Abstract:
A haptic interface for a remote manipulator uses a tunable spring to provide force reflection. The remote manipulator has an operating member coupled to the tunable spring. The operating member is also coupled to a manipulator member. A controller monitors the force with which an operator so moves the operating member and varies a spring constant of the tunable spring to keep the force exerted by the manipulator member on an object at a desired level. The haptic interface allows simultaneous control over the maximum force exerted by the manipulator member as well as the transmission ratio between the operating member and the manipulator member. The remote manipulator may be a surgical grasper, for example. A tunable spring can be smaller and lighter than the high torque actuators used in some prior remote manipulators which provide force feedback.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of provisional application serial No. 60/062,987 filed Oct. 22 1997 and entitled HAPTIC INTERFACE THROUGH TUNABLE SPRINGS. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to remote manipulators which provide force-feedback to a user. The invention has application, for example, in graspers which may be used in surgery or used for the remote manipulation of delicate parts. 
     BACKGROUND OF THE INVENTION 
     Remote manipulators are used in a wide variety of applications. One of the most demanding applications for remote graspers is in surgery. The use of remote manipulators in surgery is becoming increasingly important. For example, various surgical operations can now be performed laparoscopic,ally through the use of appropriate laparoscopic graspers. These surgical procedures would be impossible without elongated graspers which can act as extensions of the surgeon&#39;s hand for remote manipulations at locations inside a patient&#39;s body. 
     A laparoscopic grasper typically comprises a lever which can be moved by a surgeon. The lever is typically mounted adjacent to a fixed handle so that the surgeon can control the lever with one hand by squeezing the lever toward the handle. The lever is connected to the jaws of a grasper by a mechanical linkage. Typically the linkage comprises a number of links which are connected to provide a ratio of lever movement to grasper movement (or “transmission ratio”) which is less than 1:1. 
     One problem with such graspers is that they do not provide the surgeon with very good force feedback. The surgeon often cannot tell how tightly the grasper is gripping an object because the transmission ratio of the linkage is not 1:1. Furthermore, friction is inherent in the mechanical linkage. Free play which generally occurs in the joints of any mechanical linkage also deleteriously affects the force feedback to a surgeon. These problems are compounded because the ratio of the force being applied to the handle by the surgeon to the force being applied to the grasper tends to vary significantly with the mechanical properties of the object being grasped as well as with the degree of opening of the grasper. As a result, of these factors, surgeons have less control over the forces exerted by remote graspers than is desirable. There have been a number of injuries to patients undergoing laparoscopic surgical procedures. Some of these injuries can be attributed, at least in part, to the lack of accurate force feedback in currently used laparoscopic graspers. 
     Various attempts have been made to design mechanical remote graspers which have low friction losses and have force transmission functions which are nearly constant. Such designs yield improvements in some areas. These designs are still not optimal because they do not allow the force transmission function to be easily adjusted. Preferably the force transmission function can be adjusted so that the forces exerted at the handle to yield a desired range of forces at the grasper lie in the range where the surgeon&#39;s hand has the greatest force sensitivity. Another problem with such mechanical graspers is that they provide no mechanism for limiting the maximum force that can be applied by a grasper. 
     Others have provided systems for operating a remote grasper completely under computed closed loop feedback control. Such systems are often very complex and suffer from the additional disadvantage that the systems fail completely if their computer controllers malfunction. Further, such systems often use bulky and/or very expensive actuators to drive the motion of the grasper. Two examples of such systems are U.S. Pat. Nos. 5,623,582 Rosenberg and 5,625,576 Massie et al. 
     Remote manipulators have many applications other than surgery. For example, remote manipulators may be used to manipulate hazardous materials or to service parts of machinery which cannot be reached with a human hand. Many of these applications also require a remote manipulator which provides force feedback to a user, adjustable force transmission function and a mechanism for preventing excessive forces from being applied to the output portion of the remote manipulator. 
     There is a continuing need for a remote grasper in which the maximum amount of output force can be limited. There is also a continuing need for remote manipulators which provide adjustable force transmission functions. There is a particular need for such remote manipulators which are compact, light in weight, and simple in construction. 
