Problem with moving robotic arm in sketchy physics


In my attached sketchy physics file, I am trying to move a robotic arm that has two segments that move from 1m to 0.5m. I was able to get the horizontal arm to work with the servo motor, however I cannot seem to be able to connect the vertical arm to the horizontal arm without the vertical arm falling off the screen. Can you assist me in getting the vertical arm connected to the horizontal arm? Thank You. robot_new2.skp (126.0 KB)

Work from the last element in the chain. In your model, the vertical arm is affected by the vertical piston, so link those with the joint connector. Now that piston is going to be affected as part of a group for the next arm section. So group the first piston to the second arm element. When you connect the second piston to the group, everything moves relative to that connector. You then group the primary arm segment with the second piston, and tie the servo to the primary arm group. So now the servo acts on the arm segments and the pistons, and they all rotate around the servo axis.

You need to sort out the leftover tape measure marks (get rid of them), make that secondary arm section (L shaped bit) a single object, and also make your piston min/max values much bigger. You are going from 0 to -0.5 movement range. Go big, then reduce to suit. I went +5 to -5, and got about half length of the arm movement in each direction. And then look at how the sketchy shape options change the behavior. The shape dictates how the object will move, and also how it interacts with other objects it comes into contact with. Take a simple cube, place it a distance above a floor, then run the animation. I should move/roll/bounce as expected of a cube shape. Now change the shape value to a cylinder and repeat. It took me a long time to get the hang of using right mix of shapes, link sequence and collision settings to make a decent animation. I did a 6 cylinder in-line car engine, complete with camshafts and valve train, and the animation setup didn’t do a lot for my general mood and well-being! Experiment, experiment and more experimenting…you’ll get the hang of it.

Hello, I was able to finally get the robot to move the way that I wanted it to. Thank You! Now, can you help me figure out how to move the arms in the given limits so that the robot draws a small circle? I sincerely appreciate your time and help. I have attached my updated drawing. Thanks.test13.skp (121.0 KB)

Much better model to work with, and good to see you have managed to get the connection and grouping sequence working.

Before I attempt to answer your question about drawing a circle, is this going to be a looping, running simulation, or do you intend to change the piston values to demo some other function?

There are a couple of ways to do this depending on your answer. You could attempt to script the movement of the pistons and servo (beyond my current skill level), or you can ‘cheat’. Attach the end of the arm to an invisible pivot, apply a motor to the pivot, change your pistons to sliders or springs and your servo into a hinge. Let the motor on the end effector dictate the movement of the entire arm. You will most likely get rid of that gravity induced sag in the arm when the animation first runs. See attached to get you started

test13.skp (169.5 KB)

Thanks again for your time and help!

OK, your second option sounds like the way I would like to go. Also, I want to stick with your current skill level as it is much further along than mines and you can direct me as to the way you think is feasible and doable in a short time frame. Moving along, I used your solution to get me going. I’ve just added the pivot with a motor. How do I tie that now to the smaller circle on the end effector arm?

Also, in order to group another element to a group, like say the small circle that I added to the end effector, do I have to explode everything and then start all over again?

Here is what I have so far (using your solution as the base line). How to go about getting the attached file to work so that as the motor turns, the arms move accordingly to created the circle?test13update.skp (175.8 KB)


Since your rotating pin (the one that will dictate the movement) is now the last element in the chain, you have to rebuild the links and groups. Essentially you are creating a crank pin that is attached to the end effector, then you hide it. That affects only one end of the object. You than have to create another object at the other end of the arm for that to cling onto and be attached to the slider. So one end is rotating, and the other is sliding.

I’m no expert on these complex chains and still to work out an easy ‘general rule of thumb’. There must be some sketchers in here that has this process sussed out reaching out to all sketchy physics gurus Maybe the infamous Box can shed some light on this problem? :slight_smile:

To help you get the hang of what I am describing above, see attached (play it, watch it, then view hidden…the cheat will be revealed)
rotaryDemo.skp (66.3 KB)

OK…after a few more minutes messing about with your test 13, here is the working model. Open the UI and inspect each component and you’ll see what I had to modify to get the thing spinning. Would work well if in zero G, but gravity is a reality (I personally think there’s not such thing as gravity…the earth sucks :slight_smile: )…so hence the backwards lean on the whole arm assembly. If it stalls, grab the top pivot point with your mouse and give it a nudge in the direction of spin. Kind of like crank starting a car…not that I remember that…just seen it in old movies LOL

Anyhoo…here’s your working model:
test13update.skp (178.5 KB)

Thank you so much for your help! I sincerely appreciate all your time and help.

As for the earth…I like the saying from Scarface with Al Pachino…Every day above ground is a good day…enjoy yourself!

Thanks again.

Happy Thanksgiving.


I know that I am asking much of you and taking up a lot or your time. But can you add one more item…I wanted to show the second arm also moving. We have the end effector (thanks to you I learnt a few things - this word being one of them!), working as described, but you can tell me how to also get the second arm to show movement. Do we have to make the circle larger so that the second arm also moves?

I looked at your demo and also your example. I am almost there. It seems that I have to hold the arm on the hinge up so show that the second arm moves also. Is there a way to fix the file so that the second arm doesn’t look like it going through the arm with the hinge? I’m struggling to get the second arm from going into the arm with the hinge. Do we have to draw a larger circle? Hope you can help me figure it out soon.

Gobble Gobble!

See your cross post for my reply…
Here in New Zealand, we don’t celebrate thanksgiving. We’re celebrating the end of winter and the start of the long hot summer days :smile: