Curtain¶
This sample project includes a robot which consists of a row of actuators driving suspended passive chains. This model demonstrates several key concepts: underactuated systems, generative movement, and scripted modeling.
This model is demonstrated in the curtain.wbt world. The base link for all
actuators has a NULL Physics object so it does not move, simulating a rigid
connection to the ground. Each link has a NULL boundingObject so it does not
incur collision detection calculations; as a result each body needs specified
mass properties in the Physics node.
Screenshot of Webots model of the curtain robot.¶
Sample Robot Control Code¶
The controller implements keyboard input to trigger generative poses and movements. The structures uses the position controllers implemented in Webots for driven modes, and may also invoke a zero-torque model for free dynamics.
1# curtain.py
2#
3# Sample Webots controller file for driving a
4# 'curtain' of actuated hanging chains.
5#
6# No copyright, 2020-2022, Garth Zeglin. This file is
7# explicitly placed in the public domain.
8
9print("loading curtain.py...")
10
11# Import the Webots simulator API.
12from controller import Robot
13from controller import Keyboard
14
15# Import the standard Python math library.
16import math
17
18# Define the time step in milliseconds between
19# controller updates.
20EVENT_LOOP_DT = 20
21
22################################################################
23
24# Request a proxy object representing the robot to control.
25robot = Robot()
26name = robot.getName()
27print(f"curtain.py waking up for {name}...")
28
29# Query the number of devices. The curtain.proto model has one joint actuator
30# and one sensor per chain.
31num_devices = robot.getNumberOfDevices()
32chains = num_devices // 2
33print(f"Found {num_devices} devices, assuming {chains} hanging chains.")
34
35# Enable computer keyboard input for user control.
36keyboard = Keyboard()
37keyboard.enable(EVENT_LOOP_DT)
38
39# Fetch handles for the joint sensors. The names are generated by the curtain.proto scripting.
40joints = [robot.getDevice('joint%d' % (jnum+1)) for jnum in range(chains)]
41
42# Specify the sampling rate for the joint sensors.
43for j in joints:
44 j.enable(EVENT_LOOP_DT)
45
46# Fetch handles for the position actuator at the top of each chain.
47motors = [robot.getDevice('motor%d' % (jnum+1)) for jnum in range(chains)]
48for m in motors:
49 m.setPosition(0.0)
50
51################################################################
52# Run an event loop until the simulation quits,
53# indicated by the step function returning -1.
54
55while robot.step(EVENT_LOOP_DT) != -1:
56
57 # Read simulator clock time.
58 t = robot.getTime()
59
60 # Read the new joint positions.
61 q = [j.getValue() for j in joints]
62
63 # Read any computer keyboard keypresses. Returns -1 or an integer keycode while a key is held down.
64 key = keyboard.getKey()
65 if key != -1:
66 # convert the integer key number to a lowercase single-character string
67 letter = chr(key).lower()
68
69 # special case: 'p' will enter a passive zero-torque mode
70 if letter == 'p':
71 for m in motors:
72 m.setTorque(0.0)
73
74 # drive to downward reference position
75 elif letter == 'd':
76 for m in motors:
77 m.setPosition(0.0)
78
79 # drive all to front
80 elif letter == 'f':
81 for m in motors:
82 m.setPosition(-0.5)
83
84 # drive all to back
85 elif letter == 'b':
86 for m in motors:
87 m.setPosition(0.5)
88
89 # drive to alternating positions
90 elif letter == 'l':
91 for i, m in enumerate(motors):
92 p = 0.5 if (i&1) == 0 else -0.5
93 m.setPosition(p)
94
95 # drive to opposite alternating positions
96 elif letter == 'r':
97 for i, m in enumerate(motors):
98 p = 0.5 if (i&1) == 1 else -0.5
99 m.setPosition(p)
100
101 # generate a traveling wave (while 'w' is held down)
102 elif letter == 'w':
103 for i, m in enumerate(motors):
104 p = 0.5 * math.sin(1.5 * t + 0.75 * i)
105 m.setPosition(p)
Proto File¶
The robot is modeled in a proto file to allow scripted generation of the chains. The number of chains can be varied after creation and the robot model will be regenerated. The proto file is VRML with embedded Lua scripting.
1#VRML_SIM R2022a utf8
2# documentation url: https://courses.ideate.cmu.edu/16-375
3# Curtain. A variable number of hanging chains with a single position actuator at top.
4# license: No copyright, 2020-2022 Garth Zeglin. This file is explicitly placed in the public domain.
