Open Access

Prototyping Advanced Control Systems on FPGA

  • Stéphane Simard1,
  • Jean-Gabriel Mailloux1 and
  • Rachid Beguenane1Email author
EURASIP Journal on Embedded Systems20092009:897023

DOI: 10.1155/2009/897023

Received: 19 June 2008

Accepted: 3 March 2009

Published: 22 June 2009

Abstract

In advanced digital control and mechatronics, FPGA-based systems on a chip (SoCs) promise to supplant older technologies, such as microcontrollers and DSPs. However, the tackling of FPGA technology by control specialists is complicated by the need for skilled hardware/software partitioning and design in order to match the performance requirements of more and more complex algorithms while minimizing cost. Currently, without adequate software support to provide a straightforward design flow, the amount of time and efforts required is prohibitive. In this paper, we discuss our choice, adaptation, and use of a rapid prototyping platform and design flow suitable for the design of on-chip motion controllers and other SoCs with a need for analog interfacing. The platform consists of a customized FPGA design for the Amirix AP1000 PCI FPGA board coupled with a multichannel analog I/O daughter card. The design flow uses Xilinx System Generator in Matlab/Simulink for system design and test, and Xilinx Platform Studio for SoC integration. This approach has been applied to the analysis, design, and hardware implementation of a vector controller for 3-phase AC induction motors. It also has contributed to the development of CMC's MEMS prototyping platform, now used by several Canadian laboratories.

1. Introduction

The use of advanced control algorithms depends upon being able to perform complex calculations within demanding timing constraints, where system dynamics can require feedback response in as short as a couple tens of microseconds. Developing and implementing such capable feedback controllers is currently a hard goal to achieve, and there is much technological challenge in making it more affordable. Thanks to major technological breakthroughs in recent years, and to sustained rapid progress in the fields of very large scale integration (VLSI) and electronic design automation (EDA), electronic systems are increasingly powerful [1, 2]. In the latter paper, it is rightly stated that FPGA devices have reached a level of development that puts them on the edge of microelectronics fabrication technology advancements. They provide many advantages with respect to their nonreconfigurable counterparts such as the general purpose micropocessors and DSP processors. In fact, FPGA-based digital processing systems achieve better performance-cost compromise, and with a moderate design effort they can afford the implementation of a powerful and flexible embedded SoCs. Exploiting the FPGA technology benefits for industrial electrical control systems has been the source of intensive research investigations during last decade in order to boost their performances at lower cost [3, 4]. There is still, however, much work to be done to bring such power in the hands of control specialists. In [5], it is stated that the potential of implementing one FPGA chip-based controller has not been fully exploited in the complicated motor control or complex converter control applications. Until now, most related research works using FPGA devices are focusing on designing specific parts mainly to control power electronic devices such as space vector pulse width modulation (SVPWM) and power factor correction [6, 7]. Usually these are implemented on small FPGAs while the main control tasks are realised sequentially by the supervising processor system, basically the DSP. Important and constant improvement in FPGA devices, synthesis, place-and-route tools, and debug capabilities has made FPGA prototyping more available and practical to ASIC/SoC designers than ever before. The validation of their hardware and software on a common platform can be accomplished using FPGA-based prototypes. Thanks to the existing and mature tools that provide automation while maintaining flexibility, the FPGA prototypes make it now possible for ASIC/SoC designs to be delivered on time at minimal budget. Consequently, FPGA-based prototypes could be efficiently exploited for motion control applications to permit an easy modification of the advanced control algorithms through short-design cycles, simple simulation, and rapid verification. Still the implementation of FPGA-based SoCs for motion control results in very complex tasks involving SW and HW skilled developers. The efficient IP integration constitutes the main difficulty from hardware perspective while in software side the issue is the complexity of debugging the software that runs under real-time operating system (RTOS), in real hardware. This paper discusses the choice, adaptation, and use of a rapid prototyping platform and design flow suitable for the design of on-chip motion controllers and other SoCs with a need for analog interfacing. Section 2 describes the chosen prototyping platform and the methodology that supports embedded application software coupled with custom FPGA logic and analog interfacing. Section 3 presents the strategy for simulating and prototyping any control algorithm using Xilinx system Generator (XSG) along with Matlab/Simulink. A vector control for induction motor is taken as a running example to explain some features related to the cosimulation. Section 4 describes the process of integrating the designed controller, once completely debugged, within an SoC architecture using Xilinx Platform Studio (XPS) and targeting the chosen FPGA-based platform. Section 5 discusses the complex task of PCI initialization of the analog I/O card and controller setup by software under embedded Linux operating system. Section 6 takes the induction motor vector control algorithm as an application basis to demonstrate the usefulness of the chosen FPGA-based SoC platform to design/verify on-chip motion controllers. The last section concludes the paper.

