REMOTELY CONTROLLED EXPERIMENT FOR GANTRY CRANE

REV 2007 - www.rev-conference.org 1

Remotely controlled experiment for Gantry Crane

V.M. Cvjetkovi�1, M. S. Matijevi�2 and M. Ž. Stefanovi�2 1 Faculty of Science/Department of Physics, Kragujevac, Serbia

2 Faculty of Mechanical Engineering/Department of Automatics, Kragujevac, Serbia

Abstract — Education of engineers and specialists for control systems relays greatly on practice that can show the effectiveness and results of control procedures and algorithms. Remote experiment for Gantry Crane described in this paper can be set up, controlled and observed remotely using standard network protocols and web user interface. It offers secure experimental environment for remote studying of the electromechanical system to be controlled and for testing of the user defined control algorithm on that system. User can test the designed algorithm on the real system by transparently integrating it in remote experimental system only by using the agreed interface, without taking care of any other technical details. This experiment was added as the independent module to already existing infrastructure for remote experiments.

Index Controller design, Gantry Crane, Remote laboratory

I INTRODUCTION

Education of engineers and specialists for control systems relays greatly on practice that can show the effectiveness and results of control procedures and algorithms. Control problem includes many tasks such as system analysis, choosing of the proper controller type for the system to be controlled, estimation of the controller parameters, implementation of the controller and testing in the real situation. Convenient real world model is necessary for all mentioned tasks and especially for the phase of testing, as it requires an in advance designed and implemented environment suitable for easy use on one side, and which also has all the main characteristics of the typical real world system that is to be controlled. Experiments for practicing of the control require sophisticated experimental system that can offer possibilities for flexible control that can also hide many unimportant technical details enabling thus the student to concentrate on crucial concepts of the controller design. As a rule, such experimental systems if existing at all, are single in educational institutions - faculties and can serve just one or small group of students at a time. Experiments with mechanical systems that have moving parts with significant masses are potentially dangerous and require adequate protection for extreme cases and conditions that can result from mistakes and inadequate control. Remote setup and control of experimental equipment is adequate solution that provides wide availability, safety and protection of the equipment itself. Further, it can be operated without supervisor assistance. Even with remote access,

experiment can be controlled by only one user at a time, while the other users can observe the on going experiment. Experiment in action can be remotely observed in real time by the web cam and measured sampled values of the positions, corresponding velocities and generated output control voltage can be observed numerically and graphically. The task of the experiment is to make a control algorithm for controlled movement of the mechanical system with desired characteristics. Control algorithm is implemented in the form of the dll that can be uploaded from the local users PC to the system to control the experiment. Experiment is part of the previously designed and developed RemoteLab system [1], [2] and [3]. Each experiment including experiment for Gantry Crane, has allocated equipment that can be operated and controlled programmatically. Experiments are fully controlled by the data acquisition system on a separate or shared PC for acquisition and control. Acquisition PCs are connected by the computer network with the web server that is common for all online experiments. Various experiments can be situated in different spatial locations as needed, being quite transparent for the remote user. This architecture also allows easy integration of many different remote experiments in one system. RemoteLab enables easy use of experimental equipment by many users from the same or different institutions that have system control as common educational field or subject.

II DESCRIPTION OF THE EXPERIMENTAL SETUP

UML deployment diagram in Fig 1 shows the structure of the RemoteLab system before the experiment for Gantry crane was added. Experiment for Gantry Crane was added as independent hardware and software module to an already existing system, but the software structure for this experiment is slightly different. The main difference is in that the software component for experiment control is not

Figure 1 UML diagram of the RemoteLab system

REMOTELY CONTROLLED EXPERIMENT FOR GANTRY CRANE

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on the web server, as it is for other experiments, but on PC for data acquisition, as shown in Fig 2. Web cam was also added to the acquisition PC, as it is important to see live the movement of the cart. New hardware for this experiment includes new PC with experimental setup and new USB NI 6009 [4] data acquisition system (DAS) for control of the experimental equipment. Remote experiment for Gantry Crane described in this paper can be set up, controlled and observed remotely using standard network protocols. Experimental set up for this experiment is shown in Fig 3. It consists of supporting wooden framework fixed on the laboratory wall and pieces of equipment that are mounted on it. Cart with pendulum can be moved horizontally along the supporting framework in both directions and powered by the electric motor with steel string around two wheels. Electrical transducer for position and transducer for linear velocity are mounted on wheel that is driven by electric motor. Pendulum that can freely rotate in vertical plane is fixed to cart by the shaft on which are electrical transducers for the angle and angular velocity. Electric motor is controlled by the analog output voltage from data acquisition system that is used for measurement of the fore mentioned linear and angular positions and velocities. Schematic diagram of the setup is presented in Fig 4. All the time, experimental setup is monitored by the web cam and live picture can be viewed on the web on RemoteLab web site. As the safety measure, security switches that turn off the power supply for the electric motor are positioned at both ends of the cart moving path, and are activated by the cart approaching either

