Updating a complex control system

senior design team from the New Mexico Institute of Mining and Technology, located in Socorro, New Mexico, was faced with the challenge of redesigning the control system for a light gas gun facility. The facility is located at Los Alamos National Laboratory (LANL) in Los Alamos, New Mexico. The senior design team worked with the Materials Dynamics Group at LANL, which encompasses cradle-to-grave aspects of energetic materials and dynamic properties of other materials. The Materials Dynamics Group is involved in the conceptualization, synthesis and remediation, formulation, sensitivity testing, characterization, and performance evaluation of high explosives. They also work with other materials such as metals, polymers, polymeric foams, and composites to study their responses to extreme temperatures and pressure conditions that occur during high velocity impacts. The light gas guns are an integral part of the Materials Dynamics Group’s experimental scientific toolset and are used to generate planar shock waves in materials. The control system for these gas guns was last updated in 1999 and was in danger of becoming outdated in the near future.

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requiring three individual firing programs. By the use of a light gas (in this case helium) at high pressures, a projectile is accelerated to an extreme velocity and collided with a test material. Figure 1 shows a basic diagram of the two-stage light gas gun. These guns test solid or liquid materials, including some explosives. The pressure of the gas in the breech can reach up to 15,000 psi, and projectile velocities of about 0.3–3.4 km/s are obtained. The group uses a vast array of measurement equipment to analyze the effects of impact on test materials. The firing sequence for the two-stage light gas gun starts with the release of the piston loaded into the pump tube. This piston is held in place by a vacuum system. The breech pressure accelerates the piston forward; as a result, the gas in the front of the piston is compressed. When the pressure in front of the piston reaches the desired pressure, a rupture disk in the transition section breaks and allows the pressure to accelerate the projectile down the launch tube into the target. After the projectile impacts the target, the target and projectile are decelerated and contained in the catch tank. Figure 2 is a picture of the two-stage light gas gun located at LANL. Due to the hazards represented by high pressures, extreme velocities, and explosives, the gun must be operated from an adjacent control room. Safety is ensured by not

Cole T. Brinkley, Alejandro G. Gauna, John Montoya, Micah J. Yates, Mehdi Zizah, Hector Erives, and Dan E. Hooks

Light Gas Gun Operation The gun facility operates both a two-stage gun and a singlestage gun, which can be operated in two configurations, thus October 2006

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Transition (High Pressure) Section

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Fig. 1. Two-stage light gas gun diagram.

allowing the operator to fire the gun, unless predefined parameters of the system have been met. The control system software and data acquisition (DAQ) hardware that currently operate both the single-stage and the two-stage gas guns have become outdated, therefore redesign was necessary for better functionality. The outdated system operated via four National Instruments (NI) PCI DAQ cards, with over 170 I/Os. One challenge the team faced was connecting all of the signals to a new control system. The method of integration had to be modular to ensure that the new system could be developed without impeding ongoing research. This was an essential constraint since the guns are in constant use and a delay in the transition of the control systems would result in costly delay to the entire staff of the Materials Dynamics Group, which is made up of more than 50 people. This modular design also allowed for simpler integration of the updated control system. The hardware and software chosen for the new system had to accommodate all of these requirements.

The Hardware for a New System The team researched hardware options and decided the most suitable solution was to use NI DAQ hardware. Although there are other types of computer-based DAQ

