Use sigmoid function to choose a delta that will help smoothly steer from current angle to target angle.

def angvel(target, current, scale):
    '''Use sigmoid function to choose a delta that will help smoothly steer from current angle to target angle.'''
    delta = target - current
    while delta < -180:
        delta += 360;
    while delta > 180:
        delta -= 360;
    return (old_div(2.0, (1.0 + math.exp(old_div(-delta,scale))))) - 1.0