Force detection device and robot having a voltage generating circuit

A force detection device includes: a force sensor that has a piezoelectric element receiving a force and outputting a charge; a pre-compression unit that pre-compresses the force sensor; and a conversion output circuit that receives the charge from the force sensor and outputs a voltage. The pre-compression unit is short-circuited with a first ground of the conversion output circuit. The pre-compression unit has the same potential as the first ground. The force detection device includes a casing that accommodates the force sensor and the conversion output circuit. The casing is short-circuited with a second ground.

BACKGROUND

1. Technical Field

The present invention relates to a force detection device and a robot.

2. Related Art

For example, as a force detection device detecting a received force, a configuration disclosed in JP-A-2015-1384 is known. A force detection device disclosed in JP-A-2015-1384 includes a first base unit, a second base unit, a sensor device interposed between the first and second base units, a pre-compression bolt that connects the first and second base units to each other and pre-compresses the sensor device between the first and second base units, and a circuit substrate (a charge amplifier circuit) that is electrically connected to the sensor device.

A device that deals with a minute signal as in the force detection device disclosed in JP-A-2015-1384 generally includes a signal GND that serves as a reference potential of a circuit and a frame GND that serves as a reference positional of the first and second base units in order to shield disturbance noise. The signal GND and the frame GND are connected via a master GND of low impedance so that disturbance noise flowing in the frame GND does not propagate to the signal GND.

An operation current of a circuit normally flows in a wiring resistor present between the signal GND and the master GND and a contact resistor of a connector, and thus the signal GND has a high potential (for example, about several mV to about tens of mV) with respect to the master GND. On the other hand, a minute current merely flows at a moment at which disturbance noise flows between the frame GND and the master GND, and thus a steady current is almost 0 (zero). Therefore, the frame GND has the same potential as the master GND. In this way, the signal GND has a higher potential as the frame GND, for example. Thus, a leakage current may start flowing steadily in the frame GND from an input wiring of a charge amplifier circuit. Then, since the flowing leakage current is amplified by the charge amplifier circuit and is output, an output voltage of the charge amplifier circuit deviates from an output voltage corresponding to an actually received force (a drift may occur). Therefore, in the force detection device disclosed in JP-A-2015-1384, it is difficult to detect a force with high precision.

SUMMARY

An advantage of some aspects of the invention is that it provides a force detection unit and a robot that reduce a drift and have high force detection characteristics.

The advantage can be achieved by the following configurations.

A force detection device according to an aspect of the invention includes: a force sensor that has a piezoelectric element receiving a force and outputting a charge; a pre-compression unit that pre-compresses the force sensor; and a conversion output circuit that receives the charge from the force sensor and outputs a voltage. The pre-compression unit is short-circuited with a first ground of the conversion output circuit.

With this configuration, it is possible to obtain the force detection device that reduces a drift and has high force detection characteristics.

In the force detection device according to the aspect of the invention, it is preferable that the pre-compression unit has a same potential as the first ground.

With this configuration, it is possible to effectively reduce the drift and have the high force detection characteristics.

It is preferable that the force detection device according to the aspect of the invention further includes a casing that accommodates the force sensor and the conversion output circuit. The casing is preferably short-circuited with a second ground.

With this configuration, the force detection device can shield disturbance noise, and thus detect a received force with higher precision.

A force detection device according to another aspect of the invention includes: a force sensor that has a piezoelectric element receiving a force and outputting a charge; a pre-compression unit that pre-compresses the force sensor; a conversion output circuit that receives the charge from the force sensor and outputs a voltage; and a voltage generation circuit that applies the voltage to the pre-compression unit.

With this configuration, it is possible to obtain the force detection device that reduces the drift and has high force detection characteristics.

In the force detection device according to the aspect of the invention, it is preferable that the voltage generation circuit includes a voltage source and an attenuation circuit that drops a voltage of the voltage source.

With this configuration, it is easy to apply a feeble voltage to the pre-compression unit.

In the force detection device according to the aspect of the invention, it is preferable that the conversion output circuit includes a charge amplifier unit and a wiring electrically connecting the force sensor to the charge amplifier unit, and the voltage generation circuit applies the voltage to the pre-compression unit so that a difference between a voltage of the wiring and the voltage of the pre-compression unit is small.

With this configuration, it is possible to reduce the drift more reliably.

In the force detection device according to the aspect of the invention, it is preferable that the conversion output circuit includes a charge amplifier unit and a wiring electrically connecting the force sensor to the charge amplifier unit, and the voltage generation circuit generates a leakage current between the wiring and the pre-compression unit so that a leakage current generated in the conversion output circuit is offset by applying the voltage to the pre-compression unit.

With this configuration, it is possible to reduce the drift more reliably.

A force detection device according to another aspect of the invention includes: a force sensor that has a piezoelectric element receiving a force and outputting a charge; a pre-compression unit that pre-compresses the force sensor; a conversion output circuit that receives the charge from the force sensor and outputs a voltage; and a voltage generation circuit that applies the voltage to the pre-compression unit. The conversion output circuit includes a charge amplifier unit and a wiring electrically connecting the force sensor to the charge amplifier unit. The voltage generation circuit generates a leakage current between the wiring and the pre-compression unit so that a leakage current generated in the conversion output circuit is offset by applying the voltage to the pre-compression unit.

With this configuration, it is possible to obtain the force detection device that reduces a drift and has high force detection characteristics.

In the force detection device according to the aspect of the invention, it is preferable that the pre-compression unit includes first and second base units disposed to interpose the force sensor.

