Document MT0605P.E
© Xsens Technologies B.V.
MTi User Manual
55
information about triggering see section 5.5. Note that sampling of the sensors cannot be externally
triggered because of the high sampling rate of 10 kHz. It is possible to adjust the internal sampling
clock though by using the ClockSync functionality (see section 0).
The time delay between a physical event (e.g. an orientation change or acceleration) is dictated by two
factors;
1. Internal acquisition, calculation time and message generation (signal processing duration)
2. Serial transmission time
Thanks to the system architecture of the Xsens sensor fusion algorithm, the
signal processing
duration
is independent of the filter profile. Using a multi-core processing unit, it is possible to bring
down the
total
time from physical event to data transmission on the USB or serial output to far below 2
ms.
The
serial transmission time
can easily be calculated when the byte message and the baud rate is
known:
(𝑡𝑜𝑡𝑎𝑙 𝑏𝑦𝑡𝑒𝑠 𝑖𝑛 𝑚𝑒𝑠𝑠𝑎𝑔𝑒) ∗ 10 𝑏𝑖𝑡𝑠/𝑏𝑦𝑡𝑒
𝑐𝑜𝑚𝑚𝑢𝑛𝑖𝑐𝑎𝑡𝑖𝑜𝑛 𝑏𝑎𝑢𝑑𝑟𝑎𝑡𝑒 (𝑏𝑖𝑡𝑠/𝑠)
= transmission time
These factors will be discussed using the example of two common output configurations of the MTi.
The bytes in the message consist of the Preamble, BusID, MessageID, length indicator, data itself and
the checksum:
The Preamble, BusID, MesssageID, length indicator and checksum together is always 5 bytes. The
length of the various data messages is discussed in
[LLCP]
.
Example 1: Euler angels orientation data at 400 Hz and SDI data (delta_q and delta_v) at 100 Hz with
a baud rate of 230400 bps (RS232).
Euler angles is 12 bytes, SDI data is 24 bytes. This means that there will be one message of 41 bytes,
followed by three messages of 17 bytes, and then one message of 41 bytes again.
transmission time Euler angles and SDI =
(36 + 5) ∗ 10 𝑏𝑖𝑡𝑠/𝑏𝑦𝑡𝑒
230400 𝑏𝑖𝑡𝑠/𝑠
= 1.78 𝑚𝑠
transmission time SDI only =
(12 + 5) ∗ 10 𝑏𝑖𝑡𝑠/𝑏𝑦𝑡𝑒
230400 𝑏𝑖𝑡𝑠/𝑠
= 0.74𝑚𝑠
Note that, although the average data stream is lower than the baud rate, it is not possible to choose a
baud rate lower than 230400 bps in this particular case, as data comes at 400 Hz (every 2.5 ms) and
the longest transmission time at a baud rate of 115200 bps would be 3.56 ms.
Example 2: Quaternion data output at 100 Hz with a baud rate of 921600 bps (RS232).
Quaternion data is 16 bytes.
transmission time =
(16 + 5) ∗ 10 𝑏𝑖𝑡𝑠/𝑏𝑦𝑡𝑒
921600 𝑏𝑖𝑡𝑠/𝑠
= 0.23 𝑚𝑠
PREAMBLE
BID
MID
LEN
DATA
CHECKSUM