Patent Publication Number: US-10786321-B2

Title: Instrument force sensor using strain gauges in a faraday cage

Description:
RELATED APPLICATIONS 
     This patent application is the U.S. national phase of International Patent Application No. PCT/US2017/015795, filed Jan. 31, 2017, which designated the U.S. and which claims priority to and the benefit of the filing date of U.S. Provisional Patent Application No. 62/290,010 (filed Feb. 2, 2016), each of which is incorporated by reference herein in its entirety. 
    
    
     BACKGROUND 
     Field of the Invention 
     The present invention relates generally to surgical instruments for computer-assisted surgical system, and more particularly to strain gauges used in surgical instruments for computer-assisted surgical systems. 
     Description of Related Art 
       FIG. 1  is a perspective illustration of a surgical instrument  100  used in a computer-assisted surgical system. Arrow  195  shows the proximal direction and the distal direction in  FIG. 1 . 
     Surgical instrument  100  includes a housing  101 , a shaft  102 , a force sensor apparatus  103 , a joint  104 , and an end component  105 . End component  105 , such as a surgical end effector, is coupled to force sensor apparatus  303  via joint  304 , e.g., a wrist joint. Force sensor apparatus  103  is coupled to a distal end of a shaft  102  and is coupled to joint  104 . Housing  101  is coupled to a proximal end of shaft  102 , and housing  101  includes an interface which mechanically, electrically, and optically couples instrument  100  to an instrument manipulator assembly. 
     Force sensor apparatus  103  measured the flexing of instrument shaft  102  by forces acting on end component  105 . Force sensor apparatus  103  used optical fibers including Fiber Bragg Gratings. Two rings of Fiber Bragg Gratings separated apart were used. Each ring included four Fiber Bragg Gratings separated by ninety degrees. 
     The signals from the Fiber Bragg Gratings were routed through the optical fibers to a fiber interrogator that was located remotely from surgical instrument  100 . The signals from the fiber interrogator were combined arithmetically in various sums and differences to obtain measures of three perpendicular forces exerted on end component  105  and to obtain torques about two axes perpendicular to the axis of shaft  102 . See U.S. Pat. No. 8,375,808 B2, which is incorporated herein by reference in its entirety, for a more complete description of the Fiber Bragg Grating sensors. 
     The Fiber Bragg Grating sensors reflect light based on the bending of the sensor. The Fiber Bragg Grating sensors were immune to cautery noise. Unfortunately, the epoxy glue used to attach the fibers to shaft  102  degrades when cleaned in an autoclave machine. This may result in a need to recalibrate the sensors, and also may limit the useful life of the surgical instrument with respect to force sensing if the epoxy glue fails. 
     SUMMARY 
     A surgical instrument includes a force sensor apparatus. The force sensor apparatus includes a sensor capsule. The sensor capsule is part of a Faraday cage. A strain gauge is mounted within the sensor capsule. 
     The surgical instrument also includes a tube and an electronics enclosure. The tube has a first end and a second end. The first end of the tube is connected to the sensor capsule. A second end of the tube is connected to the electronics enclosure. The tube and the electronics enclosure complete the Faraday cage. As used herein, a tube does is not limited to having a circular cross section. 
     In one aspect, the sensor capsule is configured as a cantilever beam. The strain gauge is mounted on an interior wall of the cantilever beam. As used herein, a capsule does not imply only a closed body or any particular shape, for example, a capsule having an open end is disclosed. 
     In another aspect, the sensor capsule is a cylindrical tube. The cylindrical tube is mounted in the force sensor apparatus as a cantilever beam. In this aspect, the force sensor apparatus also includes a first plurality of strain gauges and a second plurality of strain gauges. 
     The cylindrical tube has an inner wall, a first end and a second end, and a lengthwise axis. The lengthwise axis is defined between the first end and the second end of the cylindrical tube. The first plurality of strain gauges is affixed to the inner wall of the cylindrical tube with a center of each gauge of the first set of strain gauges is in a first plane perpendicular to a first location on the lengthwise axis. The second plurality of strain gauges is affixed to the inner wall of the cylindrical tube, with a center of each gauge of the second set of strain gauges is in a second plane perpendicular to a second location on the lengthwise axis. The second location is different from the first location. 
