Patent Publication Number: US-2023150117-A1

Title: Six degree-of-freedom and three degree-of-freedom robotic systems for automatic and/or collaborative fastening operations

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of Chinese Patent Application No. 202111363486.1, filed on Nov. 17, 2021. The entire disclosure of the application referenced above is incorporated herein by reference. 
     INTRODUCTION 
     The information provided in this section is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure. 
     The present disclosure relates to robotic systems used for fasteners during production. 
     During production of, for example, a vehicle, numerous fasteners (e.g., nuts, screws, bolts, etc.) are fastened to vehicle devices, assemblies, components, and structures. The fasteners may be fastened manually or using a fully automatic robotic system. When attached manually, a considerable amount of time is associated with setting, tightening (referred to herein as “running”), and properly torqueing down the fasteners. Cross-threading errors can occur when the fasteners are fastened manually, which slows production and increases costs due to the repair and/or replacement of the parts involved. At the same time, the operator needs to hold an electric tightening gun, which can take great strength to hold. If this process is repeated continuously, it can cause fatigue. 
     Although a fully automated robotic system can save time installing fasteners, the fully automated system is configured for a particular application and a particular device and/or component. For example, if nuts are being installed on an engine, the automated robotic system includes a one stop station that is configured for the particular engine and nuts involved. The nuts are typically the same size. The automated robotic system is not applicable to other devices and/or components. In addition, the fully automated system may include multiple fastening tools (e.g., nut runners) for fastening the nuts. A fully automated robotic system is bulky, complex and expensive. 
     SUMMARY 
     A robotic system is provided and includes a support structure, a movable platform, a center serial chain, outer serial chains, motors, a sensor, and a control module. The center serial chain connects directly or indirectly a center of the movable platform to the support structure and includes first joints connected directly or indirectly to a linear sliding shaft. The outer serial chains are disposed radially outward of the center serial chain. Each of the outer serial chains includes second joints connecting a bar directly or indirectly to the moveable platform and the supporting structure. The motors are connected to the outer serial chains. The sensor is connected to the movable platform and configured to detect at least one of force or torque applied by a human operator on the movable platform and generate a signal indicative of the at least one of force or torque applied. The control module is configured to control the motors based on the signal to assist the human operator in at least one of moving or rotating the movable platform. 
     In other features, the outer serial chains include at least three outer serial chains. 
     In other features, the outer serial chains include three pairs of outer serial chains. Each pair of outer serial chains includes two outer serial chains. 
     In other features, the outer serial chains includes six outer serial chains. 
     In other features, the second joints of each of the outer serial chains includes a first joint and a second joint. The first joint of each of the outer serial chains is a universal joint. The second joint of each of the outer serial chains is a spherical joint. 
     In other features, the outer serial chains include three pairs of chains. The control module is configured to independently actuate each of the outer serial chains. 
     In other features, the robotic system further includes linear sliders. The outer serial chains include three pairs of chains. Each of the three pairs of chains is connected to a respective one of the linear sliders. 
     In other features, the first joints of the center serial chain include a first joint and a second joint. The first joint and the second joint are universal joints. 
     In other features, a drive fork of the first joint is in alignment with a drive fork of the second joint. A driven fork of the first joint is in alignment with a driven fork of the second joint. 
     In other features, the outer serial chains and the center serial chain provide three degrees-of-freedom motion for the movable platform or six degrees-of-freedom motion for the movable platform. 
     In other features, the outer serial chains include: a first outer serial chain, a second outer serial chain, and a third outer serial chain. The second outer serial chain is disposed 120° of separation azimuthally from the first outer serial chain relative to a center line extending through the center serial chain. The third outer serial chain is disposed 120° of separation azimuthally from the first outer serial chain and the second outer serial chain relative to the center line extending through the center serial chain. 
     In other features, the outer serial chains include: a first outer serial chain; a second outer serial chain and a third outer serial chain. The second outer serial chain is disposed 90° of separation azimuthally from the first outer serial chain relative to a center line extending through the center serial chain. The third outer serial chain is disposed 180° of separation azimuthally from the first outer serial chain and the second outer serial chain relative to the center line extending through the center serial chain. 
     In other features, the robotic system further includes: a fastening tool; and a bearing disposed between the movable platform and the fastening tool. 
     In other features, the robotic system further includes a fastening tool attached to the center serial chain and rotating at least a portion of the center serial chain. 
     In other features, a robotic system is provided and includes a support structure, a movable platform, a universal-prismatic-universal serial chain, motors, a sensor and a control module. The universal-prismatic-universal serial chain directly or indirectly connects a center of the movable platform to the support structure. The outer serial chains are disposed radially outward of the universal-prismatic-universal serial chain Each of the outer serial chains is a prismatic-universal-spherical serial chain. Each of the outer serial chains connects the movable platform to the support structure. The motors are connected to the outer serial chains. The sensor is connected to the movable platform and configured to detect at least one of force or torque applied by a human operator on the movable platform and generate a signal indicative of the at least one of force or torque applied. The control module is configured to control the motors based on the signal to assist the human operator in at least one of moving or rotating the movable platform. 
