Caster Wheels

steerable caster wheels .
This is a contradiction in terms do you mean steerable wheels or caster wheels?
Caster type wheels as are generally used allow the user to steer by manipulating the two large main wheels.
There is no need for 'steerable wheels' as the above arrangement works fine.
Front wheels with no caster would make steering more difficult without wheelieing to change direction.
Del

GA for "Wheelieing" Milo

Cheers, yeah, wheelieing is only an option for the younger fitter users and is commonly used for mounting small kerbs, going over rough terrain etc.
The obvious adaption of this would be a segway style stabilisation system.
The front casters are a pain, too big and they snag everything and loose manouverability, too small and the jam on the smallest obstacle.
Del

well, in this case the wheelchair is power driven and a servo motor is used for steering the front casters through joystick.

If you mean why you can't steer the wheelchair from a yoke instead, it's because the yoke will get in the way of the person riding they chair when he/she tries to get in or out of it.

Joystick controlled steerable castor wheels won't present this problem.

The problem is you need a differential on the rear axle if you have one, and then having steerable front wheels.
Or you can drive the rear wheels individually, in which case the front wheels don't need to be steered (turning is acheived by slowing or reversing one rear wheel, allowing you to spin on the spot).
Del

It is that the caster wheel always follows the trailing arm of the caster bracket which is at an angle to perpendicular. Moving the wheelchair backward, the casters have to turn 180 degrees to align. When the wheelchair is in a narrow passage about a few inches wider than the wheelchair, a reversal is impossible. also when moving away from a wall the ordinary casters tends to hit the wall. now a steerable caster wheel holds its position and the wheelchair can back out. As for the caster wheels instead of normal wheels, it helps to maneuver the wheelchair by an attendant in case of battery failure.