13 research outputs found
A Review of Pneumatic Actuators Used for the Design of Medical Simulators and Medical Tools
International audienc
Modeling and control of a pneumatic muscle actuator
This thesis presents the theoretical and experimental study of pneumatic servo position control systems based on pneumatic muscle actuators (PMAs). Pneumatic muscle is a novel type of actuator which has been developed to address the control and compliance issues of conventional cylindrical actuators. Compared to industrial pneumatic cylinders, muscle actuators have many ideal properties for robotic applications providing an interesting alternative for many advanced applications. However, the disadvantage is that muscle actuators are highly nonlinear making accurate control a real challenge.
Traditionally, servo-pneumatic systems use relatively expensive servo or proportional valve for controlling the mass flow rate of the actuator. This has inspired the research of using on/off valves instead of servo valves providing a low-cost option for servo-pneumatic systems. A pulse width modulation (PWM) technique, where the mass flow is provided in discrete packets of air, enables the use of similar control approaches as with servo valves. Although, the on/off valve based servo-pneumatics has shown its potential, it still lacks of analytical methods for control design and system analysis. In addition, the literature still lacks of studies where the performance characteristics of on/off valve controlled pneumatic systems are clearly compared with servo valve approaches.
The focus of this thesis has been on modeling and control of the pneumatic muscle actuator with PWM on/off valves. First, the modeling of pneumatic muscle actuator system controlled by a single on/off valve is presented. The majority of the effort focused on the modeling of muscle actuator nonlinear force characteristics and valve mass flow rate modeling. A novel force model was developed and valve flow model for both simulation and control design were identified and presented.
The derived system models (linear and nonlinear), were used for both control design and utilized also in simulation based system analysis. Due to highly nonlinear characteristics and uncertainties of the system, a sliding mode control (SMC) was chosen for a control law. SMC strategy has been proven to be an efficient and robust control strategy for highly nonlinear pneumatic actuator applications. Different variations of sliding mode control, SMC with linear model (SMCL) and nonlinear model (SMCNL) as well as SMC with integral sliding surface (SMCI) were compared with a traditional proportional plus velocity plus acceleration control with feed-forward (PVA+FF) compensation. Also, the effects of PWM frequency on the system performance were studied.
Different valve configurations, single 3/2, dual 2/2, and servo valve, for controlling a single muscle actuator system were studied. System models for each case were formulated in a manner to have a direct comparison of the configuration and enabling the use of same sliding mode control design. The analysis of performance included the sinusoidal tracking precision and robustness to parameter variations and external disturbances. In a similar manner, a comparison of muscle actuators in an opposing pair configuration controlled by four 2/2 valves and servo valve was executed.
Finally, a comparison of a position servo realized with pneumatic muscle actuators to the one realized with traditional cylinder was presented. In these cases, servo valve with SMC and SMCI were used to control the systems. The analysis of performance included steady-state error in point-to-point positioning, the RMSE of sinusoidal tracking precision, and robustness to parameter variations
Modeling and control of a pneumatic muscle actuator
This thesis presents the theoretical and experimental study of pneumatic servo position control systems based on pneumatic muscle actuators (PMAs). Pneumatic muscle is a novel type of actuator which has been developed to address the control and compliance issues of conventional cylindrical actuators. Compared to industrial pneumatic cylinders, muscle actuators have many ideal properties for robotic applications providing an interesting alternative for many advanced applications. However, the disadvantage is that muscle actuators are highly nonlinear making accurate control a real challenge.
Traditionally, servo-pneumatic systems use relatively expensive servo or proportional valve for controlling the mass flow rate of the actuator. This has inspired the research of using on/off valves instead of servo valves providing a low-cost option for servo-pneumatic systems. A pulse width modulation (PWM) technique, where the mass flow is provided in discrete packets of air, enables the use of similar control approaches as with servo valves. Although, the on/off valve based servo-pneumatics has shown its potential, it still lacks of analytical methods for control design and system analysis. In addition, the literature still lacks of studies where the performance characteristics of on/off valve controlled pneumatic systems are clearly compared with servo valve approaches.
