I've motorised an indoor (lawn) bowls wheelchair (using geared steppers) and really struggled to find examples of control code for differential speed steering. I mainly looked to tracked robots for info on skid steering but the write-ups I found only really discussed stop-skid/slew-forward motion, rather than slowing/accelerating one side relative to the other. Any help or pointers gratefully received.
 I've now published my first code for this on Github (https://github.com/SimonMerrett/StepperMobility). I make apologies for the state of it but it might help to have something to look at. I promise to try and tidy it up but I'm no coding expert.
Here's a link to the project that is the intended application for solutions to this request https://hackaday.io/project/20734-stepper-motorised-wheelchair