GB2560021A

GB2560021A - Self-configuring electronic driver for remote instrumentation

Info

Publication number: GB2560021A
Application number: GB1703127.9A
Authority: GB
Inventors: David Todman Andrew; Laming Graham
Original assignee: LONDON ELECTRONICS Ltd
Current assignee: LONDON ELECTRONICS Ltd
Priority date: 2017-02-27
Filing date: 2017-02-27
Publication date: 2018-08-29
Anticipated expiration: 2037-02-27
Also published as: GB2560021B; GB201703127D0

Abstract

A self-configuring electronic instrumentation driver 2 has first and second input terminals and first and second excitation terminals for connection to a remote transducer 6, with a high impedance voltage measuring circuit coupled to the first and second input terminals. The driver includes a programmable excitation voltage source 2.8 to produce a voltage across the excitation terminals, a high impedance output signal voltage measurement circuit coupled to the first and second excitation terminals, and a programmable current source 2.18 arranged to control current flowing through the first and second excitation terminals. During a calibration phase, resistance characteristics of the remote transducer and intermediate cabling 4 are derived by passing test voltages and currents through the remote transducer and recording values of the voltage and current. The transducer may be a load cell, transmitter, potentiometer, shunt or thermocouple. The driver may also detect loop resistance to indicate contamination or corrosion before failure occurs.

Description

(54) Title of the Invention: Self-configuring electronic driver for remote instrumentation Abstract Title: Instrument driver for calibrating a transducer (57) A self-configuring electronic instrumentation driver 2 has first and second input terminals and first and second excitation terminals for connection to a remote transducer 6, with a high impedance voltage measuring circuit coupled to the first and second input terminals. The driver includes a programmable excitation voltage source 2.8 to produce a voltage across the excitation terminals, a high impedance output signal voltage measurement circuit coupled to the first and second excitation terminals, and a programmable current source 2.18 arranged to control current flowing through the first and second excitation terminals. During a calibration phase, resistance characteristics of the remote transducer and intermediate cabling 4 are derived by passing test voltages and currents through the remote transducer and recording values of the voltage and current. The transducer may be a load cell, transmitter, potentiometer, shunt or thermocouple. The driver may also detect loop resistance to indicate contamination or corrosion before failure occurs.

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Self-configuring Electronic Driver for Remote Instrumentation

This invention relates to a self-configuring electronic instrumentation driver.

Industrial instrumentation and process control systems regularly involve the measurement of weight, pressure, temperature, position and other physical variables which affect product quality and repeatability.

A wide range of sensors has evolved to cater for these measurements, matched by a wide range of instrumentation.

Sensor types include transducers such as Wheatstone bridge load cells and pressure sensors, 4 wire PT100 temperature sensors, potentiometric position, length and angle sensors, thermocouples, 4-20mA amplified transmitters, 0-10V amplified transmitters, DC shunts etc.

Various techniques are used to ensure optimum precision of measurements for the various sensors. For example load cells are usually connected to the instrumentation via 6 wires in order to nullify the detrimental effect of cabling resistance. Some load cell systems use a 4 wire constant current connection, which is attractive because fewer wires are required, and in intrinisically safe installations, only 4 zener barriers are required. This constant current method is usually limited to simple systems in which the system resistance is known. This is because the voltage across the loadcells would vary, depending on the resistance of the load cells, resistance of Zener barriers and resistance of cables, thus introducing uncertainty into the measurements.

The fixed constant current method does not lend itself well to being used universally with a wide range of load cell resistances and connection schemes, when it is desirable to obtain the highest possible signal to noise ratio. This is because parallel connection of load cells causes the excitation signal to be shared amongst the cells, causing a proportional drop in signal level.

The present invention seeks to provide a universal signal processor, well suited to all these sensors and sensor combinations. Furthermore, it provides an elegant solution to the 4 wire load cell connection, allowing comparable flexibility and precision to that attained by a 6 wire system, with tolerance of a wide range of sensor resistance and cable resistance, yet using only 4 wires.

