General Purpose Controller Software for Controls Lab

Transcription

1 General Purpose Controller Software for Controls Lab Andrew J. Blauch School of Engineering Grand Valley State University Abstract Many industrial control compensators are implemented using microcontrollers. These embedded systems provide a versatile platform for the integration of various feedback signals and control algorithms. To be productive in industry, a control engineer should not only be comfortable using an embedded controller, but also familiar with the limitations and effects that a digital controller will have on the overall system performance. A Motorola 68HC11-based general purpose controller (GPC) application was developed for use in the laboratory portion of an undergraduate controls course. The 68HC11 family of processors has been incorporated into a wide variety of industrial applications and is currently used at many academic institutions. Evaluation boards are readily available at low cost. The GPC software that has been developed was designed for flexibility. It provides numerous configuration options for the type of input signal, feedback signal, output signal, and control algorithm. A simple command line interface allows for easy user interaction. This paper discusses the development of the GPC software and its use in an undergraduate controls course. Explanations of the user interface, software configuration options, and hardware interface capabilities are presented. A sample laboratory activity is also provided. The paper concludes with a summary of the educational benefits achieved as a result of incorporating the GPC software into the laboratory activities. Introduction Microcontrollers are appearing in more and more industrial applications and consumer products everyday. They have become a key component in many engineering projects. Because of the prevalence and importance of the microcontroller, it is vital that the academic community work at integrating the microcontroller into engineering courses. Many schools have incorporated embedded microcontroller-based systems into their curriculum, from introductory freshman engineering classes through senior level design courses. 1, 2 Control systems are an excellent illustration of embedded microcontroller application. Whether dealing with an industrial PLC controller or a home thermostat, most modern control systems are based on a microcontroller. In order to teach controls, it is helpful to have a system that can be easily interfaced with and can be configured for different types of control scenarios. Various control systems have been developed over the years for academic use. 3, 4

2 At Grand Valley State University, microcontrollers are being integrated into many courses across the engineering curriculum, including such courses as Introduction to Digital Systems 5, Dynamic System Modeling and Control 6, and Embedded System Design, to name a few. This paper discusses the development of an embedded general purpose controller (GPC) application for use in an undergraduate Automatic Controls course. The following sections provide a description of the development process and software operation, followed by a laboratory example and discussion of the corresponding educational benefits. GPC Development The GPC application was designed to be a flexible microcontroller-based data collection and control application. It is configurable for various types of input signals, feedback signals, output signals, and controllers. Communication between the user and the GPC is available via a serial connection and an easy to follow command line interface. The application was written primarily using C language. Only time critical and hardware interface code were written in assembly language. Structuring the program in this fashion provided an easier path for porting the application to different platforms. Currently, the GPC application is only available for the 68HC11 microcontroller. In the future, versions will be made for other microcontrollers such as the AVR and ARM processors. The latest software version and supporting documentation are available online. 7 The GPC reference manual provides complete details for the installation, interface, and operation of the GPC software. The software application is organized into five blocks as shown in Figure 1. The data collection block is for the acquisition and storage of real-time data. The input block is for reading the input, or reference, signal. It can be configured for direct user input, single/dual-ended analog input, or an internal waveform generator. The feedback block is for reading the feedback signal. It can be configured for single/dual-ended analog input, encoder input, or disabled for open-loop control. The output block is for generating the controller output, or actuating, signal. It can be configured for uni-directional or bi-directional pulse width modulation. The output block can also be used for deadband compensation. The control algorithm is implemented in the control block. It can be configured for either a proportional (P) or proportional + integral (PI) controller, with the ability to adjust the feedback, proportional, and integral gains. Input Block D in Control D ctrl Output D out Block Block Input Signal Control Signal Output Signal Feedback Block D fb Feedback Signal Data Collection Block Figure 1: GPC Application Block Diagram

