Abstract:
A robot head, for example for use in surgery, provides a back-drivable system allowing a surgeon to closely control the position of a cutter or other tool. The cutter is mounted at the end of a telescopic arm ( 20 ) which can be rotated about two independent perpendicular axes. Rotation about each axis is controlled by a separate motor ( 30 ′) which turns a lead screw ( 32 ). A bearing ( 34 ) travels along the lead screw and changes the angle of an offset crank ( 36 ) to cause the required rotation about the axis. The current rotational position about each axis is determined by a sensor at the output. A second sensor independently determines the position of the corresponding motor ( 30 ) and the two measured positions are compared. If they differ, the power to the cutter is immediately switched off.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This Application is a U.S. National filing under §371 of International Application No. PCT/GB2003/003354, with an international filing date of Aug. 1, 2003, now pending, claiming priority from Great Britain Application No. GB2002/20460.0, with a filing date of Sep. 3, 2002, now pending, and herein incorporated by reference. 
     TECHNICAL FIELD 
     The present invention relates to robot heads and particularly, although not exclusively, to a head for a surgical robot. 
     BACKGROUND OF THE INVENTION 
     In one type of robotically-assisted surgical procedure, a cutting implement (for example to cut bone) is mounted on an adjustable robot head which is itself held in position by a static gross-positioning device. The robot head has a manually-graspable handle which allows the surgeon to move the cutter. Typically, the cutter may be mounted at the end of a telescopic arm, and by applying force to the handle the surgeon may cause the arm to extend and/or to rotate about mutually-perpendicular pitch and yaw axes. Motors within the head respond to forces applied to the handle to ensure that the cutter moves smoothly to the position the surgeon requires. The head may include constraint mechanisms, implemented either in hardware or in software, which prevent the surgeon from moving the cutter into regions which have previously been defined as unsafe. Force feedback mechanisms may also be provided so that the surgeon receives tactile force feedback through the handle. 
     Of particular importance in surgical applications—although it may be of importance in other applications as well—is the precision with which the cutter can be positioned by the surgeon. Current systems are somewhat limited in this respect, because of relatively high friction in the mechanical components, along with a certain amount of “play” or backlash. A further requirement of course is safety, and concerns have been expressed as to the potentially serious injuries that could be caused to a patient in the event of a mechanical failure of a traditional robot head, or a failure in the control system or its software. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention at least to alleviate these perceived difficulties. 
     According to a first aspect of the present invention there is provided a back-drivable robot head including:
         (a) a manually-graspable driving member;   (b) a force sensor for sensing forces applied to the driving member by a user   (c) an arm for carrying a tool the position of which is to be controlled; and   (d) a first rotation control mechanism for rotating the arm about a first axis in response to the sensed forces;
 
characterised in that the first rotation control mechanism comprises a first rotational motor coupled to a first lead screw; and a bearing which moves longitudinally of the first lead screw as it rotates, the bearing being pivotally coupled to an offset crank of or secured to the arm.
       

     According to a second aspect of the present invention there is provided a back-drivable robot head including:
         (a) a manually-graspable driving member;   (b) a force sensor for sensing forces applied to the driving member by a user   (c) an arm for carrying a tool the position of which is to be controlled; and   (d) a first rotation control mechanism for rotating the arm about a first axis in response to the sensed forces;
 
characterised in that the first rotation control mechanism comprises a first rotational motor, an output of which is converted first to longitudinal motion and then back to rotational motion of the arm.
       