In accordance with a first aspect, the invention provides a self-configuring electronic instrumentation driver having first and second input terminals and first and second excitation terminals for connection to a remote transducer, a computer processor, a nonvolatile memory, a high impedance voltage measuring circuit coupled to the first and second inputs for measuring the voltage across the inputs and which is also coupled to an analogue-to-digital convertor arranged to provide the computer processor with a digital value representative of the measured input voltage the driver further including a programmable excitation voltage source arranged to produce a voltage across the excitation terminals at a level controlled by the computer processor and a high impedance output signal voltage measurement circuit coupled to the first and second excitation terminals for measuring the voltage across the terminals and which is also coupled to an analogue-to-digital convertor arranged to provide the computer processor with a digital value representative of the measured excitation voltage and a programmable current source controlled by the computer processor and arranged to control current flowing through the first and second excitation terminals, whereby during a calibration phase, resistance characteristics of a remote transducer and intermediate cabling are derived by passing test voltages and currents through the remote transducer, under control of the computer processor and recording initial values of the excitation voltage and current, or representative values of the voltage and current in the non-volatile memory.

The benefits afforded by the present invention include:1. Improved profitability of manufacture, particularly for companies which produce instrumentation for a wide range of sensor types, as only one embodiment is required, satisfying all the aforementioned sensor types.

2. Distributors of instrumentation need carry only one universal version in stock, simplifying stock planning.

3. Users of load cell systems may rely upon this 4 wire system to perform at least as well and with as much flexibility, as traditional 6 wire systems, yet with better economy, as 2 cables and 2 zener barriers (if a hazardous area installation is required) may be eliminated.

4. The present invention provides inherent diagnostic capability as part of its monitoring structure, when used with load cells and potentiometric sensors, which adds valuable prewarning of degradation of connections, contamination of junction boxes, cables or sensors by conductive liquids or dusts, damage to cables or damage to sensors.

As such, this novel method provides distinct technical and competitive advantages.

In a second aspect, the invention provides a method of making continuous diagnostic evaluations of a remote instrumentation sensor using the said self-configuring electronic instrumentation driver, and comprising the steps of periodically measuring the total circuit resistance of the sensor and its wiring, using the measured excitation current and the resultant loop voltage, using the equation loop resistance = [loop voltage/loop current] and arranging the computer processor to store the periodic values to build up a historic record of loop resistance and to compare the present loop resistance value periodically with historic values and to indicate an error if the deviation from historic values deviates outside an expected range.

Embodiments of the invention will now be described by way of example and with reference to the drawings in which:Figure 1 is a schematic diagram of the basic signal processor circuit;

Figure 2 is a schematic diagram of the signal processor in operation with a load cell; Figure 3 shows is a schematic diagram of the signal processor in operation with a 4-20mA loop powered transmitter;

Figure 4 shows the signal processor 2 in operation with a 4-20mA transmitter with 3rd party loop power.

Figure 5 is a schematic diagram of the signal processor in operation with a thermocouple; Figure 6 is a schematic diagram of the signal processor in operation with a PT100 temperature sensor;

Figure 7 is a schematic diagram of the signal processor in operation with a potentiometer; and

Figure 8 is a schematic diagram of the signal processor in operation with a shunt.

The present invention of an improved industrial signal processing system requiring only 4 field wires, able to process signals from load cells, resistances, PT100 sensors and slidewire position sensors. This system compensates for a wide range of cabling resistances whilst at the same time monitoring the total loop resistance of these connected sensors in order to provide the optimum excitation current and to provide real-time diagnostics of loop changes and their possible reasons, as an aid to fault finding and reduced down time. In addition, the signal processor can be used with 4-20mA process signals , DC current shunts and thermocouples.

A novel industrial signal processing system is described which has an adaptive constantcurrent excitation method with voltage feedback, allowing one embodiment to be connected to a wide range of sensor types and combinations, with effective compensation for variable cable resistance using only 4 wires.

The system requires minimal choice to be made by the user, as it analyses the sensor and chooses the most appropriate excitation current automatically, to give best attainable signal to noise ratio.

As an example of this process, a load cell of unknown resistance is connected to the system with unknown cabling resistance. The typical ranges of these values are known from past experience of the manufacturer, the specified use of the driver in its datasheet, and the specifications of available devices. So we can, for example, assume that the typically encountered resistance presented by a single load cell or combination of load cells in parallel can range from 43.75 Ohms (8 x 350 Ohm cells in parallel) to 700 Ohms (single 700 Ohm load cell.), representing approximately a 16:1 range. Most load cells will accept a 10V excitation voltage, some are limited to 5V, depending on their thermal mass, to minimise self heating.

The aim is to excite the load cell with a constant current of sufficient amplitude to develop a suitable level of constant voltage across the load cell, as high as possible, but not more than the cell’s ideal voltage.