2. The FPGA-Based Prototyping Platform for On-Chip Motion Controllers

With the advent of a new generation of high-performance and high-density FPGAs offering speeds in the 100 seconds of MHz and complexities of up to 2 megagates, the FPGA-based prototyping becomes appropriate for verification of SoC and ASIC designs. Consequently the increasing design complexities and the availability of high-capacity FPGAs in high-pin-count packages are motivating the need for sophisticated boards. Board development has become a task that demands unique expertise. That is one reason why commercial off-the-shelf (COTS) boards are quickly becoming the solution of choice because they are closely related to the implementation and debugging tools. During many years, and under its System-on-Chip Research Network (SOCRN) program, CMC Microsystems provided canadian universities with development tools, various DSP/Embedded Systems/multimedia boards, and SoC prototyping boards such as Amirix AP1000 PCI FPGA development platform. In order to support our research on on-chip motion controllers, we have managed the former plateform, a host PC (3.4 GHz Xeon CPU with 2.75 GB of RAM) equiped with the Amirix AP1000 PCI FPGA development board, to support a multichannel analog I/O PMC daughter card (Figure 1) to communicate with exterior world.
Figure 1

Rapid prototyping station equiped with FPGA board and multichannel analog I/O daughter card.

The AP1000 has lots of features to support complex system prototyping, including test access and expansion capabilities. The PCB is a 64-bit PCI card that can be inserted in a standard expansion slot on a PC motherboard or PCI backplane. Use of the PMC site requires a second chassis slot on the backside of the board and an optional extender card to provide access to the board I/O. The AP1000 platform includes a Xilinx Virtex-II Pro XC2VP100 FPGA and is connected to dual banks of DDR SDRAM (64 MB) and SRAM (2 MB), Flash Memory (16 MB), Ethernet and other interfaces. It is configured as a single board computer based on two embedded IBM PowerPC processors, and it is providing an advanced design starting point for the designer to improve time-to-market and reduce development costs.

The analog electronics are considered modular, and can either be external or included on the same chip (e.g., when fabricated into an ASIC). On the prototyping platform, of course, they are supplied by the PMC daughter card. It is a General Standards PMC66-16AISS8A04 analog I/O board featuring twelve 16-bit channels: eight simultaneously sampled analog inputs, and four analog outputs, with input sampling rates up to 2.0 MSPS per channel. It acts as a two-way analog interface between the FPGA and lab equipment, connected through an 80-pin ribbon cable and a breakeout board to the appropriate ports of the power module.

The application software is compiled with the free Embedded Linux Development Kit (ELDK) from DENX Software Engineering. Since it runs under such a complete operating system as Linux, it can perform elaborated functions, including user interface management (via a serial link or through networking), and real-time supervision and adaptation of a process such as adaptive control.

The overall platform is very well suited to FPGA-in-the-loop control and SoC controller prototyping (Figure 2). The controller can either be implemented completely in digital hardware, or executed on an application-specific instruction set processor (ASIP). The hardware approach has a familiar design flow, using the Xilinx System Generator (XSG) blockset and hardware/software cosimulation features in Matlab/Simulink. An ASIP specially devised for advanced control applications is currently under development within our laboratory.
Figure 2

Architecture of the embedded platform driving a power system (schematic not to scale).

3. Matlab/Simulink/XSG Controller Design

It is well known that simulation of large systems within system analysis and modelling software environments takes a prohibitive amount of time. The main advantage of a rapid prototyping design flow with hardware/software cosimulation is that it provides the best of a system analysis and modelling environment while offering adequate hardware acceleration.

Hardware/software cosimulation has been introduced by major EDA vendors around year 2000, combining Matlab/Simulink, the computing, and Model-Based Design software, with synthesizable blocksets and automated hardware synthesis software such as DSP Builder from Altera, and System Generator from Xilinx (XSG). Such a design flow reduces the learning time and development risk for DSP developers, shortens the path from design concept to working hardware, and enables engineers to rapidly create and implement innovative, high-performance DSP designs.

