Week 12: Computer and/or Fabrication Lab

The TA will be in the computer labs and I will be in E1-141 to help you with your CAD drawings for your Bioinspired Design.
Continuous Servos If your design requires a continuous servo, then please take a look at the following instructions on how to modify your servo. Be aware that it is impossible to sync two continuous servos together. For example, if you designed a robot with two continuous servos that control two legs, the legs would eventually become out of sync with each other. Save yourself some trouble and use a single servo with a mechanical linkage.
  1.  Remove the back cover as shown in Fig. 3 by loosening all four screws.
  2. Move the circuit board out of the way.
  3. There is another small screw in there that holds the potentiometer in place. The potentiometer provides position feedback on the location of the output shaft. We need to get rid of the potentiometer so that the microprocessor in the servo thinks the shaft is not moving. Remove the screw.
  4. Pull out the potentiometer.
  5. Pull off the gear cover.
  6. There will be a small protrusion on the gear where the screwdriver is pointing in Fig. 4. Remove that with a Dremel. Be sure to get all of it. If you don’t, the gears will bind.
  7. Replace the gear cover, but don’t replace the potentiometer yet. Plug in your servo to the Arduino and command it to go to 90. Now play with the potentiometer. You should be able to adjust the potentiometer so that there is no motion in the servo. That is your zero point.
  8. You can do a few different things now. You can drop a bit of hot glue on the potentiometer to keep it in place. You could then stuff it in upside down in the servo or have it dangling outside the servo. You could cut off the top of the potentiometer and put it back in the correct way. You could completely remove the potentiometer and replace it with a fixed resistor. Your choice.
  9. Wrap a piece of tape around or otherwise mark the servo to indicate that it is a continuous servo.
Creating Linkages in Inventor One of the easiest ways “play” with linkages is to do so in the Sketch mode of a Inventor part file (see Fig. 5). To dimension each segment, click on dimension, click on the line, right click, select “Aligned.” Now click on the vertices between two line segments and move it around. You should be able to get a good idea of how the linkage moves and if there are any problem areas that are close to a singularity.

Creating a Dynamic Simulation You will need to create a dynamic simulation of your linkage.
  1. To do this, first create your mechanism as an assembly (see Fig. 6).
  2. I suggest using the “Joint” command instead of constraints in your assembly.
  3. When you do create a Joint, I have had better luck specifying a rotational joint as opposed to using the automatic settings.
  4. Click on the “Environments” ribbon and then on “Dynamic Simulation” on the far left.
  5. If this is your first time doing a simulation, a dialog box will appear asking you if you would like to do the dynamic simulation tutorial. I strongly suggest that you do the tutorial. It actually provides you with more information than you will need, but it is also fairly short.
  6. Click on “construction mode” inside the simulation player.”
  7. Right click one of your joints in the left-side tree view. Select properties.
  8. Select the DOF tab.
  9. Click “Edit imposed motion.”
  10. Select velocity or play around with the different options to choose the pattern that corresponds to what you want your servo to do.
  11. Click “play” on your simulation player.