The Expanded Role of Software in a PC-Based Control System

Author(s):
Joseph Damico - Sandia National Laboratories

Industry:
Government/Defense

Products:
LabVIEW

The Challenge:
Developing an extremely flexible, software-based control system for an experimental SCWO reactor.

The Solution:
Using DAQ boards to monitor and generate control signals running under LabVIEW.

Introduction
Sandia National Laboratories needed an extremely flexible control system for an experimental Supercritical Water Oxidation (SCWO) reactor. The Engineering Evaluation Reactor (EER) is a test bed for SCWO disposal of hazardous materials, such as chemical waste and munitions. The EER is controlled with a LabVIEW-based system that uses National Instruments data acquisition (DAQ) boards and RS-485 for I/O. In the past, such a control system would have been custom built from hardware components. The EER control system, however, implements as many functions as possible in software, giving greater flexibility. This article discusses the implementation and advantages of the EER software-based control system.

The Engineering Evaluation Reactor
The EER combines the technologies of high-pressure systems, combustion science, and chemistry. In the reactor, hazardous wastes dissolve easily into supercritical water (4000 psi, 500°-550° C) and react with oxidizers to produce harmless byproducts. Three high-pressure pumps supply water, oxidant, and waste solution to the reactor. The reaction takes place in a 40 ft (12.19 m.) section of high- pressure tubing. The control system must keep the reactor temperature and pressure within the supercritical range and also operate sampling valves to collect effluent for analysis.

The Control System Hardware
The EER control system uses a Macintosh Quadra 950 computer equipped with four DAQ boards. SCXI modules multiplex 160 transducer signals into one analog input on the NB-MIO-16. Thermocouple measurements use the reference junctions on the SCXI-1100 modules for cold-junction compensation. The NB-DMA2800 board provides timing control for the system and DMA service for the NB-MIO-16. An NB-AO-6 board provides voltage or current control signals. An NB-DIO-32F board provides on/off control for the valves and pump controllers. The control system hardware also includes several signal input panels and three chassis.

The temperature controller chassis, which contains the 16 dedicated temperature control loops that control the heaters via SSRs, interfaces to the computer over the RS-485 bus. The analog output chassis breaks out signals from the NB-AO-6 into current outputs that control the three pumps and a voltage output that controls system pressure. The DIO chassis breaks out signals from the DIO board to control air-operated valves and pump starters that plug into SSR-controlled AC outlets.

To implement the expanded use of software in the EER control system, all power outlets are SSR controlled. There are no hardware switches or controls in the EER control system; all components are controlled from the computer via TTL logic, analog setpoint signals, or the RS-485 bus. In a hardware-oriented system, there would be relays or other logic to interlock various reactor functions. Such hardwired logic limits system flexibility. The EER safety interlocks, implemented in Boolean software functions, eliminate the need for logic hardware and are easy to modify.

The Control System Software
The LabVIEW-based EER control system consists of 144 subroutines combined into seven processes. Other than sharing data through global variables, the processes execute independently. The independent software processes share CPU cycles based on priority. To ensure safe operation, the control processes for the setpoints and valves have highest priority, while the display-related processes have lower priorities.
Control processes drive the reactor valves and setpoints by interpreting mouse clicks into commands for the interface boards. The software controls the reactor in supervisory control and data acquisition (SCADA) mode. Using the interface boards or the RS-485 link, the computer communicates a user-selected setpoint to dedicated controllers, such as the 16 temperature control loops. Each dedicated controller controls its parameter to reach the desired setpoint. The SCADA control system provides such great flexibility that you could install different controllers in the system with little or no change to the software. While the dedicated controllers control the temperature, pump rates, and system pressure, the computer concentrates on data acquisition and the user interface.

The data acquisition process collects data from a continuous multibuffered acquisition and stores it in a global array. When data is needed, the acquisition process accesses the latest data in its buffer. Other processes running parallel to the acquisition process access the global data when needed. For example, the display process accesses the global sensor data and control status information to update the user interface.
The controls and displays in the user interface overlay a schematic of the EER. Valves open and close with mouse clicks on their schematic representation. Pump speeds and flows change as the user adjusts virtual dials and slide controls. The user interface displays key reactor parameters on the schematic. With the software-multiplexed displays, any data channel can appear in any display, so that users can easily make modifications to the user inter-face during operation.

Conclusions
As EER development progressed, users requested many changes not anticipated in the original design. The inherent flexibility of the software-based system, however, made it easy to satisfy these requests. For example, users requested a duplicate set of reactor controls in a different location. Implementing these additional controls required only the addition of a touch screen monitor and a one-day programming effort that was largely a cut-and-paste effort from the existing software.

Another request for a real-time reactor temperature profile required indexing the chosen temperature channels from the global sensor data array and building them into an array for plotting. Implementing this display took less than an hour. The design philosophy of using global structures to store and access system data provides  the flexibility to quickly make additions, such as the duplicate controls and temperature profile display.

The EER control system clearly demonstrates the power of a LabVIEW software-based system - benefits include the intuitive graphical user interface, the cost savings of eliminating many hardware components, and the flexibility of implementing hardware functionality in software.

This work was supported by the U. S. Department of Energy under contract DE-AC04-94AL85000. Reference herein to any specific commercial product does not necessarily constitute or imply its endorsement by the United States Government, any agency thereof, or any of their contractors or subcontractors.

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