Process control system

Claim 1

In a process control system of the type having a central station with a digital computer; said system including a plurality of field devices located remotely from said central station and including both field sensing devices for developing measurement signals reflecting process conditions such as temperature, and also field control devices responsive to control signals for affecting respective process variables such as fluid flow rate; and means interconnecting said field devices with said central station to provide said computer with data signals corresponding to said measurement signals, and to provide for sending to said field control devices control signals responsive to the digital output signals developed by said computer representing commands for said field control devices; said interconnecting means comprising: (a) means at said central station responsive to said digital output signals for developing intermediate control signals of common format, each intermediate control signal serving to convey a command for a respective field control device; (b) a plurality of signal converting units each assigned to a corresponding field control device and serving to transform the intermediate control signal for its assigned device to an analog control signal in the form of a variable current the magnitude of which represents the command for the respective field control device; (c) a first plurality of analog signal transmission lines each connecting the output of a respective one of said converters to its corresponding field control device to provide for the actuation of that device in accordance with the magnitude of the corresponding analog control signal; the system further including maintenance backup apparatus comprising: (1) an analog signal generator located at said central station and operable to develop a manually-settable backup analog signal; (2) a second plurality of analog signal transmission lines each assigned to a corresponding one of said field control devices; (3) a selector unit for said analog signal generator including means for connecting said back-up analog signal to one end of any selected one of said second plurality of analog signal transmission lines assigned to a selected one of said field control devices; and (4) switch means operable to interrupt the connection between the output of any said selected converter unit and the corresponding one of said first plurality of transmission lines, and to establish a connection between said corresponding one transmission line and the other end of said selected one of said second plurality of transmission lines, said settable backup analog signal thereby being transmitted from said analog signal generator to the particular field control device normally supplied with an analog control signal by said selected converter unit, whereby that selected converter unit can if necessary be removed for servicing while its assigned field control device is actuated by the analog backup signal from said signal generator.