Fail-safe instrument system
First Claim
1. An admittance-responsive two-wire instrument system comprising a power supply and a receiver in one location, an active transmitter and an admittance-sensitive probe in another location, said probe being operatively connected to said transmitter, and a pair of signal wires connecting the transmitter with the receiver to form a loop, wherein the transmitter comprises means responsive to the admittance sensed by said probe for controlling the current in the loop independent of the resistance of the loop, said means for controlling being adapted to control the current to take values falling within at least one first predetermined current range within a second larger predetermined current range, said second current range not including the short and open circuit currents in said loop, and wherein the receiver comprises means for distinguishing between currents in the loop falling within the at least one first range, currents within the second larger current range but outside of the at least one first range, and the short and open circuit currents in said loop.
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Accused Products
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.
19 Citations
12 Claims
- 1. An admittance-responsive two-wire instrument system comprising a power supply and a receiver in one location, an active transmitter and an admittance-sensitive probe in another location, said probe being operatively connected to said transmitter, and a pair of signal wires connecting the transmitter with the receiver to form a loop, wherein the transmitter comprises means responsive to the admittance sensed by said probe for controlling the current in the loop independent of the resistance of the loop, said means for controlling being adapted to control the current to take values falling within at least one first predetermined current range within a second larger predetermined current range, said second current range not including the short and open circuit currents in said loop, and wherein the receiver comprises means for distinguishing between currents in the loop falling within the at least one first range, currents within the second larger current range but outside of the at least one first range, and the short and open circuit currents in said loop.
Specification