An initial calibration phase might thus be:5

1. Connect the loadcell and switch on the instrumentation. Select ‘Load Cell’ from the choice of sensor types and then select the chosen ideal cell voltage. For this example, we will assume we have chosen 10V.

2. The instrument will set its excitation voltage source to be higher than 10V by an amount considered to offer adequate tolerance for expected increases in loop resistance over the course of the life of the system. For example, long cable runs could create a sizeable change in resistance if ambient temperature rises, and the system should not voltagesaturate under the highest expected normal cable resistance excursion.

The voltage can be increased further by a generous margin, but any increase will result in increased wasted power, in the form of heat in the power supply. A level of 12-15V might be appropriate.

The instrument will inject a trial current into the excitation loop, at a level which will ensure that no more than 10V will be developed across the sensor if it were to be of the highest expected resistance type.

For a 700 Ohm load cell, this would be 10/700 A = 14.29 mA

3. The driver system will measure the resulting loop voltage and will determine what level of excitation current it should inject thereafter, in order to develop a loop voltage of no more than 10 V.

If it measured 2.5V, it would increase the level of excitation by 10/2.5 * (14.29 mA), being 57.14mA.

It would monitor the loop voltage and store the value of loop voltage, which should now be close to 10V, and the value of excitation current it chose.

4. Thereafter, whenever the driver is powered, it will inject the memorised fixed current of 57.14mA and will monitor the returned voltage against the memorised initial value. It can make decisions, based on the amount of deviation of this voltage, whether to alert the user to a possible fault.

For example, a voltage which is markedly higher than the initial conditions would signify a higher loop resistance, possibly caused by a poor connection, stretched cable or faulty sensor.

A markedly lower level could indicate that there is an additional path of parallel current flow, perhaps caused by water in the connectors.

In this way, when used for load cell, position potentiometer, PT100 or resistance measurements, it is able to make continuous diagnostic evaluations of the sensor and wiring, and is able to alert the user to any change in system conditions which might be symptomatic of an impending failure, with prompts as to possible sources of the problem. It does this by measuring the total circuit resistance of the sensor and its wiring, as we know both the excitation current and the resultant loop voltage, so loop resistance = [loop voltage/loop current].

As noted above, a gradual reduction in measured loop resistance could signify that the sensor and/or its connector, is contaminated with a conductive media such as water and that a parallel path of conduction exists, which was not there at the time of installation. Alternatively, if loop resistance rises, the user could be prompted to check for corrosion in connectors, stretched cables etc. By providing this early warning, operators are given time to arrange for convenient servicing of the system before total failure occurs.

Another benefit of this novel system is that it allows stock holding of one item, which can be sold into many common industrial instrumentation applications and adapted to different roles easily.

Manufacturing costs are reduced, as one common format is manufactured, suiting all applications, rather than requiring multiple lower volume individual formats for different applications. This affords a valuable technical and commercial benefit over competitive products.

The basic signal processor circuit is shown in Figure 1. The 4-wire connections between the signal processor 2 and a remote transducer are a programmable constant current differential pair 2.36 and 2.38, and a pair of differential input signal lines 2.40 and 2.42. The output 8.0 of the signal processor would typically be coupled to a recording or display device such as a display, data logger, alarm units, signal transmitters, network gateway devices or to a generic digital output for example via a serial COM Port. The output data could be in serial or parallel format, using standards such as RS232, RS485, TTL, 5V logic, 3V logic , I2C, USB, Wire, PCI, optical data, parallel BCD, or parallel binary.

The operation of the signal processor with some example sensor types is now described in detail, below.

Figure 2 shows the signal processor in operation with a load cell 6.

The load cell 6 is connectible to the signal processor 2 through field wiring 4, having individual wire resistances 4.2, 4.4, 4.6 and 4.8.

