I’m not exactly sure what I had in mind when I built this thing a few years ago, but I’m sure it was extremely important at the time. I know that I built it from balsa because I had nothing else laying around(That turned out to be a rather expensive decision). It’s not finished, and probably won’t ever be- I don’t think it does what it was supposed to do, but the drive base and actuated arm work, so I might as well document it.
From an engineering standpoint, this thing is pretty stupid. I built it mainly for the chassis-I was experimenting with servos, and I needed an application. I modified some standard 180 degree rotation servos to rotate continuously. Here’s how I did it
Servos use potentiometers to locate their stopping points at either end. When the output gear rotates, the potentiometer turns with it. To allow the servo to rotate continuously, the potentiometer needs to stay in a neutral position- I removed it and put back the gears into the case. I powered the servo on a receiver using the aileron channel and allowed it to rotate without any input. then I just used a pair of pliers to gently turn the shaft to the neutral position. It took quite a bit of fiddling to get it just right- if you here the motor buzz even a little, the potentiometer needs adjustment. When it stopped completely, I super-glued the potentiometer into place. The output gear has a little plastic disc that has the semicircle shape as the shaft of the potentiometer. I removed that too, so that the output did not engage the potentiometer at all.
All that’s left to do is to cut off the little plastic nub that stops the output gear from rotating past the compound gear underneath it, and the servo will rotate continuously.
I had some large plastic gears that I took out of a toy car a long time ago, so I put them to use. I initially intended to gear the servos to rotate the wheels, but I thought reverse-gearing the servos to turn wheels quickly was a stupid idea- the servos are slow, but have enough torque to rotate big wheels, so I just geared them in a belt drive 1 to 1. I probably could have just attached the wheels to the servo output head, but I believe I was inspired by my FIRST team (it was my first year) to do a belt drive. Another dumb decision.
I didn’t have a belt of any kind, so I just used some rubber bands that I had laying around-this was probably was the worst decision of the entire build. Belts stretch for tension, but rubber bands stretch for stretchiness- applying tension doesn’t do anything, so they kept slipping. At very slow speeds, however, they work just fine.
The arm servo is also continuous rotation (I have no idea why I did that) but unlike the chassis servos, it is geared down, which gives the arm unnecessarily high torque. Here’s where I made another huge mistake; I made the servo continuous rotation with my radio’s trim tab all the way to the left. In other words, the arm would only stay put when it was held left as much as the transmitter would let it. I never built a clamping hand for this project, so it didn’t matter too much, but it meant that the arm couldn’t be adjusted at all.
The arm needed to fold to fit within the small frame. I considered a slew of methods, including adding another servo, but I decided against it because I didn’t want to complicate the middle joint and lengthen the amount of time required to extend it. Instead, I just hinged it to fold one way, so that it could fold into the frame sides but still hold load from the top; it works the same way your elbow does. If I had built a hand for it, I might have been able to test how much load it could take. For now, I’ll say it works in theory.
Although I never finished this project, it was a fun one to build. In the future, I might super-size it and properly design and build a robot for a specific purpose. Probably the most important part of the project was that it did give me a ton of experience with digital servos-maybe one day I’ll build a camera pan/tilt mechanism for aerial video.
DIY Robotics 