More reliable testing: how Kistler is supporting South Korea’s space research and development

The Korea Aerospace Research Institute relies on measurement technology from Kistler to perform force-limited vibration tests on satellites. 24 force sensors – aligned in a ring structure and connected to LabAmp charge amplifiers and data acquisition units – provide the basis for integrated acceleration control to prevent damage caused by overtesting.

South Korea is one of the young member nations in the space exploration community. Activities started back in 1989, when the Korea Aerospace Research Institute (KARI) was established. Located in the central city of Daejeon, KARI is part of the Daedeok Innopolis science and research cluster where over 20,000 researchers are at work. After developing its first rockets for space vehicles in the 1990s, KARI currently focuses on developing smart unmanned aerial vehicles (UAVs), satellite programs, and – in collaboration with NASA – lunar exploration.

In an ongoing project launched in June 2018, engineers at KARI have pursued the goal of implementing an infrastructure to perform flexible vibration tests for large payloads. Force-limited vibration testing (FLVT) is a proven procedure for simulating the mechanical stresses caused by vibrations during the launch and in flight. The UUT (unit under test) is placed on a shaker that can trigger defined and controlled excitation of masses. It is important to prevent overtesting, which could possibly lead to severe damage or even destruction of the UUT: to achieve this, acceleration levels are often controlled with additional force sensors. “This method has proven more sensitive, reliable and practical than monitoring acceleration only,” according to Sung-Hyun Woo, Director and Principal Researcher in the Space Environment Test Division at KARI. “Our goal was to create a control that automatically notches the excitation in response to the feedback from the force sensors.” Notching is the technical term for a reduction of acceleration input in narrow frequency bands, and it is usually applied in frequency bands where a test object has resonances.

Highly accurate force and moment measurements

To achieve this goal, KARI’s engineers decided to integrate 24 triaxial force transducers (9377C) from Kistler. These piezoelectric (PE) sensing elements accurately measure force components up to 150 kN in three axes (Fx, Fy, Fz). Jong-Min Im, Principal Researcher at KARI and team leader for this project, explains: “We chose the preloaded triaxial force sensors from Kistler because they are easy to implement, and there’s no need for calibration after installation.” For mounting on the slip table (3.25 x 3.25 m) and to ensure accurate measurements, the 24 load cells are arranged in a ring with one large circular plate at the bottom (connected to the shaker) and another on top (connected to the UUT). “When we came to Switzerland, engineers from Kistler shared their experience from similar projects and showed us the best way to build the ring structure – that was very helpful,” Im continues. “Due to the distances between the 24 force sensors, the upper and lower rings – with a diameter of 2.3 m – must be perfectly aligned to achieve maximum accuracy.”

To ensure efficient data acquisition and transfer, the KARI engineers also integrated 20 LabAmp charge amplifiers (5165A) from Kistler. 18 of them – with four channels each – are used to process the sensor signals from the load cells; two more can be used flexibly for acceleration measurements if necessary. “These high-end, low-noise laboratory DAQ solutions made it easy to obtain the data from the transducers,” Im recalls. “Although this part of the measuring chain was realized quite quickly, it was only now that we were confronted with the real challenges. With 72 channels to be processed simultaneously, you always have to keep a close watch on the calculation time for the resulting moments. It was difficult to configure the whole hardware and software setup so as to minimize signal delay. With a longer signal delay, there is more risk of decreasing the excitation too late.”

Reliable data and fast processing enable automation

The KARI engineers’ efforts were ultimately rewarded with success. They implemented an FPGA (field-programmable gate array) that is capable of processing calculations simultaneously and independently of the number of operations. Im comments: “The resulting time for the complete force-moment calculation is now only 0.12 ms, in three loops of 0.04 ms. Many parameters and variables had to be considered before the interface between force measurement and acceleration control was established – but finally, we managed it!”

The next step was to perform a comparative test on the new control. As Im explains: “We wanted to verify whether our system really is better than a manual procedure, so we carried out a lateral vibration test with the structure in place, and with high moments acting on the base of the satellite dummy.” The test clearly showed that the system performed well. Thanks to automatic notching – which decreased the acceleration inputs from 0.15 to 0.03 g in the resonance frequency range – the moment limit of 60 kNm was not exceeded. Under manual control, on the other hand, higher values of up to 71 kNm occurred, leading to significant overtesting. The KARI engineers then went on to perform force-limited vibration tests with integrated automatic notching and limits of up to 300 kNm. The results were convincing, so they are now ready to go ahead and apply the new system to real satellites.

“Kistler has given us incredible support throughout the whole project – in terms of product quality and data reliability as well as consulting. They assisted us by sharing their vast knowledge of piezoelectric measuring chains, and they helped us to get everything going,” Sung-Hyun Woo sums up. “We also see them as a valuable partner for future projects. As well as force sensors and DAQ systems, we could benefit from Kistler's accelerometers in upcoming projects. And last but not least, another major advantage for us is the local technical support we get from Kistler Korea whenever we need it.”