     A first pair of strain gauges in the first plurality of strain gauges is configured in one leg (a first leg) of a Wheatstone Bridge. A first pair of strain gauges in the second plurality of strain gauges is configured in another leg (a second leg) of the Wheatstone Bridge. 
     The force sensor apparatus also includes an amplifier mounted within the cylindrical tube. The amplifier has a first input terminal, a second input terminal, and an output terminal. The first input terminal is connected to an output of the one leg (the first leg) of the Wheatstone Bridge. The second output terminal is connected to an output of the another leg (the second leg) of the Wheatstone Bridge. The amplifier is configured to subtract a signal on the second input terminal from a signal on the first input terminal, and is configured to provide on the output terminal a signal representative of a force acting on the cylindrical tube. 
     A force sensor apparatus includes a cantilever beam, a first plurality of strain gauges, a second plurality of strain gauges, and an amplifier circuit. The cantilever beam includes an interior wall, a first end, a second end, and a lengthwise axis. The interior wall bounds an interior volume. The lengthwise axis is defined between the first end and the second end of the cantilever beam. The cantilever beam is part of a Faraday cage. 
     The first plurality of strain gauges is mounted on the interior wall of the cantilever beam. A center of each gauge of the first plurality of strain gauges is in a first plane perpendicular to a first location on the lengthwise axis. The second plurality of strain gauges is also mounted on the interior wall of the cantilever beam. A center of each gauge of the second plurality of strain gauges is in a second plane perpendicular to a second location on the lengthwise axis of the cantilever beam. The second location is different from the first location. 
     The amplifier circuit is mounted in the interior volume of the cantilever beam. The amplifier circuit is connected to the first plurality of strain gauges and to the second plurality of strain gauges. The amplifier circuit is configured to output a first signal representative of a force in a first direction on the cantilever beam and to output a second signal representative of a force in a second direction on the cantilever beam. The first direction is perpendicular to the second direction. 
     In one aspect, a first pair of strain gauges in the first plurality of strain gauges is configured as one leg of a Wheatstone Bridge. A first pair of strain gauges in the second plurality of strain gauges is configured as another leg of the Wheatstone Bridge. 
     The amplifier circuit includes a first amplifier having a first input terminal, a second input terminal, and an output terminal. The first input terminal is connected to an output of the one leg of the Wheatstone Bridge. The second input terminal is connected to an output of the another leg of the Wheatstone Bridge. The amplifier is configured to subtract a signal on the second input terminal from a signal on the first input terminal, and is configured to provide on the output terminal the first signal representative of the force in the first direction on the cantilever beam. 
     In one aspect, a method includes mounting a strain gauge within a sensor capsule, and coupling the sensor capsule to a Faraday cage within a surgical instrument. The method also includes mounting the sensor capsule in the surgical instrument as a cantilever beam. 
     In one aspect, the mounting a strain gauge further includes mounting a first plurality of strain gauges on an interior wall of a cylindrical tube, the cylindrical tube being a part of the sensor capsule, and mounting a second plurality of strain gauges on the interior wall of the cylindrical tube. The first plurality of strain gauges is separated from the second plurality of strain gauges along a lengthwise axis of the cylindrical tube. 
     In this aspect, the method further includes configuring a pair of strain gauges in the first plurality of strain gauges as a first leg of a full Wheatstone Bridge, the full Wheatstone Bridge being within the cylindrical tube, and configuring a pair of strain gauges in the second plurality of strain gauges as a second leg of the full Wheatstone Bridge. An output of the first leg of the full Wheatstone Bridge is connected to a first input terminal of an amplifier and an output of the second leg of the full Wheatstone Bridge is connected to a second input terminal of the amplifier. The amplifier is within the cylindrical tube. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective illustration of a prior art surgical instrument used in a computer-assisted surgical system. 
         FIG. 2  is a perspective illustration of a surgical instrument  200  that is used in a computer-assisted surgical system and that includes a force sensor apparatus that is immune to noise from arcing cautery and that is autoclabable. 
         FIGS. 3A to 3C  are illustrations of one aspect of a Faraday cage that includes a sensor capsule, a tube, and an electronics enclosure that are included within a surgical instrument, such as the surgical instrument of  FIG. 2 . 
