FP_B-MEASUREMENTS

Description:

For streaming input measurements, such as wheel speed, to the Vision-RTK2 via any UART serial or TCP port, the message format FP_B-MEASUREMENTS, introduced by Fixposition, is used. Note that the maximum number of measurements in the same message is 10 (n ≤ 10). The message format consists of a fixed structure using little-endian signed and unsigned integers of 8, 16, and 32 bits and IEEE 754 single (32 bits) precision. The format is as follows:

Notes:

The message ID for FP_B-MEASUREMENTS is 2001 (0x07d1).
For input messages, set the message_time to 0.

Payload:

The structure of the FP_B message payload is:

#	Offset	Field	Type	Unit	Description
1	0	`version`	uint8_t	-	Version of the FP_B_MEASUREMENTS message (currently 1)
2	1	`num_meas`	uint8_t	-	Number of measurements present in the body of the message (1…10)
3	2	`reserved0`	uint8_t[6]	-	Reserved for future use. Set to 0.
					The following fields are repeated `num_meas` times (i = 0…`num_meas`-1):
4 + i·12	8 + i·28	`meas_x`	int32_t	*	Measurement x axis (for example, [mm/s])
5 + i·12	12 + i·28	`meas_y`	int32_t	*	Measurement y axis (for example, [mm/s])
6 + i·12	16 + i·28	`meas_z`	int32_t	*	Measurement z axis (for example, [mm/s])
7 + i·12	20 + i·28	`meas_x_valid`	uint8_t	-	Validity of `meas_x` (1 = valid data, 0 = invalid data or n/a)
8 + i·12	21 + i·28	`meas_y_valid`	uint8_t	-	Validity of `meas_y` (1 = valid data, 0 = invalid data or n/a)
9 + i·12	22 + i·28	`meas_z_valid`	uint8_t	-	Validity of `meas_z` (1 = valid data, 0 = invalid data or n/a)
10 + i·12	23 + i·28	`meas_type`	uint8_t	-	Type of measurement (see below)
11 + i·12	24 + i·28	`meas_loc`	uint8_t	-	Location of measurement (see below)
12 + i·12	25 + i·28	`reserved1`	uint8_t[4]	-	Reserved for future use. Set to 0.
13 + i·12	29 + i·28	`timestamp_type`	uint8_t	-	Type of timestamp (see below)
14 + i·12	30 + i·28	`gps_wno`	uint16_t	-	GPS week number
15 + i·12	32 + i·28	`gps_tow`	uint32_t	*	GPS time of week [ms] or monotonic time [-]

The valid meas_type values are:

Value	Description
`0`	Unspecified
`1`	Velocity (wheel speed)

The valid meas_loc values are:

Value	Description
`0`	Unspecified
`1`	Measurement of a sensor at the rear-center (RC)
`2`	Measurement of a sensor at the front-right (FR)
`3`	Measurement of a sensor at the front-left (FL)
`4`	Measurement of a sensor at the rear-right (RR)
`5`	Measurement of a sensor at the rear-left (RL)

The valid timestamp_type values are:

Value	Description
`0`	Unspecified
`1`	Use time of arrival of the measurement (ignore `gps_wno` and `gps_tow`)
`2`	Use monotonic time [any] (stored in the `gps_tow` field)
`3`	Use GPS time (stored in `gps_wno` [-] and `gps_tow` [ms] fields)

Code:

Some example programs to create a FP_B-MEASUREMENTS message:

C++:

make_fpb_measurements.c

Python:

make_fpb_measurements.py

Example message:

A hexdump of the fp_b_rc.bin generated by make_fpb_measurements.c (see above):

CODE

00000000  66 21 d1 07 24 00 00 00  01 01 00 00 00 00 00 00
00000010  66 00 00 00 c2 00 00 00  dd ff ff ff 01 01 01 01
00000020  01 00 00 00 00 01 00 00  00 00 00 00 4e dd f9 a6

Let’s decompose it:

First we have the start of the FP_B frame and its meta data:

CODE

66 21 d1 07 24 00 00 00
^^^^^ ^^^^^ ^^^^^ ^^^^^
|     |     |     |
|     |     |     Msg time = 0x0000 = 0 (as expected for input messages)
|     |     Payload size = 0x0024 = 36
|     Message ID = 0x07d1 = 2001 = FP_B-MEASUREMENTS
FP_B sync chars

Then follows payload, which starts with a fixed-sized part:

CODE

                         01 01 00 00 00 00 00 00
                         ^^ ^^ ^^^^^^^^^^^^^^^^^
                         |  |  |
                         |  |  .reserved0
                         |  .num_meas = 0x01 = 1 (this message contains one measurement)
                         .version = 0x01 = 1 (as expected for this message)

Then follows num_meas times a measurement, consisting of a couple of fields with the data:

CODE

66 00 00 00 c2 00 00 00  dd ff ff ff 01 01 01 01
^^^^^^^^^^^ ^^^^^^^^^^^  ^^^^^^^^^^^ ^^ ^^ ^^ ^^
|           |            |           |  |  |  |
|           |            |           |  |  |  .meas_type = 0x01 = 1 = velocity
|           |            |           |  |  .meas_z_valid
|           |            |           |  .meas_y_valid
|           |            |           .meas_x_valid
|           |            .meas_z = 0xffffffdd = -35 (two's complement)
|           .meas_y = 0x000000c2 = 194
.meas_x = 0x00000066 = 102
01 00 00 00 00 01 00 00  00 00 00 00
^^ ^^^^^^^^^^^ ^^ ^^^^^  ^^^^^^^^^^^
|  |           |  |      |
|  |           |  |      .gps_tow = 0x00000000 = 0 (as expected for timestamp_type = 1)
|  |           |  .gps_wno = 0x0000 = 0 (as expected for timestamp_type = 1
|  |           .timestamp_type = 0x01 = 1 = time of arrival
|  .reserved1
.meas_loc = 0x01 = 1 = rear-centre (RC)

Finally, the message ends with a checksum across all of the above:

CODE

                                     4e dd f9 a6
                                     ^^^^^^^^^^^
                                     |
                                     Checksum