The focus of this thesis has been on modeling and control of the pneumatic muscle actuator with PWM on/off valves. First, the modeling of pneumatic muscle actuator system controlled by a single on/off valve is presented. The majority of the effort focused on the modeling of muscle actuator nonlinear force characteristics and valve mass flow rate modeling. A novel force model was developed and valve flow model for both simulation and control design were identified and presented.
The derived system models (linear and nonlinear), were used for both control design and utilized also in simulation based system analysis. Due to highly nonlinear characteristics and uncertainties of the system, a sliding mode control (SMC) was chosen for a control law. SMC strategy has been proven to be an efficient and robust control strategy for highly nonlinear pneumatic actuator applications. Different variations of sliding mode control, SMC with linear model (SMCL) and nonlinear model (SMCNL) as well as SMC with integral sliding surface (SMCI) were compared with a traditional proportional plus velocity plus acceleration control with feed-forward (PVA+FF) compensation. Also, the effects of PWM frequency on the system performance were studied.
Different valve configurations, single 3/2, dual 2/2, and servo valve, for controlling a single muscle actuator system were studied. System models for each case were formulated in a manner to have a direct comparison of the configuration and enabling the use of same sliding mode control design. The analysis of performance included the sinusoidal tracking precision and robustness to parameter variations and external disturbances. In a similar manner, a comparison of muscle actuators in an opposing pair configuration controlled by four 2/2 valves and servo valve was executed.
Finally, a comparison of a position servo realized with pneumatic muscle actuators to the one realized with traditional cylinder was presented. In these cases, servo valve with SMC and SMCI were used to control the systems. The analysis of performance included steady-state error in point-to-point positioning, the RMSE of sinusoidal tracking precision, and robustness to parameter variations
Control of a hydraulically actuated mechanism using a proportional valve and a linearizing feedforward controller
A common problem encountered in mobile hydraulics is the desire to automate motion control functions in a restricted-cost and restricted-sensor environment. In this thesis a solution to this problem is presented. A velocity control scheme based on a novel single component pressure compensated ow controller was developed and evaluated. The development of the controller involved solving several distinct technical challenges. First, a model reference control scheme was developed to provide control of the valve spool displacement for a particular electrohydraulic proportional valve. The control scheme had the effect of desensitizing the transient behaviour of the valve dynamics to changes in operating condition. Next, the pressure/flow relationship of the same valve was examined. A general approach for the mathematical characterization of this relationship was developed. This method was based on a modification of the so-called turbulent orifice equation. The general approach included a self-tuning algorithm. Next, the modified turbulent orifice equation was applied in conjunction with the model reference valve controller to create a single component pressure compensated flow control device. This required an inverse solution to the modified orifice equation. Finally, the kinematics of a specific single link hydraulically actuated mechanism were solved. Integration of the kinematic solution with the flow control device allowed for predictive velocity control of the single link mechanism
Component-based mixed reality environment for the control and design of servo-pneumatic system
Synopsis
Considerable research efforts have been spent over the last two decades on improving the
design, control, and modelling of pneumatic servo drive systems including the
development of dedicated controllers and control valves. However, the commercial
updates in employing pneumatic servos are still largely limited to laboratory research
usage and the initiatives in developing seem to have lost their momentums. Although this
situation has some to do with the rapid development and availability of cost effective
electric servo technologies, one reason is considered to be a lack of design and simulation
tools for employing pneumatic servo drives. This research has therefore been conducted
to address these concerns, and to demonstrate how appropriate tools and environments
can be developed and used to aid in the design, control and commissioning of pneumatic
servo drives. Because of the inherent high nonlinearities associated with pneumatic
systems, it would be highly desirable if the simulation environment could be run in time
domain so that it can be mixed with the real system. This would make the simulation
more accurate and reliable especially when dealing with such nonlinear systems.
