xpc Target an Option for Position Control of Robotic Manipulators

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1 xpc Target an Option for Position Control of Robotic Manipulators O. Pinzón-Ardila, L. Angel, M. Useche Universidad Pontificia Bolivariana, UPB Bucaramanga Colombia (omar.pinzon, luis.angel, Abstract Real time operating systems have been evolving in gigantic steps, allowing, each time to develop applications where some restrictions on task execution time are needed. In this article xpc Target is shown as a new option for control, going through basic concepts of real time to come up with a highly used application in robotics as well as in general industry; which is a speed control for a DC motor. This application links xpc Target as a real time operating system in charge of executing a control task, besides it integrates a based FPGA I/O board which interacts with the controller and outside environment. This is how xpc Target delivers a form of systems optimization, since, rapid prototyping platforms, embedded systems, real time operating systems and also programming algorithms from MatLab and Simulink are integrated. Key words- xpc Target, real time, embedded systems, Simulink, prototyping. I. INTRODUCTION Technology development has allowed us to have smaller, faster, low cost and low energy consumption computers, having a broader way of application. But computer evolution it is not the only that has been merging, also at the same time of this powerful machines there can be found high level programming algorithms that chained with computers and ruled by a operating system give us real time systems. In general, a real time system is a set of components that cooperate to achive a common purpose, based on a coputer which process different information simultaneously and also performs a dedicated action over some kind of system. II. REAL TIME SYSTEMS A. Real time system The basic idea of a real time system is based on a computer giving response on time, it means that the system must be fast enough depending on the context in which the system is operating. A real time computational system not only has to give a rapid response, but it has to do it in an efficient and precise way. Therefore, real time programming consists on designing reliable systems capable of finding designed times whether on arbitrary or asynchronous events. B. Strict and non-strict real time systems The proper functioning of a non-strict real-time system is based in the logical correction of its results []. In the contrary, real time systems require accurate results by one side but also, that these results come in a given time, calling this correction as temporary correction []. In strict real-time systems the temporary response is a critical issue, it means that response timing is very important and it cannot be sacrificed by improving other aspects of the system []. A typical example is the autopilot in an airplane, failing at checking an altitude measure in a given time may have catastrophic consequences. In non-strict real time systems the temporary response is an important but not critical variable. Occasional failing on giving results within the systems fixed times does not have serious consequences on the overall system's functioning []. Non-strict real-time tasks are executed as fast as possible, but they are not forced by absolute times limits, this way, temporary correction can sacrificed down under certain circumstances. C. Systems controlled by event or time. Another classification of real-time system is based in the use of events or clock cycles as a control element in triggering a task execution. The architecture controlled by events or interruptions establishes the execution of a component or task, taking as a base the appearance of an interruption or a signal generated by an external event []. This way the system generates a computer s interruption in such way that the event can be managed by the routine in charge of that control. The architecture controlled by time operates in a function of the system clock cycles. This kind of architectures, operate by treating regular clock pulses as if they were interruption signals. When the clock reaches certain preset values, a proper action is chosen for its execution. This kind of systems is used

2 when periodic task execution or task execution by timers are needed []. D. Study of control oriented RTOS The features of a typical RTOS (Real Time Operating System) written by Stankovic and Ramamritham in 99, are justified in great measure for most of RTOS today, for Licenced RTOS as well as Open source RTOS. These features include: Multitasking with anticipative priority programming. Context efficient Switching. Efficient interruption service. Memory model without support for virtual memory. Synchronization and communication primitives. Service application at synchronization level. Small memory steps. A RTOS with the features mentioned above ensures a right sharing of the system resources and CPU timing, and it has a small knowledge if any of any task that is being executed on it. The model addressed to events, measures success and applicability of RTOS trough the whole system's average response time. Previous sections have shown that response time measurement neglects some synchronization limitations that are found in control applications. Licensed and Open source general purpose RTOS can evolve slowly; nevertheless, the results of research on the state of the art design of RTOS are visible in a few RTOS, the most remarkable are the Spring core, real time Asterix core, S.Ha.R.K RTOS and Linux's Fiasco micro-core. These research cores can have small commercial support, but they include features such as an anti-block synchronization methodology, some programming algorithms, aperiodic servers and new control protocols for resource access. III. REAL TIME EMBEDDED SYSTEMS CONTROLLED THROUGH MATLAB, SIMULINK AND XPC-TARGET While feedback control strategy increases its complexity, it becomes more difficult to use analogic components for its implementation. Dynamics on an analogic control loop is always interacting, making harder each time to tune in parameters in a controller. On an embedded control system, controller operates in the low power electronic domain and the plant operates in the physical domains of electric, hydraulics, mechanics, thermals and others of high power. For this purpose are necessary transducers that transform controller and plant signals, these transducers are actuators to operate the plant or sensors to sense within the electronic domain of low power. A. Fast Prototyping Many of the researches are devoted of analysis, design and synthesis of a controller based on the plant model. Nevertheless, a fast prototyping is an agile form to validate controller s code by executing it with the current plant, sensors and actuators. The purpose of fast prototyping is to achieve confidence and locate accurately defects and mistakes within a partial design of a plant before to take the next step to a full design. In scientific and educational research