Mitsubishi Heavy Industries Technical Review Vol. 58 No. 4 (December 2021)
1Digital Twin Technology to Achieve High Reliability andLow Cost in Full-Scale Testing of Aircraft

KUNIHIRO MORISHITA*1 HIDETOSHI ARAMAKI*2MASARU TONEGAWA*3 YOSHIYUKI TAHARA*3TADASHI MURATA*4KEISUKE MOCHIZUKI*5

When developing new aircraft, it is necessary to conduct, at the final stage, static strength
testing and fatigue testing for the entire aircraft structure and verify and ensure its strength
performance. In fatigue testing, aircraft loads corresponding to multiple flight patterns arerepeatedly applied from several thousands to several tens of thousands of times, using more than
one hundred different types of loading equipment. Interruptions of the test due to abnormalities
caused not only in the test piece, but also the equipment being tested, results in longer lead times
for development. This report presents a case of digital twin technology applied to a loading
equipment as a system for the early detection of abnormality signs in equipment to minimize delaysin testing. Using such systems, it becomes possible to conduct testing with enhanced reliability.

|1. Introduction
Aircraft are developed based on the concept of building blocks as shown in Figure 1.
Specifically, material testing, element testing, component testing and full-scale testing are
performed to evaluate and ensure the strength performance of the product that is being developed.
In full-scale testing, which comes last, the following two tests are carried out: static strength testingin which the ultimate load, 1.5 times higher than the limit load, is applied to validate the integrity
and fatigue testing in which aircraft loads corresponding to multiple flight patterns based on theexpected operations are repeatedly applied from several thousands to several tens of thousands oftimes to verify the fatigue strength. As the period of fatigue testing is long, interruption in themiddle of the process owing to test piece or equipment abnormalities significantly affects the leadtime for development. Among the equipment used in testing, we focused on the loading equipment,
the number of which used in a single test can total more than one hundred. Digital twin technology
using physical models was employed to build a system enabling the early detection of abnormality
signs in the loading equipment and the identification of their locations and the developed system
was actually used in testing. This report presents the results of this application, as well as amonitoring system in which a wireless measurement system is used to detect abnormality signs of
the jigs attached to the test piece for load application.

*1

Manager, Strength Research Department, Research & Innovation Center, Mitsubishi Heavy Industries, Ltd.

*2

Chief Staff Researcher, Strength Research Department, Research & Innovation Center, Mitsubishi Heavy Industries, Ltd.

*3

Strength Research Department, Research & Innovation Center, Mitsubishi Heavy Industries, Ltd.

*4Manager, CIS Department, ICT Solution Headquarters, Mitsubishi Heavy Industries, Ltd.

*5CIS Department, ICT Solution Headquarters, Mitsubishi Heavy Industries, Ltd.

Mitsubishi Heavy Industries Technical Review Vol. 58 No. 4 (December 2021)
2Figure 1

Building blocks for aircraft development

The figure indicates that full-scale testing is conducted as the final verification test.

|2. Abnormality sign detection system using digital twin technology
2.1

Digital twin system overview

As shown in Figure 2, the digital twin system, the applicability of which is under discussion
in this report, represents each of the more than one hundred of loading equipment (actuators) usedin full-scale testing. In this system, individual actuators are created by mathematical models(non-linear equations of motion). The internal state of these mathematically-modeled actuators iscontinuously estimated and the mathematical models used are kept updated in such a way as tominimize the difference between the measured values for a given input command (i.e.,measurement data of load and displacement sensors) and the estimated values calculated by themathematical models (load and displacement). Considered within the mathematical models, theinternal state is estimated based on the flow coefficient, which strongly correlates with
dysfunctional spool valves of hydraulic actuators and the friction force, which is strongly correlated
to cylinder seizure or the deterioration of sealing elements. The internal state, which graduallychanges with factors such as time-related changes, can thus be recorded numerically. This enables
abnormality signs to be identified by monitoring whether the recorded value exceeds the preset
threshold value. An unscented Kalman filter was used to estimate the state in the system, because
non-linear problems can be solved without linearizing a non-linear function and it was suitable foronline calculation.