     SUMMARY OF THE INVENTION 
     This invention provides a remote manipulator having a “force feedback” or “haptic” user interface. The remote manipulator can provide an adjustable force transmission function, a limited maximum output force, or both. Preferred embodiments of the invention provide both an adjustable force transmission function and a limited maximum output force. 
     Accordingly, a first aspect of the invention provides a remote manipulator comprising: a manipulable operating member pivotally movable about a pivot axis; a manipulator member coupled to the operating member by a linkage, the linkage causing the manipulator member to move in response to movements of the operating member; a tunable spring having a variable spring constant, the tunable spring coupled between a connection point spaced apart from the pivot axis on the manipulator member and a mount; a force sensor coupled to the operating member, the force sensor producing a signal representing a force applied to the operating member; and, a control circuit connected to receive the signal and to vary the spring constant of the tunable spring in response to the signal, The tunable spring preferably comprises a leaf spring. Most preferably the tunable spring comprises a resilient leaf supported by two spaced apart supports which cause the portions of the leaf supported by the supports to have a constant deflection. 
     Preferably the remote manipulator controller comprises: means for comparing the force sensor signal to a threshold value; and, means for increasing the spring constant of the spring if the force sensor signal exceeds the threshold value. 
     A more general aspect of the invention provides a remote manipulator comprising: an operating member, the operating member movable against a force of a tunable spring coupled to the operating member, the tunable spring having a variable spring constant; a manipulator member; a linkage coupling the operating member and the manipulator member, the linkage causing the manipulator member to move in response to motion of the operating member; a sensor coupled to detect and generate a signal representing a force applied to the operating member; a controller connected to receive the signal; and, an actuator operable by the controller to vary the spring constant. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In drawings which illustrate embodiments of the invention, but which not be construed so as to limit the invention on in any way: 
     FIG. 1 is a schematic illustration of a generalized remote manipulator according to the invention; 
     FIG. 2 is a partially schematic view of a prototype remote grasper for use in laparoscopic surgery; 
     FIG. 3 is a diagrammatic representation of the remote grasp of FIG. 2; 
     FIG. 4 is a detailed view of the tunable spring of the remote grasper of FIG. 2; 
     FIG. 5 is a block diagram illustrating the control system of the invention; 
     FIGS. 6A and 6B are schematic views of two alternative forms of leaf spring which may be used with the invention; and, 
     FIG. 7 is a schematic block diagram illustrating a prototype system according to the invention which uses a computer to control the stiffness of a tunable spring. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a schematic generalized view of a remote manipulator  10  according to the invention. Remote manipulator  10  has an operating member  12  which can be moved by a human operator or user. The term “operating member” is intended to encompass levers, buttons handles or other members which can be moved by an operator to control a remote manipulator. Operating member  12  is coupled to a manipulator member  14  by a linkage  16 . The term “manipulator member” is a general term which encompasses movable members in a remote manipulator which interact with objects in the environment. The pincer remembers on a grasper are one species of manipulator member. A rod which pushes on an object and an arm that bears against an object are other non-limiting examples of manipulator members. 
     A user can move operating member  12 . Linkage  16  causes manipulator member  14  to make corresponding movements. If the motion of manipulator member  14  is blocked by an object O then manipulator member  14  will apply a force to the object. The amount of force applied to the object is determined by the amount of force applied to operating member  12  and the force transmission function of remote manipulator  10 . 
     A tunable spring  20  is coupled between operating member  12  and a support  22 . Tunable spring  20  is connected so that a user must move operating member  12  to do work against a force exerted by tunable spring  20  in order to cause manipulator member  14  to apply a force to an object O. Any linkage connecting spring  20  and operating member  12  should either have very little backlash or be pre-loaded so that motions of operating member  12  are transmitted directly to spring  20 . 
