Computerized remote resistance measurement system with fault detection

• US 5,317,520 A
• Filed: 07/01/1991
• Issued: 05/31/1994
• Est. Priority Date: 07/01/1991
• Status: Expired due to Term

• Current Assignee: Moore Industries International, Inc.
• Original Assignee: Moore Industries International, Inc.
• Inventors: Castle, Jonathan
• Primary Examiner(s): Black, Thomas G.
• Assistant Examiner(s): Walker, Tyrone V.
• Application Number:
  US07/724,120
• Time in Patent Office: 1,065 Days
• Field of Search: 364/482, 364/577, 374/172, 374/183
• US Class Current: 702/58
• CPC Class Codes: G01K 15/007 Testing
• CPC Class Codes: G01K 7/20 in a specially-adapted circuit, e.g. bridge circuit
• CPC Class Codes: G01R 27/02 Measuring real or complex resistance, reactance, impedance, or other two-pole characteristics derived therefrom, e.g. time constant by measuring phase angle only G01R25/00

Abstract

A microprocessor-controlled remote resistance measurement system is disclosed wherein the connection leads to the three- or four-wire resistance temperature devices (RTDs) are multiplexed via a four-channel analog multiplexer at the input of the unit. A separate two-channel multiplexer is also used to multiplex a fifth input for measurement of a reference resistor. The output of the multiplexer is coupled to a voltage-to-frequency converter, wherein the frequency output is utilized as an input to the microprocessor-based controller. The microcontroller can check for broken wires by addressing the multiplexers to individually isolate any of the connecting wires to the remote RTD sensors. The output of the multiplexer is monitored in a Test Mode by connecting a known impedance to the multiplexer output to determine if any RTD connections are defective. If one of the voltage sensing wires is faulty, the known impedance will cause an erroneous frequency reading into the microcontroller, which will then provide an indication on the display for determining exactly which RTD wire is broken. Only two address lines are used to control the five multiplexer channels through the use of a function selector circuit and a two-stage measurement cycle.

Claims

1. A control circuit for a remote two-wire transmitter connected to at least one resistance temperature device (RTD), said RTD having at least three sensor wires connected to said transmitter, said control circuit comprising:

a four-channel analog multiplexer having at least three of its input terminals connected to said RTD sensor wires, having at least two address lines, and having at least one output port;

current source means for applying current to said sensor through two of said three sensor wires;

a switched impedance circuit, comprising at least a resistor and a switching device, for connecting a known impedance to said output port in response to a test signal;

a flip-flop connected to at least two of said address lines for providing said test signal;

one of said address lines is connected to a clock input port of said flip-flop, and another of said address lines is connected to a data input port of said flip-flop, wherein the phase relationship between said first and second address lines determines whether said known impedance is connected to said output port; and

means for controlling said address lines to operate said multiplexer and said flip-flop in a measurement mode, wherein said known impedance is not connected to said output port, and in a test mode, wherein said known impedance is connected to said output port, and for determining whether a faulty connection exists between said multiplexer and said RTD.