JPH0143249B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an electronic scale that combines a first weight measuring device using a spring as a weight measuring element and a second weight measuring device using a load cell as a weight measuring element.

Generally, the accuracy of a scale is expressed as the ratio of sensitivity (minimum scale) to weighing capacity. For example, when the weight is 10 kg and the sensitivity is 10 g, the accuracy of the scale is 1/1000. However, in this case, the accuracy of 1/1000 is the maximum accuracy of the scale, which is the accuracy when weighing a 10 kg object. For example, when you measure a 100g object using a scale with a weighing capacity of 10kg and a sensitivity of 10g, the accuracy is only 1/10. In this case, if you want to measure an object weighing less than 100g with an accuracy of 1/1000, you must use a scale with a weighing capacity of 100g and a sensitivity of 0.1g, for example.

In this way, scales are most accurate when measuring objects with a weight close to the weighing capacity.
It is necessary to prepare several scales depending on the weight of the object to be measured.

On the other hand, electronic scales using conventional load cells,
The structure was such that the weighing pan and the load cell were directly connected by a mechanical member. In such a structure, the electrical output of the load cell is a combination of mechanical and electrical factors. Therefore, even in an unloaded state with no object to be weighed on the weighing pan, the weight of the mechanism acts on the load cell, and an offset exists in the load cell and the electric circuit that amplifies or calculates its output. Therefore, it was not possible to provide automatic adjustment means for weighing value and zero point display.
In particular, when load cells are used in general public scales with small weighing values, the problem is serious because the electrical output level is so small that it is comparable to the noise level, and the lowest digit is displayed digitally. .

The main purpose of the present invention is to combine the advantage of springs, which have stable characteristics under small loads near zero, and the advantage of load cells, which have problems with offset but have excellent linearity of output signals with respect to load. Therefore, it is an object of the present invention to provide an electronic scale that can measure with high precision even when the weight of an article to be weighed is smaller than the weight of the weighed article without performing any switching operation.

Another object of the present invention is to provide an electronic scale that reduces the influence of load cell offset signals as much as possible.

In order to achieve the above object, the electronic scale of the present invention has the following features:
When the weight is relatively small, only the spring scale functions, and when the weight is relatively large, the spring is used as an element so that the expansion of the spring scale is suppressed to a constant value and the scale using the load cell functions. It is characterized by a combination of a first weight measuring device and a second weight measuring device having a load cell as an element.

Furthermore, the electronic scale of the present invention stores the offset value of the load cell electrical system when only the spring scale functions and the load cell is in an unloaded state, and corrects this offset value from the output of the second weight measuring device. It is characterized by displaying

The load cell in the present invention is a general term for a device that directly converts pressing force into an electric signal, and includes a so-called strain gauge, which is an electric resistance attached to or embedded in an elastic member, as well as a magnetostrictive type and a capacitor type.

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a diagram showing an embodiment of the mechanism section of the present invention.

A Roberval mechanism is supported on fulcrums a and b on a column 2 that stands upright on the base 1, and by this mechanism, a dish support rod 4 directly connected to the weighing pan 3 is supported by the fulcrums a' and b' so as to be able to move up and down, A tension spring 6 is provided between the upper end arm 2A of the support column 2 and the lower end arm 4A of the dish support rod 4, and the tension of this spring can be adjusted by a spring hook 7 and a tension ball 8. In addition, in order to detect the amount of displacement of the plate receiving rod 4, a rack rod 9 integrally connected to the plate receiving rod 4 and a pinion 1 engaged with this rack rod are used.
A disk 11 is provided coaxially with the pinion 10 and a photoelectric sensor 12 that reads a digital code marked on the outer periphery of the disk 11 to convert the displacement of the rack rod 9 into a digital code signal. That's what I do.

Although the digital code conversion mechanism described above is an example of a mechanical displacement magnification device for convenience of explanation, the present invention can be implemented by an optical displacement magnification device or an image sensor that does not particularly magnify displacement.

On the other hand, a pressurizing part 13 is provided at the lower end of the plate receiving rod 4, and a gap is provided below the pressure part 13 to form a load cell 1.
4 is arranged so that when the plate receiving rod 4 is displaced downward, the pressure portion 13 of the plate receiving rod comes into contact with the pressure receiving portion 15 of the load cell. Furthermore, a detector 17 is provided which is activated by a contactor 16 that moves together with the plate receiving rod 4, and when the pressure part 13 comes into contact with or comes close to the pressure receiving part 15, the contact of the detector is turned ON and the load cell is disabled. It is designed to emit a signal to determine whether it is in a loaded state or not.

