Developing a Real-Time System for Controlling a Universal Test Machine

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"We chose to use a complete NI PXI hardware platform running the LabVIEW Real-Time Module because of the tight hardware and software integration as well as the flexibility and scalability offered."

- Tom Savu, DOLSAT Consult, Romania

The Challenge:
Controlling the force and displacement on a 60 ton Schenk universal test machine that is used for static and dynamic tests of aircrafts and large structures or specimens for aeronautical and non-aeronautical applications.

The Solution:
Developing an NI PXI system powered by the NI LabVIEW Real-Time Module that can run eight synchronized proportional integral derivative (PID) loops.

Author(s):
Tom Savu - DOLSAT Consult, Romania
Daniel Cazacu - DOLSAT Consult, Romania

What Needed to Be Done

DOLSAT Consult, established in 2005, is an NI value added reseller that specializes in developing measurement and control solutions for a wide range of industries based on NI hardware and software. The Institute for Theoretical and Experimental Analysis of Aeronautical Structures (STRAERO), based in Bucharest, Romania, chose DOLSAT to replace its obsolete Schenck hydraulic PID system (Figure 2), which controls two Schenck Hydropuls testing machines (Figure 1) and six servo-hydraulic actuators. STRAERO wanted to be able to perform more complex, reliable tests on structures and specimens in a more user-friendly way.

As a provider of aerospace research and development services, STRAERO offers specific strength and stress resistance tests of mechanical structures, including tensile and compressive stress, fatigue, yield, and impact strength with loads up to 60 tons.

    

                         Figure 1. Schenck PID/Control/Monitoring System         Figure 2. Schenck Hydropuls Machine

The Schenck hydraulic position control system has been a reliable solution for the past 20 years and has received upgrades, including a high-accuracy, high-stability Rigol waveform generator and an HBM MGCplus data acquisition system. However, the PID control system’s limitations, combined with the complexity of integrating the new hardware upgrades from a software point of view, became obvious problems, even for performing simple tests.

The New Solution

We needed the new automation system (Figure 3) to integrate the PID capabilities for controlling the eight hydraulic servo valves serving the Hydropuls machines and actuators and to replace the old, relay-based control and monitoring system for the hydraulic infrastructure.

Figure 3: Basic Block Diagram of the Automation System

We considered solutions from MoogInc. and Parker Hannifin, but we chose to use a complete NI PXI hardware platform (Figure 4) running the LabVIEW Real-Time Module because of the tight hardware and software integration as well as the flexibility and scalability offered.

Figure 4: PXI System Configuration

The displacement transducers used in the Hydropuls machines are non-linear variable differential transformers (LVDT), HBM W Series, half-bridge inductive transducers. For compatibility, we used an HBM Clip AE501 signal conditioning module with a ±10 V output. To overcome some limitations of the delta-sigma analog-to-digital converters (ADCs) in hardware-timed single point data acquisition for the load cell signal conditioning, we used an HBM–AE101 measuring amplifier, also with a ±10 V output. To acquire the ±10 V signals from th HBM modules we used the eight differential analog input channels on the NI PXI-6220 multifunction M Series DAQ board, which proved to be more than adequate for the task.

To drive the servo valve, we used a PEES COMPONENTS AN405 servo amplifier with a standard ±10 V input that is capable of providing a ±100 mA current. Even though the system operates in a noisy electrical environment, using the NI PXIe-4322 isolated analog output module, we could drive the servo valve amplifier without any common mode voltage problems, which was a key factor for developing a reliable and deterministic control application. To acquire the 24 V logic signals from the Schenck hydraulic monitoring unit, including the oil temperature limits, pressure drops, and heating operation, we used the NI PXI-6511 bank isolated digital input module. To control the draining pumps and the safety signals we used the ABB R600 range interface solid state relays, controlled by the NI PXI-6513 isolated digital output interface module.

From the start of the project, developing the software application was a straightforward process. The new LabVIEW Real-Time Control sample project together with the extensive documentation helped us quickly prototype a fully functional application (Figure 5).The real-time application uses four parallel loops with one timed loop for managing the critical processes with hardware–timed, single-point data acquisition and three loops for managing the host PC communication, data logging, and system monitoring. We implemented the communication between the real-time application and the host PC (Figure 6) using the network streams features for both high-throughput and normal message communication.

 

Figure. 5.: Testing a flat metal bar to mechanical stress using ±5KN force sine wave(white: process variable; red: set point)

Figure 6. Basic UI Used for Testing

Conclusion

Using the NI hardware and software platform, and with support from the NI team, we started implementing a scalable, reliable solution to update a 20-year-old, industry-proven, Schenck Hydropuls system.

Author Information:
Tom Savu
DOLSAT Consult, Romania
Tel: +(40 72) 489 2180
dolsat@dolsat.com

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