JPH0835891A - Inner force sensor - Google Patents

Abstract

PURPOSE:To automate the bridge balance adjustment of an inner force sensor. CONSTITUTION:When the initial value of an index (k) which represents a set value to a D/A converter 205 is set to '0' and its process is started, a CPU 206 sets a digital value (k) in a register in the converter 205. Since a certain voltage value Vk is output to an amplifier 202, the CPU 206 obtains an output value alphak corresponding to the value Vk from an A/D converter 203. The alphak represents the amplitude of a deviation from the balanced state of a bridge circuit 201. The CPU 206 temporarily stores data set in a RAM 207. Then, these process is repeated until the value (k) reaches the maximum value kmax settable by the converter 205. For the data set of the stored kmax+1 set, the k value k0 for applying the minimum alphaa is obtained, and set to the converter 205. Thus, the state for applying the smallest bridge output is realized.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a force sensor, and more particularly to a force sensor used in the field of measuring force or torque using a strain gauge. INDUSTRIAL APPLICABILITY The force sensor of the present invention is applied to, for example, an application for measuring stress of a structural element of a building or an application for mounting a force control robot between a wrist and an end effector to measure force and torque. .

[0002]

2. Description of the Related Art A force sensor is a type of detection that converts a stress of a strain gauge or the like into a change of an electric resistance value (hereinafter simply referred to as "resistance value") in a mechanical elastic body made of metal or the like. It is a sensor that detects the force or torque acting on the elastic body as a change in resistance value by combining the devices. In this way, when using a detector that uses a conversion element that converts a measured amount into a minute change in resistance value, a change in resistance value corresponding to the measured amount (stress in a strain gauge) is extracted as a voltage value. Therefore, a method using a Hoystone bridge incorporating the conversion element is widely adopted.

In this system, one or more of four resistors of a Hoystone bridge (hereinafter, simply referred to as "bridge") are represented by the conversion element (hereinafter, strain gauge). ) Is replaced with a bridge configuration, and an excitation voltage is applied to the bridge, whereby the electrical resistance change of the strain gauge is output as a voltage value.

The advantage of this method is that the component corresponding to the original resistance value of the resistor (strain gauge) forming the bridge is not output as a voltage value, but only the minute change in the resistance value is output as a voltage value. Moreover, the output is proportional to the resistance change amount. However, as a premise, it is required that the four resistors forming the bridge have the same resistance value (original resistance value for the strain gauge).

However, since it is practically impossible to make the resistance values of the four resistors equal, it is practically possible to use a variable resistor so that the output of the bridge should be zero (for example, the applied stress is zero). State, hereinafter referred to as "reference state".)
The resistance value of the variable resistor is adjusted so that the output of the bridge becomes 0 below.

FIG. 1 is a diagram for explaining balance adjustment of a bridge using such a variable resistor. In the figure, reference numerals 1 to 5 represent resistors forming a Wheatstone bridge. In this example, resistors 1 and 2 are
It is a strain gauge attached to an elastic body W such as metal,
When strain due to stress occurs, the resistance value is the original resistance value R
Change from 1, R2. The resistors 3 and 4 have a resistance value R
A fixed resistor having 3 and R4. And resistor 5
Is a variable resistor incorporated to perform balance adjustment.

The resistance values R1 to R5 of the respective resistors 1 to 5 are selected so that the following relationship is already established at least approximately in the state where balance adjustment is not performed. R1 = R2 = (R3 + R5 / 2) = (R4 + R5 / 2) ... [1] When the excitation voltage Ve is applied to this bridge, the voltage Vo is output. The balance adjustment of the bridge is carried out such that this voltage Vo is zero (Vo = 0) under the reference conditions.

[0008]

As described above, in the prior art, the balance adjustment of the bridge is most commonly performed by using a variable resistor. However, the balance adjustment by such a variable resistor is performed. When the bridge for performing the above is adopted as the force sensor, the following problems occur.

(1) In order to perform balance adjustment, it is necessary to manually adjust an adjusting member (knob, screw, etc.) for adjusting the resistance value of the variable resistor (see reference numeral 5 in FIG. 1). Often, the sensor's detector is not installed in a location that is easily accessible by people (eg, inside a building).

In particular, in the case where the reference state must be frequently changed according to the stress applied to the strain gauge and the balance adjustment must be performed many times, the variable resistor can be manually adjusted each time. What is required. Such a situation is not only extremely inconvenient, but in some cases, imbalance adjustment is impossible.

