Data output rate of FT Sensor is less than expected 100 Hz

david_khosid 2018-01-22

I'm using FT 150 with ROS and see that the data output rate of the sensor is lower than expected.
In Robotiq's spec, the data output rate of the sensor is 100 Hz, but in real we measure stable 62.5 Hz. I can't explain the divergence with the expectation.

Setup:
Ubuntu 14.04 and ROS-Indigo
Sensor driver: https://github.com/ros-industrial/robotiq

I start the driver with
$ rosrun robotiq_force_torque_sensor rq_sensor

and check the ROS topic with
$ rostopic hz /robotiq_force_torque_sensor
... average rate: 62.604

Etienne_Samson 2018-01-22

@davidkh what is your physical setup? How are you plugged, converter, how do you get the data?

david_khosid 2018-01-22

We have figured out the root cause. The reason for a 62.5 Hz (16 ms interval) was from using a USB interface to the sensor. By default, a common FTDI chip introduces a delay of up to 16 ms when reading from a USB port. On Linux machine, they can check and adjust it.

# checking for the delay

> cat /sys/bus/usb-serial/devices/ttyUSB0/latency_timer

16

# changing the delay

> sudo -i
# echo 1 > /sys/bus/usb-serial/devices/ttyUSB0/latency_timer

# cat /sys/bus/usb-serial/devices/ttyUSB0/latency_timer

1
# logout

Even without restarting the sensor, the output rate is 100 Hz now.

$ rostopic hz /robotiq_force_torque_sensor

subscribed to [/robotiq_force_torque_sensor]

average rate: 100.016