With this configuration, the configuration of the pre-compression is simplified.

In the force detection device according to the aspect of the invention, it is preferable that the conversion output circuit is disposed between the first and second base units.

With this configuration, it is possible to protect the conversion output circuit by the pre-compression unit. A space between the first and second base units can be effectively utilized, and thus it is possible to achieve miniaturization of the force detection device.

A robot according to another aspect of the invention includes the force detection device according to the aspect of the invention.

With this configuration, it is possible to obtain the advantages of the force detection device according to the aspect of the invention, and thus the robot has high reliability.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, a force detection device and a robot according to the invention will be described in detail with reference to the appended drawings according to preferred embodiments.

First Embodiment

FIG. 1is a sectional view illustrating a force detection device according to a first embodiment of the invention.FIG. 2is an enlarged sectional view illustrating a force sensor included in the force detection device illustrated inFIG. 1.FIG. 3is a sectional view illustrating a force sensor element included in the force sensor illustrated inFIG. 2.FIG. 4is a plan view illustrating a modification example of the force detection device illustrated inFIG. 1.FIG. 5is a circuit diagram illustrating a circuit system included in the force detection device illustrated inFIG. 1.FIG. 6is a circuit diagram illustrating a circuit system included in a force detection device of the related art.FIG. 7is a circuit diagram illustrating a circuit system included in the force detection device illustrated inFIG. 1. Hereinafter, to facilitate description, the upper side is referred to as a “top” and the lower side is referred to as a “bottom” inFIGS. 1 and 2. As illustrated inFIGS. 1 to 4, hereinafter, three axes perpendicular to each other are referred to as α, β, and γ axes, a direction parallel to the α axis is referred to as an “α axis direction”, a direction parallel to the β axis is referred to as a “β axis direction”, and a direction parallel to the γ axis is referred to as a “γ axis direction”.

A force detection device1illustrated inFIG. 1has a function of detecting an external force (including a moment) and, specifically, a function of detecting an external force added along the perpendicular three axes, the α, β, and γ axes. The force detection device1includes a first base unit2, a second base unit3disposed to be opposite to the first base unit2, a force sensor6disposed between the first base unit2and the second base unit3, an analog circuit substrate4(a circuit substrate), a digital circuit substrate5, and pre-compression bolts9connecting the first base unit2to the second base unit3. In the force detection device1, the first base unit2and the second base unit3are disposed to interpose the force sensor6and a pre-compression unit10pre-compresses the force sensor6.

The first base unit2includes a tabular bottom plate21and a projection unit22that projects from the middle of the bottom plate21to the upper side (the side of the second base unit3). As will be described below in a fifth embodiment, a bottom surface211of the bottom plate21serves as a fitting surface (a first fitting surface) for a robot1000when the force detection device1is fixed to the robot1000for use. The shape of the bottom plate21in a plan view is a circle. Here, the shape of the bottom plate21in the plan view is not particularly limited and is, for example, a polygon such as a triangle or a quadrangle. In the case of the polygon, its corners may be rounded.

A top surface221of the projection unit22is configured as a flat surface parallel to the bottom surface211. Here, the direction of the top surface221of the projection unit22is not particularly limited and may be not parallel to the bottom surface211.

As illustrated inFIG. 1, the second base unit3includes a tabular top plate31and a projection unit32that projects from the middle of the top plate31to the lower side (the side of the first base unit2). As will be described below in the fifth embodiment, a top surface311of the top plate31is a fitting surface (a second fitting surface) for the robot1000when the force detection device1is fixed to the robot1000for use. The top surface311of the top plate31is parallel to the bottom surface211of the bottom plate21in a natural state in which no external force is applied. Here, the top surface311and the bottom surface211may be not parallel to each other in the natural state. The shape of the top plate31in the plan view is substantially the same as the shape of the bottom plate21in the plane view, and thus is a circle. Here, the shape of the top plate31in the plan view is not particularly limited and may be, for example, a polygon such as a triangle or a quadrangle. The shape of the top plate31in the plan view may be different from the shape of the bottom plate21in the plan view.

A top surface321of the projection unit32is configured as a flat surface parallel to the top surface311. Here, the direction of the top surface321of the projection unit32is not particularly limited and may be not parallel to the top surface311.

The first base unit2and the second base unit3have been described above. A material of the first base unit2and the second base unit3is not particularly limited as long as the material has conductivity. In particular, a hard metal material is preferable. Examples of the material include various metals such as iron, nickel, cobalt, gold, platinum, silver, copper, manganese, aluminum, magnesium, zinc, lead, tin, titanium, and tungsten.

Next, the force sensor6will be described. As illustrated inFIG. 1, the force sensor6is disposed between the projection unit22of the first base unit2and the projection unit32of the second base unit3. The force sensor6has a function of outputting three charges Qa, Qb, and Qc according to an external force applied along perpendicular three axes (a, b, and c axes). As illustrated inFIG. 2, the force sensor6includes a force sensor element7(a piezoelectric element) and a package8that accommodates the force sensor element7.

As illustrated inFIG. 3, the force sensor element7includes four ground electrode layers71that are grounded to a signal ground SGND, a first sensor72that outputs a charge Qa according to an external force (shearing force) parallel to the a axis, a second sensor73that outputs a charge Qc according to an external force (condensing/tensile force) parallel to the c axis, and a third sensor74that outputs a charge Qb according to an external force (shearing force) parallel to the b axis. The ground electrode layers71and the sensors72,73, and74are alternately laminated.