An excitation voltage source 2.8 is controlled by a microprocessor 2.10 to provide slightly more than the optimum excitation voltage for the sensor type connected, in order to attain optimum power efficiency. The excitation voltage presented to the field wiring 4 is monitored through a high input impedance signal conditioner stage 2.6, which feeds an A/D converter 2.28, which in turn provides a digital input to the microprocessor 2.10, representative of the excitation voltage. The impedance of the signal conditioner stage 2.6 should be sufficiently high in comparison to the impedance of the sensor so as not to shunt the sensor’s impedance to such an extent as to reduce the voltage across the sensor. For example, if the sensor is a 700 Ohm sensor, the upper expected range for a load cell, we would aim to influence its apparent resistance by less than 0.1% in order to minimise the masking of any resistance changes it might exhibit, (699.3 Ohms minimum) thus an impedance of 700 kilohms or more would be appropriate. In practice, a value much higher than this is typically used,

The current being drawn by the sensor 6, through wire resistances 4.2 and 4.4, is regulated by a current source 2.18 which is controlled by microprocessor 2.10

Voltage drops will exist across inter-connection resistances 4.2 and 4.4, which will be affected by excitation current, cable length, cable temperature, resistance of the load cell or combined resistance of parallel connected load cells and resistance of any connections between the load cells and the processor.

The voltage between points 2.24 to 2.26 at the inputs to the high input impedance signal conditioner stage 2.6, will thus be liable to variation from installation to installation and can change within an installation if there are temperature changes, connector resistance changes, sensor failures or other similar time-varying changes to the installation or ambient environment.

The voltage between points 6.2 to 6.4 on the loadcell bridge, will remain constant however, provided the sensor 6.0 remains intact, due to the constant current being passed through the sensor, controlled by current source 2.18.

The signal generated by the sensor at points 6.6 and 6.8 of the bridge 6, is fed to the processor 2 through wire resistances 4.6 and 4.8

A signal pre-processor stage 2.30, (which operates to normalise input signals to lie within a prescribed range, regardless of sensor type, typically +/-50mV) accepts this signal. It has a high input impedance, thus negligible current flows through resistances 4.6 and 4.8, and thus negligible signal loss (voltage drop) occurs between the load cell 6 and the signal processor inputs 2.40 and 2.42. The processed, normalised signal is then passed to a signal conditioner 2.2, which acts as an amplifier and anti-alias filter. The anti-alias filter is used to eliminate the effects of seismic fluctuations from local vibration sources when measuring parameters such as load or weight. This is especially evident in processes involving mixing, crushing or sieving. The frequency of this noise signal can often be comparable to that of the A/D sample rate, giving aliasing errors.

A similar effect is common when measuring pressure, where a compressor might be a piston type, producing pulses of varying pressure and quite constant frequencies. The filter also helps with temperature measurement, where the sensors are producing only millivolts of signal which with long cable lengths, can be of similar order to electrical noise induced by nearby electrical machinery. An anti-alias filter is therefore valuable for all instrumentation to be used in an industrial environment.

This then feeds an A/D converter 2.4, which sends its data to microprocessor 2.10 as a digital representation of the bridge output voltage.

The system is calibrated usually at two or more known loads, usually zero load and full scale load, to produce a range of scaled outputs to destination device 8.0, which may be a display, a COM port, data logger etc.

The system conditions at the time of calibration are stored in non-volatile memory, which may be part of the microprocessor 2.10 or which may be provided by a separate memory device. This system is capable of compensating for a wide range of load cell and cabling resistances.

Its prime benefit is that it requires only 4 wires, and can adapt its excitation conditions automatically, as described above, to optimally suit a wide range of sensor resistances and cable resistances. This is important when the aim is to sell a standard instrument to a wide range of industries and applications where sensor resistance and cable resistance are likely to vary over a wide range.

Additionally, the processor can detect changes in connection quality, by comparing the stored system conditions such as initial excitation current and voltagesaved at the time of initial calibration, to the instantaneous value at any time. These parameters reveal changes in input sensor circuit resistance, as R=V/I and I is constant. This is valuable in highlighting impending failures due to connector corrosion, damaged cabling, failure of one or more load cells in a parallel connected group of load cells, or similar.

An on-board temperature sensor 2.32 connects to an A/D converter 2.34, which sends its value to the microprocessor 2.10. This can be used to provide temperature compensation of the system if high precision operation is required over a wide operating temperature range.

As an example, consider a load cell system requiring a nominal 10V DC excitation voltage. An excitation voltage source 2.8 is set to a voltage somewhat higher than 10V to accommodate voltage drops in the cabling, but low enough to minimise heat dissipation. Other voltages can be catered for through a menu system in a user programming interface which changes variables in the software operating on the microprocessor 2.10.