5PROTO curtain [
6 field SFVec3f translation 0 0 0
7 field SFRotation rotation 0 1 0 0
8 field SFString controller "curtain"
9 field SFString name "curtain"
10 field SFInt32 numchains 6
11 field SFString customData ""
12]
13{
14 Robot {
15 # connect properties to user-visible data fields
16 translation IS translation
17 rotation IS rotation
18 controller IS controller
19 name IS name
20 customData IS customData
21
22 # Calculate derived parameters
23 %{
24 local chain_y_spacing = 0.25
25 local link_y_width = 0.2
26 local basewidth = (fields.numchains.value - 1) * chain_y_spacing + link_y_width
27 local chain1_y = (-0.5 * basewidth) + (0.5 * link_y_width)
28 }%
29 children [
30 # define the non-moving hanging support
31 Transform {
32 translation 0 0 1.6
33 children [
34 Shape {
35 appearance DEF baseColor PaintedWood {
36 colorOverride 0.21529 0.543008 0.99855
37 }
38 geometry Box {
39 size 0.02 %{= basewidth }% 0.18
40 }
41 }
42 ]
43 }
44
45 # loop to create each chain
46 %{ for c = 1, fields.numchains.value do }%
47 %{ local motor_name = "\"motor" .. c .. "\"" }%
48 %{ local sensor_name = "\"joint" .. c .. "\"" }%
49
50 # template defining an individual chain
51 HingeJoint {
52 jointParameters HingeJointParameters {
53 axis 0 1 0
54 anchor 0 0 1.5
55 }
56 device [
57 PositionSensor {
58 name %{= sensor_name }%
59 }
60 RotationalMotor {
61 name %{= motor_name }%
62 controlPID 10 0 0
63 maxVelocity 3.14
64 minPosition -10
65 maxPosition 10
66 maxTorque 2
67 }
68 ]
69 endPoint Solid {
70 translation 0 %{= chain1_y + (c-1) * chain_y_spacing }% 1.5
71 rotation 0 1 0 0
72 children [
73 Transform {
74 translation 0 0 -0.15
75 children [
76 Shape {
77 appearance DEF linkColor GlossyPaint {
78 baseColor 1 0.975219 0.328771
79 }
80 geometry Box {
81 size 0.02 0.2 0.28
82 }
83 }
84 ]
85 }
86 HingeJoint {
87 jointParameters HingeJointParameters {
88 axis 0 1 0
89 anchor 0 0 -0.3
90 dampingConstant 0.1
91 }
92 device [
93 # PositionSensor { name "joint1B" }
94 ]
95 endPoint Solid {
96 translation 5.816463393668452e-06 0 -0.30015172239714644
97 rotation 0 -1 0 0.0016977587231221368
98 children [
99 Transform {
100 translation 0 0 -0.15
101 children [
102 Shape {
103 appearance USE linkColor
104 geometry Box {
105 size 0.02 0.2 0.28
106 }
107 }
108 ]
109 }
110 HingeJoint {
111 jointParameters HingeJointParameters {
112 axis 0 1 0
113 anchor 0 0 -0.3
114 dampingConstant 0.1
115 }
116 device [
117 # PositionSensor { name "joint1C" }
118 ]
119 endPoint Solid {
120 translation 0 0 -0.3
121 rotation 0 1 0 0
122 children [
123 Transform {
124 translation 0 0 -0.15
125 children [
126 Shape {
127 appearance USE linkColor
128 geometry Box {
129 size 0.02 0.2 0.28
130 }
131 }
132 ]
133 }
134 HingeJoint {
135 jointParameters HingeJointParameters {
136 axis 0 1 0
137 anchor 0 0 -0.3
138 dampingConstant 0.1
139 }
140 device [
141 # PositionSensor { name "joint1D" }
142 ]
143 endPoint Solid {
144 translation 0 0 -0.3
145 rotation 0 1 0 0
146 children [
147 Transform {
148 translation 0 0 -0.15
149 children [
150 Shape {
151 appearance USE linkColor
152 geometry Box {
153 size 0.02 0.2 0.28
154 }
155 }
156 ]
157 }
158 ]
159 name %{= "\"link" .. c .. "_4\"" }%
160 physics Physics {
161 density -1
162 mass 0.5
163 centerOfMass [
164 0 0 -0.15
165 ]
166 inertiaMatrix [
167 0.006 0.004 0.002
168 0 0 0
169 ]
170 }
171 }
172 }
173 ]
174 name %{= "\"link" .. c .. "_3\"" }%
175 physics Physics {
176 density -1
177 mass 0.5
178 centerOfMass [
179 0 0 -0.15
180 ]
181 inertiaMatrix [
182 0.006 0.004 0.002
183 0 0 0
184 ]
185 }
186 }
187 }
188 ]
189 name %{= "\"link" .. c .. "_2\"" }%
190 physics Physics {
191 density -1
192 mass 0.5
193 centerOfMass [
194 0 0 -0.15
195 ]
196 inertiaMatrix [
197 0.006 0.004 0.002
198 0 0 0
199 ]
200 }
201 }
202 }
203 ]
204 name %{= "\"link" .. c .. "_1\"" }%
205 physics Physics {
206 density -1
207 mass 0.5
208 centerOfMass [
209 0 0 -0.15
210 ]
211 inertiaMatrix [
212 0.006 0.004 0.002
213 0 0 0
214 ]
215 }
216 }
217 } # end definition of individual chain
218 %{ end }% # end loop to create each chain
219 ] # end children of Robot
220 } # end Robot
221}
World File¶
1#VRML_SIM R2022a utf8
2WorldInfo {
3}
4Viewpoint {
5 orientation 0.12859871179887852 -0.023940037157748277 -0.9914077092420426 3.8572616925142666
6 position 4.0633189701268275 -3.320537956126432 1.983958187570635
7 followType "None"
8}
9Background {
10 skyColor [
11 0.1 0.1 0.1
12 ]
13}
14DirectionalLight {
15 direction -0.4 -0.5 -1
16 intensity 3
17 castShadows TRUE
18}
19RectangleArena {
20 floorSize 2 2
21}
22curtain {
23}