Unfortunately, the tools that are available in the market such as Propneu (Festo, 2005)
and Hypneu (Bardyne, 2006) are dedicated for pneumatic circuit design only.
This research is aimed at developing a mixed reality environment for the control and
design of servo-pneumatic systems. Working with a mixed reality environment would
include both the capability to model the system entirely as a simulation, the so-called
"off-line", as well as being able to use real components running against simulations of
others "on-line", or in a Mixed Reality (MR) manner. Component-based paradigm has
been adopted, and hence the entire pneumatic system is viewed as a series of component
modules with standardised linking variables. The mathematical model of each individual
component has been implemented in simulation software which produces time domain
responses in order to allow for mixing the simulation with the real system.
The main outcome of this research can be seen as a successful development and
demonstration of the Component-based Mixed Reality Environment (CMRE), which
would facilitate the control and design of servo-pneumatic systems. On the one hand, the
CMRE facilitates the identification of some nonlinear parameters such as frictional
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ynopsis
parameters. These parameters could cause great difficulties in servo-pneumatic modelling
and control. Accurate friction parameters would give the ability to attain an accurate
model, and therefore provide more flexibility in applying different control and tuning
strategies on the real system. On the other hand, the CMRE facilitates the design process
by enabling the designer to evaluate the servo-pneumatic system off-line prior to building
the system. This would reduce the design time, increase the reliability of the design, and
minimize the design cost.
The concept of the CMRE was validated by tests carried out on laboratory-based
prototype servo-drive. Close agreement between the experimental and simulated
responses was obtained showing that the models have represented the real system
adequately. Case studies were then conducted to demonstrate the validity of the proposed
methodology and environment. In these case studies, PIDVF controller and cascade
control structure were successfully implemented, synthesised, and tuned. The results
revealed that the CMRE is an easy, accurate and robust way of implementing different
control and tuning strategies on servo-pneumatic systems. Furthermore, the research has
shown how the CMRE can lead to significant improvements in certain life cycle phases
of the system, e.g. commissioning, maintenance, etc.
This research has contributed to knowledge in the following:
(1) Adopting the mixed reality concept and the component-based approach in order to
create a CMRE in facilitating the control and design of servo-pneumatic systems.
(2) A method to identify the friction parameters of a single-axis pneumatic machine,
(3) Encapsulate existing control methods within the CMRE to be applied on the real
system.
(4) A scheme for controller tuning, in which the controller is tuned off-line and then
applied on the real system, and hence avoided on-line tuning which can be
troublesome and time consuming.
It is anticipated that the concept of the CMRE can be extended to include multi-axes
servo-pneumatic system, servo-hydraulic, and servo-electric drives. Therefore,
conceptual model structures have been introduced in this research which can be
considered as the foundation for creating similar environments for those systems
Modelling of servo-controlled pneumatic drives: a generalised approach to pneumatic modelling and applications in servo-drive design
The primary objective of this research is to develop a
general modelling facility for modular pneumatic servo-drives.
The component-oriented approach has been adopted as the modelling
technique to provide the flexibility of modelling a wide variety
of components and the segmentation of the non-linear system to
less complex uncoupled component modules.
A significant part of the research work has been devoted to
identify a series of component modules of the single axis linear
pneumatic servomechanism with standardised linking variables. The
mathematical models have been implemented in a simulation software
which produces time domain responses for design evaluation
purposes. Alternative components for different servomechanism
design were modelled as mutually exclusive modules which could be
selected for assembly as if they were real physical entities. The
philosophy of the approach was validated by tests on prototype
servo-drives with matching components. Design analysis could be
performed by simulating and comparing the performance of
alternative system structures. [Continues.