The first sensor72includes a first piezoelectric substrate721that has a first crystal axis CA1aligned in the negative direction of the a axis, a second piezoelectric substrate723that has a second crystal axis CA2aligned in the positive direction of the a axis, and an output electrode layer722that is installed between the first piezoelectric substrate721and the second piezoelectric substrate723and outputs the charge Qa. The first piezoelectric substrate721and the second piezoelectric substrate723can be formed of, for example, Y-cut quartz crystal substrates.

The second sensor73includes a third piezoelectric substrate731that has a third crystal axis CA3aligned in the positive direction of the c axis, a fourth piezoelectric substrate733that has a fourth crystal axis CA4aligned in the negative direction of the c axis, and an output electrode layer732that is installed between the third piezoelectric substrate731and the fourth piezoelectric substrate733and outputs the charge Qc. The third piezoelectric substrate731and the fourth piezoelectric substrate733can be formed of, for example, X-cut quartz crystal substrates.

The third sensor74includes a fifth piezoelectric substrate741that has a fifth crystal axis CA5aligned in the positive direction of the b axis, a sixth piezoelectric substrate743that has a sixth crystal axis CA6aligned in the negative direction of the b axis, and an output electrode layer742that is installed between the fifth piezoelectric substrate741and the sixth piezoelectric substrate743and outputs the charge Qb. The fifth piezoelectric substrate741and the sixth piezoelectric substrate743can be formed of, for example, Y-cut quartz crystal substrates. When viewed in the lamination direction of the sensors72,73, and74, each X axis (the electrical axis of quartz crystal) of the first piezoelectric substrate721and the second piezoelectric substrate723intersects (in the embodiment, is perpendicular to) each X axis (the electric axis of quartz crystal) of the fifth piezoelectric substrate741and the sixth piezoelectric substrate743.

In the force detection device1, a translational force component in the X axis direction, a translational force component in the Y axis direction, a translational force component in the Z axis direction, a rotational force component around the X axis, a rotational force component around the Y axis, and a rotational force component around the Z axis can be detected based on the charges Qa, Qb, and Qc output from the force sensor element7.

In this way, the force sensor element7includes the piezoelectric substrates721,723,731,733,741, and743. Therefore, the force sensor6can detect a received force with high precision. In particular, in the embodiment, a material of each of the piezoelectric substrates721,723,731,733,741, and743is quartz crystal. Thus, the force sensor6can have better temperature characteristics, a higher mechanical strength (rigidity and load bearing), and a higher dynamic range than when another piezoelectric substance is used. Therefore, the received force can be detected in a wider range and with higher precision. Accordingly, the force sensor6can have more excellent detection characteristics.

As illustrated inFIG. 2, the package8includes a base81including a recessed portion811and a lid82joined to the base81to cover an opening of the recessed portion811. An airtight accommodation space S1is formed inside the package8and the force sensor element7is accommodated in the accommodation space S1. Atmosphere of the accommodation space S1is not particularly limited. The accommodation space can be filled with, for example, a rare gas such as nitrogen, argon, helium. Thus, the atmosphere of the accommodation space S1is stabilized, and thus it is possible to suppress deterioration, erosion, or the like of an electrode. The accommodation space S1may be in, for example, a depressurized state (preferably, a vacuum state).

A material of the base81is not particularly limited. For example, any of various ceramics such as aluminum oxide (alumina) and zirconium oxide (zirconia) can be used. For example, a bottom portion (a portion on which the force sensor element7is placed) of the base81and a side wall portion (a portion erect from an outer frame of the bottom portion) of the base81may be formed of different materials. In this case, for example, the bottom portion can be formed of any of various metal materials such as stainless steel, Kovar, copper, iron, and carbon steel and the side wall portion can be formed of any of various ceramics. For example, as the material of the bottom portion, an alloy of Kovar with a close coefficient of thermal expansion to ceramics is preferable. Thus, thermal strain rarely occurs in the package8, and thus it is possible to reduce application of unnecessary stress (an external force other than pre-compression and a force to be detected) to the force sensor element7.

As illustrated inFIG. 2, a terminal813connecting the outside and the inside of the accommodation space S1is formed in the base81. The terminal813is electrically connected to the force sensor element7via a connection portion814. The connection portion814is not particularly limited. For example, a conductive paste such as an Ag paste, a Cu paste, or an Au paste can be used.

The lid82is located in a central portion and includes a middle portion821that is in contact with the force sensor element7, an outer edge portion822that is located in the outer edge and is in contact with the base81, and a tapered connection portion823that is located between the middle portion821and the outer edge portion822and connects the middle portion821to the outer edge portion822. The middle portion821is installed to protrude from the outer edge portion822.

The material of the lid82is not particularly limited and may be a member that has an approximate coefficient of thermal expansion to the material of the base81. For example, when the above-described ceramic is used as the material of the base81, the material of the lid82is preferably a metal material (for example, an alloy of Kovar or the like). Thus, thermal strain rarely occurs in the package8, and thus it is possible to reduce application of unnecessary stress (an external force other than pre-compression and a force to be detected) to the force sensor element7. Therefore, the force detection device1has higher force detection precision.

The force sensor6has been described above. The configuration of the force sensor6is not particularly limited. For example, the package8may be omitted. The configuration of the force sensor element7is not particularly limited as long as the force sensor element7includes piezoelectric elements. For example, the second sensor73may be omitted from the force sensor element7. In the embodiment, the force detection device1includes one force sensor6, but the number of force sensors6included in the force detection device1is not particularly limited. Two or more force sensors6may be used. Specifically, for example, as illustrated inFIG. 4, four force sensors6may be disposed at an equal interval around a central axis J of the force detection device in a plan view when viewed in the γ axis direction.

