JP3479325B2 - Digital tilt measuring instrument - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital inclination measuring instrument having both the performances of a high precision digital level and a general purpose digital inclination measuring instrument.

[0002]

2. Description of the Related Art Conventionally, in a wide range of fields such as civil engineering and mechanical engineering, precision measurement of horizontality and inclination measurement can be controlled by the same measuring instrument, and direct measurement and remote measurement on an object to be measured. There has been a demand for an absolute levelness-guaranteed tilt measuring device capable of performing the above.

The servo-type tilt sensor previously developed by the present inventor has higher accuracy than the conventional tilt sensors as disclosed in, for example, Japanese Patent Laid-Open No. 4-110607.
It is considered that it will become the mainstream of the tilt sensor in the future because of its relatively simple structure and excellent characteristics that it is hardly affected by an external magnetic field and the like. However, the large zero change is an obstacle to the practical use of the servo-type tilt sensor. Generally, a zero change is a phenomenon in which the indicator value changes from the previous horizontal time when the level or tilt sensor is leveled, the indicated value is adjusted to the minimum value, and then it is tilted largely and then placed horizontally again. That is.

Most of the conventional inclination measuring devices cause a zero change due to a temperature change because of poor temperature characteristics. Therefore, a zero adjusting mechanism is provided without exception. As a result of research by the present inventor, the mechanism of the zero change occurrence of the servo type tilt sensor, which has not been clarified yet, can be explained as follows.

First, a general structure of a servo type tilt sensor and a servo mechanism which is the basis of its operation will be described. As shown in FIGS. 6 and 7, the servo-type tilt sensor includes a magnet 15 that generates a magnetic field and a magnetic flux φ from the magnet 15.
And the magnetic fluxes φ1, φ
A movable coil 17 which is crossed with 2 and is rotatably supported by a span band and attached rotatably, shutters 18 and 19 as two reflectors attached to the left and right ends of the movable coil 17, and a reflective photoreflector 20. Two
1 and the bridge circuit 22 shown in FIG.

The reflection type photoreflectors 20 and 21 are fixed inside a case forming an outer shell of the servo tilt type sensor and are tilted following the case. Therefore, the gaps g1 and g2 between the reflective photoreflectors 20 and 21 and the rotatable shutters 18 and 19 are variable. The span band is a thin band strip material made of a platinum-nickel alloy and having a rectangular cross section, and its rotational force is imparted by its twisting force. Since the span band support method has less rattling and friction than the pivot support method, even a slight vibration received by the movable coil 17 can be accurately detected.

As shown in FIG. 7, Δ from the center O in the Y direction
The position of the movable coil 17 is adjusted so that the center of gravity G of the movable coil 17 is arranged at a position lowered by y. That is, the movable coil 17 is rotatably supported in the shape of a pendulum suspended by the span band. By thus supporting the movable coil in a pendulum shape, the torque balance of the servo mechanism functions and the tilt angle can be measured.

The reflection type photoreflectors 20 and 21 are composed of a photodiode and a phototransistor. The photodiodes of the reflection type photoreflectors 20 and 21 output infrared rays toward the shutters 18 and 19, respectively.
The infrared rays reflected by 8 and 19 enter the base of each phototransistor, and the photocurrent Ic flows.

The photocurrent Ic of each phototransistor of the reflection type photoreflectors 20 and 21 changes as shown in FIG. 9 depending on the distance from the shutter. In the servo type tilt sensor, since the straight line portion of the relative photocurrent-distance curve is used, the gaps g1 and g2 are about 0.5 mm. In this portion, the photocurrent becomes small when the reflective photoreflector and the shutter are close to each other. Further, FIG. 10 shows a voltage-current curve representing the relationship between the collector-emitter voltage V _CE of the phototransistor and the photocurrent Ic, with the light amount Ee (mW / cm ² ) of infrared rays as a parameter.

The bridge circuit 22 includes the phototransistors Tra and Trd of the reflective photoreflectors 20 and 21, and
Resistors R1 and R2, moving coil 17, output resistor R
and power source Vs. A connection point between the emitters of the phototransistors Tra and Trd and a resistor R
The power supply Vs is connected between the connection point of 1 and R2. Also, the collector of the phototransistor Tra and the resistor R1
A series circuit of the movable coil 17 and the output resistor Ro is connected between the connection point of the moving transistor 17 and the connection point of the collector of the phototransistor Trd and the resistor R2.

