Design

• Sensor reference frames
• Origin of measurements
• Physical connections robust trackers
  • Cables
• Vibrations
  • Magnetic materials and magnets

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

Sensor reference frames

The Xsens Sirius 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 Sirius series Datasheet and Family Reference Manual.

Default sensor coordinate system for the Xsens Sirius 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 Sirius series module and robust trackers.

Physical connections robust trackers

The Xsens Sirius series only supports wire connectors. The devices only feature the 12-pin ODU connector for main supply and communication.

All robust trackers have three (M3) mounting holes to mount them on a flat surface. The figures below show the connector and mounting hole positions on the Xsens Sirius devices.

Xsens Sirius series connector and mounting hole positions (dimensions in mm)

The figures below show examples of the Xsens Sirius series mounted on a surface.

Xsens Sirius series mounting example (dimensions in mm)

Cables

The following cables can be ordered from Xsens.

CA-MP-USB

The CA-MP-USB cable is a RS232 to USB cable that has the USB converter integrated at the end of the cable and consists of the following parts:

• ODU connector: A10WAM-P12XMM0-0000; AMC HD, break-away plug, 12 circuits.
• Cable: 2.9m length, 8 core, AWG28, shielded, UL, 40ºC ± 85ºC, 5.1mm diameter.
• USB converter

The table below shows the pinning of the connections.

The figures below show the cable length definition (from connector end-to-end).

Host Interface cable CA-MP-USB length definition

CA-MP-USB-422

The CA-MP-USB-422 cable is a RS422 to USB cable that has the USB converter integrated at the end of the cable and consists of the following parts:

• ODU connector: A10WAM-P12XMM0-0000; AMC HD, break-away plug, 12 circuits.
• Cable: 2.9m length, 8 core, AWG28, shielded, UL, 40ºC ± 85ºC, 5.1mm diameter.
• USB converter

The table below shows the pinning of the connections.

The figures below show the cable length definition (from connector end-to-end).

Host Interface cable CA-MP-USB-422 length definition

CA-MP-12-OPEN

The CA-MP-12-OPEN is a 12-pin Host Interface pigtail cable that consists of the following parts:

• ODU connector: A10WAM-P12XMM0-0000; AMC HD, break-away plug, 12 circuits
• Cable: 2.9 m, 12 core, AWG28, shielded, UL, 40ºC - +85ºC, 6.1 mm diameter

The table below shows the pinning of the connections. The shield of the cable is only connected on the ODU connector side.

The figures below show the cable length definition (from connector end-to-end) and the pinning of the ODU connector.

Host Interface cable length definition

Numbering 12-pin ODU connector (solder cup view).
 

Vibrations

The Xsens Sirius 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 Sirius 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 Sirius 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 Sirius series to be mounted on an object without a direct metal to metal connection that transduces vibrations from the object to the Xsens Sirius series. The vibration dampeners have been tested with frequencies up to 1200 Hz that caused aliasing when the Xsens Sirius 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 Sirius 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.