     Tunable spring  20  has a spring constant which can be varied. The force, F, exerted by tunable spring  20  is given by the equation: 
     
       
         F=K S x  (1) 
       
     
     where K S  is the variable spring constant and x is the displacement of tunable spring  20  from a reference position. It can be appreciated that the ratio of the force applied by a user to operating member  12  to the force applied by manipulator member  14  to an object O can be varied by varying the spring constant K S . The force applied by manipulator member  14  to object O can also be limited so that it does not exceed a maximum value F LIM  by causing the spring constant K S  to increase to a very large value as the force applied by manipulator member  14  to object O approaches the maximum value F LIM . 
     Remote manipulator  10  includes a sensor  29  which produces an output signal  31  representing the force being applied to operating member  12  by a user. Signal  31  is provided as input to a controller  30 . Controller  30 , in turn, produces an output signal  32  which controls an actuator  34 . Actuator  34  serts the spring constant K S  of tunable spring  20  to an instantaneously desired value, as determined by controller  30 , to achieve the desired force transmission to manipulator member  14 . 
     The components of remote manipulator  10  can each be realized in many ways without departing from the broad parameters of the invention. FIG. 2 illustrates a specific embodiment of the invention. The embodiment of FIG. 2 is a prototype remote manipulator  40  for use in laparoscopic surgery. Commercial embodiments of the invention would likely differ in details of implementation. 
     Remote manipulator  40  comprises an operating lever  42  which is pivotally mounted adjacent a fixed handle  44 . A user can squeeze a first end  42 A of operating lever  42  toward handle  44  to operate manipulator  40 . 
     Operating lever  42  is pivotally mounted to handle  44  by a pivot pin  46 . When a user squeezes end  42 A of operating lever  42  toward handle  44 , a second end  42 B of operating lever  42  pulls on a first; end  48 A of a rod  48  which extends through the bore  50  of a tube  52  affixed to handle  44 . Rod  48  actuates a pincer  54  at a remote end of tube  52 . 
     Pincer  54  comprises a pair of pincer members  56  and  57  which are pivotally connected by a pivot pin  58  to the remote end of tube  52 . Pincer members  56  and  57  have opposed first ends  56 A and  57 A respectively which can grasp an object O between themselves. Second ends  56 B and  57 B of pincer members  56  and  57  are each connected to a second end  483  of rod  48  by a link  60 . 
     A tunable compression spring  20  is mounted on a mounting plate  62  which is rigidly coupled to handle  44 . Spring  20  is coupled to operating lever  42  by a link  64 . Spring  20  should be connected to handle  44  in a way which is sufficiently stiff that spring  20  does not move significantly when the force exerted by spring  20  on link  64  changes. Link  64  is pivotally coupled to operating lever  42  at a point  66  which is spaced apart from pivot pin  46  by a distance D (FIG.  4 ). The spring constant K S  of tunable spring  20  is adjusted by a motor  68 . 
     A force sensor  29  is coupled so as to measure the force F IN  applied by an operator to end  42 A of operating member  42 . Force sensor  29  generates an analog signal  31  which is provided to controller  30  Controller  30  comprises an amplifier  69 , an analog to digital converter (“ADC”)  70 , a processor  72  and an interface  74  which drives motor  68  in response to commands from processor  72 . 
     Preferably a force sensor  26  is also coupled so as to measure the force F OUT  being applied to an object O by end  57 A of pincer member  57 . A signal  28  representative of F OUT  is also provided to controller  30  for monitoring and comparison with F IN . Force sensors  26  and  29  may comprise strain gauges on pincer member  57  and operating lever  42  respectively. 
     FIG. 3 shows a preferred embodiment of tunable spring  20  which comprises a resilient leaf  80 . Link  64  is connected at a midpoint  78  of leaf  80 . Leaf  80  is supported on either side of midpoint  78  by supports  82  and  83  which are equally spaced by the distance Z from midpoint  78 . Supports  82  and  83  are slidably mounted to base  62 . 
     A threaded rod  86  is driven by motor  68 . Motor  68  drives the rotation of threaded rod  86  through a transmission  69 . Threaded rod  86  has sections  86 A and  86 B in which the pitch of the threads are opposite. Support  82  is threadedly engaged with section  86 A of rod  86 . Support  83  is threadedly engaged with section  86 B of rod  86 . When motor  68  turns rod  86  in a first sense about its longitudinal axis supports  82  and  83  move toward one another (reducing the distance Z). This increases the spring constant K S . of tunable spring  20 . When motor  68  turns rod  68  in a sense opposite to the first sense, supports  82  and  83  move apart. This reduces the spring constant K S  of tunable spring  20 . 