         FIG. 4A  is a cut-away view of another force sensor apparatus suitable for use in the surgical instrument of  FIG. 2 . 
         FIG. 4B  is an illustration of a plurality of strain gauges, where the strain gauges are mounted in a row on a printed circuit assembly to form a strip used in the force sensor apparatus of  FIG. 4A . 
         FIG. 4C  is a cross-section view of the strip of  FIG. 4B  mounted in the cantilever beam of  FIG. 4A . 
         FIGS. 5A and 5B  are schematics of Wheatstone Bridges and amplifiers used to generate signals representative of the forces on the cantilever beam of  FIG. 4A . 
         FIGS. 5C and 5D  illustrate the orientation of the strain gauges in force sensor apparatus of  FIG. 4A . 
         FIG. 5E  is an illustration of the strain gauge and amplifier configuration that is inserted within the cantilever beam of  FIG. 4A . 
     
    
    
     In the drawings, the first digit of a three digit reference numeral is the figure number in which the element having that reference numeral first appeared. 
     DETAILED DESCRIPTION 
     A surgical instrument  200  includes a force sensor apparatus  203  that is immune to noise from arcing cautery without relying on fiber optic strain gauges and that is autoclabable. Surgical instrument  200  includes a housing  201 , a shaft  202 , a force sensor apparatus  203 , a joint  204 , and an end component  205 . End component  205 , such as a surgical end effector, is coupled to force sensor apparatus  203  via joint  204 , e.g., a wrist joint. Force sensor apparatus  203  is coupled to a distal end of a shaft  202  and is coupled to joint  204 . Housing  201  is coupled to a proximal end of shaft  202 , and housing  201  includes an interface which mechanically and electrically couples instrument  200  to an instrument manipulator assembly. In  FIG. 2 , arrow  295  defines the proximal and distal directions. 
     As explained more completely below, force sensor apparatus  203  includes at least one strain gauge that is enclosed in a Faraday cage  250 . Faraday cage  250  includes a sensor capsule  210  that includes one or more strain gauges, a tube  220 , and an enclosure  230 . In one aspect, each of sensor capsule  210 , tube  220 , and enclosure  230  is made of metal, e.g., stainless steel. While examples are presented herein of a capsule, these examples do not limit a capsule to only a closed body or to any particular shape. Also, as used herein, a tube is not limited to a tube having a circular cross section. 
     A first end of tube  220  is attached to sensor capsule  210  and a second end of tube  220  is attached to enclosure  230  so that any voltage or current on the exterior surface of Faraday cage  250  cannot penetrate inside Faraday cage  250 . 
     Force sensor apparatus  203  includes one or more resistance strain gauges mounted on an inside wall, e.g., mounted in an interior volume, of sensor capsule  210 . The resistance strain gauge or gauges sense the bending of sensor capsule  210 , and so the resistance strain gauges measure torques on sensor capsule  210 . 
     Amplifiers mounted in the interior volume of the sensor capsule  210  are connected to the resistance strain gauges. The amplifiers take signals from the resistance strain gauges and generate signals representative of forces acting on sensor capsule  210 . These signal are driven on wires that run through an inner channel of tube  220  to circuits housed in enclosure  230  within housing  201 . The electronics housed within enclosure  230  process the signals on the signal lines from the amplifiers. 
     Thus, the resistance strain gauges and the associated amplifiers that are included in sensor capsule  210  are immune from any common-mode voltages or currents induced by the arcing cautery, e.g., the reported resistance strain gauge data does not change when arcing cautery is present. Faraday cage  250  is important because the strain gauge signals are at the hundreds-of-microvolts level while cautery discharges are at the thousands-of-volts level. The arcing cautery interference is orders of magnitude larger than strain gauge signal levels. 
     Enclosures around the strain gauges, signal wires, and electronic circuits act not only as a Faraday cage, but also act as a sealed chamber preventing the steam and high pressures of the autoclave process from damaging the strain gauges and electronics. However, the strain gauges and the electronics are selected and built to survive the elevated temperature (140° C.) present during autoclave cleaning. 