where a designed system is rarely taken into production, fast prototyping has a wide application. In the industry, fast prototyping allows testing of a partial design before including robustness and optimization measures to the design. Three different configurations of fast prototyping are shown in Figure : Functional, cycling and on target. Fast functional prototyping is used to test new ideas and research works where there is no controller, or the controller is instead too primitive to carry advanced control strategies. In such case, the fast prototyping controller controls the whole system. Given that the focus of attention is on proving the concept, then the size of the generated code and the features of fixed comma of the software are not important. In this case a compatible PC is often used as hardware because the intention it is not to use it for production of control applications. Flexibility of this system when adding I/O hardware is important in fast prototyping, although in this configuration hardware contingences cannot be considered. Fast cycling prototyping substitutes only a portion of the existent control system with a new controller. This is useful if the control system requires controllers or a high capacity current driver that are not usually found in fast prototyping controllers or, if on the contrary, these controllers only form part of the controller s main functionality. fast On-target prototyping uses production hardware and directly captures all hardware dependencies and I/O's limitations. This allows engineers to assess algorithm capabilities to control a device under some testing conditions, especially those that are difficult to simulate. B. Fast prototyping on the design of embedded control systems Classic control design methods are founded on determining a plant s models. The plant s model can be determined using basic sciences laws, but they always face unknown parameters. Therefore, experiments must be performed to adjust information of the dynamic behavior of the plant and this way they help to estimate unknown parameters. Once the plant s model is found a new controller can be designed by means of simulation or synthesis, using methods like poles localization, reverse dynamics, total power control, H and predictive control models. Fast prototyping tools support this design paradigm. At the beginning of the control design process an engineer could have an inaccurate model or he could have no model at all. In this stage, a framework of the control system is developed to stabilize a system and obtain a desired behavior. Experiments, therefore, can be designed and performed to obtain response from the system under different operating conditions. Data acquired can be used to improve the plant s model and to design a new control model based on a more accurate plant model. Simulating the control system combined with the model obtained from the plant, the designer can study and optimize the control system s performance. Finally, the

3 control system can be implemented in a fast prototyping system. If the system doesn t find the performance specifications analyzed previously in the simulation, the model and the control system s design are tuned until desired performance is achieved. Figure. Fast Prototyping Configurations IV. XPC TARGET A. General Purpose Hardware A fast prototyping platform needs to be more powerful and flexible than an eventual target processor. For example, if the software has not been optimized yet, then it is not efficiently executed. To achieve a real time behavior a more powerful processor is required. Furthermore, additional measures can be required in order to obtain a view of the controller's performance. The flexibility, computational capacity and memory needed, can make fast prototyping platforms to be much more expensive than the software that is used in production. Due to the costs, fast prototyping platforms are used for more than a single job, with this approach is shown the flexible and inherent nature of these platforms. Meanwhile, xpc-target provides the means to turn a general purpose's PC hardware into a system that allows data acquisition, fast prototyping, and hardware simulation, all in a single loop. B. PC form factor A device s form factor is determined by the form and physical size of the PC. There is a specificc number of form factors available in PC based systems. A form factor can cover component design such as type of connectors, bus protocols, board sizes, power consumption specifications and mechanical contours. The form factor rarely influences directly in the processors selection, but a particular form factor is associated to a family of processors. Thus, it is common to adopt the election of a form factor with that of a chosen processor. This can be a Desktop PC, a Workstation or an industrial PC. The advantage of using PC based platforms is their power scalability, flexibility and expansibility of its computing capacity. The xpc-target can be used with any PC which integrates Intel 386/486, Pentium, or AMD K5/K6/Athlon processors. This also includes desktop PCs, industrial PCs such as xpctargetbox, PC/04, PC/04+, CompactPCI, all in one embedded PC, or any form factor compatible with PC. Thanks to scale economy and competition, these devices have been developed in the order of millions of floating point operation per second (MFLOPS). Furthermore, the wide range of availability of form factors allows the xpc-target to be used in small PC/04 systems as well as in most of PCI expanded systems. For example, is common to make previous design works using a desktop PC and then immediately the control algorithm is conditioned to a industrial PC for field tests. C. Real time Operating System The xpc-target core provides a real time operating system that supports interruption handling and that adjusts itself to provide maximum performance with minimum limits. The high performance hardware allows sampling rates that reach 00 khz. D. Drivers An important step in software transformation within a real time system is the requirement of having devices controllers or drivers that communicate between I/O devices, the target Pc and the code that has been executing within applications. These drivers allow real time interaction between the application and the real physic system. The driver contains the code that is executed in the hardware to make the interface with the I/O devices such as analogic to digital signal converters (A/D), encoders, digital signals and communication ports. Each driver is implemented as a S function of Simulink that is written fundamentally in C code and it is compiled in a MatLab s MEX proprietary format. E. xpc Target Configurationn Figure 2, shows a xpc-target host-target architecture. Within the host the following applications can be found: MatLab, Simulink, Real Time workshop and xpc-target. xpc- by Simulink and a C code Target uses the code generated compiler to build a real time target application. Within the target hardware the core is initialized in real time. Figure 2. xpc-target architecture.