     Each of supports  82  and  83  comprises two closely spaced apart pairs guides  88 . One guide  88  in each pair of guides bears against a front face  80 A of leaf  80 . The second guide  88  in each pair of guides bears against a rear face  80 B of leaf  80 . Guides  88  cause both the deflection and slope of leaf  80  to be essentially zero at a distance Z on either side of midpoint  78 . 
     FIG. 4 shows a diagrammatic view of remote grasper  40 . Moving a point  59  on operating lever through a distance X IN  causes the separation of the ends of pincer members  56  and  57  to move through a corresponding distance X OUT . Applying a force F IN  at point  59  produces a corresponding force F OUT  between the ends of pincers  56  and  57 . 
     When tunable spring  20  is not connected to grasper  40 , r is given by:              r   =         X   IN       X   OUT       =       F   OUT       F   IN                 (   2   )                                
     For the embodiment of FIG. 2, r is given by:              r   =       D3   D2          (             C   2     +     2      AB                 cos                 γ     -     B   2         +   A       2      D                   sin        (     γ   -     γ   0       )           )               (   3   )                                
     where A, B, C and D are dimensions shown in FIG. 4, γ is the angle shown in FIG. 4, and γ 0  is the value of γ when X IN  is equal to zero. In some typical surgical graspers A is about 5 mm, B is about 4 mm, C is about 5 mm, and D is about 32 mm. For typical design parameters of at least some commonly available laparoscopic graspers the function r can be approximated reasonably closely by a linear function. In some currently available surgical graspers r is about 0.19. The invention is, of course, not limited to these, or any, specific dimensions. 
     C 0  is given by:                C   0     =       X   OUT       X   IN               (   4   )                                
     For the embodiment of FIGS. 2-4, r s  is the ratio of distances D 1  and D 3 . 
     For the tunable spring  20  of FIG. 3 it can be shown that K S  is given by:                K   S     =       24      EI       F   3               (   5   )                                
     where E is Young&#39;s !modulus, I is the moment of inertia of leaf  80 , and Z is the distance between midpoint  78  of leaf  80  and the innermost guides  88  of supports  82  and  83 . For a leaf  80  having a rectangular cross section of width b and thickness t, I is given by:              I   =       bt   3     12             (   6   )                                
     Controller,  30  controls tunable spring  20  so that F OUT  is equal to a desired value F DESIRED  which is given by:                F   DESIRED     =     {             r   ′          F   IN                 if:                     r   ′          F   IN       ≤     F   LIM                 F   LIM               if:                     r   ′          F   IN       &gt;     F   LIM             }             (   7   )                                
     where F LIM  is the maximum value desired for F OUT  and r′ is the desired forte transmission function from operating lever  42  to grasper  54  (when F OUT  is not being limited). It can be shown that:                r   ′     =         F   OUT       F   IN       =     r     1   +       r   2          r   s   2          C   0          K   s                     (   8   )                                
     where r is the mechanical transmission function between operating handle  42  and grasper  54 , r s  is the transmission function between operating lever  54  and tunable spring  20 , K S  is the spring constant of tunable spring  20  and C 0  is the compliance of the environment in which grasper  54  is operating. 
     The above equations can be used to provide a relationship which yields a desired value for Z as a function of a measured input force F IN , and a desired value for r′. With some simplifying assumptions one can obtain the relationship:              Z   =       (     L       r          F   IN       F   OUT         -   1       )       1   /   3               (   9   )                                
     where L is a constant give by: 
     
       
         L=24EIr 2 r S   2 C 0   (10) 
       
     
     The above relationships may be used in controller  30  to control tunable spring  20  so as to cause grasper  40  to operate according to equation (2). 
     FIG. 5 is a block control diagram which provides a functional illustration of a control system for grasper  40 . Section  100  represents the mechanical linkages of grasper  40 , section  102  represents processes in controller  30  and section  104  represents processes in the actuator which adjusts the spring constant of tunable spring  20 . 