     In other features, the universal-prismatic-universal serial chain includes a first universal joint, a linear sliding shaft, and a second universal joint. 
     In other features, each of the outer serial chains comprise a linear slider, a universal joint, a bar and a spherical joint. 
     In other features, a robotic system is provided and includes a support structure, a movable platform, a universal-prismatic-universal serial chain, revolute-universal-spherical serial chains, motors, a sensor, and a control module. The universal-prismatic-universal serial chain directly or indirectly connects a center of the movable platform to the support structure. The revolute-universal-spherical serial chains are disposed radially outward of the universal-prismatic-universal serial chain. Each of the revolute-universal-spherical serial chains connects the movable platform to the support structure. The motors are connected to the revolute-universal-spherical serial chains. The sensor is connected to the movable platform and configured to detect at least one of force or torque applied by a human operator on the movable platform and generate a signal indicative of the at least one of force or torque applied. The control module is configured to control the motors based on the signal to assist the human operator in at least one of moving or rotating the movable platform. 
     In other features, the universal-prismatic-universal serial chain includes a first universal joint, a linear sliding shaft, and a second universal joint. 
     In other features, each of the revolute-universal-spherical serial chains includes a revolute joint, a first bar, a universal joint, a second bar and a spherical joint. 
     Further areas of applicability of the present disclosure will become apparent from the detailed description, the claims and the drawings. The detailed description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG.  1    is a front perspective view of an example of a six degree-of-freedom (6-DOF) robotic system mounted on a stand in accordance with the present disclosure; 
         FIG.  2    is a top front perspective view of the 6-DOF robotic system of  FIG.  1   ; 
         FIG.  3    is a bottom front perspective view of the 6-DOF robotic system of  FIG.  1   ; 
         FIG.  4    is a top perspective view of a portion of the 6-DOF robotic system of 
         FIG.  1    without a supporting frame; 
         FIG.  5    is a bottom perspective view of a portion of the 6-DOF robotic system of  FIG.  1    without a supporting frame; 
         FIG.  6    is a bottom view of the 6-DOF robotic system of  FIG.  1   ; 
         FIG.  7    is a top perspective view of a center serial chain of the 6-DOF robotic system of  FIG.  1   ; 
         FIG.  8    is a bottom perspective view of a center serial chain of the 6-DOF robotic system of  FIG.  1   ; 
         FIG.  9    is a side block representative view of the center serial chain of  FIGS.  7 - 8   ; 
         FIG.  10    is a bottom perspective view of a 6-DOF robotic system including a top mounted fastening motor in accordance with the present disclosure; 
         FIG.  11    is a front perspective view of an example of a 6-DOF robotic system mounted on a stand and including six motors for rotating six bars in accordance with the present disclosure; 
         FIG.  12    is a top rear perspective view of the 6-DOF robotic system of  FIG.  11   ; 
         FIG.  13    is a bottom front perspective view of the 6-DOF robotic system of  FIG.  11   ; 
         FIG.  14    is a top perspective view of a center serial chain of the 6-DOF robotic system of  FIG.  11   ; 
         FIG.  15    is a bottom perspective view of a center serial chain of the 6-DOF robotic system of  FIG.  11   ; 
         FIG.  16    is a front perspective view of an example of a 3-DOF robotic system mounted on a stand and including three rotary motors in accordance with the present disclosure; 
         FIG.  17    is a side view of the 3-DOF robotic system and stand of  FIG.  16   ; 
         FIG.  18    is a top view of the 3-DOF robotic system and stand of  FIG.  16   ; 
         FIG.  19    is a top front perspective view of the 3-DOF robotic system of  FIG.  16   ; 
         FIG.  20    is a bottom front perspective view of the 3-DOF robotic system of  FIG.  16   ; 
         FIG.  21    is a side view of the 3-DOF robotic system of  FIG.  16   ; 
         FIG.  22    is a bottom view of the 3-DOF robotic system of  FIG.  16   ; 
         FIG.  23    is a top perspective view of a center serial chain of the 3-DOF robotic system of  FIG.  16   ; 
         FIG.  24    is a bottom perspective view of a center serial chain of the 3-DOF robotic system of  FIG.  16   ; and 
         FIG.  25    illustrates a method of operating a robotic system in accordance with the present disclosure. 
     
    
    
     In the drawings, reference numbers may be reused to identify similar and/or identical elements. 
     DETAILED DESCRIPTION 
     Fully automated robotic systems typically include controllers, motors, arms, end effectors, sensors, etc. for automatically positioning, setting, attaching and/or fastening components. No human interaction is involved. Each of the fully automatic robotic systems are application limited, complex, expensive and require a considerable amount of space. 
