Steps towards a space simulator

I am quite interested in how simulators such as the Orbiter space simulator are implemented. A spacecraft can be seen as a rigid object with a moments of inertia tensor. Without any forces acting on the object, the rotational moment of the object does not change. In general the moments of inertia tensor causes the direction of the rotation vector to be different at each point in time even if the rotational moment is not changing. This motion can be numerically simulated using a higher order integration method such as 4th order Runge-Kutta. Here is a video showing the resulting simulation of a cuboid tumbling in space:

Brian Vincent Mirtich's thesis demonstrates how to simulate collisions of two convex polyhedra. Furthermore micro-collisions are used as a simple but powerful method to simulate resting contacts. If the micro-collisions are sufficietly small, a resting object can be approximated with sufficient accuracy:

One still needs to implement friction (also shown in Mirtich's thesis) which requires a numerical integral to compute the friction occuring during a micro-collision. Collisions of polyhedra are demonstrated in Mirtich's thesis as well, however it might be simpler to make use of the GJK algorithm. Planetary bodies, spacecraft, and other non-convex objects could be handled by dividing them into multiple convex objects. It would also be interesting to integrate soft body physics as shown in Rigs of Rods. However the accuracy of Rigs of Rods is not sufficiently high for space simulation. E.g. an object tumbling in space would not preserve its momentum.

Update:

In the following examples, dynamic Coloumb friction with the ground is simulated.

This website is now based on Jekyll

Hi, I decided to update the design of my homepage to a responsive HTML design. This makes the web page look nicer on mobile phones. I ended up using the Hyde theme which is based on Poole which in turn is based on Jekyll. I hope you will enjoy the new layout. Here is a demonstration of what Jekyll can do (also to remind myself). See Hyde example for more.

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The text can be italic or bold.

Quoted text looks like this.

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Inline code looks like this.

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Here is a Gist from Github:

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Raspberry Pi Zumo robot

Pimoroni sells a Zumo chassis kit which is a low-cost tracked mobile platform. To build a robot, only the following additional components are required (see Github repository for more details):

  • micro metal gear motors
  • Raspberry Pi Zero W and Micro SD card
  • portable power bank
  • 4 AA rechargable batteries
  • H bridge motor driver
  • Raspberry Pi camera module
  • wires

In his seminal publication Neural Network Vision for Robot Driving Dean A. Pomerleau shows that one can train a neural network to drive using low-resolution camera images (also see video). In a similar fashion the Zumo robot can first be controlled using an XBox controller. 10 times a second a video frame together with the current motor settings are recorded. The images are downsampled to 32x24 pixels. A neural network with two hidden layers with 20 units each is trained. The output of the neural network are two vectors (one for the left and one for the right motor) with 11 units each. The first unit corresponds to driving backwards with full speed. The last unit corresponds to driving forward with full speed. Initially the robot frequently looses track and a manual override is required. Each time the manual override is used, new training data is recorded. After several iterations the robot finally achieves the desired behaviour and it can patrol the kitchen as shown in the video below.

Regularisation was used to reduce the variance. The bias of the network is quite high but this is probably due to conflicting training data (i.e. over time different drive speeds are used at the same position in the kitchen). Note that the experiment works best in controlled lighting conditions. Otherwise much more training data is required to cope with changes in illumination.

See also:

Sainsmart 6-dof robot arm

Sainsmart is selling a 6-dof robot arm. One can control the robot arm using an Arduino controller and a DFRobot I/O expansion shield. I mounted a Sunfounder Rollpaw gripper on the robot (note that the servo shaft is not compatible with the Sainsmart servos and I had to replace the wrist servo of the Sainsmart robot). I developed some software to perform smooth and synchronized motion with the robot drives. The robot can be controlled using a serial terminal. Alternatively one can use an XBox controller. The software performing inverse kinematics is demonstrated in the following video.

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Orbiter 2016

Orbiter 2016 is a space simulator for PC which works even on graphics cards which are considered low-end these days. The simulator can be used to simulate launch, ascent, orbiting, space station dockings, flight to different moons and planets, atmospheric reentry, vertical landings, and landings on a runway. Click on the picture below for more screenshots:

Orbiter 2016 simulation of XR2 orbiting the Moon

Together the downloads are about 3GByte:

If you have a lot of disk space, you can install high-resolution textures.

Here is a video I made showing SCRAM ascent and ISS docking

I also made a set of videos showing a simulated flight to the Moon and back.

If you have time for some space training, I can recommend to check it out. Enjoy!

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