(2) Since the manual adjustment of balance makes it easy to make individual differences in adjustment accuracy, the objectivity and reliability of measured values are limited.

SUMMARY OF THE INVENTION An object of the present invention is to overcome the above problems and provide a force sensor capable of automatically adjusting the balance of a bridge without manual operation. Further, it is to improve the ease of use of the force sensor and the objectivity and reliability of the measurement result.

[0013]

SUMMARY OF THE INVENTION The present invention includes "at least one resistor attached to a mechanical elastic body, the resistance value of which changes according to the stress acting on the elastic body,
A force sensor comprising a Hoystone bridge circuit for extracting a change in the resistance value due to the stress as a voltage value, and a balance adjusting unit for performing balance adjustment of a bridge output of the Hoystone bridge circuit. Is a D / A converter that evenly divides the output voltage of the Hoystone bridge circuit within a certain range and outputs an arbitrary voltage within the range as an actual output voltage, and a digital setting of the D / A converter. Means for sequentially switching the inputs, means for determining a digital setting input value that gives the minimum value of the bridge output under the sequentially switched digital setting input, and the determined digital setting input value for the D / A
The above-mentioned technical problem is solved by the above-mentioned force sensor characterized in that it is provided with a means for setting in the converter.

[0014]

The present invention is basically based on the use of a DA converter instead of the variable resistor which has been conventionally used as the balance output adjusting means for the bridge output of the Hoystone bridge circuit equipped in the force sensor. There are various characteristics. This D / A converter is connected to a position where the output voltage of the Hoystone bridge circuit is evenly divided within a certain range, and any of the voltages is output as an actual output voltage. The value of the bridge output after the division adjustment can be adjusted by the digital setting input of the D / A converter.

The digital setting input value is sequentially switched by a control device equipped with, for example, a microprocessor and a memory. Then, the balance setting of the bridge output is achieved by obtaining the digital setting input value that gives the minimum value among the bridge outputs under the sequentially switched digital setting inputs and setting it in the D / A converter. .

When the reference state for which zero output is desired to be changed is changed, if the above-described balance adjustment processing is re-executed for the force sensor placed in the changed environment, optimum balance adjustment in the changed reference state is achieved. To be done.

As described above, in the force sensor of the present invention,
Since the balance adjustment of the bridge can be automated, the restriction of the installation location of the force sensor is remarkably relaxed, and the balance adjustment is difficult even when the reference state (the state where the measured value is 0) is frequently changed. Not be. In addition, instability factors such as individual differences in balance adjustment accuracy disappear, and the objectivity of measurement
Improves reliability.

[0018]

FIG. 2 shows the basic construction of a D / A converter used as a bridge balance adjusting means in a force sensor according to the present invention. To explain this, first, the symbols D0 to Dn represent digital input signals. When the signals D0 to Dn are input to the register designated by the reference numeral 6, the register 6 causes the logical states D0 'to ...
Temporarily store Dn '.

This temporary storage can be performed at any timing. Temporarily stored logic states D0 'to Dn'
Are switches S0 to Sn connected to the subsequent stage of the register 6.
Used to control the. For example, when the i-th digital signal Di has a positive logic, the i-th switch Si is connected to the upper line in the circuit display in the figure, and conversely, when it has a negative logic, it is connected to the lower line.

The upper line and the lower line of each of the switches S0 to Sn are connected to terminals 8 and 9, respectively. Further, the resistance value R is provided on the rear side of each switch S0 to Sn
And a resistor network 7 composed of an element group having 2R are connected to connect the switches S0 to Sn to the terminal 10.