     The examples set forth herein include 6-DOF and 3-DOF robotic systems (referred to as the “robotic systems”) that are automatic and/or collaborative. Fastening operations may be performed automatically and/or collaboratively. The robotic systems utilize human senses and intelligence to ensure fast and accurate fastening at the beginning of an operation while leaving the majority of operations with the robotic system alone. The robotic systems include platforms that are moveable by a human system operator with little resistance and include fastening tools that once positioned perform fastening operations without aid of the system operator. The robotic systems have high payload capability and are low cost and flexible, such that each robotic system is applicable to many different devices and components. The disclosed robotic systems may be used on various vehicle and non-vehicle systems, assemblies, devices, etc. The robotic systems may be used on, for example, vehicle systems, vehicle sub-systems, engines, instrument panels, wheels, doors, panels, etc. Although the following robotic systems are shown in  FIGS.  1 - 24    in a vertical upright arrangement. The robotic systems may be arranged at angles, horizontally, and upside down. 
       FIGS.  1 - 8    show a 6-DOF robotic system  100  mounted on a stand  102 . The stand  102  includes a platform (or table)  104  that supports a device (e.g., an engine)  106  set thereon. Although the device  106  is shown, other worked on objects may be disposed on the platform  104 . An operator  108  stands in a front open area of the stand  102  and may move a lower end  109  of the 6-DOF robotic system  100  to set a fastener on the device  106 . The operator  108  may move the lower end  109  via handles  110  to move a fastening tool (e.g., a nut runner)  112  having a fastener holding tip  114  to the location on the device  106  where the fastener is to be attached and fastened to the device  106 . The fastening tool  112  may hold various fastener holding tips for various types and styles of fasteners. Each fastener holding tip may be adjustable for different types and styles of fasteners. 
     The 6-DOF robotic system  100  includes a frame  120  that has a top plate  122  and is supported by the stand  102 , six prismatic-universal-spherical serial chains (referred to as the outer serial chains)  124 , and a center universal-prismatic-universal serial chain (referred to as the center serial chain)  126 . 
     The six outer serial chains  124  include three pairs of chains. The chains of each pair may be connected in parallel or not. For example, if the tool is used to work on a nut that is not on a flat surface, the two links on a pair of chains might be twisted (spatially) relatively to each other. The six outer serial chains  124  include six linear sliders  128  mounted on the frame  120  and attached to a movable platform  130  via six bars (or links)  132 . The bars  132  are attached to the linear sliders  128  via universal joints  134 , each having 2-DOF. The linear sliders  128  are attached to three vertical plates  135 , which are attached to the frame  120 . The bars  132  are attached to the platform  130  via spherical joints  136 , which have 3-DOF. Each of the linear sliders  128  may include a respective ball screw (one ball screw  138  is shown in  FIG.  3   ) that is rotated by a respective rotary motor (rotary motors  140  are shown). The rotary motors  140  are attached to the linear sliders  128  via brackets  141 . The term “prismatic-universal-spherical” refers to the linear sliders  128 , the universal joints  134 , and the spherical joints  136 . The linear slider  128  may be referred to as a prismatic joint. The six outer serial chains  124  provide six individually driven serial chains for 6-DOF motion control. 
     The center serial chain  126  may be directly or indirectly coupled to the frame  120  and/or the stand  102  and/or other supporting structure, such as the top plate  122 . The center serial chain  126  includes: a first universal joint  150  mounted to the top plate  122  via a coupling  154 ; a linear telescopic sliding shaft (or first shaft)  156  including an inner member  158  and an outer member  160 ; a second universal joint  162 ; a second shaft  164 ; the platform  130 ; and the fastening tool (e.g., a nut runner)  112 . The term “universal-prismatic-universal” or “UPU” refers to the first universal joint  150 , the first shaft  156 , and the second universal joint  162 . The first shaft  156  may be referred to as a prismatic joint. The second universal joint  162  is in alignment with the first universal joint  150 , such that the axis along the pair of forks of the first universal joint  150  mounted on the first shaft  156  (on inner member  158 ) is parallel to the axis along the pair of forks of the second universal joint  162  mounted on the first shaft  156  (on outer member  160 ). The center serial chain  126  is able to extend or retract due to the first shaft  156 , where the outer member  160  is free to slide relative to the inner member  158 , which allows vertical movement of the platform  130 . The center chain (or the UPU chain) provides 5DOF, where the rotational motion around the chain axis (vertical axis at the initial position) is limited. This allows the chain to counter the twist torque of the nutrunner when in passive mode, or transfer torque to the tip end when in active mode. When in active mode, a motor may be implemented behind the first U joint. The flange of the motor is fixed on the top plate. The motor shaft is connected to the first U joint via a coupling. The center serial chain  126  allows for 6-DOF motion while in an active state and applying torque only. More specifically, the 6 th  DOF is provided when driving actively, where the first universal joint is able to rotate by the driving motor and the other 5DOF are provided when using the center serial chain for torque transferring and resistance. In the 5DOF case, the first universal joint is locked from rotations, thus one less DOF. The universal joints  150 ,  162  resist and/or counteract torque associated with running the fastening tool  112 . 
