Thread: 4x4 software differential
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Unread 24-06-2002, 00:21
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#0047 (ChiefDelphi)
  
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System approach
Posted by Mike Gray.

Engineer on team #67, H.O.T., from Huron Valley Schools and GM MIlford Proving Ground.

Posted on 11/6/2000 6:38 AM MST


In Reply to: Use Feedback on the steering... posted by Joe Johnson on 11/5/2000 7:26 PM MST:



: I think that Joe is right about reevaluating the system. I'm not real clear on the approach you are taking, but it sounds as though you have CV or universal joints on all four steering knuckles. All four wheels turn the same amount for a given x axis value.

For turning without scuffing, a line projected through each of the axles must meet all the other lines at the
instantaneous center of rotation. In a car, this is accomplished by using a relay rod that is shorter than the distance between the 'kingpins' (cars don't actually have kingpins, but they once did, and it makes for a good analogy). The steering components form a four-bar linkage, with the axle as ground link and the relay rod as the coupler. Getting the dimensions right can be very difficult, but getting close will improve the steering a bunch.

Articulated loaders don't have the wheel scuffing problem because they pivot in the middle. Power transmission becomes a ploblem on center-pivot machines. Systems like this can be very unstable at high speeds too.

You might try two steering motors, one per side. Compute the proper angle for one side with respect to the other using the 'law of cosines'. OR use one motor and connect fore and aft four-bar steering linkages with a transverse relay rod. Effective steering angle will be limited to about 45-50 degrees
because the linkage will 'lock up' at some point which depends on the coupler and rocker lengths.

Joe already warned about using positive integers for complex calculations. Its difficult and often confusing, but it is possible to evaluate angular relationships realtime. It may slow the system down quite a bit though...I don't know how much.

Don't let these comments dissuade you from doing something that would be really cool. Just be warned
that it takes much more than it appears on the surface to make it work effectively.

Best wishes!


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