CN114577381B - Robot joint torque sensor and torque measurement method thereof - Google Patents
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Abstract
A robot joint torque sensor and a torque measuring method thereof belong to the technical field of sensors. The strain beam comprises a strain surface and support columns, V-shaped strain gauges are arranged on the strain surface and are arranged between the support columns, and joint connecting flanges are arranged between the top ends of the support columns of adjacent strain beams; the strain beams corresponding to the same diameter are a group of full-bridge strain beams, and the V-shaped strain gauge corresponding to the same diameter is a Wheatstone full-bridge circuit. The method comprises the following steps: solving a Wheatstone full-bridge circuit output voltage signal; amplifying and analog-to-digital converting the output voltage signal to obtain a digital signal; calibrating the sensor; fitting out a load output coefficient: obtaining a fusion coefficient; and obtaining the torque. The invention improves the sensitivity of torque measurement, improves the integral rigidity and strength of the force sensor, and reduces the crosstalk error of the sensor.
Description
Technical Field
The invention relates to a robot joint torque sensor and a torque measurement method thereof, belonging to the technical field of sensors.
Background
The cooperative robot is an important research direction of the mechanical arm, and can be in flexible and safe contact with the outside through a force control function. For the robot adopting the high reduction ratio reduction gearbox, the external force is not easy to accurately sense due to the friction damping influence of the reduction gearbox, so that the force control is realized mainly by adopting a scheme of adding a force sensor in more scenes. The torque of each joint can be sensed by adopting the joint force sensor, and the flexible control and force control functions of each joint can be realized by combining a control algorithm.
The force sensor usually detects the external force by detecting the deformation of the elastic body, and for the force sensor with higher rigidity, a strain measurement mode is adopted. The force sensor reduces joint stiffness while ensuring a high sensitivity in force sensing applications. Stiffness and sensitivity are often a set of conflicting parameters, and for the same sensor solution, stiffness increases and then tends to decrease sensitivity. In addition, the joint itself is subjected to forces and moment loads of five degrees of freedom in addition to torsional loads, which may cause the force sensor to output erroneous torque information, known as crosstalk. To reduce the effects of crosstalk while increasing joint stiffness, crossed roller bearings are typically used to support the force sensors. However, when the robot joint adopts other speed reducers except the harmonic speed reducer, the joint structure is complicated and the weight is heavier due to the additional crossed roller bearings.
Disclosure of Invention
In order to solve the problems in the background technology, the invention provides a robot joint torque sensor and a torque measuring method thereof.
The invention adopts the following technical scheme: a robot joint torque sensor comprises a joint connecting flange, a fixing flange, four strain beams and four V-shaped strain gauges; the upper end of the fixed flange is provided with four strain beams, each strain beam comprises a strain surface and two support columns, the strain surface is fixedly arranged between the two support columns, each strain surface is provided with a V-shaped strain gauge, each V-shaped strain gauge comprises two strain gauges which are vertically and symmetrically arranged in the 45-degree direction, and the V-shaped strain gauges are used for detecting the strain of the strain beams and realizing the measurement of torque; joint connecting flanges are arranged between the top ends of two support columns of each two adjacent strain beams; the two strain beams corresponding to the same diameter are a group of full-bridge strain beams, and the two V-shaped strain gauges corresponding to the same diameter are a Wheatstone full-bridge circuit.