3 GPC Software Operation When the application first starts, the version information is displayed followed by the command prompt as shown in Figure 2. The user can control and monitor the system by entering commands at the prompt. GPCImageBoot Version: 3.01 Created: Aug , 08:54:04 Type? for help GPC> Figure 2: GPC Sign-on and Command Prompt Table 1 provides a summary of the available commands. Commands are grouped into several categories. The first set of commands provides the user with general information and control. The configuration commands provide the user with control over displaying and manipulating numeric and string parameters. The data collection commands, along with the data collection parameters, provide the user with control over the data collection subsystem. The macro commands provide the user with more complex control options. All commands are entered at the prompt. The entire command syntax does not need to be entered for the command to execute. Only a sufficient number of characters to ensure a unique command match must be entered. For commands that take arguments, if no argument is specified a default value is used. Table 1: Command Summary Category Command Parameters Description General DELAY <Time> DELAY for fixed time interval QUIT QUIT application RESET RESETs system to default settings VER Display application VERsion Configuration Data Collection Macro NPGET <Name> Numeric Parameters, GET value NPMON <Name> Numeric Parameters, MONitor value NPSET <Name> <Value> Numeric Parameters, SET value SPGET <Name> String Parameters, GET value SPSET <Name> <Value> String Parameters, SET value DCARM DCGET Data Collection, ARM trigger Data Collection, GET values DBCOMP <X1> <X2> DeadBand COMPensation algorithm INFO Display configuration INFOrmation MSTEP <Amplitude> Motion STEP input

4 The configuration of the control system is based on numeric and string parameter settings. The user can modify these parameters by using the appropriate configuration commands. Table 2 provides a summary of the numeric and string parameters. Detailed explanations of the commands and parameters are available in the GPC reference manual. 7 Table 2: Parameter Summary Name Description AN1, AN2, AN3, AN4 Analog input signals INPUT, FEEDBACK, OUTPUT, CONTROL Input, feedback, output, and control signals DBNEG, DBPOS Deadband compensation values DCINC, DCPTS Data collection parameters GFB, GKI, GKP Controller gains ITYPE, FTYPE, OTYPE, CTYPE, WTYPE Block configuration parameters TCLOCK, TEXEC, TUPDATE Interrupt parameters WCONT, WINC, WOFFSET, WSCALE Waveform generation parameters GPC Hardware Operation The GPC application has been developed for the 68HC11 microcontroller, with the intent of providing ports for other microcontrollers in the future. Table 3 provides a summary of the microcontroller I/O pin usage. The signal name, pin, and I/O type are listed along with the signal usage. The schematic of the I/O pin usage for the 68HC11 is shown in Figure 3. Examples of hardware interface circuits are available in the GPC reference manual. 7 Table 3: Microcontroller I/O Pin Usage Summary Signal Name Pin I/O Type Usage SER_RX PD0 Digital Input Serial Communication SER_TX PD1 Digital Output Serial Communication PWM PA6 Digital Output Pulse Width Modulation PWM_POS PD2 Digital Output Pulse Width Modulation PWM_NEG PD3 Digital Output Pulse Width Modulation ENC_A PD4 Digital Input Encoder ENC_B PD5 Digital Input Encoder ANALOG1/INPUT(HI) PE4 Analog Input Analog-to-Digital Converter ANALOG2/INPUT(LO) PE5 Analog Input Analog-to-Digital Converter ANALOG3/FEEDBACK(HI) PE6 Analog Input Analog-to-Digital Converter ANALOG4/FEEDBACK(LO) PE7 Analog Input Analog-to-Digital Converter The SCI peripheral on the 68HC11 is used for serial communication purposes. It is configured for 9600 BAUD, 8 data bits, no parity, and 1 stop bit. The pins PD0 (SER_RX) and PD1 (SER_TX) are the serial receiver and transmitter signals, respectively. The 68HC11 does not have a built-in pulse-width modulation (PWM) circuit. Therefore, it must be generated using interrupts and the output compare peripherals. The actual PWM signal is created on pin PA6 (PWM) using the OC1 and OC2 timer features/interrupts. The software provides a full range of duty cycles with glitch free transitions. The period of the PWM signal is

5 127.5 μsec. The pulse width is set in 0.5 μsec steps (0 255 output value corresponds to 0 100% duty cycle). When enabled, the PWM consumes approximately 30% of the processor time. This has a significant effect on the minimum required update time of the control system. Bi-directional output signal capability is provided by the generation of two directional signals. The pins PD2 (PWM_POS) and PD3 (PWM_NEG) are the active high positive and negative directional signals, respectively. The 68HC11 has a single 8-bit analog-to-digital converter (ADC). The evaluation boards used are wired for a 0 to 5 volt analog input range. While different voltage ranges could be used, the software assumes a positive voltage range [0,V rh ]. The ADC is configured for continuous scan of multiple channels (PE4 PE7). In this configuration, each channel is scanned and converted one at a time. A single conversion takes 16 μsec. Therefore, each channel is converted every 64 μsec. Figure 3: Microcontroller Interface Connections

6 Laboratory Example The GPC application has been integrated into the laboratory activities of the EGR455 Automatic Control course. A motor and tachometer closed-loop control system is used for experimental modeling, analysis, and design activities. An illustration of the hardware setup is shown in Figure 4. A bi-directional driver circuit is used to interface the microcontroller to the motor. A second motor is mounted to the first motor to act as a tachometer. The tachometer is connected to a low pass filter and protection circuit to interface back to the microcontroller. The schematics for each of the hardware interface circuits are available in the GPC reference manual. 7 Figure 4: Example of Laboratory Setup The GPC application is configured for direct user input, dual-ended analog feedback, and bidirectional PWM output. Once the hardware and software setups are completed, the students can run experiments and collect data for various control scenarios using the appropriate GPC commands. As an example, Figure 5 shows the results for a closed-loop configuration with a PI controller. The step response was generated by executing the MSTEP command. The signals D fb, D ctrl, and D out correspond to the GPC variables illustrated in Figure 1 and are automatically recorded by the data collection subsystem during the MSTEP command. Once the step response was completed, the experimental results were downloaded using the DCGET command. The theoretical results were derived from the linearized model of the system. The simulated results were obtained from a non-linear Simulink model of the system.

7 60 Step Response: Closed-Loop K p =1 K i =100 D fb [u] 40 Input Experimental 20 Theoretical Simulation D ctrl [u] D out [u] Time [s] Figure 5: Example of Laboratory Results For this particular configuration, the only significant difference appears in the D out plot between the experimental/simulation data and the theoretical data. This difference is a result of the deadband compensation employed by the GPC software to counter the deadband in the DC motor. The GPC software was configured to compensate for the deadband in the DC motor by adding an offset to the output. The Simulink model created also takes both the DC motor deadband and the GPC deadband compensation into account. The theoretical model, however, assumes a linearized model for each component. Educational Benefits By incorporating the microcontroller-based system into the Automatic Controls course, students were introduced to a physical embedded control system, similar in many ways to the ones they are likely to encounter in industry. The flexible control interface allowed the students to test, analyze, and design different control configurations for the given system. During the laboratory activities, a number of practical embedded control topics arose that were not part of the traditional classical control topics covered in lecture. Table 4 lists some of the issues that arose as a result of incorporating the GPC embedded application into the control system. The topics listed were discussed and explained in the laboratory environment as they were encountered.

8 Table 4: Embedded Control Topics Sampling noise quantization of time effects on open-loop and closed-loop configuration Pulse Width Modulation analog signal from digital output frequency range constraints output saturation Analog-to-digital Conversion quantization of value sampling rate constraints input saturation Software execution time/delay interrupt starvation integer/floating-point math trade-offs numeric limitations As mentioned, these embedded topics are not typically covered in a traditional classical controls course. However, they are important issues for a practicing control engineer to understand. By incorporating the GPC embedded application into the course, the students were exposed to these issues in the laboratory environment and experienced their effects first hand. As problems came up, the issues were discussed along with their associated impact on the control system. In addition, the simulation models were expanded to include these digital and non-linear effects. Conclusion This paper provided an overview of the GPC software and its application in an undergraduate controls course. The software has been used for the past three years in the course with positive results. Development of the software is still ongoing. Comments, corrections, and suggestions are welcomed and should be sent directly to the author. Bibliography 1. E. Montanez, Microcontrollers in Education: Embedded Control Everywhere and Everyday, Proceedings of the 2005 ASEE Annual Conference and Exposition, Portland, OR. 2. L.F. Ferreira, E.L. Matos, L.M. Menendez, and E. Mandado, MILES: A Microcontroller Learning System combining Hardware and Software tools, Proceedings of the 2005 ASEE/IEEE Frontiers in Education Conference, Indianapolis, IN. 3. N. Krouglicof, Development of a Universal Controller for Pedagogical Applications Involving Data Acquisition, Data Logging and Control, Proceedings of the 2002 ASEE Annual Conference and Exposition, Montreal, Quebec Canada. 4. S.S. Moor, P.R. Piergiovanni, and M. Metzger, Process Control Kits: A Hardware and Software Resource, Proceedings of the 2005 ASEE/IEEE Frontiers in Education Conference, Indianapolis, IN. 5. A.J. Blauch and A. Sterian, A Practical Application Digital Systems Course For All Engineering Majors, Proceedings of the 2002 ASEE Annual Conference and Exposition, Montreal, Quebec Canada. 6. H. Jack and A. J. Blauch, A Modeling and Controls Course using Microcontrollers, Proceedings of the 2004 ASEE Annual Conference and Exposition, Salt Lake City, UT. 7. Software,