As illustrated inFIG. 1, the force sensor6is disposed between the projection unit22of the first base unit2and the projection unit32of the second base unit3. The force sensor6is disposed to be interposed between the projection units22and32so that the base81is oriented toward the projection unit22and the lid82is oriented toward the projection unit32. When the lamination direction of the sensors72,73, and74is a “lamination direction LD”, the lamination direction LD matches (is parallel to) the central axis J of the force detection device1. Here, the lamination direction LD is not particularly limited. The lamination direction LD may be sloped with respect to a maintenance direction SD or may be perpendicular to the maintenance direction SD.

As illustrated inFIG. 1, the first base unit2and the second base unit3are connected and fixed by the pre-compression bolts9. The force sensor element7is pre-compressed by the pre-compression bolts9so that the force sensor element7is compressed in the maintenance direction SD (the lamination direction LD). More specifically, head portions of the pre-compression bolts9engage with the second base unit3and thread portions of the pre-compression bolts9are screwed with the first base unit2to pre-compress the force sensor element7. Therefore, by adjusting a fastening amount of the pre-compression bolts9, it is possible to adjust the magnitude of the pre-compression given to the force sensor element7. In this way, by pre-compressing the force sensor element7, an output at the time of applying an external force is stabilized and the applied external force can be detected with high precision. In a state in which the first base unit2and the second base unit3are fixed by the pre-compression bolts9, at least one of the first base unit2and the second base unit3can be displaced with respect to the other within a predetermined range. Thus, it is possible to deliver the received external force to the force sensor element7more reliably.

The number of pre-compression bolts9is 2. As illustrated inFIG. 1, the pre-compression bolts9interpose the force sensor6and are disposed on both sides of the force sensor6. Thus, the pre-compression bolts9can pre-compress the force sensor6on both sides with good balance.

As illustrated inFIG. 1, female screw portions212screwed with thread portions91of the pre-compression bolts9are installed in the bottom plate21of the first base unit2. Each pre-compression bolt9is inserted into the first base unit2from the side of the second base unit3. The thread portion91of each pre-compression bolt9is screwed with the female screw portion212and a pressure with a predetermined magnitude, that is, pre-compression, is applied to the force sensor element7in the maintenance direction SD. Thus, when a shearing force is applied to the force sensor element7, a frictional force occurs between the piezoelectric substrates included in the force sensor element7and a charge is reliably output from the force sensor element7. By adjusting the fastening amount of the pre-compression bolts9, it is possible to adjust the magnitude of the pre-compression.

A material of the pre-compression bolts9is not particularly limited. For example, any of various metal materials and various resin materials can be used. When the pre-compression bolts9are formed of a metal material, the first base unit2and the second base unit3are electrically connected (short-circuited) by the pre-compression bolts9, and all of the pre-compression bolts9, the first base unit2, and the second base unit3can have the same potential.

Next, the analog circuit substrate4and the digital circuit substrate5will be described. As illustrated inFIG. 1, the analog circuit substrate4and the digital circuit substrate5are disposed between the first base unit2and the second base unit3. In the analog circuit substrate4and the digital circuit substrate5, through holes are disposed so that the through holes avoid and do not interfere with the projection unit22and the pre-compression bolts9. Here, the disposition of the analog circuit substrate4and the digital circuit substrate5is not particularly limited.

As illustrated inFIG. 5, the analog circuit substrate4includes a conversion output circuit40a(a charge amplifier) that converts the charge Qa output from the force sensor element7into a voltage Va, a conversion output circuit40b(a charge amplifier) that converts the charge Qb output from the force sensor element7into a voltage Vb, and a conversion output circuit40c(a charge amplifier) that converts the charge Qc output from the force sensor element7into a voltage Vc. Further, each of the conversion output circuits40a,40b, and40cincludes an operational amplifier41, a capacitor42, and a switching element43.

As illustrated inFIG. 5, the digital circuit substrate5includes an external force detection circuit50that detects an applied external force. The external force detection circuit50has a function of detecting an applied external force based on the voltage Va output from the conversion output circuit40a, the voltage Vb output from the conversion output circuit40b, and the voltage Vc output from the conversion output circuit40c. The external force detection circuit50includes an AD converter51connected to the conversion output circuits40a,40b, and40cand an arithmetic unit52(an arithmetic circuit) connected to the AD converter51.

The AD converter51has a function of converting the voltages Va, Vb, and Vc from analog signals to digital signals. Then, the voltages Va, Vb, and Vc converted digitally by the AD converter51are input to the arithmetic unit52. The arithmetic unit52detects the translational force component in the α axis direction, the translational force component in the β axis direction, the translational force component in the γ axis direction, the rotational force component around the α axis, the rotational force component around the β axis, and the rotational force component around the γ axis based on the digitally converted voltages Va, Vb, and Vc.

As the conversion output circuits40a,40b, and40cwill be described below in detail, the conversion output circuits40a,40b, and40care the same. Thus, the conversion output circuit40awill be described representatively and the description of the conversion output circuits40band40cwill be omitted.

The conversion output circuit40ahas a function of converting the charge Qa output from the force sensor element7into the voltage Va and outputting the voltage Va. As described above, the conversion output circuit40aincludes the operational amplifier41, the capacitor42, and the switching element43. An inverted input terminal (a negative input) of the operational amplifier41is connected to the output electrode layer722of the force sensor element7via the input wiring49and the charge Qa from the force sensor6flows to the inverted input terminal. On the other hand, a non-inverted input terminal (a positive input) is connected to the signal ground SGND (a first ground) which has a reference potential of the conversion output circuit40a. An output terminal of the operational amplifier41is connected to the external force detection circuit50.

