Joint Trajectory Playback Example

Jump to: navigation , search

Enable the robot joint trajectory interface, parse a file created using the joint position recorder example, and send the resulting joint trajectory to the action server.


A commonly used ROS method for robot arm motion control is the joint trajectory action interface. The trajectory_controller and it's corresponding joint trajectory action server is the intera_interface implementation to support this action interface. This example shows usage for launching the joint trajectory action server, and parsing of a text file describing timestamped joint position goals into a joint trajectory action call to be commanded to the joint trajectory action server.


This example demonstrates the usage of the Joint Trajectory Action server to command raw joint position commands. The main() creates and instance of the Trajectory class and calls the parse_file() method. This method is responsible for parsing the input file and packing them as Request message for the Joint Trajectory Action server. The start() method is then called, which sends the Request messages to the action server. Finally, wait() is then called, which verifies if the trajectory execution was successful. Action Servers - robot/limb/right/follow_joint_trajectory. If you would like to follow along with the actual source code for the example on GitHub, it can be found through this link for joint trajectory file playback example.


Verify that the robot is enabled from an SDK terminal session, if the robot is not enabled run:

$ rosrun intera_interface

Record a joint position file using the example, ex:

$ rosrun intera_examples -f <position_file_name>

The recorder is now recording joint positions with corresponding timestamps for robot arm. Move the arms while holding the cuffs. If the position file name exists, the function will overwrite existing file.

NOTE: You can open and close the gripper while recording by using the robot's cuff buttons: Oval(lower) = Close, Circle(Upper) = Open

Start the joint trajectory controller, ex:

$ rosrun intera_interface --mode velocity

In another RSDK terminal session, Run the joint trajectory playback example program, ex:

$ rosrun intera_examples -f <position_file_name>

The robot arm will then be commanded to repeat the trajectory recorded during the joint position recording. The difference between this example and the joint_position playback example is that the trajectory controller has the ability to honor the velocities (due to the timestamps) to more accurately repeating the recorded trajectory.


Important Arguments: Arguments for

See the trajectory controller's usage on the command line by passing the -h, help argument:

$ rosrun intera_interface -h
Intera SDK Joint Trajectory Controller

    Unlike other robots running ROS, this is not a Motor Controller plugin,
    but a regular node using the SDK interface.    

optional arguments:
  -h, --help            show this help message and exit
  -l {right}, --limb {right}
                        joint trajectory action server limb (default: right)
  -r RATE, --rate RATE  trajectory control rate (Hz) (default: 100.0)
  -m {position_w_id,position,velocity}, --mode {position_w_id,position,velocity}
                        control mode for trajectory execution (default:

Important Arguments: Arguments for

See the joint_trajectory_file_playback's usage on the command line by passing the -h, help argument:

$ rosrun intera_interface -h
RSDK Joint Trajectory Example: File Playback

    Plays back joint positions honoring timestamps recorded
    via the joint_recorder example.

    Run the example first to create a recording
    file for use with this example. Then make sure to start the
    joint_trajectory_action_server before running this example.

    This example will use the joint trajectory action server
    with velocity control to follow the positions and times of
    the recorded motion, accurately replicating movement speed
    necessary to hit each trajectory point on time.

required arguments:
  -f FILE, --file FILE   path to input file
optional arguments:
  -h, --help            show this help message and exit
  -l LOOPS, --loops LOOPS
                         number of playback loops. 0=infinite. (default: 1)



See the API Reference page for details: ROS APIs

Joint Trajectory Action Server - /robot/limb/right/follow_joint_trajectory [control_msgs/FollowJointTrajectoryAction]

Intera_interface APIs:

JointTrajectoryActionServer class:

Code Walkthrough

Now, let's break down the code.

33 import argparse
34 import operator
35 import sys
36 import threading
38 from bisect import bisect
39 from copy import copy
40 from os import path
42 import rospy
44 import actionlib
46 from control_msgs.msg import (
47     FollowJointTrajectoryAction,
48     FollowJointTrajectoryGoal,
49 )
50 from trajectory_msgs.msg import (
51     JointTrajectoryPoint,
52 )
54 import intera_interface
56 from intera_interface import CHECK_VERSION

This imports the intera_interface for accessing the limb and the gripper class. The actionlib is imported to use its Action Server class.The CHECK_VERSION is imported to check if the software running on the robot would be compatible with this local version. It is not necessary to check the version in custom programs.