     The platform  130  is held in place by the six outer serial chains (or legs)  124  and the center serial chain (or leg)  126  and is able to be moved with little resistance by an operator via the handles  110 . A control module  170  is connected to the rotary motors  140  of the linear sliders  128 , a motor  171  of the fastening tool  112 , and a sensor  172  and controls positioning of the platform  130  and thus the fastening tool  112  relative to the frame  120 , the supporting platform  104 , and the device  106 . The control module  170  may detect force applied on the handles  110  via the sensor  172  and in response provide active compliance by assisting an operator  108  in movement of the platform  130  in the direction of the applied force based on feedback from the sensor  172 . The platform  130  may be moved in x, y, z directions and may be tilted about the x, y, z axes. 
     In an embodiment, the 6-DOF system  100  operates as a collaborative system by which (i) sensing, movement of the platform  130  to a start position, and closed loop feedback is provided by the operator  108 , and (ii) sensing, movement of the platform  130  to a start position, and fastening (or torqueing down) a fastener is performed by the robotic system  100 . In one embodiment, the operator  108  attaches a fastener to the tip of the fastening tool  112 , moves the platform  130  with the assistance of the robotic system  100  to a start position, indicates to start fastening the fastener, and waits to hear and/or see a completion indication. The indication to start fastening may be provided by the operator  108  touching an input device  173 , such as a start button on the platform  130  or elsewhere. The input device  173  may be located on the robotic system  100 , the stand  102 , or elsewhere. The completion indication may be provided by an indicator  174 . The indicator  174  may include a light, a speaker, a clicking device configured to generate a “click” sound when a predetermined torque level has been reached on the corresponding fastener, a message on a display, etc. In one embodiment, the fastening tool  112  generates the click sound when a fastener has been torqued down to the predetermined level. In another embodiment, the control module  170  automatically controls initial positioning of the fastening tool to set fastening locations and fastening of fasteners. 
     The control module  170  controls operation of the rotary motors  140  and the motor  171  of the fastening tool  112  based on feedback from the sensor  172 . The sensor  172  may be mounted to the platform  130  as shown and provides feedback to the control module  170 . In one embodiment, the sensor  172  is a 6-dimensional force and torque sensor that measures force and torque exerted on the platform  130  by the operator  108  and the fastening tool  112 . The sensor  172  measures forces and torques in Cartesian coordinate directions (x, y, z) and corresponding angular torques about the x, y, z axes. 
       FIG.  9    shows a block representative view of the center serial chain  126  of  FIGS.  1 - 8    (referred to as the center serial chain  126 ′). The center serial chain  126 ′ includes: the first universal joint  150 ′ mounted to the top plate  122 ′; the linear telescopic sliding shaft (or first shaft)  156 ′; the second universal joint  162 ′; the second shaft  164 ′; the platform (or end effector)  130 ′; and the fastening tool (e.g., a nut runner)  112 ′ with tip  114 ′.  FIG.  9    shows the case when the center chain is used passively resisting the fastening torque from the torque gun mounted center of  130 . The first universal joint  150 ′ may also be driven by a torque generator/wrench from top plate  122 ′ if used to actively provide fastening torque from the top, as further described below). The fastening tool  112 ′ includes motor  171 ′ that rotates a shaft  900 , which in turn rotates the tip  114 ′. In the example shown, the fastening tool  112 ′ extends through the platform  130 ′, the sensor  172 ′, and rides on a bearing  902  mounted in the platform  130 ′. The platform  130 ′ may include two plates with the sensor  172 ′ disposed between and in contact with the two plates, as shown. Platform  130 ′ is connected to linear sliders  128 ′ via bars  132 ′. The universal joints  150 ′,  162 ′ are aligned with each other as described above. The universal joints  150 ′,  162 ′ resist and/or counteract torque associated with running the fastening tool  112 ′. The outer serial chains  124  associated with the bars  132 ′ do not resist and/or counteract torque associated with running the fastening tool  112 ′ due to the bearing  902 , which allows the fastening tool  112 ′ to rotate relative to the platform  130 ′ and an axis of rotation  906 . The fastening tool  112 ′ is separated from the platform  130 ′ by the bearing  902 . 
