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Epson IMU ROS1 Driver Node

Overview

• The Epson IMU ROS software is ROS C++ wrapper of a Linux C driver for communicating on a ROS system

• The original Linux C driver can be found at github

• This driver software supports either UART or SPI interface connection to the Epson device

• The user should be familiar with building ROS packages using the catkin_make process

• This is NOT detailed step by step instructions

• Refer to the README.md inside the ROS package or Github repository for more detailed up-to-date info

Interface Connections

UART Connection & Configuration

• For direct connection, the target UART port on the host must be compatible with CMOS 3.3V I/O

• Connection is also possible through a USB-UART 3.3V converter IC such as on the M-G32EV041 evaluation board

• Connecting GPIO output for asserting RESET# pin on the Epson device is optional, but recommended where possible (i.e. on embedded platforms)
• Below is an example UART connection when using a Raspberry Pi based system:

  • Epson IMU	Raspberry Pi
    EPSON_SIN Input	RPI_GPIO_P1_8 (GPIO14/UART_TXD) Output
    EPSON_SOUT Output	RPI_GPIO_P1_10 (GPIO15/UART_RXD) Input
    EPSON_RESET Input	RPI_GPIO_P1_15 (GPIO22) Output *Optional, but recommended*

• When connecting the Epson G552PRx to a ROS system, the underlying UART connection must be compatible with RS422 I/O signal levels
• Below is an example RS422 connection to a RS422 Host:

  • Epson IMU	RS422 Host
    EPSON_TD+ Output	HOST_RD+ Input
    EPSON_TD- Output	HOST_RD- Input
    EPSON_RD+ Input	HOST_TD+ Output
    EPSON_RD- Input	HOST_TD- Output

SPI Connection & Configuration

• Interfacing the Epson device using SPI is typically applicable only to embedded Linux platforms
• Direct connection to SPI port must be compatible with CMOS 3.3V I/O

• Connection between the host & Epson device is possible using the M-G32EV031 evaluation breakout board

• This driver uses the external library Unofficial wiringPi library to generate low level SPI communication, GPIO, & accurate time delays

  • Other libraries can be used to generate timer delays and SPI messages but requires changes to redirect the low-level function calls

• Accurate timing delays are necessary to ensure throughput for high dout_rate settings (refer to IMU datasheets for timing specifications)

• Below is an example SPI connection when using a Raspberry Pi:

  • Epson IMU	Raspberry Pi
    EPSON_RESET Input	RPI_GPIO_P1_15 (GPIO22) Output
    EPSON_DRDY Output	RPI_GPIO_P1_18 (GPIO24) Input
    EPSON_CS Input	RPI_GPIO_P1_16 (GPIO23) Output
    EPSON_SCLK Input	RPI_GPIO_P1_23 (GPIO11) Output
    EPSON_DOUT Output	RPI_GPIO_P1_19 (GPIO10) Input
    EPSON_DIN Input	RPI_GPIO_P1_21 (GPIO9) Output

SPI Configuration for Timer Delays

• This software contains wrapper functions for time delays in millisecond and microseconds using seDelayMS() and seDelayMicroSecs(), respectively.

• On embedded Linux platforms, these function calls are redirected to platform-specific HW delay routines

• For example on RaspberryPi, the time delay functions for millisecond and microseconds is redirected to WiringPi library delay() and delayMicroseconds()

• If a hardware delay is not available from a library, then a custom software delay loop is an option (but not preferred)

SPI Configuration for GPIO Control

• This software uses the following 3 GPIO pins on the host for connection to the Epson device

  • GPIO output for asserting CS# on the Epson device (NOTE: Alternatively, pin SPI0_CE0 on the RaspberryPi can be used)

  • GPIO input for reading the logic status on the Epson device DRDY (GPIO1) pin

  • GPIO output for asserting RESET# on the Epson device

• Connecting CS# and DRDY to the Epson device is mandatory

• RESET# is recommended for asserting Hardware Reset during every IMU initialization for better robustness

ROS1 Node

• This driver publishes IMU messages as per REP 145 Conventions for IMU Sensor Drivers

• Imu Message - contains header, orientation, angular velocity and linear acceleration

• Temperature Message - contains header, temperature in Degrees Celsius

Orientation Data

• For models that support quaternion output, Imu Message publish on imu/data, and the orientation field can update with valid data

• For models that do not support quaternion output, Imu Message publish on imu/data_raw, and the orientation field will not update with valid data

Launch File

• The roslaunch/XML in the launch folder is used to pass the initialization parameters to configure the Epson device at runtime using roslaunch

