Tuesday, September 21, 2004

Laser Cut Parts Arrive

Outline files

The activities this week are to test the variables saving features of the ServoMojo board. This includes trim or centering, home position, acceleraion and speed, etc. Also, I'm putting togeather a spreadsheet to store servo locations for all the general motions required. This includes stand, sit, turn R / L, walk forward / back / arc, and transitions between all of the above.

All of these should be easily settable using either narrow or wide mode. Have you settled on which mode you will be using, or perhaps are you using a combination of the two modes?
One way to move to the "home" position is to generate a reset, which will cause the saved positions to be reloaded. Note, though, that the servos will snap back to their home positions as fast as they can at that time. This is due to not being able to know the actual servo positions at reset time. I could easily implement a "Home" command, though, that moves all the servos to their saved positions, without having to perform a reset, which also implies about a 3 second delay as the stored configuration is loaded from the on-board EEPROM. That command would allow the servos to be positioned and also honor the current velocity and acceleration settings, and eliminate the startup delay following a reset. The firmware you have does not support that - but it should be easy to add. I can send you a new chip that you can replace on the board to have that feature. The processor is in a socket to the replacement should be straight-forward.

There's a sample of how the Japanese guys do it here...

Unfortunately, I cannot find a description of the RCB-1's communication protocol in either the software nor the hardware documentation. Without that, I cannot say how easy or difficult it
would be to match it. However, their PC side software is similar to what I envision for ServoMojo. But I don't expect to have that piece in place for some time. I do think it is an important component, and one that I plan to have in place before I present the ServoMojo board for general availability. One hold-up for me has been the lack of a PC virtual machine that runs on my G5 Mac in order to do the development - the current version of Virtual PC supports up to the G4 processor, but not the G5. Just starting recently, however, Microsoft has begun taking pre-orders for the latest version of Virtual PC which finally supports the G5, so at least there is light at the end of the tunnel - but they still don't say exactly when it will be available.

I like the name of the software, "Heart to Heart". I don't know the data output format, but I suspect it is'nt compatible with the ServoMojo. Do you know if this is the case Brian? Is conversion worthwhile?

I ordered parts from Pololu in polycarbonate as a test of the material and the service. The Pololu laser cutting service is very inexpensive and their operator Jan Malasek is very helpfull and conscientious. http://www.pololu.com/laser_cutting.html

Not so much can be said about the aluminum watter cutting service DC waterjet. These guys didn't respond to emails, and after bugging them a few times they finally came back with estimates. http://www.dcwaterjet.com/index.shtml

Pololu can't cut metal nor mat'l thicker than 3/16". So I only sent them the parts that were 2D and 1/8" thick. Their charge for these parts was $28, and that includes the polycarbonate material, checking the file for errors and getting back to me on fixing them...I really like using them. I've not yet ordered the parts for the 1/4" material. Stuart is considering making them locally so we'll have one all aluminum bot and another plastic/aluminum hybrid.
The same parts cut in aluminum from DC water jet is $90. The 1/4" parts are $114. (not including material) Emachineshop.com was $118. All the 1/4" parts require extra machining after cutting out the outline.

The polycarbonate is considered bullet proof by the battlebot guys, but not suprisingly, when you cut it this thin, it's flexible compared to metal. Dale suggested that this could be an advantage, absorb shock, etc. Re-Ordering in 3/16" mat'l is an option. Stuart's aluminum version will kick this one's butt.


The following is the central body made up of the back plate, battery retainer, balance servo, hip/turn servo, shoulder bearings w/mounting flanges, the neck standoffs, and the batteries



The batteries and motors fit nice and compact. Dale made the 3 bearing mounts in aluminum, two shoulder and one for the neck. That's the two shinny parts in the top view. Thanks Dale.