     According to a third aspect of the present invention there is provided a back-drivable robot head including:
         (a) a manually-graspable driving member,   (b) a force sensor for sensing forces applied to the driving member by a user   (c) an arm for carrying a tool the position of which is to be controlled; and   (d) a first rotation control mechanism for rotating the arm about a first axis in response to the sensed forces;
 
characterised in that the first rotation control mechanism comprises a first rotational motor, an output of which is converted first to longitudinal motion and then back to rotational motion of the arm; the head further including a first input encoder for measuring rotation of the first motor, a first output encoder for measuring the angular position of the arm about the first axis, and in which the measurement from the first output position encoder is compared with an expected arm position based on the measurement from the first input position encoder, an alarm being raised if the expected position is inconsistent with the actual position.
       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may be carried into practice in a number of ways, and one specific embodiment will now be described, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic view of a preferred surgical robot head, with the covers removed; 
         FIG. 2  shows the rear mounting, for mounting the head onto a gross positioning device; 
         FIG. 3  is a view from below, showing rotational control of the telescopic arm about a vertical axis; 
         FIG. 4  shows the telescopic arm at one extreme end of its range of rotation; 
         FIG. 5  shows the arm at the other extreme end of its range; 
         FIG. 6  shows the pivotal connection between the offset crank and the lead screw; 
         FIG. 7  shows the mounting of the motor that drives the lead screw; 
         FIG. 8  shows the primary sensor which determines rotational position; 
         FIG. 9  shows the telescopic arm at the two extreme limits of its range; and, 
         FIG. 10  shows the tracks on which the telescopic arm moves. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a surgical robot head in accordance with a preferred embodiment of the invention. The head consists of a generally L-shaped frame  10  having an upper portion  12  and a lower portion  14 . The upper portion  12  has a rotatable mounting  16 , best shown in  FIG. 2 , having a rear mounting plate  17  which allows the head to be bolted to a static gross positioning device (not shown). Once so mounted, the whole of the robot head can then rotate about a horizontal pitch axis, as shown by the arrows  18 . 
     Mounted to the lower portion  14  of the frame is a telescopic arm  20 , capable of extending and retracting by means of a motor  28 , as shown by the arrows  22 . The arm is mounted for rotation about a vertical yaw axis, as shown by the arrows  24 . The horizontal pitch axis and the vertical yaw axis intersect on the longitudinal axis of the arm  20 . 
     In use, a cutter (not shown) is inserted into a bore  26  at one end of the arm, and is locked into place by means of a locking handle  27 . 
     The surgeon operating the device grasps a handle  30 , and manually guides the cutter through the bone as required. Sensors within the handle  30  or between the handle and the body detect the forces that are being applied, and adjust the pitch, yaw and in/out motions accordingly, as will be described in more detail below. It will be understood of course that in the operating theatre most of the mechanical parts displayed in  FIG. 1  will be hidden behind smooth external covers; these have been omitted from  FIG. 1  to expose the workings of the head. 
       FIG. 3  is a view from below, showing the mechanism for controlling rotational movement of the arm  20  about the vertical yaw axis. Rotation of a lead screw  32  by means of a pancake or other motor  30 ′ causes a ball screw or bearing  34  to move up and down the lead screw. The ball screw  34  is connected to an arm or crank  36  which is itself connected to the telescopic arm  20 . Accordingly, the yaw position of the arm  20  is controlled by the linear position of the ball screw  34  on the lead screw  32 . 
     The crank arm  36  connects to the lead screw  32  by means of a pivoting linkage, to allow for the different angles of the crank arm as the ball screw  34  moves along. Movement also causes the lead screw  32  to rotate slightly about a pivot bearing  40  adjacent the motor  30 ′. 
       FIGS. 4 and 5  show, respectively, the arm  20  at each end of its range of movement. As may be seen, in both of these extreme positions, the lead screw  32  is substantially horizontal in the drawing; compare this with  FIG. 3 , in which the lead screw  32  has been pushed downwards slightly due to the length of the crank  36 . 
       FIG. 6  is a close-up view showing in more detail the pivotal coupling between the crank arm  36  and the lead screw  32 .  FIG. 7  is a further close-up showing the pivotal coupling of the motor  30 ′ and the crank arm  32  with respect to the lower part  14  of the frame. 
