Robot Arm That Record and Repeat Movements


This is a robot arm is 5 Degree of Freedom and is similar to the industrial ones, but only it can record movements and repeat very precisely. It’s entirely built from Styrene.

[original post from http://letsmakerobots.com/node/490]

About Styrene

The main construction material used is Polystyrene (or simply Styrene), which is widely available on most hobby stores. For bulk quantities or for those coming from the States, you can contact Evergreen Models – they will stock every styrene sheet / tubing you will ever need.

Styrene is a soft, normally white plastic, which is very easy to handle and will bond very nicely when glued with polystyrene glue. I normally design the part I want to make directly onto the styrene sheet and cut it off using scissors. For internal cut-outs I use my Dremmel with a 2.5mm diameter Carbide cutter. If I need my part to be thicker than the sheet’s thickness, I make multiple identical parts and glue them together. When you make parts like that, make sure you leave the drilling for the end – this will ensure holes are aligned across all glued sheets.

Robot Arm

Material and Tools

Materials List:

  • Polystyrene sheets:       0.5mm, 1.5mm and 2mm thick (Evergreen Models).
  • Polystyrene tubing:        Various
  • Brass sheet:                   0.5mm thick
  • Brass tubing:                  Various
  • Servos & Motors:              2 x HS-805BB (Shoulder yaw & pitch joints)
  • 1 x HS-485HB (Elbow joint)
  •                                             1 x HS-322HD (Wrist joint)
  • DC motor with 300:1 gearbox (Gripper)
  • Tactile Feedback:         Quantum Tunnelling Composite (QTC)
  • 1 x custom-made Electrical Rotary Union (Elbow-to-Wrist joint)
  • 1 x SKF Thrust Bearing (AXK 2542) + 2 x SKF Thrust Washers (AS 2542)
  • Various springs
  • Polystyrene glue
  • Epoxy glue
  • Heat glue
  • Multi-strand wire
  • Heat-shrink

Tools List

  • Scalpel
  • Studding knife
  • Small saw
  • Small files – various cross-sections
  • Sanding paper
  • Dremmel
  • Hand-drill
  • Vice
  • M3 & M4 taps
  • Heat-gun

The Grabber

This manipulator arm has been built without CAD drawings; entirely with the method described above. It comprises 5 DOFs: a yaw and pitch root (shoulder joints), an elbow joint, a wrist joint and a gripper. The wrist joint is fitted with a custom-made electrical rotary union that will allow for continuous rotation of the end-effector (gripper) without electrical connectivity problems. Basically the electrical rotary union eliminates the need for cables between rotating mechanical assemblies.

The gripper comprises two fingers each fitted with a Quantum Tunneling Composite (QTC) pressure sensor. These sensors are basically pressure sensitive resistors that under pressure change from insulators (R >10 MΩ) to conductors (R < few Ohms). The two sensors are wired in series and the signal is fed directly to the controller of the wrist joint.

Robot Arm Robot Arm

 

 

The Control System

Each axis of motion is controlled via a ‘Supermodified’ combo. The arm was built using the first generation of Supermodified controllers which work in conjunction with a magnetic encoder (1024ppr). The second generation comes with 4 times the resolution (4096ppr) (http://letsmakerobots.com/node/18615) so i am really looking forward to try out those babes on the second generation 6DOF manipulator. Supermodified controllers are direct replacement boards for the original servo electronics, that allow full PID speed and position control of the servo. Based on the ATMega 368P (fully Arduino compatible with half the footprint of Arduino nano and 4MHz faster) also allow for controlling 4 digital IOs, as well as 4 analog inputs.

So for example the gripper’s pressure sensors need not be fed back to the main controller, since the Supermodified controller board of the gripper can handle the analog signal from the sensors and report back to the main controller (in the video that would be the PC). The Arduino merely plays the role of a USB to I2C bridge. Btw the gripper’s DC motor has been a recent addition since its predecessor – a tower pro 90 micro – proved inadequate. The tiny DC motor powers the dual parallel-link finger assembly by using a tiny pulley system having an extra-strong thread acting as its timing belt :))).

The remaining servos have all been “Supermodified”. The joint hierarchy is as follows:

Shoulder (yaw) -> Shoulder (pitch) -> Elbow -> Wrist -> Gripper

The wiring is fairly simple and straightforward since each Supermodified controller can be ‘daisy-chained’ with other Supermodified controllers, minimizing complexity and offering a neat end-result. The whole manipulator arm wiring effectively comprises a single 4 core cable (Power and I2C comms), running from one servo to the next according to the hierarchy above. I2C comms are then fed to the Arduino, which for the time being acts as a USB (com) to I2C communication-bridge between the host computer and the Supermodified servo controller network.

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