OWI-007 Robotic Arm

Overview

The OWI 007 Robotic Arm Trainer is an educational kit that builds a 5 degree-of-freedom (DOF) arm with a wired controller. It requires no soldering, and can be assembled in a few hours using just a screwdriver and (perhaps) a pair of needlenose pliers. The arm features a translucent plastic shell that allows one to see the mechanisms inside, and a small light associated with each motor to draw attention to the one that is moving. Every motor is protected with a clutch mechanism, to prevent damage even when an attempt is made to drive a joint beyond its limits. This makes the arm reasonably safe for children to operate.

I found this kit available for $50 from Kits USA; a more typical price elsewhere is in the $75 range. Don't be fooled by their listing of it as a "Programmable" arm kit; it is not programmable at all. (An add-on controller card is available for Windows PCs, but that is a separate item.)

Packaging and Instructions

The kit comes packed very neatly, with all parts tied down to a cardboard insert that identifies each piece, using the same terminology as the instructions. This made it very easy to find the parts, and to keep the workspace neat, as I could just snip the tie on the next part needed, and leave everything else in place.

The manual starts with an introductory section about the history of robotics, which is interesting, though poorly written; most likely it's a rough translation from the Japanese. The actual assembly instructions, however, were top-notch. Every step was clearly explained and illustrated, and included warnings against common mistakes. The instructions also described how to test at various steps (e.g., after the installation of each motor), with suggestions on how to correct possible problems.

After the assembly instructions is some general discussion on the principles behind the robot arm. This material was also quite good.

Assembly

Assembly of the arm took a few hours, and went smoothly for the most part. There was just one hitch: connections of wires to the battery plates are done via solderless terminals, which are supposed to fit over tabs on the battery plates. The manual suggests gripping these with needlenose pliers and pushing them straight on. I found this to not work too well; the fit was far too tight. I bent several of the terminals in my attempts to get them on, and broke off one terminal completely. I had better luck pushing the terminals on with my bare fingers, though it was still difficult and several of them refused to go on all the way.

Apart from this step (near the end of the assembly process), all went well. If you're building this with anyone more experienced around, I recommend having them assist with this step, and be prepared to deal with problems.

A proper fix for the broken terminal would require either soldering, or replacing it with a new terminal (no spares are included in the kit). I chose to just jam it in with its neighbors (five different wires are connecting to the same battery plate at this point) and hope for the best. This has worked so far, but it wouldn't surprise me if one day one of my motors stops working because that wire has shaken loose.

Functionality

The assembled arm works pretty much as advertised. The controller offers only one speed in each direction: slow. But that does make it easier to control, and it's sufficiently fun to be controlling a robot arm that nobody in my family seems to mind the wait.

The only problems I've had with it are with the shoulder joint; this one is weak, and often can't lift even the arm itself if the elbow is fully extended. This is not due to insufficient torque in the motor, but rather due to the clutch which is designed to prevent damage. The clutch is adjustable via a tightening screw; if I tighten this all the way (far more than the manual seems to recommend), it's able to lift the arm for a while, but then later I find I need to tighten it again. I could never get it tight enough to lift a D-cell battery as the manual suggests.

So, among the many educational lessons learned from this kit:

Clutches are annoying fiddly things.

They're force-transmission parts that are basically designed to slip, right before enough force is applied to other parts (e.g. arm shell, gears, motor mount, etc.) to break them. The problem is when they slip too soon, you can't get a decent amount of force where you want it (e.g. lifting the arm). Getting the clutch to that "just right" point is difficult, and getting it to stay there is even harder. For my own robots, I'll try to avoid them and find other ways to protect against damage (e.g. using springs as linkages).

On the base, where the whole arm is rotated, we have a similar problem. The clutch mechanism here is different: the motor (attached to the arm) rotates a gear that's inside a circular toothed track. This track can also rotate relative to the base. The idea is that under normal circumstances, there should be more friction holding the track to the base than holding the arm to the track. This usually works, but again, sometimes it doesn't, and I see the track rotating inside the base instead of the arm rotating. Sometimes it's something in between, with the arm rotating in fits and starts rather than smoothly. See above comments about fiddly clutches.

Hacking Potential

Each of the five motors in the kit is integrated into a gearbox which reduces the rotations per minute (RPM) substantially. The gears move well and are reasonably quiet, and in my test, the motors draw about 170 mA at 3V under no load. It wouldn't be hard to repurpose these motors and gearboxes for something else, though changing the gear ratio would require some hacking.

The plastic pieces that comprise the arm shell are also well engineered; they fit together nicely, move smoothly, and look cool. There isn't a lot of room inside, but probably enough to add some sensors, or rip out the gearboxes and put in your own (perhaps with encoders for positional control). In the latter case, you might be able to rip out the clutches too, resulting in much more reliable movement.

Finally, the control box connects to the arm base via a simple ribbon cable and plug. It shouldn't be hard to find a similar cable, connected to a microcontroller or a motor controller. In fact, I'm planning to try exactly that, allowing me to control the arm from my computer. The cool thing is, I'll be able to just unplug my controller board and plug the OWI controller back in, whenever I want to control it manually again -- no changes to the internals of the arm will be required.

Conclusion

The OWI Robotic Arm kit is a nice piece of engineering. No, it's not the most reliable or dextrous arm you could find, and the control is crude (no positional feedback of any sort). But hey, it's fifty bucks. I think the designers have done a great job for the price, and it fulfilled its purpose of teaching me a thing or two about robotics. My boys (ages 1 and 5) also enjoy it quite a bit; my 5-year-old has had fun picking up and putting down plastic blocks, and now understands things like "motor" and "joint" and "clutch" (the thing I grumble at and take a screwdriver to every time the shoulder stops moving).

I recommend this kit for anyone who's new to robotics, or perhaps even experienced robot hackers looking for a good arm framework.