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Universal measuring instrument with signal processing algorithm encapsulated into interchangeable intelligent detectors

Abstract

A meter (10) is capable of use with a plurality of different sensor heads (16, 16b). The reading meter (12) includes a digital display (20), a processor (22) and memory (23), and an analog-to-digital converter (ADC 41). Each different sensor head includes a memory (32a) which carries at least the processing algorithm for the particular sensor. In operation, the reading meter (12) reads the detector head memory (32a), and loads the information so read into its processor memory (23). The raw sensor data is converted to digital form, and processed in accordance with the stored algorithm, to produce the desired display. In one embodiment, the detector head memory (32a) is read, and the information is serially coupled to the reading meter. In another embodiment, the serial data is converted to optical signals (634) for transmission. The processor memory (23) may include other data which is held in semipermanent storage, such as calibration data for an ADC within the reading meter, and housekeeping information for the display (20). A count-down counter (52) in each sensor head is preprogrammed with a number representing the number of days remaining in the calibration period, and is decremented daily.

FIELD OF THE INVENTION

This invention relates to universal measuring instruments or meters which use interchangeable detectors, and more particularly to those instruments in which the detectors carry the algorithms required to process the sensor output signal, such as for linearization and offset correction.

BACKGROUND OF THE INVENTION

Electronic measuring instruments are commonly used to measure and display a wide variety of parameters, such as voltages, temperature, pressure, speed, frequency, acceleration, sound intensity, and illumination levels, to name a few. Such measuring instruments commonly include a sensor which generates an analog signal having a magnitude which varies in accordance with variations in the parameter being measured. While such analog signals can be used to directly drive analog display meters, it is now common to convert the analog signal, using an analog-to-digital converter, into a digital signal which can be displayed digitally on a display panel. An engineer or technician who is required to measure a variety of different parameters such as those listed above ordinarily requires a corresponding number of measuring instruments. However, the cost of maintaining a separate measuring instrument for each type of parameter to be measured can be significant. Many electronic measuring instruments in current use include a gain amplifier positioned between the sensor and the analog-to-digital converter. For use with some sensors, the gain amplifier must be calibrated for zero signal strength (offset adjustment) and for full-scale signal strength (gain adjustment) in order to achieve reliable measurements or reading. Most electronic measuring instruments must be recalibrated periodically in order to assure accurate measurements. A calibration laboratory normally maintains a log of the date on which each measuring instrument was last calibrated, and the date on which the next calibration is due. Unless the calibration log is reviewed on a schedule, it may happen that an instrument will continue to be used beyond its scheduled recalibration date, which results in readings which may be in error. Also, particular types of sensing elements can produce electrical signals representing the measured parameter, which electrical signals vary in different ways for a given change in the parameter being measured. For example, a temperature sensitive element might produce an electrical voltage that varies linearly with changes in the temperature being measured. By contrast, a pressure sensitive element might generate its signal in the form of an electrical resistance which ideally varies in accordance with a quadratic formula. Accordingly, for a measuring meter designed to be used with different types of sensors, it is necessary to know the type of sensor which is in use at any particular time in order to determine the best algorithm for computing the measured value. This requirement, however, limits the possibility of use of new types of sensors or detectors with a meter which does not recognize the sensor.

SUMMARY OF THE INVENTION

Briefly described, and in accordance with a preferred embodiment thereof, the present invention relates to an instrumentation system which includes a meter with a central processor for computing a measurement of one or more parameters such as temperature, pressure, voltage or the like. Most generally, the meter is arranged to accept a number of different sensor modules or detector heads (detectors). In order that the meter may be used with any detector, each detector carries a memory which includes information about the detector sensor, including the processing algorithm, and the display formatting information, which must be used to process the raw sensor reading for display or recording. As a result, any meter according to the invention can make use of any detector head, even a detector head designed after the manufacture of the meter itself, and measuring parameters unknown at the time of the fabrication of the meter. This is a distinct advantage over conventional meter/detector arrangements, in which the meter itself contains a memory preprogrammed with algorithms for all of the detectors known at the time of its manufacture. In one embodiment of the invention, the detector head memory not only includes the algorithm required for processing of the raw sensor reading, but it also includes information relating to the units and characters which are to be displayed by the meter. This is necessary, since the meter displays, if limited to only those known by the meter, might not be appropriate to a new type of sensor. In an embodiment of the invention, the memory within the detector head is a commercially available two-terminal unit which includes the keep-alive battery, and which responds to a serial digital data stream.

More particularly, a display is coupled to the central processor for numerically displaying the measurement computed by the central processor. The instrumentation system also includes interchangeable sensor modules or detectors which are adapted for use with the meter, with each sensor module being designed to sense a parameter such as temperature, pressure, voltage or the like, and to provide an output signal indicative of the parameter being sensed. Some of the sensor modules include a sensor for the main parameter to be measured, and other auxiliary sensors, such as temperature sensors, which are used to process the raw signal to compensate for the auxiliary parameter. A coupling mechanism is provided for selectively coupling one of the sensor modules to the measuring meter, to measure and display the parameter being sensed by the selected sensor module. The coupling mechanism mechanically couples the sensor module to the meter, and couples the output signal provided by the selected sensor module to the measuring meter. In addition, the coupling mechanism may also provide supplemental information to the measuring meter, as described below. In a first embodiment, each sensor module provides a processing algorithm to the measuring meter, which includes not only the general characteristics of the class of sensors, but which also incorporates within the algorithm the calibration information for the particular sensor associated with the sensor module, so that when the algorithm is applied to the raw sensor data, a signal is obtained which represents the actual value of the sensed parameter. For example, if the selected sensor module is a temperature sensor, its on-board memory might store an algorithm which includes coefficients which, when the algorithm is applied to the sensed signal, provides temperature-representative signals which are corrected for the nonlinearities and other vagaries of the particular temperature sensor, based upon the prior calibration of the sensor under known or laboratory-controlled conditions. The measuring meter accesses, from the temperature sensor module memory, the algorithm, with its inherent correction, as previously stored in the sensor module, and uses such information to calculate the sensed temperature. The memory on-board each sensor module is preferably a CMOS random access memory (RAM) with a battery backup which stores the data in a nonvolatile manner, and allows the memory to be changed when desired, to include recalibration data values. An electrically erasable, programmable memory can also be used. Such recalibrations, of course, may be performed at desired intervals.

In a particular embodiment of the invention, the memory located in the detector head is arranged to produce serial digital data rather than parallel data, so that the cable extending between the sensor head and the measuring meter can have a smaller number of conductors, which in turn advantageously reduces the bulk and stiffness of the cable. In yet another embodiment of the invention, a fiber-optic cable is used to transmit the data.