JPH0146810B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a force measuring device, and particularly to a device that measures force by converting it into an electrical signal.

Conventionally, there is a so-called load cell as a force measuring device as described above. This method involves attaching a strain gauge to the surface of a strain-generating elastic body that distorts when subjected to force, and measuring the change in resistance of this strain gauge to measure the magnitude of the force. It is possible to measure forces of various magnitudes by changing the , but since there are large errors due to the physical properties of the strain-elastic body, such as thermal expansion, it is necessary to compensate for this, which is technically difficult. In addition, there was a problem in that the costs involved increased and the product price became expensive.

SUMMARY OF THE INVENTION An object of the present invention is to provide a force measuring device that can compensate for errors caused by thermal expansion without providing a compensation circuit and can measure force with high accuracy and a simple configuration.

Hereinafter, the present invention will be explained in detail based on two illustrated embodiments. The first embodiment has a main strain elastic body 1 and a sub strain elastic body 2, as shown in FIG. It is fixed on the table 4 with bolts 5,5. That is, both the strain elastic bodies 1 and 2 are of a cantilever type. Both of these strain elastic bodies 1 and 2 are made of the same material or materials having the same temperature coefficient, and are formed into shapes in which the bending portions (strain components) 1a and 2a have the same maximum stress. Note that 6 is an insulating material for insulating both the elastic bodies 1 and 2.

The other ends of both elastic bodies 1 and 2 are connected by a string 7, which has an effective length l equal to the length l of the member 3 and has the same coefficient of linear expansion as the member 3. It is made of a material that has

When a downward load W is applied to the other end of the principal strain elastic body 1, a deflection Δl1 proportional to the load W is generated in the principal strain elastic body 1, as shown in FIG. pull downward. Tension P applied to string 7
acts on the other end of the secondary elastic elastic body 2, causing the other end to bend downward by Δl2. Here, if the spring constant of the primary strain elastic body 1 is K1 and the spring constant of the secondary strain elastic body 2 is K2, and the elongation of the string 7 is ignored, then P=Δl2・K2 holds, and Δl1=Δl2=Δl Therefore, W=Δ1(K1+K2) P=W・K2/(K1+K2) It can be seen that the tension P is proportional to the load W.

A magnetic field is applied to the string 7 perpendicularly to its length direction by a magnetic field generator 8 provided on the principal strain elastic body 1, and the string 7 is connected to an amplifier 9 as shown in FIG. Therefore, string 7 vibrates. That is,
When string 7 bends slightly in the direction of cutting the magnetic field due to the applied load, string 7 bends according to Fleming's right-hand rule.
This current is supplied to the amplifier 9 via the capacitor 10 and amplified, and the amplified output is supplied to the string 7 via the resistor 11. This output flows in a direction that causes string 7 to further bend in the same direction,
The string 7 is further bent in the direction of cutting the magnetic field. The string 7 is bent to a position where the energy applied from the amplifier 9 and the bending reaction force of the string 7 are balanced, and then returns in the opposite direction. This causes a current in the opposite direction to flow through the string 7, and this reverse current is supplied to the amplifier 9 via the capacitor 10 and amplified, and an amplified reverse current is supplied to the string 7. Flex string 7 in the opposite direction. After that, this is repeated at the frequency f
vibrates. This frequency f is is required. However, n is the harmonic number of vibration, l
is the effective length of the string 7, g is the gravitational acceleration, and r is the mass per unit length of the string 7. Therefore, by measuring the frequency f, the tension P can be measured, and from this, the load W can be determined. A circuit for measuring the frequency f is shown in FIG. In the same figure,
12 is an oscillator including the circuit shown in FIG. 3; 14 is a frequency counter; and 16 is a time gate, which controls the frequency counter 14. Reference numeral 18 denotes a calculation section which performs calculations of actual load, zero adjustment, tare subtraction, etc. based on the counter output of the frequency counter 14. 20 is an actual load display section.