On connecting the cabling, with any zener barriers which may be required, plus the loadcells , (whose combined resistance is currently unknown) and commencing calibration of the system, the processor 2.10 will initially feed a small test current into the sensor circuit, the level of which is controlled by the constant current source 2.18. The level is set to be low enough to ensure that the voltage generated across the sensor and field cabling will be less than the voltage produced by the source 2.8, to ensure that the excitation circuit does not become saturated.

As an example, if we set the voltage from voltage source 2.8 to be somewhat greater than 10V and assume the highest resistance of loadcell we might expect is 5 kilohms, with 1 kilohms of field wiring resistance including Zener barriers, we should limit the test excitation current to no more than 10V/6kilohms = 1.66mA.

The voltage across the sensor can be measured via high input impedance ports 2.6 and the A/D 2.28. This allows the microprocessor to determine the optimum excitation current for the sensor and cabling resistance conditions present at this time, which it then sets through current source 2.18 and stores this value for use thereafter.

Let us assume we develop a voltage of 583mV across the sensor cabling whilst the 1.66mA test current flows. We can compute the ideal excitation current by calculating the ratio of desired to actual voltage and multiplying it by the test current. Thus (10V/0.583V)X1,66mA = Ideal current = 28.47mA, and a voltage close to 10V should exist at the excitation output of the signal processor.

This excitation current value and the voltage measured by 2.6 and A/D 2.28 are then stored in the microprocessor 2.10 and calibration can proceed.

In use, the microprocessor 2.10 will compare the excitation voltage being developed across the field wiring with that which was measured during initial calibration, and will be able to alert the user to a possible fault in the system if this voltage is seen to change by more than a predetermined amount.

Figure 3 shows the signal processor 2 in operation with a 4-20mA loop powered transmitter.

A transmitter 10 connects to the signal processor 2 through field wiring 4, having individual wire resistances 4.2 and 4.6.

The excitation voltage source 2.8 is controlled by the microprocessor 2.10 to provide approximately 24VDC to suit the industry standard for loop excitation voltage (other voltages could be used). The excitation voltage presented to the field wiring is monitored through the high input impedance signal conditioner stage 2.6, which feeds the A/D converter 2.28, which feeds microprocessor 2.10 . The impedance of the stage 2.6 should be sufficiently high such that it does not add appreciable current to that produced by the 4-20mA sensor.

For example, we would aim to influence its apparent signal by less than 0.02%. 0.02% of the 16mA range of a 4-20mA signal is 3.2 microamperes. With 24V Excitation, the minimum resistance would therefore be 7.5 Megohms. And a value of 20 Megohms might be appropriate.

The current being modulated by the sensor 10, through wire resistances 4.2 and 4.6, is regulated by the current source 2.18 which is controlled by microprocessor 2.10. The regulation level is set to a value slightly higher than the normally expected operational loop current; 25mA for example.

The transmitter 10 will produce a lower current limiting effect in the range 4-20mA, which will be the actual current flowing in the loop. Voltage drops will exist across the interconnection resistances 4.2 and 4.8, which will be affected by transmitter current, cable length, cable temperature and quality of interconnections, for example.

The voltage between points 2.24 to 2.26 at the input to conditioner stage 2.6 will thus be liable to variation from installation to installation and can change within an installation if there are temperature changes or connector resistance changes or sensor failures, for example.

The signal pre-processor stage 2.30 accepts the 4-20mA signal. It has a low impedance, thus negligible loop voltage drop occurs. The processed signal is then passed to signal conditioner 2.2, which acts as an amplifier and anti-alias filter. This then feeds A/D converter 2.4, which sends its data to microprocessor 2.10

The system is calibrated usually at two or more known signal levels, usually 4mA and 20mA, to produce a range of scaled outputs to destination device 8.0, which may be a display, a COM port, data logger etc.

The system conditions at the time of calibration are stored in non-volatile memory, which may be part of microprocessor 2.10 or which may be a separate memory device.

The signal processor 2 can detect changes in connection quality, by using the microprocessor 2.10 to compare the stored system conditions saved at the time of initial calibration to the instantaneous value at any time. This is possible because the signal processor 2 can monitor the input current and loop voltage at all times.

This is valuable in highlighting impending failures due to connector corrosion or damaged cabling. Additionally, the current limiting effect of 2.18 protects the signal processor 2 and transmitter 10 from damage caused by inappropriate connection of the transmitter, for example by reverse connection, or by connecting to a system which already incorporates an excitation supply. This is valuable in minimising the chance of damage to the processor, something which is quite common in practice.