If a potential Va is applied to the terminal 8 and a potential Vb (> Va) higher than the potential is applied to the terminal 9, the digital signals D0 to Dn and the register 6 are controlled to control the terminal 10. The potential difference ΔV = Vb−Va is 2 ⁿ
It is possible to output an arbitrary potential Vc divided by -1.
That is, the potential Vc of the terminal 10 is Vc = Va + k. (Vb-Va) / ( ^2n- 1) ... [2]. However, k is an arbitrary integer of 0 ≦ k ≦ 2 ⁿ .
4 and subsequent figures, the D / A converter shown in FIG. 2 will be represented by the abbreviation symbols shown in FIG.
The terminals designated by reference numerals 8 ', 9'and 10' in FIG.
Corresponding to terminals 8, 9 and 10 in FIG. 2, the output voltage of the bridge is applied to terminals 8'and 9 '
0'is evenly divided within a certain range, and an arbitrary voltage within the range is output as an actual output voltage. FIG. 4 shows an outline of the circuit configuration of the bridge of the force sensor according to the present invention, which uses the D / A converter having the basic configuration shown in FIG. In the figure, reference numerals 101 and 10
Reference numerals 2, 103 and 104 denote resistors that form a bridge, and strain gauges are used for the resistors 101 and 102 in this example. The strain gauges 101 and 102 are attached to the elastic body W ′ such as metal as in the case of FIG. 1. Also, the resistance values of each resistor R1 ', R2', R
The method of selecting 3 ′, R4 ′, and R5 ′ is also the same as that described in the related description of FIG. 1 (see the above-mentioned formula [1]).

In the present embodiment, the amplifier 10 whose potential (intermediate output) across the resistor 105 is an operational amplifier, which is usually called a voltage follower, and has a voltage amplification factor of 1.
6 and 107, and input to terminals 8'and 9'corresponding to terminals 8 and 9 of the D / A converter shown in FIG. The terminal 10 '(corresponding to the terminal 10 in FIG. 2) is one of the bridge output terminals.

The amplifiers 106 and 107 increase the apparent input impedance of the D / A converter and lower the apparent output impedance of the bridge circuit, and
It is installed so that the internal circuit of the A converter and the bridge circuit do not affect each other. In addition, this amplifier 1
A configuration in which the portions corresponding to 06 and 107 are included in the internal circuit of the D / A converter is also conceivable.

With such a connection, it is possible to obtain the split-adjusted bridge output voltage Vo '. The value that the bridge output voltage Vo 'can take is discrete, and the adjusting capability of the bridge balance depends on the fineness of the discrete value.

Since the fineness of the step of the discrete value has been described with reference to FIG. 2 (see the above equation [2]),
It can be arbitrarily set by selecting the bit number n of the digital signals D0 to Dn. That is, by designing a sufficiently large n corresponding to the required adjustment ability, a bridge circuit having the ability to perform balance adjustment with satisfactory smoothness can be incorporated in the force sensor.

Next, FIG. 5 shows an example of the structure of a force sensor system in which the bridge circuit shown in FIG. 4 is incorporated so that the balance of the bridge can be automatically adjusted.
The entire force sensor system is a central processing unit (hereinafter referred to as “CP” including a bridge circuit 201, an amplifier 202, an A / D converter 203, and a microprocessor shown in FIG.
Say U ”. ) 206 and volatile memory (hereinafter referred to as “RA
M memory ”. ) 207, non-volatile memory (hereinafter,
It is called "ROM memory". ) 208.
Then, the RAM memory 207, the ROM memory 208, A
The digital signal input terminal of the D / A converter 203 and the D / A converter 205 for balance adjustment of the bridge circuit 201 (FIG. 2D0
To Dn) are connected to the CPU 206 via a bus line 204 indicated by a thick line.

The output voltage from the bridge circuit 201 is amplified by the amplifier 202 and input to the A / D converter 203. The A / D converter 203 converts an analog value converted into a voltage representing a force or torque exerted on the force sensor into a digital value and outputs the digital value to the bus line 204.

The CPU 206 performs a process for controlling the entire system, a process for effectively utilizing the force or torque data detected by the bridge circuit 201 according to an application, and a process for adjusting the bridge balance. To do. Since the former process is not particularly related to the present invention, the description of the contents is omitted here. RAM2
A register for storing an index k representing a digital value set by the CPU 206 in the D / A converter 205 is set in 07. The RAM 207 is also used for temporary storage of other data necessary for the CPU 206 to execute processing. The ROM 208 stores a program for causing the CPU 206 to execute each of the above processes together with required setting values.