In the force measuring device configured in this way, if the temperature coefficient of the primary strain elastic body 1 is α1 and the temperature coefficient of the auxiliary strain elastic body 2 is α2, the tension P is as follows: P=W・K2 (1+α2)/K1 (1+α1
)+K2(1+α2). Since the main and sub elastic bodies 1 and 2 are made of the same material or are made of materials whose elastic coefficients change with the same temperature, α1 = α2, and the distance between the main and sub elastic bodies is 20 to 30 mm. Therefore, the temperature conditions are the same. Therefore, P becomes P=W·K2/K1+K2, and complete temperature compensation is achieved.

In addition, distortion remains in the main and auxiliary strain elastic bodies 1 and 2 even after the load is removed (this is called hysteresis), but this strain is caused by the deflection Δl1, Δl2 of the main and auxiliary strain elastic bodies 1 and 2. This affects the tension P, but the magnitude of hysteresis is the same regardless of the size of the shape when the stress of the flexible portion is equal. Therefore, the flexible parts 1a and 2 are made of the same material.
In this force measuring device in which the main and sub strain elastic bodies 1 and 2 are formed so that the stress applied to a is equal, hysteresis can be canceled out. The same can be said about creep. In other words, the amount of creep is defined as a function of the stress applied to the elastic body and time,
Although each material has a unique value depending on the internal structure after heat treatment, the main and sub-strain elastic bodies 1 and 2 are made of the same material, and the main and sub-strain elastic bodies are shaped to have the same maximum stress. This is offset by forming the elastic bodies 1 and 2.

Furthermore, since the effective length of the string 7 and the length of the member 3 are made the same and both are made of materials with the same coefficient of linear expansion, the linear expansion is relatively the same and changes in the tension P are suppressed. I'm setting it to zero. Note that even if the linear expansion coefficients are not the same, the member 3 may be made of a material and have a length such that the relative linear expansion becomes zero.

In the second embodiment, as shown in FIG. 5, a known balamegram type elastic body is used as the principal strain elastic body 1, and a measuring pan 22 is provided at the tip of the principal strain elastic body 1. It is something. Note that the same parts are given the same reference numerals and the description thereof will be omitted.

As described above, the force measuring device according to the present invention has a simple configuration consisting of the main strain elastic body 1, the auxiliary strain elastic body 2, the string 7, etc., and the dimensions of the principal strain elastic body 1 can be changed. This makes it possible to measure loads of any magnitude. Moreover, the string 7 and the secondary elastic elastic body 2
and member 3 can be of the same size regardless of the magnitude of the load to be measured, and standards can be standardized. Furthermore, since the effective length of the string 7 and the length of the member 3 are the same l, and are made of the same material or material with the same linear expansion coefficient, no compensation circuit is required to compensate for errors due to changes in the linear expansion coefficient. Compensation can be performed without the need for a force measuring device, and a highly accurate force measuring device can be realized at low cost.

In the above embodiment, the tension P was measured using the string 7, but any device that can sense force may be used instead, such as a crystal sensor, a fork sensor, or the like. In addition, although the force detector is configured to apply tension, the main strain elastic body 1 and the secondary strain elastic body 2
The configuration may be such that pressure is applied to the force detector by swapping the positions of the two.

[Brief explanation of drawings]

FIG. 1 is a side view of a first embodiment of the force measuring device according to the present invention, FIG. 2 is a principle diagram of the first embodiment,
FIG. 3 is a diagram of the principle of string vibration in the first embodiment, FIG. 4 is a circuit diagram of the first embodiment, and FIG. 5 is a side view of the second embodiment. 1...Primary strain elastic body, 2...Secondary strain elastic body, 3
... Member, 4 ... Fixed part, 7, 8 ... Force detector.

Claims (1)

[Claims] 1. A cantilever-type primary strain elastic body with a fixed base end, and a cantilever-type secondary strain elastic body with a fixed base end that is spaced apart from the primary strain elastic body. Both the strain elastic bodies are connected so that when the main strain elastic body is deformed by receiving a force, the secondary elastic body also shares the force, and the combined restoring force of both the strain elastic bodies is and a force detector that balances the force with the force and detects the force applied to the secondary elastic body, and that deforms little when receiving the force, and the proximal end of both the strain elastic bodies is connected to the force detector. A force measuring device characterized in that both base end portions are fixed by being fixed to a fixed portion via a member having an effective length, and the linear expansion coefficients of the force detector and the member are made equal.