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Velodyne VLP16 frequency too low

asked 2019-05-21 22:17:25 -0500

agurman gravatar image

updated 2019-05-23 00:49:07 -0500

Hey guys,

Im running a velodyne VLP16, however i am having issues with it's publish frequency, this lidar is spinning at 600 rpm with a packet rate of 754. The /diagnostics topic for the velodyne_nodelet_manager currently show's this:

values: 
  - 
    key: "Events in window"
    value: "46"
  - 
    key: "Events since startup"
    value: "2774"
  - 
    key: "Duration of window (s)"
    value: "10.884267"
  - 
    key: "Actual frequency (Hz)"
    value: "4.226284"
  - 
    key: "Target frequency (Hz)"
    value: "9.921053"
  - 
    key: "Minimum acceptable frequency (Hz)"
    value: "8.928947"
  - 
    key: "Maximum acceptable frequency (Hz)"
    value: "10.913158"
  - 
    key: "Earliest timestamp delay:"
    value: "0.000624"
  - 
    key: "Latest timestamp delay:"
    value: "0.000779"
  - 
    key: "Earliest acceptable timestamp delay:"
    value: "-1.000000"
  - 
    key: "Latest acceptable timestamp delay:"
    value: "5.000000"
  - 
    key: "Late diagnostic update count:"
    value: "0"
  - 
    key: "Early diagnostic update count:"
    value: "0"
  - 
    key: "Zero seen diagnostic update count:"
    value: "0"

You can see that i should be getting a frequency of around 10hz, however the actual frequency is only around 4.1hz. Does anyone have any idea what could be the cause of this?

Here are some more outputs:

rostopic bw /velodyne_points

average: 3.77MB/s
    mean: 0.90MB min: 0.89MB max: 0.90MB window: 29

rostopic hz /velodyne_points

average rate: 4.196
    min: 0.216s max: 0.276s std dev: 0.02812s window: 20

rostopic bw /velodyne_packets

average: 434.13KB/s
    mean: 92.29KB min: 92.29KB max: 92.29KB window: 9

The velodyne_nodelet_manager seems to be using about 0.6% of memory and 2% of cpu, and the computer i am streaming the lidar too does not show any signs of computational burden

I should also note from a hardware layer perspective the lidar run's through a gigabit switch

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answered 2019-05-28 00:25:49 -0500

agurman gravatar image

I managed to resolve this issue. For anyone encountering this in the future, within the configuration web interface for the VLP-16 there is a parameter called FOV Start which allows you to crop the data being sent between two angles. If you are using this parameter there seems to be either a bug, or performance issue where it drastically drops the frequency. Restoring this to the default 0 to 359 range allowed me to attain the desired 10hz frequency.

The velodyne ros package has a few parameters that allow you to crop the range of the data within the node itself without impacting negatively on frequency performance.

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Asked: 2019-05-21 22:17:25 -0500

Seen: 83 times

Last updated: May 28