       FIG.  10    shows a 6-DOF robotic system  1000  that is similar to the 6-DOF robotic system  100  of  FIGS.  1 - 8   , except instead of including a fastening tool attached near the tip of the center serial chain, a top mounted fastening motor  1002  is included. The top mounted fastening motor  1002  is attached to a first universal joint  1004 , which is attached to a prismatic joint (or first shaft)  1006 . The first shaft  1006  may include inner and outer members similar to the members  158 ,  160  of  FIGS.  1 - 8   . The prismatic joint  1006  is attached to a second universal joint  1008 , which is attached to a second shaft  1010 . The second shaft  1010  is attached to a platform  1012 , which includes a sensor  1014 . The sensor  1014  may be configured and operate similarly as the sensor  172  of  FIGS.  1 - 8   . The second shaft  1010  rotates the end shaft  1016  that is connected to a tip  1018 . 
     The 6-DOF robotic system  1000  also includes a frame  1020 , a top plate  1022 , rotary motors  1024  that actuate linear sliders  1026 , which in turn move bars  1028  of respective outer serial chains. The motors  1002 ,  1024  are controlled via a control module (e.g., the control module shown in  FIG.  4   ) based on output of the sensor  1014 . 
     Although the six outer serial chains are shown as including three pairs of outer serial chains, where each pair is positioned 120° apart from each other relative to the centerline  166 , the pairs of outer serial chains may have different angles of separation. For example, two of the pairs may be 180° apart, where the third pair is 90° apart from the other two pairs. See, for example, the arrangement shown in  FIGS.  11 - 13   . By having two pairs 180° apart, the pairs of outer serial chains are out of the way and allow an operator to better access the device being worked on and the movable platform  130 . 
       FIGS.  11 - 15    show a 6-DOF robotic system  1100  mounted on a stand  1102 . The stand  1102  includes a platform (or table)  1104  that supports a device (e.g., an engine)  1106  set thereon. An operator  1108  stands in a front open area of the stand  1102  and may move a lower end  1109  of the 6-DOF robotic system  1100  to set a fastener on the device  1106 . The operator  1108  may move the lower end  1109  to move a fastening tool (e.g., a nut runner)  1112  having a fastener holding tip  1114  to the location on the device  1106  where the fastener is to be attached and fastened to the device  1106 . 
     The 6-DOF robotic system  1100  includes a top plate  1120  that is attached to the stand  1102 , six revolute-universal-spherical serial chains (referred to as the outer serial chains)  1124 , and a center universal-prismatic-universal serial chain (referred to as the center serial chain)  1126 . 
     The six outer serial chains  1124  include six bars  1128  rotated by six rotary motors  1129  that are mounted to the top plate  1120 . The six bars  1128  are attached to another six bars  1132  via universal joints  1134 . The six bars  1132  are attached to a movable platform  1136  via spherical joints  1138 . Each of the universal joints  1134  has 2-DOF motion. The spherical joints  1138  have 3-DOF motion. 
     The term “revolute-universal-spherical” refers to (i) the connections between the rotary motors  1129  and bars  1128 , (ii) the universal joints  1134 , and (iii) the spherical joints  1138 . The connection of each pair of one of the rotary motors  1129  and one of the bars  1128  is a revolute joint  1140 , which allows for a swinging motion of the bars  1128 . The six outer serial chains  124  provide six individually driven serial chains for 6-DOF motion control. 
     The center serial chain  1126  includes a first universal joint  1150  mounted to a cap  1152 . The cap  1152  is mounted to the top plate  1120 . The center serial chain  1126  further includes: a linear telescopic sliding shaft (or first shaft)  1156  including an inner member  1158  and an outer member  1160 ; a second universal joint  1162 ; a second shaft  1164 ; the platform  1136 ; and the fastening tool (e.g., a nut runner)  1112 . The term “universal-prismatic-universal” refers to the first universal joint  1150 , the first shaft  1156 , and the second universal joint  1162 . The first shaft  1156  may be referred to as a prismatic joint. The second universal joint  1162  is in alignment with the first universal joint  1150 , such that the axis along the pair of forks of the first universal joint  1150  mounted on the first shaft  1156  (on inner member  1158 ) is parallel to the axis along the pair of forks of the second universal joint  1162  mounted on the first shaft  1156  (on outer member  160 ). The center serial chain  1126  is able to extend or retract due to the first shaft  1156 , where the outer member  1160  is free to slide relative to the inner member  1158 , which allows vertical movement of the platform  1136 . The center chain (or the UPU chain) provides 5DOF, where the rotational motion around the chain axis (vertical axis at the initial position) is limited. This allows the chain to counter the twist torque of the nutrunner when in passive mode, or transfer torque to the tip end when in active mode. When in active mode, a motor may be implemented behind the first U joint. The flange of the motor is fixed on the top plate. The motor shaft is connected to the first U joint via a coupling. The center serial chain  1126  allows for 6-DOF motion only in an active state and applying torque, while 5DOF are allowed in a passive state and countering driving torque of the fastening tool  1112 . More specifically, 6DOF are provided when driving actively, where the first universal joint is able to rotate by the driving motor to provide an additional DOF and the other 5DOF are provided when using the center serial chain for torque transferring and resistance. In the 5DOF case, the first universal joint is locked from rotations, thus one less DOF. The universal joints  1150 ,  1162  resist and/or counteract torque associated with running the fastening tool  1112 . 
