When choosing an inertial product, finding the right one can be uneasy. The wide range of available products, even on a same grade, can be confusing. So the right question is: how to differentiate them?
Most inertial systems are specified on datasheets in terms of measurement range, accuracy, noise, bandwidth and so on.
These specifications are the main element to differentiate products. For in-depth comparison, other elements can be taken into account, such as components used, sensor calibration and test reports.
Specifications can be more or less optimistic depending on manufacturers and test conditions. Therefore, it is sometimes difficult to understand which product is best suited for which application.
This article will give hints and pieces of advice to read between the lines of inertial systems data-sheets.
What comparison elements do I have?
There are several elements available to anyone willing to compare two products:
- Datasheets: sometimes the only comparison element used
- Sensor grade: reveals what you can expect of a sensor
- Tests reports: shows if the expectations are met in realistic conditions
All theses elements can be used to differentiate products that may look similar at first look.
Datasheets are necessary to compare two products, but relying only on this element may lead to take biased decisions, and ending with a product that doesn't entirely comply with your needs.
Indeed, inertial systems datasheets usually specify optimal performance, in defined test conditions (such as, for 25°C). This is why performance might be degraded in harsh environments. It usually lacks information such as behaviour of the system under stress conditions.
The sensor grade represents the quality of the component, which is a good indicator of its reliability. It is a scale that is usually separated into the following categories:
It can be summarized in the following sentence “The higher, the more accurate it is”. For inertial devices, this classification is based on the drift in g for the accelerometers and in degrees per second for the gyroscopes.
Why does sensor grade matters?
As the quality increases with the grade, so does the price...
This is a trade-off between cost and reliability. Maintaining good performance in all conditions comes with a price, and every inertial devices can not afford this. Different sensor grades are available, each one answering to different kind of applications. However there is a real gap of performance, even if not clearly stated on datasheets.
This is especially obvious on low cost products without a proper embedded calibration. Even if consumer grade inertial systems show good accuracy on paper, their performance will quickly drop at any temperature change or very short GPS outages, while industrial and tactical products will handle it without significant loss of accuracy.
Below is an example of the main differences between typical consumer and industrial grade MEMS gyroscopes:
Various applications have very different specifications. Consumer and industrial grade sensors answer to different needs. To summarize the differences, the main strength of Industrial grade sensors is their reliability, when consumer grade sensors have a wider measurement range for a lower price.
These remarks are similar on accelerometers than on gyroscopes, but we won't get into the details here, as consumer grade sensors are now generally provided as "combo" sensors, featuring 3 axis gyroscopes + 3 axis accelerometer. So by finding which kind of gyroscope is used, you should also get which accelerometer is used.
Reports include test reports and calibration reports. Both documents are complementary of the datasheet and help characterizing a product.
Product specification can be showed on datasheets, but its behaviour in real situation, such as under a changing environment, can only be proven with tests results.
Many applications are subject to vibrations. Sources are multiples: engines, equipment, nature of the environment, etc. This is a frequent problem that cannot easily be avoided. This is why SBG Systems performs vibration tests on its sensors. Sensors behaviour is characterized, so the level and frequency of vibration they can stand is known, and specifically tuned algorithms can be designed. This is a critical part of the sensor selection to ensure providing high quality products.
Consequences and prevention of vibrations is detailed in this article.
All sensors components are different: manufacturing process including electronics implies mechanical stress on sensor elements. This stress modifies each sensor behaviour in an unpredictable way. This means the repeatability of inertial sensors may be compromised without a proper compensation.
Sensors are also affected by temperature variations. They can be more or less affected depending on their quality, but it will always have an impact on accuracy. If the application is airborne, marine or ground type, an inertial device is expected to keep working the same way, no matter if the temperature drops below 0°C or rises up to 40°C.
This is why an individual sensor calibration is the only way to ensure the best performance in a wide range of environments.
SBG Systems calibration procedure is one of the most advanced calibration in the MEMS sensor industry, compensating most sensors errors. In order to certify that the calibration has been applied, and that the sensor is within specifications, we deliver a calibration report with every single product. This shows the residual error we can expect in realistic conditions, not only the performance expected in the lab for a test.
You can find what is calibrated in SBG Systems products in this article.