Developing the Control Center for a New Green Rocket Propellant Test Bench at the German Aerospace Research Centre

Author(s):
Sven Petersen - S.E.A. Datentechnik, GmbH
Wolfram Koerver - S.E.A. Datentechnik GmbH

Industry:
Research, Aerospace/Avionics

Products:
CompactRIO, LabVIEW, PXI/CompactPCI, LabVIEW FPGA Module

The Challenge:
Running a priority unit system and an emergency stop system in a green propellant fuel-testing facility to monitor signals from a nominal test bench control system, and performing all the necessary actions to take the test bench to a safe state in case of signal failure.

The Solution:
Using PXI measurement and control hardware to run a measurement and control center alongside a priority unit, powered by National Instruments CompactRIO, to select and decide which device is currently in control of routing the control signals between the different systems; and an emergency stop system, also powered by NI CompactRIO, to guarantee direct control of the rocket test bench in any emergency situation.

The Institute of Space Propulsion of the German Aerospace Research Centre (DLR) contributes to worldwide research efforts on the utilization of environmentally friendly, nontoxic propellants for rocket propulsion applications. Our new green propellant test facility (GPP), currently under construction in Lampoldshausen, is designed to perform engine tests with DLR experimental rocket engines under operating conditions similar to real service conditions. The GPP provides combustion chamber pressures up to 6 MPa and hot gas temperatures exceeding 3,000 °C. Our research activities at the GPP use liquid oxygen as an oxidizer and liquid methane – the most favored green propellant in Europe today – as fuel.

We operate the test bench and route all control and monitoring signals from a control room at the GPP. To run the rocket tests, an FPGA handles up to 60 digital channels to control different elements of the test bench, including valves for the fuel inlet among other signals required for sensor and actuator monitoring. The FPGA processes, visualizes, and stores the test bench signals during test execution. PXI FPGA controller devices in the measurement and control center (MCC) supervise and control performance during operation, ensuring that all safety procedures are in place and functional under all conditions.

In case the nominal control system fails to operate, there are two additional independent subsystems that also perform all safety tasks – the emergency stop system (ESS) and the priority unit (PU).

While the PU monitors watchdog signals from the MCC, the ESS performs all necessary actions to take the test bench to a safe state in case any of the watchdog signals fail.

We also needed a stand-alone real-time system that would allow programmatic shutdown of the test bench depending on the current control state, for ESS safety shutdown in particular. We needed test personnel to be able to program this system according to test-specific needs, so the software had to be flexible and open.

Solution

The test bench control and measurement electronics are placed in five 19-inch racks with separate, redundant power supplies. The control and supervisory systems are integrated in two of the five racks and consist of four major parts – three control components and an intelligent switching unit:

• The MCC (which was developed by National Instruments Alliance Partner Werum Software & Systems AG) is based on PXI measurement and control hardware.
• The manual switching panel (MSP) allows manual control of the test bench without any active system in between (such as computers or FPGAs).
• The ESS runs on CompactRIO and guarantees direct control of the rocket test bench in any emergency situation, even if the MCC is not working properly or the watchdog fails.
• The PU, which also runs on CompactRIO, selects and decides which device is currently in charge of control and routes the control signals between the different systems (manual control, PC control, and the emergency system).

The PU is responsible for safely routing the control signal lines into the systems via a switching matrix built by NI Alliance Partner S.E.A. Datentechnik. The switching matrix shifts control between the three components (the MCC, ESS, and MSP). The PU monitors an external key switch and controls the system into the different startup and shutdown modes.

The PU is built into the 19-inch, five-rack unit, which also contains the control signal switching matrix. The unit is based on a CompactRIO system, with an NI cRIO-9101 4-slot chassis with 1 M gate reconfigurable I/O FPGA, an NI cRIO-9002 real-time controller, an NI 9425 32-channel digital input module, and an NI 9476 32-channel digital output module. The NI 9425 and NI 9476 control the switching matrix and monitor the watchdogs and key switches. The matrix routes 3 x 60 inputs to 60 outputs using special switching relays to guarantee the required switching time. We can access and program the CompactRIO system during experiment preparation and setup phases via a service plate. The procedures and reaction logics to any watchdog failure from external systems are programmed into the FPGA unit to ensure the guaranteed reaction within milliseconds.

Based on the settings of the key switch or the emergency stop switch, the FPGA logic sets the output routing to the system in control at the time to perform the desired action. The test bench can only switch from manual MSP to computer-controlled MCC operation of the test bench when certain conditions – indicated by the watchdogs and switch settings – allow a safe change. In case one of the watchdog signals gets lost or the emergency stop button is pressed during MCC operation, system control switches to the ESS within a precisely defined time interval.

The ESS is also based on a CompactRIO system, with an NI 9104 8-slot chassis with 3 M gate FPGA, a cRIO-9004 real-time controller, and NI 9425 and NI 9476 digital I/O modules. The NI 9425 and NI 9476 modules handle 180 digital I/O signals. The system is integrated into a 19-inch electronic drawer, designed by S.E.A. Datentechnik.

As the backup system for the nominal MCC system, the ESS continually measures and mirrors the current state of the system’s 60 digital control lines to its 60 digital outputs, allowing the system to shut down from the test bench’s last reliable state. The shutdown itself depends on the MCC’s current state, which the ESS also monitors. Depending on the MCC’s five digital status lines, the ESS starts one of nine shutdown sequences to bring the test bench into a safe state. The experiment preparation team can easily adapt these sequences to the specific experiment needs. The FPGA software includes templates for further customer-specific sequence development.

We program the CompactRIO system with LabVIEW FPGA via an Ethernet connector behind the service plate. Besides the key switches and LED indicators, we need no additional operation consoles or PCs to perform the critical safety functions of the PU or the ESS. We achieve the overall performance, test, qualification, and validation of the system with the LabVIEW Real-Time and FPGA modules and with S.E.A. TestMaster using simulation hardware and software to simulate the environment.

Summary

Using National Instruments PXI and CompactRIO real-time technology, we designed a test bench solution to control, monitor, and switch signals and perform critical safety operations without CPU intervention. By separating different safety-critical functions into independent FPGA-controlled systems, researchers and scientists can achieve fast and simple programming of FPGA functions in changing operation environments, enabling the preparation and fail-safe performance of critical rocket experiments.

For more information, contact:

Wolfram Koerver

S.E.A. Datentechnik GmbH

51147 Köln, Germany

++49 22 03 9 80 07 0

info@sea-gmbh.com

www.sea-gmbh.com

 

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