of the end positions. As the physical limit for the movement of the cart, rubber bumpers are positioned at both ends behind the security switches.

III THEORETICAL BACKGROUND

For studying of the described experimental setup system for Gantry Crane, the simplified mathematical model can be made. Mathematical equations describing the dynamic model of the simplified system in Fig 4 can be obtained from the LaGrange equations of motion:

, .k k k kx

E E E Ed dQ Q

dt dt x xϕϕ ϕ� ∂ � ∂ ∂ ∂� �− = − =� � � �∂ ∂ ∂ ∂� �� �� �

(1)

Substituting the kinetic energies and generalized forces, we obtain following differential equations:

22

cos sin

1 sgn( ) cos sin

L x g

FI Mx x L L

ma m ma mµ

ϕ ϕ ϕ

ϕ ϕ ϕ ϕ

= −

� �+ + = − + −� �� �

�� ��

�� ��� �M

(2)

Those equations can be made linear in approximation for small angle 0ϕ ≈

2 1 sgn( )

L x g

FI Mx x L

ma m ma mµ

ϕ ϕ

ϕ

= −

� �+ + = − +� �� �

�� ��

���� �M (3)

If we take the state vector as

[ ]Tx x L Lϕ ϕ=x �� (4)

Then the equations can be represented in the form:

�

2 21 0

20

0 1 0 0 00 0 0 1

( )0 0 0 1 00 0 0 1

x x

x xdu f

L LdtL L

ω ωϕ ϕ

ωϕ ϕ

� �� � � � � � − = + − − � � � � � �� �

BA

� �

� �� � � �� � � ��

(5)

�������� ��� �����

�����

Figure 4 Schematic diagram of the experimental setup

Figure 3 Experimental setup for Gantry Crane

Figure 2 UML deployment diagram for Gantry Crane experiment on RemoteLab system

REMOTELY CONTROLLED EXPERIMENT FOR GANTRY CRANE

REV 2007 - www.rev-conference.org 3

If the friction is neglected, then we can obtain the state space model

u= +x Ax B� (6)

Experimental set up can also serve as the inverted pendulum system shown in Fig 5. In the case of inverted pendulum, linear approximation of the dynamical model can be obtained with ϕ π≈ and

introducing β π ϕ= − that will give the

desired 0β ≈ . Differential equations become:

2 1 sgn( )

L x g

FI Mx x L

ma m ma mµ

β β

β

= +

� �+ + = − +� �� �

�� ��

���� �M (7)

The importance of experimentally studying the

behavior of Gantry Crane is that it is quite frequently used in many real situations of moving the load from one place to another. It is desired to move the load from one position to another position accurately, as fast as possible, without or with some minimum swaying of the load, acting as pendulum in that case. Inverted pendulum poses the problem of dynamical keeping the equilibrium state of unstable system which inverted pendulum is. The real life case of the similar system is for instance, keeping the rocket in desired direction while traveling to desired location.

Previous equations describing the system are continuous, and for the purpose of controlling the system with digital computer, it is necessary to make transition to discrete domain, i.e. to describe the system with difference instead of differential equations. The transition to discrete system can be performed by taking the Lap lace transform of the previous continuous equations, and then by Tustin or bilinear transformation [5] to discrete z domain.

IV DATA ACQUISITION AND CONTROL

Integral part of the experimental setup for Gantry Crane is also the supporting electronics specifically designed and developed to control the experiment. The electronics is mainly used to enable control of the electric motor as the actuator, by the DAS. Electric motor, electrical connections for DAS, electronics and cart are shown in Fig 6. DAS for this experiment has characteristics of used A/D – D/A sections that satisfy the requirements of controlling the experimental setup – having enough channels, fast enough max sampling frequency, adequate voltage ranges for input and output analog sections.

