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SweetieBot Herkulex servos interface

This package contains OROCOS components to control Herkulex DRS 101/202 servos. OROCOS is C++ framework targeting the implementation of real-time control systems. It is compatible with ROS and OROCOS components can be easily accessed from ROS nodes, see rtt_ros_integration package. Full documentation is available in Russian here.

Package contains three OROCOS components HerkulexDriver, HerkulexArray and HerkulexSched, startup scripts and launch files.

HerkulexDriver component implements data link layer. It handles serial port interaction, checks packet checksums, translates data frames to sweetie_bot_herkulex_msgs::HerkulexPacket messages and backward.

HerkulexArray is high-level configuration and monitoring subsystem. This component is not meant to work in real-time environment. It can be used to configure servos array (associate names with hardware ID, automatically set servo register to specific values at startup, configure reverse flag, offsets for each servos). Also HerkulexArray provides operations to access registers, read servo statuses, control movements and etc.

HerkulexSched provides real-time interface. It operates in control cycle (discretization period). Each control cycle is divided into three rounds:

  1. RT_JOG. At this round set goal (Herkulex SJOG) command is sent to servos. Target positions are read from in_goals port (herkulex_msgs::ServoGoal).

  2. RT_read. During this round HerkulexSched sends read request to servos which are listed in poll_list parameter. It requests positions and velocities form each servo and publish them using out_joints port (sensor_msgs::JointState). If detailed_state parameter is set also pwm and trajectory generator variables are acqured and publised. See component interface for more information.

    If poll_round_size property less then zero component attempts to request states of all servos. If poll_round_size greate or equal to zero HerkulexSched requests states only of poll_round_size servos and on the next RT_read round proceeds to next group of poll_round_size servos from poll list.

  3. CM. During this round requests from HerkulexArray component are forwarded to data link layer (HerkulexDriver). So real-time and non real-time submodules can function simulteniously.

The start of next control cycle is triggered by sync port messages. Duration of rounds are chosen via HerkulexSched component properties.

  • period_RT_JOG --- duration of RT_JOG round. At it beginning JOG command is sent. It should be longer then transit time of JOG command plus processing time of servo MC.
  • period_RT_read --- during this round read request are being sent to the servos from poll list. The next request is sent each req_period seconds. If response to previous request is recieved before req_period timer fires then the next request is sent immediately.
  • period_CM --- during this period requests from HerkulexArray is being forwarded to servos.
  • timeout --- how long scheduler is waiting for servo response during RT_READ round before assume that packet is missing,
  • req_period --- maximal time between two requests during RT_READ round. If it is less then timeout scheduler do not wait for the response before it starts the next request. It allows use duplex link with servos effectively.

period_CM, period_RT_read, period_RT_JOG properties declare time periods when a command can be send to servo. So actual duration of time rounds may be on timeout longer. timeout declare maximal duration of one exchange operation (send command to and receive answer from servo).

min_control_cyle_duration = period_RT_JOG + (period_RT_read + timeout) + (period_CM + timeout) 
                                               RT read exchange round     CM exchange round

Typically during RT_READ round the scheduler is able to poll

ceil( period_RT_read / req_period )

servos.

Servo array configuration can be loaded to HerkulexArray properties from OROCOS *.cpf file. Example configuration for two servos is provided in scripts/test_servo_array.cpf

Configuration hints:

  • At 115600 baurdrate use 5 ms timeout value and 2-3 ms req_period.
  • It is better to set period_RT_JOG to SJOG command duration (depends on number of servos) plus 2-3 ms. During this time servos would be able process received command.
  • Set period_RT_read to (poll_round_size-1)*timeout + 1 ms. So maximal duration of RT_read round would be poll_round_size*timeout + 1 ms
  • Control cycle duration can be controlled using external timer component from OCL library.

Limitations

  1. HerkulexDriver waitSendPacket() operation does not function properly with USB-to-serial converters due to tcdrain() syscall returns control prematurely. So if actual JOG request execution time is larger then period_RT_JOG this can cause next read operation timeout.
  2. HerkulexArray read in_goals port one time per control cycle. This can cause unwanted behaviour in presence of multiple writers or if port bufferization is turned on.

Konown issues

If reset_delay property of HerkulexArray is too small it is unable to reset and discover servos. Sometimes it can case unpredictable servo behavior probably due command misinterpretation.

Usage

TODO: rttlua deployment scripts (from sweetie_bot_deploy).

TODO: launch as separate ROS node.

Test array must contains two servos with hardware ID 0 and 1. Serial port /dev/ttyUSB0 is used with baudrate 115600. See files in script folder for more details.

To test configuration non-realtime confuguration (without HerkulexSched) run:

$ roslaunch sweetie_bot_herkulex_control test_herkulex_array.launch

Test script loads two components herk_driver (HerkulexDriver) и herk_array (HerkulexArray). Configuration file test_servos.cpf contains description of two servo with ID=0 and ID=1. Your can call various HerkulexArray operations from deployer (use ls herk_array to browse HerkulexArray interface):

> herk_array.printAllServosStatuses()
> herk_array.setGoal("servo", a.JOG_SPEED, 3.14 / 4, 0) 
> herk_array.setGoal("servo", a.JOG_POSITION, 0, 0.5)
> herk_array.setRegisterRAM("test_servo", "led_control", 2)
> herk_array.printAllRegistersRAM("test_servo")

Servo setting can be modified via properties of herk_array. After modification call cleanup and configure operations to apply changes.

To test real-time configuration run:

$ roslaunch sweetie_bot_herkulex_control test_herkulex_sched.launch

The script loads following components:

  1. sweetie_bot_hardware_herkulex components: herk_driver (HerkulexDriver), herk_array (HerkulexArray) and herk_sched (HerkulexSched).
  2. herk_timer (OCL::TimerComponent) which generates sync messages for herk_sched.
  3. reporter_herk_state and reporter_herk_stat (OCL::TimerComponent) store servo positions and communication statistics to files.

Configuration file test_servos.cpf contains description of two servo with ID=0 and ID=1. Script sequentiality sets different goal positions for servo1.

Time rounds setting can be modified via properties of herk_sched. After modifications reconfuguration is necessary.
Control cycle period is controlled by herk_timer settings.