Automated Control and Measurement of Chemical Agent Penetration

Author(s):
Edward Delaplaine - Mink Hollow Systems, Inc.

Industry:
Aerospace/Avionics

Products:
Compact FieldPoint, LabVIEW

The Challenge:
Controlling the concentration of a chemical agent gas simulant in a simulant challenge test chamber while measuring temperatures, pressures, and penetration levels of a unit under test.

The Solution:
Using Compact FieldPoint hardware to monitor and control the chamber – a laptop with wireless connection provides a user interface for control and datalogging while a LabVIEW real-time application maintains safe operating conditions.

Introduction

Protecting U.S. troops from chemical agents is of utmost importance for the U.S. Armed Forces. To help achieve this goal, cutting-edge filters, filtration systems, and protective materials require rigorous testing at the design and manufacture phases. Legacy chemical test chambers routinely require manual control and operation and logging of data by hand. An essential element to facilitate testing goals is the modernization and automation of these older chambers. Mink Hollow was contracted by the Chemical Biological Center at the APG to update one such chamber using off-the-shelf hardware and custom software to control and monitor various user-configurable input sensors. Hardware requirements and low software development cost led us to select National Instruments Compact FieldPoint hardware and the National Instruments LabVIEW programming environment as the system workhorses. With rapid hardware configuration and software development, we provided the Chemical Biological Center with a custom control system in a relatively short time within the constraints of a modest budget.

Concentration Control

Various tests require controlling agent simulant concentration in the test chamber to a specific level. To accomplish this, three outputs act in concert to pump, vaporize, and disperse simulant throughout the chamber. The first element, a powerful blower, is controlled without feedback to disperse vaporized chemical. The blower provides the critical airflow needed to pump and disperse the chemical, minimizing chemical gradients in the chamber.

Vaporization of the simulant occurs when the pumped chemical stream hits the second element, heating coils, at a specific temperature. Thermal accuracy is critical because over temperature condition has the potential to oxidize and alter the chemical makeup, and an under temperature condition would not properly vaporize the chemical, a critical for proper dispersion. The heating coils are thermally controlled via a PID loop with thermocouple feedback. If the system detects that the heating coils are not heating, the chemical pump is disabled as vaporization cannot occur. The third control element, also PID controlled, is the chemical pump, which uses a concentration sensor for feedback.

Tuning the three-output, two-controller system would have been a difficult task if it were not for the National Instruments PID toolkit with auto-tuning. Mink Hollow coded in the software for the two controllers the ability to enable or disable an auto-tuning mode. At system installation, the application was run in auto-tuning mode, and within a few hours both controllers were stable. Without this time saving feature, the tuning process would have taken significantly longer owing to the formidably long time constant of the concentration loop. (More than a minute is required for the concentration sensor to measure a change resulting from a single drop of pumped chemical, whereas pumping chemical for just 60 seconds oversaturates the chamber.) Once the temperature and concentration controllers were tuned, the parameters were saved to a configuration file that can be modified if necessary.

User Interface

Process Control Panel: The control and data acquisition are configured using a laptop computer wirelessly connected to the Compact FieldPoint system. The main front panel allows the user to control the blower, the temperature, and the concentration of the chamber for challenge testing. Concentration is controlling to 100mg/m3 with temperature control point set to 140 degrees F. (Note: in this instance, the thermal set point is out of the achievable range for the heating elements, yet chemical vaporization is occurring and concentration control is achieved.)

Data Viewer Panel: The second, “data viewer”, is the interface for data logging configuration. The same interface is used both to log current data and to view archived data. Stored in the log file, each channel can be configured with its own name, units, and scaling. Log files are tab delimited for easy export to Microsoft Excel for test reports, etc. When viewing or logging, users can add any channel to any plot and time-synchronize all plots, easily facilitating the simultaneous viewing of a specific time event on multiple plots.

Single Pot Panel: The last tab provides a full screen plot showing the data of any of the four plots visible in the “Data Viewer” panel.

Convenient Yet Safe Control

Because typical tests in the simulant challenge test chamber can routinely take eight hours, it was imperative for the customer to monitor the system from different offices within the control building. Wireless connectivity to the control computer achieves this goal, allowing for the monitoring of the system from any number of locations without the need for dedicated cables or network infrastructure.

Wireless connection between the laptop and the control hardware has the potential for dropped signals and is a potential problem especially during critical chamber heating and chemical pumping periods. To help safeguard against this potential, we included a LabVIEW real-time application running on the Compact FieldPoint controller, which checks for a watchdog pulse from the laptop. If a pulse is not received, the connection is assumed to be disrupted and the chamber is put into a safe operating state where the chemical pump stops and the heating element is turned off.

Because the development of a communications protocol using TCP/IP would have been time consuming, the National Instruments technology DataSocket (built on TCP/IP) was employed. After the configuration of the DataSocket clusters were completed, the laptop and Compact FieldPoint were communicating in minutes saving both development and system debug time.

Conclusion

Mink Hollow selected National Instruments hardware and software as a base platform to provide the Chemical Biological Center at the APG with a robust custom chamber control system in relatively short time within a modest budget, resulting in a successful timely and cost-effective installation. Mink Hollow Systems is looking forward to providing similar control and monitoring systems in many more legacy test chambers.

For more information, contact:

Edward Delaplaine

Mink Hollow Systems, Inc.

6880 Mink Hollow Rd.

Highland, MD 20777

Tel: (301) 854-1579

E-mail: tdelaplaine@minkhollowsystems.com

 

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