www.mentor.com/systemvision 

Combining ModelSim™ and Simulink® with SystemVision™ in an Integrated Simulation Environment

R. Scott Cooper Mentor Graphics Corporation

Combining ModelSim and Simulink with SystemVision - 2 -

Introduction Multi-technology system development poses many challenges for conventional design tools. Multi-technology designs are complex because they can incorporate many different technologies within a single system. These systems typically include analog (continuous) and digital (discrete) effects, often spread across several areas of engineering specialization. As a result, such systems are commonly designed and verified with a combination of CAD/CAE tools.

Simulation environments that can directly use such tools as resources for system-level design and verification are essential for two main reasons: 1) to allow the integration of individual components and subsystems into a single, simulatable system, and 2) to allow effective communication and design collaboration among individuals involved in the development of such systems.

This paper discusses one such environment, the SystemVision™ Modeling Solution. SystemVision has the ability to effectively integrate compiled ModelSim libraries, Simulink block diagrams, and additional multi-technology design elements into a single simulatable system.

This was demonstrated by developing an Unmanned Aerial Vehicle (UAV) system with the combined power of these tools. The role of these tools was as follows:

• SystemVision – this tool provided the ability to model detailed physics-based devices in the electrical and mechanical domains. SystemVision was also used as the integration environment for the entire system simulation.

• ModelSim – this tool was used to develop and test two digital subsystems to be ultimately implemented as FPGAs.

• Simulink – this tool was used to develop a servo controller that is embedded as part of the UAV Rudder subsystem.

Unmanned Aerial Vehicle (UAV) Overview Referring to the system implementation illustrated in the cover figure, the unmanned aerial vehicle system works as follows: the vehicle operator,

stationed on the ground, operates a hand-held joystick device (shown in the Command block). This joystick device produces two analog commands that control the throttle (propeller speed) and rudder (steering), respectively.

Both the throttle and rudder commands are multiplexed and sampled by an A/D converter and the resulting digital words are sent to the Encoder block. The digital words are then combined to form a serial bitstream. These actions are controlled by a state machine within the Encoder block.

Data is transmitted to the Flight Vehicle in a standard RF format, where it is demodulated and decoded back into the original binary logic format of the bitstream.

The two channels within the bitstream are identified and converted back into the digital word representations of each joystick command. This digital information is then used to generate voltage pulses, whose width is proportional to the value of the commanded data. A second state machine, this one located in the Decoder block, sequences all of these actions.

The pulse width data for each channel is converted back into an analog representation of the input command and filtered in the Conditioning block before being driven into the Throttle and Rudder subsystems. The Throttle subsystem is not discussed in this paper.

The Rudder subsystem consists of a servo controller, power amplifier (not shown), DC motor and rudder assembly. The servo controller ensures that the actual rudder position closely matches the commanded profile. The rudder assembly consists of two control horns to convert between rotational and translational movement. Rudder positioning is achieved via these control horns, coupled through a translational linkage.

The Impact of Specialists In an ideal world, each subsystem of this design would be developed with a common, universal tool that could also be used to perform integrated systems-based analysis. For example, all of the subsystems in the UAV design can be fully developed and analyzed using SystemVision alone. Unfortunately, this is not the approach typically

Combining ModelSim and Simulink with SystemVision - 3 -

taken in practice. A more likely scenario is one in which technology specialists each develop design subsystems using their own preferred tools, and then contribute these subsystems to the overall design.

The UAV design could easily involve three such specialists: an analog and mixed-analog/digital specialist; a digital/firmware specialist; and a controls specialist.

The SystemVision environment itself would be a natural fit for the analog and mixed-analog/digital specialist. The ability to freely combine analog and digital effects—both at the schematic level as well as within various models themselves—is essential for this type of system.

The digital/firmware specialist would likely be familiar with tools focused exclusively on digital or firmware design. Many such tools exist, one of the most popular being ModelSim, the digital simulator of choice for a variety of front-end tools.

The controls specialist would likely be comfortable in the MATLAB®/Simulink environment, as this tool dominates the controls field.

So in the real world, the three specialists would likely use three separate tools to develop their various subsystems. And this approach works fine while the subsystems are initially under development. But what happens when the subsystems need to be integrated together in order to analyze systemic interactions between them? This is where some problems arise.

In many cases, the problem of integrating subsystems is left to the prototyping phase of the design. One major drawback to this approach (and there are many) is that none but the most obvious problems will be detected in a single prototype, and it can be quite expensive (in terms of both time and money) to make major design changes during this phase.

A better approach would be to simulate the subsystems together in a “virtual prototyping” environment, prior to creating any actual hardware. This was the approach taken here. How the subsystems of the UAV design were virtually integrated together is discussed next.