In the bridge circuit 22, the collector-emitter voltage V _CE of each of the phototransistors Tra and Trd is set to V
_CE a, V _CE d, and the respective voltages of the resistors R1, R2 are V _A ,
If V _D , the output voltage Vo of the movable coil 17 is Vo.
= A _{V CE a-V CE d =} V A -V D. Further, the equilibrium condition of the bridge is V _A · V _CE d = V _D · V _CE a.

The operation of the servo type tilt sensor having the above structure will be described. When the case of the servo type tilt sensor is tilted to the right, for example, when the power is not turned on, the reflection type photoreflectors 20 and 21 are arranged as shown in FIG. Although tilted with the case, the shutters 18 and 19 remain horizontal until they strike the surfaces of the reflective photoreflectors 20 and 21. Then, when the power is turned on, the relative photocurrent Ia of the reflective photoreflector 20 increases and the voltage V _CE a decreases, while the relative photocurrent Id of the reflective photoreflector 21 decreases and the voltage V _CE d increases. . Therefore, V _CE d−V _CE a = −
Moving coil 17 depending on Vo or V _CE a−V _CE d = Vo
A current Io flows through the current Io, and a torque that rotates the movable coil 17 in the arrow direction is generated by the current Io.

The magnitude T of the torque is T = ± A ・ B ・ N ・
It is represented by Io. However, A is the area of the movable coil 17,
B is the magnetic flux density, and N is the number of turns of the coil. The movable coil 17 rotates until the torque T and the torque due to the inclination of the case are balanced, so-called torque balance occurs, and the gap g1 and the gap g2 are automatically controlled to be equal as shown in FIG.

When the servo type tilt sensor is horizontal, the output current Io oscillates alternately in the positive and negative directions, and the difference between the positive peak value Io ⁺ and the negative peak value I ^- is zero. As a result, the movable coil 17 is pulled in the X-axis direction. This state is called the state where the servo works, and shutters 18 and 19
Is ready to rotate following the rotation of the case.

When the case is tilted from this state, the voltage V _CE a of the phototransistor Tra and the phototransistor T are increased.
A difference occurs between the voltage rd and the voltage V _CE d, and a current Io due to this difference Vo flows through the movable coil 17, and as a result, the shutters 18 and 19 equalize the gaps g1 and g2 with the reflective photoreflectors 20 and 21. To rotate.

Therefore, even if the case is rotated by 0 to 360 degrees, the shutters 18 and 19 are the reflection type photoreflector 2.
Following the rotation of the case, the gaps g1 and g2 between 0 and 21 are kept equal. On the other hand, the output voltage Vo outputs a voltage corresponding to the tilt angle. This torque balance is due to the pendulum-like structure in which the center of gravity G of the movable coil is displaced.

As described above, the servo-type tilt sensor operates by the servo mechanism. In this case, the servo-type tilt sensor is made horizontal and the indicated value is set to zero. Then, it is tilted by a certain angle and then made horizontal again. When returned, the indicated value should return to zero, but why does the so-called zero change that does not return occur?

In order to elucidate the mechanism of occurrence of the zero change in the servo type tilt sensor, the conventional servo type tilt sensor has a horizontal angle of 10 °, a horizontal angle of 15 ° and a horizontal angle of 30 °.
Bridge circuit 22 for each case of inclination
FIG. 13 shows the results of measuring the voltage and current of each part of the.
The point of particular interest in this measurement data is V _CE a when horizontal.
And V _CE d are both 8.45 V, and Ia and I
d shows almost the same value as -4.23 mA and -4.18 mA.

Although the variation in the performance of the phototransistor is larger than that of a general transistor, the reason why the measurement result is such is that the movable coil 17 automatically selects the distance between the shutter and the reflection type photoreflector by the servo mechanism. Then, by making a difference in the relative light conversion rate of each reflection type photoreflector, V _CE a and V _CE d and I
It is considered that the operation is performed so that a and Id are equal to each other.

Based on the above measurement data, as shown in FIG. 10, a load line corresponding to each inclination angle is drawn on the V _CE -Ic curve. As a result, it can be seen that when the servo type tilt sensor is tilted, the operating point of the load line is deviated. This delusion of the operating point of the load line means that the load line does not return to its original position when the servo type tilt sensor is returned from the tilted state to the horizontal state. That is, this is considered to be the cause of the zero change.

