MRDS: MOBILE BASE + ROBOT ARM

Today I would like to explain how to attach a robot arm to a mobile base using the simulation environment of Microsoft Robotics Developer Studio. It is a very simple task but it can get a bit tricky.

I have been following the examples and explanations on the book Professional Microsoft Robotics Developer Studio (you can find the source code of the examples at http://www.promrds.com ). In this post I will explain how to attach a Simulated LynxL6Arm to a simulated Corobot. 

First of all, you should open in Visual Studio the project where the Corobot Entity is defined and add a reference to SimulatedLynxL6Arm.Y2007.M07. 

Then edit the file Corobot.cs and locate the definition of the constructor public CorobotEntity(string name, Vector3 initialPos). At the end of the constructor insert an instance of the robotic arm like this:

//Insert the LynxL6Arm
LynxL6Arm.SimulatedLynxL6Arm l6arm = new LynxL6Arm.SimulatedLynxL6Arm(name + "_arm", new Vector3(0,0,0));
l6arm.State.Pose = new Pose(
        new Vector3(0, 0.23f, 0),
        Quaternion.FromAxisAngle(0, 1, 0, (float)(-Math.PI / 2)));


InsertEntityGlobal(l6arm);

Now, compile the project and run it. You should see something like this:

That was easy, wasn't it? But now, if you try to drive the Corobot around you will find that IT WONT MOVE! You will see the wheels spinning, but the robot doesn't move an inch from its position. WTF!

Now is when one becomes crazy trying to find out what is going on. You start reading MRDS's forums and find answers as crazy as "copy the source code of the definition of the robot arm inside the definition of the mobile base"... WHAT?? Come on! There must be a more elegant solution!!

And, indeed, there is. It took me one whole day to find out what was happening but, if you take a deep breath and focus, you eventually find the solution.

If you look to the picture above (the one with the Yellow Lynx L6 Arm on top of the Corobot) you will notice that it has a cylindric base. Well, that base is, by default, defined to be Kinematic, that means that it will not follow the physic laws and will stay like sticked to the ground. That is why the robot will not move!

Now, the solution to this problem is to remove the kinematic flag from the arm instance that we just created. So the correct source code looks like this:

//Insert the LynxL6Arm
LynxL6Arm.SimulatedLynxL6Arm l6arm = new LynxL6Arm.SimulatedLynxL6Arm(name + "_arm", new Vector3(0,0,0));
l6arm.State.Pose = new Pose(
        new Vector3(0, 0.23f, 0),
        Quaternion.FromAxisAngle(0, 1, 0, (float)(-Math.PI / 2)));

//Remove the Kinematic flag!! Otherwise the mobile base gets stuck!
l6arm.State.Flags = l6arm.State.Flags & ~(EntitySimulationModifiers.Kinematic);
InsertEntityGlobal(l6arm);