     The platform  1136  is held in place by the six outer serial chains (or legs)  1124  and the center serial chain (or leg)  1126  and is able to be moved with little resistance by an operator. A control module (e.g., the control module  170  of  FIG.  4   ) connected to the rotary motors  1129 , a motor of the fastening tool  1112 , and a sensor  1172  and controls positioning of the platform  1136  and thus the fastening tool  1112  relative to the stand  1102 , the top plate  1120 , the supporting platform  1104 , and the device  1106 . The control module may detect force applied on the platform  1136  via the sensor  1172  and in response assist the operator  1108  in movement of the platform  1136  in the direction of the applied force based on feedback from the sensor  1172 . Thus, active compliance is provided as described above. The sensor  1172  may be configured and operate similarly as the sensor  172  of  FIGS.  1 - 8   . The platform  1136  may be moved in x, y, z directions and may be tilted about the x, y, z axes. The platform  1136  may include handles as described above for movement of the platform  1136 . 
     The above-described 6-DOF robotic systems provide freedom of operations in different x, y, z positions and angles relative to the x, y, z axes. Parallel robot serial chains provide high load capacity and system rigidity. The outer serial chains may be arranged at various angles for improved operator access. 
       FIGS.  16 - 24    show a 3-DOF robotic system  1600  mounted on a stand  1602 . The stand  1602  includes a platform (or table)  1604  that supports a device (e.g., an engine)  1606  set thereon. An operator  1608  stands in a front open area of the stand  1602  and may move a lower end  1609  of the 3-DOF robotic system  1600  to set a fastener on the device  1606 . The operator  1608  may move the lower end  1609  to move a fastening tool (e.g., a nut runner)  1612  having a fastener holding tip  1614  to the location on the device  1606  where the fastener is to be attached and fastened to the device  1606 . 
     The 3-DOF robotic system  1600  includes a frame  1601  that is attached to the stand  1602 , a top plate  1620 , three prismatic-universal-universal (PUU) serial chains (referred to as the outer serial chains)  1624 , and a center universal-prismatic-universal serial chain (referred to as the center serial chain)  1626 . Although described as PUU serial chains, the chains may be prismatic-universal-spherical (PUS) serial chains, where the last joint in each chain is a spherical joint rather than a universal joint. To provide the 3-DOF, universal joints may be used, however to make manufacturing and assembly of the robotic system  1600  easier spherical joints may be used. 
     The three outer serial chains  1624  include six bars  1628  attached to three linear sliders  1629 , which are actuated by three rotary motors  1630  that are mounted to the top plate  1620 . Three pairs of the six bars  1628  are attached to the three linear sliders  1629  (referred to as prismatic joints) via six universal joints  1634 . Each pair of the bars  1628  are connected in parallel. The linear sliders  1629  may include respective ball screws  1631 . The six bars  1628  are also attached to a movable platform  1636  via universal or spherical joints  1638 . Each of the joints  1634 ,  1638  has 2-DOF motion. 
     The terms “prismatic-universal-universal” and “prismatic-universal-spherical” refers to the linear sliders  1629 , the universal joints  1634 , and the joints  1638 , which may be universal or spherical joints. The three outer serial chains  1624  provide three individually driven serial chains for 3-DOF motion control. Each of the outer serial chains  1624  includes a sub-mechanism including two parallel universal-universal chains provided by the joints  1634 ,  1638  and the bars  1628 . The three outer serial chains  1624  maintains orientation of the end effector (or platform)  1636 . 
     The center serial chain  1626  includes a first universal joint  1650  mounted to a cap  1652 . The cap  1652  is mounted to the top plate  1620 . The center serial chain  1626  further includes: a linear telescopic sliding shaft (or first shaft)  1656  including an inner member  1658  and an outer member  1660 ; a second universal joint  1662 ; a second shaft  1664 ; the platform  1636 ; and the fastening tool (e.g., a nut runner)  1612 . The term “universal-prismatic-universal” refers to the first universal joint  1650 , the first shaft  1656 , and the second universal joint  1662 . The first shaft  1656  may be referred to as a prismatic joint. The second universal joint  1662  is in alignment with the first universal joint  1650 , such that the axis along the pair of forks of the first universal joint  1650  mounted on the first shaft  1656  (on inner member  1658 ) is parallel to the axis along the pair of forks of the second universal joint  1662  mounted on the first shaft  1656  (on outer member  160 ). The center serial chain  1626  is able to extend or retract due to the first shaft  1656 , where the outer member  1660  is free to slide relative to the inner member  1658 , which allows vertical movement of the platform  1636 . The center chain (or the UPU chain) provides 5DOF, where the rotational motion around the chain axis (vertical axis at the initial position) is limited. This allows the chain to counter the twist torque of the nutrunner when in passive mode, or transfer torque to the tip end when in active mode. When in active mode, a motor may be implemented behind the first U joint. The flange of the motor is fixed on the top plate. The motor shaft is connected to the first U joint via a coupling. The center serial chain  1626  allows for 6-DOF motion in an active state and applying torque; it allows 5DOF motion in a passive state and countering driving torque of the fastening tool  1612 . More specifically, 6DOF are provided when driving actively, where the first universal joint is able to rotate and 5DOF are provided when using the center serial chain for torque resistance. In the 5DOF case, the first universal joint is locked from rotations, thus one less DOF. The universal joints  1650 ,  1662  resist and/or counteract torque associated with running the fastening tool  1612 . 