59 class Trajectory(object):
60     def __init__(self, limb="right"):
61         #create our action server clients
62         self._limb_client = actionlib.SimpleActionClient(
63             'robot/limb/right/follow_joint_trajectory',
64             FollowJointTrajectoryAction,
65         )
67         #verify joint trajectory action servers are available
68         is_server_up = self._limb_client.wait_for_server(rospy.Duration(10.0))
69         if not is_server_up:
70             msg = ("Action server not available."
71                    " Verify action server availability.")
72             rospy.logerr(msg)
73             rospy.signal_shutdown(msg)
74             sys.exit(1)

Action server clients for the right limb is created. The existence of action server for right limb is checked with a timeout of 10 seconds. If it is not available, a shutdown signal is sent and the program exits.

76         #create our goal request
77         self.goal = FollowJointTrajectoryGoal()
79         #limb interface - current angles needed for start move
80         self.arm = intera_interface.Limb(limb)
82         self.limb = limb
83         self.gripper_name = '_'.join([limb, 'gripper'])
84         #gripper interface - for gripper command playback
85         try:
86             self.gripper = intera_interface.Gripper(limb)
87             self.has_gripper = True
88         except:
89             self.has_gripper = False
90             rospy.loginfo("Did not detect a connected electric gripper.")
92         #flag to signify the arm trajectories have begun executing
93         self._arm_trajectory_started = False
94         #reentrant lock to prevent same-thread lockout
95         self._lock = threading.RLock()

The request message that will hold the goal position for right limbs' action server is created. The current joint is also captured. If there is a electric gripper connected, the gripper positions will be captured as well.

 97         # Verify Grippers Have No Errors and are Calibrated
 98         if self.has_gripper:
 99             if self.gripper.has_error():
100                 self.gripper.reboot()
101             if not self.gripper.is_calibrated():
102                 self.gripper.calibrate()
104             #gripper goal trajectories
105             self.grip = FollowJointTrajectoryGoal()
107             #gripper control rate
108             self._gripper_rate = 20.0  # Hz
110         # Timing offset to prevent gripper playback before trajectory has started
111         self._slow_move_offset = 0.0
112         self._trajectory_start_offset = rospy.Duration(0.0)
113         self._trajectory_actual_offset = rospy.Duration(0.0)
115         #param namespace
116         self._param_ns = '/rsdk_joint_trajectory_action_server/'

The error() method returns whether or not the gripper is in an error state. The possible cause of errors might be over/under-voltage, over/under-current, motor faults, etc. The calibrated() method returns a boolean, true if the gripper is already calibrated. The request message that will hold the goal position for the gripper is created. The namespace is modified and the gripper's control rates is modified.

118     def _execute_gripper_commands(self):
119         start_time = rospy.get_time() - self._trajectory_actual_offset.to_sec()
120         grip_cmd = self.grip.trajectory.points
121         pnt_times = [pnt.time_from_start.to_sec() for pnt in grip_cmd]
122         end_time = pnt_times[-1]
123         rate = rospy.Rate(self._gripper_rate)
124         now_from_start = rospy.get_time() - start_time
125         while(now_from_start < end_time + (1.0 / self._gripper_rate) and
126               not rospy.is_shutdown()):
127             idx = bisect(pnt_times, now_from_start) - 1
128             if self.has_gripper:
129                 self.gripper.set_position(grip_cmd[idx].positions[0])
130             rate.sleep()
131             now_from_start = rospy.get_time() - start_time

The grip_cmd variable holds the gripper' position that was parsed from the file. The corresponding time stamps are read into pnt_times variable. idx holds the index of the timestamp from the file, relative to the current one. It is important to note that the velocity at which the Joint Positions was traversed is preserved during the playback. The corresponding gripper's position is then commanded to the action server.

133     def _clean_line(self, line, joint_names):
134         """
135         Cleans a single line of recorded joint positions
137         @param line: the line described in a list to process
138         @param joint_names: joint name keys
140         @return command: returns dictionary {joint: value} of valid commands
141         @return line: returns list of current line values stripped of commas
142         """
143         def try_float(x):
144             try:
145                 return float(x)
146             except ValueError:
147                 return None
148         #convert the line of strings to a float or None
149         line = [try_float(x) for x in line.rstrip().split(',')]
150         #zip the values with the joint names
151         combined = zip(joint_names[1:], line[1:])
152         #take out any tuples that have a none value
153         cleaned = [x for x in combined if x[1] is not None]
154         #convert it to a dictionary with only valid commands
155         command = dict(cleaned)
156         return (command, line,)

This try_float() function replaces the contents of the variable passed with float and none values. Then, a tuple is constructed by zipping the names passed with the values from the variable line. This may contain valid values as well as none values . After removing the none values, a dictionary is constructed. This dictionary is then parsed to create valid Joint commands.