The XSG cosimulation feature allows the user to run a design on the FPGA found on a certain platform. An important advantage of XSG is that it allows for quick evaluation of system response when making changes (e.g., changing coefficient and data widths). As the AP1000 is not supported by XSG among the preprogrammed cosimulation targets, we use the Virtex-4 ML402 SX XtremeDSP Evaluation Platform instead (Figure 3). The AP1000 is only targetted at the SoC integration step (see Section 4).
Figure 3

Virtex-4 ML402 SX XtremeDSP evaluation platform.

We begin with a conventional, floating-point, simulated control system model, and corresponding fixed-point hardware representation is then constructed using the XSG blockset, leading to a bit-accurate FPGA hardware model (Figure 4), and XSG generates synthesizable HDL targetting Xilinx FPGAs. The XSG design, simulation, and test procedure is briefly outlined below. Power systems including motor drives can be simulated using the SimPowerSystems (SPS) blockset in Simulink.
Figure 4

Controller-on-chip design flow.

(1)Start by coding each system module individually with the XSG blockset.

(2)Import any user-designed HDL cores.

(3)Adjust the fixed-point bit precisions (including bit widths and binary point position) for each XSG block of the system.

(4)Use the Xilinx Gateway blocks to interface a floating-point Simulink model with a fixed-point XSG design. The Gateway-in and Gateway-out blocks, respectively, convert inputs from Simulink to XSG and outputs from XSG to Simulink.

(5)Test system response using the same input stimuli for an equivalent XSG design and Simulink model with automatic comparision of their respective outputs.

Commonly, software simulation of a complete drive model, for a few seconds of results, could take a couple of days of computer time. Hardware/software cosimulation can be used to accelerate the process of controller simulation, thus reducing the computing time to about a couple of hours. It also ensures that the design will respond correctly once implemented in hardware.

4. System-on-Chip Integration in Xilinx Platform Studio

FPGA design and the SoC architecture are managed with Xilinx Platform Studio (XPS), targetting the AP1000. We have customized the CMC-modified Amirix baseline design to support analog interfacing, user logic on the Processor Local Bus (PLB), and communication with application software under embedded Linux. XPS generates the corresponding .bin file, which is then transferred to the Flash configuration memory on the AP1000. The contents of this memory is used to reconfigure the FPGA. We have found an undocumented fact that, on the AP1000, this approach is the only practicable way to program the FPGA. JTAG programming is proved inconvenient, because it suppresses the embedded Linux, which is essential to us for PCI initialization. Once programmed, user logic awaits a start signal from our application software following analog I/O card initialization.

To accelerate the logic synthesis process, the mapper and place and route options are set to STD (standard) in the implementation options file (etc/fast_runtime.opt), found in the Project Files menu. If the user wants a more aggressive effort, these options should be changed to HIGH, which requires much more time. Our experiments have shown that it typically amounts to several hours.

4.1. Bus Interfacing

The busses implemented in FPGA logic follow the IBM CoreConnect standard. It provides master and slave operation modes for any instanciated hardware module. The most important system busses are the Processor Local Bus (PLB), and the On-chip Peripheral Bus (OPB).

The implementation of the vector control scheme requires much less of generality, and deletes some communication stages that might be used in other applications. It is easier to start from such a generic design, dropping unneeded features, than to start from scratch. This way, one can quickly progress from SoC architecture in XPS down to a working controller on the AP1000.

4.1.1. Slave Model Register Read Mux

The baseline XPS design provides the developer with a slave model register read multiplexer. This allows to decide which data is provided when a read request is sent to the user logic peripheral by another peripheral in the system. While a greater number may be used, our pilot application, the vector control, only use four slave registers. The user logic peripheral has a specific base address (C_BASEADDR), and the four 32-bit registers are accessed through C_BASEADDR + register offset. In this example, C_BASEADDR + 0x0 corresponds to the control and status register, which is composed of the following bits:

0–7: the DIP switches on the AP1000 for

debugging purposes,
  1. 8

    : used by user software to reset, start, or

     

stop the controller,

9–31   : reserved.