4 The real time core begins the host-target communication, activates an application loader and waits until the target application downloads itself into the host PC. The host-target communication can be developed by TCP/IP serial communication protocols. Once the target application is downloaded into the target PC, the application can be controlled and modified from the host PC. V. DC MOTOR SPEED CONTROL Figure 3, shows the speed control system components of a direct current motor that has been developed with base on a FPGA that is connected to a PCI bus of a general purpose PC. Figure 3. Direct Current Motor Speed Control System. A. Host PC The host Pc is an equipment which uses MatLab software and its tools: Simulink and Real-time Workshop, to create the control program in a block diagram. The host PC communicates with the so called target equipment by means of a TCP/IP protocols; this means that it is possible to communicate through an intranet or the internet. Once the control program is running into the target it is possible to tune the applications parameters and visualize the different system variables to verify their functioning in a remote way. B. Target PC The target PC is composed of an embedded system where a proprietary MatLab s xpc-target real-time operating system is executed. The control program created in the host adapts to the real time operating system to execute in real time. C. PCI Card The control system integrates itself by means of a PCI card (Peripheral Component Interconnect) programmed as an I/O device that allows connectivity between Target and the direct current motor. Figure 4, shows the PCI card used in this work. The Dragon PCI card from Fpga4fun Company, has a FPGA which is connected to the PCI bus and to a USB port, the last one provides card s power supply and configuration for the FPGA. Figure 4. Dragón PCI target The FPGA is programmed by means of the ISE WebPACK 9.2 and using the Verilog programming language. The following modules are integrated within the FPGA: PCI bus communication, pulse width modulation and quadrature encoder reading. D. DC Motor The dc motor is a system element which is controlled by means of a PWM input signal. The implemented motor is a 24 VDC HITACHI motor, reference HITACHI TYPE- D04A32E, 2W. There is an 00 P/R encoder atached to the motor in its lower portion, the encoder has a 5 VDC power supply to generate the signals. E. Encoder It is the sensor or measurement element which generates digital signals that are proportional to the control s variable magnitude. The encoder used is an incremental codifier which is used to transform the rotational displacement of de DC motor into a digital code or pulse signals. The encoder is provided with two channels that generate quadrature signals, that is, that one signal has a 90 degree gap in respect to the other signal. Depending on the direction of the motor rotation, encoder signals will change their sense. F. DC Motor Mathematical Model Identification A direct current motor model can be approximated to a first order transfer function. Ω Where Kb is the motor static gain (rpm/%), is the system's time constant, Ω(s) is the DC motor speed (rpm) and U(S) is the pulse width modulation's duty cycle applied in a percentage to the direct current motor (%). Figure 5 shows the open loop system's experimental response to a test signal which changes its duty cycle between 20% and 80%. In this figure the rise and downfall establishment times were determined to be as 0.3 s and.4 s respectively, the static rise and downfall gains were 39.5 rpm/% and 38.7/% respectively. ()

5 3500 RESPUESTA DEL MOTOR EN LAZO ABIERTO X: 0.3 Y: 332 velocidad de salida velocidad deseada Where Kp is the proportional constant, Ki is the integral constant, and Ts is the sampling time. The controller s purpose is to achieve a closed loop transient response with a estab Velocidad del Motor [RPM] lishment time less or equal to.4 s and an overshoot percentage less or equal to 5%. Figure 6 shows the system s roots place. To accomplish with the design specifications, the controller must locate a z c zero in , a p c pole in and a K c gain in , giving the closed loop poles in / i with a 7.3 rad/s frequency as a result. Root Locus Editor for Open Loop (OL) X: 0 Y: tiempo (s) Figure 5. Response of open loop motor In the practice, establishment time estimation is the amount of time needed by the response to reach the 2% line of the final value or four time constants. With the last statement a s time constant is obtained. Im ag Ax is The DC motor s transfer function in discrete time is obtained by MatLab s c2d order, using a sampling rate equal to 0. s and a ZOH (Zero Order Hold) Ω 28,83 0,2636 (2) Real Axis G. PI Controller Design for Speed Control The Pi controller improves the system s closed loop performance. Assuming that PI controller s transfer function in discrete time has the following structure: (3) Figure 6. Location of closed loop poles H. Speed Control Experimental Results Figure 7 shows experimental design implemented with MatLab s xpc-target tool to control motor s speed. A PI Controller is used in the closed loop control system's block diagram. In this diagram, SALIDA PWM and ENCODER blocks developed by the authors, are used to make the PCI card to physically interact with the plant in real time. Target Scope Id : Scope (Target PC ) [RPM] 200 CICLO DUTY [%] SET POINT - SALIDA SET POINT VELOCIDAD [RPM] SUMADOR SUMADOR 2 PI CONTROLADOR PI SATURACION 2 DRAGON _PCI FPGA4FUN Digital Output SALIDA PWM DRAGON _PCI FPGA4FUN Digital Input ENCODER VELOCIDAD [RPM ] GENERADOR DE PULSOS RUN STOP Figure 7. Experimental System s Block Diagram

6 Figure 8, shows speed control system's experimental results. The tryout is performed in order to make the system follow speed changes between 200 rpm and 3000 rpm. The system error signal feeds the PI controller input; the PI controller is in charge to produce the control action. The block which generates PWM, varies the applied duty cycle to the power transistor in order to supply controlled power to the direct current motor. It is to be noted that the system has a saturation block which prevents PWM from surpassing 0% and 00% limits. From figure 8 it can be determined that establishment time is in the [0.9.3] s range and that overshoot percentage is between [ ] % range. A PI controller has been designed and implemented, which has provided with a satisfactory response in matters of following a reference signal. Furthermore, it delivers a great reliability in the presence of external mechanical and electrical perturbations to the DC motor. This controller has been designed by using a compatible PC and open architecture software such as MatLab along with its Simulink tool, which allow designing a block diagram of the didactic module. Implementation of xpc Target as the speed control systems interface allows the interaction of a great quantity of users to manipulate applications parameters. ACKNOWLEDGMENTS The authors thank the administrative department of science, technology and innovation COLCIENCIAS- Colombia project financing DESARROLLO DE UN LABORATORIO VIRTUAL DE CONTROL POR VISIÓN PARA TELEOPERAR ROBOTS EN EL ESPACIO OPERACIONAL through 487 of 2009 call for the conformation of the project bank research, technological development and innovation that make use and infrastructure and services of the national academic network of technology RENATA-. The work presented in this article relate the progress achieved in the aforementioned project. Velocidad del Motor [RPM] RESPUESTA DEL MOTOR EN LAZO CERRADO X:.3 Y: 3060 velocidad de salida velocidad deseada tiempo (s) Figure 8. Motor response of the experimental system. CONCLUTIONS With this article, new ways of analyzing and implant real time control systems are established, clarifying some conceptual mistakes that have spread through pass of time. REFERENCES [] L. Jiménez, R. Manchón, y O. Garcia, Sistemas Informáticos de Tiempo Real, Miguel. Hernandez. España, vol., pp. 6, April 200. [2] J. Astrom, Computer-Controlled Systems Theory and Desing, Tercera Edicion, Editorial Prentice Hall, Pag 2. [3] R. Couglhin, y F. Driscoll, Amplificadores operacionales y circuitos integrados lineales. Editorial Prentice Hall, 999. Pags , [4] F. Heath, Embedded Systems Design, Second Edition. Butterworth- Heinemann, p. [5] D. Varsakelis, Handbook of networked and embedded control systems, Editorial Board. Pags [6] The Mathworks. Getting Started with xpc Target, Version 2 ed., The Mathworks, Inc., Natick, MA, July 2002.