The on-board temperature sensor 2.32 connects to A/D converter 2.34, which sends its value to the microprocessor 2.10. This can be used to provide temperature compensation of the system if high precision operation is required over a wide operating temperature range.

A transmitter 12 connects to the signal processor 2 through field wiring 4, having individual wire resistances 4.6 and 4.8. The excitation voltage for the transmitter 12.4 is provided by an external power source, 12.2.

The signal pre-processor stage 2.30, accepts the 4-20mA signal. It has a low impedance, thus negligible loop voltage drop occurs. The processed signal is then passed to signal conditioner 2.2, which acts as an amplifier and anti-alias filter. This then feeds A/D converter 2.4, which sends its data to microprocessor 2.10.

The system is calibrated usually at two or more known signal levels, usually 4mA and 20mA, to produce a range of scaled outputs to destination device 8.0, which may be a display, a COM port, data logger etc. In this case, calibration proceeds differently due to the external excitation source. In this case, only the current flowing through 2.30 is monitored.

When calibration is performed, the transmitter 12.4 will be set typically to an initial low current level such as 4mA and the system will take a snapshot of the A/D signal at this level and scale to produce a numeric value representing the actual physical measurement, for example Opsi.

The transmitter 12.4 will then be set to a higher current level, typically to 20mA, and the system will take a snapshot of the A/D signal at this level and scale to produce a numeric value representing the actual physical measurement, for example 250psi.

We will still record the voltage appearing across 2.24 to 2.26 but this reading has no influence on the calibration, as we are not in a position to monitor 12.2 in this arrangement.

An on-board temperature sensor 2.32 connects to A/D converter 2.34, which sends its value to the microprocessor 2.10. This can be used to provide temperature compensation of the system if high precision operation is required over a wide operating temperature range.

In other respects this is similar to the operation described in connection with Figure 3, but with no need to activate the voltage source 2.8, and associated circuitry.

Figure 5 shows the signal processor 2 in operation with a thermocouple.

A thermocouple 14 connects to the signal processor 2 through field wiring 4, having individual wire resistances 4.6 and 4.8 which will normally be compensating cable. The signal pre-processor stage 2.30, accepts the thermocouple signal.

The processed signal is then passed to signal conditioner 2.2, which acts as an amplifier and anti-alias filter. This then feeds A/D converter 2.4, which sends its data to microprocessor 2.10. The system is calibrated usually at two or more known signal levels, usually at the lower and upper extremities of the temperature range of interest, to produce a range of scaled and linearised outputs to destination device 8.0, which may be a display, a COM port, data logger etc.

An on-board temperature sensor 2.32 connects to A/D converter 2.34, which sends its value to the microprocessor 2.10. This is used to provide error cancellation of the cold junction temperature error signal.

Figure 6 shows the signal processor 2 in operation with a PT100 temperature sensor.

A PT100 temperature sensor 16 connects to the signal processor 2 through field wiring 4, having individual wire resistances 4.2, 4.4, 4.6 and 4.8.

The signal pre-processor stage 2.30, accepts the thermocouple signal. The processed signal is then passed to signal conditioner 2.2, which acts as an amplifier and anti-alias filter. This then feeds A/D converter 2.4, which sends its data to microprocessor 2.10.

The system is calibrated usually at two or more known signal levels, usually at the lower and upper extremities of the temperature range of interest, to produce a range of scaled and linearised outputs to destination device 8.0, which may be a display, a COM port, data logger etc.

Figure 7 shows the signal processor 2 in operation with a potentiometer.

A potentiometer 18 connects to the signal processor 2 through field wiring 4, having individual wire resistances 4.2, 4.4, 4.6 and 4.8.

The excitation voltage source 2.8 is controlled by microprocessor 2.10 to provide slightly more than the optimum excitation voltage for the sensor type connected, in order to attain optimum power efficiency.

The excitation voltage presented to the field wiring is monitored through a high input impedance signal conditioner stage 2.6, which feeds A/D converter 2.28, which feeds microprocessor 2.10. The current being drawn by the sensor 6, through wire resistances 4.2 and 4.4, is regulated by current source 2.18 which is controlled by microprocessor 2.10 Voltage drops will exist across inter-connection resistances 4.2 and 4.4, which will be affected by excitation current, cable length, cable temperature, resistance of the sensor and resistance of any connections between the sensor and the processor.

