Older blog entries for kerwin (starting at number 3)

I have made some progress on my homemade robot arm. See http://www.ranchbots.com/robot_arm.html for some photos and movie clips of the motion that I have acheived. The site lists all of the updates and design changes, but I'll summarize them here.

1) I decided to stick with unmodified servo motors on 3 of the joints. I was concerned before that they would move to quickly if I just sent them a position signal. And that was a valid concern as there was danger of shaking the thing to pieces at the normal speed of those motors. So I "fooled" it by using an incremental signal approach. The short version is that the SW watches the state of the control switches and as long as they are active for a given motor in a given direction then the SW adds or subtracts a small amount of time from the pulse that is sent to the servo. This works surprisingly well and the code turns out to be just a few nested if statements. Source code is available on the site.

2) I added springs to the "elbow" joint to help the elbow joint motor deal with the load and to help it maintain a static position. Took some fooling with the mounting to find a good stable position, but I got it. Works great.

3) I haven't figured out a controller yet. I'm considering several joysticks. The simplest version is just 2 switches for each joint. One is the enable switch, the other is direction.

4) Since I'm using servo and DC motors I have to power them all continuously or the arm will go limp and possibly get damaged. This turns out to be more current draw than I planned. The servos draw 200 mA continuous current with load, and peak at about 400 mA, each. I have 3 of those. The DC motors have slightly more continuous, but their peak current levels hit near an amp. My solution for this is to use SW to "park" the arm when it is not being used. Uh, oh. Let me back up a bit.

5) The arm is going to be mounted on a mobile platform, so I'm designing the whole thing to be battery powered. If I run the arm all the time, I'll get about 15 minutes at best out of the batteries I'll be using (Radio Shack NiMH rechargeable packs). So the idea is that when I need the arm to collect samples or whatever I bring it out of the parked position by turning it on, do my business then when I'm done I hit the "park" switch, and SW will index all the motors then return the arm to a resting and safe position THEN power down all the motors.

This is a really fun project. Lots of lessons. The main lesson I have learned is that for this application you really want to use a wormgear motor if possible. Wormgear motors hold their last position when you remove power. Much better for battery operation.

Another lesson is one I already knew but took a chance on. The lower arm and hand assembly is the heaviest part of the arm almost and it is at the end of the moment arm. The lower arm and hand then should be shorter. But it works now as long as I don't try to pick up a brick (which would tip over the whole vehicle anyway).

Feel free to give the site a look and send comments.


Denver Area Robotics Club: We had our first meeting on Sat Feb 23. It went well with 13 in attendance. We agreed to meet on last Sat of each month, for now at the Highlands Ranch (Colorado, USA) Library at 2PM. We have a line following robot contest scheduled for the next meeting. I've heard from 2 new people since that meeting that are interested, so we'll keep seeing growth I hope.

As a club we are buying a basic bot board from a guy that has developed a circuit board using the Atmel micro (90S2313). I've never used Atmel, preferring Motorola, but Atmel has several free compilers and is a little less intimidating. Plus, this guy has a layout ready to go that is tested, so that's what we're doing. I have a goal of standardizing technology in the club to minimize duplication of effort, particularly software.

Other items: I got a Gameboy camera that I look forward to hacking into for robot vision. I worked out some mechanical problems on my robot arm that I've been struggling with for over a week now. that's a relief. Two of my bot boards (Motorola 68HC08GP32 based) are dead right now and boy am I ticked about that one. That is crippling me right now for development. I got the new improved version of my "tankcam" TV remote controlled vehicle going, though it's dead until I get its control board going. Blah, blah, blah. Lots of fun still, even with all the problems.

And some big news, my web site can be found on Google now. I submitted it 3 times. Not sure if 6 weeks was standard to get that in their crawler or if I just had to get more hits, or what. But it comes up now. Kind of neat.

2/18/2002: The first meeting of the Robotics Club of Denver / Highlands Ranch (so far everyone interested is from Highlands Ranch) is set for Sat, Feb 23, 2PM at the Highlands Ranch Library. It is just behind the Safeway at corner of Broadway and Highlands Ranch Parkway. All are welcome. About 10 interested folk so far, with 5 having some robotics experience. I hope it will grow quite a bit.

Meanwhile, I've done some more work on my robot arm. Photos and other info at www.ranchbots.com/robot_arm.html. The motor for the shoulder is powerful but I'm using a rubber belt that I made myself to translate energy from that motor to the shoulder joint. The belt slips quite a bit. So I'm working on rebuilding the pulleys with a strip of rubber fixed to them with glue and then building up the edge to prevent slippage. If that doesn't work out, I may punt and go to a hi torque servo motor on that joint.

I also added some potentiometers to the pivot joints so that I can get some feedback on position. I'm still plinking away on mechanicals. I haven't written any software for this yet. I also have to do a layout for the interface circuit board. I have a buddy working out details on the power supply portion. I don't know DC-DC converters that well, so I asked him to do some consulting for me. I should have that layout done by end of Feb. But then again, I should be rich too.

I revisited my very first project, a tank type steering toy that i fitted with a UHF remote control system that I removed from a satellite TV receiver. I added to it an X10 camera so that i can watch what it is up to. I also gave it a "weapon", a laser pointer that is mounted on the camera base so that it points where the camera does. This will be one of 2 or more vehicles armed with the laser blasters. 2 users can then pilot them around using the TV images being sent back and "hunting" the other. Each vehicle also has a "target" mounted on it that will register a hit from the opponent's laser. Laser tag, on vehicles, by remote control, over a TV. I have some photos and details on that on my site also. www.ranchbots.com/tankbot.html.

2/4/2002: email: kerwin@ranchbots.com - This entry: my homemade robot arm and still searching for those interested in a Denver Robotics Club.

I have looked high and low for a robot arm to hack into. ebay has several listings but the price always hits 60 - 150 bucks depending on what the thing is. I haven't seen anything surplus, so finally I decided to just build one. So I did. Took me about 3 weekends. I have finished the mechanical construction phase. Now I'm working on the electrical control system.

I put up some photos of it on my site. www.ranchbots.com. I also put up some technical details on the control system as I have designed it thus far. It will use one microcontroller (68HC908) to control the arm alone as it will have multiple feedback inputs for position sensing as well as inputs from a master control micro or switch system to drive it.

The construction was challenging. I resolved to use what I had on hand to get it going since I didn't want to spend a bunch on a hackers special in the first place. I met that goal pretty well, using parts that i have removed from copiers and printers and other items in my junk box. I did cheat and buy a micro servo for the gripper motor as this was at the end of the moment arm and thus would create the most torque. It came out pretty well. I have tested each joint piecemeal and all of them work okay.

I still have to add pots to the joints at their rotation points, limit switches, connectors, a wiring harness that will keep the wires from turning into a bird's nest, etc. Much work to do still but I'm happy with it. I'll put up video when it's running.

If you have comments or design ideas, shoot them at me.

As far as the Denver Robotics Club goes, I'm still looking for interested folks. I have a little club going down in Highlands Ranch (5 people and growing slowly), but I would like to really get a big organization going. If you live in Denver area and like robotics, drop me an email at kerwin@ranchbots.com.

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