WO1996004524A1

WO1996004524A1 - Method for compensating measuring errors in inclination sensor

Info

Publication number: WO1996004524A1
Application number: PCT/FI1994/000332
Authority: WO
Inventors: Viljo Lukkarinen
Original assignee: NOFA INDUSTRIAL Oy
Current assignee: NOFA INDUSTRIAL Oy
Priority date: 1994-08-01
Filing date: 1994-08-01
Publication date: 1996-02-15
Anticipated expiration: 1997-02-01

Abstract

A method to eliminate measuring error induced by variations of the temperature and variations of the size of a bubble level (6) bubble in a tilt-measuring instrument, which has a bubble filled with conductive fluid (3) and where, in connection with the bubble level, mutually comparable and tilt-indicating condensators (2, 4) and (2, 5) are formed, the output voltages of which are compared and by means of which tilt data is formed. A tilt rate substantially independent both of the temperature of the bubble level and the size of the bubble (6), is formed with voltages acting between the output voltages (U1, U2) of differential amplifier (V1, V2), used as amplifier of the sensor (4, 5) signals, and the supply voltage (u) by means of a voltage dividing coupling, where the inputs of comparison amplifier (V3, V4) are coupled to outputs (U1 and U2) of the said differential amplifier in order to compare the level of one said output with level (U3) or (4) formed by means of another output level and supply voltage (U).

Description

METHOD FOR COMPENSATING MEASURING ERRORS IN INCLINATION SENSOR

This invention relates to a bubble level based tilt sensor, where the impacts of temperature variations and impacts of variations in the bubble size are compensated for in a circuit that measures the rate of tilt by means of a method according to patent claim 1.

The problem with a known capacitive tilt sensor based on con¬ ductive bubble level fluid is that the different coefficients of heat expansion in the bubble level housing and in the bubble fluid induce a strong change of the bubble size along with the change of temperature. The bubble grows when the temperature falls and vice versa. This fact has no impact on determing only the mere spot of balance, which is, generally, the horizontal position of the bubble but in practical appli¬ cations, for instance, the horizontal condition is a sector determined suitable for the respective application, for the crossing of whose limits a control/alarm control impuls or rate measuring data of tilt can be given.

If the sensor signals are compared with fixed reference rates, as at known technical level, error arise in indicating the limits of the said angular sector when the bubble size changes. This is due to following facts:

When a non-steady signal is fed to electrode 2 of the capacit¬ ive sensor (Fig.l), a strong signal is attained from poles 4 and 5 if the bubble is small, because the capasitances formed by bubble level fluid 3 and electrodes 4 and 5 are then big and the passages of conductive fluid wide (small resistance). When the bubble is big the capacitances are smaller and the resistance of the fluid passages is bigger leading to reduct¬ ion of the signal levels in poles 4 and 5. Accordingly, with the same tilt different signal levels from the outputs are attained when the temparature changes. Almost as an ideal solution based on practical experience the following method was invented, which is characterized in what is presented in the patent claims.

The major advantage of the invention can be considered the fact that by means of the configuration set forth in this method, the tilt rates are almost completely independent of the variations in the supply voltage and variations in bubble size and, on the other hand, of the variations in the bubble level temperature during measuring. For instance, when the bubble size diminishes, the output levels of the sensor sig¬ nals and the differential amplifier rise and, simultaneously, the refence levels U3 and U4 of the comparison amplifiers grow just as much as to remain in right relation while the bubble size changes all the time along with the temperature.

The method is ideal since the reference levels change with absolute dependency according to the variation of the bubble size or the variation of supply voltage compared, for intance, with a method, where one would try to make compensation by NTC resistors or similar components, the matching of whose heat mass and performance curves with the masses and properties of the bubble level tube/fluid would be complicated and imper¬ fect.

In the following the method is disclosed with reference to the enclosed drawing, figure 1.

A bubble level, filled with conductive fluid 3, illustrated as a tilt sensor that contains also an air bubble 6. In the lower bubble surface a conductive strip 2 is fixed and in the upper surface there are strips 4 and 5. The supply voltage is U. Depending on the bubble level tilts, the position of bubble changes with regard to strips 4 and 5 inducing a difference in the capacitance between condensators formed by means of said strips. From strips 4 and 5 signals depart toward differential resistor VI,V2. The resistor is "floating" depending on the voltage levels from strips 4 and 5. The signals are rectified by diods DI and D2. Rl and R2 are dumping resistors of the sensor capacitances and D3,D4 are the dumping diods. Cl and C2 are charging condensators and resistors R3 and R4 dump-ing resistors of the charging condensators. The differential amplifier VI,V2 amplifies the output signals Ul and U2 that start forward to comparison rectifiers V3 and V4. Ml and M2 are meters of the tilt rate and can be added to the configur¬ ation. The amplification of the differential amplifier VI,V2 is set by restrictors R5,R6,R7. Reference voltages U3 and U4 of comparison amplifiers V3 and V4 are taken, by means of resistance coupling R8,R9 and RIO, Rll from supply voltage U and from output voltages Ul and U2 of the differential ampli¬ fier. With the bubble level in tilted position, output data is attained from the comparison amplifiers V3 and V4 independent of the supply voltage and the bubble size.

In the configuration in figure 1 OUT 1 and OUT 2 are 0, when the sensor is horizontal by selected accuracy. When the tilt grows to a certain rate, OUT 1 and OUT 2 turn to 1 depending on the direction of tilt. Accordingly, by means of the two comparison amplifiers V3 and V4 of the configuration in the figure both limits in the chosen tilt sector can be defined.

The invention is not restricted to the enclosed embodiment but a number of other modifications are possible within the limits of the inventional concept determined in the pateht claims. In addition to indication of a pre-determined tilt rate, the con¬ figuration aϊtd method can be applied to compensation of measuring errors of tiltmeters.

Claims

PATENT CLAIMS

1. A method to compensate measuring error induced by variat¬ ions of the temperature and variations of the size of bubble in bubble level (6) in a tilt measuring instrument, which has a bubble filled with conductive fluid (3) and where, in con¬ nection with the bubble level, mutually comparable and tilt- indicating condensators (2,4) and (2,5) are formed, the output voltages of which are compared and by means of which tilt data is formed, characterized in that a tilt rate, substantially independent both of the temperature of the bubble level and the size of the bubble (6), is formed with voltages acting between the output voltages U1,U2 of differential amplifier (V1,V2), used as amplifier of the sensor (4,5) signals, and the supply voltage (u) by means of a voltage dividing coup¬ ling, where the inputs of comparison amplifier (V3),(V4) are coupled to outputs (Ul) and (U2) of the said differential amplifier in order to compare the level of one said output with level (U3) or (U4) formed by means of another output level and supply voltage (U) .

2. A method according to patent claim 1 characterized in that the reference voltages of the comparison amplifiers are formed by a voltage dividing coupling of the voltage acting between the outputs of the supply voltage (U) and of the differential ap plifier (U1),(U2).

3. A method according to patent claims 1 or 2 characterized in that the voltage indicating the rate of tilt is measured from between the sensor outputs (4) , (5) or between outputs of the different amplifier (V1,V2).

4. A method according to any of the patent claims 1 - 3 characterized in that the output signals of the capacitive sensor are rectified.

5. A method according to any of the patent claims 1 - 3 characterized in that the dumping resistors (R1,R2) or dumping diods (D3,D4) of the sensor capacitance are coupled to the output electrodes of the capacitive sensor.