WO2000060558A1

WO2000060558A1 - Multiplexing sensor signals

Info

Publication number: WO2000060558A1
Application number: PCT/US2000/009050
Authority: WO
Inventors: Al-Thaddeus Avestruz; Richard D. Thornton; Robert A. Venditti
Original assignee: Thornton Associates, Inc.
Priority date: 1999-04-06
Filing date: 2000-04-06
Publication date: 2000-10-12
Also published as: WO2000060558A9; AU4200300A

Abstract

Two or more parameters are sensed and separate analog electronic signals representing values of the respective sensed parameters are provided. The analog electronic signals are multiplexed at a location from an electronic device (14) that uses the analog electronic signals. The multiplexer (18) is controlled by signals from the electronic device (14).

Description

MULTIPLEXING SENSOR SIGNALS Background

This invention relates to multiplexing sensor signals.

Typically, a sensor converts a change in a parameter (e.g., a physical value such as conductivity or temperature) into a change in an electrical parameter such as voltage, current, resistance, capacitance, or inductance. The change in the electrical parameter is sent as an analog signal to a remote instrument where it is measured and converted into a digital signal that can be displayed or sent to a higher level controller for other purposes.

The instrument may use sophisticated analog-to-digital converters and digital signal processing to perform the measurement and conversion.

The result of the processing need not be a simple parameter value, such as conductivity of a liquid, but may be generated from a combination of parameter values, and yield a value such as the temperature-compensated conductivity of a solution. In such a case, the sensor probe measures both temperature and conductivity. The instrument combines the two measurements into a single result. Because the conductivity may vary over a wide range, the instrument must be able to change ranges in accordance with the measured values.

In one common measurement technique, an analog current signal in the range 4 to 20 ma is sent from the probe sensor to the instrument. Because the signal is never less than 4 ma, the instrument can transmit DC power to the sensor on the same pair of wires that are used for transmitting the analog signal.

Each sensor in a multiple sensor probe typically is served by dedicated conductors that carry its analog signal separately from the analog signals carried from the other sensors. The conductors are bundled in a potentially long cable leading to the instrument. A multiplexer may be used in the instrument to connect each of the signals in turn to a measuring circuit.

It is also known to convert the different analog sensor signals into digital signals in the probe using special purpose electronics and then to transmit the different digital signals serially to the remote instrument. Sensor probe assemblies may include digital memory elements that store identification and calibration information, as described in U.S. Patent 4,672,306.

Summary

The invention delivers multiple analog signals from a sensor probe assembly to a remote instrument over a relatively small number of wires using an analog multiplexer near the sensors. Digital techniques allow the instrument to control the multiplexer and to read and write identification and calibration information in a digital memory device in the sensor probe. Much of the signal processing is done in the instrument.

Thus, in general, in one aspect, the invention features a probe that senses two or more parameters and provides separate analog electronic signals representing values of the respective sensed parameters. An electronic device is configured to use the analog electronic signals. A multiplexer is located remotely from the electronic device and has analog inputs and at least one analog output. The multiplexer is configured to multiplex the analog electronic signals appearing at a number of the inputs to a smaller number of the outputs. A communication medium carries the multiplexed analog signals from the outputs to the electronic device.

Implementations of the invention may include one or more of the following features. The electronic device may include a measurement instrument. The multiplexer may be controlled by digital control signals. The communication medium may carry the digital control signals from the electronic device to the multiplexer. The communication medium may include wires connecting the electronic device to the multiplexer. The electronic device may include signal processing circuitry for processing the analog signals. There may also be a digital memory remote from the electronic device and linked to the electronic device by the communication medium. The digital memory may be in a common housing with the multiplexer. The digital memory may store calibration and identification information associated with the probe. The electronic medium may carry digital commands and data between the electronic device and the memory.

The probe may sense conductivity and temperature or pH and temperature, for example. The probe may sense two related parameters and the electronic device may generate a measurement based on the two parameters. Wires may respectively carry each of the analog electronic signals to one of the inputs.