The invention relates to a torque measuring method of a robot joint torque sensor, which comprises the following steps:
s1: solving two output voltage signals of two groups of Wheatstone full-bridge circuits />
S2: two output voltage signals of two groups of Wheatstone full bridge circuits/>Amplifying and analog-to-digital conversion are carried out, the amplification factor is the same as the conversion resolution, two digital signals D A and D B output by the two groups of Wheatstone full-bridge circuits are obtained, and the two digital signals D A and D B are used as output data;
s3: calibrating the sensor;
s4: the sensor is loaded with independent bending moment load, axial load or radial load respectively, output data corresponding to two groups of Wheatstone full-bridge circuits are recorded, and after zero offset is corrected respectively, corresponding linear coefficients are fitted, namely: load output coefficient K:
In the formula (9):
m x is the bending moment load about the x-axis;
M y is the moment load about the y-axis;
M z is the torque load about the z-axis;
F x is the radial load along the x-axis;
f y is the radial load along the y-axis;
F z is the axial load;
k is the load output coefficient of the corresponding digital signal and the corresponding load;
because the sensor bears six loads simultaneously, the real output data of the two groups of Wheatstone full-bridge circuits after zero offset is eliminated is as follows:
S5: the fusion method of the output data with the average and minimum of the two-axis bending moment crosstalk as an index defines a fusion coefficient as K m, and the fused data is as follows:
For the fused data output components corresponding to a single directional load, there are:
evaluating crosstalk with a ratio of a load factor of a load other than the torque load around the z-axis to a load factor of the torque load around the z-axis; the crosstalk defining the moment load about the x-axis and the moment load about the y-axis is:
Taking out The two solutions of the fusion coefficient obtained by solving are:
Substituting the two solutions into the formula (13) respectively to obtain two different resultant cross-talk of bending moment loads around the x-axis, comparing the two cross-talk, and obtaining a fusion coefficient with a small cross-talk value as a final fusion coefficient;
s6: the torque load about the z-axis, i.e. the torque, is determined as follows:
Compared with the prior art, the invention has the beneficial effects that:
1. Aiming at the problems of the existing joint torque sensor, the invention designs the bearingless torque sensor which is convenient to integrate into different types of robot joints. Firstly, the support beams uniformly distributed in the circumferential direction are additionally arranged, so that the anti-crosstalk capacity of the sensor is improved, the bending moment rigidity is improved, high bending moment rigidity and low crosstalk are structurally realized under the condition of no bearing support, and the sensitivity and rigidity can be adjusted by adjusting the strain beams and the support beams which are circumferentially arranged. And secondly, the design of being directly connected to the shell is adopted, so that the strain beam is conveniently integrated into various robot joints, a larger strain beam distribution radius is obtained, and the torsional rigidity is improved under the condition of unchanged sensitivity. And secondly, strain signals are acquired by using two groups of Wheatstone full-bridge circuits which are orthogonal to each other, and on the basis of considering the mounting error of the strain gauge and the error of the amplifying circuit, the two groups of Wheatstone full-bridge circuit torque measuring methods for realizing the uniform distribution of bending moment crosstalk two axes are provided, so that the crosstalk is further reduced. Compared with the bearingless scheme in the research of the prior art, the cross talk is obviously reduced, the anti-cross talk effect is close to that of the research of adopting the bearing to support, the requirements of robot joint compliance control can be met, and the method is expected to be applied to the robot joint design schemes of various non-harmonic reducers.
2. The invention adopts a vertical structure, and the strain surface is arranged at a position far away from the axis, so that the sensitivity of torque measurement is improved.
3. The design that the supporting beams are arranged around the strain surface is provided, so that the integral rigidity and strength of the force sensor are improved, and the force sensor can be directly integrated into a robot joint without an additional supporting structure.
4. The invention provides a design that the supporting beam is not contacted with the strain surface, so that the crosstalk error of the sensor is reduced, and a high-precision torque measurement value can be obtained under the condition of no bearing support.
5. The invention provides a signal acquisition method and a data processing method for a torque sensor, which reduce the crosstalk error of the sensor.
6. The invention provides a parameter adjusting method for a torque sensor, which can adjust design parameters according to requirements in a design stage and meet different rigidity and sensitivity design requirements.
Drawings
FIG. 1 is a schematic diagram of the torque sensor of the present invention;
FIG. 2 is a front view of FIG. 1;
FIG. 3 is a top view of FIG. 1;
FIG. 4 is a schematic structural view of a strain beam;
FIG. 5 is a schematic diagram of a Wheatstone full bridge circuit;
FIG. 6 is a force diagram of the present invention.
