The Otto Arm

The Otto Arm

The Otto arm is a 3d printed 4 degrees of freedom robotic arm. It uses only the hardware that comes with your servos plus three little screws. It is a great intro to small robotic arms and it fits right into the Otto ecosystem as it is Otto trailer based. This means you can now tow a fully functional mini robotic arm behind your Otto robot allowing Otto to work as a third hand!

Parts

1. Filament: 200g is plenty 

2.  9 gram servos with included hardware x4 and servo arm (I am using Miuzei MG90S 9G Micro Servo) 

3. IR remote

4. IR sensor

5. 2 small screws to attach hitch to trailer approximately m2x7

6. A m3x16 screw for the gripper

7. micro controller such as a Arduino Nano, Esp32, Leonardo, or other small micro controller (Uno r3 or similar size will not work fit in the base as it is too big, but externally routing wires works.)

8. 6mm power jack

9. Some way to regulate power from 5mm jack such as a buck converter (If you have 5v to the power jack this part is unnecessary.)

10.  Power source- power adapter with 6mm jack for wired power OR 4 AA or rechargeable battery with 6mm jack for wireless power- I used this and it worked great https://www.printables.com/model/435931-stackable-battery-holder-for-4-aa-batteries/files

11. Wire to extend one servo cable. 

12. Wires suitable for servo power

13. Pin headers are helpful for wiring. 

14. Enough skill or courage to make a wiring harness for 4 servos and an IR sensor. 

Printing

• All parts will fit on a Prusa mini

• I printed in 3 platters with a .4mm nozzle, 15% infill, .2mm layer height, speed setting out of PLA

• Print profiles for a Prusa mini are included in the files portion 

• Platter 2 needs custom support 3 places. (sorry) If printing in individual parts; 1st arm and robot arm base/top of trailer need the supports. To place these supports, select painting, then smart fill, then select the 3 regions

  shown in this picture.

• Everything else is support free YAY!

Wiring 

• Wiring is pretty simple, connect both power and ground to the 4 servos, the IR sensor, and the micro-controller. (look up correct pin for powering your micro-controller externally, on a nano it is labeled VIN.) Than connect pin 2 to the sensor output, pin 3 to servo 1, pin 5 to servo 2, pin 6 to servo 3, and pin 9 to servo 4. DO NOT power your micro controller via both the pins and the USB port this will fry your micro controller. (I speak from experience) DO NOT have your Servos plugged in when you are uploading code or have micro-controller plugged in. This will fry your board. (I speak from experience; again) Here is a diagram.  I was in a hurry and my wiring harness looks like this. (I am sure you can do better)

  

Code

• I have mapped side movent to fast forward and fast backward, shoulder movment to up and down, elbow to vol+ and vol-, and gripper to 1 and 0. 

• I wrote this code (With the help of chat GPT) to work with my Arduino Nano Every and uno r3 but it should work (or almost work) with many other micro controllers.

• If you tweak the code for a different micro controller it would be great if you would upload that code so other makers can borrow it. 

• This code is VERY BAD I am an amateur coder and this code works but it is not great. The code is in the files. If anyone makes a better version please share it. Thank you.

Assembly

Step 1: Plug all servos in to put them in there neutral position  

1. This allows you to assemble the arm in it home position without worrying about servos hitting stops.

Step 2: Remove supports form the base and the first arm

1. This is all the parts and tools needed for the arm itself, except the wiring and control electronics. The Hot Glue gun is only to mount electronics not for the arm itself.

Step 3: Place all three servos in there respective spots. 

1. The Output gear on the servos should both be facing towards the axial they are controlling.  For servo 1 the output gear should be facing towards the left hand side. It should look like this.  

Step 4: Assemble the main pivot.

1. Take the vertical pivot pin, the main pivot itself, and break off one of the C clips. 

2. Slide the pin down between the two support arms and make sure it is flush.

3. Push the exposed end of the pin into the hole in the arm base. 

4. Flip the Base over and fasten it in place with a clip. It should look like this.

Step 5: Assemble the 1st arm. 

1. Take the 1st arm, the horizontal base pin and gear, and a C clip.

2. Put the arm into place between the support arms, making sure the servo outputs are pacing towards the square cutout in the main pivot. 

3. Push the horizontal pin and gear into place from the square cutout side. It is meant to be tight.

4. Clip it into place with a C clip. It should look like this. 

Step 6: Assemble the 2nd arm 

1. Take the second arm, the elbow pin, the last clip, and the printed washer.

2. Insert the elbow pin through the 2nd arm from the non-gear side. 

3. Put the flat washer on the end of the pin. It should look like this. 

1. Slide this assembly through the first arm from the servo output side.

2. Then clip it in place with a C clip. It should look like this. 

Step 7: Attach the gears 

1. Take all three gears and place a servo horn through them. I did not use any glue but you could if you wanted to. They should look like this. 

1. Place the servo gears in there respective spots. I numbered 2 of them but the numbers are tiny.

2. If you can see the numbers the one without numbers goes on servo 1, controling side to side movement, the one with a 1 goes to servo 2 controlling the first arm, and the one with a 2 goes on servo 3 controlling arm 2 moment.

3. If you can't see the numbers the one with The largest teeth goes on servo 3, the one with middle sized teeth goes on servo 1, and the one with small teeth goes on servo 2. (Sorry if this is confusing)

4. NOTE: When placing them follow these instructions. For servo 1 face the horn backwards and the arm forwards. For servo 2 face the horn as far Clockwise as it can go without hitting the other gear while the arm is fully down. For servo 3 face the horn down arm 1 while arm two is facing directly the other way. Here are pictures of the placement respectively. 

St

Step 8: Assembling the Trailer

1. Take the trailer, the four wheels, the two small screws, and the hitch.

2. Screw the hitch to the trailer.

3. Pop the wheels onto the trailer with the bulged side facing in. It should look like this.

Step 9: Assemble the Gripper. 

1. Take all the gripper parts.

2. Place the gripper fingers onto the base plate making sure the two pillars on the left gripper are facing up.  

3. Put the servo in its holder and attach a servo horn. 

4. Mount the servo holder on top of the base plate using an m3X10 screw from the bottom. 

5. Run the wires through the channel in the 2nd arm. (It is tight but it fits) Then down the bottom of the 1st arm through the hoops. You might need some extension to get it down the whole way. 

Step 10: Finish wiring. 

1. Run the wires through the rectangular hole and plug them into your wiring harness.

2. Glue the IR sensor through the round hole. 

3. Glue 6mm power Jack into the cutout in the trailer

4. Fasten micro controller down.

5. Hook up all loose wires.

6. Put the top on. 

7. AND YOU ARE DONE!!!

NOTES: 

1. You can print another trailer and place the 4 AA battery pack in it for completely wire free power.

2. There is a arm base plate in the files that has 2 m3 holes which are 10mm apart so you can make your own attachments easily. 

3. PLEASE Remix this. This is not a perfect design and I would love to see your upgrades.

4. PLEASE Modify the code to make it work better or upgrade it. I personal would love to see an esp32 code with Bluetooth control. 

5. THANK YOU for printed this model and or just reading the instructions, I hope you enjoy it. 

Thank You!