As for the other 3 registers, they correspond to

C_BASEADDR + 0x4: Output to analog

channel 1

C_BASEADDR + 0x8: Output to analog

channel 2

C_BASEADDR + 0xC: Reserved (often used for

debugging purposes)

4.1.2. Master Model Control

The master model control state machine is used to control the requests and responses between the user logic peripheral and the analog I/O card. The latter is used to read the input currents and voltages for vector control operation. The start signal previously mentioned in slave register 0 is what gets the state machine out of IDLE mode, and thus starts the data acquisition process. In this specific example, the I/O card is previously initialized by the embedded application software, relieving the state machine of any initialization code. Analog I/O initialization sets a lot of parameters, including how many active channels are to be read.

The state machine operates in the following way (Figure 5).
Figure 5

Master model state machine.

(1)The user logic waits for a start signal from the user through slave register 0.

(2)The different addresses to access the right AIO card fields are set up, namely, the BCR and read buffer.

(3)A trigger is sent to the AIO card to buffer the values of all desired analog channels.

(4)A read cycle is repeated for the number of active channels previously defined.

(5)Once all channels have been read, the state machine falls back to trigger state, unless the user chooses to stop the process using slave register 0.

4.2. Creating or Importing User Cores

User-designed logic and other IPs can be created or imported into the XPS design following this procedure.

(1)Select Create or Import Peripheral from the Hardware menu, and follow the wizard (unless otherwise stated below, the default options should be accepted).

(2)Choose the preferred bus. In the case of our vector controller, it is connected to the PLB.

(3)For PLB interfacing, select the following IPIF services:

(a)burst and cacheline transaction support,

(b)master support,

(c)S/W register support.

(4)The User S/W Regitser data width should be 32.

(5)Accept the other wizard options as default, then click Finish.

(6)You should find your newly created/imported core in the Project Repository of the IP Catalog; right click on it, and select Add IP.

(7)Finally go to the Assembly tab in the main System Assembly View, and set the base address (e.g., 0x2a001000), the memory size (e.g., 512), and the bus connection (e.g., plb_bus).

4.3. Instantiating a Netlist Core

Using HDL generated by System Generator may be inconvenient for large control systems described with the XSG blockset, as it can require a couple of days of synthesis time. System Generator can be asked to produce a corresponding NGC binary netlist file instead, which is then treated as a black box to be imported and integrated into an XPS project. This considerably reduces the synthesis time needed. The process of instantiating a Netlist Core in a custom peripheral (e.g., user_logic.vhd), performed following the steps documented in XPS user guide.

4.4. BIN File Generation and FPGA Configuration

To configure the FPGA, a BIN file must be generated from the XSG project. Since JTAG programming disables the embedded Linux, the BIN file must be downloaded directly to onboard Flash memory. There are two Intel Strataflash Flash memory devices on the AP1000, one for the configuration, and one for the U-boot bootstrap code (which should not be crushed) (Table 1).
Table 1

The Two Intel StrataFlash Flash memory devices.

Bank

Address

Size

Mode

Description

1

(16 MB)

16

Program Flash

2

(16 MB)

8

Config. Flash

The configuration memory (Table 2) is divided into three sections. Section 2 is the default Amirix configuration, and should not be crushed. Downloading the BIN file to memory is done through a network cable using the TFTP protocol. For this purpose, a TFTP server must be set up on the host PC. The remote side of the protocol is managed by U-boot on the AP1000. Commands to U-boot to initiate the transfer and to trigger FPGA reconfiguration from a designated region are entered by the user through a serial link terminal program. Here is the complete U-boot command sequence:

setenvserverip 132.212.202.166

setenv ipaddr 132.212.201.223

erase 2 : 039

Send tftp 00100000 download.bin

Send cp.b 00100000 24000000 00500000

Send swrecon

Table 2

AP1000 flash configurations.

Region

Bank

Sectors

Description

0

2

0–39

Configuration 0

1

2

40–79

Configuration 1

2

2

80–127

Configuration 2 (Default Config.)

5. Application Software and Drivers

One of the main advantages of using an embedded Linux system is the ability to perform the complex task of PCI initialization. In addition, it allows for application software to provide elaborated interfacing and user monitoring through appropriate software drivers. Initialization of the analog I/O card on the PMC site and controller setup are among such tasks that are best performed by software.

5.1. Linux Device Drivers Essentials

Appropriate device drivers have to be written in order to use daughter cards (such as an analog I/O board) or custom hardware components on a bus internal to the SoC, and be able to communicate with them from the embedded Linux. Drivers and application software for the AP1000 can be developed with the free Embedded Linux Development Kit (ELDK) from DENX Software Engineering, Germany. The ELDK includes the GNU cross development tools, along with prebuilt target tools and libraries to support the target system. It comes with full source code, including all patches, extensions, programs, and scripts used to build the tools. A complete discussion on writing Linux device drivers is beyond the scope of this paper, and this information may be found elsewhere, such as in [8]. Here, we only mention a few important issues relevant to the pilot application.