     The platform  1636  is held in place by the three outer serial chains (or legs, with each chain/leg consisting of two parallel links)  1624  and the center serial chain (or leg)  1626  and is able to be moved with little resistance by an operator. A control module (e.g., the control module  170  of  FIG.  4   ) connected to the rotary motors  1630 , a motor of the fastening tool  1612 , and a sensor  1672  and controls positioning of the platform  1636  and thus the fastening tool  1612  relative to the stand  1602 , the top plate  1620 , the supporting platform  1604 , and the device  1606 . The control module may detect force applied on the platform  1636  via the sensor  1672  and in response assist the operator  1608  in movement of the platform  1636  in the direction of the applied force based on feedback from the sensor  1672 . Thus, active compliance is provided as described above. The sensor  1672  may be configured and operate similarly as the sensor  172  of  FIGS.  1 - 8   . The platform  1636  may be moved in x, y, z directions. The platform  1636  is maintained in a parallel state relative to the supporting platform  1604 , the top plate  1620 , and/or a ground (or floor) on which the stand  1602  is set. The platform  1636  is not rotatable via the robotic system  1600 . The platform  1636  may include handles as described above for movement of the platform  1636 . 
     The above-described robotic systems include 6-DOF and 3-DOF systems. The 6-DOF systems provide freedom of operations in different x, y, z directions and angles relative to the x, y, z axes. The 3-DOF systems provide freedom of operations in different x, y, z directions. Parallel robot serial chains provide high load capacity and system rigidity. Outer serial chains may be arranged at various angles for improved operator access. The 3-DOF systems are simplified versions of the 6-DOF systems. The 3-DOF system can save build costs but loses the flexibility of fastening, for example, nuts at tilted/angled orientations. 
       FIG.  25    shows a method of operating a robotic system, such as any of the robotic systems disclosed herein. The method may begin at  2500 . At  2502 , a fastening tool may grab a fastener, as described above. At  2504 , a sensor (e.g., the sensor  172  of  FIG.  4    or other sensor disclosed herein) of a platform (e.g., platform  130  of  FIG.  4    or other movable platform) of the robotic system may detect force(s) and/or torque(s) applied to the platform. 
     At  2506 , a control module (e.g., the control module  170  of  FIG.  4   ) may generate one or more control signals respectively for one or more motors based on the output of the sensor. In one embodiment, the operations performed by the control module are implemented as machine-executable instructions stored on a non-transitory computer-readable medium. At  2508 , the control module provides active compliance by controlling output of the one or more motors to assist in movement of the platform based on the one or more motor control signals. The assisted movement may be in x, y, z directions and/or about x, y, z axes. 
     At  2510 , the control module may receive or generate an indication to begin fastening a fastener. This may be based on an input received from a user via the input device (e.g., the input device  173  of  FIG.  4   ) and/or based on a location and/or orientation of the platform. At  2512 , the control module via the fastening tool torques down the fastener to a predetermined torque level. At  2514 , the control module generates an indication that the fastener is torqued down and releases the fastener. If there is another fastener to torque down, operation  2502  may be performed, otherwise the method may end at  2018 . 
     The foregoing description is merely illustrative in nature and is in no way intended to limit the disclosure, its application, or uses. The broad teachings of the disclosure can be implemented in a variety of forms. Therefore, while this disclosure includes particular examples, the true scope of the disclosure should not be so limited since other modifications will become apparent upon a study of the drawings, the specification, and the following claims. It should be understood that one or more steps within a method may be executed in different order (or concurrently) without altering the principles of the present disclosure. Further, although each of the embodiments is described above as having certain features, any one or more of those features described with respect to any embodiment of the disclosure can be implemented in and/or combined with features of any of the other embodiments, even if that combination is not explicitly described. In other words, the described embodiments are not mutually exclusive, and permutations of one or more embodiments with one another remain within the scope of this disclosure. 
