CA2058356A1 - Calibration system for measurement instruments - Google Patents

Abstract

ABSTRACT

A measurement instrument for providing a signal for remote reading has a local readout of a parameter being measured and a two button calibration system, one button being used to set each end of a range of values to be transmitted by the remote signal, thus setting the calibration and span of the signal. The buttons may be used to enter currently displayed values, or by pressing both buttons at once, a setting mode may be entered in which one button increases and the other increases the displayed value to be entered.

Description

20~8~6 This invention relates to instruments ~or the continuous measurement of the value of a line~r variable between presettable upper and lower limits, and is particularly although not exclusively application to echo-ranging instruments for measuring distances, for example from the top oS a tank or silo to the surface of its contents.

Instruments of the type referred to above are commonly installed as to provide monitoring of a particular variable, for example the depth of material in a tank, which may vary between predetermined limits determined by ` the ;tank dimensions and its particular installation.
Particularly where a signal representing the value measured by the instrument is to be transmitted to a remote location, it is desirabIe that upper and`lower values of this slgnal not only do not fall too far beyond the span of the range to be measured, thus sacrificing resolution, but also~that the signal value bears an accurately calibrated 20 ~relatlonship to~the value being measured.

Existing instruments have tackled this problem in various ways. Older, purely analog instruments were provided with calibration and span controls, usually electrical potentiometers, which were adjusted during 25~ installation or recalibration to achieve desired results.
Ad~ustment reguired some~degree of skill due to interaction ~` ` between the controls, and the adjustment obtained was often subject to some degree of drift due to known stability ~-~ problems in analog instruments. More precise and in some ways easier adjustment became available with the advent of microprocessor controlled digital instruments, with digital values of various parameters such as span, and full or empty levels being entered by an operator at a control ~`~ unit. This required an adequate understanding by the ~ ~ ~ 35 operator of the relevant portions of a fairly complex ,, .

, , :

20383~6 calibration procedure, much of which related to othsr aspects of the instrument display or operation, and also required the operator to be aware in advance of the settings to be entered; for example, if calibration for the empty level of a tank was required, the operator needed to know the actual value of this empty level, and of the span.
Additionally, a multikey keypad was needed for data entry, which is space consuming, and potentially bewildering to an inexperienced operator. A further problem in acoustic ranging systems is that the range measured will be significantly affected by the temperature and composition of the atmosphere in which the measurement is conducted, due to changes in the speed of sound. Whilst automatic temperature compensation is customarily provided, special - 15 calibration problems arise when making measurements in tanks in which the empty volume is filled with vapours rather than air. This will render the readings inaccurate, but assuming that the vapour is always present, the inaccuracy will be a constant proportion of the reading.

In many applications there is a need for an instrument in which the span and calibration of an output signal can be set up in a very simple and fool-resistant manner, with a minimum number of controls and a simple numeric display.

We seek to tackle these problems by providing a measurement instrument which measures a parameter and displays its value in fixed units, and also provides an electrical output signal whose magnitude varies linearly between minimum and maximum values in proportion to the variations of the measured parameter between first and second values. The first value is set by causing the instrument to measure a desired minimum value of the parameter and to display a value accordingly, and selecting the displayed value as the first value, and the second 20~83~6 value is set by causing the instrument to measure a desired maximum value of the parameter, to display a value accordingly, and selecting the displayed value as the second value. Typically, separate keys are utilized to enter each value. After such setting, the first and second values are associated with specific displayed values;
whilst these displayed values may not accurately represent the absolute value of the parameter being measured due to offsets, or errors such as those discussed above, the minimum and maximum values of the output signal should continue to represent the absolute values of the parameter at the time of setting so that the output signal will vary in the desired manner.

The invention as set forth in the appended claims is described further below with reference to the accompanying drawings.

IN THE DRAWINGS:
Figure 1 is a simplified block diagram of a pulse-echo ranging system incorporating the invention;
Figure 2 shows a control panel of the system; and Figures 3A and 3B are a flow diagram illustrating relevant portions of a computer program utilized to operate the system.

Referring to Figure 1, the invention is described as applied to an acoustic pulse-echo ranging system operating and programmed generally as described in my United States Patents Nos. 4,831,S65; 4,890,266, and 4,992,998, but with a simplified user interface as described further below. Under control of a microcontroller 2, a transmitter 4 generates pulses or shots of alternating current utilized to drive an acoustic transducer 6 aimed towards a surface whose movement is to be monitored. Typically this will be the surface 8 of a 2~83~6 fluent solid or a liquid in a tank, bin or other vesssl 10, above or at the top of which the transducer is mounted.
The alternating current pulses are converted by the transducer into acoustic energy which is projected towards the surface 8 and echoed back towards the transducer 6.
The transducer 6 is also connected to a receiver 12 through an interface circuit 14 which damps the transducer during the transmit pulse and limits the signal amplitude applied to the receiver. The receiver filters and logarithmically amplifies the return echo signals which are digitized by an analog to digital converter 16 and applied to an input port of the microcontroller for further processing as described in the above mentioned U.S. patents. The converter 16 is also used to process signals from a thermistor 50 associated with the transducer. The microcontroller operates under control of a program stored in read only memory (ROM) 18, utilizing parameters stored in non-volatile random memory, in this example electrically erasable read-only memory (EEPROM) 20, random access memory (RAM) 22 providing working memory and temporary data storage.