Detailed Description
The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention, and it is apparent that the described embodiments are only some embodiments of the invention, but not all embodiments, and all other embodiments obtained by those skilled in the art without making creative efforts based on the embodiments of the present invention are all within the protection scope of the present invention.
A robot joint torque sensor and a torque measuring method thereof adopt a strain measuring principle, and comprise a joint connecting flange 1, a fixed flange 3, four strain beams 2 and four V-shaped strain gauges 5; the fixed flange 3 is connected with the robot body so as to measure the torque load of the external force on the robot joint, four strain beams 2 are uniformly distributed at the upper end of the fixed flange 3 along the circumferential direction of the fixed flange, each strain beam 2 comprises a strain surface 6 and two support columns 7, the strain surface 6 is fixedly arranged between the two support columns 7, each strain surface 6 is provided with a V-shaped strain gauge 5, each V-shaped strain gauge 5 comprises two strain gauges which are vertically and symmetrically arranged in the 45-degree direction, and the V-shaped strain gauge 5 is used for detecting the micro strain of the strain beam 2 so as to realize the measurement of the torque; a joint connecting flange 1 is arranged between the top ends of two support columns 7 of each two adjacent strain beams 2, and the two support columns are connected with a robot joint through the joint connecting flange 1; the two strain beams 2 corresponding to the same diameter are a group of full-bridge strain beams, and the two V-shaped strain gauges 5 corresponding to the same diameter are a Wheatstone full-bridge circuit, as shown in FIG. 5. Setting E as power supply voltage, E o as output voltage signal, amplifying the output voltage signal, collecting by analog-to-digital conversion circuit, and multiplying by corresponding coefficient to obtain the torque of sensor, wherein the specific coefficient and the sensitivity coefficient of strain gauge are related to circuit amplification factor and actual calibration result. Through the Wheatstone full bridge circuit, crosstalk errors can be effectively reduced. The four groups of strain beams 2 can form two groups of full-bridge strains, namely two signal data can be obtained, crosstalk experiments are respectively carried out on the two groups of full-bridge strains, and the weighted average of the two groups of signal data is taken as final data according to experimental results. The crosstalk error and torque ripple can be further reduced by a weighted average of the two sets of full-bridge data.
Four groups of supporting beams 4 are uniformly distributed between the joint connecting flange 1 and the fixing flange 3 along the circumferential direction of the fixing flange 3, the four groups of supporting beams 4 and the four strain beams 2 are alternately arranged along the circumferential direction of the fixing flange 3 one by one, each group of supporting beams 4 comprises a plurality of supporting beams 4 with gaps between the supporting beams, the arrangement of the supporting beams 4 can improve the rigidity and the strength of the support, and the support rigidity and the strength of the support are better under the condition of no bearing support; in addition, since the deformation of the support beam 4 is more sensitive to torsional loads, the measurement of torsion is not affected, while providing good support in the non-torsion direction, low crosstalk can be further ensured. The support beams 4 may or may not be uniformly distributed.
The outer wall of the strain surface 6 and the outer wall of the fixed flange 3 are arranged in the same diameter, and the vertical structure is adopted, so that the distance between the strain surface 6 and the axis is as far as possible, and the sensitivity can be improved.
Gaps exist between the support columns 7 and the corresponding support beams 4, and the support beams 4 and the strain body 2 are not in contact, so that crosstalk errors are reduced.
By adjusting the thickness of the strain face 6 and the width of the support beam 4, the sensitivity and stiffness of the sensor can be adjusted. The relationship between sensitivity and rigidity can be further adjusted by adjusting the number and distribution of the support beams 4 under the condition that the total width of the support beams 4 is unchanged.
The V-shaped strain gauge 5 is arranged on the inner wall or the outer wall of the corresponding strain surface 6.