       FIGS. 3A to 3C  are illustrations of one aspect of a Faraday cage  350  that includes a sensor capsule  310 , a tube  320 , and an electronics enclosure  330  that are included within a surgical instrument, such as surgical instrument  200 . Sensor capsule  310  includes a top portion  310 - 1 , a lid, and a bottom portion  310 - 2 , a base. In one aspect, top portion  310 - 1  and bottom half portion are each made of stainless steel. 
     Bottom portion  310 - 2  has a flat bottom and round sides. Strain gauge  340  and related circuitry  348  are mounted on the interior surface of the flat bottom of bottom portion  310 - 2 . 
     Strain gauge  340 , in this aspect, is a surface sputtered on the inner flat bottom surface of bottom portion  310 - 2 . A first set of wires  341  connect strain gauge  340  to a Wheatstone Bridge amplifier  342 , which in one aspect is a wire bonded integrated circuit. Wheatstone Bridge amplifier  342  is connected by a second set of wires to an analog-to-digital converter  343 , which in one aspect also is a wire bonded integrated circuit. Analog-to-digital converter  343  is connected to an isolation and differential driver integrated circuit  344  by a third set of wires. A fourth set of wires, e.g., pair of wires  345 , are connected to the output of differential driver integrated circuit  344  and routed through tube  320 , a hypotube in one aspect. Tube  320  functions as a cable shield. In one aspect, Wheatstone Bridge amplifier  342 , analog-to-digital converter  343 , isolation and differential driver integrated circuit  344  are connected to each other by gold wire-bonding. 
     Tube  320 , in one aspect, is a stainless steel hypotube that is laser welded to bottom portion  301 - 2  of sensor capsule  310 . The proximal end of tube  320  is welded to a metallic enclosure  330  that is housed within the surgical instrument body. As shown in  FIG. 3B , top portion  310 - 1  is laser welded to bottom portion  310 - 2  to form a sealed capsule that is sensor capsule  310 . In this configuration, force sensor apparatus  303  measures a force that bends capsule  310  away from a lengthwise axis  390  of force sensor apparatus  303 . 
       FIG. 3C  is a cut-away view that shows force sensor apparatus  303  mounted on a distal end of shaft  302 . Arrow  395  shows the proximal and distal directions in  FIGS. 3A, 3B, and 3C . A wrist joint  304  and an end effector  305  are coupled to force sensor apparatus  303 . Circuitry  348  represents Wheatstone Bridge amplifier  342 , analog-to-digital converter  343 , and isolation and differential driver integrated circuit  344 . Capsule  310  is mounted as a cantilever beam in force sensor apparatus  303  to measure forces in one direction acting on end effector  305 . 
       FIG. 4A  is a cut-away view of another force sensor apparatus  403  suitable for use in surgical instrument  200 . In  FIG. 4A , force sensor apparatus  403  is mounted on a distal end of shaft  402 . A wrist joint  404  is mounted on the distal end of force sensor apparatus  403 , and an end component  405  is coupled to wrist joint  404 . 
     The distal end of shaft  402  has a reduced outer diameter so that a first cylindrical tube  450  that is the outer wall of force sensor apparatus  403  slides over that reduced diameter. Arrow  495  defines the proximal and distal directions in  FIG. 4A . 
     Sensor capsule  410  in force sensor apparatus  403  is a second cylindrical tube  410 . The outer diameter of second cylindrical tube  410  is smaller than the inner diameter of first cylindrical tube  450 , sometimes referred to as tube  450 , so that second cylindrical tube  410  fits inside first cylindrical tube  450 . In one aspect, tube  450  and tube  410  are each a metal tube, such as a stainless steel tube. 
     Second cylindrical tube  410 , sometimes referred to as tube  410  or sensor capsule  410 , is mounted inside first cylindrical tube  450  so that second cylindrical tube  410  functions a cantilever beam. Specifically, a proximal end portion of tube  410  is fixedly anchored in a proximal end of force sensor apparatus  403  by a rigid elongate bushing  451 . The remaining portion of tube  410  is the cantilever beam, which extends from bushing  451  in the distal direction with the free distal end of tube  410  being inside the housing of wrist joint  404 . 
