NT Data Logging

A data logger can be connected to the NT bus, which allows us to record on a micro SD card a substantial amount of data for each cycle of the controller loop. It thus records the data with the maximal possible time resolution. Furthermore, the logger is quite small and can be easily mounted on-board. As a result one can get highly accurate and informative data taken in the real flight condition. This hopefully will allow use to better diagnose any troubles, such as jello or micro vibrations in the video, and find solutions to resolve them.

The data logger option also suggests many other applications. For instance, it can be used to dynamically balance the motors and propellers.

NT Logger Module

The NT logger module is connected to the NT bus, as any other NT module.

The micro SD card must be formatted with FAT32. The firmware author uses the Windows tool. Using the SD Association formatting tool didn't yielded any noticeable advantage.

The micro SD card should be of very high speed. Unfortunately, not any card works equally well. Little experience is available so far:

micro SD card	experience	reported by
SanDisk Extreme SDHC 16GB UHS-I Class 10 U3	very good, missing frames just every once in a while	OlliW
Kingston 8GB HC class 4	don't work	OlliW
Kingston 32GB HC class 4	don't work	OlliW

NTLoggerTool

For analyzing the recorded data, the NTLoggerTool is available, providing tools such as FFT, and others. It also allows us to convert the data into other formats, most notably into .cfl files. The .cfl files are suitable for the program BlackboxExplorer for STorM32, which is a decendant of CleanFlight's BlackboxExplorer. The NTLoggerTool also allows us to record directly the data on the NT bus usiang a USB-TTL adapter, which enables many work-bench tests and procedures.

The NTLoggerTool is steadily advancing with fast pace, and any manual would be quickly outdated. For a discussion it is best to join the STorM32 NT thread at rcgroups.

The NTLoggerTool is open source (GPL3). Anyone is highly welcome to contribute to it.

Logged Data

The data logger "spies" on the STorM32's Tx line, i.e., is capable of recording all data transmitted from the STorM32 to the NT modules. The points in the controller loop at which data are "sampled" are indicated in this sketch:

GUI Settings

However, not all data samples may be send out. Which data are emitted by the STorM32 can be controlled by the parameter NT Logging, found in the [GUI:Expert Tool] window.

NT Logging	basic	pid	accgyro	accgyro_raw
	SetLogger CmdAhrs SetMotorAll	CmdPid	CmdAccGyro	CmdAccGyroRaw Imu1 & Imu2 & Imu3
“off”	-	-	-	-
“basic”	x	-	-	-
“basic + pid”	x	x	-	-
“basic + accgyro”	x	-	x	-
“basic + accgyro_raw”	x	-	-	x
“basic + pid + accgyro”	x	x	x	-
“basic + pid + ag_raw”	x	x	-	x
“full”	x	x	x	x

It can happen that not all data can be transmitted in one cycle, if the computational load and data rate are too high. This mostly occurs with both 2nd IMU and full data logging enabled. In that case the raw data is distributed over several cycles.

Also the data rate of the micro SD card represents an obvious limitation. If the SD card can't keep up, frames (i.e. the data of one controller cycle) will be lost. Lowering the logging can alleviate that a lot.

Live Recording

It is also possible to "spy" on the STorM32's NT Tx line using a USB-TTL adapter, and record the data using the NTLoggerTool. The feature is analogous to the DataDisplay in the STorM32 GUI, except that it records much more data and moreover at full speed, with the maximal time resolution.

The USB-TTL adapter must be capable of handling 2.000.000 bauds. Adapters with FT232 (FTDI) work out of the box. Adapters with CP2102 may be configured to do so, see How to configure CP2102 USB adapters for high baud rates.

The USB-TTL adapter is then connected to the NT bus as follows:

USB-TTL adapter GND -> STorM32 NT GND
USB-TTL adapter Rx  -> STorM32 NT Tx

The USB-TTL adapter's Tx line must not be connected the STorM32's NT Rx line, i.e., should be left floating.