Figure 2

Digital twin system overview

The figure shows the flow of data processing in the digital twin system with regard to actuators(test equipment).

Mitsubishi Heavy Industries Technical Review Vol. 58 No. 4 (December 2021)
32.2

Configuration of the abnormality sign detection system

Figure 3 shows a system configuration for abnormality sign detection. A full-scale testing
network mainly consists of the test equipment, control unit, data collection unit, server and PC fortest control. With the addition of a digital twin system-enabled PC for monitoring, data are sent andprocessed every time one cycle of the test is completed, thereby monitoring whether anyabnormality values are observed.
As shown in Figure 4, the information displayable on the monitoring screen includes thenormal or abnormality status of each actuator installed on the aircraft (e.g., normal: yellow,
warning: pink and critical: red), the load being applied, the length of cylinder stroke and thetemporal change of estimated values for the internal state of mathematically-modelled actuators.
The screen display has been made visually easy to notice abnormality conditions when they occur.
In particular, with regard to the estimated values of the internal state, importing past test data
makes it possible to check on how it has changed with time, helping to understand the tendency ofabnormality occurrence.

Figure 3

Configuration of the abnormality sign detection system

The figure shows the positioning of digital twin system in the abnormality sign detection system infull-scale testing together with the flow of data processing.

Figure 4An example screen display of the digital twin system

The figure gives an actual example of the digital twin system showing the display state and the trend of theinternal state of an actuator with an abnormality.

Mitsubishi Heavy Industries Technical Review Vol. 58 No. 4 (December 2021)
42.3

An example of detected abnormality conditions

After the digital twin system was introduced, no abnormality events occurred as a result of
the change in the internal state of actuators, as originally anticipated. However, as shown in
Figure 5, the system identified a temporary failure of a sensor (loosened connectors) as anabnormality condition, thereby demonstrating its capability of detecting abnormality signs at an
early stage even if they were not caused by changes over time.

Figure 5

An example of detection of abnormality conditions

The figure shows a case in which the digital twin system detected
abnormality conditions resulting from loosened connectors, instead ofchanges in the internal state.

|3. Wireless measurement monitoring system
3.1

Wireless measurement system overview

The main purpose of the aforementioned digital twin system is to detect abnormality signs
based on the estimated internal state of actuators. As shown in Figure 6, in the case of load
application in full-scale testing, the loading jigs are attached to the test piece to transfer load on to it
and the load is induced by actuators that are connected to these jigs. As it is not possible to directly
bolt the jigs to the test piece, a rubber pad is adhered and sandwiched between the test piece and amounting plate to which a jig is connected using mounting bolts. Long-term testing has the risk of
causing issues such as the degradation of adhesion strength, as well as the loosening of mountingbolts. The monitoring system described in this section, therefore, aims to detect such abnormality
signs (based on the change in the displacement).
While the stroke of an actuator can be used to monitor the overall displacement of a loading
jig, it is not sensitive enough to measure extremely small changes caused in the aforementioned
adhered parts and bolted joints. Therefore, a clip gauge was introduced as shown in Figure 7,
thereby enabling the detection of minute changes on the order of several tenths of millimeters. To
make it possible to install wherever monitoring is considered necessary, a wireless measurement
system with a wireless logger as shown in Figure 8 is employed for data collection. This eliminates
the necessity of work for cable connection between a sensor and the measurement device, which isotherwise necessary for every installation. Prompt start of monitoring has thus become possible.

Mitsubishi Heavy Industries Technical Review Vol. 58 No. 4 (December 2021)
5Figure 6 Illustration of the structure of load application
to the test piece
The figure shows the mechanism of transferring load on to the testpiece via the loading jig and a rubber pad from the actuator.

Figure 7

How to install a clip gauge

The figure shows an example of how to install a clip gauge for themeasurement of minute changes in different directions.

