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Description  |
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BACKGROUND OF THE INVENTION
The present invention relates to electronic test and measurement systems in
general, and in particular to modular electronic instrument systems having
automated calibration capability.
Calibration of electronic test and measurement instruments in regular
intervals is essential for maintaining proper operation and accuracy.
Because values of internal components of such instruments change with age,
temperature, and other effects, including catastrophic failure and
replacement, recalibration of instruments is required from time to time.
The periods of time between recalibration may vary, depending on
instrument complexity, use, and other factors.
Recalibration of instruments has traditionally been a time consuming and
somewhat tedious task, requiring that a calibration technician assemble
proper calibration equipment and then follow a procedure in a manual for
the particular instrument to be calibrated. He also has to keep detailed
records of equipment model numbers and serial numbers, and the actions he
performed. In large calibration depots or laboratories, technicians have
often prepared their own calibration procedures, either to match equipment
on hand to do the calibration, or to streamline the manufacturer's
calibration procedure where possible.
Automated calibration systems, and, in some instances, self-calibrating
microprocessor-based instruments, have become commercially available from
a number of suppliers. Such automated calibration systems typically
include a number of pieces of calibration equipment in a rack along with
an instrument controller/computer interconnected by a common interface
such as a standard IEEE-488 bus or the like. The calibration equipment
comprises precise signal sources and sensors, e.g., signal generators and
highly accurate digital multimeters, respectively, used as references for
other instruments to be calibrated. The references themselves may be
calibrated at predetermined intervals against even more accurate standards
such as those which typically reside in a so called "primary standards
laboratory" or are present at the National Bureau of Standards.
In prior art automated calibration systems, software controls the display
of messages, and where applicable, provides the control of instruments.
The instrument controller executes programs specifically written for the
instrument to be calibrated, and generally matches the steps described in
the manual for that target instrument. Thus the programs simply display
messages to instruct a calibration technician to set front-panel controls
on the instrument to be calibrated, hook up cables and test leads, control
calibration instruments to provide stimulus or a measurement, and to
verify operation or make circuit adjustments as required for calibrating a
target instrument. After each step is completed, the technician presses a
button to allow the calibration system to proceed to the next step. If the
target instrument is remote controllable, the automated calibration system
may supply an appropriate signal or voltage if instructed to do so at any
given step, and to inform the technician whether the specified tolerance
is met. Automated calibration systems also automatically keep records of
calibration so that, if necessary, detailed information regarding the
accuracy of that instrument is available. This is a requirement, for
example, in the nuclear power industry. Additionally, some modern
microprocessor-controlled target instruments may store correction
constants in their memories instead of using variable resistors and
capacitors to make necessary adjustments. The correction constants, which
may be stored in the target instruments under remote control by the
calibration system, may be used as multipliers for each of a number of
ranges so that displayed readings are correct for each appropriate range.
Examples of situations where correction constants may be appropriate
include the voltage ranges of digital voltmeters and the vertical and
horizontal deflection factors of oscilloscopes.
While the prior art automated calibration systems have greatly reduced the
time required to calibrate instruments, and in many cases have allowed
calibration to be carried out by lower skilled personnel, some major
drawbacks yet exist. It is necessary that for every target instrument to
be calibrated, a specific calibration procedure must be written which
matches the requirements for calibration by the maintenance manual applied
by the manufacturer. Some of the instructions to the operator can be
standardized, such as function, range, and connection messages; however,
instructions as to which variable resistor to adjust and within what
tolerance must be explicitly created. Moreover, such instructions must be
created in the program language for the system controller. Also, such
instructions are rather specific as to which calibration instruments, by
model number, are used in the system since they must be explicitly
programmed to provide the appropriate stimulus or quantity to be measured.
As a rule, calibration instruments are not readily interchangeable among
different models or among different manufacturers, and care must be taken
to ensure that replacement equipment has the necessary functions and
accuracy to perform the calibration and to operate properly in the
calibration procedure. It is recognized that, as a rule, calibration
equipment must be at least four times more accurate than the instrument to
be calibrated, and it is typically very difficult to locate a single
replacement for a calibration instrument which adequately provides the
required accuracy, particularly if the calibration instrument is to
provide several functions, each with different ranges and accuracies.
SUMMARY OF THE INVENTION
In accordance with the present invention, an automated calibration system
for an instrument to be calibrated (target instrument) comprises an
instrument controller/computer and a plurality of accurate calibration
instruments, some of which are remotely controllable by the computer.
