The technical field relates generally to systems and methods for measurement of a resistor thermal device and more specifically to measurement using a three-wire device.
A three-wire resistance temperature detector (RTD) when compared to a four-wire RTD requires more complex measurement circuits to compensate for wire voltage drop due to the fact that a Kelvin connection cannot be made with fewer than four wires. Several compensation methods exist: The first method creates one excitation current and makes two voltage measurements. A calculation must be made either in hardware (error amplifiers) or software to combine the voltages. Both voltages must be measured, and one current must be well-known or measureable.
A second method uses two equal currents and makes one voltage measurement. A calculation is not required because the currents cancel the wire drops, but two currents must be matched and voltage must be measured and the current must be known or measureable. Other methods exist with several variations in which one current is time multiplexed with various switches so that a time multiplexed voltage measurement is capable of measuring RTD voltage and wire drop voltage. This method requires the hardware or software calculation for compensating.
The second method of using two equal currents is generally preferred because it does not require complex calculation. Attempts have been made to realize measurements using the second method. One approach creates two current sources that are well matched and well known and then makes a voltage measurement. Another approach uses two current sources that are well matched but not well known and then makes a voltage measurement and a current measurement. These two approaches require two well matched current sources supported by complex circuitries or rely upon IC manufacturing processes to adjust parameters that are difficult to control with high accuracy.
Therefore, it is to a system and method that enables measurement of a RTD without requiring complex calculation or two well matched current sources, the present invention is primarily directed.
In one embodiment, the present invention is an apparatus, for measurement of a resistance temperature detector (RTD). The apparatus comprises a current splitter. The current splitter is connected to a current source and receives a source current from the current source. The current splitter also provides a first current on a first current path and a second current on a second current path. A first current path is connected to a first end of the RTD and a second current path is connected to a second end of the RTD. The first current and the second current are adjusted by the current splitter. A control signal may he used to control the current splitter.
In another embodiment, the present invention is a DC current splitter used for measurement of a RTD device. The DC current splitter comprises a third resistor connected to a current source, a first transistor connected to the third resistor and the first resistor and controlled by the control signal from the external source, a fourth resistor connected to the current source, a second transistor connected to the fourth resistor and the second resistor, and an operational amplifier connected to the third resistor and to the fourth resistor and outputting an output voltage to control the second transistor.
In another embodiment, the present invention is an AC current splitter used for measurement of a RTD device. The AC current splitter comprises a first switch connected to a current source, a second switch connected to the current source, an input for receiving the control signal, and an inverter tor receiving the control signal and outputting an inverted control signal to the second switch. The control switch controls the first switch and the inverted control signal controls the second switch.
In yet another embodiment, the present invention is a method for measuring a resistor-thermal device (RTD). The method comprises receiving a source current by a current splitter, generating a first current and a second current by the current splitter, adjusting the first current and the second current by the current splitter, measuring the first current, and measuring a voltage across the RTD.
The foregoing has broadly outlined some of the aspects and features of the various embodiments, which should be construed to be merely illustrative of various potential applications of the disclosure. Other beneficial results can be obtained by applying the disclosed information in a different manner or by combining various aspects of the disclosed embodiments. Accordingly, other aspects and a more comprehensive understanding may be obtained by referring to the detailed description of the exemplary embodiments taken in conjunction with the accompanying drawings, in addition to the scope defined by the claims.
As required, detailed embodiments are disclosed herein. It must be understood that the disclosed embodiments are merely exemplary of various and alternative forms. As used herein, the word “exemplary” is used expansively to refer to embodiments that serve as illustrations, specimens, models, or patterns. The operational amplifier (op amp) and error amplifier are used interchangeably in this specification. The figures are not necessarily to scale and some features may be exaggerated or minimized to show details of particular components. In other instances, well-known components, systems, materials, or methods that are known to those having ordinary skill in the art have not been described in detail in order to avoid obscuring the present disclosure. Therefore, specific structural and functional details disclosed herein are not to he interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art.
The present invention introduces a system and method that connects to a single current source and splits the single source current into two currents. The system continuously adjusts the currents to ensure two currents are substantially the same. The first current passes through a RTD and merges with the second current at a node after the RTD. The first current is measured and the voltage across the RTD is also measured. After knowing the first current and the voltage across the RTD, the resistance of the RTD is easily determined and the temperature of the RTD is obtained through a chart using the resistance of the RTD.
When using the circuit of the
V=0.5*i*RW+0.5*i*RRTD−0.5*i*RW; (1)
wherein i—current from the current source 102;
RW—resistance of the wire between the screw and the RTD;
RRTD—resistance of the RTD;
The RTD wires have equal length and the resistance of three wires is substantially the same. The equation (1) can be simplified to:
V=0.5*i*(RW+RRTD−RW); (2)
V=0.5*i*RRTD; (3)
RRTD=V/(0.5*i); (4)
After RRTD is determined, the temperature of the RTD can be obtained based on the thermal characteristics of the RTD.
The current from the first current path passes through a resistor 218, a screw 106, and a RTD 112. The current from the second current path passes a resistor 220 and a screw 108 and merges with the current from the first current path. The current i flowing through resistor 218 is measured and the voltage V across screws 106 and 108 is also measured. After knowing the current i and the voltage V, the resistance value R of the RTD can be easily determined and the temperature T of the RTD can be obtained from the thermal characteristics of the RTD.
When MOSFET 212 is disabled by the external control logic (not shown), the current on the first current path is interrupted and ceases to flow into the RTD. Bias resistors 206 and 208 tip the error amplifier input so that the error amplifier 210 output disables the MOSFET 218 which interrupts the current on the second current path. Diodes 214 and 216 complete the bidirectional blocking operation of 212 and 218.
The MOSFET 212 can be optionally removed as shown in schematic 600 in
The current i flowing through the resistor 218 can be measured with a current meter equipped with a low pass filter to filter out the switching aspect of the measurement result. The voltage V across the screws 106 and 108 is also measured with a voltage meter equipped with a low pass filter to filter out the switching aspect of the measurement result. Similar to the circuit shown in
If the first current switch has been turned off, which causes the first current to stop, the current controller measures the difference between the first current and the second current, step 514, and the current controller outputs a control signal, step 516, which turns off the second current switch, step 518.
This invention allows a single, standard error amplifier to create two equal currents which is a hybrid of the first and second methods of the prior art, single current source and dual current source methods, respectively. The two current method from the present invention is capable of shared-wire, grounded RTD connection methods used by heavy duty gas turbines. The accuracy of the circuit 200 of the present invention is limited only by the matching of resistors 202 and 204, the offset error voltage of the error amplifier 210, and the triode mode of the MOSFET 218.
The present invention is a hybrid method and it is simpler and improves accuracy of RTD measurement. A single source current is required and must be well known or measurable. A single op amp (error amplifier) circuit creates a current splitter that creates two current paths, each of half the magnitude of the source current. One voltage is measured. Alternatively, a time-multiplexed current (AC) may also be used to create two current paths. The advantage of this improved, hybrid method is that for the cost of a single op amp, no compensation math is required (one or more op amps required), only one voltage must be measured, and only one current must he known or measureable.
This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims or if they include equivalent structural elements with insubstantial differences from the literal language of the claims. It is within the scope of the present invention that the features and devices described in different embodiments may be combined or interchanged.
Filing Document | Filing Date | Country | Kind |
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PCT/US2014/058083 | 9/29/2014 | WO | 00 |