The voltage between points 2.24 to 2.26 will thus be liable to variation from installation to installation and can change within an installation if there are temperature changes of connector resistance changes or sensor failures. The voltage between points 18.2 to 18.4 will remain constant however, provided the sensor 18.0 remains intact, due to the constant current being passed through the sensor, controlled by 2.18.

The signal generated by the sensor is fed to the signal processor 2 through wire resistances 4.6 and 4.8. The signal pre-processor stage 2.30, accepts the potentiometer wiper voltage signal.

The processed signal is then passed to signal conditioner 2.2, which acts as an amplifier and anti-alias filter. This then feeds A/D converter 2.4, which sends its data to microprocessor 2.10.

The system is calibrated usually at two or more known signal levels, usually at the lower and upper extremities of the measurement range of interest, to produce a range of scaled outputs to destination device 8.0, which may be a display, a COM port, data logger etc.

Figure 8 shows the signal processor 2 in operation with a shunt.

A shunt 20 connects to the signal processor 2 through field wiring 4, having individual wire resistances 4.2, 4.4, 4.6 and 4.8.

The signal pre-processor stage 2.30, accepts the shunt signal, which can be used to measure a current 20.2 passing though the shunt. The processed signal is then passed to signal conditioner 2.2, which acts as an amplifier and anti-alias filter. This then feeds A/D converter 2.4, which sends its data to microprocessor 2.10.

The system is calibrated usually at two or more known signal levels, usually at the lower and upper extremities of the temperature range of interest, to produce a range of scaled outputs to destination device 8.0, which may be a display, a COM port, data logger etc. An on-board temperature sensor 2.32 connects to A/D converter 2.34, which sends its value to the microprocessor 2.10. This is used to provide error cancellation of the cold junction temperature error signal.

List of Parts

2.2 Signal Amplifier and anti-alias filter 2.4 A/D converter

2.6 Loop voltage monitoring signal conditioner

2.8 Raw variable loop voltage supply 2.10 Main system microprocessor 2.14 Loop voltage positive 2.18 Programmable constant current source

2.20 Positive processed signal

2.22 Negative processed signal 2.26 Loop voltage negative 2.28 Loop voltage A/D converter 2.30 Pre-processor to normalise signals to +/- 50mV

2.32 On board temperature sensor

2.34 Temperature sensor A/D converter 2.36 Programmable constant Current + Output 2.38 Programmable constant Current - Output 2.40 Input Signal +

2.42 Input Signal8.0 Destination of the processed signal. Could be a display, data logger, COM Port etc.

Claims

Claims

A self-configuring electronic instrumentation driver having first and second input terminals for connection to a remote transducer and first and second excitation terminals for connection to a remote transducer, a computer processor, a nonvolatile memory, a high impedance voltage measuring circuit coupled to the first and second inputs for measuring the voltage across the inputs and which is also coupled to an analogue-to-digital convertor arranged to provide the computer processor with a digital value representative of the measured input voltage the driver further including a programmable excitation voltage source arranged to produce a voltage across the excitation terminals at a level controlled by the computer processor and a high impedance output signal voltage measurement circuit coupled to the first and second excitation terminals for measuring the voltage across the terminals and which is also coupled to an analogue-to-digital convertor arranged to provide the computer processor with a digital value representative of the measured excitation voltage and a programmable current source controlled by the computer processor and arranged to control current flowing through the first and second excitation terminals, whereby during a calibration phase, resistance characteristics of a remote transducer and intermediate cabling are derived by passing test voltages and currents through the remote transducer, under control of the computer processor and recording initial values of the excitation voltage and current, or representative values of the voltage and current in the non-volatile memory.

A driver as claimed in claim 1 including an on-board temperature sensor connected to an analogue-to-digital converter, which passes a value representative of ambient or board temperature to the computer processor, whereby the computer processor may provide temperature compensation of measured values.

A method of making continuous diagnostic evaluations of a remote instrumentation sensor using the driver of any preceding claim, comprising the steps of periodically measuring the total circuit resistance of the sensor and its wiring, using the measured excitation current and the resultant loop voltage, using the equation loop resistance =[loop voltage/loop current] and arranging the computer processor to store the periodic values to build up a historic record of loop resistance and to compare the present loop resistance value periodically with historic values and to indicate an error if the deviation from historic values deviates outside an expected range..

Intellectual

Property

Office

Application No: GB1703127.9 Examiner: Ian Rees