Data generated by the microcontroller 2 is displayed by a liquid crystal display 24 driven by output ports of the microcontroller, and is also output to a 4-20 ma interface 26 which converts digital output data from the microcontroller into analog current levels ranging between 4 and 20 milliamps. The output produced by the interface 26 is suitable for transmission to a remote display, recording device or programmable controller adapted to operate with this type of interface. Whilst such an interface is widely adopted in industry, other standardized instrumentation interfaces could of course be used in place of that described. User input to t~e microcontroller is limited to two push-buttons or keys 28 and 30 whose condition is sensed by input ports of the microcontroller, 20~3;~g and a further output line from the controller controls an alarm relay 32 used to warn of abnormal conditions sensed by the microcontroller.

A suitable microcontroller is the 68HCll, available from Motorola, which in fact incorporates certain of the separately shown peripheral functions discussed above such as the converter 16, the EEPROM 20 and part of the RAM 22.

Referring now to Figure 2, the display 24 consists simply of a three digit seven segment numerical display 38, and a logo, in this case a trade mark of the applicant, different segments 40 and 42 of which are independently controlled to provide various indications to the operator, supplemented by four auxiliary indicators 44, 46, 48 and 50. The push-buttons 2~ and 30 are labelled 20 and 4 respectively and located on a control panel 34 together with the display 24 and terminal bloc~ 36 providing output connections from the interface 26, power supply connections to the apparatus, and connections to the circuit controlled by the relay 32.

Referring to Figure 3, the control program of the microcontroller 2, after an initialization sequence at start-up, enters a main execution loop in which it controls the transmitter to cause the transducer to generate a shot or shots of acoustic energy, and processes the digitized ~5 echo signal from the A/D converter 16 to identify a true echo generally in accordance with the procedures of the prior patents identified above, and to determine the elapsed time from the beginning of the shot to the receipt of the echo ("measure echo time" in Figure 3). The time is then converted to distance, utilizing stored data relating the speed of sound in air to temperature, as sensed by the thermistor 50, and this distance is displayed on the numeric display 38. It should be understood that the 20583~6 distance displayed is based upon the distance between the transducer 6 and the sur~ace 8, without compensation e~ther for any offset between the transducer and the effective top level of the tank occasioned for example by the tank having a domed top, or for the transducer being mounted above or below the top of the tank, or any error due to the tank being filled with a vapour other than air. It does however provide verification to a user that the device is operating normally to determine ranges. For example, a very small constant reading on an empty or near empty tank, or despite varying levels, would be an indication that the device was responding to a strong spurious echo from structure near the top of the tank.

The program next relates the calculated distance to preset first and second distance values ee_4ma_dist and ee_20ma_dist defining a range span, and corresponding to 4 and 20 milliamp values respectively. Until these distance values are set as further described below, default values stored in EEPROM 20 are used defining a range which encompasses any likely distance reading. The actual distance reading is translated to a milliamp value by linear interpolation between the preset distance values, and the digitized milliamp value generated is translated into an actual current in a current loop attached to the interface 26.

The program then tests the status of the keys 28 and 30 and if it detects a press of one or both keys enters different subroutines accordingly prior to returning to the beginning of the loop. If both keys are pressed, the seven segment display is caused to display the characters "~bL", the arrows representing turning on of the indicators 44 and 46. When the keys are released, the seven segment display then displays the blanking distance, i.e. the distance corresponding to a period following a shot during which the 20583~6 receiver signal is ignored for the purpose of identifying a true echo. A default value of this distance i8 sufficient that ringing of the transducer following a shot has decayed to the point that the receiver i8 no longer saturated and can therefore respond to echo signals, but a larger value may be desirable if a strong short range spurious echo prevents proper operation of the instrument, as discussed above. Whilst the blanking distance is displayed, it may be increased or decreased, as indicated by indicator 44 or 46, by pressing on the key 28 or the key respectively. If no keys are pressed for a predetermined period, the displayed value i9 stored in EEPROM 20 in place of the previously stored value, and then the display reverts to showing the measured distance.

If the key 30 (which bears the designation 4) is pressed, the existing value of ee_4ma_dist is displayed until the key is released, when the seven segment display is caused to display the characters "c4", and a further key press is awaited.

~0 If both keys are pressed, the display shows " bL"
and then the value of ee_4ma_dist when the keys are released, after which the keys 28 and 30 may be used to adjust this value as described above for the blanking distance. This enables the value to be set manually.