The capacitor42is connected between a first inverted input terminal and the output terminal of the operational amplifier41. The switching element43is connected between the inverted input terminal and the output terminal of the operational amplifier41and is connected to the capacitor42in parallel. The switching element43is connected to a driving circuit (not illustrated) and operates according to an on/off signal from the driving circuit. When the switching element43is turned off, the charge Qa output from the force sensor element7is stored in the capacitor42and the voltage Va obtained with a voltage of the capacitor42(that is, a quotient value of the charge by capacitance of the capacitor42) is output to the external force detection circuit50. Conversely, when the switching element43is turned on, both terminals of the capacitor42are short-circuited, the charge Qa stored in the capacitor42is discharged to become 0 coulomb, and the voltage Va to be output to the external force detection circuit50becomes 0 volts. In this way, by turning the switching element43on, it is possible to reset the conversion output circuit40a. Thus, it is possible to reduce an influence by a drift. The voltage Va to be output from the conversion output circuit40ais ideally proportional to a storage amount of the charge Qa to be output from the force sensor element7.

Here, in the related art, as illustrated inFIG. 6, in order to shield disturbance noise, the pre-compression unit10(the first base unit2and the second base unit3) is connected to a frame ground FGND (a second ground) that has a reference potential. The signal ground SGND (the first ground) and the frame ground FGND are connected via a main ground MGND (a third ground) so that the disturbance noise flowing in the frame ground FGND does not directly propagate to the signal ground SGND.

Here, when an operational current I of a different circuit CR (for example, the above-described AD converter51) from the conversion output circuit40asteadily flows in a resistor R1(a wiring resistor or a contact resistor of a connector) located between the signal ground SGND and the main ground MGND, the signal ground SGND has a higher potential (for example, about several mV to tens of mV) than the main ground MGND. On the other hand, a minute current merely flows at the moment at which disturbance noise flows between the frame ground FGND and the main ground MGND, and a steady current is almost 0 (zero). Therefore, the frame ground FGND has the same potential as the main ground MGND.

The input wiring49and the pre-compression unit10are isolated from each other. However, insulation resistance is not infinite and is typically equal to or greater than about 109Ω and equal to or less than about 1012Ω, which differs depending on an environmental temperature. Therefore, as described above, when the signal ground SGND has a higher potential than the frame ground FGND, for example, a leakage current I′ equal to or greater than about 10−12A and equal to or less than 10−15A may start flowing steadily from the input wiring49to the frame ground FGND.

Then, since the flowing leakage current I′ is amplified and output by the conversion output circuit40a, a drift may occur. Thus, it is difficult to detect the received force with high precision. Since the insulation resistance between the input wiring49and the pre-compression unit10varies depending on the environmental temperature, the degree of drift differs depending on the environmental temperature. Thus, excellent temperature characteristics may not be achieved. In order to maintain high force detection precision, it is necessary to frequently turn on and reset the switching element43. Thus, the force detection device1may not be used continuously for a long time.

With regard to this problem, in the force detection device1according to the embodiment, the pre-compression unit10is connected to the signal ground SGND (the first ground), as illustrated inFIG. 7. That is, the pre-compression unit10and the signal ground SGND are short-circuited. Thus, each compression unit10becomes 0 V with respect to the signal ground SGND serving as a reference. A potential difference between the input wiring49and the pre-compression unit10is substantially 0 (zero), the leakage current I′ from the input wiring49to the frame ground FGND is reduced, and thus the drift occurring in a configuration of the related art described above is suppressed. Accordingly, the force detection device1can detect the received force with high precision. In such a configuration, since the potential difference between the input wiring49and the pre-compression unit10can be maintained to be substantially 0 (zero) without receiving an influence of an environmental temperature, the excellent temperature characteristics can be achieved. Since a reset interval can be lengthened compared to the configuration of the related art, the force detection device1can be used continuously for a long time.

In the embodiment, the first base unit2and the second base unit3included in the pre-compression unit10are each connected to the signal ground SGND. At least part of the pre-compression unit10may be connected to the signal ground SGND. For example, one of the first base unit2and the second base unit3may be connected to the signal ground SGND.

The force detection device1according to the embodiment has been described above. As described above, the force detection device1includes the force sensor6that includes the force sensor element7(the piezoelectric element) receiving a force and outputting a charge, the pre-compression unit10that pre-compresses the force sensor6, and the conversion output circuit40athat receives the charge Qa from the force sensor6and outputs the voltage Va. Then, the potential of the pre-compression unit10becomes the same as the positional of the signal ground SGND (the first ground) from which the conversion output circuit40ais short-circuited. That is, the pre-compression unit10is short-circuited with the signal ground SGND (the first ground). Thus, it is possible to prevent the leakage current I′ from starting steadily flowing from the input wiring49connecting the force sensor6to the conversion output circuit40ato the pre-compression unit10. Therefore, the drift can be reduced, and thus the force detection device1can detect the received force with high precision. In such a configuration, the force detection device1can achieve excellent temperature characteristics since the environmental temperature is rarely influenced. Since the reset interval can be further lengthened than in the configuration of the related art, the force detection device1can be used continuously for a long time.

