Title: Tractor hitch control system having safety features

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Abstract: A control system is provided for a hydraulically operated tractor hitch assembly including a rockshaft operated by a main cylinder which is controlled through a spool valve operated by a pilot cylinder which is controlled, in turn, through a solenoid operated pilot valve. The pilot valve is controlled in response to a command signal and first and second feedback signals which correspond to the position of the rockshaft and to the position of the spool valve member. The velocity of the downward movement is limited by limiting the amplitude of a first error signal which is developed by a first comparator responsive to the command and first feedback signals with the first error signal and the second feedback signal being applied to a second comparator. Such velocity-limiting operations are controlled through a manually operated switch or by a motion sense circuit responsive to the rate of change of the command signal. A command signal which has a fixed magnitude corresponding to a fully raised position is applied in a raise position of a raise-lower switch or in a transport lock position of a control lever. Error detector means are provided for sensing the loss of the first feedback signal and for applying a signal to cause centering of the main valve to lock the hitch against movement.


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