     A metal plug is laser welded inside the distal end of tube  410 . Alternatively the tube can be sealed with a water-proof adhesive such as a room temperature vulcanization silicone, commonly referred to as Silicone RTV. When Silicone RTV is used to seal the distal end of tube  410 , a small non-metallic hole is left that has the size of the inner diameter of tube  410 . However, cautery tests showed that for inner diameters of tubes used within a force sensor apparatus of a surgical instrument, the opening in the tube when sealed with Silicone RTV resulted in no cautery interference with the strain gauge data. Thus, the open-ended tube is an effective part of the Faraday cage. A distal end, a first end, of cable shield tube  420  extends through the proximal end of tube  410  and is welded or sealed with adhesive. A proximal end, a second end, of cable shield tube  420  is welded or sealed to an enclosure in the body of the surgical instrument, which is equivalent to enclosure  330 . Thus, sensor capsule  410  and cable shield tube  420  are part of a Faraday cage. 
     For convenience, an X-Y-Z coordinate system is defined for  FIG. 4A . The Z-axis is in a direction of the lengthwise axis of shaft  402  and force sensor apparatus  403 . The Y-axis  492  is the up and down direction in  FIG. 4A  and the X-axis  491  (not shown) is into and out of the page. The Z direction of force sensor apparatus  403  is sometimes referred to as the lengthwise direction as opposed to a radial direction, the x-direction, and the y-direction. The proximal and distal directions are along the Z direction. 
     A first set of strain gauges includes a first plurality of strain gauges  440 - 1 . First plurality of strain gauges  440 - 1  is mounted on an inner surface of tube  410  at a first location  481 . A second set of strain gauges includes a second plurality of strain gauges  440 - 2 . Second plurality of strain gauges  440 - 2  is mounted on the inner surface of tube  410  at a second location  482 , which is a fixed distance in the lengthwise direction of force sensor apparatus  403  from first location  481  of first plurality of strain gauges  440 - 1 . 
     A center of each of first plurality of strain gauges  440 - 1  is separated from a center of another of first plurality of strain gauges  440 - 1  by a fixed angle so that first plurality of strain gauges  440 - 1  are uniformly spaced around the inner diameter of tube  410 . The result is a center of each of first plurality of strain gauges is in a first plane, which is perpendicular to first location  481  on the lengthwise axis. 
     Similarly, a center of each of the second plurality of strain gauges  440 - 2  is separated from a center of another of second plurality of strain gauges  440 - 2  by a fixed angle so that the second plurality of strain gauges is uniformly spaced around the inner diameter of tube  410 . The result is a center of each of second plurality of strain gauges  440 - 2  is in a second plane, which is perpendicular to second location  482  on the lengthwise axis. 
     In one aspect, the second plurality of strain gauges  440 - 2  includes four strain gauges, for example, see  FIG. 4C , where second plurality of strain gauges  440 - 2  includes strain gauges RA, RB, RC, and RD that are uniformly spaced about the inner surface of tube  410 . A center of each strain gauge is separated from the center of another strain gauge by a ninety degree angle. Also, the center of each of strain gauges RA, RB, RC, and RD is in an X-Y plane, which is perpendicular to the lengthwise axis of force sensor apparatus  403 . (Herein and in the drawings, a bold reference numeral and a un-bolded reference numeral are the same reference numeral. Bolding is used only to help distinguish reference numerals from regular text in the description, and is not intended to distinguish between different elements.) 
     The strain gauges in first plurality of strain gauges  440 - 1  are similarly spaced as shown in  FIG. 4C . See also,  FIGS. 5C and 5D . 
     First plurality of strain gauges  440 - 1  is connected to strain gauge amplifiers  448  by first set of leads  441 - 1 . Strain gauge amplifiers  448  are included within an interior volume of tube  410 . The interior volume is bounded by the inner wall of tube  410 . Second plurality of strain gauges  440 - 2  is connected to strain gauge amplifiers  448  by second set of leads  441 - 2 . 
     Leads  445  connect strain gauge amplifiers  448  to electronics in the electronic enclosure in the body of the surgical instrument. Leads  445  pass though shield tube  420 . In one aspect, leads  445  include five leads, a power lead, a ground lead, an X-axis force lead, a Y-axis force lead, and a temperature lead. 