In such a configuration, when the weight of the object on the weighing pan 3 is zero or less than a predetermined value, the pressure part 13 of the pan support rod 4 does not press the pressure receiving part 15 of the load cell yet, as shown in FIG. , pressure applying section 13 and pressure receiving section 1
There is a gap in 5, and the contact of detector 17 is OFF.
in a state. The tension spring 6 exhibits a displacement that is balanced with the load acting on the spring, and this amount of displacement is converted into an electrical signal by the photoelectric sensor 12. When the weight of the object on the weighing pan 3 exceeds a predetermined value K, as shown in FIG.
5 and presses the pressure receiving part 15, and the load cell 1
4 is in a loaded state, and the contact of the detector 17 is in an ON state. In this state, the displacement of the tension spring 6 remains stuck at the upper limit, and the load cell 14
A force obtained by subtracting the force exerted by the tension spring from the weight of the object to be weighed and the weighing mechanism acts as a load on the weighing body. The relationship between the opening/closing of the contact of the detector 17 and the load/unload of the load cell 14 is adjusted so that when the weighing value increases, the contact closes first, and then the load is applied to the load cell with a delay. .

FIG. 3 shows an embodiment of the electronic circuit section of the present invention.
In the figure, 14 is a load cell, 17 is a detector,
18 is an amplifier, 19 is an A-D converter, 20 is an
21 is a data latch, 12 is a photoelectric sensor, 22 is an arithmetic unit, 23 is a display unit, and 24 is a code converter. The digital code signal obtained from the photoelectric sensor 12 is a code such as a gray code that cannot be treated as a binary number as it is, so it is converted to a binary number by the code converter 24 and the first
is inputted to the calculation section 22 as the output A of the weight measuring device. The output signal of the load cell 14 is sent to the amplifier 18
After being amplified by A-D converter 19 and digitally converted by A-D converter 19, the data is introduced into calculation unit 22 as output B of the second weight measuring device. The data latch 21 reads the contents of data B and updates the stored contents only when the data update signal E is input, and the stored contents are always updated by the offset signal G.
It is introduced into the arithmetic unit 22 as follows. When the contact of the detector 17 is OFF, that is, when the load cell 14 is in an unloaded state, the discrimination signal D is at H level and the AND gate 20 is opened. A data update signal E is output in synchronization with a conversion end signal C issued by the -D converter 19. The calculation unit 22 adds the second measurement data D _B from the load cell 14 to the first measurement data D _A from the photoelectric sensor 12 and obtains the latest offset value G stored in the data latch 21 .
, and outputs the calculated value H, which is displayed on the display unit 23. The data latch 21 is backed up by a battery (not shown) and retains its contents even when the commercial power is turned off.

Next, the operation of the structure of the above-mentioned mechanism section and electronic circuit section will be explained.

Generally, in electronic circuits, the unnecessary DC output that appears when there is no signal is called offset, and it is usually expressed as a value converted to the input side. In the present invention, when the load cell 14 is separated from the mechanism section and is in an unloaded state, the A-D conversion output data B should originally indicate a value corresponding to 0 load, but the load cell, the amplifier, and the Since there is an offset in the circuit system including the D converter, the digital data B normally indicates a small measured value even when the detector 17 is off. The data latch 21 always reads the latest offset value B and stores it when the power is turned on and the load cell 14 is unloaded.
Even when a load is applied to the load cell 14 or the supply of commercial power is cut off, the final offset value B is memorized and retained.

When the power is turned on and there is nothing on the weighing pan, the gate 20 opens and instructs the data latch 21 to latch signal B, and the A-D converter 1
9 outputs only the offset D _pff and introduces it into the data latch 21 and the calculation section 22. Therefore, the calculation unit 22 calculates H ₀ =(value related to signal B)−(value related to signal G)=D _pff −D _pff =0, and the display unit 23 displays 0. Even if a drift occurs and the value of D _pff changes, the displayed value always remains 0.

Next, an object of weight W ₁ is placed on the weighing pan 3,
If this W ₁ is less than the predetermined value K, the load cell 1
4 still maintains an unloaded state, the tension spring 6 is displaced and the disk 11 is rotated by an angle that balances with the weight _W1 , and the photoelectric sensor 12 detects this and the calculation unit 22
Introduce metric data D _A indicating weight W ₁ to . Therefore, the output H ₁ of the calculation section is H ₁ =D _A +D _pff -D _pff =D _A , and displays the weighed value obtained only by the first weight measuring device, which is not affected by the offset.

When the weight of the object on the weighing pan 3 is exactly the predetermined value K,
The tension spring 6 is in equilibrium in a state where it is stretched to the upper limit, and the pressure applying part 13 contacts the pressure receiving part 15 with a pressing force of 0.