ModelSim and SystemVision Both the Ground Controller (Encoder) and Flight Vehicle (Decoder) blocks are composed entirely of digital components, and would likely be developed by the digital/firmware specialist using a tool like ModelSim. Compiled “resource libraries” would be generated from ModelSim, and then read by SystemVision.

How is this accomplished? Along with its mixed-analog/digital solver, SystemVision also incorporates a tightly-integrated version of the ModelSim solver at the kernel level, such that both simulation solvers run in the same Windows® process. This means that the simulation is quite efficient, and that the user doesn’t need to explicitly direct the ModelSim portion of the simulation through ModelSim itself.

In the cover figure, the Encoder and Decoder block symbols simply “point” to their respective design units that have been stored in ModelSim resource libraries. These design units then become part of the overall system simulation.

Simulink and SystemVision For the Rudder subsystem’s servo design, SystemVision co-simulates with Simulink over a virtual bus. Although this communication occurs synchronously between the two tools, each tool can simulate its portion of the design independently between synchronization points, using either fixed or variable time steps. Both continuous and discrete designs are supported.

The tools communicating over the virtual bus can be located on a single computer or implemented over a network or the internet. This allows co-simulation to be achieved across geographically different locations, if desired.

How SystemVision and Simulink connect to the virtual bus is depicted in Figure 1. Interface components are added to the desired ports in the SystemVision design, and corresponding components are added to ports in the Simulink design. These components also allow users to set communication rates between the tools.

Combining ModelSim and Simulink with SystemVision - 4 -

Figure 1 - Interface Between SystemVision and Simulink

Simulation results for this system in response to arbitrary rudder positioning commands are shown in Figure 2. The waveforms given include the commanded profile from the Ground Controller (Rudder Command), the PW2Analog converter output (PW2Analog Output) and the actual rudder position (Rudder Output) from the Flight Vehicle.

Figure 2 – UAV Simulation Results

Summary Complex multi-technology system development requires specialists who use preferred “technology-specific” tools to develop their designs. It is often the case that these designs are actually subsystems of a larger system. It then becomes essential to have a simulation environment that can use these technology-specific tools as resources to allow simulations of the larger system. The SystemVision Modeling Solution was used as such a simulation environment to incorporate ModelSim design units and a Simulink block as part of a complex multi-technology UAV design.

For More Information

• For a detailed description of the UAV system discussed in this paper, please refer to: “Design Team Collaboration within a Modeling and Analysis Environment,” as well as related case studies. These can be found at the following web location: http://www.mentor.com/system/tech_pubs/

• Download SystemVision for hands-on system modeling and analysis. All of the UAV project files are available with the download: www.mentor.com/systemvision/downloads.html

• Learn more about VHDL-AMS by reading “The System Designer’s Guide to VHDL-AMS: Analog, Mixed-Signal, and Mixed-Technology Modeling.” Written by Peter Ashenden, University of Adelaide, Gregory Peterson, University of Tennessee, and Darrell Teegarden, Mentor Graphics. http://books.elsevier.com/mk/default.asp?isbn=1558607498

 www.mentor.com/systemvision  TECH8870‐w

For more information, call us or visit www.mentor.com/systemvision Copyright © 2010 Mentor Graphics Corporation. This document contains information that is proprietary to Mentor Graphics Corporation and may be duplicated in whole or in part by the original recipient for internal business purposed only, provided that this entire notice appears in all copies. In accepting this document, the recipient agrees to make every reasonable effort to prevent the unauthorized use of this information. Corporate Headquarters Mentor Graphics Corporation 8005 S.W. Boeckman Road Wilsonville, Oregon 97070 USA Phone: 503-685-7000

North American Support Center Phone: 800-547-4303 Fax: 800-684-1795

Europe Mentor Graphics Deutschland GmbH Arnulfstrasse 201 80634 Munich Germany Phone: +49.89.57096.0 Fax: +49.89.57096.400

Japan Mentor Graphics Japan Co., Ltd. Gotenyama Hills 7-35, Kita-Shinagawa 4-chome Shinagawa-Ku, Tokyo 140 Japan Phone: 81-3-5488-3030 Fax: 81-3-5488-3031

Silicon Valley Mentor Graphics Corporation 1001 Ridder Park Drive San Jose, California 95131 USA Phone: 408-436-1500 Fax: 408-436-1501

Pacific Rim Mentor Graphics Taiwan Room 1603, 16F, International Trade Building No. 333, Section 1, Keelung Road Taipei, Taiwan, ROC Phone: 886-2-27576020 Fax: 886-2-27576027