The invention relates to a torque measuring method of a robot joint torque sensor, which comprises the following steps:
s1: solving two output voltage signals of two groups of Wheatstone full-bridge circuits />
S101: according to the output principle of the Wheatstone full-bridge circuit, the output voltage signal of the strain gauge of one of the Wheatstone full-bridge circuits is set as follows:
In the formula (1):
e o is the output voltage signal;
E is the power supply voltage;
R n is the resistance of the corresponding strain gauge in the wheatstone full bridge circuit, n=1 to 4;
S102: setting the initial resistance of one strain gauge as R i, preferably R i =350Ω, defining the resistance increment of each strain gauge due to strain as R d, and obtaining the actual resistance R of each single strain gauge as:
Rn＝Rdn+Rin，n＝1～4 (2)
bringing equation (2) into equation (1) yields an output voltage signal of:
taking R i1＝Ri2＝Ri3＝Ri4 =r, the relationship between the output voltage and the actual resistance change can be obtained:
Neglecting high order errors, and simplifying to obtain:
The characteristics of the strain gage resistance in response to strain are considered as follows:
in formula (6):
K s is the sensitivity system of the strain gauge;
epsilon is the strain produced on the strain gauge;
the method further comprises the following steps:
in the formula (7):
Epsilon n is the strain generated on the corresponding strain gauge, n=1 to 4;
When a torque load M s around the z-axis is loaded, there is ε 1＝-ε2＝ε3＝-ε4 =ε, when it is available:
eo＝EKsε (8)
When subjected to bending moment load, axial load or radial load, e o =0, thereby reducing crosstalk.
S2: two output voltage signals of two groups of Wheatstone full bridge circuits/>Amplifying and analog-to-digital conversion are carried out, the amplification factor is the same as the conversion resolution, two digital signals D A and D B output by the two groups of Wheatstone full-bridge circuits are obtained, and the two digital signals D A and D B are used as output data;
in practice, the output data of the wheatstone full-bridge circuit cannot perfectly eliminate the crosstalk effect, and due to the reasons of installation asymmetry, resistance error of strain gauges and the like, components caused by crosstalk loads are still included in the output data of the two sets of wheatstone full-bridge circuits, so that in order to describe the effect, the output data can be decomposed into six components, and a linear relationship exists between the output data corresponding to each load and the load.
S3: calibrating the sensor;
S4: the sensor is loaded with independent bending moment load, axial load or radial load respectively, output data corresponding to two groups of Wheatstone full-bridge circuits are recorded, and after zero offset is corrected respectively, corresponding linear coefficients are fitted, namely: the load output coefficient K, the formula (9) is the linear coefficient of six loads corresponding to the output data of two groups of Wheatstone full bridge circuits:
In the formula (9):
m x is the bending moment load about the x-axis;
M y is the moment load about the y-axis;
M z is the torque load about the z-axis;
F x is the radial load along the x-axis;
f y is the radial load along the y-axis;
F z is the axial load;
k is the load output coefficient of the corresponding digital signal and the corresponding load;
because the sensor bears six loads simultaneously, the real output data of the two groups of Wheatstone full-bridge circuits after zero offset is eliminated is as follows:
If the two Wheatstone full-bridge circuits only have data about the torque load of the z-axis and the other two are 0, the output data D A and D B of the two Wheatstone full-bridge circuits are the torque data to be measured. In an ideal state, the load output coefficients except the torque load output coefficient around the z-axis are close to 0 and are far smaller than the torque load output coefficient around the z-axis, and the torque load output coefficients around the z-axis of the two Wheatstone full bridge circuits are equal. However, in practice, the other load output coefficients than the torque load output coefficient around the z-axis are not 0 due to errors such as mounting errors, amplification circuit magnification errors, structural strain errors, and the like, and the torque load output coefficients around the z-axis of the two wheatstone full bridge circuits are not equal.
Therefore, in practice, since only the data of D A and D B and the respective load coefficients previously specified are available, the situation of the respective loads in the specific load is not known, and it is necessary to obtain torque measurement data as accurate as possible from the data of D A and D B.
