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Little Cat 4 X 4DOF Quadruped

#1

[size=150]Little Cat 4 X 4DOF Quadruped Robot Project[/size]

This is the new home of my quadruped robot project.

http://www.marconettengineering.com/Walk16b.jpg
Chassis

The Quadruped chassis can handle 2DOF, 3DOF or 4DOF legs, either in a sprawled (insect) or Mammalian (cat or dog) layout.
due to the manner in which the four yaw servos are mounted, only a single chassis plate is required to construct the 'bot. The more familiar use of 3DOF legs will still require two chassis plates.

Although for a 4 x 4DOF 'bot, using tubes and flanges on the corner servo horn mounts makes for an interesting and very attractive spacer for adding an second deck (chassis plate).

A standard cutout for mounting a BB2/SSC-32 stack is incorporated into the design, as well as cutouts for four (4) yaw servos.

Lithium Polymer or LiMH batteries up to 5.7" x 2.75" can be accommodated on the chassis.

http://www.marconettengineering.com/4DOF_Leg_B.jpg

4DOF LEG

The 4DOF leg allows rotation in the roll axis in addition to pitch (2 joints) and yaw.

Alan KM6VV

#2

Nice looking chassis. Will you use IK for the control of the legs?

#3

Oh Yes! Actually a combination of IK and FK to get the job done.

Alan KM6VV

#4

Hi Alan,

Looking forward to see more of your sophisticated cat project!
Any progress?

#5

HI KÃ¥re,

I’ve got the legs and boards on a new metal chassis, and I’m working out the “neutral” position for the legs, and getting them set up properly.

Seems like the legs should have a little bend in them when at rest, what do you think? I saw your “Fexix the dog” walking in a video yesterday. Impressive! Looks like you’ve got a bit of a roll in the gait, to help with the COG maybe?

Hopefully this weekend I’ll be able to “power up” and test some idle positions on my little cat this weekend. Not the proper battery pack yet, I’m waiting for a check (actually Paypal) to be able to order some LiPo batteries and appropriate charger. Thanks for the battery info!

I should grab a few pix too while I’m at it to post.

Alan KM6VV

#6

I’ve found some useful tools for quadruped gait on a web site of a friend I’ve been emailing for quite some time.

Peter has a program to allow you to study some gaits:

peterbalch.btinternet.co.uk/quad.htm

http://www.peterbalch.btinternet.co.uk/quad.gif

Download:

peterbalch.btinternet.co.uk/quad.zip

#7

Do you have any pictures of Little Cat?

#8

Hi Adam,

Here’s a picture of Little Cat on my office chair.

http://www.marconettengineering.com/LittleCat.jpg
Little Cat on chair

I really should get some more pictures taken. I’m about to make a LiPo battery/BEC tray for it, perhaps then.

Alan KM6VV

#9

Hi Alan,

How’s it coming? Had been a long time since your last post. Are you still working on the quad?

#10

It’s been slow. I’ve got some mods into the 1.3 Phoenix code.

Finally got the LiPo batteries, and a charger for them. Two months wait for the batteries on back order. Also got a pair of BEC’s.

A Lexan chassis is seen in this old pix, I currently have a nice .080" aluminum chassis with the yaw (geared) servo horns running in pockets to get the proper gear alignment.

I’m also trying to get a Micromouse up and running for our local HBRobotics club meetings.

Alan KM6VV

#11

Hi Alan,

Very interesting to see a picture!
But may I ask why one of the hip servos (front left?) are mounted the oposite way (the C-bracket pointed downwards)?

#12

hi alan great to see some pictures. looking really good.
good to see progress.

it also seems the rotation of the servo should be facing the other way. servo horn inwards?

we will stop picking now… :wink:

great work keep it up.
J

#13

Hi Kåre,

I wondered if anyone would notice! I noticed it after I hoisted and posted the pix. At the time, I was experimenting with a different leg arrangement, one that would give a little more rotational travel to the new “roll” joint. It worked, but made adjustment of the force on the LP bearing (backlash) impossible to adjust.

