Platform Control

To keep the communication with the Care-O-Bot platform variable (as different motors or CAN-hardware could be used), cob_base_drive_chain::CanCtrlPltfCOb3 implements the communication to different motors through a generic interface cob_canopen_motor::CanDriveItf and for different CAN modules through cob_generic_can::CanItf. The used CAN module is specified in an .ini-File, as well as motor-specific parameters. Each motor-object is loaded with appropriate parameters and during initialization each wheel is homed.

After initialization, the user can easily command the platform using the provided functions of cob_base_drive_chain::CanCtrlPltfCOb3 like setVelGearRadS.

To bring these communication facilities to the ROS network, the node cob_base_driove_chain_node uses that communication class and provides its functions to the ROS network as services. This makes it quite easy to create different controllers (e.g. joystick, keyboard, vision-based...) without taking care of the basic communication.


This node provides access to the platform control of the Care-O-Bot (steer and drive motors) to the ROS network.

Subscribed Topics

joint_command (pr2_controllers_msgs/JointTrajectoryControllerState)
  • it listenes to motor commands on this topic

Published Topics

JointState (sensor_msgs/JointState)
  • every time you request joint states via ROS service, these states are also published on this topic
Diagnostic (diagnostic_msgs/DiagnosticStatus)
  • it publishes states and error information of the platform


Init (cob_srvs/Trigger)
  • initializes the platform and executes homing procedure of motors
Reset (cob_srvs/Trigger)
  • re-initializes the platform
Shutdown (cob_srvs/Trigger)
  • stops motors and shuts down the platform
GetJointState (cob_srvs/GetJointState)
  • get joint joint states
ElmoRecorderConfig (cob_srvs/ElmoRecorderConfig)
  • configures the Elmo specific recorder (see below)
ElmoRecorderReadout (cob_srvs/ElmoRecorderReadout)
  • reads out and stores previous recorded drive information (see below)

Using the Elmo Recorder

Elmo controllers (like the Harmonica drive) have the functionality to record drive information (like position,velocity, current) at a very high frequency into their own flash memory. Later on, the user can read out that data to precisely analyse the data. The described tool makes it easy for the user to use this feature. It is preconfigured to record the values velocity, position, active current, commanded velocity.

It is implemented in cob_base_drive_chain::CanCtrlPltfCOb3 and again provided to the ROS network via services. To use the Elmo Recorder without the services, you can use the interface of cob_base_drive_chain::CanCtrlPltfCOb3 directly as desribed below.

ROS service interface

In order to record and extract data using cob_base_drive_chain node in the ROS environment, the only thing you have to do is calling two services offered by base_drive_chain_node:

  • ElmoRecorderConfig cob_srvs/ElmoRecorderConfig
    int64 recordinggap: With that setting, you can specify the time gap between two data points. The entered number is multiplied with 4 * 90µsec. Always 1024 data points are recorded in total, so the total recording time is RG * 4 * 90*1024 µsec. Configuring the Recorder this way, the recording process starts immediately.

  • ElmoRecorderReadout cob_srvs/ElmoRecorderReadout
    int64 subindex: Specify the recorded value you want to download and save. Predefined records are 1: Main Speed, 2: Main Position, 10: Active Current Command, 16: Speed Command. string fileprefix: path and file-prefix of log files. The record-object and motor Ids are automatically attached.

You always get a success code and an answer message. When requesting a read-out, you can't relay on the success-code, as the availability of the recorder first has to be checked.

All the operations (configuring and reading out the Recorder) are done for all available motors at once. You will find one file for each motor in the specified log-directory.

In the log file, the first column contains the time of the data point, the second contains the exponential representation of the data point of the selected record source.

Developer interface

There are two other levels of “user”-interfaces, to keep the drive interface-classes slight. The function CanCtrlPltfCOb3::ElmoRecordings basically is just a wrapper function for the main Elmo-Recorder interface in cob_canopen_motor::CanDriveHarmonica::setRecorder(int iFlag, int iParam, std::string sParam). With these arguments you can control all of the Recorders features. iFlag specifies the general task, you are going to set up:

  • iFlag = 0: Configure Elmo Recorder for new Record

iParam = iRecordingGap, which specifies every which time quantum (4*90usec) a new data point is recorded

  • iFlag = 1: Query upload of previous recorded data, data is being proceeded after complete upload-data collection and then syved into a log file (named filename + Drive_ID + record_value_ID + “.log”) iParam = recorded ID filename = path + fileprefix

  • iFlag = 2: After initiating the upload process, read the state of data collection and processing

  • iFlag = 99: Cancel any ongoing Segmented SDO Data transfer.

Return values are 0: Everything is fine, 1: Recorder hasn't been configured yet, 2: Data collection still in progress.

To keep all the CAN communication the process going, be sure to evaluate the CAN-buffer continuous. Also take care of the watchdogs using the heartbeat message.


The most difficult point of the Recorder read-out process is the asynchronous and delayed collection of data. If you wanted to give all the data back (like in a return) to the calling function, you had to create a giant loop, that would block all the program (and therefore also the communication facilities to the respective motor).

To avoid that effect, the user only has to start-up the read-out process. From then, the transfer process is running autonomously, triggered by the receive of data packets, which are led to the appropriate functions after having been fetched from the CAN-buffer. During such a segmented SDO transfer, the data is collected in the cob_canopen_motor::SDOSegmented collection class. It also holds some SDO-transfer-specific features like the state of the transmission. When receiving the last segment of a transfer, the collected data is handed to an adequate processing function. In our case, it's ElmoRecorder::processData(). Finally, the collected data is saved to a file; you also could imagine of keeping it temporarily in a variable to give the calling function the opportunity to process the data itself.

How to read out data from the Elmo Recorder is described in cob_canopen_motor.

Wiki: cob_base_drive_chain (last edited 2012-12-19 09:53:41 by MatthiasGruhler)