the embodiment , illustrated in fig1 - 4 , has a support frame 10 which is a flat base plate upon which the remaining structures may be mounted . a pair of independently operable , linear actuators 12 and 14 are mounted on the support frame 10 . each of the linear actuators 12 and 14 are independently connected to a microprocessor control circuit 16 so each may be independently operated by the control circuit 16 . the preferred actuators are preferably linear actuators and are preferably identical , and therefore only the linear actuator 14 is described in further detail . while a variety of linear actuators may be used , such as hydraulic rams , compressed air or pneumatic cylinders , or a rack and pinion , the preferred linear actuator comprises a lead screw 18 which is rotatably mounted to a pair of thrust bearings 20 and 22 , bolted to the support frame 10 . the lead screw 18 is preferably an acme screw and is drivingly connected to a controllable position motor , such as a conventional stepper motor 24 . as is known to those skilled in the art , a stepper motor is a controllable positioned motor and is actuated by pulses , each of which turn the motor through a preselected , angular displacement . therefore , the angular position of the motor is known by the number and polarity of the electrical pulses which have been applied to the motor . the preferred stepper motor provides 200 steps per 360 degrees of rotation , and can operate as high as 16 , 000 steps per second . the stepper motor is , therefore , easily and accurately controlled and provides a wide range of angular velocity . as will be apparent to those skilled in the art , a variety of other controllable position motors are available for use with the actuators in place of the stepper motors . for example , a dc motor , combined with a shaft encoder , can also be used . a variety of alternative position detector systems can also be applied to embodiments of the present invention . for example , a linear encoder could be utilized , positioning , for example , a series of phototransistors along and parallel to the path of each nut member 26 and by mounting a light emitting diode upon each nut member 26 to actuate the nearby photo transistor . a nut member 26 may be threadedly engaged on the lead screw 18 . a pair of rotatable wheels 28 and 30 are mounted to an axle to protrude downwardly from the nut member 26 and roll along the top surface of the support frame 10 . these wheels provide a bearing which prevent rotation of the nut member 26 and also support the vertically downward component of force applied to the nut member 26 . consequently , rotation of the stepper motor 24 in one direction , translates the nut member 26 in one direction along the support frame 10 , while rotation of the stepper motor in the opposite direction translates the nut member in the opposite direction . in both cases , the horizontal displacement is directly proportional to the algebraic total of the angular displacement of the stepper motor 24 . therefore , the number and polarity of the pulses applied to the stepper motors 24 and 25 determines the position of the nut members 26 and 27 . the two identical linear actuators 12 and 14 are independently operable along approximately parallel axes . a movable support platform 32 may be mounted to permit pivotal movement about two pivot axes . the first pivot axis for the support platform 32 may be the axis of a clevis pin 34 which extends through a clevis 36 to pivotally mount the clevis 36 to a support block 38 , which in turn may be fixed to the support frame 10 . the clevis 36 , support block 38 and clevis pin 34 together form a first hinge with a pivot axis which is preferably perpendicular to the parallel displacement paths of the linear actuators 12 and 14 and is approximately horizontal . a support axle 40 may be oriented perpendicularly to the clevis pin 34 and fixed to the clevis 36 . the axle 40 may be pivotally connected to support platform bearings 42 and 44 , which in turn are fixed to the support platform 32 so that the axis of the axle 40 provides a second axis about which the support platform 32 is free to pivot . consequently , the bearings 42 and 44 and pivot axle 40 , together with the clevis 36 , form a second hinge having a pivot axis substantially perpendicular to the first pivot axis through the clevis pin 34 . as a result of this mounting of the support platform 32 to the support frame 10 , the inclination or pitch of the support platform 32 may be varied about the axis of the clevis pin 34 to allow for such motions as dorsiflexion and plantar flexion . similarly , pivotal movement of the support platform 32 about the axis of the support axle 40 allows for roll of the support platform 32 to permit , for example , inversion and eversion of a foot 46 , supported on the support platform 32 . the foot is preferably held in place by a binding 64 . the support platform 32 may be drivingly linked to the linear actuators 12 and 14 by means of a pair of drive links 50 and 52 . each of the drive links includes a universal hinge at each of