Design, Development, and Evaluation of a Teleoperated Master-Slave Surgical System for Breast Biopsy under Continuous MRI Guidance
The goal of this project is to design and develop a teleoperated master-slave surgical system that can potentially assist the physician in performing breast biopsy with a magnetic resonance imaging (MRI) compatible robotic system. MRI provides superior soft-tissue contrast compared to other imaging modalities such as computed tomography or ultrasound and is used for both diagnostic and therapeutic procedures. The strong magnetic field and the limited space inside the MRI bore, however, restrict direct means of breast biopsy while performing real-time imaging. Therefore, current breast biopsy procedures employ a blind targeting approach based on magnetic resonance (MR) images obtained a priori. Due to possible patient involuntary motion or inaccurate insertion through the registration grid, such approach could lead to tool tip positioning errors thereby affecting diagnostic accuracy and leading to a long and painful process, if repeated procedures are required. Hence, it is desired to develop the aforementioned teleoperation system to take advantages of real-time MR imaging and avoid multiple biopsy needle insertions, improving the procedure accuracy as well as reducing the sampling errors.
The design, implementation, and evaluation of the teleoperation system is presented in this dissertation. A MRI-compatible slave robot is implemented, which consists of a 1 degree of freedom (DOF) needle driver, a 3-DOF parallel mechanism, and a 2-DOF X-Y stage. This slave robot is actuated with pneumatic cylinders through long transmission lines except the 1-DOF needle driver is actuated with a piezo motor. Pneumatic actuation through long transmission lines is then investigated using proportional pressure valves and controllers based on sliding mode control are presented. A dedicated master robot is also developed, and the kinematic map between the master and the slave robot is established. The two robots are integrated into a teleoperation system and a graphical user interface is developed to provide visual feedback to the physician. MRI experiment shows that the slave robot is MRI-compatible, and the ex vivo test shows over 85%success rate in targeting with the MRI-compatible robotic system. The success in performing in vivo animal experiments further confirm the potential of further developing the proposed robotic system for clinical applications
Implementation of Iterative Learning Control on a Pneumatic Actuator.
Masters Degree. University of KwaZulu-Natal, Durban.Pneumatic systems play a pivotal role in many industrial applications, such as in
petrochemical industries, steel manufacturing, car manufacturing and food industries. Besides
industrial applications, pneumatic systems have also been used in many robotic systems.
Nevertheless, a pneumatic system contains different nonlinear and uncertain behaviour due to
gas compression, gas leakage, attenuation of the air in pipes and frictional forces in mechanical
parts, which increase the system’s dynamic orders. Therefore, modelling a pneumatic system
tends to be complicated and challenges the design of the controller for such a system. As a
result, employing an effective control mechanism to precisely control a pneumatic system for
achieving the required performance is essential.
A desirable controller for a pneumatic system should be capable of learning the dynamics of
the system and adjusting the control signal accordingly. In this study, a learning control scheme
to overcome the highlighted nonlinearity problems is suggested. Many industrial processes are
repetitive, and it is reasonable to make use of previously acquired data to improve a controller’s
convergence and robustness. An Iterative Learning Control (ILC) algorithm uses information
from previous repetitions to learn about the system’s dynamics. The ILC algorithm
characteristics are beneficial in real-time control given its short time requirements for
responding to input changes.
Cylinder-piston actuators are the most common pneumatic systems, which translate the air
pressure force into a linear mechanical motion. In industrial automation and robotics, linear
pneumatic actuators have a wide range of applications, from load positioning to pneumatic
muscles in robots. Therefore, the aim of this research is to study the performance of ILC
techniques in position control of the rod in a pneumatic position-cylinder system. Based on
theoretical analysis, the design of an ILC is discussed, showing that the controller can
satisfactorily overcome nonlinearities and uncertainties in the system without needing any prior
knowledge of the system’s model. The controller has been designed in such a way to even work
on non-iterative processes. The performance of the ILC-controlled system is compared with a
well-tuned PID controller, showing a faster and more accurate response