CD Robot, Version 2

Although the first CD Robot worked well enough to run for a week and rip nearly 800 CDs, it lacked style. The robot was slow, temperamental, and had a limited range with halting, jerky movements. I wanted a robot to be proud of.

The most trouble some aspect of the original robot was the vertical slide. It used two rails, one an acme screw and the other the linear slide from a printer. The assembly was not rigid and tended to bind, besides being bulky and ugly.

The new design puts the screw inside a slotted tube that is concentric with the rotary table. The tube has two slots, on opposite sides, where flanges attached to the nut on the screw extend through the tube. These flanges are made of Teflon and run on both sides of the slot, so the nut does not wobble as the screw turns

Because the screw is concentric with the rotary table, the system is much more compact than the old robot, and the stepper that drives the screw can be fixed to the base, rather than rotating with the vertical assembly. This removes two-thirds of the mass from the rotary table.

This design uses two thrust bearings and a few oil-impregnated bronze bearings on the acme screw and a larger thrust bearing and a hacked, 3-inch lazy-susan bearing on the rotary table.

By April 2003 I had not decided on what type of gripper to use. The simplest system may be to use suction cups like in the original robot. This system will employ a venturi vacuum generator and a half-homemade vacuum switch ( converted from a pressure switch ) so it will be much smaller than the previous pneumatics. But, I’d still rather use a mechanical gripper.

The proposed mechanical gripper uses the rose that holds a CD in a CD jewel case on the end of a post. This works OK, but it is hard to align on the CD. Worse, it requires that the CDs be placed in a stack without a post through the center hole of the CDs. This makes it very hard to stack more than 50 CDs and still have then aligned to a reference point well enough for the robot to insert the gripper in the center hole.

By Fall of 2003 it was clear that the pneumatic gripper was not right for this project. It would work, but is inelegant. The pump is noisy, and the action is imprecise. I needed a purely mechanical gripper.

The joint result of a better design and my inept machine work is a motor-driven pair of arms that rotate towards and away from each other. The drive involves three gears, to keep the arms equidistant from the center of the gripper, and a belt that can slip to allow the motor to over-drive. To open or close the gripper, the controller just turns the motor in one direction or the other for a few seconds. The gripper works wonderfully.

In the original CD Robot, the stepper motor controllers were driven directly from the serial parallel port of the same computer that was attached to the CDROM drive. The new electronics puts a set of buffers and inverters between the Gecko Drive controller and the parallel port.These are not required for isolation, because the Gecko Drive stepper controllers also has opto inputs, but on standard PC parallel ports some of the outputs have different current draw characteristics ( some are open collector, some are not ) and the opto couplers allow me to have consistent operation across all of the outputs. My electronics also includes an H-bridge to control the gripper motor and some opto-isoloators to seperate the limit switches from the parallel port.

Stepper motors run with higher power when you feed them a voltage that is much higher than the name plate value, as long as the stepper motor controller limits the current. The Geck Drive can handle 80 volts, but I wanted to use a standard ATX power supply. So, I used a set of DC-DC converters to step the voltage up to 36 volts.

Miraculously, the robot and all of the electronics fit inside of the wooden case that I built. In the photo at right you can see the ATX powersupply. I originally mounted it outside the case, but with a bit a stomping and sawing I got it into the case.

And that completes this robot. The control software is a combination of python and bash, and it worked well enough to rip another few hundred CDs. Now that I have the bulk of my collection ripped, I can more easily convert new CDs with iTunes, so the robot is now a reminder of an enjoyable, but very long, project.