FastForward- Overlapping Footprint Biped


This is my second Nagata style differential based biped. My goal was to create better foot clearance than CAN. I redesigned the legs and used a liftarm of length 4 to drive the main liftarm of the legs. 

It has very good foot clearance. 

Normal Nagata style walkers always have the feet 180 degrees out of phase of each other. When one is all the way in front, the other is all the way to the back. The differential and slip limiter allow the feet to range anywhere from 90 degrees out of phase to 270 degrees out of phase. 

My original goal has been acheived quite nicely in this robot. My goal was to use a differential to allow for fast foot recovery. When one foot becomes weight bearing, and the other not, the differential provides all the motor power to the non-weight bearing foot. This lets the non-weight bearing foot move while the weight bearing foot stays still.

I have made an *MLCAD DAT file of FastForward's design. 

One time when I was playing with it I noticed that it walks faster one direction than the other. It is easy to defeat the differential in the design, by hooking the two 40T gears together using an axle. I measured the time it took to walk the length of a yardstick walking both forwards and back. The time was the same no matter which way the robot walked. 

I then reenabled the differential mechanism and measured the walking pace both forwards and backwards. The robot walks forward about 10% faster than without the differential. It walks backwards about 15% slower than without the differential. 

It took me a while to figure out why it walked faster one direction than the other. After staring at the robot walking for a while I realized why. In the forward direction, the weight bearing foot just starts into its downward thrust when the differential transfers power to the non-weight bearing foot. When the non-weight bearing foot gets through most of its foot recovery, it catches up with the weight bearing foot and both legs proceed. When both proceed, the weight bearing foot lifts the body of the biped by pusing down on the ground. This lifts the non-weight bearing leg in the process and thus extends the stride. You can see this effect by watching the lead foot in this 400K movie.

The reverse is true when going in reverse. The weight bearing leg takes on all the weight of the biped when it is most of the way through its weight bearing cycle. This means that when the non-weight bearing leg catches up to the weight bearing leg, the weighbearing leg lowers the robot's body. This shortens the stride of the non-weight bearing foot and makes the robot walk slower. You can see this effect by watching the leading foot in this 92K movie. Also notice that this allows the feet to get tangled more easily than walking forwards.

This last 124K movie shows the limited slip differential (the differential combined with the 40T gears) in action. You can kind of see one 40T gear stop moving for a while until the other gear races ahead and comes up from behind of the other 40T gear.