Remotely controllable instruments may be interconnected with each other
through an interface such as an IEEE-488 bus.
Each instrument in the system has its own characteristics file, which may
be organized generally in three major sections. The first section, which
may be referred to as resource capabilities, includes information as to
specifications, e.g., functions, ranges, and accuracies. The second
section includes information relating to remote protocol, e.g., which
IEEE-488 commands provide a desired operation by the instrument. The third
section includes calibration instructions in a standardized format. All
instruments do not necessarily have all three sections in their respective
characteristics files; however, calibration instruments should have
associated therewith at least resource capability information, and target
instruments should have associated therewith at least calibration
information. This information may be provided to the instrument
controller/computer from software, preferably provided by the instrument
manufacturer, in any form, such as floppy disks or ROMs resident in the
instruments. An instrument controller/computer program can match
calibration instruction requirements of a target instrument with resource
capabilities/specifications of calibration instruments, and can configure
a calibration system independent of specific model numbers of instruments.
A procedure can be created using the available resources, and, in fact,
the calibration steps can be performed immediately without writing a
specific calibration procedure using specific calibration instruments.
The target instrument may or may not be remotely controllable by the
controller/computer, as evidenced by the presence or lack of remote
protocol information (the second section of the characteristics file
mentioned above), and the calibration of the target instrument may or may
not be remotely controllable. However, the target instrument has
associated therewith calibration information, as mentioned above, which
may be used by the controller/computer to effect either automatic or
manual calibration, and if the target instrument is not remotely
controllable, the controller/computer can still automatically generate
instructions to a calibration technician.
It is therefore one object of the present invention to provide a novel
modular electronic instrument system having automated calibration
capability.
It is another object of the invention to provide an automated instrument
calibration system that takes advantage of standardized database formats,
which represent the capabilities, remote control command set, and
calibration information.
It is a further object of the invention to provide an automated instrument
calibration system capable of automatically configuring appropriate
calibration instruments for an instrument to be calibrated, independent of
specific instrument model numbers.
It is still another object of the invention to provide a computerized
instrument calibration system wherein an instrument to be calibrated has
associated therewith a set of calibration instructions that can be read
and implemented by the system's computer, and each calibration instrument
has associated therewith resource capability information that the system
computer can automatically match with the requirements of the instrument
to be calibrated.
Other objects, advantages, features, and attainments of the present
invention will become obvious to those having ordinary skill in the art
upon a reading of the following description when taken in conjunction with
the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a generalized block diagram of an automated calibration system in
accordance with the present invention;
FIG. 2 depicts a characteristics file for each instrument of the system
shown in FIG. 1; and
FIG. 3 is a schematic diagram of a two-lead circuit for reading the
contents of a ROM.
DETAILED DESCRIPTION OF THE INVENTION
Referring to FIG. 1, there is shown an automated calibration system
comprising an instrument controller 10 and a plurality of accurate
calibration instruments 12-1 through 12-N, some of which may be remotely
controllable by the instrument controller 10. Remotely controllable
instruments may be interconnected with each other, as well as with the
instrument controller 10, through an interface bus 14. Interface bus 14
may suitably be a General Purpose Interface Bus (GPIB) in accordance with
IEEE-488. A target instrument 16, for which calibration is desired, may or
may not be connected to interface bus 14, depending on whether or not the
target instrument is remotely controllable.
Instrument controller 10 suitably may include a computer or a
microprocessor, or indeed be a computer. Instruments 12-1 through 12-N may
comprise any of a number of calibration instruments in any desired
combination. Examples of such calibration instruments include square-wave
and sine-wave generators, time mark generators, universal counter/timers,
programmable power supplies, digital multimeters, oscilloscopes, and other
such well known instruments.
The present invention may suitably take advantage of standardized (or
proposed as standards) database formats to represent instrument
characteristics currently under development by the Institute of Electrical
and Electronic Engineers (IEEE) P981 Committee, and standardized
programming, such as ATLAS (Abbreviated Test Language for All Systems),
currently being used by the U.S. Department of Defense and its
contractors. The instrument characteristics description methods under
development by the IEEE P981 Committee cover the instrument specification
in terms of function, accuracy, resolution, and range, and also cover
remote control protocol. This information, which may be referred to as a
"characteristics file," may be provided in an ASCII file to facilitate
system integration and execution of ATLAS or other test programs.