158     def _add_point(self, positions, side, time):
159         """
160         Appends trajectory with new point
162         @param positions: joint positions
163         @param side: limb to command point
164         @param time: time from start for point in seconds
165         """
166         #creates a point in trajectory with time_from_start and positions
167         point = JointTrajectoryPoint()
168         point.positions = copy(positions)
169         point.time_from_start = rospy.Duration(time)
170         if side == self.limb:
171             self.goal.trajectory.points.append(point)
172         elif self.has_gripper and side == self.gripper_name:
173             self.grip.trajectory.points.append(point)

This method creates a request message of the JointTrajectoryPoint type and appends the goal position based on the side of the limb/gripper that is requesting. It compiles a list of Joint Positions as the trajectory.

175     def parse_file(self, filename):
176         """
177         Parses input file into FollowJointTrajectoryGoal format
179         @param filename: input filename
180         """
181         #open recorded file
182         with open(filename, 'r') as f:
183             lines = f.readlines()
184         #read joint names specified in file
185         joint_names = lines[0].rstrip().split(',')
186         #parse joint names for right limb
187         for name in joint_names:
188             if self.limb == name[:-3]:
189                 self.goal.trajectory.joint_names.append(name)

The lines are split into a list with ',' as the delimiter to extract the joint names. The arm of the current joint, found by stripping the "_<joint>" (last three characters) from the joint_name, is checked and stored.

191         def find_start_offset(pos):
192             #create empty lists
193             cur = []
194             cmd = []
195             dflt_vel = []
196             vel_param = self._param_ns + "%s_default_velocity"
197             #for all joints find our current and first commanded position
198             #reading default velocities from the parameter server if specified
199             for name in joint_names:
200                 if self.limb == name[:-3]:
201                     cmd.append(pos[name])
202                     cur.append(self.arm.joint_angle(name))
203                     prm = rospy.get_param(vel_param % name, 0.25)
204                     dflt_vel.append(prm)
205             diffs = map(operator.sub, cmd, cur)
206             diffs = map(operator.abs, diffs)
207             #determine the largest time offset necessary across all joints
208             offset = max(map(operator.div, diffs, dflt_vel))
209             return offset

The first commanded position and the current position for all the joints are captured. The default values that were loaded into the param server are read. The largest time offset necessary across all the joints is calculated.

211         for idx, values in enumerate(lines[1:]):
212             #clean each line of file
213             cmd, values = self._clean_line(values, joint_names)
214             #find allowable time offset for move to start position
215             if idx == 0:
216                 # Set the initial position to be the current pose.
217                 # This ensures we move slowly to the starting point of the
218                 # trajectory from the current pose - The user may have moved
219                 # arm since recording
220                 cur_cmd = [self.arm.joint_angle(jnt) for jnt in self.goal.trajectory.joint_names]
221                 self._add_point(cur_cmd, self.limb, 0.0)
222                 start_offset = find_start_offset(cmd)
223                 # Gripper playback won't start until the starting movement's
224                 # duration has passed, and the actual trajectory playback begins
225                 self._slow_move_offset = start_offset
226                 self._trajectory_start_offset = rospy.Duration(start_offset + values[0])
227             #add a point for this set of commands with recorded time
228             cur_cmd = [cmd[jnt] for jnt in self.goal.trajectory.joint_names]
229             self._add_point(cur_cmd, self.limb, values[0] + start_offset)
230             if self.has_gripper:
231                 cur_cmd = [cmd[self.gripper_name]]
232                 self._add_point(cur_cmd, self.gripper_name, values[0] + start_offset)

The non-float values are cleaned and a dictionary of valid Joint Commands is returned in the _clean_line() function as explained above. The time offset for move to start position is also calculated. The parsed set of recorded commands along with their recorded time is added to the goal list as well as gripper's command and time.

234     def _feedback(self, data):
235         # Test to see if the actual playback time has exceeded
236         # the move-to-start-pose timing offset
237         if (not self._get_trajectory_flag() and
238               data.actual.time_from_start >= self._trajectory_start_offset):
239             self._set_trajectory_flag(value=True)
240             self._trajectory_actual_offset = data.actual.time_from_start

Called in start function, test to see if the actual playback time has exceeded the move-to-start-pose timing offset.

242     def _set_trajectory_flag(self, value=False):
243         with self._lock:
244             # Assign a value to the flag
245             self._arm_trajectory_started = value
247     def _get_trajectory_flag(self):
248         temp_flag = False
249         with self._lock:
250             # Copy to external variable
251             temp_flag = self._arm_trajectory_started
252         return temp_flag

Assign the value to the trajectory flag function and get the trajectory flag value function.

