Current Measurement Circuit and Method of Diagnosing Faults in Same

• US 20100039118A1
• Filed: 09/01/2006
• Published: 02/18/2010
• Est. Priority Date: 09/07/2005
• Status: Active Grant

• Current Assignee: TRW Limited (Zeppelin - Stiftung der Stadt Friedrichshafen)
• Original Assignee: TRW Limited (Zeppelin - Stiftung der Stadt Friedrichshafen)
• Inventors: Crozier, Stephen Edwin, Williams, Andrew Stephen James
• Granted Patent: US 9,322,871 B2
• Time in Patent Office: Days
• US Class Current: 324/538
• CPC Class Codes: G01R 19/0023 Measuring currents or voltages from sources with high internal resistance by means of measuring circuits with high input impedance, e.g. OP-amplifiers electrostatic instruments G01R5/28; measuring electrostatic potential G01R15/165; measuring electrostatic fields G01R29/12; amplifiers per se H03F
• CPC Class Codes: G01R 31/2829 Testing of circuits in sensor or actuator systems testing of apparatus for measuring electric or magnetic variables G01R35/00; testing of indicating or recording apparatus G01D; in airbag systems B60R21/0173; checking gas analysers G01N33/007; monitoring or fail-safe circuits for electromagnets H01F7/1844

Abstract

A current measurement circuit includes first and second input nodes which receive the respective voltages at either side of a resistive element through which electrical current can flow, a differential amplifier means which produces an output signal indicative of the difference in voltage between its input terminals, electrical path forming means providing a connection between each of the input nodes and a respective input terminal of the differential amplifiers, a controllable voltage source for selectively applying an offset voltage to at least one input of the differential amplifier in response to a control signal, a control signal generator which generates the control signal applied to the voltage source such that at a first instance an offset voltage is applied which is greater than that applied at a second instance, and a diagnostic means which is adapted to identify faults present in the electrical path between the first input node and the respective input to the amplifier by comparing the output from the amplifier at the first instant when the offset voltage is applied and at the second instant without the offset voltage.

Claims

1. A current measurement circuit comprising:

a first input node and second input node which respectively receive a voltage from either side of a resistive element through which electrical current can flow.

a differential amplifier, which produces an output signal indicative of the difference in voltage between its input terminals,

electrical path forming means providing a connection between each of said input nodes and a respective input terminal of said differential amplifier;

a controllable voltage source for selectively applying an offset voltage to at least one input of said differential amplifier in response to a control signal;

a control signal generator which generates said control signal applied to said voltage source such that at a second instance an offset voltage is applied which is greater than the offset voltage applied at a first instance, and

a diagnostic means which identifies faults present in said electrical path between said first input node and said respective input to said amplifier by comparing the output from said amplifier at said first instant with the output at said second instant and further in which said diagnostic means produces an error signal indicating a fault in the event that said difference between the output of said amplifier at said first instant and said output at the second instant is different from a predetermined threshold level.

2. The circuit according to claim 1 wherein said electrical path forming means comprises a low pass filter.

3. The circuit according to claim 1 wherein said voltage source is adapted to apply an offset voltage to just one input terminal of said amplifier.

4. The circuit according to claim 3, wherein said controllable voltage source is controlled by a single control signal.

5. The circuit according to claim 1 wherein said controllable voltage source is controlled by at least two signals allowing an offset voltage to be independently applied to said positive terminal and said negative terminal.

6. The circuit according to claim 7 wherein said diagnostic means is embodied within a microprocessor which also embodies said control signal generator, a program running on said microprocessor causing said control signals to be generated and said output signals obtained from said amplifier to be diagnosed by said processor.

7. The circuit according to claim 5 wherein said control means causes said circuit to perform at least one of the following functions:

(a) applying an offset voltage to said positive terminal at said second instant but not to said negative terminal, and to apply no offset voltage at said first instant,

(b) applying an offset voltage to said negative terminal at said second instant but not to said positive terminal, and to apply no offset voltage at said first instant;

(c) applying an offset voltage to both said positive terminal and said negative terminal at said second instant but apply no offset voltage at said first instant.

15. The circuit according to claim 3 wherein said error signal is provided when the difference between the output of said amplifier at said second instant and the output of said amplifier at said first instant is less than said predetermined threshold.

16. The circuit according to claim 7 wherein said error signal is provided when the difference between the output of said amplifier at said second instant resulting from one of functions (a) and (b) and the output of said amplifier at said first instant is less than said predetermined threshold.

17. The circuit according to claim 7 wherein said error signal is provided when the difference between the output of said amplifier at said second instant resulting from function (c) and the output of said amplifier at said first instant is greater than said predetermined threshold.

8. A method of diagnosing faults present in a current sensing apparatus in which a differential amplifier having a positive input and a negative input, the differential amplifier also having an output and being operable to produce an output indicative of a potential drop across a current sense resistor connected to the inputs, the method comprising the following steps:

(a) measuring the output of the amplifier at a first instant;

(b) applying an offset voltage to at least one input of the differential amplifier at a second instant;

(c) measuring the output of the amplifier at a second instant;

(d) comparing the output of the amplifier at the second instant to that at the first instant.

(e) applying an offset voltage to at least one other input of the differential amplifier at a third instant;

(f) measuring the output of the amplifier at the third instant;

(g) comparing the output of the amplifier at the third instant to that at the first instant;

(h) applying an offset voltage to both the positive and the negative inputs of the amplifier;

(i)

comparing the output of the amplifier at the fourth instant to that at the first instant; and

(k) determining the difference between the outputs of the amplifier for at least one of the preceding comparisons of the amplifier outputs and comparing the resulting difference to a predetermined limit to determine whether a fault should be indicated.

9. The method according to claim 8 wherein wherein a fault is indicated in step (k) upon determining that the difference between the outputs of said amplifier at the first instant and one of the second and third instants is less than the predetermined limit for indicating a fault.

10. The method according to claim 8 wherein a fault is indicated in step (k) upon determining that the difference between the outputs of the amplifier at the first instant and the fourth instant is greater than the predetermined limit for indicating a fault.

11. The method according to claim 8 which further comprises wherein said offset voltage is generated by alternately connecting an input of the amplifier to a voltage source when an offset voltage is to be applied and a high impedance source when an offset voltage is not required.

12. A method of diagnosing faults present in a current sensing apparatus in which a differential amplifier having a positive input and a negative input, the differential amplifier also having an output and being operable to produce an output indicative of a potential drop across a current sense resistor connected to the inputs, the method comprising the following steps:

(a) measuring the output of the amplifier at a first instant;

(b) applying an offset voltage to at least one input of the differential amplifier at a second instant;

(c) measuring the output of the amplifier at a second instant;

(d) comparing the output of the amplifier at the second instant to that at the first instant; and

(e) determining the difference between the outputs of the amplifier that are compared in step (d) and comparing the resulting difference to a predetermined limit to determine whether a fault should be indicated.

13. The method according to claim 12 wherein, during step (b), the offset voltage is applied to only one input of the differential amplifier at the second instant and further wherein, during step (e) a fault is indicated upon the difference being less that the predetermined limit for indicating a fault.

14. The method according to claim 12 wherein, during step (b), the offset voltage is applied to both inputs of the differential amplifier at the second instant and further wherein, during step (e) a fault is indicated upon the difference being greater that the predetermined limit for indicating a fault.