In order to verify the above hypothesis, as shown in FIG. 14, the relative light conversion rate-distance characteristic curve Q1 of each of the left and right reflection type photoreflectors of the conventional servo type inclination sensor,
I drew Q2 symmetrically with respect to the center of the moving coil. In FIG. 14, the straight line m indicates a horizontal reference line in which the distances g1 and g2 are equal and both are 0.5 mm. Also, the straight line n is
When the servo type tilt sensor is tilted, the movable coil is tilted with respect to the horizontal reference line m by automatic control.

Here, the straight line n and the left and right characteristic curves Q1, Q
When straight lines that pass through the intersections q1, q2, q3, and q4 with 2 and are parallel to the horizontal reference line m are drawn, those straight lines fall within the shaded area. This area is a dead zone area, that is, an area where an operating point offset occurs. If the intersection points q1 to q4 are not at symmetrical positions, it is impossible to obtain zero by subtraction. Therefore, it is considered that the larger the width of the dead zone region, the larger the offset and the larger the shift of the operating point.

[0023]

DISCLOSURE OF THE INVENTION The present invention provides a digital tilt measuring device capable of performing ultra-precision horizontal level measurement and tilt measuring with the same measuring device, and also capable of direct mounting and remote measuring. The purpose of the present invention is to provide a servo-type tilt sensor that guarantees absolute horizontality without any change in zero, and thereby provides a digital tilt measuring instrument with an accuracy of 1% or less at a minimum reading value of 0.001 degrees. There is a technical problem to be solved in realizing.

[0024]

In order to solve the above-mentioned problems, a digital inclination measuring instrument according to the present invention comprises a movable coil (17) rotatably supported in a pendulum form by a span band so as to shift the center of gravity downward. And reflectors (18, 19) fixed to both sides of the movable coil (17), and reflectors (1
8 and 19) and fixed to the reflector (1
A pair of phototransistors (20, 21) for emitting light toward the reflector plate (18, 19) by reflected light and a pair of phototransistors (2
0, 21) and a bridge circuit (22) formed by two resistors (R1, R2). A pair of phototransistors (2
A moving coil (17) is connected between each of the resistors (R1, R2) and each of the two resistors (R1, R2). The first phototransistor (20) and the second phototransistor (2
1) a variable resistor (VR) connected between the variable resistor (VR), the center tap of the variable resistor (VR) and two resistors (R1, R).
A power supply (VS) is connected between the connection point of 2). First
The collector-emitter voltage of the phototransistor (20) and the second phototransistor (21) of
_CE a, V _CE d, the voltages on both sides of the center tap of the variable resistor (VR) are V1, V2, and one phototransistor (2
0) to one resistor (R1) and the voltage between the center tap of the variable resistor (VR) is Va, and the other photo transistor (21) to the other resistor (R2). When the voltage with the intermediate tap of the variable resistor (VR) is Vd, Va = V1 + V _CE a, Vd = V2 + V _CE d, and the variable resistor VR is adjusted so that Va = Vd.

In addition, in the digital inclination measuring device according to the present invention, after adjusting the variable resistor (VR), the variable resistance is adjusted by two fixed resistors having the same resistance values as those provided on both sides of the intermediate tap. Can replace the VR.

[0026]

As described above, Va = V1 + V _CE a, Vd = V
2 + V _CE d, and by adjusting the resistance values on both sides of the intermediate tap of the variable resistor (VR) so that Va = Vd, the minimum value of the output voltage is obtained, and the two phototransistors (20, 21) are It eliminates the delusion of the operating point of the load line drawn on the voltage-current curve, thus eliminating the zero change. After adjusting the variable resistor (VR), the variable resistor (VR)
Are replaced by two fixed resistors, each equal to the adjusted resistance on either side of the center tap. The digital tilt measuring device having the servo-type tilt sensor having the above-described configuration can measure the horizontality with a minimum reading value of 0.001 degree and an error of 1% with high accuracy.

When the case of the servo type tilt sensor is tilted, the phototransistors (20, 21) rotate following the case. Then, the one phototransistor (20, 21) tries to reduce the distance from the facing shutter, and the other phototransistor (20, 21) tries to increase the distance to the facing shutter. As a result, an imbalance occurs between the photoelectric conversion outputs of the two phototransistors (20, 21), and a current corresponding to the tilt angle flows through the movable coil (17).

