We're using 1500 lb 18" industrial linear actuators and a rotational 'servo' of our own design which is a 12vDC 100 AMP motor with a 60:1 step-down gear box. (It is designed with POT feedback). The rotational one is a brush motor, so in theory, it's infinitely variable as it's just a mater of the precision of the feedback pot and the voltage we apply to the motor.
We're currently using RoboteQ AX2550 controllers, but after discussion with RoboteQ, we're upgrading to their newer controllers which is supposed to solve this problem, the HDC2400