This paper presents the control system development of the Rutgers Ankle rehabilitation device. The "Rutgers Ankle" is a haptic interface with a Stewart platform structure driven by six double-acting pneumatic actuators. Each cylinder input is controlled by one set of on/off solenoid valves. For overall system control, an interrupt handler loop is designed based on hardware interrupts. For precise flow rate control of the solenoid valves, a PWM scheme is proposed for a set of two on/off valves. The proposed PWM logic compensates the dead band of the valve response time. A distribution method for the desired pressure differential of the two air chambers of an actuator is proposed. The method is based on the minimization of the difference between the desired and measured pressure. During experiments, one actuator achieved a rise time of 90 ms and 0.3 mm resolution in an independent position control loop. The Rutgers Ankle has a 7 Hz mechanical bandwidth and allows stiffness control for haptic interaction during rehabilitation exercises. The measured performance is believed to be sufficient for ankle rehabilitation purposes.