Figure 8 Monitoring system configuration with a wireless logger
The figure illustrates a monitoring system consisting of the wireless logger, relaying
device and receiver.

3.2

Abnormality sign detection method by the monitoring system

In full-scale testing, aircraft loads corresponding to multiple flight patterns are repeatedly
applied from several thousands to several tens of thousands of times. Monitoring was conducted
regarding the flight patterns for which the loads were frequently applied in testing. For thispurpose, we adopted a system in which abnormality signs are detected by monitoring the rate ofMitsubishi Heavy Industries Technical Review Vol. 58 No. 4 (December 2021)
6

change in displacement as shown in Figure 9. Compared with the unaffected starting state at theinitial stage of testing, a series of displacement changes during the first application of load are used
to calculate the difference between the maximum and minimum values (i.e., amplitude) and theobtained amplitude is set as the reference value. Whenever load is applied for the Nth time, theamplitude is calculated and the rate of its deviation from the reference value is assessed to examine
the conditions for abnormalities. Setting a threshold value for the rate of deviation enables thealarm to sound automatically when abnormality signs are detected.

Figure 9 Conceptual diagram of how to detect abnormality signs based on the change indisplacement
The figure shows how abnormality signs are detected based on the change in the amplitude of displacement
using the test results of repeated applications of the same load pattern.

3.3

An example of detected abnormality conditions

After the displacement monitoring system was introduced, no abnormality events occurred asa result of the change in the displacement of actuator stroke or clip gauge. As a reference,
Figure 10 gives an example of a case in which abnormality signs were detected, when loosening of
the adhered parts and bolted joints between the test piece and the loading jig was intentionallyinduced prior to the official introduction of the system. This has demonstrated that testing can beeffectively stopped by the monitoring system, when the rate of deviation exceeds the preset
threshold after being increased slightly by gradually-increasing degree of loosening of the joints byjudging the test result as abnormality.

Figure 10 An example of detection of abnormality signs by the monitoringsystem
The figure shows the results of a case conducted to assess the accuracy of detection under
the intentionally induced abnormality conditions, before the system was officiallyintroduced.

|4. Estimation of the effect of test process shortening
After the aforementioned abnormality sign detection system was introduced in actual
full-scale testing, no serious events were detected. However, it has been indicated that the early
detection of abnormality signs by the system can shorten the process of full-scale testing by nearly
10% within the test period covered by this test, when compared with the entire process containing
the interrupted periods which were referred to estimation based on past cases, as shown in
Figure 11. When the scheduled test period is longer, the probability of the occurrence of equipment
abnormality is expected to increase and as a result the effect can be greater.

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Figure 11 Shortening effect on the test process by application ofabnormality sign detection system.
The figure illustrates a shortening effect of the system on the test process, when comparedbased on past cases. In the case of the full-scale testing in this report, it has been indicatedthat the use of the system can shorten the test process by nearly 10% within the test period
covered by this test.

|5. Conclusion
This report summarizes the abnormality sign detection systems for the long, continuouslyoperating test equipment in full-scale fatigue testing conducted in the process of developing
aircraft. Their demonstrated effectiveness for the detection of abnormality signs by this system wasalso presented. The two systems described in this report are the digital twin system to detect
abnormality signs shown by the internal state of actuators and the wireless measurementmonitoring system to identify an extremely small, but abnormality change in the displacement ofloading jigs. The digital twin system can be deployed with the actuator information, load and stroke
measurement data. The wireless measurement monitoring system is applicable without cabling
work, if sensor installment is spatially allowable.
In this testing, in addition to these two systems, various other types of monitoring systems
such as hydraulic power unit monitoring and actuator hydraulic valve monitoring are also
employed as precautions for long-term continuous operation, in order to execute testing in a stablemanner. While promoting the adoption of these systems for similar tests, we will further improve
the reliability and cost effectiveness, as well as continue to develop technologies to help realize ashorter lead time for development.