As described above, in the force detection device1, the pre-compression unit10is short-circuited with the signal ground SGND (the first ground). Thus, the signal ground SGND and the pre-compression unit10can have the same potential with the simple configuration. The pre-compression unit10has the same potential as the signal ground SGND. Thus, the drift can be further reduced, and thus the force detection device1can detect the received force with high precision.

As described above, the pre-compression unit10includes the first base unit2and the second base unit3interposing the force sensor6. Thus, the force sensor6can be pre-compressed with the simple configuration.

As described above, the analog circuit substrate4(the conversion output circuit40a) and the digital circuit substrate5(the external force detection circuit50) are disposed between the first base unit2and the second base unit3. Thus, the analog circuit substrate4and the digital circuit substrate5can be protected by the first base unit2and the second base unit3. Since a space between the first base unit2and the second base unit3can be effectively utilized, it is possible to miniaturize the force detection device1.

Second Embodiment

FIG. 8is a sectional view illustrating a force detection device according to a second embodiment of the invention.FIG. 9is a circuit diagram illustrating a circuit system included in the force detection device illustrated inFIG. 8.

A force detection device1according to the embodiment has the same as the force detection device1according to the above-described first embodiment except that a casing100is included.

In the following description, differences between the force detection devices according to the second embodiment and the above-described first embodiment will be mainly described and the description of the same factors will be omitted.

As illustrated inFIG. 8, the force detection device1according to the embodiment includes the casing100that accommodates the pre-compression unit10, the force sensor6, the analog circuit substrate4, and the digital circuit substrate5. The casing100includes a base110and a cover120. Here, the cover120functions as a lid body or a cover body. The base110is screwed to be fixed to the bottom surface211of the first base unit2in an insulated state (with an insulation layer (not illustrated) interposed therebetween). The cover120is screwed to be fixed to the top surface311of the second base unit3in an insulated state (with an insulation layer (not illustrated) interposed therebetween). That is, the pre-compression unit10is accommodated inside the package100in the state insulated from the package100. The base110and the cover120are installed so that delivery of an external force to the force sensor6is not interrupted. By installing the casing100, it is possible to protect the force sensor6, the analog circuit substrate4, and the digital circuit substrate5.

A material of each of the base110and the cover120is not particularly limited. A material with conductivity is preferable. For example, examples of the material include various metals such as iron, nickel, cobalt, gold, platinum, silver, copper, manganese, aluminum, magnesium, zinc, lead, tin, titanium, and tungsten.

The configuration of the casing100is not particularly limited. For example, the base110may be installed to be integrated with the first base unit2. In other words, the first base unit2may also serve as the base110. For example, the cover120may be installed to be integrated with the second base unit3. In other words, the second base unit3may serve as the cover120.

As illustrated inFIG. 9, the casing100is connected to the frame ground FGND (the second ground) that has a reference potential. Thus, disturbance noise can be blocked (reduced) by the casing100. Therefore, the force detection device1can detect a received force with higher precision. The signal ground SGND and the frame ground FGND are connected via the main ground MGND. Therefore, it is possible to prevent disturbance noise flowing in the frame ground FGND from propagating to the signal ground SGND.

The force detection device1according to the embodiment has been described above. As described above, the force detection device1includes the casing100that accommodates the force sensor6and the conversion output circuits40a,40b, and40c(the analog circuit substrate4). The casing100is electrically connected (short-circuited) with the frame ground FGND (the second ground). Since the frame ground FGND is a different ground from the signal ground SGND, the casing100can block (reduce) disturbance noise which is likely to propagate to the pre-compression unit10. Thus, the force detection device1can detect a received force with higher precision.

Even in the second embodiment, it is possible to achieve the same advantages as those of the above-described first embodiment.

Third Embodiment

FIG. 10is a circuit diagram illustrating a circuit system included in a force detection device according to a third embodiment of the invention.

A force detection device1according to the embodiment has the same as the force detection device1according to the above-described first embodiment except that a method of causing the input wiring49and the pre-compression unit10to have the same potential is different.

In the following description, differences between the force detection devices according to the third embodiment and the above-described first embodiment will be mainly described and the description of the same factors will be omitted.

As illustrated inFIG. 10, the force detection device1according to the embodiment includes a voltage generation circuit48disposed in the analog circuit substrate4. The signal ground SGND (the first ground) and the pre-compression unit10are electrically connected via the voltage generation circuit48. Therefore, in the force detection device1according to the embodiment, a voltage with any magnitude generated by the voltage generation circuit48is applied to the pre-compression unit10.

The voltage generation circuit48includes a voltage source481and an attenuation circuit482that attenuates a voltage supplied from the voltage source481. A positive voltage is supplied from the voltage source481. The attenuation circuit482is a circuit that attenuates a voltage at a predetermined attenuation factor and includes two resistant elements482aand482bconnected in series. One end of the attenuation circuit482is connected to the voltage source481and the other end thereof is connected to the signal ground SGND. In the attenuation circuit482, when Ra is resistance of the resistant element482aand Rb is resistance of the resistant element482b, a voltage can be attenuated at an attenuation factor of Rb/(Ra+Rb). In the embodiment, a variable resistor capable of varying a resistant value is used as the resistant element482a. Therefore, by adjusting the resistant value of the resistant element482a, it is possible to control the magnitude of the voltage to be applied to the pre-compression unit10.

In the above-described first embodiment, the potential difference between the input wiring49and the pre-compression unit10is substantially 0 (zero) by short-circuiting the pre-compression unit10and the signal ground SGND. However, more specifically, a voltage of the input wiring49deviates slightly (for example, less than about ±hundreds of μV) from a voltage of the signal ground SGND. This deviation is, for example, deviation that depends on an input offset voltage, a gain, and an output voltage of the operational amplifier41. Therefore, even when the pre-compression unit10and the signal ground SGND are short-circuited, a potential difference between the input wiring49and the pre-compression unit10slightly deviate from each other.