Each instrument in the automated calibration system of FIG. 1, including
both the calibration instruments 12-1 through 12-N and the target
instrument 16 since it may also be used as a calibration instrument, has
associated therewith its own characteristics file. The characteristics
file may be provided, preferably but not necessarily by the instrument
manufacturer, as software to be loaded into the database of instrument
controller 10. Such software may be, of course, in the form of recorded
media, such as a floppy disk, or in the form a read-only memory (ROM)
resident in the instrument.
The characteristics file for each instrument may be organized generally in
three major sections. See FIG. 2. The first section, which may be referred
to as resource capabilities, includes information as to instrument
specifications, e.g., functions, ranges, and accuracies. The second
section includes information relating to remote protocol, e.g., IEEE-488
command set. The third section includes calibration instructions in a
standardized format. All instruments do not necessarily have all three
sections in their respective characteristics files; however, calibration
instruments should have associated therewith at least resource capability
information, and target instruments should have associated therewith at
least calibration information. The calibration software running on the
instrument controller 10 can match calibration instruction requirements of
a target instrument with resource capabilities/specifications of
calibration instruments, and can configure a calibration system
independent of specific model numbers of instruments. A procedure can be
created using the available resources, and, in fact, the calibration steps
can be performed immediately without writing a specific calibration
procedure using specific instruments.
For a general-purpose automated calibration system, it would be desirable
to not have to rely upon specific instruments suggested in calibration
instructions of a target instrument 16. For example, it may be desirable
under certain conditions, such as equipment malfunction and lack of
availability of another unit, to replace a particular type of instrument
from one manufacturer with a similar type of instrument from another
manufacturer, or even with a number of instruments from a number of
manufacturers, if necessary, wherein the replacement instrument or set of
instruments is capable of satisfying the requirements of measuring signals
from the target instrument 16 or providing calibration stimulus thereto.
While changes could be made to the test program to accommodate the
replacement instrument, it would be preferable to have a completely
automated closed-loop system in which instruments could be replaced with
similar instruments without regard to specific model numbers or
manufacturers.
The instrument controller 10, in configuring the system to perform
calibration on some target instrument 16, uses target instrument software
to review the calibration instruction requirements in the characteristics
file for the target instrument, as discussed hereinabove, and then uses
calibration instrument software to search the resource capability section
of characteristics files of the calibration instruments by reviewing a
stored library or by polling the instruments in the system to locate
instruments having the functions, ranges, and accuracies to find the
instruments able to perform the calibration. Generally, the calibrating
instrument should have an accuracy at least four times that of a target
instrument. Having identified the appropriate calibration instruments, the
controller 10 then checks the characteristics file of the target
instrument for the presence or absence of remote protocol information to
determine whether the calibration step can be performed under remote
control or not. If it can, the remote control instructions will be sent
over the interface (remote control) bus 14. If the calibration step cannot
be performed under remote control, instrument controller 10 will
automatically generate instructions in the form of messages displayed on a
display device 16 such as a video screen to prompt or instruct a
calibration technician to perform manual steps, such as setting
front-panel switches and adjusting internal variable components.
Even target instruments not having a remote interface may be calibrated
using the system of the present invention, as long as a characteristics
file is created or available. For example, consider a small hand-held
digital multimeter having only two test leads. The manufacturer could
provide a characteristics file in a ROM resident in the multimeter, and by
connecting the ground lead to a control line and the measurement lead to a
serial data line, the system can read the characteristics file and then
prompt a technician throughout a calibration procedure. FIG. 3 shows a
simple circuit that allows a characteristics file of a small instrument
having only two leads to be read by the system. Terminal 20 is connected
via a test lead to a control line in the system over which an enable
signal is provided. The enable signal activates a clock 22, which applies
clock pulses via a divide-by-N counter 24 to an address counter 26. The
address counter 26 addresses storage locations in ROM 28, and the data is
output in N-bit parallel format from ROM 28 to a parallel-to-serial
converter 30. The clock pulses from clock 22 are also applied undivided to
the clock input of parallel-to-serial converter 30 to clock the data out
in a serial stream to the system controller 10 via a terminal 32. The data
is reassembled in parallel format in the instrument controller 30 for use
in generating the calibration procedure.
