This example shows joint torque control usage. After moving to neutral, the robot will enter torque control mode, applying torques representing virtual springs holding the joints to their start position. The example calculates and applies torques linearly relative to offset from the start position creating the illusion that springs of configurable stiffness are attached to each joint.
Introduction
The Joint Torque Springs Example demonstrates the usage of torque controller to apply torque commands to robot's joints. In the main() function, the dynamic reconfigure from server is initialized and passed to an object of the JointSprings class. On initialization, the object creates an instance of the limb class for the particular side that is passed, and captures the default spring and damping constants from the dynamic reconfigure server. The main() function calls the method, move_to_neutral() to send the limbs to a neutral position. Then, the attach_spring() method captures the initial joint angles of the limb and calls the update_forces() method which calculates the necessary torques based on Hooke's law and publishes them as joint commands.
Interfaces -
- Limb.set_joint_torques()
- Limb.move_to_neutral()
- Limb.joint_angles()
- Limb.joint_velocities()
- Limb.set_command_timeout()
- Limb.exit_control_mode()
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 torque spring example.
Usage
Start the joint torque springs example from an RSDK terminal session using:
$ rosrun intera_examples joint_torque_springs.py
Messages will appear after start:
Initializing node...
Getting robot state...
Enabling robot...
Running. Ctrl-c to quit
The robot will move to neutral pose first, when done moving to neutral pose, you can interact with the robot by grabbing, pushing, and rotating each joint to feel the torques applied that represent the virtual springs attached.
Pressing Control-C at any time will stop torque mode and exit the example.
Arguments
Important Arguments:
-l or --limb : The robot arm name (default="right")
See the joint torque spring's available arguments on the command line by passing joint_torque_springs.py the -h, help argument:
$ rosrun intera_examples joint_torque_springs.py -h
usage: joint_torque_springs.py [-h] [-l LIMB]
RSDK Joint Torque Example: Joint Springs
Moves the default limb to a neutral location and enters
torque control mode, attaching virtual springs (Hooke.s Law)
to each joint maintaining the start position.
Run this example and interact by grabbing, pushing, and rotating
each joint to feel the torques applied that represent the
virtual springs attached. You can adjust the spring
constant and damping coefficient for each joint using
dynamic_reconfigure.
optional arguments:
-h, --help show this help message and exit
-l LIMB, --limb LIMB
limb on which to attach joint springs
Code Walkthrough
Now, let's break down the code.
33 import argparse
34 import importlib
35
36 import rospy
37 from dynamic_reconfigure.server import (
38 Server,
39 )
40 from std_msgs.msg import (
41 Empty,
42 )
43
44 import intera_interface
45 from intera_interface import CHECK_VERSION
This imports the intera interface for accessing the limb 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.
43 class JointSprings(object):
44 """
45 Virtual Joint Springs class for torque example.
46
47 @param limb: limb on which to run joint springs example
48 @param reconfig_server: dynamic reconfigure server
49
50 JointSprings class contains methods for the joint torque example allowing
51 moving the limb to a neutral location, entering torque mode, and attaching
52 virtual springs.
53 """
54 def __init__(self, reconfig_server, limb = "right"):
55 self._dyn = reconfig_server
56
57 # control parameters
58 self._rate = 1000.0 # Hz
59 self._missed_cmds = 20.0 # Missed cycles before triggering timeout
60
61 # create our limb instance
62 self._limb = intera_interface.Limb(limb)
63
64 # initialize parameters
65 self._springs = dict()
66 self._damping = dict()
67 self._start_angles = dict()
The JointSprings class object is initialized by passing the side of the limb under interest, an object of the reconfigure server that holds the default spring and damping constants for the limb. These default values can be modified at runtime through the dynamic reconfigure server.
69 # create cuff disable publisher
70 cuff_ns = 'robot/limb/' + limb + '/suppress_cuff_interaction'
71 self._pub_cuff_disable = rospy.Publisher(cuff_ns, Empty, queue_size=1)
72
73 # verify robot is enabled
74 print("Getting robot state... ")
75 self._rs = intera_interface.RobotEnable(CHECK_VERSION)
76 self._init_state = self._rs.state().enabled
77 print("Enabling robot... ")
78 self._rs.enable()
79 print("Running. Ctrl-c to quit")
The zero gravity mode can be enabled by pressing the cuff and so it is important to disable the cuff. A publisher for the cuff interaction suppress topic is created. It is also important to note that the robot should be enabled before trying to manipulate its joints. So, the robot is checked if it is enabled and if not, it is enabled. The initial state of the variable is recorded in _init_state variable. This is to ensure that the robot is sent back to that state once the example program ends.
81 def _update_parameters(self):
82 for joint in self._limb.joint_names():
83 self._springs[joint] = self._dyn.config[joint[-2:] + '_spring_stiffness']
84 self._damping[joint] = self._dyn.config[joint[-2:] + '_damping_coefficient']
The _update_parameters() method captures the dynamically entered user's input for the spring and damping constants through the dynamic reconfigure server.
86 def _update_forces(self):
87 """
88 Calculates the current angular difference between the start position
89 and the current joint positions applying the joint torque spring forces
90 as defined on the dynamic reconfigure server.