     Spatial and functional relationships between elements (for example, between modules, circuit elements, semiconductor layers, etc.) are described using various terms, including “connected,” “engaged,” “coupled,” “adjacent,” “next to,” “on top of,” “above,” “below,” and “disposed.” Unless explicitly described as being “direct,” when a relationship between first and second elements is described in the above disclosure, that relationship can be a direct relationship where no other intervening elements are present between the first and second elements, but can also be an indirect relationship where one or more intervening elements are present (either spatially or functionally) between the first and second elements. As used herein, the phrase at least one of A, B, and C should be construed to mean a logical (A OR B OR C), using a non-exclusive logical OR, and should not be construed to mean “at least one of A, at least one of B, and at least one of C.” 
     In the figures, the direction of an arrow, as indicated by the arrowhead, generally demonstrates the flow of information (such as data or instructions) that is of interest to the illustration. For example, when element A and element B exchange a variety of information but information transmitted from element A to element B is relevant to the illustration, the arrow may point from element A to element B. This unidirectional arrow does not imply that no other information is transmitted from element B to element A. Further, for information sent from element A to element B, element B may send requests for, or receipt acknowledgements of, the information to element A. 
     In this application, including the definitions below, the term “module” or the term “controller” may be replaced with the term “circuit.” The term “module” may refer to, be part of, or include: an Application Specific Integrated Circuit (ASIC); a digital, analog, or mixed analog/digital discrete circuit; a digital, analog, or mixed analog/digital integrated circuit; a combinational logic circuit; a field programmable gate array (FPGA); a processor circuit (shared, dedicated, or group) that executes code; a memory circuit (shared, dedicated, or group) that stores code executed by the processor circuit; other suitable hardware components that provide the described functionality; or a combination of some or all of the above, such as in a system-on-chip. 
     The module may include one or more interface circuits. In some examples, the interface circuits may include wired or wireless interfaces that are connected to a local area network (LAN), the Internet, a wide area network (WAN), or combinations thereof. The functionality of any given module of the present disclosure may be distributed among multiple modules that are connected via interface circuits. For example, multiple modules may allow load balancing. In a further example, a server (also known as remote, or cloud) module may accomplish some functionality on behalf of a client module. 
     The term code, as used above, may include software, firmware, and/or microcode, and may refer to programs, routines, functions, classes, data structures, and/or objects. The term shared processor circuit encompasses a single processor circuit that executes some or all code from multiple modules. The term group processor circuit encompasses a processor circuit that, in combination with additional processor circuits, executes some or all code from one or more modules. References to multiple processor circuits encompass multiple processor circuits on discrete dies, multiple processor circuits on a single die, multiple cores of a single processor circuit, multiple threads of a single processor circuit, or a combination of the above. The term shared memory circuit encompasses a single memory circuit that stores some or all code from multiple modules. The term group memory circuit encompasses a memory circuit that, in combination with additional memories, stores some or all code from one or more modules. 
     The term memory circuit is a subset of the term computer-readable medium (CRM). The term CRM, as used herein, does not encompass transitory electrical or electromagnetic signals propagating through a medium (such as on a carrier wave); the term computer-readable medium may therefore be considered tangible and non-transitory. Non-limiting examples of a non-transitory, tangible CRM are nonvolatile memory circuits (such as a flash memory circuit, an erasable programmable read-only memory circuit, or a mask read-only memory circuit), volatile memory circuits (such as a static random access memory circuit or a dynamic random access memory circuit), magnetic storage media (such as an analog or digital magnetic tape or a hard disk drive), and optical storage media (such as a CD, a DVD, or a Blu-ray Disc). 
     The apparatuses and methods described in this application may be partially or fully implemented by a special purpose computer created by configuring a general purpose computer to execute one or more particular functions embodied in computer programs. The functional blocks, flowchart components, and other elements described above serve as software specifications, which can be translated into the computer programs by the routine work of a skilled technician or programmer. 
     The computer programs include processor-executable instructions that are stored on at least one non-transitory, tangible CRM. The computer programs may also include or rely on stored data. The computer programs may encompass a basic input/output system (BIOS) that interacts with hardware of the special purpose computer, device drivers that interact with particular devices of the special purpose computer, one or more operating systems, user applications, background services, background applications, etc. 
     The computer programs may include: (i) descriptive text to be parsed, such as HTML (hypertext markup language), XML (extensible markup language), or JSON (JavaScript Object Notation) (ii) assembly code, (iii) object code generated from source code by a compiler, (iv) source code for execution by an interpreter, (v) source code for compilation and execution by a just-in-time compiler, etc. As examples only, source code may be written using syntax from languages including C, C++, C#, Objective-C, Swift, Haskell, Go, SQL, R, Lisp, Java®, Fortran, Perl, Pascal, Curl, OCaml, Javascript®, HTML5 (Hypertext Markup Language 5th revision), Ada, ASP (Active Server Pages), PHP (PHP: Hypertext Preprocessor), Scala, Eiffel, Smalltalk, Erlang, Ruby, Flash®, Visual Basic®, Lua, MATLAB, SIMULINK, and Python®.