its ends , such as a ball joint , universal joint , flexible connecting shaft or any other kind of joint which allows free pivotal movement in all angles of direction about a central pivot point . for example , the drive link 50 is connected to the support platform 30 by a ball joint 53 and to the nut member 26 by a ball joint 54 . such a universal hinge or joint is necessary because roll of the support platform 32 about the axle 40 for inducing eversion and inversion will cause the upper ends of both drive links 50 and 52 to move back and forth relatively closer to and further from a central , vertical plane passing through the support axle 40 . in the operation of the preferred embodiment , actuation of the stepper motors 24 and 25 in the identical direction from the same initial position and for the identical displacement will vary only the inclination or pitch of the support platform 32 over a range of angles about the clevis pin 34 . the nut members 26 and 27 translate horizontally from left to right , as illustrated in fig3 to accomplish such motion as plantar flexion and dorsiflexion over a desired angular range . the angular limits over which the dorsiflexion and plantar flexion occur are determined by the linear displacement limits of the nut members 26 and 27 , which , in turn , are determined by the angular displacement of the stepper motors 24 and 25 . the roll motion for inducing inversion and eversion is a function of the difference between the linear displacements of one nut member from the other nut member along the parallel axes along which they reciprocate to provide different roll angles , as illustrated in fig5 and 6 . consequently , both inversion and eversion angles , as well as dorsiflexion and plantar flexion angles may be controlled and smoothly varied to provide a gentle rolling , pivoting movement by independently controlling , selecting and varying the linear positions of the nut members 26 and 27 . both of these motions may be simultaneously and smoothly blended by continuously displacing the nut members 26 and 27 along their respective lead screws 18 and 19 and simultaneously varying the difference between their displacements . while a variety of actuators , and particularly linear actuators , may be utilized with embodiments of the present invention , the lead screw and nut arrangement illustrated is preferred . it is simple , easily controlled , and , because of the mechanical advantage , combined with friction , forces exerted during use , for example by a foot on the support platform 32 , cannot be transmitted back to cause rotation of the lead screws 18 and 19 , although if necessary a stepper motor can be locked in place . the mathematical relationships relating the angular displacement of the stepper motors 24 and 25 to the pitch and roll of the support platform 32 will vary somewhat , depending on the particular embodiment of the invention which is constructed and may be determined by the application of well known principles of algebra , geometry and trigonometry or by testing to determine the particular relationship which may be used for controlling a preferred embodiment of the invention . it is desirable in some embodiments to initialize the control circuit for the particular embodiment before proceeding with motion of the support platform 32 . one manner of accomplishing this is to provide a pair of microswitches 60 and 61 , located , for example , at one end of the linear translation range for the nut members 26 and 27 . these microswitches are connected to the microprocessor control circuit 16 . typically , upon initial actuation of the microprocessor control circuit 16 , the stepper motors are rotated to translate the nut members into contact with their respective microswitches 60 and 61 . upon actuation of its microswitch , the associated nut member is stopped and when both are stopped , the microprocessor then may store in memory this initial position . thereafter the number of pulses and their polarity , which are applied to the stepper motors 24 and 25 , may be maintained in memory so that the microprocessor is continuously aware of the position of the nut members 26 and 27 . thereafter , the microprocessor drives the stepper motors 24 and 25 according to any desired control relationship to cause the nut members 26 and 27 to reciprocate back and forth along the lead screws 18 and 19 to obtain the desired motion of the support platform 32 . it should be apparent that embodiments of the present invention may be utilized beyond the field of physical therapy . the present invention may be used , for example , for supporting and varying the inclination and orientation of other types of work pieces . while certain embodiments of the present invention have been disclosed in detail , it is to be understood that various modifications may be adopted without departing from the spirit of the invention or scope of the following claims .
an apparatus is described which is capable of producing multiple motions . the apparatus is useful in one embodiment as a continuous passive motion foot or hand machine .