The general scheme of the measurement and control by the DAS and software that controls the DAS is given in Fig 7. Fig 7 directly shows the connections of the experimental setup shown in Fig 6. Experimental setup has 4 transducers for measured mechanical quantities of the angle, angular velocity, position and linear velocity. Voltages produced by transducers for those quantities are measured by 4 analog input channels in differential mode. All transducers are linear and allow easy calculation of the measured quantity from the measured voltage. One analog output channel is used for continuous control of the electric motor. Output voltage that controls the electric motor is calculated by the software module that is used for control. As there are many different algorithms that can be used for the control of experimental setup, control software was designed as the module in such a way that it can be easily replaced by another module that implements some other control algorithm. In that way, this experimental system

Figure 7 Scheme of experimental measurement and control Figure 6 Electric motor, connections, supporting electronics and cart

Figure 5 System with inverted pendulum

REMOTELY CONTROLLED EXPERIMENT FOR GANTRY CRANE

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allows testing of different controller types that implement different control algorithms.

V SOFTWARE SUPPORT FOR THE EXPERIMENT

Software support for this experiment has the general

structure as presented in deployment diagram in Fig 2. It consists of the WEB user interface, experiment control, and software control for data acquisition system. Software for the WEB user interface is physically located on the WEB server which is separate PC and serves also for all other experiments within RemoteLab system. Other two important software parts are physically located on the separate PC for data acquisition. Software configuration for previous experiments as shown in Fig 1 has component for experiment control on the WEB server and not on acquisition PC. The main reason for that is the demand for the manual control of the Gantry Crane experimental equipment, which was not important in previous experiments within the RemoteLab system. It was easier to implement the manual control on the local PC for data acquisition, than on the WEB server. The control is much more critical in this experiment, as it is required to control the electromechanical system with significant masses, while in previous experiments, pure electrical systems were controlled with no moving parts. The experimental “knowledge” is coded on data acquisition PC in this case. The manual control of the experiment is possible just from the local PC connected to DAS. Experiment control was designed with modular software controller that has standard interface for connection with Experiment control part of the software.

Starting the experiment with automatic control is possible both remotely using the web interface and locally.

The third software component, the software control of the DAS, uses the NI DAQmx 8.0 [6] library to access the functions of the DAS. All software components were implemented in C# .NET in version 2003 on DAS PC, and in version 2005 on the WEB server. Corresponding versions of the Microsoft Visual Studio (2003 and 2005) were used for software development.

VI MEASUREMENTS ON THE SYSTEM

Experimental setup can be controlled manually and automatically, from the local PC and remotely using the WEB interface. As already mentioned, manual control is possible only from the local PC. The windows form of the software for experiment control, which is equivalent of the front panel of the LabVIEW [7] virtual instruments, is presented in Fig 8. Manual control of the cart can be performed by the upper slider control in the Manual control frame. On the left, there is a check box named Motor control. Middle position of the slider is neutral, and corresponds to 2.5 V that is neutral for the motor control. Slider movement on the left or right produces lower or higher voltage that activates the motor and starts motor rotation clockwise or anticlockwise. Speed of rotation is proportional to the voltage difference from 2.5, and it is maximal at 0 V and 5V that are extreme values of the DAS A/D converter. Lower slider with check box da1 controls the other analog output of the DAS which is not used in this experiment. Text boxes between the two sliders show the current output voltage on analog outputs. Using the manual control, cart can be moved to the desired position. Check boxes on the left from sliders enable and disable the corresponding sliders. Automatic control can be local and remote. Local automatic control always uses the proportional - P type of controller, while other types of controllers can be used remotely. Local manual and automatic control are provided mainly for testing the equipment that can quickly and simply show if everything is OK. Some parameters can be specified for the automatic control, and these correspond to the names of text boxes. Cart position specifies the desired position to which the cart should be moved from the current position. Sampling frequency sets the measurement speed at which the data from transducers are measured and output voltage for motor control is generated. Constant is the value of the P controller gain, and Experiment duration sets the time length of the experiment in seconds. Button Experiment start, starts the experiment under local control. Table named Measured data contains all measured data from transducers and generated output control voltages. The number of measurements - rows of the table is determined as the product of the sampling frequency and experiment duration. Measured data from the table can be written to file with file name specified in the long text box just above the table. Finally, the button with the text Remote on, transfers the control of the experiment to the WEB server.