In general, in another aspect, the invention features a probe assembly. In the probe, a probe senses two or more parameters and provides separate analog electronic signals representing values of the respective sensed parameters. A multiplexer has analog inputs, at least one analog output, and a control port, and is configured to multiplex the analog electronic signals appearing at a number of the inputs to a smaller number of the outputs in accordance with digital control signals appearing on the control port. A port of the probe assembly is configured to couple the probe assembly to a communication medium carrying the multiplexed analog signals from the outputs to a remote electronic device.

In general, in another aspect, the invention features a bus configuration for use between a measurement instrument and sensor electronics. In this aspect of the invention, the bus has exactly nine wires including two power-carrying wires, three digital signaling wires, and four analog signaling wires. In implementations, the digital signaling wires may carry clock and data signals and the analog signaling wires may carry drive signals from the instrument and sensor signals from the sensor electronics. Among its advantages, the invention enables an instrument to make a complex measurement based on multiple sensor measurements using a reduced number of wires (and connector pins) between the instrument and the probe. The probe electronics may be kept compact and inexpensive. The measurement signals remain in analog form all the way back to the instrument while many of the advantages of digital communication and control are achieved. A variety of measurement features can be implemented and changed in the instrument without redesigning the probe assembly. For example, a given sensor probe may be used with simple analog electronics when cost is more important than performance, but may be used with sophisticated analog and digital signal processing for critical applications.

Other advantages and features will become apparent from the following description and from the claims.

Description Figure 1 is a block diagram of a measurement system. Figure 2 is a schematic diagram of a probe for making a four terminal resistance measurement.

Figure 3 is a schematic diagram of probe circuitry. Figure 4 is a block diagram of a self-calibrating scheme. Figure 5 is a schematic diagram of an isolation circuit. As seen in Figure 1, measurement system 10 includes a probe assembly 12 and an instrument 14 connected at opposite ends of a connecting cable 16. The instrument 14 and the probe assembly are "remote" from one another in that they are separated by the connecting cable, which has a typical length in the range of 1 to 100 meters. Longer lengths are desirable in some applications. The probe assembly includes probe electronics 18 and multiple sensors 20. The separation of the probe assembly from the instrument by the cable permits the probe sensors to be located in an environment where sensor measurements are to be made (such as a mixing vat in a chemical plant) while the instrument is accessible to a user located in a friendlier environment.

Each of the sensors is connected by a set of wires (four in this example), 22, 24, which carry analog signals generated by the sensor to the probe electronics 18. The analog signals carried on the respective sets of four analog connections 22, 24 coming from the different sensors are multiplexed (by an analog multiplexer in the probe electronics) over a single set of four wires 26. Wires 26 are part of cable 16 and connect the probe electronics to the remote instrument 14. The signals carried on wires 26 are in analog form. Wires 26 may be relatively short, such as 0 to 2 m, or longer in other applications. In some cases, the probe electronics and the sensors could be mounted in the same housing.

Conventional electrical connectors are used to join the cable 16 at one end to the instrument and at the other end to the probe electronics.

Cable 16 also includes a set of two or three wires 28 that provide paths for digital control and data signals and a pair of wires 30 that carry DC power from the instrument to the probe electronics. In other examples, other numbers of wires 28 and 30 could be used.

The digital signals that are carried on wires 28 are used for two purposes. One purpose is to control which sensor 20 is the one sending analog signals through the probe electronics to the instrument at a given time. The second purpose is to write to and read from a digital memory in the probe electronics, which contains identification and calibration information for each of the sensors 20. More generally, the digital signals can carry a variety of information in both directions between the instrument and the probe electronics. The instrument and probe electronics use conventional digital communication technology to provide bi-directional serial communication on wires 28.

As shown in figure 2, to make a measurement using one of the sensors 20, e.g., a four-terminal resistor, a microprocessor (not shown in figure 2) in the instrument sends a digital signal to the probe electronics to cause the sensor to be connected to wires 26, by closing switches 27 in the probe assembly. The analog signal may pass through amplifiers 40, such as when the sensor has high impedance, but in other cases, it will connect directly to wires 26. The microprocessor also sends a digital signal to the memory element (not shown in figure 2) to request identification information about the type of sensor that is connected and to determine proper calibration constants to use with the sensor.