The torque generated by this current balances the tilting torque of the case and, as a result, the moving coil (1
7) rotates following the case. The output current of the moving coil (17) is extracted and displayed as a voltage proportional to the angle. Therefore, the servo-type tilt sensor operates as a normal inclinometer. As described above, the digital inclination measuring device according to the present invention can perform high-precision measurement of horizontality as a level and can also perform normal inclination measurement as an inclinometer.

[0029]

Embodiments of the digital inclination measuring device according to the present invention will be described below. As shown in FIG. 1, a digital tilt measuring device according to the present invention includes a servo-type tilt sensor 1 having no zero change, which is a feature of the present invention, a chopper stabilizer-type differential amplifier circuit 2, an LP filter circuit 3, Linearization circuit 4, polarity inversion circuit 5, and VF conversion circuit 6
, A frequency divider 7, a counter circuit 8, a multiplexer 9, a decoder 10, a display circuit 11, a power transformer 12, and a voltage stabilizing circuit 13 as main components. In addition to the above, a transmitter / receiver 14 for telemetry is optionally provided.

The minimum reading of the digital tilt measuring instrument according to this embodiment is ± 0.001 degrees (3.6 seconds) at a temperature of 0 to 70 ° C.
This error is guaranteed within 1%. Also,
The temperature drift shows an astonishing value of 00 sec 04 / ° C. Such high performance is realized by the present invention for the first time without a zero change, a servo type inclination sensor 1, a differential amplifier circuit 2 with a small temperature drift, and an LP filter circuit 3.
It is due to.

The output of the servo-type tilt sensor 1 is 3 V / 30 degrees, 6.2 when the series resistance Ro of the moving coil is 8.2 KΩ.
V / 75 degrees, and the minimum value is 0.1 mV / 0.001 degrees. The differential amplifier circuit 2 is a chopper stabilizer amplifier using two operational amplifiers and has a temperature error of ΔVos /
ΔT = 0.01 to 0.05 μV / ° C. Selected from components.
If the temperature error is 0.02 μV / ° C, the increase in ΔVos will be 0.02 when the temperature rises from 25 ° C to 70 ° C by 45 ° C.
× 45 = 0.9 μV. Therefore, the measurement error of the minimum reading value 0.001 degree at the horizontal time is 0.9 μV / 0.1 mV = 0.9%
Is. This level value could not be obtained by the conventional servo type tilt sensor.

The LP filter circuit 3 removes the chopper noise generated in the differential amplifier circuit 2. The output voltage from the LP filter circuit 3 is converted by the linearization circuit 4 into an angular voltage in the range of ± 90 degrees by the voltage / angle curve 4a.

The polarity reversing circuit 5 inverts the polarity of the voltage depending on the direction of the inclination angle. The output voltage of the polarity inverting circuit 5 is converted into a frequency by the VF conversion circuit 6. The output of the polarity reversing circuit 5 is supplied to the display circuit 11 to form the sign of the angle.

The VF conversion circuit 6 has an angle of 0 to 74 degrees 59.
Minutes are converted at 6.2V, and angles 0 to 59 minutes 59 seconds are converted at 1V. The output of the VF conversion circuit 6 is supplied to the degree display portion 11a of the display circuit 11, and the frequency divider 7 outputs 3.6.
The frequency is divided into / 100 and supplied to the counter circuit group 8.
The counter circuit group 8 is composed of a decimal counter for each first digit of the indicated values of the angle minutes and seconds and a hexadecimal counter for each second digit of the indicated values of minutes and seconds. The count value of each counter of the counter group 8 is sent to the decoder 10 via the multiplexer 9. The decoder 10 decodes each count value of the minute and second of the angle and sends it to the display circuit 11.

The display circuit 11 digitally displays the angle in 6 digits (99 ° 59'59 ") according to the count value. The display circuit 11 has a portion for displaying degrees and a portion for displaying minutes and seconds. With the above configuration, the servo-type tilt sensor 1 without zero change is formed.
By using the differential amplifier circuit 2 having a small temperature error and the LP filter circuit 3, it is possible to realize a high-precision digital inclination measuring instrument that guarantees absolute horizontality within 1%.

