Servo - May 2011

A Single Axis Robotic Arm With

End Effector

by Paul Verhage

www.servomagazine.com/index.php?/magazine/article/may2011_Verhage

Getting a robot arm to lift up and down is pretty easy; just connect it to a servo. The challenge is trying to get it to pick up an object. Taking a clue from the end effector on the Space Shuttle’s robotic arm, I came up with a “snare” of an idea. Now, my CheapBot robots carry a practical robotic arm.

The single axis robotic arm described here lifts and lowers targets captured within its end effector. The end effector is a wire snare that wraps around targets, and is extended and retracted by a miniature servo. Because of a momentary pushbutton switch at the end of the arm, a robot carrying this arm can determine when it has successfully captured and released its target. Combined with a little logic, I was able to program a CheapBot to follow a line and use its arm to pick up plastic jacks at one end, and deposit them at the other.

To keep the arm practical, I selected components that were available at local businesses, including hardware stores, hobby shops, and craft stores. I ordered the few remaining items from Jameco. Here are the necessary parts: - 1 inch diameter hidden pillar*

- Miniature servo (like a GWS Pico or JR Sport Micro)

- Momentary pushbutton switch (Jameco part number 106112)

- Piano wire, 20 mil diameter, 36 inch long - Mini EZ connector****

- 4.7K ohm resistor (1/4W)

- Heat shrink - #24 AWG stranded wire - 2-56 machine screws - 2-56 nylocks (use nylocks so the arm can’t fall apart from use)

- Connectors to your robot controller (like three-pin headers)

*Hidden pillars are fluted tubes used in wedding cakes; I got mine from George’s Hobbies in Lawrence, KS in their cake decorating aisle. **Syntra is a 1/8 inch thick sheet of foamed PVC (poly-vinyl chloride). Since it’s foamed (filled with air bubbles), it’s softer and easier to machine than regular PVC (which is a very hard plastic). Syntra is available from online robot stores, local plastic companies, and some sign companies. ***Correplast is corrugated plastic and very popular as a sign material. I was able to purchase some from a local sign company. ****Mini EZ connectors attach piano wire push rods to servo horns in model airplanes. You’ll find them in hobby shops that carry RC airplanes.