If the key 30 is pressed, the display shows ~c4~, and the "measure echo time" and "convert time to distance"
routines are performed. A check is then made as to whether a confidence factor, calculated as described in our prior patents, is sufficient for the echo to be considered reliable. If the echo is not reliable, a low echo indication is provided by causing the seven segment display to display the characters LOE, and the subroutine terminates. Otherwise, the measured target distance is 2~3~6 displayed, and also written into EEPROM 20 as a new value of ee_4ma_dist, before the subroutine terminates.

If the key 28 (which bears the designation 20) is pressed, a similar subroutine is followed, except that the characters "c20" are displayed instead of "c4", and the value of ee_2Oma_dist is updated.

In addition to the indications discussed above, various additional indications are provided by the logo segments 40 and 42 and the auxiliary indicators 44, 46, 48 and 50. During normal ranginq operation of the instrument the segments forming the whole of the logo are turned on.
Loss of a reliable echo is signified by the eye segments 40 of the logo being turned off, leaving only the pulse symbol segment 42 turned on. During the calibration subroutines just described, the entire logo is turned off. Detection by the program of a fault in the operation of the instrument such as a memory error or failure to detect any response from the receiver will result in activation of the alarm relay 32 and also the indicators 48 and 50. During execution of the subroutine following pressing of the key 30, the indicator 46 is turned on, and during execution of the subroutine following pressing of the key 28, the indicator 44 is turned on.

It should be noted that the key 28 may be utilized to set a range or target distance which is either greater or less than the distance set by key 30. Assuming tha~ the distances set represent maximum and minimum levels to be monitored in a tank using the key 28 to set the minimum level (i.e. the maximum target range) will result in an output representing the space remaining in the tank, whilst using it to set the maximum level (i.e. the minimum targPt range) will result in an output representing the degree of filling of the tank. This avoids the necessity for any 20~83,~6 separate control or input to lnitiate these two difrerent functions.

In a modification, the simultaneous pressing of both buttons could be utilized to set a level which would result in energization of the alarm relay, instead of changing the blanking delay, with the direction of alarm sensing depending on which key was used to make the final setting; or such a function could be initiated using an additional key. The provision of additional keys however would compromise the essential simplicity of operation of the unit. It would also be possible for the keys to be programmed to perform different functions seguentially, accompanied by appropriate display indications, but this again would prejudice simplicity. The two, upper and - 15 lower, operating keys instinctively represent upper and lower settings, and upward and downward directions of adjustment.

Variations in the display are of course also possible. It should be understood that the primary data output of the instrument is that through the interface 26, and that it is this output which is being calibrated during the procedures discussed above. No means is provided for calibration of the distances displayed by display 24; these are solely intended for operator guidance during the process of adjusting the output span, and represent the distance between the transducer and a target sensed by the transducer, assuming the intervening medium to be air. The auxiliary indications are also intended for operator guidance. The full display of the manufacturer's trademark during normal operation is a confirmation of this condition; such a full display will not occur if no reliable echo can be detected, or calibration is in progress. In a variation, upper and lower portions o~ the logo segments 40 could be separately controlled to indicate . ~

20~8~6 echoes out of range respectively beyond the limits set by the buttons 28 and 30, or to replace the indicator~ 44 and 46.
Whilst the invention has been described with reference to an acoustic ranging instrument, the invention can be used in echo or reflection ranging instruments using other forms of radiation sources such as infra-red lasers, or in other measurement applications. For example, a belt scale weight sensing device or a flowmeter may be similarly equipped to provide an analog signal to a remote integrator incorporated in a measurement or control device.

Claims (7)

1. A measurement instrument comprising:
means to measure a parameter and provide an electrical output signal whose magnitude varies linearly between minimum and maximum magnitudes in proportion to variations of the parameter between first and second values of the parameter;
means to display an actual value as measured by said measurement means;
first means to store the displayed value while the instrument is measuring said first value;
second means to store the displayed value when said instrument is measuring said second value; and computing means to translate an intermediate displayed value into said electrical output signal by linear interpolation between the output signal magnitudes associated with said stored values.

2. An instrument according to claim 1, wherein the first and second means are pushbuttons whose conditions are sensed by said computing means.

3. An instrument according to claim 2, wherein said computing means implements a routine initiated by simultaneous actuation of said pushbuttons to increment or decrement an operating parameter of the instrument by selective actuation of one or the other pushbutton.

4. An instrument according to any one of the preceding claims, wherein the display means includes auxiliary indications of the status of the instrument.

5. An instrument according to claim 4 in which at least some of the auxiliary indications are provided by selectively turning on different portions of a display of a design trademark identifying the source of the instrument.

6. An instrument according to any of claims 1, 2, 3 or 5, wherein the first value of the parameter may be either greater than or less than the second value, thereby determining the direction of change of the output signal magnitude relative to an increase in the measured parameter.

7. An instrument according to any of claims 1, 2, 3 or 5, wherein the instrument is an acoustic pulse-echo ranging instrument.