     As is best seen in  FIG. 6 , the lead screw  32  is formed with a high lead angle: this allows for low gear ratios to be used, as well as allowing the system to be back drivable (in other words, the surgeon can simply pull the arm  20  around by grasping the handle  30  shown in  FIG. 1 ). With the arrangement described, no gear box is required, and the motor  30 ′ ( FIG. 3 ) is simply attached directly to the end of the lead screw  32 . 
     Turning back now to  FIG. 3 , it will be seen that surrounding the vertical yaw axis is a cylindrical structure  37 . This is used in order to determine the exact rotational position of the arm  20 , in conjunction with an encoder generally indicated at  38 . As is best shown in  FIG. 8 , the cylindrical structure  37  defines a circumferential cam surface  40  onto which is secured a thin reflective strip  42 . A sensor  44  picks up patterns (not shown) on the strip, from which the angular position of the arm  20  may be accurately determined. In this embodiment, a stop  46  defines a nominal zero position, with the actual position at any time simply being determined by counting the number of pulses the sensor  44  has detected as the arm moves away from the zero position. The use of a circumferential strip  42  as described substantially eliminates errors due to backlash. 
     In order to protect against mechanical or other fault, an additional safety sensor (not shown) is built into the motor  30 ′. Position signals from the motor&#39;s sensor and from the main sensor  44  are compared and, if there is any discrepancy, an alarm is raised and the power to the cutter is switched off immediately. Because of the changing angles of the crank arm  36 , there is not a linear relationship between the pulses detected by the motor sensor and those detected by the main sensor  44 . Accordingly, it is convenient for the comparison to be carried out in software. Suitable software will not be described here, as it is well within the capabilities of a skilled person in the field to construct a function or a mapping defining the non-linear relationship, and then setting up a comparison with appropriate trigger points for switching off the power. 
     Turning back to  FIG. 1 , it will be seen that the mechanism for controlling rotation of the head about the horizontal pitch axis, on the mount  16 , is virtually identical to the mechanism already described for rotation about the vertical yaw axis. The mechanisms within the upper part  12  of the frame  10 , and surrounding the mount  16 , will not therefore be described separately. 
       FIG. 9  shows the arm  20  in its retracted position  50  and in an extended position  52 . Extension is effected by means of the motor  28  ( FIG. 1 ) which turns a lead screw  54 . Unlike the motors for the yaw/pitch actions, this motor is fixed in position. As the motor rotates and the screw turns, a barrel portion  56  of the arm, mounted to a carriage, is moved along the guide tracks  58 . 
     Between the rails  58  is a positioning strip  60 . The position of the carriage with respect to this strip is sensed by means of a position sensor (not visible in the drawings) positioned beneath the barrel  56 . Just visible at the left hand edge of  FIG. 10  is a carriage stop which acts as a zero-point indicator. The exact location of the carriage along the rails  50  is determined by the number of pulses received by the sensor from corresponding markings on the strip as the carriage moves away from the zero point. 
     For additional security, a secondary sensor (not shown) is provided in association with the motor  28 . A hardware or software comparison is made between the measured position of the barrel  56  as determined by the main sensor, and the position as determined by the secondary sensor. If the sensors do not agree, an alarm is raised and power to the cutter is immediately switched off. 
     The manually-graspable knob  30 , best seen in  FIG. 1 , has a force sensor (not shown) mounted within it, along with associated wiring and electronics. The outer part of the knob is connected to the sensor which is itself connected to the arm  20 . Hence, any force the surgeon applies to the knob  30 , in any direction, will automatically be sensed by the sensor. The sensor generates control signals based upon the sensed forces which are used, along with details of the current head position and cutter constraints, to control the pitch and yaw motors  30 ′, along with the arm extension motor  28 . The motors are controlled so that the surgeon feels an increasing resistance as he pushes towards a constraint boundary, and decreasing resistance as he moves away. In an unconstrained region, the motors are controlled to give an equal low resistance to movement in any direction. For the present purpose, an unconstrained region means either:
         (a) when the constraints are switched off (e.g. during registration), or   (b) far away from any boundary, inside the constraint region.       

     When the surgeon needs to cut bone, an appropriate cutter is pushed into the bore  26 , and locked in place by the locking handle  27  ( FIG. 1 ). Alternatively, other surgical or medical instruments may be placed within the bore  26 , depending upon the application. 
     The robot head described may also be used in non-surgical applications.