To support all the required functions when creating a Linux device driver, the following includes are needed:

include linux/config.h

include linux/module.h

include linux/pci.h

include linux/init.h

include linux/kernel.h

include linux/slab.h

include linux/fs.h

include linux/ioport.h

include linux/ioctl.h

include linux/byteorder/

big_endian.h

include asm/io.h

include asm/system.h

include asm/uaccess.h

5.2. PCI Access to the Analog I/O Board

The pci_find_device() function begins or continues searching for a PCI device by vendor/device ID. It iterates through the list of known PCI devices, and if a PCI device is found with a matching vendor and device, a pointer to its device structure is returned. Otherwise, NULL is returned. For the PMC66-16AISS8A04, the vendor ID is 0x10e3, and the device ID is 0x8260. The device must then be initialized with pci_initialize_device() before it can be used by the driver. The start address of the base address registers (BARs) can be obtained using pci_resource_start(). In the example, we get BAR 2 which gives access to the main control registers of the PMC66-16AISS8A04.

volatile u32 *base_addr;

struct pci_dev *dev;

struc resource *ctrl_res;

dev = pci_find_device(VENDORID,

DEVICEID, NULL);

.

.

.

pci_enable_device (dev);

get_revision (dev);

base_addr = (volatile u32 *)

pci_resource_start (dev, 2);

ctrl_res = request_mem_region (

(unsigned long)base_addr,

0x80L, "control");

bcr = (u32 *) ioremap_nocache (

(unsigned long)base_addr,

0x80L);

The readl() and writel() functions are defined to access PCI memory space in units of 32 bits. Since the PowerPC is big-endian while the PCI bus is by definition little-endian, a byte swap occurs when reading and writing PCI data. To ensure correct byte order, the le32_to_cpu() and cpu_to_le32() functions are used on incoming and outgoing data. The following code example defines some macros to read and write the Board Control Register, to read data from the analog input buffer, and to write to one of the four analog output channels.

volatile u32 *bcr;

define GET_BCR() (le32_to_cpu(

readl (bcr)

define SET_BCR(x) writel(

cpu_to_le32(x),bcr)

define ANALOG_IN() le32_to_cpu(

readl (&bcr[ANALOG_INPUT_BUF])

define ANALOG_OUT(x,c) writel(

cpu_to_le32(x),

&bcr[ANALOG_OUTPUT_CHAN_00+c])

5.3. Cross-Compilation with the ELDK

To properly compile with the ELDK, a makefile is required. Kernel source code should be available in KERNELDIR to provide for essential includes. The version of the preinstalled kernel on the AP1000 is Linux 1.4. Example of a minimal makefile:

TARGET= thetarget

OBJS= myobj.o

EDIT THE FOLLOWING TO POINT TO

THE TOP OF THE KERNEL SOURCE TREE

KERNELDIR = kernel-sw-003996-01

CC = ppc_4xx−gcc

LD = ppc_4xx−ld

DEFINES = − −DMODULE

−DEXPORT_SYMTAB

INCLUDES= −I$(KERNELDIR)/include

−I$(KERNELDIR)/include/Linux

−I$(KERNELDIR)/include/asm

FLAGS = −fno-strict-aliasing

−fno-common

−fomit-frame-pointer

−fsigned-char

CFLAGS = $(DEFINES) $(WARNINGS)

$(INCLUDES) $(SWITCHES)

$(FLAGS)

all: $(TARGET).o Makefile

$(TARGET).o: $(OBJS)

$(LD) −r -o $@

5.4. Software and Driver Installation on the AP1000

For ease of manipulation, user software and drivers are best carried on a CompactFlash card, which is then inserted in the back slot of the AP1000 and mounted into the Linux file system. The drivers are then intalled, and the application software started, as follows:

mount /dev/discs/disc0/part1 /mnt

insmod /mnt/logic2/hwlogic.o

insmo /mnt/aio.o

cd /dev

mknod hwlogic c 254 0

mknod aio c 253 0

/mnt/cvecapp

6. Application: AC Induction Motor Control

Given their well-known qualities of being cheap, highly robust, efficient, and reliable, AC induction motors currently constitute the bulk of the motion industry park. From the control point of view, however, these motors have highly nonlinear behavior.