Figure 8 Windows form of the Experiment control program

REMOTELY CONTROLLED EXPERIMENT FOR GANTRY CRANE

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In order to uniquely define who has the control of the

experiment, control permission logic was built in the program for Experiment control that works on the local PC. When the button Remote on is active, as it is the case in Figure 8, manual and local automatic control are disabled, and therefore the check box Motor control and button Experiment start are gray. Pressing the button Remote on again, transfers the control back to the local, the button title changes to Remote off, and previously disabled check box Motor control and button Experiment start are enabled again. Setting the check box Motor control disables the two buttons. Pressing the button Experiment start disables the other two for the time period specified in Experiment duration. Such built in logic for the transfer of experiment control allows just one source to be in control of the experiment. The concept of remote control in this experiment is similar to previous experiments shown in Fig 1. Local acquisition PC is controlled by the WEB server using Windows Sockets standard for data transfer and control. Web user interface for this experiment is presented in Fig 9. In the upper part of the web user interface are text boxes for the same data as on the local PC – Sampling frequency (Hz), Cart position (cm), Constant of the prop. regulator and Experiment duration (s). Additionally, there are option – radio buttons for selecting the internal P controller or the Custom user defined controller in the form of the dll file.

User can define his own controller for the Gantry Crane, respecting the defined interface and test it on the experimental setup system. The dll file with implemented interface on the local user PC can be uploaded to the web server using the web control for file upload. Button Experiment start begins the experiment with automatic control with selected controller. Live view of the experiment with the WEB cam shows the movement of the cart for the selected controller. Text box Experiment status shows the current status of the experiment – “Experiment started” and “Experiment over”. When the experiment is over, the user can review the measured data from each transducer and output voltage data produced by the controller in graphical and numerical form.

Figure 10 Graphical presentation of the position data

Figure 9 Web user interface for Gantry Crane experiment

REMOTELY CONTROLLED EXPERIMENT FOR GANTRY CRANE

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For each transducer, measured data in the graphical

form can be seen by clicking on the appropriate hyperlink on the left. Graphical presentation of the measured data for the position is presented in Fig 10. Graphic in Fig 10 shows the movement of the cart from the right to left, as the final position is 20 cm. The first value on the left is the starting position, while the last value on the right is the final measured position. Oscillations mean that cart passes the desired position and goes back and forth before reaching the final position. Live view with the web cam has some delay, of the order of a second, and it is the additional reason for the absence of the manual control in remote operation. Graphic with the measured data for the velocity is presented in Fig 11. Typical for the velocity diagram is that the first point corresponds to the zero velocity and also the last measured point, as the measurement starts just before the cart begins to move, and ends after the cart reaches final position, therefore the first and the last measured point are on the same height. Comparing the graphics in Fig. 10 and Fig 11, it can be seen that the measured curve in Fig 11 is mathematical derivation of the curve in Fig 10 as the velocity is derivation of distance over time. Figure 12 shows the graphic of the measured data for the angle of the pendulum. From the Fig 12 it can be seen that as the consequence of the cart movement, pendulum starts to oscillate and continues oscillations after the cart stops. That is undesired behavior, and control

algorithm should be changed in order to prevent it or decrease as much as possible. Corresponding diagram for the angular speed of the pendulum is given in Fig 13. Graphics of the measured data for the angular speed of the pendulum in Fig 13 is mathematical derivation of the graphics in Fig 12, as the angular speed is mathematical derivation of the angle over time.

Graphical presentation is also available for the output voltage data for controlling the electric motor, generated by the controller used in experiment. Fig 14 presents the graphic of the corresponding output voltage data for the measurements presented in previous figures. Horizontal parts correspond to extreme voltages of the D/A section. Zero voltage on the left, corresponds to movement of the cart to the left, with control signal exceeding the limit in the negative domain which cannot be produced by the D/A, and other saturation corresponding to movement to the right with control signal exceeding the maximum value of the D/A.

Measured numerical values in Fig 8 correspond to figures with graphical presentations. Measured numerical values can also be accessed from the web, with hyperlinks on the right side of the experiment web page in Fig 9. Those hyperlinks correspond to text files with numerical measured data. Fig 15 presents the web page with measured numerical data for position. Other hyperlinks give access to measured data from other transducers and have the same form on the web page.

