Xsens DOT Sampling Rate
Xsens DOT internal sampling rate is 800Hz, meaning that its sensing elements, mainly accelerometer and gyroscope, sample data each 1/800s (different case for the magnetometer, not topic of this article). The strapdown integration (SDI) algorithm receives the high-rate sensor input (of 800Hz) and processes it into a lower-rate signal at 60 Hz. The main advantage of using the SDI algorithm is that the accuracy is maintained, and information is preserved to ensure precise orientation tracking. For additional information on Xsens SDI algorithm, check out our MTw whitepaper. This large amount of data is constantly fed into the Xsens XKF-Core for additional processing. As a result, its output is constant and equal to 60Hz for each data output and cannot be changed. This yields data packets being output every 1/60s or 16.666667ms. Different values from this might indicate that data loss has occurred during your measurement.
How to check for data loss
To quickly assess whether you might have encountered data loss, it is good practice to check the size of your recordings files, when you use multiple sensors, and make sure the file sizes are comparable. In the example below, one file within the first take had half the data of the others.
Another easy way to assess that is to compute the difference in Timestamp between two subsequent packets which should equal 16.666667ms.
If higher values are encountered, the reason is because data loss has occurred during retransmission between the sensor and the phone.
This might depends on multiple factors including:
- How well the device handles connection and data retransmission, making data transmission device-specific & OS specific. Certain brands of mobile phones might handle Bluetooth data transmission better than others
- Bluetooth bandwidth is relatively smaller when compared to other means of data transmission such as Wi-Fi. Even though Bluetooth Low Energy (BLE) and Data Length Extension (DLE) have recently been introduced and improved upon this factor, it could still happen that some data packets are lost during transmission.
- Proximity between the sensor and phone: increasing the distance between the phone and the sensors will increase the chance of encountering data loss; for good performance we suggest that you keep the distance to less than 10m.
- Bluetooth transmission environment: the more crowded the Bluetooth transmission band, the greater the possibility of data packet loss.
Mitigate Data Loss
In order to mitigate the effects of data loss, here are some possibilities:
- Use Recording Mode: by storing data on the sensor, there is no data transmission. It is always recommended using Recording over Real-time streaming to prevent data loss. Here is a guide on how to use Recording.
- Use a lower-payload measurement mode: by reducing the amount of data being streamed, the chance of encountering data loss is diminished. For instance, when choosing a Sensor Fusion measurement mode, uncheck free acceleration if only orientation data is needed to reduce payload.
- Connect less sensors: lowering the number of sensors connected will reduce the overall amount of data being transferred, hence lowering the chance of data loss.
- Keep the phone closer to the sensors: reducing the distance between transmitter and receiver will reduce the chance of data loss.
- Avoid using the sensors in a crowded radio frequency environment.