To make a resistance measurement, the measuring circuit 29 in the instrument can measure the voltage across a resistor R_x and also the voltage across sensor 20. The resistance of the sensor 20 is the ratio of the measured voltages times the value of resistor R^ In other cases, the sensor output may be a voltage (for example) and the direct measurement of this voltage is a measurement of the sensed physical parameter.

In figure 3, an example "smart board" that makes up the probe electronics 18 allows four-terminal measurements of temperature and conductivity (appearing in the analog signal sets 22 and 24) with a total of nine wires connected to the instrument: four for each of the multiplexed analog signals 26, two for power 30, and three for digital signals 28. This circuit could be expanded to allow additional measurements from additional sensors without any increase in the number of wires connecting the sensor system to the instrument.

The three digital wires 28 carry, respectively, serial data, a serial clock, and a chip select bit. The serial clock is generated at the instrument and provided to both a nonvolatile memory 50 and a multiplexer 52. The serial data can be digital data (such as calibration or identification data) that is written to or read from the non- volatile memory by the instrument, or digital data that controls the multiplexer to switch among possible analog sensor sources. The chip select bit is used by the instrument to disable the multiplexer when the data on the serial data line is intended for the non-volatile memory. Otherwise, the data is used to control the multiplexer switching. In the example of figure 3, the multiplexer controls ten switches Ml through

M10 based on the serial data. The analog lines 26 to the instrument include a drive line, signal÷ and signal- lines, and a drive return or ground line. For example, to make a four- wire measurement of the signals on lines 24, the multiplexer would close switches Ml, M4, M7 and M10 and open all other switches. The drive signal from the instrument would then be applied to the signal 1++ and the signal 1— lines. The analog signal from the sensor would be returned to the instrument on lines signall+ and signall-.

By connecting jumpers Jl and J3, a two-wire measurement could be made using a two electrode cell.

In the example of figure 3, the second set of analog lines 22 also makes two or four wire measurements (depending on whether J2 and J4 are installed or not) using switches M2, M3, M8 and M9.

Switches M5 and M6 can be used to short out the input signal to zero any offset errors in the input measurement circuitry.

In other schemes, the digital data could be sent to and from a microprocessor or other signal processing or storage devices in the sensor electronics.

As seen in figure 4, the measurement system also enables automatic calibration of each of the sensors, for example, by measuring analog signals generated by a known value of resistance 50 connected in place of the sensor 22. This may be important when the sensor resistance has temperature sensitivity. The calibrating resistor 50 can be constructed of the same material as the sensor 22 so that it has the same temperature dependence. Remote calibration may also be important because it includes the cable and remote electronics in the calibration process.

As seen in figure 5, a charge coupling arrangement can be used to electrically isolate sensor measurements. A capacitor Cl is connected by switches SW1 (controlled by the multiplexer) to a source Ns and is allowed to charge to the full value of the sensor voltage, even when the source resistance Rs (21) is large.

To make a measurement, capacitor Cl is disconnected from the source and connected by switches SW2 to C2, which in turn is connected to an amplifier AMP and then to cable 16. By connecting Cl alternately to the source and to C2 many times, the voltage on C2 will essentially equal the voltage of the source. The high isolation impedance of the switches allows the signal source to float relative to the instrument as long as the multiplexer voltage range is not exceeded. This has important advantages when the instrument is used to measure both pH and conductivity for the same solution. In this case, the two sensors typically interact if the pH sensor is forced to have the same common potential as the conductivity sensor.

Thus, the scheme of figure 5 provides high common mode rejection in addition to isolation. Isolation is determined by the common-mode range of the switches. In addition to standard MOSFET or JFET analog switches, mechanical relays and optoMOS relays may be used if a large common mode voltage is expected.