I am still not satisfied with the adjustment of the bearing load. I’ve tried to find the Bellville washers that I’ve used elsewhere, but nothing of the appropriate size. Still looking!

Alan KM6VV

#14

Hi Jonny,

Aren’t the servo horns all facing inwards? l guess I’m not following you.

Oh, the “roll” servo! Yes, they should be as well; it would give me more clearance. I have new '654 servos to replace the hip and knee '475 servos, so I’ll make the changes. Thanks for that catch!

On the LP (thrust) bearings, I think I may need a bigger diameter. Any chance of a new part Jim?

#15

Just taking a first look at your project, I predict you will have issues with the gearing at the top of the leg. If there is any slop in the top bearing then the small thin gears with fine teeth will slip and not hold under load. I suggest you go with gearing with larger thicker teeth that will be more tolerant of alignment holding issues with the top bearing setup.

#16

I’m not seeing any slippage as of yet. These are the parts of the LP bearing, and my mounting the gears and hubs on my much larger .080" chassis instead of LM’s .060" aluminum plate should make it more stable. I have some play, that the Belleville washers would theoretically help take out.

smallparts.com/Belleville-Spring-Washer-0150-Thick/dp/B001DCUHA2?ie=UTF8&qid=1260042673&sr=1-2&pf_rd_r=0CDBP953KQESF1KA76NQ&pf_rd_m=AIUBT5HP6PMAF&pf_rd_t=301&pf_rd_i=0&pf_rd_p=467590031&pf_rd_s=center-3

But I take your point. I would like a larger diameter bearing as I’ve mentioned. Maybe a larger one would be good addition to the 1/4 scale SES parts!

But before doing the engineering for new LP’s, I want to see this little kitty walk!

I have been looking at some gears from Servo City:

servocity.com/html/48_pitch_hitec_servo_gears.html

And perhaps I can machine up a set of larger bearings as well.

Thanks for your observations.

Alan KM6VV

#17

Probably would be good to power up the gear drive servo and do some load testing to see what happens. A chain drive might be a possibility, but has its own issues. Since the rotation range is limited, a simple dual rod servo arm push/pull setup might be workable.

#18

Pushrods probably can’t make the 180 degree swing.

That does give me an idea. With a little gearing, like for the tilt/pan from Servo City, I could simply mount a new precision pot driven with a 2:1 gear from the servo. That would work!

Alan KM6VV

#19

http://www.marconettengineering.com/LittleCat.jpg
Little Cat on chair

Alan is this the Neutral stance “home pos” or just standing for the camera?
you will need to have the legs bent to make it more stable. you could get it walking without changing the legs.
as long as you keep (##PosX at 0) the code will know the legs are underneath and inline to the body.

e.g:
; Initial Start positions of the feet.
;Right Front leg

[code]RFPosX = 0
RFPosY = 55
RFPosZ = 40

; Right Rear leg
RRPosX = 0
RRPosY = 55
RRPosZ = 40

; Left Front leg
LFPosX = 0
LFPosY = 55
LFPosZ = 40

; Left Rear leg
LRPosX = 0
LRPosY = 55
LRPosZ = 40 [/code]

in order to use the HipRotate Servos you would need to do some changes:
In ServoDriver it looks like this:

  serout SSC_OUT,SSC_BAUD,"#",dec LFHipRotatePin,"P",dec TOINT(TOFLOAT(HipRotateAngle)/0.10588238)+650]

**
it should be:**

  serout SSC_OUT,SSC_BAUD,"#",dec LFHipRotatePin,"P",dec TOINT(TOFLOAT(IKHipRotateAngle+90)/0.10588238)+650]

:wink:

#20

Hi Jonny,

It’s just standing. I was planning on a bend in the legs. Although the “triangles” for the IK are hard to work out. I pulled out a few angled servo brackets. Not sure if they help my legs.

You’re saying a 90deg rotation would help the hip servos? Not exactly following. It’s probably been too long since I worked on my IK last to remember.

I appreciate your help!

Alan KM6VV