By visiting the LUGNET TECHNIC section, found a new post from Philo presenting his own design for an adder-subtractor, and it made me to "anticipate" into TBs something that I had also in the queue to write about, since longtime (the adder-subtractor)...
The adder-subtractor modules are frequently used for Dual Differential Drive (DDD) drivetrains [1, pp 136-138] and consists of a tied dual differential gear setup which turns two inputs (lets say 'd, drive' and 's, steering') into a pair of differentiated outputs: 'd+s' and 'd-s'.
This could be for instance used to build a treaded vehicle like an excavator, where two motors are connected to the drive train inputs as described in the first reference I know, about a LEGO TECHNIC adder-subtractor design .
One motor (d) drives the vehicle making it to move forward/backward, while the other one(s) makes the steering function, turning the vehicle to the left or to the right. It is the different speed achieved on each tread, that allows to control the vehicle direction.
While motor 's' is stopped (s=0), just the first differential is driven and vehicle moves straight (each tread moves under control of 'd', at identical velocity). If just motor 's' is running, then only the 2nd differential is driven and vehicle rotates in place.
As soon as both 's' and 'd' motors starts to run simultaneously the subtractor comes to action, both treads don't run at the same velocity any longer ('d+s' not equal to 'd-s') and the vehicle turns while running (just like a car).
The great advantage of this setup is that by actuating only motor 'd', you can now guarantee that the two sides of the treaded vehicle run at the same speed, which does not happen with 2 independent motors (they always have a slight power difference). Also, you could use a less powerful motor for the steering than for the driving, if you wanted to save some weight/space in the vehicle.
Below some interesting designs for DDD drivetrains, using an 'adder-subtractor' mechanism. The first two from Sariel different buildings and a last one from jovel (LowLUG, NL) as used in his TechBall vehicle.
The images redirect to the correspondent folders at BS.
These used the most common LEGO gear differential among the several which have been released and the principles described are perfectly illustrated at the video below from leggor3.
Now back on topic,... it looks like Philippe realized on someone stating that new 3L differential is not suitable for building an adder-subtractor and decided to give it a try. Fortunately it proved to be wrong and results might be seen here.
It is a fact that the new differential lacks some of the characteristics from its predecessor (read "The new differential" section at this TBs previous post) which may be contributing for this opinion. Also the video below from Philippe, makes everything clearer.
Thanks Philippe for once more sharing with us your demonstrative skills.