hardware in existence, the software support is limited or nonexistent for other brands. The main difficulty with most other types of DAQ hardware is that driver software would have to be written to interface the DAQ hardware to the desktop PC. With NI DAQ hardware, the drivers for LabVIEW are provided. In addition, the Materials Dynamics Group was already familiar with LabVIEW software and NI DAQ hardware. Once the hardware was chosen, the type of PC interface to be used for the DAQ hardware needed to be addressed. The viable options included PCI, USB 2.0, PXI, and SCXI. The existing system used four PCI DAQ devices, and most new desktop PCs have only three PCI slots on the motherboard. This was a key variable in planning, as PCI slots are being phased out, to avoid further upgrade difficulties in the future. The Materials Dynamics Group was not comfortable with the specialized PXI system, which is basically a Windowsbased computer with a special interface for NI DAQ hardware built into the computer case. The PXI system is also very costly. The remaining options of USB and SCXI interface protocols are very similar because they both use USB to interface to the computer. The important difference is that SCXI is a chassis that holds all the DAQ hardware and then connects to the computer via USB, whereas with the USB DAQ hardware, each device has its own USB interface to the computer. The choice boiled down to cost. It would have cost about twice as much to implement the system with SCXI because of the additional cost of the SCXI chassis. As for deciding on the particular devices that were needed to design the new control system, a detailed set of documentation for each signal in the existing system had to be created first so that appropriate devices could be chosen. Based on the number of digital and analog I/Os documented, the configuration was reduced from four PCI devices to three USB devices. Figure 3 shows the connections to the system equipment.

Interfacing the Updated Control System

Fig. 2. Two-stage light gas gun.

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A means for interfacing the updated control system into the overall system needed to be identified. We first considered designing and building a large interface card that would have connectors that matched the existing ribbon cables on one side and a large block of clamp-down screw terminals on the other side. However, the cost and time constraints of designing and

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October 2006

building such an interface were reduced by simply purchasing signal breakout boards from NI. These boards are designed to interface to a system using the same PCI DAQ devices that are part of the existing system. An ideal solution was to use them in reverse of their intended function to interface with the overall system through the existing ribbon cables. This interface allowed the new system to be tested and integrated by unplugging the ribbon cables from the back of the current desktop PC and then plugging them into the appropriate locations on the breakout boards. To create this interface, every signal had to be located and wired between the USB DAQ hardware and the breakout boards. This had to be done in such a manner that all of the signals would be wired to the correct terminal on the USB DAQ hardware. Determining the correct wire pattern was accomplished by working through all of the existing programs and identifying what was connected to each terminal on the existing PCI DAQ hardware. Next the wiring pattern was translated onto the USB DAQ hardware. This was a very long process because there were over 170 terminals on the existing system spread out over three control software programs, all of which had to be documented and translated onto the new USB DAQ hardware. Since there was some redundancy in I/O wiring across, the three programs, splitting the I/O mapping duties across the team by program and comparing results, was an effective error-checking technique. Figure 4 shows the new DAQsystem interface. The only problem that was found in the testing phase of the design was that not all digital input channels worked the same on all devices. The digital input channels on the existing PCI DAQ hardware were implemented using internal pull-down resistors that would read false into the control software until a 5-V signal was generated at the Fig. 3. I/O lines.

input terminal, which would subsequently read true into the control system software. However, on the new USB DAQ hardware, the digital input channels were designed using internal pull-up resistors that would read true into the control system software until the input terminal was grounded at which time the control system software would read false. This fundamental difference in the design of the digital input channels meant that external pulldown resistors of a significantly lesser value than the internal pull-up resistors had to be wired between each digital input terminal and the signal ground.

The gun facility operates both a two-stage gun and a single-stage gun, which can be operated in two configurations, thus requiring three individual firing programs.

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The New Software The existing light gas gun control system was designed using the LabVIEW 6.i development package that interacted with the PCI DAQ hardware. The main objective was to implement, with no down time, an updated control system that included a new desktop PC, and the USB DAQ hardware. The updated system is faster, more powerful, and simplifies the subsystem interfaces, such as the interface between the light gas gun equipment and the DAQ hardware as well as the interface between the DAQ hardware and the PC. The change in DAQ hardware required a compatible update in the control system software package. The latest version of LabVIEW, LabVIEW 8.0, was chosen because of its advanced

Fig. 4. Updated control system interface.