Figure 14 Graphical presentation of the control signal

Figure 13 Graphical presentation of the angular velocity data

Figure 12 Graphical presentation of the angle data

Figure 11 Graphical presentation of the velocity data

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VII USAGE OF THE EXPERIMENTAL SETUP

Experimental setup for Gantry Crane can be used for various educational purposes at various levels of complexity. The direct and the most important usage of the experimental setup is for exercises in the design of various controllers and testing the effectiveness of these controllers in practice on the experimental setup. Some introductory exercises can be made, organized, for identification of the static and dynamic characteristics of the system that are important for the controller design. Existing hardware can be studied from the point of view of used components and their characteristics. System can be programmed with different software, LabVIEW for instance, which is the powerful graphical software system for measurement and control, or some other kind of software for measurement. It can be studied from the point of view of general control theory and how to achieve the best control of that system. Besides the point of view on the system as crane, it can be viewed also as the inverse pendulum and specific control requirements can be examined for that case. Existing software support enables the user to concentrate just on controller design and testing without having to study or pay attention to details of the system design and implementation. If it is of benefit to the user, he can study and experiment with any other aspect of the laboratory system for the Gantry Crane.

VIII CONCLUSION

The system for testing of the Gantry Crane controller design is presented in this paper. Various aspects of the system consisting of laboratory experimental setup, hardware and software are discussed. System enables the remote experimenting with Gantry Crane thus offering wide and quite safe access to this unique system intended to serve as play ground for future engineers. Some basic theoretical background is given for mathematical description of the dynamic model of the experimental setup. Description of the signals produced by the system, their measurement and generation of signals for the system control is

explained with close relation to measurement hardware – data acquisition system that was actually used. General architecture of the wider system – RemoteLab is described, and also the specifics of the software support system for the Gantry Crane experiment that is the part of the RemoteLab system, is given. Experiment can be performed both locally on the PC that controls the laboratory setup, and remotely using the web user interface with web browser. All signals can be measured and displayed both locally and remotely in the form of graphics and numerically including the generated control signal. Controller for the Gantry Crane can be implemented in the form of a dll with strictly defined interface. That gives the designer the opportunity to concentrate just on the design of the controller without paying attention on the implementation of the system. Through the specified interface, the user gets the measured signals from the system and generates the control signal. There is no limitation on how to design the controller, he can design it completely on his own as long as he strictly respects the required interface of the controller with the rest of the system. Some examples of the measured signals and generated control signal are given, that can serve for the analysis of the controller performance. Besides the suggested main usage of the system, some other educational possibilities for using this system are also presented.

REFERENCES

[1] Vladimir M Cvjetkovic, and Yevgeniya S Sulema, Remote Laboratory for Supporting e-Studies in Electronics, iJoe, Volume 2, No. 1, 2006 [2] Vladimir M Cvjetkovic, Dragoljub Stevanovic and Milan Matijevic, Remote System for Development, Implementation and Testing of Control Algorithms, iJoe, Vol. 3, No. 1 (2007) [3] Vladimir M. Cvjetkovic, On Line Experiments with linear analog systems from the first to nth order, REV 2006, 29 - 30 June 2006, Maribor, Slovenia [4] National Instruments, User guide and specifications, USB 6008/6009 http://www.ni.com/pdf/manuals/371303e.pdf [5] A. M. Zikic, Practical digital control, Ellis Horwood Series, 1989 [6] National Instruments, NI-DAQmx Version 8.0.1 for Windows 2000/XP, http://digital.ni.com/softlib.nsf/websearch/A0D9C8120EADC9A786257131005BF69E?opendocument&node=132050_US [7] National Instruments, http://www.ni.com/labview/

AUTHORS

V. M. Cvjetkovi� is with the Faculty of Science, Radoja Domanovi�a 12, 34000 Kragujevac, Serbia, (email: vladimir@kg.ac.yu) M. S. Matijevi� is with the Faculty of Mechanical Engineering, Sestre Janji� 6, 34000 Kragujevac, Serbia (email: milan.matijevi�@gmail.com) M. Ž. Stefanovi� is with the Faculty of Mechanical Engineering, Sestre Janji� 6, 34000 Kragujevac, Serbia (email: miladin@kg.ac.yu)

This work was fully supported and financed by the Austrian Cooperation through WUS Austria -- within the eLearning project No.: 010/06 - ���� 8093-01/2005

Figure 15 Measured numerical data for position