     When a force is applied (perpendicularly to the lengthwise axis) anywhere along the distal end of sensor capsule  410 , which, as just described, is the distal end of a cantilever beam, sensor capsule  410  bends. First and second pluralities of strain gauges  440 - 1 ,  440 - 2  inside tube  410  sense the strain on tube  410  at several locations to measure the bending stress on tube  410 . The stress is a measure of the torque on tube  410  (or bending moment: caused by the length of the beam and the force applied). If the force is applied along tube  410  at a different location along its length (or if the length of tube  410  changes), the torque changes value. However, two pluralities of strain gauges  440 - 1 ,  440 - 2  are separated from each other along the length of tube  410 , as illustrated in  FIG. 4A , and measure the torque at each of the two locations. Algebraic subtracting of the measured torques yields a value for the force on tube  410 , independent of where the force is applied along the length of the tube. This assumes that the force acting on the shaft is at location that is distal to the most distal set of strain gauges. 
     U.S. Pat. No. 8,375,808 B2, which was previously incorporated by reference, demonstrates that for strain gauges mounted on the outer surface of a tube, transverse forces can be measured independent of variations in the effective lever arm length and independent of changes in z-axis forces. While the derivation of the forces and torques in U.S. Pat. No. 8,375,808 B2 is for strain gauges located on the outside of a tube, the derivation is applicable to the strain gauges on the inner surface of the tube also, and so is not considered in further detail herein. 
     First plurality of strain gauges  440 - 1  measures the strain on the cantilever beam, i.e., tube  410 , along the X and Y axes of shaft  402  at first location  481  on the Z-axis of the shaft  402 . Second plurality of strain gauges  440 - 2  measures the strain on the cantilever beam, i.e., tube  410 , along the X and Y axes of shaft  402  at second location  482  on the Z-axis of shaft  402 , where the first location is different from the second location. 
     Two strain gauges in first plurality of strain gauges  440 - 1  that are located on the X-axis at first location  481  and two strain gauges in second plurality of strain gauges  440 - 2  that are located on the X-axis at second location  482  interconnected to form a full Wheatstone bridge arrangement. The output from the two X-axis strain gauges in first plurality of strain gauges  440 - 1  at first location  481  and the output from the two X-axis strain gauges in second plurality of strain gauges  440 - 2  at second location  482  go to an amplifier in strain gauge amplifiers  448 , where the two outputs are subtracted and amplified, yielding the X-axis force on shaft  402 . Herein, when it is said that a strain gauge is located on an axis, it means that the center of the strain gauge is positioned approximately on that axis, i.e., the center is on the axis to within manufacturing tolerances. 
     Similarly, two strain gauges in first plurality of strain gauges  440 - 1  that are located on the Y-axis at first location  481  and two strain gauges in second plurality of strain gauges  440 - 2  that are located on the Y-axis at second location  482  are interconnected to form a full Wheatstone bridge arrangement. The output from the two Y-axis strain gauges in first plurality of strain gauges  440 - 1  at first location  481  and the output from the two Y-axis strain gauges in second plurality of strain gauges  440 - 2  at second location  482  go to a second amplifier in strain gauge amplifiers  448 , where the two outputs are subtracted and amplified, yielding a signal that represents the Y-axis force on shaft  402 . 
     Hence, unlike prior configurations that output the torques to electronics in the housing of the surgical instrument or elsewhere in the surgical system to obtain the X-axis and Y-axis forces, force sensor apparatus  403  generates signals representing the X-axis and Y-axis forces directly, and so is said to generate the X-axis and Y-axis forces directly. 
     This has several advantages. It reduces the number of full Wheatstone Bridges that are required in force sensor apparatus  403 . It reduces the number of leads that are required to carry signals off force sensor apparatus  403  and it reduces the number of amplifiers needed. It also eliminates the need for processor time to generate the forces from the measured torques. The reduction in the number of Wheatstone bridges, leads, and amplifiers required makes it feasible to implement force sensor apparatus  403  in a smaller volume, which is advantageous in a restricted volume such as that in the distal end of a surgical instrument shaft. 
     To facilitate mounting a set of strain gauges inside tube  410 , in one aspect, the strain gauges are made as a strip of four strain gauges in a row.  FIG. 4B  is an illustration of second plurality of strain gauges  440 - 2 , where strain gauges RA, RB, RC, and RD are mounted in a row on a printed circuit assembly to form strip  443 - 2 , which in this example is a rectangular strip. First plurality of strain gauges  440 - 1  has the same configuration. See  FIG. 5E . 