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ability to communicate with the USB DAQ hardware. Moreover, the Materials Dynamics Group was more familiar and comfortable with LabVIEW and NI hardware than any other type of control software and DAQ hardware. Figure 5 is a screen shot of the control system graphical user interface (GUI). Before any code was rewritten, the existing programs needed to be completely understood. Every button, indicator, and gauge displayed on the GUI was examined and documented according to the existing setup. Each signal was documented according to what it controlled or monitored, the PCI card to which it connected, and its pin location on the PCI card. Again, this was a very long process because of the large number of connections that had to be documented. When the control system was first designed, it was fairly well documented. A previous revision of DAQ hardware, several revisions of control software programs, and computing platforms have been made since the birth of this system over ten years ago. However, with these system updates, the documentation was not updated to match the revisions. With this new exhaustive documentation, the system is fully updated and more than ready for the next revision. Good documentation is necessary for any control system update. Figure 6 is an illustration of the state representation of the control system programs. After completely understanding the old programs and how they were set up, they were modified for the new hardware. To reprogram the existing software, the parts of the existing code that communicated with the PCI DAQ hardware needed to be identified. After these parts were identified, the additional functionality of NI-DAQmx was used to create functionally equivalent sections of code to communicate with the new USB DAQ hardware. For example, in the existing light gas gun con-

trol system, an initial configuration of the DAQ hardware was setup in the first part of each program. Also, some particular subblocks of code were used to communicate with the DAQ hardware. These sections of code were analyzed and redesigned to be functionally equivalent but communicate with the USB DAQ hardware. One particularly useful feature of LabVIEW was the ability to use local and global variables to store data that needed to be sent out to the DAQ hardware. Figure 7 is a screen shot of the code used to change the data on the digital output lines on the USB DAQ hardware.

The system is fully updated and more than ready for the next revision. Good documentation is necessary for any control system update.

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Improvements Added to the System In addition to redesigning the existing control system, extra features were added to make the system more reliable. These extra features also help streamline work on the light gas guns and will reduce unexplained failures in the system. One key problem with the old system is that it did not allow the operator to know if an error occurred. If an error occurred, the system went into abort mode, which released all of the pressures and vacuums in the gas gun and damaged experimental setups as well as several wasted days of preparation time. The only way to prevent future errors was to understand the problems that currently exist. To determine any errors in the light gas gun operation, we created a data log of the key system variables. System settings were previously hand written during each test, which was not always convenient or accurate. These new computer-based data logs also help the members of the Materials Dynamics Group recreate successful experiments by giving them the exact system data at the time of each test. To make identifying problems with the gun

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Fig. 5. The GUI for the two-stage light gas gun control system.

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operation easier, the software during the operation of the light checks the safety parameters of gas gun by creating a pop-up the system and delivers a cusmessage whenever an error tom pop-up message to the occurs. Pop-up messages will user at the end of each test. occur if any of the safety paramThe updated system coleters shown on the front panel lects all of the important sysare broken, such as when a tem variables into global manual abort is triggered or variables during the normal when a safety door is opened. operation of the control softThis was accomplished by modware. After the control software enters the shutdown ifying a sample program called MessageBox.vi [1]. All safety sequence, all of the data in the global variables are convert- parameters as shown on the front panel are collected into ed into string format and subsequently saved in an Excel file, properly named with the date and Initialize and Monitor the the gun name. This was accomStart the Configure DAQ Pressures of plished by using two LabVIEW Control Loop Hardware the System functions. The first function, FormatDate/TimeString, is used to create a custom file name for each data log. The second function, Decide to Proceed Output the Monitor the with the Control Control Signal Position of All WritetoText File, is used to create System or Enter the to the Pressure Valves and the Excel file. Each system variable Shutdown Sequence Intensifier Interlocks is named in the first column and the corresponding data are given in the second column. The Excel file is organized by first listing gun pressures Release the Turn Off Turn Off Power and then all of the safety parameters. Pressures and Wait Pressure to All System All of the Excel files are saved in the for the System to Intensifier Equipment Reach Equilibrium same directory on the PC. The updated system also alerts the operator to any errors that occur Fig. 6. A state diagram of the control programs.

In addition to redesigning the existing control system, extra features were added to make the system more reliable.