6.1. FPGA-Based Induction Motor Vector Control

The selected control algorithm for our pilot application is the rotor-flux oriented vector control of a three-phase AC induction motor of the squirrel-cage type. It is the first method which makes it possible to artificially give some linearity to the torque control of induction motors [9].

RFOC algorithm consists in partial linearization of the physical model of the induction motor by breaking up the stator current into its components in a suitable reference frame . This frame is synchronously revolving along with the rotor flux space vector in order to get a separate control of the torque and rotor flux. The overall strategy then consists in regulating the speed while maintaining the rotor flux constant (e.g., 1 Wb). The RFOC algorithm is directly derived from the electromechanical model of a three-phase, Y-connected, squirrel-cage induction motor. This is described by equations in the synchronously rotating reference frame as
(1)

where and are and components of stator voltage , , and are and components of stator current , is the modulus of rotor flux modulus, and is the angular position of rotor flux, is the synchronous angular speed of the reference frame , and are stator, rotor, and mutual inductances, , are stator and rotor resistances, is the leakage coefficient of the motor, and is the number of pole pairs, is the mechanical rotor speed, is damping coefficient, is the inertial momentum, and is torque load.

6.2. RFOC Algorithm

The derived expressions for each block composing the induction motor RFOC scheme, as shown in Figure 6, are given as follows:
Figure 6

Conceptual block diagram of the system.

Speed PI Controller:

(2)

Rotor Flux PI Controller:

(3)

Rotor Flux Estimator:

(4)
(5)
with
(6)
(7)
and using Clarke transformation
(8)
(9)

To be noticed that sine and cosine, of (5), sum up to a division, and therefore do not have to be directly calculated.

Current PI Controller:

(10)
(11)

Decoupling:

(12)
with
(13)

Omega ( ) Estimator:

(14)

Park Transformation:

(15)

Inverse Park Transformation:

(16)

In the above equations, for standing for any variable such as voltage , current or rotor flux , we have the following.

Input reference corresponding to

Error signal corresponding to

Proportional and integral parameters corresponding to the PI controller of

, , and three-phase components of in the stationary reference frame.

and two-phase components of in the stationary reference frame.

and components of in the synchronously rotating frame.

The RFOC scheme features vector transformations (Clarke and Park), 4 IP regulators, and space-vector PWM generator (SVPWM). This algorithm is of interest for its good performances, and because it has a fair level of complexity which benefits from a very-high-performance FPGA implementation. In fact, FPGAs make it possible to execute the loop of a complicated control algorithm in a matter of a few microseconds. The first prototype of such a controller has been developed using the method and platform described here, and has been implemented entirely in FPGA logic [10].

Commonly used mediums prior to the advent of today's large FPGAs, including the use of DSPs alone and/or specialized microcontrollers, led to a total cycle time of more than 100  s for vector control. This lead to switching frequencies in the range of 1–5 kHz, which produced disturbing noise in the audible band. With today's FPGAs, it becomes possible to fit a very large control system on a single chip, and to support very high switching frequencies.

6.3. Validation of RFOC Using Cosimulation with XSG

A strong hardware/software cosimulation environment and methodology is necessary to allow validation of the hardware design against a theoretical control system model.

As mentioned is Section 3, the design flow which has been adopted in this research uses the XSG blockset in Matlab/Simulink. XSG model of RFOC block is built up from (2) to (16) and the global system architecture is shown in Figure 8 where Gateway-in and Gateway-out blocks provide the necessary interface between the fixed-point FPGA hardware that include the RFOC and Space Vector Pulse Width Modulation (SVPWM) algorithms and the floating-point Simulink blocksets mainly the SimPowerSystems (SPS) models. In fact to make the simulations more realistic, the three-phase AC induction motor and the corresponding Voltage Source Inverter were modelled in Simulink using the SPS blockset, which is robust and well proven. To be noticed that SVPWM is a widely used technique for three-phase voltage-source inverters (VSI), and is well suited for AC induction motors.

At runtime, the hardware design (RFOC and SVPWM) is automatically downloaded into the actual FPGA device, and its response can then be verified in real-time against that of the theoretical model simulation done with floating-point Simulink blocksets. An arbitrary load is induced by varying the torque load variable as a time function. SPS receives a reference voltage from the control through the inverse Park transformation module. This voltage consists of two quadrature voltages ( , ), plus the angle (sine/cosine) of the voltage phasor corresponding to the rotor flux orientation (Figure 6).

