WEBVTT 00:00:01.580 --> 00:00:05.900 Hello and welcome to the video tutorials of NeuroMechFly. 00:00:06.560 --> 00:00:11.460 NeuroMechFly is a neuromechanical simulation of the fruit fly Drosophila melanogaster. 00:00:11.640 --> 00:00:17.440 It is developed here at the Neuroengineering Laboratory at EPFL, the Swiss Federal Institute 00:00:17.440 --> 00:00:19.180 of Technology in Lausanne. 00:00:20.020 --> 00:00:25.080 In this first tutorial, we'll briefly go through the body model of NeuroMechFly. 00:00:27.840 --> 00:00:34.620 You can start by visiting the NeuroMechFly website at neuromechfly.org. 00:00:36.160 --> 00:00:41.440 On the front page, you'll see a summary of the features and capabilities of our model, 00:00:42.580 --> 00:00:46.080 as well as an interactive viewer of NeuroMechFly. 00:00:46.840 --> 00:00:49.300 Let's view this in full screen. 00:00:52.320 --> 00:00:59.260 As indicated at the bottom left corner, you can drag an interactive viewer to orbit, 00:01:00.260 --> 00:01:07.080 scroll to zoom, and right drag to pan, 00:01:08.080 --> 00:01:17.100 and while pressing shift, you can drag different body segments to apply external perturbations. 00:01:19.820 --> 00:01:27.400 You can reset the physics simulation here, and reset the camera here. 00:01:28.360 --> 00:01:40.480 Lower down, in the position actuator section, you'll find sliders that control the joint 00:01:40.480 --> 00:01:44.360 angles on the six legs. 00:01:45.360 --> 00:01:55.732 For example, looking at the left front leg, we can move the sliders to change the joint angles. 00:01:57.400 --> 00:02:03.660 The positions of the sliders indicate the desired joint angles, in this case, specified 00:02:03.660 --> 00:02:05.980 by the user in the web interface. 00:02:07.280 --> 00:02:13.120 In simulations, typically, it is the target joint angles desired by the controller. 00:02:15.880 --> 00:02:20.980 And the green triangles indicate the current actual angles of the joints. 00:02:21.770 --> 00:02:28.000 You'll notice that the green triangle and the slider don't always overlap, 00:02:28.620 --> 00:02:33.100 and that is because the position actuators have a finite gain. 00:02:34.100 --> 00:02:42.020 That is, position actuators apply a finite amount of force to make the joint angles consistent 00:02:42.020 --> 00:02:43.760 with what the user desires. 00:02:46.940 --> 00:02:55.620 The more drastic the pose of the fly is, the greater the distance between the target position 00:02:55.620 --> 00:02:57.720 and the actual position. 00:03:00.380 --> 00:03:06.580 Higher up, in the physics parameters section, you'll see, in this case, three important 00:03:06.580 --> 00:03:08.640 physics parameters that you can tune. 00:03:09.560 --> 00:03:11.120 We'll start with actuator gain. 00:03:11.120 --> 00:03:18.440 This is the ability or the amount of force that the simulation applies to make the 00:03:18.440 --> 00:03:20.060 green arrow follow the slider. 00:03:20.700 --> 00:03:28.860 The greater the actuator gain, the closer the green triangle is to the slider. 00:03:32.740 --> 00:03:34.440 Let's reset the simulation. 00:03:37.000 --> 00:03:41.299 It's important to note that 00:03:41.299 --> 00:03:52.010 the NeuroMechFly model in this demo has many joints indicated here, 00:03:52.010 --> 00:03:56.340 but not every joint is actively actuated. 00:03:57.560 --> 00:04:06.140 Here the pink markers indicate the active joints, where there is an actuator that actually 00:04:06.140 --> 00:04:13.780 applies a force, whereas the yellow ones are only subjected to passive dynamics, such as 00:04:13.780 --> 00:04:15.540 joint stiffness and joint damping. 00:04:20.020 --> 00:04:31.540 If we reduce the joint stiffness, we'll observe that the passive joints become basically entirely 00:04:31.540 --> 00:04:37.660 soft in that, not soft in that they deform, but soft in the sense that they 00:04:37.660 --> 00:04:41.020 become more subjected to gravity, 00:04:41.020 --> 00:04:50.320 whereas you increase the stiffness the closer they are to the preset neutral angles. 00:04:56.940 --> 00:05:04.540 If you shift drag a body segment to apply a perturbation force, the fly will eventually 00:05:04.540 --> 00:05:07.460 recover to the neutral position. 00:05:07.460 --> 00:05:14.040 In fact, we can visualize this by looking at the contact points and contact forces. 00:05:17.200 --> 00:05:24.340 As we push this leg downward, you can see more contact points between this leg and the ground, 00:05:24.340 --> 00:05:34.220 and if we enable contact forces, we'll see that these forces are also being visualized. 00:05:37.460 --> 00:05:39.720 Let's reset the physics. 00:05:44.300 --> 00:05:51.460 The timescale of this restoration to the neutral pose is determined or impacted by 00:05:51.460 --> 00:05:53.060 the joint damping parameter. 00:05:53.500 --> 00:05:58.960 If we increase the damping, then this process becomes slower, 00:06:06.280 --> 00:06:15.280 whereas if we decrease damping, if we set it to literally zero, I suspect the physics 00:06:15.280 --> 00:06:17.040 simulation is simply going to crash, 00:06:17.100 --> 00:06:19.540 so let's keep it at 0.01, 00:06:20.240 --> 00:06:23.180 then this process becomes very fast. 00:06:29.820 --> 00:06:35.060 It's important to set sensible values to this physics parameter. 00:06:36.060 --> 00:06:41.820 Unfortunately, it's not always possible to experimentally calibrate every single one 00:06:41.820 --> 00:06:42.400 of them, 00:06:43.140 --> 00:06:49.020 but we have to somehow make an educated guess, 00:06:49.020 --> 00:06:59.800 and if we choose very obviously wrong parameters such as a combination of very 00:06:59.800 --> 00:07:05.040 high stiffness and very low damping, then we get this kind of behavior. Let me 00:07:07.880 --> 00:07:09.240 reset the simulation. 00:07:11.040 --> 00:07:15.400 This kind of fast oscillation is just not very realistic at all. 00:07:15.400 --> 00:07:20.260 So maybe the lesson learned here is that NeuroMechFly is a physics simulation. 00:07:20.500 --> 00:07:21.860 It is not an animation. 00:07:22.740 --> 00:07:26.080 The fly in simulation is subjected to the laws of physics, 00:07:26.420 --> 00:07:27.880 not everything is possible, 00:07:28.520 --> 00:07:32.220 and it's important to tune the physics parameters correctly.