than v 4 and the control system calls for the valve operating member 28 of the actuator to move to the right in the last row ve 2 is less than v 4 and the control system calls for the actuator to move to the left the other elements in the control system see fig2 consisting of the or gate 53 the exclusive or gate xor 54 and the sample and hold unit 51 are used to ensure that if during the clock period the actuator 22 reaches the desired position as indicated by solenoids 38 and 39 being both on or off that position is held until the next sampling interval this ensures that the solenoid valves 38 39 are turned on and off at most only once during each clock period this results in an increased life cycle for the solenoid valves the sampling frequency may be set at 20 h z which is sufficient for the hydraulic power lift system since the implement 11 is locked either in the raised or lowered position with the engine shut off the control handle or quadrant lever 13 may be inadvertently moved by the tractor operator resulting in the control handle being at a different setting with respect to the rockshaft 15 to prevent any unexpected and possibly unsafe movement of the lift system and the implement when the tractor is again started a safety start unit 55 is inserted in the control system in fig2 the safety start unit 55 is shown between the two summing devices 32 and 49 when the tractor is turned off the safety start.