6.4. Reducing Cosimulation Times

In a closed loop setting, such as RFOC, hardware acceleration is only possible as long as the replaced block does not require a lot of steps for completion. If the XSG design requires more steps to process the data which is sent than what is necessary for the next data to be ready for processing, a costly (time wise) adjustment has to be made. The Simulink period for a given simulated FPGA clock (one XSG design step) must be reduced, while the rest of the Simulink system runs at the same speed as before. In a fixed step Simulink simulation environment, this means that the fixed step size must be reduced enough so that the XSG system has plenty of time to complete between two data acquisitions. Obviously, such lenghty simulations should only be launched once the debugging process is finished and the controller is ready to be thouroughly tested.

Once the control algorithm is designed with XSG, the HW/SW cosimulation procedure consists of the following.

(1)Building the interface between Simulink and FPGA-Based Cosimulation board.

(2)Making a hardware cosimulation design.

(3)Executing hardware cosimulation.

When using Simulink environment for cosimulation, one should distinguish between the single-step and free-running modes, in order for debugging purposes, to get much shorter simulations times.

Single-step cosimulation can improve simulation time when replacing one part of a bigger system. This is especially true when replacing blocks that cannot be natively accelerated by Simulink, like embedded Matlab functions. Replacing a block with an XSG cosimulated design shifts the burden from Matlab to the FPGA, and the block no longer remains the simulation's bottleneck.

Free-running cosimulation means that the FPGA will always be running at full speed. Simulink will no longer be dictating the speed of an XSG step as was the case in single-step cosimulation. With the Virtex-4 ML402 SX XtremeDSP Evaluation Platform, that step will now be a fixed 10 nanoseconds. Therefore, even a very complicated system requiring many steps for completion should have ample time to process its data before the rest of the Simulink system does its work. Nevertheless, a synchronization mechanism should always be used for linking the free-running cosimulation block with the rest of the design to ensure an exterior start signal will not be mistakenly interpreted as more than one start pulse. Table 3 shows the decrease of simulation time afforded by the free-running mode for the induction motor vector control. This has been implemented using XSG with the motor and its SVPWM-based drive being modeled using SPS blockset from Simulink. For the same precision and the same amount of data to be simulated (speed variations over a period of 7 seconds), a single-step approach would require 100.7 times longer to complete, thus being an ineffective approach. A more complete discussion of our methodology for rapid testing of an XSG-based controller using free-running cosimulation and SPS, has been given in [11].
Table 3

Simulation times and methods

Type of simulation

Simulation time

Free-running cosimulation

1734 s

Single-step cosimulation

174610 s (48 hours)

6.5. Timing Analysis

Before actually generating a BIT file to reconfigure the FPGA, and whether the cosimulation is done through JTAG or Ethernet, the design must be able to run at 100 MHz (10 nanoseconds step time). As long as the design is running inside Simulink, there are never any issues with meeting timing requirements for the XSG model. Once completed, the design will be synthesized, and simulated on FPGA. If the user launches the cosimulation block generation process, the timing errors will be mentioned quite far into the operation. This means that, after waiting for a relatively long delay (sometimes 20–30 minutes depending on the complexity of a design and the speed of the host computer), the user notices the failure to meet timing requirements with no extra information to quickly identify the problem. This is why the timing analysis tool must always be run prior to cosimulation. While it might seem a bit time-consuming, this tool will not simply tell you that your design does not meet requirements, but it will give you the insight required to fix the timing problems. The control algorithm once being fully designed, analysed (timing wise), and debugged through the aforementioned FPGA-in-the-loop simulation platform, the corresponding NGC binary netlist file or VHDL/Verilog code are automatically generated. These could then be integrated within the SoC architecture using Xilinx Platform Studio (XPS) and targetting the AP1000 platform. Next section describes the related steps.

6.6. Experimental Setup

Figure 7 shows the experimental setup with power electronics, induction motor, and loads. The power supply is taken from a 220 V outlet. The high voltage power module, from Microchip, is connected to the analog I/O card through the rainbow flex cable, and to the expansion digital I/Os of the AP1000 through another parallel cable. Signals from a 1000-line optical speed encoder are among the digital signals fed to the FPGA. As for the loads, there is both a manually-controlled resistive load box, and a dynamo coupled to the motor shaft.
Figure 7

Experimental setup with power electronics, induction motor, and loads.

