Fail-safe instrument system
AssigneeDrexelbrook Engineering Company
InventorsFrederick L. Maltby, L. Jonathan Kramer
Life of this patent
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Abstract
In an instrument system consisting of a transmitter in one location and a receiver in another location connected by a transmission medium, fail-safe operation is obtained by restricting the output of the transmitter to well-defined levels and interpreting any substantial departure of the received signal from these well-defined levels as a failure. When a failure is detected, alarms or other equipment may be activated, including a control circuit which causes replacement of the defective transmitter or transmission medium, thereby restoring the system to service. An embodiment wherein the transmitter is an admittance-responsive transmitter in a two-wire current loop for monitoring the condition of materials is described.
Claims30
A two-wire system comprising a power supply and a receiver in one location and a transmitter in another location, a pair of signal wires connecting said transmitter with said receiver and carrying a varying signal, wherein said transmitter produces a first signal falling within a first predetermined signal range in response to a first set of detected input data, and a second signal falling within a second predetermined signal range in response to a second set of detected input data, said first and second signal ranges being spaced from one another and substantially different from short circuit and open circuit signals.
A transmitter adapted to receive power from and to transmit signals over a pair of conductors, said signals being responsive to detected data, said transmitter being adapted to transmit plural predetermined signals in response to plural sets of possible detected data, the nominal values of said predetermined signals being spaced from one another, said signals being controlled to vary from their nominal values by amounts less than their spacing from one another, all of said signals being chosen to be substantially different from the signal(s) likely to be generated in the event said transmitter and/or the medium over which said signals are transmitted should fail.
A receiver for receiving transmitted signals corresponding to detected data, a first predetermined signal being transmitted in response to a predetermined set of possible data and a second predetermined signal being transmitted in response to other data, comprising means for outputting a first signal indicative of validity when said received signal corresponds to one of said predetermined transmitted signal(s), and means for outputting a first signal indicative of invalidity when said received signal corresponds to other transmitted signal(s) wherein said predetermined signals are spaced at intervals in a continuum of possible received signals, said receiver comprising means for comparing said signals with spaced windows in said continuum.
An instrument system comprising:
- transmitter means for transmitting one of predetermined transmitted signals in response to predetermined detected input data;
- and
- receiver means and power supply means, for supplying power to said transmitter means, and for receiving all transmitted signals and for producing predetermined outputs in response to all said transmitted signals, wherein said receiver means indicates a valid received signal if the received signal is one of a set of predetermined received signals and indicating an invalid received signal if the received signal is not one of said predetermined received signals;
- wherein said predetermined transmitted signals are a subset of said predetermined received signals.
A two wire instrument system comprising:
- transmitter means producing in normal operation predetermined transmitted signals in response to predetermined detected input data;
- and
- receiver and power supply means for powering said transmitter means, for receiving signals and for producing predetermined control output signals in response to predetermined received signals;
- wherein said receiver means enters a fail-safe state and produces a selected one of said predetermined output control signals in response to a received signal which is not one of said predetermined received signals.
Dependencies are inferred from the claim text.