applications it is essential that the tracking error is extremely low, whereas other applications require a time optimal tracking. Independent of the application, the controller is a crucial part of the robot system. The most common controller configuration uses only measurements of the motor angular positions and velocities, instead of the position and velocity of the end-effector.The development of new cost optimised robots have introduced unwanted flexibilities in the joints and the links. It is no longer possible to get the desired performance and robustness by only measuring the motor angular positions. This thesis investigates if it is possible to estimate the end-effector position when an accelerometer is mounted at the end-effector. The main focus is to investigate Bayesian estimation methods for state estimation, here represented by the extended Kalman filter (EKF) and the particle filter (PF).A simulation study is performed on a two degrees of freedom industrial robot model using an EKF. The study emphasises three important problems to take care of in order to get a good performance. The first one is related to model errors which in general requires better identification methods. The second problem is about tuning of the EKF, i.e., the choice of covariance matrices for the measurement and process noise. It is desirable to have an automatic tuning procedure which minimises the estimation error and is robust to initial conditions of the tuned parameters. A variant of the expectation maximisation (EM) algorithm is proposed for estimation of the process noise covariance matrix Q. The EM algorithm iteratively estimates the unobserved state sequence and the matrix Q based on the observations of the process, where the extended Kalman smoother (EKS) is the instrument to find the unobserved state sequence.The third problem considers the orientation and position of the accelerometer mounted to the end-effector. A novel method to find the orientation and position of the triaxial accelerometer is proposed and evaluated on experimental data. The method consists of two consecutive steps, where the first is to estimate the orientation of the sensor from static experiments. In the second step the sensor position relative to the robot base is identified using sensor readings when the sensor moves in a circular path and where the sensor orientation is kept constant in a path fixed coordinate system.Finally, experimental evaluations are performed on an ABB IRB4600 robot. Different observers using the EKF, EKS and PF with different estimation models are proposed. The estimated paths are compared to the true path measured by a laser tracking system. There is no significant difference in performance between the six observers. Instead, execution time, model complexities and implementation issues have to be considered when choosing the method.
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| 6. |
- Axelsson, Patrik, 1985-, et al.
(författare)
-
Single Joint Control of a Flexible Industrial Manipulator using H∞ Loop Shaping
- 2012
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Control of a flexible joint of an industrial manipulator using H∞ loop shaping design is presented. Two controllers are proposed; 1) H∞ loop shaping using the actuator position, and 2) H∞ loop shaping using the actuator position and the acceleration of end-effector. The two controllers are compared to a standard PID controller where only the actuator position is measured. Using the acceleration of the end-effector improves the nominal performance. The performance of the proposed controllers is not significantly decreased in the case of model error consisting of an increased time delay or a gain error.
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