     Strain gauges RA, RB, RC, and RD are soldered to a printed circuit assembly and the result is referred to as strip  443 - 2 . Strip  443 - 2  is curled into a cylinder to be placed inside tube  410 . X-axis strain gauges and Y-axis strain gauges alternate on strip  443 - 2 , so that when strip  443 - 2  is placed inside tube  410 , the X-axis strain gauges are centered over the X-axis of tube  410  and the Y-axis strain gauges are centered over the Y-axis of tube  410 . This quad design helps position the strain gauges at the correct spot on the tube&#39;s inside surface. 
     A length L of strip  443 - 2  is made as small as possible, leaving a gap  438  between the two ends of strip  443 - 2  when strip  443 - 2  is mounted within tube  410 , as illustrated in  FIG. 4C . The purpose of gap  438  is to allow strip  443 - 2  to be curled-up tighter (with no gap) during insertion, making the curled strip diameter smaller than the tube inside diameter. The outer surface of curled strip  443 - 2  is tacky with adhesive and requires a reduced diameter during insertion into the tube. Thus, length L of strip  443 - 2  is smaller than an inner circumference of the inner surface of tube  410 . 
       FIGS. 5A and 5B  are schematics of Wheatstone Bridges and amplifiers used to generate signals representative of the forces on cantilever beam-type second cylindrical tube  410 .  FIGS. 5C and 5D  illustrate the orientation of the strain gauges in force sensor apparatus  403 .  FIG. 5E  is an illustration of the strain gauge and amplifier configuration that is inserted within tube  410 . 
     A center of each of X-axis strain gauges R 1  and R 3  in first plurality of strain gauges  440 - 1  ( FIG. 5D ) is located on X-axis  491 - 1  in a plane, which extends through first location  481  along the lengthwise axis of shaft  402 . A center of each of strain gauges RA and RC in second plurality of strain gauges  440 - 2  ( FIG. 5C ) is located on X-axis  491 - 2 , in a plane, which extends through second location  482 . 
     Strain gauges R 1  and R 3  and strain gauges RA and RC are connected to form a full X-axis Wheatstone bridge as illustrated in  FIG. 5A . Resister RL is the resistance of a lead RL in  FIG. 5E . (In  FIGS. 5A and 5B , the same reference numeral is used for an element as for the resistance of that element.) A bridge excitation voltage supply  545 -X powers the two sets of strain gauges in the full X-axis Wheatstone Bridge. 
     X-axis strain gauges R 1  and R 3  form a first leg of the full X-axis Wheatstone Bridge. The output from X-axis strain gauges R 1  and R 3  is a first input to a first amplifier  546 -X. X-axis strain gauges RA and RC form a second leg of the full X-axis Wheatstone Bridge. The output from X-axis strain gauges RA and RC is a second input to amplifier  546 -X of strain gauge amplifiers  448 . Amplifier  546 -X subtracts outputs from the two legs of the Wheatstone Bridge and drives a signal on X-axis force output line  445 -X that represents the X-axis force on shaft  402 . 
     Similarly, a center of each of Y-axis strain gauges R 2  and R 4  in first plurality of strain gauges  440 - 1  ( FIG. 5D ) is located on Y-axis  491 - 1  in the plane, which extends through first location  481  along the lengthwise axis of shaft  402 . A center of each of strain gauges RB and RD in second plurality of strain gauges  440 - 2  ( FIG. 5C ) is located on X-axis  491 - 2  in the plane, which extends through second location  482 . 
     Strain gauges R 2  and R 4  and strain gauges RB and RD are connected to form a full Y-axis Wheatstone bridge as illustrated in  FIG. 5B . Strain gauges R 2  and R 4  form a first leg of the full Y-axis Wheatstone bridge, and strain gauges RB and RD form a second leg of the full Y-axis Wheatstone bridge. Resister RL is the resistance of a lead RL in  FIG. 5E . 
     A bridge excitation voltage supply  545 -Y powers the two sets of strain gauges in the full Y-axis Wheatstone bridge. The output from Y-axis strain gauges R 2  and R 4  is a first input to a second amplifier  546 -Y and the output from Y-axis strain gauges RB and RD is a second input to amplifier  546 -Y of strain gauge amplifiers  448 . 