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Fig. 7. LabVIEW code used.

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global variables. Then, at the end of the control system shutdown sequence, the safety parameters in the global variables are searched for unexpected data. If any unexpected data is found, the software generates a pop up message explaining the nature of the event. Figure 8 is an example of a pop-up message generated after the light gas gun shuts down.

Not all digital input channels worked the same on all devices.

[4] T. George, A Simplified Technique of Control System Engineering; Graphical Methods of Understanding and Improving Process Control. Philadelphia: MinneapolisHoneywell Regulator Co., 1958.

Cole T. Brinkley has spent the last five years working on his B.S. degree in electrical engineering at New Mexico Institute of Mining and Technology. He plans to continue his work with the Materials Dynamics Group at Los Alamos National Laboratory as Post B.S. Tech and pursue an M.S. degree in control system engineering. Alejandro G. Gauna received his electrical engineering degree from New Mexico Tech in 2006. He held previous job positions as engineering technician, research assistant, and manufacturing engineer. He is currently working for Procter and Gamble as a product supply engineer.

Fig. 8. An example of a pop-up message.

Conclusions Ultimately, the upgrade of the control system was a success. The Materials Dynamics Group was left with a GUI with which they were already familiar as well as hardware with faster and easier connectivity. The documentation provided gives an explanation of what applications each port serves, and where a single signal can be located on any particular DAQ card, which will allow for an easy understanding of the program as a whole. The additions to the old system give the customer increased reliability, increased ability to troubleshoot problems, and accurately logged information to help repeat successful tests. This case study can easily be applied to other control systems of similar applications. Although the methods and tools of implementation may vary, the presented systematic approach explains the thought process that you must go through to provide the most appropriate solution. You must consider not only the hardware and software aspects of the project, but also the potential applications the system may be used for in the future. By acknowledging each issue a customer may have, an approach can be taken to not only address each concern, but also to include additional features that enhance the overall control system.

References [1] J. Johnston, “www.ni.com,” May 5, 2006 [Online]. Available: http://sine.ni.com/apps/utf8/niepd_web_display.display_epd

John Montoya will be continuing his education as a graduate student in electrical engineering, researching the field of fiber-optic chemical sensors with NMT professor Dr. Xiao. His special interest is in fiber optics. Micah J. Yates received his electrical engineering degree from New Mexico Tech in 2006. He has held positions at Sandia National Laboratories as an undergrad. Mehdi Zizah graduated with bachelor degrees in both electrical engineering and mathematics from New Mexico Tech.. He is currently working with Halliburton as a logging field engineer. Hector Erives received his bachelor’s degree from Instituto Tecnologico de Chihuahua in 1988, his masters degree from University of Texas at El Paso in 1990, and his doctorate degree from New Mexico State University in 1996. From 1997–2002 he worked as a staff scientist at Opto-Knowledge Systems, Inc., where he was involved in the calibration of imaging instruments and the development of spectral analysis algorithms. From 2002–2004, he worked as a lead calibration engineer at Science Systems and Applications, Inc., where he was leading the calibration efforts of two NASA satellite sensors. He is currently an assistant professor at the Electrical Engineering Department at New Mexico Institute of Mining and Technology.

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For Further Reading [1] B. Rick, M. Taqi, and N. Matthew., LabVIEW Advanced Programming Techniques. Boca Raton, FL: CRC Press, 2000. [2] E. Nesimi, LabVIEW for Electric Circuits, Machines, Drives, and Laboratories. Englewood Cliffs, NJ: Prentice Hall, 2002. [3] S. Stanley, Modern Control System Theory and Application. Reading, MA: Addison-Wesley, 1978.

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Dan E. Hooks is a technical staff member at Los Alamos National Laboratory. His research is focused on understanding the properties and detonation characteristics of crystalline molecular explosives, and he performs experiments that include light gas guns. He received his B.S. degree from the University of Wisconsin—Madison and his Ph.D. from the University of Minnesota, both in materials science and engineering.

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October 2006