Design

• Sensor reference frames
• Origin of measurements
• Physical connections OEM trackers
  • Footprint for PCB layout
  • Ribbon Cable
• Vibrations
  • Magnetic materials and magnets

This section describes the (mechanical) design and hardware integration considerations of the Xsens Avior series devices. 3D models of the robust trackers are available and found on BASE.

Sensor reference frames

The Xsens Avior series uses a right-handed coordinate system as the basis of the sensor frame. The default sensor coordinate system is printed on the side of the device and is indicated in the figures below. More details regarding (the modification of) the reference frames of the device can be found in the Xsens Avior series Datasheet and Family Reference Manual.

Default sensor coordinate system for the Xsens Avior series device

Origin of measurements

The accelerometer determines the origin of measurements. The in the figures below shows the location of the accelerometer of the Xsens Avior series module and robust trackers.

Physical connections OEM trackers

The connector on the Xsens Avior series is a 20-pin, 1.27 mm pitch male connector of Samtec (FTSH-110-04-F-DV-A). This connector supports an SMD counterpart that can be soldered onto a PCB as well as a ribbon cable (IDC) counterpart (ribbon cable not included in the development kit). In order to mount the Xsens Avior series onto a PCB, the connector should be facing down and the housing should be supported with M3 spacers that can be soldered onto the PCB (the spacers must be placed on the bottom, i.e., the other side of the PCB). When using a ribbon cable, the Xsens Avior series can be mounted upside-down to create easy access to the connector. 

The figure below shows the PCB mounting option. The module is fixated with three M3 screws with a length of at least 18 mm. It is recommended to use screws and spacers with weak magnetic properties to reduce the influence on the internal magnetometer. A tightening torque of 0.1 Nm is recommended to avoid mechanical stress on the module.

Xsens Avior series connection on a PCB (dimensions in mm)

Footprint for PCB layout

The figure below shows the recommended footprint of the Xsens Avior series counterpart connector, together with the three spacers.

The table below shows the recommended parts for this mounting option.

Layout footprint example (dimensions in mm)

Ribbon Cable

A ribbon cable (for standalone configuration) is not included with the development kit. However, the following cable (or equivalent) should fit:

Vibrations

The Xsens Avior series samples IMU signals at 10kHz per channel, processing them using a strapdown integration algorithm with coning/sculling compensation. Proper coning/sculling compensation already mitigates errors that poorly designed signal processing pipelines introduce when the device is under vibration. For best results, however, it is recommended that the Avior products be mechanically isolated from vibrations as much as possible: since vibrations are measured directly by the accelerometers, the following two conditions can make the readings from the accelerometers invalid;

1. The magnitude of the vibration is larger than the measurement range of the accelerometer. This will cause the accelerometer to saturate, which may be observed as a “drift” in the zero-level of the accelerometer. This will show up as an erroneous roll/pitch.
2. The frequency of the vibration is higher than the bandwidth of the accelerometer. In theory, such vibrations are rejected, but in practice they can still give rise to aliasing, especially if close to the bandwidth limit. This can be observed as a low frequency oscillation. Further, high frequency vibrations often tend to have large acceleration amplitudes (see item 1).

There is an effect on the gyroscopes as well and, especially when the vibrations include high-frequent coning motion, the gyroscope readings may become invalid. The Xsens Avior series features mechanical vibration rejecting gyroscopes, designed to better cope with these specific conditions.

Xsens has tested a set of vibration dampeners. Vibration dampeners are low-profile rubber cylinders that allow the Xsens Avior series to be mounted on an object without a direct metal to metal connection that transduces vibrations from the object to the Xsens Avior series. The vibration dampeners have been tested with frequencies up to 1200 Hz that caused aliasing when the Xsens Avior series was mounted directly on the vibration table. These vibrations had no effect with the vibration dampeners fitted. The dampeners tested are manufactured by Norelem and have part number 26102-00800855, www.norelem.com

Magnetic materials and magnets

When the Xsens Avior motion tracker is placed close to or on an object that is either magnetic or contains ferromagnetic materials, the measured magnetic field is distorted (warped) and causes an error in the computed yaw. The earth's magnetic field is altered by the presence of ferromagnetic materials, permanent magnets or power lines with strong currents (several amperes) in the vicinity of the device. The distance to the object and the amount of ferromagnetic material determines the magnitude of disturbance introduced. Errors in estimated yaw due to such distortions can be quite large, since the earth's magnetic field is very weak in comparison to the magnitude of the sources of distortion.

For more information on how to mitigate the detrimental effects of magnetic distortion, refer to the BASE article Estimating Yaw in magnetically disturbed environments.