254     def start(self):
255         """
256         Sends FollowJointTrajectoryAction request
257         """
258         self._limb_client.send_goal(self.goal, feedback_cb=self._feedback)
259         # Syncronize playback by waiting for the trajectories to start
260         while not rospy.is_shutdown() and not self._get_trajectory_flag():
261             rospy.sleep(0.05)
262         if self.has_gripper:
263             self._execute_gripper_commands()

This function sends the FollowJointTrajectoryAction request message which includes the recorded positions and their corresponding time, to the Joint Trajectory Action Server.

265     def stop(self):
266         """
267         Preempts trajectory execution by sending cancel goals
268         """
269         if ( is not None and
270             self._limb_client.get_state() == actionlib.GoalStatus.ACTIVE):
271             self._limb_client.cancel_goal()
273         #delay to allow for terminating handshake
274         rospy.sleep(0.1)

The Trajectory execution is stopped by sending cancel goals to the Joint trajectory action server.

276     def wait(self):
277         """
278         Waits for and verifies trajectory execution result
279         """
280         #create a timeout for our trajectory execution
281         #total time trajectory expected for trajectory execution plus a buffer
282         last_time = self.goal.trajectory.points[-1].time_from_start.to_sec()
283         time_buffer = rospy.get_param(self._param_ns + 'goal_time', 0.0) + 1.5
284         timeout = rospy.Duration(self._slow_move_offset +
285                                  last_time +
286                                  time_buffer)
288         finish = self._limb_client.wait_for_result(timeout)
289         result = (self._limb_client.get_result().error_code == 0)
291         #verify result
292         if all([finish, result]):
293             return True
294         else:
295             msg = ("Trajectory action failed or did not finish before "
296                    "timeout/interrupt.")
297             rospy.logwarn(msg)
298             return False

This method waits for the completion of the trajectory execution by Joint trajectory action server.

301 def main():
302     """RSDK Joint Trajectory Example: File Playback
304     Plays back joint positions honoring timestamps recorded
305     via the joint_recorder example.
307     Run the example first to create a recording
308     file for use with this example. Then make sure to start the
309     joint_trajectory_action_server before running this example.
311     This example will use the joint trajectory action server
312     with velocity control to follow the positions and times of
313     the recorded motion, accurately replicating movement speed
314     necessary to hit each trajectory point on time.
315     """
316     epilog = """
317 Related examples:
319     """
320     arg_fmt = argparse.RawDescriptionHelpFormatter
321     parser = argparse.ArgumentParser(formatter_class=arg_fmt,
322                                      description=main.__doc__,
323                                      epilog=epilog)
324     parser.add_argument(
325         '-f', '--file', metavar='PATH', required=True,
326         help='path to input file'
327     )
328     parser.add_argument(
329         '-l', '--loops', type=int, default=1,
330         help='number of playback loops. 0=infinite.'
331     )
332     # remove ROS args and filename (sys.arv[0]) for argparse
333     args = parser.parse_args(rospy.myargv()[1:])

The name source file to read the Joint Position is captured along with the number of times the trajectory has to be looped from the command line.

335     print("Initializing node... ")
336     rospy.init_node("sdk_joint_trajectory_file_playback")
337     print("Getting robot state... ")
338     rs = intera_interface.RobotEnable(CHECK_VERSION)
339     print("Enabling robot... ")
340     rs.enable()
341     print("Running. Ctrl-c to quit")
343     traj = Trajectory()
344     traj.parse_file(path.expanduser(args.file))
345     #for safe interrupt handling
346     rospy.on_shutdown(traj.stop)
347     result = True
348     loop_cnt = 1
349     loopstr = str(args.loops)
350     if args.loops == 0:
351         args.loops = float('inf')
352         loopstr = "forever"
353     while (result == True and loop_cnt <= args.loops
354            and not rospy.is_shutdown()):
355         print("Playback loop %d of %s" % (loop_cnt, loopstr,))
356         traj.start()
357         result = traj.wait()
358         loop_cnt = loop_cnt + 1
359     print("Exiting - File Playback Complete")
361 if __name__ == "__main__":
362     main()

The node is initialized. An instance of the Trajectory class is created. The parse_file() method is called to extract the recorded Joint Positions and their corresponding timestamps as explained above. The start() method is called to send the FollowJointTrajectory request message to the Joint Trajectory Action server. This loops for the user specified number of times.