It’s been a while, and here’s my report.
Gripper is almost finished. Making a gripper around a rc servo seems easy at first, but there’s a pitfall: you should avoid stall (that is, signaling the servo to go to a position where it can’t go). Using a servo with metal gears can provide some protection, but stalling should be avoided anyway. Possible ways to manage this:
- Power the servo for just a short time, long enough for the servo to close or open (something like 500ms). The servo will stall, but for just a small amount of time.
- Knowing ahead where you should drive you servo depending on the size of the object you wish to grip. Doing this can be a bit tricky, so it could be wise to combine this and previous behavior.
- Add a pressure sensor. This will increase complexity, but having feedback could allow to adjust servo signal to set a constant pressure and avoid stall, without shutting down the servo.
- Add some kind of spring-tension to fingers. Even more complex, but not a bad idea at all.
Marlin scara support seems to be broken on current versions. The “newest” version I was able to make it work was 1.0.2. After doing some small modifications and adding some simple gcodes, the whole thing seems to work. I could make a post explaning how to setup Marlin for a scara, if there’s someone interested.
Now, I should say that the first firmware I tried was Wangsamas (Repeteier based), but I left it because of some erratic behavior on some tests. However, now I realize it was just a setup parameter, and indeed movement using Wangsamas firmware was more soft and fast. So, I guess I will try Wangsamas again.
I’ve used simple mechanical lever switches for homing, and they seem to work well on Marlin, but on Wangsamas sometimes I got false readings and hence bad homing. I really don’t care too much about this now; I will come back to Wangsamas a fix this later.
Aesthetics enhancements, belt tension adjust, rewiring and a better wood finish.
- I need to add some kinds of bumpers for angular motion links.
- Gripper need rubber fingertips. I guess I will go without tactile sensors for now.
- Boards need a support. I will make this out of wood, probably.
- Pen adapter: This will allow to check path travel. And have fun, of course.
- Software: I need to upload my Marlin modifications somewhere, forgot about Marlin for a while, and go with Wangsamas. I pretty sure it will work very well.
- 4th axis? Maybe.
I had put a lot of work on this first prototype (and his brothers). But, to be honest, it’s not clear where this project is actually going. Final price of those units will be probably higher than planned; probably over 3k. It’s not clear if they will sell, but bear in mind I need to sell just a few units, not thousands. Maybe I sold them as hypermodern art. I really wish to be able to fund a new, faster, larger and cheaper production run… who knows.
And now, a video.