     It can be seen that a user applies a force F IN  to operating lever  42  against a force developed by tunable spring  20  as indicated at  110 . A net force F IN  acts through the linkage of grasper  40 , indicated by  112  to produce an output force F OUT  between pincer members  56  and  57 . The compliance of object O, indicated at  114  determines the displacement X OUT  which results from the application of force F OUT . The displacement X OUT , and the mechanical transmission ratio of grasper  40  in turn, determine the displacement X IN  of operating lever  42  as indicated by  116 . The displacement X S  of tunable spring  20  is related to the displacement X IN  by r s  as indicated at  118  The force F S  exerted by tunable spring  20  is related to X S  by the spring constant K S  as indicated at  120 . Finally, the force F S  is applied to operating lever  42  as indicated at  122 . 
     Controller  30  takes as an input a measured value for F IN  and a desired value for r′ as indicated at  130 . Controller  30  then computes a desired value for F OUT  according to the rule of equation (2) as indicated at  132 . This desired value for F OUT  is used to calculate according to equation (9) a desired value for the distance Z which determines the spring constant K S  as indicated at  134 . 
     The desired value for Z is compared to the current value of Z at  140 . Any difference between these two values is amplified at  142 . The resulting signal is added to a feedback signal  144  at  146  and the result is used to compute a motor driving signal S at  148 . The motor driving signal in the exemplary embodiment shown in the drawings is calculated by the formula:              S   =       K   t         R   a          (     Js   +   M     )                 (   11   )                                
     where K t  is the torque constant of motor  68 , R a  is the electrical resistance of the armature of motor  68 , J is the moment of inertia of the rotor of motor  68 , s is the first derivative of the result entering block  148 , and M is a damping constant for rotary motion of motor  68 . The motor driving signal S is then integrated at  150  and applied through the transmission ratio of transmission  69  to effect a change in Z as indicated at  160 . Feedback signal  144  is generated from the motor driving signal at  145  by multiplying by K a , the electric constant of motor  68 . The change in Z results in a change in K S  as indicated by  162 . 
     Controller  30  may be implemented in software in a programmable controller or a computer equipped with suitable input and output interfaces or in suitable hardware. FIG. 7 shows a block diagram of a system according to the invention which uses a computer to control the spring constant of a tunable spring. A commercial embodiment of the invention could differ from the embodiment of FIG. 7 in various obvious respects. 
     In the prototype embodiment of FIG. 7, signals  28  and  31  are provided to a computer  200  by way of an amplifier  69  and an I/O card  202 . Signals  28  and  31  are preferably conditioned by passing them through a low pass filter which may be included in amplifier  69 . The low pass filter may, for example, have a cutoff frequency of about 10 Hz. Software  204  in a memory device  206  runs on computer  200 . Software  204  also receives a signal  214  which represents the current state of tunable spring  20 . Signal  214  may, for example, be provided to computer  200  from a motor encoder coupled to motor  68  through a decoder  212  and I/O card  202 . Software  204  then operates motor  68  as necessary to control tunable spring  20  by providing an output signal at I/O card  202  which is amplified by a power amplifier  218  (which may be, for example only, a pulse width modulation servo amplifier) and applied to drive motor  68 . 
     It can be appreciated that the haptic control system of the invention decouples the transmission of force from the motion of a remote manipulator. This permits a designer to simultaneously provide a variable transmission ratio and limit the maximum force output of the remote manipulator. Many variations are possible in the design of a system according to the invention. 
     While the invention has been described primarily with reference to a grasper of the type commonly used in surgery, remote manipulators of other types may also be made according to the invention. While the invention has particular advantages for use with remote manipulators of the type where an object O is grasped between a pair of members the invention may also be applied to other types of remote manipulator. For example, the remote manipulator could be of a type which simply pushes on an object O. 
     While the embodiment of FIG. 2 uses an operating lever  42  as an operating member  12 , other types of operating member could be used. The invention could be used, for example, in a situation where the operating member  12  comprises a movable push button which can be pressed against a resistance of a tunable spring  20 . 
