PDA

View Full Version : [Question(s)] How to build a driving, steering, wheel/motor mount?



jwatte
12-02-2012, 03:35 PM
My current rover is reaching its end, because it won't fit suitable encoders to know how fast I'm going and whether I'm slipping (plus the plastic truck chassis is starting to turn to mush.)
I want to build a new chassis, with four independently sprung, steered, and powered wheels.
I have wheels and motors, and I have a bunch of aluminum, bearings, springs, bolts, steel shaft stock, etc.
I don't, however, have a design I like. I started with this, but I'm growing to dislike it, because it looks wrong somehow, and it also has no good way of keeping the shaft from falling down through the plate when picking up the robot.

4333

4334

I may need something totally different to get to where I need to go. But I already bought the motors with encoders, the wheels, and some other parts.
Ideas? Anything would help -- I'm a noob at mechanics, and have been learning CNC milling over the last year.

tician
12-02-2012, 04:16 PM
I'm not quite sure how your system is intended to work. It looks like the entire assembly rotates around a hole in the chassis to steer, but I am not really clear on how the suspension assembly functions. How do the two arms connected by springs allow the motor to move up and down independently of the chassis (preferably without tilting and riding on the side of the tire)?


One of the numerous designs floating around in my head is a bit similar to yours, but the suspension assembly is instead two vertical tubes assembled one in the other with a spring along the outside between flanges on the ends of the two tubes. The entire vertical tube assembly rotates to steer (need either a non-round tube or some sort of spline or key way to prevent independent rotation of the two tubes) with the motor rigidly attached to the flange on the bottom tube. There would also need to be an internal stop of some sort to keep the two tubes from flying apart when the wheel is not on the ground or under loading.

A more conventional steering and suspension system would have the motor rotating between two rigidly connected plates stacked on top of each other with each plate linked to the chassis by an A-frame (connected by bearings and bolts/pins). There would be either a rod linking an arm to turn the motor to a servos mounted on the chassis or a servo directly mounted on the top plate to turn the motor.

jwatte
12-02-2012, 10:54 PM
Thanks!

After googling, the two a-frame solution is apparently called "double wishbone" suspension, and the single axle (vertical) with aspring is a "mcphearson strut." Or at least the moral poregenitor of the same.
Time to doodle some more.

jwatte
12-03-2012, 01:12 AM
Here's a rough sketch of the direction I'm thinking. From the eye mounts, struts (?) will connect up to the chassis at an angle. Above the thrust bearing and hole, there may be a second thrust bearing, and then there will be a servo mounted with the horn facing down to turn the motor directly. Heck, if I had enough precision, I could mill the servo spline (23T for me) into the axle itself. There ought to be very little strain on this, because the force is taken by the thrust bearing.
Hmm... or maybe I'll use a servo with an actual mechanically friendly spline, like the AX12 or Herculex 0101...
Also: I don't like the screws through the "face plate" mounting the servo mounting plate, as all force from the wheel will be communicated through those two screws. I'll come up with something better -- perhaps a curved cutout piece that contains both the "eye" and the plate somehow.

4335

Note that I don't really like ball joints, as I don't think I can create those myself very easily. Plus, if there's only two arms, the motor and the arms would interfere when the suspension moves.

Also, I may have to create my own motor mounting bracket, rather than that 16-gauge aluminum I got from Pololu with the motors. That thing doesn't seem very rigid under load...

CasperH
01-25-2013, 09:38 AM
Hi,

I would mostly look at existing (scaled) car model designs. Use either rear driven wheels (both) and one servo for steering the front wheels (connected properly for Ackerman). Which part are you stuck with? Proper mounting brackets? Additional pictures might help. :-)

On another note, if your new rover is similar to the Mars rover idea you might like the rocker-bogie: http://www.alicesastroinfo.com/2012/07/mars-rover-rocker-bogie-differential/

jwatte
01-25-2013, 12:24 PM
I need four individual wheel drive with four individual wheel encoders, to know which wheels are driving and which wheels are slipping.

However, I got some good suggestions on another forum to look for "a-frame" suspensions as used by old-style race cars. (Those suspensions are what then developed into "double wishbone" suspensions in high-performance cars.)

I ended up deciding to use ball joints, and found some cheap M6 thread ball joints from midwestproducts.com. I then milled "knuckles" that mount the motor to the wheel, to top and bottom ball joints, and to a steering rod. My next step is to make the actual A frames (probably just cut them out of a single piece of aluminum) and springs to suspend them. I'll probably also mount a steering servo on the actual A frame (so it follows the suspensions movement) for each of the turning wheels. I can do Ackerman in software, then :-)

The rocker bogie solves the suspension problem, but leaves steering out of it, and that's the part that I had real problems with, until I accepted I'd have to buy some ball joints :-)

I may have pictures soon. Although my Mech Warfare walker is taking priority :-(

CasperH
01-26-2013, 02:18 AM
Ok so the four wheels are individually driven, are all of them steering also (antiparallel?) or just the front or rear wheels. How much steering angle do you wish to have i.e. turn radius?

Currently what I'm thinking is that you can use the suspension you mention and then use servo's with (flexible) winches. Something using ball links and rods:

http://www.rcgroups.com/forums/showthread.php?t=1113273

As for the drive, you wanted 4 servo's? I'm thinking that just attaching the servo's to the wheels with a flexible coupling is out of the question due to the forces when driving over rough terrain?

jwatte
01-26-2013, 03:13 PM
Ok so the four wheels are individually driven, are all of them steering also (antiparallel?) or just the front or rear wheels.

It would be cool with four wheels steering, but I'll start with the rear wheels fixed.

I use DC gear motors, with encoders, for the propulsion. Those gear motors mount to knuckles, which mount to top and bottom ball joints, and steering links. The (digital hobby) servos are only for steering.

kito818
01-27-2013, 06:42 AM
Probably should have asked if you had a purpose for this rover build, or if you had an overall design for the whole rover before I opened my mouth. So is there a specific type of terrain, or task you want this to master or is this just a from scratch build that your going for?

jwatte
01-27-2013, 01:24 PM
The purpose is Robomagellan. Navigate park-like terrain to bump into orange cones at predetermined locations, on time.

The wheels will mount to a main chassis plate, on which I will mount batteries, controllers, a mini-ITX computer, cameras, GPS, gyro/accelerometer, and a compass. And a remote kill switch (needed by the rules.)

CasperH
02-20-2013, 09:03 AM
Hi jwatte,

Ran in to this little thing, thought it might prove usefull for your insperation:

http://blog.makezine.com/2013/02/14/3d-printing-revolution-the-complex-reality/

The car I mean, not neccesarily the 3D printing.

CasperH
02-20-2013, 09:24 AM
Ok this is embarrassing, I just noticed you actually responded there in a comment also.

jwatte
02-20-2013, 11:15 AM
Hah, yes, it seems we read the same things :-)