     Amplifier  546 -Y subtracts two outputs and drives a signal on Y-axis force output line  445 -Y that represents the Y-axis force on shaft  402 . In one aspect, bridge excitation voltage supply  545 -Y and bridge excitation voltage supply  545 -X are a common power supply. 
     As described above, strain gauges R 1 , R 2 , R 3 , and R 4  are soldered to a first printed circuit assembly, i.e., surface mounted on the printed circuit assembly, to form a first plurality of strain gauges  440 - 1  ( FIG. 5E ). Strain gauges RA, RB, RC, and RD are soldered to a second printed circuit assembly to form a second plurality of strain gauges  440 - 2 . In one aspect, each of strain gauges R 1 , R 2 , R 3 , R 4 , RA, RB, RC, and RD is a high-temperature-rated, instrument-grade strain gauge made of Karma material so that the zero-load output drifts minimally vs. autoclave cycles. 
     Strain gauge amplifiers  448  do not include any analog-to-digital converter integrated circuits or any differential driver integrated circuits. These integrated circuits are included in the electronics within the Faraday cage enclosure in the surgical instrument body. Strain gauge amplifiers  448  are mounted on a rigid printed circuit board  548 . A first integral flex circuit  541 - 1  connects rigid printed circuit board  548  to first plurality of strain gauges  440 - 1 . A second integral flex circuit  541 - 2  connects rigid printed circuit board  548  to second plurality of strain gauges  440 - 2 . A center of first plurality of strain gauges  440 - 1  is separated from a center of second plurality of strain gauges by a lengthwise distance Zsep, which in one aspect is 34 mm. 
     As used herein, “first,” “second,” “third,” etc. are adjectives used to distinguish between different components or elements. Thus, “first,” “second,” and “third” are not intended to imply any ordering of the components or elements or to imply any total number of components or elements. 
     The above description and the accompanying drawings that illustrate aspects and embodiments of the present inventions should not be taken as limiting—the claims define the protected inventions. Various mechanical, compositional, structural, electrical, and operational changes may be made without departing from the spirit and scope of this description and the claims. In some instances, well-known circuits, structures, and techniques have not been shown or described in detail to avoid obscuring the invention. 
     Further, this description&#39;s terminology is not intended to limit the invention. For example, spatially relative terms—such as “beneath”, “below”, “lower”, “above”, “upper”, “proximal”, “distal”, and the like—may be used to describe one element&#39;s or feature&#39;s relationship to another element or feature as illustrated in the figures. These spatially relative terms are intended to encompass different positions (i.e., locations) and orientations (i.e., rotational placements) of the device in use or operation in addition to the position and orientation shown in the figures. For example, if the device in the figures were turned over, elements described as “below” or “beneath” other elements or features would then be “above” or “over” the other elements or features. Thus, the exemplary term “below” can encompass both positions and orientations of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly. Likewise, descriptions of movement along and around various axes include various special device positions and orientations. 
     The singular forms “a”, “an”, and “the” are intended to include the plural forms as well, unless the context indicates otherwise. The terms “comprises”, “comprising”, “includes”, and the like specify the presence of stated features, steps, operations, elements, and/or components but do not preclude the presence or addition of one or more other features, steps, operations, elements, components, and/or groups. Components described as coupled may be electrically or mechanically directly coupled, or they may be indirectly coupled via one or more intermediate components. 
     All examples and illustrative references are non-limiting and should not be used to limit the claims to specific implementations and embodiments described herein and their equivalents. Any headings are solely for formatting and should not be used to limit the subject matter in any way, because text under one heading may cross reference or apply to text under one or more headings. Finally, in view of this disclosure, particular features described in relation to one aspect or embodiment may be applied to other disclosed aspects or embodiments of the invention, even though not specifically shown in the drawings or described in the text. 
     Embodiments described above illustrate but do not limit the disclosure. It should also be understood that numerous modifications and variations are possible in accordance with the principles of the present disclosure. For example, in many aspects the devices described herein are used as single-port devices; i.e., all components necessary to complete a surgical procedure enter the body via a single entry port. In some aspects, however, multiple devices and ports may be used.