Accordingly, in the embodiment, the voltage generation circuit48is used to apply a voltage with a predetermined magnitude to the pre-compression unit10so that the potential difference between the input wiring49and the pre-compression unit10disappears (is further reduced). In other words, the voltage generation circuit48is used to correct the magnitude of the voltage to be applied to the first base unit2and the second base unit3. Thus, the potential difference between the input wiring49and the pre-compression unit10can be further decreased, and thus it is possible to more effectively reduce the leakage current I′ flowing from the input wiring49to the pre-compression unit10. Therefore, it is possible to more effectively suppress the drift, and thus the force detection device1can detect a received force with higher precision.

For example, a deviation of the voltage of the input wiring49from the signal ground SGND may be measured in advance based on the measured deviation and the magnitude of the voltage (that is, a resistant value of the resistant element482a) to be applied to the first base unit2and the second base unit3may be determined. Alternatively, the resistant value of the resistant element482amay be gradually changed while measuring the voltage Va output from the conversion output circuit40aand the resistant value of the resistant element482amay be fixed in a state in which the drift decreases.

In the embodiment, the voltage generation circuit48is disposed in the analog circuit substrate4. In this way, a voltage can be adjusted in the pre-compression unit10and each of the conversion output circuits40a,40b, and40c. Therefore, it is possible to suppress the drift for each of the conversion output circuits40a,40b, and40c. The voltage generation circuit48may be disposed in the digital circuit substrate5. Since one voltage generation circuit48can correspond to the plurality of conversion output circuits40a,40b, and40c, it is possible to miniaturize the force detection device1. Thus, it is easier to adjust the voltage. The voltage generation circuit48may be disposed at a position different from the analog circuit substrate4and the digital circuit substrate5. In this case, a voltage to be applied to the first base unit2and the second base unit3may be set with reference to the main ground MGND serving as a reference. Thus, it is possible to prevent disturbance noise from flowing in the signal ground SGND.

The force detection device1according to the embodiment has been described above. As described above, the force detection device1includes the force sensor6that includes the force sensor element7(the piezoelectric element) receiving a force and outputting a charge, the pre-compression unit10that pre-compresses the force sensor6, the conversion output circuit40athat receives the charge Qa from the force sensor6and outputs the voltage Va, and the voltage generation circuit48that applies the voltage to the pre-compression unit10. Thus, the potential difference between the input wiring49and the pre-compression unit10can be further decreased, and thus it is possible to prevent the leakage current I′ from starting steadily flowing from the input wiring49to the pre-compression unit10. Therefore, it is possible to more effectively reduce the drift, and thus the force detection device1can detect a received force with higher precision. In such a configuration, the force detection device1can achieve excellent temperature characteristics since the environmental temperature is rarely influenced. Since the reset interval can be further lengthened than in the configuration of the related art, the force detection device1can be used continuously for a long time.

As described above, the voltage generation circuit48includes the voltage source481and the attenuation circuit482that drops (attenuates) the voltage of the voltage source481. Thus, it is easy to apply a feeble voltage to the pre-compression unit10. Therefore, it is possible to further decrease the potential difference between the input wiring49and the pre-compression unit10with high precision.

In the embodiment, the attenuation circuit482of the voltage generation circuit48is used to apply the voltage with the predetermined magnitude to the pre-compression unit10so that the potential difference between the input wiring49and the pre-compression unit10disappears. A stepdown power circuit using an amplification circuit, a reference voltage generation circuit using a band gap reference or a Zener diode, or an independent external power supply may be used.

As described above, the conversion output circuit40aincludes the operational amplifier41(a charge amplifier unit) and the input wiring49(wiring) that electrically connects the force sensor6to the operational amplifier41. The voltage generation circuit48applies a voltage to the pre-compression unit10so that a difference between the voltage of the input wiring49and the voltage of the pre-compression unit10is decreased. Thus, it is possible to further decrease the potential difference between the input wiring49and the pre-compression unit10more reliably.

Even in the third embodiment, it is possible to achieve the same advantages as those of the above-described first embodiment.

Fourth Embodiment

FIG. 11is a circuit diagram illustrating a circuit system included in a force detection device according to a fourth embodiment of the invention.

A force detection device1according to the embodiment has the same as the force detection device1according to the above-described first embodiment except that the input wiring49is caused not to have the same potential as the pre-compression unit10.

In the following description, differences between the force detection devices according to the fourth embodiment and the above-described first embodiment will be mainly described and the description of the same factors will be omitted.

As illustrated inFIG. 11, the force detection device1according to the embodiment includes the voltage generation circuit48. The signal ground SGND and the pre-compression unit10are electrically connected via the voltage generation circuit48. Therefore, in the force detection device1according to the embodiment, a voltage with any magnitude generated by the voltage generation circuit48is applied to the pre-compression unit10.

The voltage generation circuit48includes a voltage source481and an attenuation circuit482that attenuates a voltage supplied from the voltage source481. The attenuation circuit482is a circuit that attenuates a voltage at a predetermined attenuation factor and includes two resistant elements482aand482bconnected in series. One end of the attenuation circuit482is connected to the voltage source481and the other end thereof is connected to the signal ground SGND. In the embodiment, unlike the above-described third embodiment, both the resistant elements482aand482bare fixed resistors. By controlling (adjusting) the magnitude of the voltage to be supplied from the voltage source481instead, it is possible to control the magnitude of the voltage to be applied to the pre-compression unit10.