Stuart has redesigned the connector for the pinion gear on the foot, basically turning the gear around, mounting the tooth end next to the servo. This allows room in the material to put a std servo horn spline into the center of the pinion, and the pillow block on the shaft end. The image shows a recess cut into the foot to accomodate this change in the location of the servo.

Keith Rowell Design

Wednesday, September 08, 2004

Center of Gravity

Should it be higher for smooth walking motion? Or lower for stability?

A lower CG will tend to be more stable, but a higher CG will, in my opinion, give a nicer looking walk (less waddle). One of the projects I've been working on is a very small muscle wire powered biped. I was originally thinking of making a smaller version of MiniMechadon with a pair of AAA batteries mounted horizontally. I have since decided to mount the AAA's vertically and make it a humanoid (about 3" tall) to improve the walking characteristics.

That being said, if you're designing Knewt to compete in RoboOne-type competitions it would probably be better to have a lower CG to gain better inherent stability at the sacrifice of walking "style" and efficiency.



Friday, September 03, 2004

Laser and WaterJet Parts Cutting

Laser and Water Jet cutting estimates

I solicited cutting estimates from 3 sources.


Pololu can't cut all the parts. Their maximum thickness is 3/16" and they can't cut aluminum. Their cost for doing the 1/8" parts in polycarbonate was $27. I ordered those, they were more flexible than I expected. Not the ideal stiffness. Dale mentioned that this could be an advantage...they're certainly light.

DCwaterjet took several days to get their estimate to me after a couple of reminders, their cost was very close to eMachineshop at around $200 - $300 using waterjet on aluminum.

Thursday, September 02, 2004

Project updates
The new project name is "Knewt" (knowledge in a small package) This week we explored a problem with the gears intended for the foot motors, and decided they were'nt a problem after all.
There is a 5%-7% descrepancy in the tooth pitch of the bastard gears from the surplus bins. But further testing and measurement revealed that a little wear on the aluminum gear, and the errant 7% is worn away. I'm worried about backlash, but as the foot only sees resistance in one direction, there isn't much reason to worry about it. A spring may be added at some point if play becomes a nieucance.

My method for measuring the fine dimensions on these gears was to scan them at a high resolution directly on my scanner, then import the image into CAD, in this case intellicad. (oddly enough, the photoshop PSD format was the only one that made the image visible in the workspace, jpg's and bmp's came in as icons...something I didn't expect) I then used 3 point circles to outline the id and od tooth dimensions, splitting the difference between the od's of the two gears to get the pitch. I then measured the pitch of one tooth on each gear along the pitch line, and found a 7% discrepancy. Even though these are faily high resolution images, there must be some error which I estimate to be less than 3%. So to change the diameter of the pitch circle and remove the discrepancy, I mounted the steel pinion on a drill and ran it against the aluminum ring gear for a few minutes and saw marked improvement in the smoothness of their mesh. I'm hopefull that with a proper jig and some lapping compound, the gears will mesh very smoothly....the clock builders who make alot of gears said that backing off the clearance between the gears will work.
Alternately, we'll go back to linkages...


The Finer Points of Surplus Parts

7% difference in the pitch of the surplus gears I've scrounged up for the build

Keith Rowell Design

We can always fix this gear problem with other answers if they fail to please.
I've deduced from the high res scans that there's a 7% difference in the pitch. I wondered if just "running them in" with a drill motor and a jig to apply preassure, or such, would "wear" them down to fit enough to make up that meager 7%. Ultimately either the gaps in the pinion teeth are too narrow or the teeth on the ring gear are too wide, or both. I believe that 7 percent will be an easy modification to reach, because even the slightest wear moves the pitch line alot. Maybe running them on a jig with lapping compound in their teeth would match them up...(I must sound like a real hill billy!)

what do you think?

Is this a regular discussion in hill billy circles?
Don’t worry about the gears I think we can use them as they are.
See you later


The percentage of difference you're test shows suggests you have one metric and one fractional gear. Metric is measured in "Modules" and fractional is measured in "pitch".