1. Field of the Invention
The present invention generally relates to meter displays and, more specifically, to a clonable measurement display instrument, provided with calibration data, for replicating a like display instrument also having the same calibration data.
2. Description of the Prior Art
Numerous devices and liquid level gauges have been devised for measuring and displaying the level of a liquid within a storage tank reservoir. The prior art devices have involved numerous techniques for monitoring or physically detecting the level of the liquid in the tank reservoir, including the use of sight gauges, electro-optical systems using light types, external and color lights, lenses, prisms and filters, and fiber optics. These are generally complex and expensive systems. Examples of liquid measuring systems are disclosed, for example, in U.S. Pat. Nos. 4,296,472; 4,382,382, 4,487,065; 4,598,742; 4,601,201; 4,644,177; 4,745,293; 4,857,894, 4,870,292; 4,908,783; 4,994,682; 5,124,933; 5,245,869; 5,247,833 and 5,351,036.
Tanks of ships tend to be irregular. This is in contrast to an ideal tank, for purposes of measurement, which is rectangular or cylindrical. The volume of a simple tank is generally equal to the base area times the height. With the ideal tank the cross-sectional area of the tank is constant along the height, so that the height of the liquid is directly proportional to the volume taken up by the liquid in the tank. Any simple sensor can then be used to translate the height of the liquid and correlate this information to the quantity of fluid in the tank. Most tanks used aboard ships, however, are not ideal and they tend to follow the shape of the hull. Frequently, ducts, pipes, and other three dimensional spaces or regions are contained within the tank which are not used to contain fluid. This further complicates the geometric configuration of the tank for purposes of liquid level monitoring. Therefore, in most tanks aboard ships, a tank response curve is generally not linear, and the height of the column of liquid is not directly proportional to the volume of the liquid in the tank. Not only does one have to contend with the shape of a hull, but also the exact position of the tank level sensor. The volume inside tanks through which pipes, ducts, gates, crawl spaces, tunnels or the like extend is quite common and, as suggested, complicates the liquid level measurements.
In view of the foregoing, the tank response curve of an ideal tank is generally a straight line which correlates a given height of the liquid to the volume in the tank. One of the greatest problems of the older systems is inaccuracy, and tank level readings could not be trusted. It is for this reason that certain critical tank level readings were performed manually. Such inaccuracies had been introduced by the meters themselves. If a meter had a perfect linear scale, the error that just the instrument introduced is 2%, the error introduced by electronics, such as voltage source drift, being negligible.
As indicated, aboard a ship, most liquid level scales that are used are non-linear. There are several errors that come into play. The system is calibrated by simulating several fluid heights corresponding to known fluid volumes. At each height, the voltmeter is marked accordingly. The resulting response curve defines the conversion transform for the particular tank involved. A plot of such response curves indicates, however, that considerable amounts of error can be introduced at points along steep slopes of the curve where small changes in height of the fluid surface produce large changes in the volume or liquid levels.
Considering the human reading errors, meter errors and calibration errors, the total error can be significant.
On a conventional meter scale, one can record a limited number of "break points". This number is limited because physically one can only place a limited amount of nomenclature on the display scale. Such nomenclature is also usually represented in round numbers (0, 10, 20, 30 etc.), even though the outcome of the calibration procedure may not have produced round numbers. Combined, this introduces a considerable error.
While numerous techniques for calibrating of electronic instrumentation have been disclosed, these systems are typically designed for linearizing non-linear transducers which monitor some variable or parameter (e.g. thermistor or other system circuit components). See, for example, U.S. Pat. No. 4,253,155, which is concerned with non-linearities of system circuit components generally, and U.S. Pat. No. 4,713,783, which is concerned with thermistors specifically.
While fluid level measurements are important, on ships and elsewhere, specially calibrated meters are used in numerous other applications, including displaying rudder control, pressures, temperatures, etc. Many of these meters must be loaded with special calibration data. Such calibration data is typically stored in a memory device. However, when a meter containing such calibration data becomes defective and must be replaced it has up to now been a problem to do so since each meter is, in essence, customized, to perform a very specific function. Therefore, such a meter is not normally interchangeable with another meter that may be available or with an "off the shelf" meter since these do not contain the special calibration data which customizes the meter to perform the desired display function for a non-linear application.
Specific examples of patents used for linearizing non-linear variables are discussed in U.S. Pat. No. 5,751,611, assigned to the assignee of the present application.
Numerous patents disclose redundant systems with automatic change-over from a defective component to a backup unit. However, such arrangements normally require that multiple similar dedicated units be maintained in place. Thus, for example, in U.S. Pat. No. 4,864,842, a method and system are disclosed for transferring calibration data between calibrated measurement instruments. In this patent, experimentally determined calibration curves are stored, which calibration data can be transferred to a plurality of dedicated field gauges. While this avoids the necessity of individually calibrating each gauge each time calibration is necessary, the field gauges must initially be cross-related to a master gauge. At a later time, when a new calibration is necessary, the master gauge is calibrated using carefully prepared samples of a test material. Using the experimentally derived calibration curves with the cross relation data provides calibration data for the field gauges.
In. U.S. Pat. No. 3,928,830, a diagnostic system is disclosed for field replaceable units, in which the system monitors functional units. In the event of failure in the operation of the system, as for example a data error, the system checks its monitors for an indication of an out-of-power condition or failure in the module or field replaceable unit inside the functional unit. The system has the capability of managing itself to deactivate a functional unit when failure sensors indicate a field replaceable unit in the functional unit has failed.
Master/slave systems typically have one microprocessor which acts as a master which provides overall system control and other microprocessors which act as responsive slaves for controlling specific functions. A system for simultaneously loading a program to master computer memory devices and corresponding slave computer memory devices is disclosed in U.S. Pat. No. 5,187,794. Thus, the use of a microprocessor to exchange data with a host station or a remotely located device is known. However, the use of two identical microprocessor-based devices in which either of the units may act as a master or slave unit in order to replicate the data stored in non-volatile memory to the other unit to, in essence, clone the first unit and make it completely interchangeable with the second unit is not disclosed in this patent. U.S. Pat. No. 4,700,292 discloses an interface circuit arrangement for transferring data from a master processor to a slave processor when the master malfunctions. The role of the malfunctioning microprocessor is assigned to its associated microprocessor while diagnostic operations are carried out to trace and correct the defect. However, this patent is mostly concerned with the efficiency of the data transfer of large quantities of data in mass memory.
Other examples of redundant-type systems include U.S. Pat. No. 4,823,256 which discloses a reconfigurable dual processor system; U.S. Pat. No. 4,941,087 which discloses a system for bumpless changeover between active units and backup units by establishing rollback points and logging write and read operations; and U.S. Pat. No. 4,797,884 which discloses for a redundant device control unit. In the last mentioned patent, two nominally identical electronic devices are connected in a manner such that one of the devices functions as an active device in the system and the other device functions as a standby device, in which each device includes a back-up control unit including an optical fiber link for establishing communications between the device and the other nominal or identical device.
It is also common to provide a system backup with microprocessor-based devices. For such backup operation, the control program and/or data is downloaded from one device to another. The backup program performs basically the same functions as the initial program. Switch over to the backup program occurs either automatically in response to a monitor, or manually by an operator when an error has been detected. Backup control systems are disclosed in U.S. Pat. Nos. 4,691,315 and 5,313,385.