In the above-described first embodiment, the leakage current I′ from the input wiring49to the frame ground FGND is reduced by short-circuiting the pre-compression unit10and the signal ground SGND. The leakage current I′ occurs not only in this route but also in the inside of the conversion output circuit40ain some cases. An example of the leakage current I′ includes a leakage current flowing in a route to a positive supply or a negative supply of the switching element43or the operational amplifier41included in the conversion output circuit40a. Therefore, even when the leakage current I′ from the input wiring49to the frame ground FGND is reduced, the drift may not still be sufficiently suppressed in some cases.

Accordingly, in the embodiment, a potential difference is given between the input wiring49and the frame ground FGND so that a leakage current occurring inside the conversion output circuit40adescribed above is offset, and the leakage current I′ is generated between the input wiring49and the frame ground FGND. That is, the voltage generation circuit48is used to apply a voltage with a predetermined magnitude to the pre-compression unit10so that the leakage current I′ that has the same magnitude and a reverse direction of the leakage current I′ generated inside the conversion output circuit40aflows between the input wiring49and the frame ground FGND. Thus, the leakage current I′ generated inside the conversion output circuit40aand the leakage current I′ flowing between the input wiring49and the frame ground FGND can be offset, and thus it is possible to suppress further effectively the drift. Accordingly, the force detection device1can detect a received force with higher precision.

Further, 80% or more of the leakage current I′ generated inside the conversion output circuit40ais preferably offset by the leakage current I′ generated between the input wiring49and the frame ground FGND, 90% or more is more preferable, and 95% or more is further more preferable, and 100% is most preferable. Thus, it is possible to sufficiently reduce the drift.

In the embodiment, by measuring temperature characteristics of insulation resistance between the input wiring49and the frame ground FGND and temperature characteristics of the leakage current generated inside the conversion output circuit40ain advance and controlling the magnitude of the voltage to be supplied from the voltage source481based on an environmental temperature, the leakage current I′ generated inside the conversion output circuit40acan be offset by the leakage current I′ flowing between the input wiring49and the frame ground FGND with higher precision.

The force detection device1according to the embodiment has been described above. As described above, the force detection device1includes the force sensor6that includes the force sensor element7(the piezoelectric element) receiving a force and outputting a charge, the pre-compression unit10that pre-compresses the force sensor6, the conversion output circuit40athat receives the charge Qa from the force sensor6and outputs the voltage Va, and the voltage generation circuit48that applies the voltage to the pre-compression unit10. The conversion output circuit40aincludes the operational amplifier41(the charge amplifier unit) and the input wiring49(the wiring) electrically connecting the force sensor6to the operational amplifier41. The voltage generation circuit48applies the voltage to the pre-compression unit10to generate the leakage current I′ between the input wiring49and the pre-compression unit10so that the leakage current I′ generated inside the conversion output circuit40ais offset. Thus, it is possible to more efficiently suppress the drift. Accordingly, the force detection device1can detect a received force with higher precision.

Fifth Embodiment

FIG. 12is a perspective view illustrating a robot according to a fifth embodiment of the invention.

A robot1000illustrated inFIG. 12is, for example, a robot that can be used for a manufacturing step of manufacturing an industrial product such as a precision device. The robot1000includes a base1100that is fixed to, for example, a floor or a ceiling, an arm1200that is connected to the base1100to be rotatable, an arm1300that is connected to the arm1200to be rotatable, an arm1400that is connected to the arm1300to be rotatable, an arm1500that is connected to the arm1400to be rotatable, an arm1600that is connected to the arm1500to be rotatable, an arm1700that is connected to the arm1600to be rotatable, and a control unit1800that controls driving of the arms1200,1300,1400,1500,1600, and1700. A hand connection unit is installed in the arm1700and an end effector1900suitable for work to be performed by the robot1000is mounted on the hand connection unit.

In the robot1000, the force detection device1that detects an external force applied to the end effector1900is installed. By feeding a force detected by the force detection device1back to the control unit1800, the robot1000can perform precise work. In accordance with the force detected by the force detection device1, the robot1000can detect contact or the like of the end effector1900to an obstacle. Therefore, an obstacle avoiding operation, a target damage avoiding operation, or the like which is difficult in position control of the related art can be performed easily, and thus the robot1000can perform work safely.

The force detection device1is disposed between the arms1600and1700. Although not illustrated, the bottom surface211of the first base unit2is connected to the arm1600and the top surface311of the second base unit3is connected to the arm1700. Here, the disposition of the force detection device1is not particularly limited. As the force detection device1, for example, any of the force detection devices1described above in the first, second, third, and fourth embodiments can be used.

In this way, the robot1000includes the force detection device1. Therefore, the robot1000can obtain the advantages of the above-described force detection device1, and thus achieve excellent reliability.

Even in the fifth embodiment, it is possible to achieve the advantages of the above-described first embodiment. The configuration of the robot1000is not particularly limited. For example, the number of arms may be different from the number of arms of the embodiment. The robot1000is not particularly limited. For example, a so-called scalar robot or two-arm robot may be used.

The force detection device and the robot according to the embodiment have been described based on the illustrated embodiments, but the invention is not limited thereto. The configuration of each unit can be substituted with any configuration with the same function. Any other constituent may be added to the invention. The embodiments may be appropriately combined.

The entire disclosure of Japanese Patent Application No. 2017-090242, filed Apr. 28, 2017 is expressly incorporated by reference herein.