S5: to obtain the final sensor measurement data, the average value of the output data of two Wheatstone full-bridge circuits is usually directly taken as the fused data in the prior art, namelyIn order to reduce the crosstalk of the finally output fused data, the invention defines a fusion coefficient as K m by using an output data fusion method of which the biaxial bending moment crosstalk is averaged and minimized as an index, and the fused data is as follows:
In order to determine K m, it is necessary to use the load output coefficients obtained in calibration, which are all measured under load in a single direction.
For the fused data output components corresponding to a single directional load, there are:
The coefficient represents the sensitivity degree of the two Wheatstone full-bridge circuits to each load independently, and the K m is expected to enable the sensitivity degree of the data fused by the two Wheatstone full-bridge circuits to crosstalk loads to be lower, and meanwhile, the sensitivity degrees of the two bending moment directions are preferably the same, so that the influence on control caused by different sensitivity degrees can be avoided.
Evaluating crosstalk with a ratio of a load factor of a load other than the torque load around the z-axis to a load factor of the torque load around the z-axis; the strain caused by force load is far smaller than the strain caused by bending moment load in practical application, so that the crosstalk caused by bending moment load is mainly considered in subsequent crosstalk evaluation. The crosstalk defining two bending moment loads is respectively:
Taking out The two solutions of the fusion coefficient obtained by solving are:
Substituting the two solutions into the formula (13) respectively to obtain two different resultant cross-talk of bending moment loads around the x-axis, comparing the two cross-talk, and obtaining a fusion coefficient with a small cross-talk value as a final fusion coefficient;
therefore, in the process of calibrating the torque sensor, the linear coefficients of six loads corresponding to data of two Wheatstone full-bridge circuit outputs are respectively and independently loaded, and the fusion coefficient K m is obtained by using the fusion method.
S6: the calibrated torque sensor obtains the torque load around the z-axis through the following steps:
It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrative embodiments, and that the present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. Any reference sign in a claim should not be construed as limiting the claim concerned.
Furthermore, it should be understood that although the present disclosure describes embodiments, not every embodiment is provided with a separate embodiment, and that this description is provided for clarity only, and that the disclosure is not limited to the embodiments described in detail below, and that the embodiments described in the examples may be combined as appropriate to form other embodiments that will be apparent to those skilled in the art.
Claims (6)
1. A robot joint torque sensor, characterized in that: comprises a joint connecting flange (1), a fixing flange (3), four strain beams (2) and four V-shaped strain gauges (5); the upper end of the fixed flange (3) is provided with four strain beams (2), each strain beam (2) comprises a strain surface (6) and two support columns (7), the strain surface (6) is fixedly arranged between the two support columns (7), each strain surface (6) is provided with a V-shaped strain gauge (5), each V-shaped strain gauge (5) comprises two strain gauges which are vertically and symmetrically arranged in the 45-degree direction, and the V-shaped strain gauges (5) are used for detecting the strain of the strain beam (2) so as to realize the measurement of torque; a joint connecting flange (1) is arranged between the top ends of two support columns (7) of every two adjacent strain beams (2); two strain beams (2) corresponding to the same diameter are a group of full-bridge strain beams, and two V-shaped strain gauges (5) corresponding to the same diameter are a Wheatstone full-bridge circuit; four groups of supporting beams (4) are arranged between the joint connecting flange (1) and the fixing flange (3) along the circumferential direction of the fixing flange (3), the four groups of supporting beams (4) and the four strain beams (2) are alternately arranged along the circumferential direction of the fixing flange (3), and each group of supporting beams (4) comprises a plurality of supporting beams (4) with gaps between each other.
2. A robotic joint torque sensor as claimed in claim 1, wherein: the outer wall of the strain surface (6) and the outer wall of the fixed flange (3) are arranged in the same diameter.
3. A robotic joint torque sensor as claimed in claim 2, wherein: gaps are reserved between the support columns (7) and the corresponding support beams (4).