     Where the operating member is a lever, tunable spring  20  need not be coupled to the lever in the identical manner shown in FIGS. 2 and 4. Tunable spring  20  could couple to an operating lever, such as lever  42  on either side of pivot pin  46 . Tunable spring  20  could be on either side of handle  44 . While tunable spring  20  is shown as being coupled to lever  42  with a single link, which is preferred, the linkage coupling tunable spring  20  could be replaced with some other design of mechanical linkage. All that is necessary for broader implementations of the invention is that tunable spring  20  be coupled to operating lever  42  so that an operator moves lever  42  against a force generated by deflection of tunable spring  42 . 
     The tunable spring  20  need not be of the type shown in FIG. 3 (although the tunable spring of FIG. 3 has advantages which make it particularly well suited for use with the invention). FIGS. 6A and 6B show, for example, two alternative tunable springs which may be used to practice the invention. The tunable spring of FIG. 6A has a leaf  80  which is supported by two guides  88  which are spaced equally on either side of a midpoint  78  of leaf  80  by a distance Z. 
     The tunable spring of FIG. 6B is the same as the spring of FIG. 6A except that the endpoints of leaf  80  are held against transverse movement by guides  88 ′. 
     The tunable spring of FIG. 3 is preferable to those shown in FIGS. 6A and 6B because, for a given length of leaf  80  it produces the greatest range of stiffnesses and also has the lowest maximum bending moment for a given deflection of the spring. A tunable spring  20  does not necessarily need to have a leaf which is supported on both sides of its midpoint. Tunable springs in which one end of the spring is coupled to an operating member  12  could also be used. 
     In a tunable spring  20  of the type shown in any of FIGS. 2,  6 A and  6 B, various mechanisms may be provided to adjust the separation of supports  82  and  83 . The invention is not limited to the use of a threaded rod  86 , as shown. 
     Tunable springs of types other than tunable leaf springs could also be used in the invention. All that is necessary in the tunable spring is that it have physical dimensions compatible with the intended application, and that it have a variable spring constant capable of being controlled by a controller  30 . In general, the tunable spring should have several attributes. The tunable spring should have a spring constant which is variable over a sufficiently large range that when the tunable spring is at its lowest stiffness setting the tunable spring presents minimal resistance to movement of the operating member and when the tunable spring is at its highest stiffness setting a large force must be applied to move the operating member against the force of the tunable spring. Preferably, when the tunable spring is at its stiffest setting the operating member feels nearly rigid. For example, in some applications it is desirable that the operating member exhibit a stiffness on the order of about 100 Newtons/mm when the tunable spring is at its stiffest setting. 
     The tunable spring should not exhibit plastic deformation even under the largest forces which are reasonably likely to be applied to the tunable spring under its normal operating conditions. 
     The action of changing the spring constant of the tunable spring should not, in itself, move the operating member. The force generated by the tunable spring should be a product of the spring constant and a displacement which depends on the position of the operating member. 
     The linkage  16  between operating member  12  and manipulator member  14  is not necessarily a mechanical linkage. The invention could be used in situations where linkage  16  includes a wired or wireless electrical remote control. 
     While controller  30  has been described as using signal  31  which represents F IN  as the basis for controlling tunable spring  20  it could be possible, in the alternative, to use F OUT  for this purpose. However, using F OUT  may result in control instabilities due to the fact that the linkage between the operating member and the manipulator member(s) will typically have some friction and backlash. Only one of sensors  26  and  29  is necessary to practise the invention although it is preferred to provide both sensors  26  and  29 . 
     In addition to one or more force sensors, a displacement sensor could be provided to measure, directly or indirectly, the displacement of the manipulator member(s). The measured displacement could be used to compute, in real time, the stiffness of the environment or object against which the manipulator member is bearing. This computed value for the stiffness of the environment may be used in place of a constant value for C 0  in order to improve the operation of controller  30 . 
     As will be apparent to those skilled in the art in the light of the foregoing disclosure, many other alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof. Accordingly, the scope of the invention is to be construed in accordance with the substance defined by the following claims.