By having each instrument in the system provide its own complete
information, many options may be provided for carrying out calibration of
a target instrument, depending upon such factors as the capabilities and
level of sophistication of the instruments themselves, the technical level
of persons performing the adjustments and verifications, and conditions
under which calibration is conducted. As an example of such an option, it
is possible to prompt the person who prepares the calibration procedure
for any changes that may be desired. For example, the calibration software
may indicate that in order to calibrate a target instrument such as a
digital voltmeter at the 10-volt D.C. range with a tolerance of 1%, a
calibration instrument with 10-volt D.C. capability and 0.1% accuracy is
recommended. The person preparing the calibration procedure may agree or
disagree with this recommendation, and override the recommendation with
another calibration instrument selected to provide the electrical signals
for this step of the procedure. Also, algorithms may be provided which
tend to promote the continued use of a selected instrument. This feature
could, of course, minimize changes to the interconnection between the
target instrument and the calibration instruments.
During the preparation phase of a particular calibration procedure, the
requirements for calibration set forth in the calibration instructions
section of the characteristics file of the target instrument are matched
with the specifications of the calibration instruments in the system as
described in the resource capabilities section of the characteristics file
of the calibration instruments. Of course, if the instrument controller
10, operating under control of calibration software, is not successful in
finding an instrument with adequate accuracy, it may suggest an instrument
with the best accuracy available and leave it up to the calibrating
technician to decide whether to continue or not. Such reduced accuracy
would, of course, reduce the quality of the calibration; however, this may
be acceptable to the owner of the target instrument. Appropriate notes
could be added to the calibration report. Also, it may happen that certain
quantities required in the calibration instructions of the target
instrument are not available. This is frequently the case when calibrating
the resistance measurement function of digital multimeters. For example,
in accordance with the calibration instructions for a particular target
instrument, the resistance function at the 200-ohm range should be
calibrated at 190 ohms, and an otherwise suitable calibration instrument
in the system provides only certain resistance values, such as 10, 100,
and 1000 ohms. In the system of the present invention, the instrument
controller 10 could make a recommendation to use the 100 ohms available
and adjust tolerance requirements so that the calibration step still may
be performed satisfactorily.
It is also possible in the system of the present invention that each
procedural step is executed as soon as the matching of requirements and
capabilities has occurred. Typically, in a manual calibration system, at
least, this will not be done since the actual execution of calibration
will be left to a lower skilled calibration technician, while the
conformation of recommendations by the calibration software to match
requirements with capabilities is done by a higher skilled metrologist.
As mentioned previously, the availability of remote interfaces target
instruments and calibration instruments impact the execution of
calibration. If the target instrument as well as all of the calibration
instruments have remote interfaces, it is possible that calibration can be
performed in a closed-loop system, totally automatically without
intervention from a human operator other than perhaps making initial
connections and disconnecting the equipment when calibration is complete.
Automatic calibration, however, requires that no changes to the wiring are
necessary, that remotely controlled switching devices are available, and
that the target instrument uses electronically controlled calibration
constants. There are already commercially available instruments having
this capability.
As a practical matter, a more typical situation would be one based on the
current technology of the installed base of electronic test instruments in
which the target instrument has a remote interface and adjustment of
variable controls are made by a calibration technician. The calibration
instructions in the characteristics file of the target instrument contain
this information which is retrieved by the instrument controller 10, as
discussed earlier, and utilized to prompt the technician during
calibration. If the target instrument does not have a remote interface,
the technician needs additional instructions for setting up front-panel
and internal control settings. While this information may be explicitly
mentioned in the calibration instructions of the target instrument, it
also may be generated automatically from the listing of calibration
requirements provided by the characteristics file of the target
instrument. The absence of a remote interface is indicated by the absence
of remote protocol information, and the instrument controller 10 can
recognize the difference and either provide remote control of the target
instrument or provide instructions to a technician as necessary.
It should be pointed out that extremely accurate calibration instruments do
not always have remote interfaces, and again, the instrument controller 10
will detect the absence of remote protocol information and provide, in
place of remote commands, instructions to a technician to indicate which
manual steps are necessary. This situation may very well occur when the
calibration instruments themselves need calibration. It could be that an
extremely accurate instrument from the primary standards laboratory is
brought to the calibration system. The description in the characteristics
file of that extremely accurate instrument may be added to the library of
instruments in the system, and the instrument then may function as either
a calibrating instrument or a target instrument.
Of course, such an automated calibration system as herein described is
capable of maintaining a complete record of calibration of a target
instrument, and such information may be directed to a printer to provide a
printed report.
As can be discerned from the foregoing description, a very flexible and
standardized calibration system executes calibration procedures developed
from the characteristics files of the instruments in the system,
independent of specific instrument models and manufacturers. It will,
accordingly, become obvious to those having ordinary skill in the art that
many changes and modifications can be made to the system of the present
invention without departing from the invention in its broader aspects.
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