Next, the configuration of the servo type tilt sensor 1 which is a feature of the digital tilt measuring device according to the present invention will be described in detail. In the servo type tilt sensor 1, like the conventional servo type tilt sensor described above, the fixed portion of the moving coil type instrument is fixed in the case, and the moving coil of the moving coil type instrument is displaced downward by a span band. It is rotatably supported like a pendulum, and reflectors 18 and 19 are fixed on both sides of the movable coil in the direction perpendicular to the span band direction.
The reflector plate 1 emits light toward 8 and 19 and is reflected by the reflected light.
A pair of reflection type photoreflectors 20 and 21 for detecting the distance to 8 and 19 are fixed in the case near the reflection plates 18 and 19, and the pair of reflection type photoreflectors 20 and 21 and two A bridge circuit 22 is formed by a resistor, and a movable coil is connected to the center of the bridge circuit 22.

The servo type tilt sensor 1 operates by the servo mechanism described above, and when the case is tilted, the imbalance of the outputs of the two reflection type photoreflectors 20 and 21 is fed back to the current of the moving coil. As a result, the torque is balanced, and the movable coil follows the rotation of the case.

A feature of the servo type tilt sensor 1 according to the present invention is that it has a bridge circuit 22 configured as shown in FIG. 2 so as to eliminate zero change.

In this bridge circuit 22, the emitters of the phototransistors Tra and Trd are connected to both ends of the variable resistor VR, and the intermediate tap of the variable resistor VR is set to 1 of the power source Vs.
Connect to the end. The other end of the power supply Vs is connected to the connection point of the resistors R1 and R2.

The voltages on both sides of the center tap of the variable resistor VR are V1 and V2, and the voltages on both arms of the bridge are V1 and V2.
Letting a and Vd be Va = V1 + V _CE a, Vd = V2 +
V _CE d. By adjusting the variable resistor VR,
The values of V1 and V2 may be adjusted so that the values of Va and Vd are equal, and as a result, the output voltage Vo may be minimized.
Adjustment of the bridge circuit 22 is extremely simple.

FIG. 3 shows voltage and current measurement data of each part of the servo type tilt sensor 1 having the bridge circuit 22 shown in FIG. In addition, Tra, Tr
d V _CE a-Ia straight line, V _CE d-Ia straight line, Va-Ia
The result of drawing the straight line and the Vd-Id straight line is shown in FIG.

As can be seen from this figure, in any load line group, the operating points a, d, and P remain stationary, and all the load lines are tilted around the operating point. That is, since the operating point does not move, each straight line returns to the original state even when the horizontal state → the inclined state → the horizontal state, and therefore zero change does not occur.

FIG. 5 shows the inclination angle-output voltage characteristics of the servo type inclination sensor 1 with the output resistance Ro as a parameter.

The digital inclination measuring device according to the present invention uses the servo type inclination sensor 1 and the chopper stabilizer type differential amplifier 2 having a small temperature drift as described above. It has excellent performance. (1) We have realized a digital inclination measuring instrument that has the functions of both a digital level and an inclination measuring instrument that guarantee absolute horizontality that did not exist in the past. (2) The minimum reading is 0.001 degrees, the error is ± 1.0%, the maximum reading is ± 90 degrees, and the error is ±
Ultra high accuracy of 0.001%. (3) As for the temperature characteristics, an astonishing value of 0 ″ 04 / ° C. was obtained. (4) The output of the servo-type tilt sensor 1 was highly stable ±
A voltage of 6.2 V / ± 90 degrees was obtained. (5) Since there is no zero change, the yield of the servo type tilt sensor 1 is high.

Furthermore, since the angle-output voltage characteristic of the digital inclination measuring device according to the present invention is 3V / 30 degrees, the minimum reading value 0.001 degrees is converted into a voltage of 0.1 mV. Therefore, if the diameter of the servo-type tilt sensor 1 is 30 mm,
Displacement due to rotation of 0.001 degrees is 15 × tan 0.001
° = 0.2618 μm. The converted value of voltage / displacement in this case is 381.97 μV / 1 μm, which is extremely high in accuracy.

Therefore, the digital inclination measuring instrument according to the present invention is not only used as a level gauge and an inclination measuring instrument transistor, but also as a highly accurate transit for displacement measurement in civil engineering, length measurement and surveying in mechanical engineering, etc. It can be widely applied to.