• Initialization parameter changes by editing the launch file does not require rebuilding with catkin_make

• The user needs to modify dout_rate & filter_sel as needed based on user requirements

Timestamping With EXT Signal (Time Correction)

• The Epson device has a 3.3V I/O EXT (GPIO2) pin which can be connected to a cyclic external sync signal such as a GNSS 1PPS signal

• When time_correction is enabled (set to 1), the driver attempts to correct the time stamp field in Imu Message using the devices internal reset counter value (measured time delay since latest GNSS 1PPS)

• To enable the time_correction, do the following:

  1. Connect a function 1PPS signal to the EXT/GPIO2 pin of the Epson device

  2. Edit the appropriate roslaunch/XML in launch/ to set ext_sel to External Counter Reset, set <param name="ext_sel" type="int" value="1" />

  3. In the same launch file, enable time_correction, set <param name="time_correction" type="int" value="1" />

• The use of time_correction is intended to give a more accurate timestamp of the inertial data by minimizing the effects of link & host processing delays

Building & Installing the ROS Node

1. Place this package (including folders) into a new folder within your catkin workspace src/ folder.

2. Modify the CMakeLists.txt to:

   • Select the PLATFORM to NONE for PC or RPI for RaspberryPi depending on your host platform

   • Select the INTERFACE to UART or SPI depending on the serial connection

   • NOTE: You *MUST* re-build using catkin_make when changing the PLATFORM or INTERFACE or making any changes in the CMakeLists.txt

3. From the catkin workspace folder run catkin_make to build all changed ROS packages located in the <catkin_workspace>/src/ folder

4. Reload the current ROS environment variables that may have changed after the catkin build process

   <catkin_workspace>/source devel/setup.bash

5. Modify the appropriate roslaunch/XML in the launch folder to set your desired initialization parameters

   • Typically, only the dout_rate & filter_sel needs to be edited

Running the ROS Node

• To start the Epson IMU ROS driver use the appropriate roslaunch/XML located in launch/ with roslaunch

• For example:

  <catkin_workspace>/roslaunch ess_imu_driver normal.launch

USB-UART Latency

• When using USB-UART bridge ICs to connect between the Linux host & Epson device, higher than expected latencies or slower than expected data rates are possible

• For example, the M-G32EV041 evaluation board uses an FTDI IC and driver

• If the connection between the Epson device UART and Linux host is by FTDI, the latency_timer setting in the FTDI Linux driver may be large i.e. typically 16 (msec)

• There are 3 methods listed below to reduce the impact of this latency by setting the latency_timer to 1msec

Modifying *latency_timer* by *udev* mechanism

• udev is a device manager for Linux that can dynamically create and remove devices in *userspace* and run commands when new devices appear

• Create a *udev* rule to automatically set the latency_timer to 1 when an FTDI USB-UART device is plugged in

• For example, below is a text file named *99-ftdi_sio.rules* that can be put in the */etc/udev/rules.d* directory

  SUBSYSTEM=="usb-serial", DRIVER=="ftdi_sio", ATTR{latency_timer}="1"

  NOTE: Root (sudo) access is required to create or copy file in */etc/udev/rules.d* NOTE: This is the preferred method because it automatically triggers when the device is plugged in. However, it affects ALL FTDI USB-UART devices on the host system

Modifying *latency_timer* by *sysfs* mechanism

• The example below reads the *latency_timer* setting for */dev/ttyUSB0* which returns 16msec.
• Then, it sets the *latency_timer* to 1msec, and confirms it by readback.

  NOTE: May require root (sudo su) access on your system

  cat /sys/bus/usb-serial/devices/ttyUSB0/latency_timer
  16
  echo 1 > /sys/bus/usb-serial/devices/ttyUSB0/latency_timer
  cat /sys/bus/usb-serial/devices/ttyUSB0/latency_timer
  1

Modifying *low_latency* flag using *setserial* utility

• The example below sets the low_latency flag for /dev/ttyUSB0

• This will have the same effect as setting the latency_timer to 1msec.

• This can be confirmed by running the setserial command again.

• NOTE: You may need to install setserial with sudo apt install setserial

  user@user:~$ setserial /dev/ttyUSB0
  /dev/ttyUSB0, UART: unknown, Port: 0x0000, IRQ: 0
  
  user@user:~$ setserial /dev/ttyUSB0 low_latency
  
  user@user:~$ setserial /dev/ttyUSB0
  /dev/ttyUSB0, UART: unknown, Port: 0x0000, IRQ: 0, Flags: low_latency

Technical Support

• For technical support or related issues using this ROS software, please email [email protected]