Mech-Spider Fail (ARRRGH!)
@ September 26, 2011
- Actuators / output devices: Polulu 37mm 1:131 gear motors
- Control method: radio control
- CPU: Dual TI MSP430
- Power source: 7.2V 1500mAh RC battery
- Programming language: C
- Target environment: indoors/outdoors/small areas
I got interested in the Klann linkage when I stumbled across MechanicalSpider.com, so I decided to play with it to see what would come of it. After designing the first leg mechanism in AutoDesk Inventor 9, I thought it would be fun to build a robot spider to hand out treats on Halloween. This is the process I used to develop this robot. I wanted to use materials I had on hand and make most of the parts myself. I started with the leg-driving mechanism shown below.
I cut the gears from 1/4-inch polypropylene sheet. I got the gear patterns from the CAD library at www.hobbing.com. After I ran the exe file from the library, I ended up with 573 gear drawings of gears with 10 to 200 teeth. The drawings were DWG files, which is a format for AutoCAD. I used a free CAD package called DraftSight (a nice 2D CAD application from www.3ds.com) to scale the gears to a size I could work with, imported the DWG drawings into Inventor, printed them out as a pattern, tacked them to the plastic sheet, and cut them out with a band saw outfitted with a metal-cutting blade.
I downloaded the enable text for the Klann linkage from www.mechanicalspider.com. The text outlines the design method to get the linkage sizes and pivot points. I used DraftSight to draw the lines for the design. I tried a 6-in step at first, but the robot would have been too big, so I went down to a 4-inch step and everything was reasonably sized. The design drawing is shown below.
The next step was to import this drawing into Inventor, isolate the pivot points, draw the linkages around the pivot points, and produce the patterns. I tacked the patterns to some 1/4-inch black polyethylene I had stockpiled and cut the linkages out on the band saw as well. I managed to put most of the first set of legs together this weekend (9/24-25/2011). So far, everything is working well. I just have to build one more set of legs and figure out how to bind them together. I'm thinking PVC so far...
Update 10/23/2011: The picture at top shows my progress so far. I should be able to get motors installed this week (hopefully). I tried to hand-hold the motor and first set of legs for testing, but all hell broke loose because the motors have too much torque. That was quite interesting. Don't do that! I had to scrap the arm for now because of time constraints. I'll make that my project for next year. I still have to make it look all spidery, so I've got some artsy work to do.
Update 11/4/2011: Well, I finished but lo, it doesn't work. Maybe the motors are too small. Maybe there is just something jamming the mechanism. I don't know. Right now I'm too frustrated to mess with it. I'll have to strip it down and start putting things back on slowly to see what the problem is.
@ Sun, 2012-01-15 05:56
This robot was 1 of 3 that
This robot was 1 of 3 that inspired me to make a walker. I too am going to use a Jansen style linkage. Though I've got different gearing on mine. I think the problem your having is you need stronger motors.
Thanks for the inspiration!
@ Thu, 2011-11-10 21:07
Dumb question perhaps?
Do you use any form of a hbridge or something? Or is it onboard on the TI board you're using? because I cant see anything like that on your video..
@ Wed, 2011-11-16 19:30
H Bridges
The controller I'm using has four integrated H-bridges that run the motors. The circuitry is under the plastic shield. It was originally designed by the robot competition group (BEST Robotics) that the team I coach belongs to. I use it when I want a quickie robot because it uses radio control and I don't have to spend a lot of time writing control code. BEST has since abandoned this controller and moved over the VEX Cortex controller.
@ Fri, 2011-11-04 11:10
Oh bummer
It is deffinetly a gearing ratio problem against the small motor.
I would suggests three solutions there:
- it feels like there is PWM going on the motors (I can make out some humming when they stall), so first thing I would try would be to disable it an see how it acts on full power
- while the gearing on the motors is 1:131 which is a lot, you loose probably half of it in the leg gearing due to the 4:1 (or so I'm just guessing) multiplication in the gear train. So I would go for gears the same size if possible instead of the big one and two smaller. Even better would be to have another reduction stage by having a small gear driving bigger ones (1:2 would be more than enough), movement speed might become an issue though.
- you could also get some beefier motors but I think they are not the real issue here
Just my 2c.
@ Fri, 2011-11-04 07:38
Does it move when you lift it off the floor?
If it does I would expect there is not enough torque from your motors. The first sprocket is quite large. You could possibly gear it down so the motor turns a small gear on the large sprocket. It would be slower but it would move I think..
Good luck.
@ Fri, 2011-11-04 05:07
Nice! It's good that you
Nice! It's good that you post the failure video. That's how we learned! Thanks for posting.
Did you try to lift it in the air? Does it work without weight? If its moving fine maybe you need bigger motor or try to down gear it for better torque.
@ Wed, 2011-11-09 00:56
Lift test
Yes, I tried that and it helped, but I think that the gear ratio is just too big for the motor.
@ Wed, 2011-10-26 17:01
Amazing!
Are you planning on posting some video?
Great job!
@ Thu, 2011-10-27 15:56
Thanks!
Yes, I plan on adding some video after I get in moving. I'm waiting for some mounting screws for the motors. I can't get the right size locally.
@ Mon, 2011-10-24 19:49
how are the legs mounted to the plywood chasssis?
Hi great job! Can you show us how are the legs mounted to the plywood chasssis?