x-IMU3 User Manual v0.11
April 6, 2022
Measurement source
Sample rate
Sample rate error
Data messages
Inertial sensor
400 Hz
±0.3%
Inertial, Quaternion, Rotation matrix
Euler angles, Linear acceleration,
Earth acceleration, Temperature
Magnetometer
20 Hz
±8%
Magnetometer
High-g accelerometer
1600 Hz
±2%
High-g accelerometer
Battery voltage
5 Hz
-
Battery
1 Hz
-
Table 28: Sample rate and associated data messages for each measurement source
9.2
Message rates
The message rate is the rate at which a data message is sent. The message rate of each data message type
is configured by a separate message rate divisor in the device settings. A message rate divisor is a positive
integer that a fixed sample rate is divided by to obtain the message rate. For example, if the inertial message
rate divisor is 4 then the inertial message rate will be 100 messages per second. A message rate divisor of
0 will disable the sending of that data message type. See Section 11.1.76 on page 49 to Section 11.1.82 on
page 52 for a detailed description and examples for each message rate divisor setting.
9.3
Sample averaging
If a message rate divisor is greater than 1 then the measurements in each data message will be the average of
the most recent
n
samples where
n
equal to the message rate divisor. The timestamp of the data message will
be that of the most recent sample. For example, if the inertial message rate divisor is 8 then the measurements
in each inertial message will be the average of 8 samples and the timestamp of the message will be that of
the 8
th
sample.
9.4
Timestamps
The timestamp of a data message indicates the time at which a measurement was obtained. For example,
when an ADC conversion completes. Timestamps are therefore not affected by the sample rate error or the
latency of a communication interface. Applications that involve time-dependent calculations such as numerical
integration or interpolation should not infer timing from the nominal sample rate and should instead use the
timestamp of each measurement. A timestamp is the number of microseconds since device start up with a
resolution of one microsecond.
9.5
Synchronisation
Multiple devices operating on the same Wi-Fi network will automatically synchronise so that the timestamps
from all devices become the number of microseconds since the device start up of the device that was switched
on first. The sample clocks of synchronised devices will remain asynchronous. If an application requires
synchronous sampling then this must be achieved in post-processing through interpolation and resampling.
10
Network announcement message
The network announcement message is used by a host to discover and connect to devices on the same
network. The message is continuously broadcast by the device on UDP port 10000 at a fixed rate of one
message per second. Each message provides the device name, serial number, Wi-Fi and battery status, as
well as the device settings required for a host to establish a TCP or UDP connection. The message is a single
JSON object. The key/value pairs are described in Table 29 on the next page.
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