An example of a processing procedure for adjusting the bridge balance in the force sensor system according to this embodiment will be described below with reference to the flowchart of FIG. The bridge balance adjustment process is executed by the following process procedure while the force sensor is kept in a desired state as a reference state (for example, a no-load state or a state in which only gravity acts). When the processing is started with the initial value of the index k representing the set value for the D / A converter 205 set to 0, first, CP
U206 sets the digital value k in the D / A converter 205 (step S1). This set value k corresponds to k in the above-mentioned expression [2] mentioned in the related description of FIG. D
When the digital value k is set in the A / A converter 205, a certain voltage value Vk is output to the amplifier 202.
The U 206 acquires a certain digital output value αk corresponding to the voltage value Vk from the A / D converter 203 (step S).
2). αk represents the amount of deviation of the bridge circuit 201 from the equilibrium state.

The CPU 206 uses the set value k for the D / A converter 205 and this value αk as one data set (k, αk).
) Is temporarily stored in the RAM 207 (step S).
3). Then, the index k is increased by 1 (step S4),
It is checked whether or not the value of the index k after 1-up exceeds the maximum value kmax that can be set in the D / A converter 205 (step S5). From the above equation [2], the value of kmax is
It is given by kmax = ^2n- 1. Here, n is the number of bits of the register (see reference numeral 6 in FIG. 2) used in the D / A converter 205.

When it is confirmed in step S5 that k> kmax = 2 ⁿ -1 is not satisfied, the process returns to step S1. After that, if the processes of steps S1 to S5 are repeated 2 ⁿ times in total, a YES determination is made in step 5, and the process proceeds to step S6 and subsequent steps. That is, 2 stored in the RAM 207
^{The n} sets of data sets (k, αk) are sequentially read, the minimum value Min {αk} is calculated (step S6), and the k value k0 corresponding to this Min {αk} is calculated (step S7).

Finally, in step S8, this digital k0
Is set in the D / A converter 205, the state in which the smallest bridge output is given is realized, and the bridge level adjustment processing is ended. When it is necessary to change the reference state of the force sensor, if the above processing is re-executed for the force sensor placed in the changed environment, the optimum balance adjustment in the changed reference state is automatically achieved. To be done.

[0033]

According to the present invention, the balance adjustment of the bridge incorporated in the force sensor can be automatically performed without manual operation, and the restriction on the installation location of the force sensor is remarkably alleviated. At the same time, even when the reference state (state where the measured value is desired to be 0) is frequently changed, balance adjustment is not difficult.

In addition, since unstable factors such as individual differences in balance adjustment accuracy disappear, the objectivity and reliability of measurement by the force sensor are improved.

[Brief description of drawings]

FIG. 1 is a diagram for explaining balanced adjustment of a bridge by a conventional method using a variable resistor.

FIG. 2 shows a basic configuration of a D / A converter used as a bridge balance adjusting means in the force sensor according to the present invention.

FIG. 3 is a schematic representation of the D / A converter shown in FIG.

4 shows an outline of a circuit configuration of a bridge of a force sensor according to the present invention, which uses a D / A converter having the basic configuration shown in FIG.

5 illustrates an example of the configuration of a force sensor system in which the bridge circuit shown in FIG. 4 is incorporated and the balance adjustment of the bridge can be automatically performed.

FIG. 6 is a flowchart outlining one example of a processing procedure for bridge balance adjustment in the force sensor system according to the present embodiment.

[Explanation of symbols]

 1, 2 resistor (strain gauge) 3, 4 resistor (fixed resistor) 5 variable resistor 6 register 7 resistor network 8, 9, 10, 8 ', 9', 10 'terminal 101, 102 resistor (strain gauge) ) 103, 104, 105 resistors (fixed resistance) 106, 107 amplifier (operational amplifier) 201 bridge circuit 202 amplifier 203 A / D converter 204 bus line 205 balance adjustment D / A converter 206 central processing unit (CPU) ) 207 RAM 208 ROM

Claims (1)

[Claims] 1. At least one resistor attached to a mechanical elastic body, the resistance value of which changes in accordance with the stress acting on the elastic body, wherein the change in the resistance value due to the stress is extracted as a voltage value. Hoiston bridge circuit,
In a force sensor having balance adjusting means for performing balance adjustment of the bridge output of the Hoystone bridge circuit, the balance adjusting means equally divides the output voltage of the Whistone bridge circuit within a certain range, A D / A converter that outputs any of the voltages as an actual output voltage,
Means for sequentially switching the digital setting inputs of the D / A converter, means for determining a digital setting input value that gives the minimum value of bridge outputs under the sequentially switched digital setting inputs, and the determined digital The force sensor, comprising means for setting a set input value in the D / A converter.