The mixed signal bus represented by the nine wires 26, 28, 30 of figure 3 is convenient because a nine-wire bus is also used in the telecommunications industry making connectors and cables readily available for the sensor application.

Other embodiments are within the scope of the following claims.

What is claimed is:

Claims

1. Apparatus comprising a probe that senses two or more parameters and provides separate analog electronic signals representing values of the respective sensed parameters, an electronic device configured to use the analog electronic signals, a multiplexer that is located remotely from the electronic device and has analog inputs and at least one analog output, the multiplexer being configured to multiplex the analog electronic signals appearing at a number of the inputs to a smaller number of the outputs, and a communication medium carrying the multiplexed analog signals from the outputs to the electronic device.

2. The apparatus of claim 1 in which the electronic device comprises a measurement instrument.

3. The apparatus of claim 1 in which the multiplexer is controlled by digital control signals.

4. The apparatus of claim 3 in which the communication medium carries the digital control signals from the electronic device to the multiplexer.

5. The apparatus of claim 1 in which the communication medium comprises wires connecting the electronic device to the multiplexer.

6. The apparatus of claim 1 in which the electronic device includes signal processing circuitry for processing the analog signals.

7. The apparatus of claim 1 further comprising a digital memory remote from the electronic device and linked to the electronic device by the communication medium.

8. The apparatus of claim 7 in which the digital memory is in a common housing with the multiplexer.

9. The apparatus of claim 7 in which the digital memory stores calibration and identification information associated with the probe.

10. The apparatus of claim 7 in which the electronic medium carries digital commands and data between the electronic device and the memory.

11. The apparatus of claim 1 in which the probe senses conductivity and temperature.

12. The apparatus of claim 1 in which the probe senses two or more related parameters and the electronic device generates a measurement based on the parameters.

13. The apparatus of claim 1 further comprising wires respectively carrying each of the analog electronic signals to one of the inputs.

14. A probe assembly comprising a probe that senses two or more parameters and provides separate analog electronic signals representing values of the respective sensed parameters, a multiplexer that has analog inputs, at least one analog output, and a control port, and is configured to multiplex the analog electronic signals appearing at a number of the inputs to a smaller number of the outputs in accordance with digital control signals appearing on the control port, and a port that is configured to couple the probe assembly to a communication medium carrying the multiplexed analog signals from the outputs to a remote electronic device.

15. The probe assembly of claim 14 further comprising a digital memory that stores calibration and identification information associated with the probe.

16. The probe assembly of claim 14 in which the probe senses conductivity and temperature.

17. The probe assembly of claim 14 in which the probe and the multiplexer are connected by wires respectively carrying each of the analog electronic signals to one of inputs.

18. A method comprising sensing two or more parameters and providing separate analog electronic signals representing values of the respective sensed parameters, multiplexing the analog electronic signals at a location remote from an electronic device that uses the analog electronic signals, and controlling the multiplexer by digital signals from the electronic device.

19. A measurement instrument comprising a probe that senses two or more parameters and provides separate analog electronic signals representing values of the respective sensed parameters, an electronic measurement device configured to use the analog electronic signals, a multiplexer and a digital memory that are located remotely from the electronic device, the multiplexer having analog inputs and at least one analog output, the digital memory storing calibration and identification information associated with the probe, wires respectively carrying each of the analog electronic signals to one of inputs. the multiplexer being configured to multiplex the analog electronic signals appearing at a number of the inputs to a smaller number of the outputs, and wires carrying the multiplexed analog signals from the outputs to the electronic device, digital control signals from the electronic device to the multiplexer, and digital commands and data between the electronic device and the digital memory.

20. A bus configuration for use between a measurement instrument and sensor electronics, the bus configuration comprising exactly nine wires including two power-carrying wires, three digital signaling wires, and four analog signaling wires.

21. The bus configuration of claim 20 in which the digital signaling wires carry clock and data signals.

22. The bus configuration of claim 20 in which the analog signaling wires carry drive signals from the instrument and sensor signals from the sensor electronics.