Next, an object of weight W ₂ is placed on the weighing pan 3,
If this W ₂ exceeds a predetermined value K, a load of (W ₂ −K) acts on the pressure receiving part 15 of the load cell 14, and the corresponding electrical output is output from the A-D converter as a data value D _B This naturally includes the offset signal D _pff . Further, the displacement of the tension spring 6 at this time is stuck at the upper limit value, and the data input A to the calculation section 22 represents the predetermined value K. Therefore, the calculation unit output H ₂ is the true weighing value W ₂ with the offset and predetermined value corrected, as H ₂ =D _B −D _pff +K = (W ₂ −K+D _pff )−D _pff +K = W ₂ is displayed.

Furthermore, if the power supply is cut off while an object such as a tare is placed on the weighing pan and the power is turned on again, the offset value immediately before the tare or other object was placed on the weighing pan is stored in the data latch 21. The weight of the object such as a tare bag is displayed after correcting the predetermined value. In this case, there may be a slight variation between the latest offset value stored in the data latch 21 and the current offset value, but the offset value itself is an extremely small value compared to the weighed value, and the data latch Since the data stored in is the latest, this is the most reliable correction method possible for the device. If an object such as a tare is removed in this state, the contents of the data latch 21 will be immediately updated to the current offset value, and the display will definitely represent zero.

FIG. 5 shows the relationship between the weight W of an object placed on the weighing pan and the output H of the calculation section. In the figure, the case where the offset is negative is illustrated, and the digitization error caused by AD conversion is omitted. The solid line olm represents the output a from the first weight measuring device whose main element is the tension spring 6. Also, a thin dashed line
pqr represents the calculation section output for only the digital conversion output signal b, and the two-dot chain line (b-g) represents the value obtained by correcting the offset value from the signal b, that is, the calculation section output for the load acting on the load cell,
This becomes the output from the second weight measuring device whose main element is the load cell 14. Therefore, the sum of the first output a from the tension spring and the second output (b-g) from the load cell is the thick broken line (a+b
-g), it is connected to the first output a, and represents the true output of the arithmetic unit with the offset corrected.

Note that the scale of the digital display can be switched depending on whether the weight of the object on the weighing pan is below a predetermined value K or exceeds K.

According to the present invention, there are the following effects.

When the weight of the item to be weighed is relatively small, a spring scale that is stable and sensitive near zero works effectively.
Since measurement automatically shifts to load cell measurement when the weight becomes relatively large, it is possible to accurately measure even small weights compared to the weighing capacity (full scale), and a single scale can be used over a wide range of areas. Can be done.

Since the load transmission mechanism section and the electronic circuit section after the load cell are completely separated, the offset value of the electronic circuit section including the load cell can be accurately recognized and corrected.

When the weight is relatively large, the offset value is updated every time the weight of the object on the weighing pan falls below a predetermined value, so that the most accurate correction calculation is always performed using the latest offset value.

[Brief explanation of drawings]

FIG. 1 shows an embodiment of the mechanical section of the present invention. FIG. 2 shows one aspect of the usage state of FIG. 1. FIG. 3 is a block diagram of an embodiment of the electronic circuit section of the present invention. FIG. 4 is an explanatory diagram of the operation of the present invention. 1... Base, 2... Support, 3... Weighing pan, 4...
...Plate support rod, 6...Tension spring, 9...Rack rod, 1
0... Pinion, 11... Disc, 12... Photoelectric sensor, 13... Pressure section, 14... Load cell, 1
5...Pressure receiving part, 17...Detector, 19...A-D
Converter, 21...Data latch, 22... Arithmetic unit.

Claims (1)

[Claims] 1. A first weight measuring device that converts the displacement of a scale mechanism engaged with a spring into an electrical signal, and a second weight measuring device that converts the load on a load cell caused by the scale mechanism into an electrical signal. When the scale mechanism is not pressing the load cell, only the output of the first weight measuring device is taken as the measured value, and when the scale mechanism is pressing the load cell, the output of the first weight measuring device is the measured value. An electronic scale characterized in that the sum of the outputs of the first and second weight measuring devices is used as the measured value. 2. A first weight measuring device that converts the displacement amount of the scale mechanism engaged with the spring into an electrical signal, a second weight measuring device that converts the load of the load cell caused by the scale mechanism into an electrical signal, and a second weight measuring device that converts the load of the load cell caused by the scale mechanism into an electrical signal; a detector for determining whether the load cell is in the no-load state or the no-load state; a storage means for storing an offset value of the load cell when the load cell is in the no-load state; and outputs of the first and second weight measuring devices and the offset value of the load cell. and an arithmetic unit that calculates a weighing value to be displayed from the value, and when the load cell is determined by the detector to be unloaded, the output of the second weight measuring device is stored and updated as an offset value. , when the detection unit determines that the load cell is in a loaded state, the calculated value is obtained by adding the outputs of the first and second weight measuring devices and subtracting the offset value while maintaining the updated offset value. The electronic scale according to claim 1, characterized in that the electronic scale is configured to display.