Document MT0605P.E
© Xsens Technologies B.V.
MTi User Manual
59
5.5.4
Clock synchronization (Clock Bias Estimation)
The MTi features clock synchronization: it is possible to adjust the bias of the MTi
’s internal clock with
an external clock of which the frequency is known. Note that the adjusted bias is also used in the
calibration of the inertial sensors, so that no additional errors are introduced. When a pulse is missed,
e.g. because it was not sent or was lost on the input line, this will not have a bad influence on the
performance. The maximum time that the pulses may be absent is 30 seconds.
The clock synchronization can be used for two distinctive use cases:
When a precise external clock is available (e.g. a GPS time pulse), this frequency can be sent
to the MTi to make sure that the time of the MTi follows the UTC time.
When an external device has a time constant that differs from the MTi, the sensor readings
will at some point no longer be aligned to each other. If the external device accepts
synchronization pulses, it is possible to use SyncOut; if the external device can send
synchronization pulses at a frequency that is the same as the required output frequency of the
MTi, it is possible to use SyncIn. If these two options are not possible, the Clock Sync is an
alternative.
Specification
Value
MTi’s internal clock accuracy
10 ppm
Input frequency
0.1
– 1000 Hz
Maximum deviation from MTi’s internal clock
900 ppm
Initialisation time (per ppm difference between
internal clock and external clock)
0.72 ms/ppm
5.5.4.1
Clock Bias Estimation from GPS
The MTi-G-700 GPS/INS uses the clock bias estimation function to synchronize the MTi-G with the
GPS time (1 ppm). This synchronization is set by default, and although not recommended, it is
possible to disable this synchronization setting.
5.5.5
StartSampling
One of the advanced timing features of the MTi is the StartSampling synchronization function.
StartSampling will trigger the MTi to start processing data, so that the start time for sampling can be
chosen. This is useful when the timing of a data needs to be aligned with an external sensor or sensor
system at an accuracy of better than 2.5 ms. Timing specification is as following:
0 ms
0.69 +/- 0.05 ms
3.19 +/-0.05 ms
10.69 +/- 0.05 ms
External pulse
received at MTi
First sample (10kHz) received
for signal processing
First inertial data available
(acc/gyr, 400 Hz)
First orientation
available (400 Hz)
It is possible to delay the “First sample received”, and with that the entire data output, with up to
0.65536 seconds. For example, setting a delay of 6810 us (6.81 ms) will output data at exactly 10 ms
after the external pulse has been received.
