Tag Archive: GVT


Sensing temperature

Like I said already a few times is that we use ultrasound for our thesis, I also mentioned that the speed of sound is reliable of temperature and humidity.

So let’s have a closer look at how we measure temperature with sensors, I found this interesting comparison between temperature sensors so I’ll summarize it for you guys.

There are two types of sensors to measure temperature, the contact and the noncontact ones.

 

 

Here are the most common contact temperature sensor types:

A thermocouple uses two metals to create a voltage that is dependent of the temperature difference between the ambient and junction temperature. This is already a big advantage because it doesn’t need extra power and it’s also very cheap. They are robust but decay over time and can measure a big range going up to 3,000°C and as low as –250°C. Thermocouples are often used in ground vibration tests.

Thermistors (NTC and PTC) work somewhat like an LDR (light dependent resistor), so it’s a resistor which his resistance value is dependent of its temperature. These are often used as inrush current limiters, temperature sensors, self-resetting overcurrent protectors, and self-regulating heating elements. So like you see they are mostly used for simple temperature measurements.

Resistance temperature detectors (RTD) are also resistors (thin-film or wire-wound) with a resistance value which is dependent of its temperature. They are very accurate, robust and stable because it’s made of platinum, which is not affected by corrosion or oxidation. Thin-film RTD elements have a thin layer of platinum for its resistor and are used for Health & Usage Monitoring Systems.

 

 

The noncontact types are represented by the infrared temperature probes.

So every object that has a temperature above absolute zero emits Infrared radiation which is linked to its surface temperature. So by using an infrared sensor you can measure it, but you still need some electronics afterwards to process the data into something which is readable for the user.ScreenHunter_4

A problem with the use of infrared probes is that the object you want to measure needs to be bigger than the measurement spot like you see here:

measurement spot

One of the big advantages is that they can be used from a big distance and they can measure high temperatures because they don’t contact. Also measuring moving objects is an option.

 

 

 

So hope you now know a bit how they measure temperature in the world of electronics. What would you use to measure the temperature at home?

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Measuring vibration without a touch

Like I said last post, I found a paper about a new type of non contact sensor for ground vibration tests.

The sensor replaces the accelerometers that need to be placed on the airplane. It’s called a NIRV (noncontact inertia reference velocity) sensor, internally it is the combination of a laser displacement sensor and an accelerometer.

airplane

The sensors are mounted on large stands that surround the airplane, the laser displacement sensors in the NIRV sensors determine the distance between the sensor and the airplane. Because the stands are very big, they will wobble a bit themselves. To compensate this wobble of the sensor, he uses the accelerometer and makes the distance measurement more accurate.

During the GVT’s, the displacement of the airplane is registered and is expressed in terms of velocity per unit force, just like the data that the normal accelerometers produce. Because the data is expressed in the same unit, the NIRV measurements can then be compared to the results of normal accelerometers, they concluded that the 2 types give an equally good result.

The NIRV sensors have a big advantage, because the sensors are mounted on the stands, these can be used on most airplanes, so this will make you save time in the setup of a measurement. This because there is no tear down or build up needed between 2 measurements, like with the normal accelerometers.

 

So guys, see you later and hope you understood everything.

Sensing vibration

There is already a post about ground vibration tests (GVT).

In these tests they normally use accelerometers, so I think this is a good thing to blog about.

There are 2 types of accelerometers, first you have the AC – and secondly the DC – coupled devices. The AC – coupled device can’t measure static acceleration, so it doesn’t register the gravity. While a DC device can measure starting from zero Hertz.

An AC device isn’t suited to follow slow motions, because of the limitations by its RC time constant which defines its high-pass characteristics. The DC on the other hand has no problem tracking a slow motion.

A typical AC sensor is a piezoelectric accelerometer. While a typical DC-accelerometer is the capacitive (used in air-bags and mobile devices) or a piezoresistive one.

I have found a good paper about this, it gives a lot of info and a good comparison.

In GVT they use AC and DC sensors, also force and temperature sensors. The fact they also use temperature sensors is because DC piezoresistive sensors are very temperature dependent.

 

But of course the technology isn’t standing still so there is a new kind of sensor they are testing for the GVT. So stay tuned for my next post if you want to know more about it.

 

See you next time followers.

Ground vibration testing

As stated in my last post this one is providing some details about ground vibration testing (GVT). These tests are performed at the end of the development process. The main goal is to collect vibration data. This data is used to validate the aircraft structure and is helpful to predict the flutter behavior of the plane. Due to their huge importance in safety-critical flight tests multiple configurations must be tested. This causes an extreme time pressure for the engineers. The GVT is economically important for aircraft manufacturers. The video below is displaying some of the GVT tests of the solar impulse airplane. Shakers excite the airplane and accelerometers placed all over the airplane collect the responses. It’s important the exact position of all the accelerometers is known accurately. An entire GVT cycles can be done in just under a month which is rather fast for placing 700 accelerometers and over 25km of wiring. (EADS-CASA completes large-scale Airbus A400M ground vibration test in less than a month’s time

You can find more information about GVT on the LMS site

You can also check out these interesting papers about GVT:

Advanced systems and services for Ground Vibration Testing –  Application for a research test on an Airbus A340-600 aircraft

Ground Vibration Testing of Large Aircraft – State-of-the-Art and Future Perspectives

flutter testing

The main application of our thesis is in aerospace industry. This industry is under continuous pressure and has to develop aircrafts in shorter timeframes and at lower costs than ever before. A large part of aircraft development time is used for testing. This means if testing time can be reduced significantly  a shorter time to market can be achieved.

An aircraft has to be flutter certified. This is a certification at maximum flight speed to determine structural integrity and safety of the airplane. The first formal flutter test was carried out by Von Schlippe in 1935 in Germany. His approach was to vibrate the aircraft at resonant frequencies at progressively higher speeds and plot amplitude as a function of airspeed. This approach was sufficient untill a Junkers JU90 crashed in 1938 during flutter tests. Next a more advanced approach was used. The typical test procedure is visible in the figure. The full explanation is available in this NASA-report. It’s still based on the Von Schlippe methods. Firstly, structural excitation of the plane. The second part is the response measurement. This is done using hundreds of accelerometers. The last part is the data analysis.

flight_flutter_test_process

Before these tests can be started a ground vibration testing(GVT) is done to predict analytically the flutter behaviour. My next post will provide more details on ground vibration testing. You can also watch the flutter test of the A380 in the youtube video.