Figure 8

Indcution motor RFOC drive, as modelled with XSG and SPS blocksets.

From the three motor phases, three currents and three voltages (all prefiltered and prescaled) are fed to the analog I/O board to be sampled. Samples are stored in an internal input buffer until fetched by the controller on FPGA. Data exchange between the FPGA and the I/O board proceeds through the PLB and the Dual Processor PCI Bus Bridge to and from the PMC site.

The process of generating SVPWM signals continuously runs in parallel with controller logic, but the speed at which these signals are generated is greater than the speed required for the vector control processing. As a consequence, these two processes are designed and tested separately before being assembled and tested together.

Power gating and motor speed decoding are continuous processes that have critical clocking constraints beyond the capabilities of bus operation to and from the I/O board. Therefore, even though the PMC66-16AISS8A04 board also provides digital I/O, both the PWM gating signals and the input pulses from the optical speed encoder are directly passed through FPGA pins to be processed by dedicated hardware logic. This is done by plugging a custom-made adapter card with Samtec CON 0.8 mm connectors into the expansion site on the AP1000. While the vector control uses data acquired from the AIO card through a state machine, the PWM signals are constantly fed to the power module (Figure 6). Those signals are sent directly through the general purpose digital outputs on the AP1000 itself instead of going through the AIO card. This ensures complete control over the speed at which these signals are generated and sent while targeting a specific operating frequency (16 kHz in our example). This way, the speed calculations required for the vector control algorithm are done using precise clocking without adding to the burden of the state machine which dictates the communications between FPGA and the AIO card. The number of transitions found on the signal lines between the FPGA and speed encoder are used to evaluate the speed at which the motor is operating.

6.7. Timing Issues

Completion of one loop cycle of our vector control design, takes 122 steps leading to a computation time of less than 1.5  s. To be noticed that for a sampling rate of 32 kHz, the SVPWM signal has 100 divisions (two zones divided by 50), which has been chosen as a good compromise between precision and simulation time. The simulation fixed-step size is then 625 nanoseconds, which is already small enough to hinder the performance of simulating the SPS model. Since PWM signal generation is divided into two zones, for every 50 steps of Simulink operations (PWM signal generation and SPS model simulation), the 122 vector control steps must complete. The period of the XSG—Simulink system must be adjusted in order for the XSG model to run 2.44 times faster than the other Simulink components. The simulation fixed-step size becomes 2.56 nanoseconds, thus prolonging simulation time. In other words, since the SPS model and PWM signals generation take little time (in terms of steps) to complete whereas the vector control scheme requires numerous steps, the coupling of the two forces the use of a very small simulation fixedstep size.

7. Conclusion

In this paper, we have discussed our choice, adaptation, and use of a rapid prototyping platform and design flow suitable for the design of on-chip motion controllers and other SoCs with a need for analog interfacing. It supports embedded application software coupled with custom FPGA logic and analog interfacing, and is very well suited to FPGA-in-the-loop control and SoC controller prototyping. Such platform is suitable for academia and research communauty that cannot afford the expensive commercial solutions for FPGA-in-the-loop simulation [12, 13].

A convenient FPGA design, simulation, and test procedure, suitable for advanced feedback controllers, has been outlined. It uses the Xilinx System Generator blockset in Matlab/Simulink and a simulated motor drive described with the SPS blockset. SoC integration of the resulting controller is done in Xilinx Platform Studio. Our custom SoC design has been described, with highlights on the state machine for bus interfacing, NGC file integration, BIN file generation, and FPGA configuration.

Application software and drivers development for embedded Linux are often needed to provide for PCI and analog I/O card initialization, interfacing, and monitoring. We have provided here some pointers along with essential information not easily found elsewhere. The proposed design flow and prototyping platform have been applied to the analysis, design, and hardware implementation of a vector controller for three-phase AC induction motors, with very good performance results. The resulting computation times, of about 1.5  s, can in fact be considered record-breaking for such a controller.

Declarations

Acknowledgments

This research is funded by a Grant from the National Sciences and Engineering Research Council of Canada (NSERC). CMC Microsystems provided development tools and support through the System-on-Chip Research Network (SOCRN) program.

Authors’ Affiliations

(1)
Department of Applied Sciences, University of Quebec at Chicoutimi

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© Stéphane Simard et al. 2009

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