91 """
92 # get latest spring constants
93 self._update_parameters()
94
95 # disable cuff interaction
96 self._pub_cuff_disable.publish()
The cuff interaction is disabled as long as the example runs and each time the forces are updated, the _pub_cuff_disable publisher publishes an empty message.
98 # create our command dict
99 cmd = dict()
100 # record current angles/velocities
101 cur_pos = self._limb.joint_angles()
102 cur_vel = self._limb.joint_velocities()
103 # calculate current forces
104 for joint in self._start_angles.keys():
105 # spring portion
106 cmd[joint] = self._springs[joint] * (self._start_angles[joint] -
107 cur_pos[joint])
108 # damping portion
109 cmd[joint] -= self._damping[joint] * cur_vel[joint]
110 # command new joint torques
111 self._limb.set_joint_torques(cmd)
Hooke's law is applied to calculate the necessary torques to be applied on each joints. The updated spring and damping constants are used in the calculation. The set_joint_torques() method of the limb interface publishes the set of joint torques and their corresponding names with the mode as 3 to the topic robot/limb/right/joint_command . The mode indicates that the joint commands are to be passed to the torque controller.
113 def move_to_neutral(self):
114 """
115 Moves the limb to neutral location.
116 """
117 self._limb.move_to_neutral()
The move_to_neutral() method of the limb interface, moves all the joints to their neutral pose. This method employs the position controller to send the joints to a pre-defined neutral position.
119 def attach_springs(self):
120 """
121 Switches to joint torque mode and attached joint springs to current
122 joint positions.
123 """
124 # record initial joint angles
125 self._start_angles = self._limb.joint_angles()
126
127 # set control rate
128 control_rate = rospy.Rate(self._rate)
129
130 # for safety purposes, set the control rate command timeout.
131 # if the specified number of command cycles are missed, the robot
132 # will timeout and disable
133 self._limb.set_command_timeout((1.0 / self._rate) * self._missed_cmds)
134
135 # loop at specified rate commanding new joint torques
136 while not rospy.is_shutdown():
137 if not self._rs.state().enabled:
138 rospy.logerr("Joint torque example failed to meet "
139 "specified control rate timeout.")
140 break
141 self._update_forces()
142 control_rate.sleep()
The start angles recorded here are used by the _update_forces method to determine the dx, which is the displacement caused from the starting position of the joints. The _update_forces method is invoked at 1000Hz, which performs the spring motion as explained above.
144 def clean_shutdown(self):
145 """
146 Switches out of joint torque mode to exit cleanly
147 """
148 print("\nExiting example...")
149 self._limb.exit_control_mode()
150 if not self._init_state and self._rs.state().enabled:
151 print("Disabling robot...")
152 self._rs.disable()
The method exit_control_mode() of the limb interface switches to position controller mode from torque/velocity controller and saves the current joint angles as the current joint position. Finally, it checks if the robot was initially disabled and if so disables it.
155 def main():
156 """RSDK Joint Torque Example: Joint Springs
157
158 Moves the default limb to a neutral location and enters
159 torque control mode, attaching virtual springs (Hooke's Law)
160 to each joint maintaining the start position.
161
162 Run this example and interact by grabbing, pushing, and rotating
163 each joint to feel the torques applied that represent the
164 virtual springs attached. You can adjust the spring
165 constant and damping coefficient for each joint using
166 dynamic_reconfigure.
167 """
168 # Querying the parameter server to determine Robot model and limb name(s)
169 rp = intera_interface.RobotParams()
170 valid_limbs = rp.get_limb_names()
171 if not valid_limbs:
172 rp.log_message(("Cannot detect any limb parameters on this robot. "
173 "Exiting."), "ERROR")
174 robot_name = intera_interface.RobotParams().get_robot_name().lower().capitalize()
175 # Parsing Input Arguments
176 arg_fmt = argparse.ArgumentDefaultsHelpFormatter
177 parser = argparse.ArgumentParser(formatter_class=arg_fmt)
178 parser.add_argument(
179 "-l", "--limb", dest="limb", default=valid_limbs[0],
180 choices=valid_limbs,
181 help='limb on which to attach joint springs'
182 )
183 args = parser.parse_args(rospy.myargv()[1:])
184 # Grabbing Robot-specific parameters for Dynamic Reconfigure
185 config_name = ''.join([robot_name,"JointSpringsExampleConfig"])
186 config_module = "intera_examples.cfg"
187 cfg = importlib.import_module('.'.join([config_module,config_name]))
188 # Starting node connection to ROS
189 print("Initializing node... ")
190 rospy.init_node("sdk_joint_torque_springs_{0}".format(args.limb))
191 dynamic_cfg_srv = Server(cfg, lambda config, level: config)
192 js = JointSprings(dynamic_cfg_srv, limb=args.limb)
193 # register shutdown callback
194 rospy.on_shutdown(js.clean_shutdown)
195 js.move_to_neutral()
196 js.attach_springs()
197
198
199 if __name__ == "__main__":
200 main()
The rospy node is initialized and then an instance of the dynamic reconfigure is created. As indicated above, this supplies the default and runtime, spring and dynamic constants. In custom programs this can be avoided by assigning spring and damping constants directly in the code.