4. A robotic joint torque sensor as claimed in claim 2, wherein: the V-shaped strain gauge (5) is arranged on the inner wall or the outer wall of the corresponding strain surface (6).
5. A torque measurement method of the robot joint torque sensor according to any one of claims 1 to 4, characterized in that: the method comprises the following steps:
s1: solving two output voltage signals of two groups of Wheatstone full-bridge circuits />
S2: two output voltage signals of two groups of Wheatstone full bridge circuits/>Amplifying and analog-to-digital conversion are carried out, the amplification factor is the same as the conversion resolution, two digital signals D A and D B output by the two groups of Wheatstone full-bridge circuits are obtained, and the two digital signals D A and D B are used as output data;
s3: calibrating the sensor;
s4: the sensor is loaded with independent bending moment load, axial load or radial load respectively, output data corresponding to two groups of Wheatstone full-bridge circuits are recorded, and after zero offset is corrected respectively, corresponding linear coefficients are fitted, namely: load output coefficient K:
In the formula (9):
m x is the bending moment load about the x-axis;
M y is the moment load about the y-axis;
M z is the torque load about the z-axis;
F x is the radial load along the x-axis;
f y is the radial load along the y-axis;
F z is the axial load;
k is the load output coefficient of the corresponding digital signal and the corresponding load;
because the sensor bears six loads simultaneously, the real output data of the two groups of Wheatstone full-bridge circuits after zero offset is eliminated is as follows:
S5: the fusion method of the output data with the average and minimum of the two-axis bending moment crosstalk as an index defines a fusion coefficient as K m, and the fused data is as follows:
For the fused data output components corresponding to a single directional load, there are:
evaluating crosstalk with a ratio of a load factor of a load other than the torque load around the z-axis to a load factor of the torque load around the z-axis; the crosstalk defining the moment load about the x-axis and the moment load about the y-axis is:
Taking out The two solutions of the fusion coefficient obtained by solving are:
Substituting the two solutions into the formula (13) respectively to obtain two different resultant cross-talk of bending moment loads around the x-axis, comparing the two cross-talk, and obtaining a fusion coefficient with a small cross-talk value as a final fusion coefficient;
s6: the torque load about the z-axis, i.e. the torque, is determined as follows:
6. The method according to claim 5, wherein: the step S1 comprises the following steps:
S101: according to the output principle of the Wheatstone full-bridge circuit, the output voltage signal of the strain gauge of one of the Wheatstone full-bridge circuits is set as follows:
In the formula (1):
e o is the output voltage signal;
E is the power supply voltage;
R n is the resistance of the corresponding strain gauge in the wheatstone full bridge circuit, n=1 to 4;
S102: setting the initial resistance of one strain gauge as R i, defining the resistance increment of each strain gauge due to strain as R d, and obtaining the actual resistance R of each single strain gauge as follows:
Rn＝Rdn+Rin，n＝1～4 (2)
bringing equation (2) into equation (1) yields an output voltage signal of:
taking R i1＝Ri2＝Ri3＝Ri4 =r, the relationship between the output voltage and the actual resistance change can be obtained:
Neglecting high order errors, and simplifying to obtain:
The characteristics of the strain gage resistance in response to strain are considered as follows:
in formula (6):
K s is the sensitivity system of the strain gauge;
epsilon is the strain produced on the strain gauge;
the method further comprises the following steps:
in the formula (7):
Epsilon n is the strain generated on the corresponding strain gauge, n=1 to 4;
When a torque load M z around the z-axis is loaded, there is ε 1＝-ε2＝ε3＝-ε4 =ε, when it is available:
eo＝EKsε (8)
When subjected to bending moment load, axial load or radial load, e o =0, thereby reducing crosstalk.
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| CN113108960A (en) * | 2021-05-07 | 2021-07-13 | 珞石（北京）科技有限公司 | Sensor for measuring joint torque |
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