[0047]

As described above, the present invention realizes a servo type inclination sensor without a zero change with a simple structure, whereby a minimum reading value is 0.001 degree at a temperature of 0 to 70 ° C., for example. It is possible to realize a highly accurate digital level and tilt measuring instrument with an error of 1.0%. As a result, it is possible to achieve an extremely excellent effect of performing highly precise horizontal level measurement and tilt measurement with a single measuring device. Further, the present invention can be widely applied to measuring instruments for displacement measurement, length measurement, and surveying in a wide range of fields, and thus contributes greatly to various measurements.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of an embodiment of a digital inclination measuring device according to the present invention.

FIG. 2 is a circuit diagram of a bridge circuit of the servo type tilt sensor in the embodiment.

FIG. 3 is a table showing measured data by the bridge circuit.

FIG. 4 is a graph showing the operation of the servo-type tilt sensor based on the measurement data shown in FIG.

FIG. 5 is a graph showing a tilt angle-output characteristic of a servo type tilt sensor.

FIG. 6 is a sectional view of a conventional servo-type tilt sensor.

FIG. 7: Side view of moving coil

FIG. 8 is a circuit diagram of a conventional servo-type tilt sensor.

FIG. 9 is a graph showing photocurrent characteristics of a conventional servo-type tilt sensor.

FIG. 10 is a graph showing voltage-current characteristics of a conventional servo-type tilt sensor.

FIG. 11 is a partial sectional view showing a reflective photoreflector.

FIG. 12 is a partial cross-sectional view of a reflection type photoreflector that holds horizontal.

FIG. 13 is a table showing voltages and currents at various parts of the bridge circuit.

FIG. 14 is a graph showing each relative light conversion rate-distance characteristic curve of the left and right reflection type photoreflectors.

[Explanation of symbols]

1 ... Servo-type tilt sensor, 2 ... Differential amplifier circuit,
3 ・ ・ LP filter circuit, 4 ・ ・ Linearization circuit, 5 ・
・ Polarity inversion circuit, 6 ・ ・ VF conversion circuit, 7 ・ ・ divider, 8 ・ ・ Counter circuit group, 9 ・ ・ Multiplexer, 10 ・ ・ Decoder, 11 ・ ・ Display circuit, 12
..Power supply transformers, 13 ... Stabilization circuits, 14 ...
Transmitter, 15 ・ ・ Magnet, 16 ・ ・ Yoke, 17 ・ ・ Movable coil, 18,19 ・ ・ Shutter, 20,21 ・ ・ Reflective photoreflector (phototransistor), 22 ・ ・ Bridge circuit, L0-L4 ・
.Load lines, Tra, Trd ... Phototransistors,
V _CE a, V _CE d ·· Collector-emitter voltage, R
o, R1, R2 ··· resistor, VR · · variable resistor, V
s ... Power supply, Va, Vd, Eo ... Voltage, Io ...
Output current, Ia, Id ... Phototransistor current,
O ... the center of the moving coil, G ... the center of gravity of the moving coil,

─────────────────────────────────────────────────── ─── Continuation of the front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) G01C 9/06

Claims (2)

(57) [Claims] 1. A movable coil rotatably supported like a pendulum with a center of gravity shifted downward by a span band, reflectors fixed to both sides of the movable coil, and fixed in close proximity to the reflector. The bridge includes a pair of phototransistors that emit light toward the reflector and detect the distance from the reflector by the reflected light, and a bridge circuit formed by the pair of phototransistors and two resistors. In a digital tilt measuring instrument in which a moving coil is connected between each of a pair of phototransistors and each of two resistors constituting a circuit, the digital inclination measuring device is connected between a first phototransistor and a second phototransistor. And a power supply connected between the intermediate tap of the variable resistor and the connection point of the two resistors, the first phototransistor and the second phototransistor Selector - emitter voltage, respectively _{V CE a, V} CE
d, the voltage on both sides of the center tap of the variable resistor is V1, V
2. The voltage at the connection point between one phototransistor and one resistor and the intermediate tap of the variable resistor is Va, and between the connection point between the other phototransistor and the other resistor and the intermediate tap of the variable resistor. If the voltage is Vd, Va = V1 + V
_CE a, Vd = V2 + V _CE d, and adjusting the variable resistor VR so that Va = Vd. 2. The digital tilt measuring instrument according to claim 1, wherein after adjusting the variable resistor, the variable resistor is replaced by two fixed resistors each having the same value as the resistance value provided on both sides of the intermediate tap. .