there will now be described , by way of example only , the best mode contemplated by the inventor for carrying out the present invention . in the following description , numerous specific details are set out in order to provide a complete understanding to the present invention . it will be apparent to those skilled in the art , that the present invention may be put into practice with variations of the specific . fig1 shows a ‘ butterfly diagram ’ wherein a sequence in time of vectors are depicted with the origins of the vectors from the contact surface in the ground ( the beginnings of the lines at the lower aspect of the diagram , the force vectors have a length ( the magnitude of the force ) and a direction ( the inclination of the vector ) and an origin in the walking or supporting surface . these diagrams are typically found in research in gait analysis and are indicative for normal and abnormal gait . it is important that these represent reliable and common patterns . fig2 shows a snapshot in time of a skeletal representation of a normal person walking where a single vector is shown arising from under the ball of the foot . the vector acts in front of the knee joint just prior to the knee engaging in the swing phase . whereas a normal leg has musculature and reflex response control , an artificial knee joint has little information to ensure that the knee is stable ( whilst weight bearing on it ) and that the movement through the knee is supple ( whilst swinging through ). the knee joint must make a decision when the stability is to be engaged and when the suppleness is called for . if the skeletal figure in fig2 were to be understood to be an artificial leg , then a vector passing in front of the knee would cause the same to extend until it meets a mechanical blockage preventing further motion . if the force vector passed posterior to the knee joint , then the knee would bend and the patient would be liable to collapse . fig3 shows a simplified leg corresponding to the skeletal leg of fig2 , although the timeframe corresponds with heel strike . vector 301 indicates a force in heel strike that has an axial component 302 ( parallel with shin 304 ), and a perpendicular component 303 . whereas in a weight stabilised joint , the force vector component 302 must be large enough to trigger the mechanism within knee 305 , the force component 303 must be small enough to not prematurely bend the knee about 305 and induce a collapse . whilst this mechanism works well within the normal range of force vectors in a normal butterfly diagram , the ground reaction vector in a stumble is causes 303 to be very large . in contrast , in a scuff action , for example , the reaction force 302 would be rather small . these conditions can induce a liability for the limb to collapse . one general problem associated with mechanical joints is that the response of the joint controller is dependent upon a load applied but not necessarily in accordance with a desired kinematic requirement independent of load . this means that a correct and safely maintained response is required in each of the two alternating operating states of a prosthetic ; namely , a low reactive torque state , such as typically in a swing phase , and a state of high reactive torque under the bearing of weight . an inappropriate torque level typically means a severe disruption of gait . fig4 shows the vector &# 39 ; s force components in a pre - toe - off condition in gait . force vector 401 can be considered as a knee bending vector component 403 and a knee straightening vector component 402 . as the leg is straightened , a sensor means in the knee joint 405 can detect a balancing of forces . the sensor means , conveniently a resilient member , enables a small displacement of the sensor to occur , which movement actuates a valve , conveniently with a memory function , whereby to enable release of the joint for swing . fig5 shows an example of a prior art mechanical memory . specifically , the memory comprises an eccentric toggle h 1 , that resumes a gravitational neutral position unless prevented by an open state of a valve member h 3 , which is energized by hydraulic pressure h 4 caused by a flex movement of the free knee . it is notable that this linkage is not instantaneous ; release of the memory function takes a time period independent of the time it takes to reapply weight on the prosthesis . in the event of an inadvertent early reapplication of weight on the prosthesis , the toggle would probably not be in a position to allow closure of the valve , which could be painful and perhaps cause an prosthesis user to fall following knee collapse in such circumstances . in effect , the memory of this prior teaching is continued for a longer period than desired whereby to cause at the very least a non - natural gait , with an increased likelihood of a fall occurring due to the time required for a change in state being far greater than desirable . a reflex style pushback in the leg by the amputated femur causes the swing to be released , which is good in normal swing but the leg causes to be very liable to collapse in downslope walking . reference is made to “ mechanisms of stumble recovery : non - microprocessor controlled compared to microprocessor - controlled prosthetic knees ” k kaufman et al , wherein it was concluded that neither weight activated stance stability , nor knee - hyperextension deactivated stance control provided a suitable degree of safety for the user , although it was reported that one microprocessor knee was reported to provide a sufficient degree of surety . the safety in the prior art microprocessor knee follows from a two condition input instead of the single condition inputs in the state of the art mechanical knee joints . in the microprocessor joint , the chosen inputs are firstly a determination of a ground reaction force passing in front of the knee joint axis through data collection by strain gauges in the distal part of the chassis relative to the joint , and , secondly ascertaining that the knee angle is 180 ° ( i . e . being straight ). these methods act as filters for particular conditions . a first embodiment of the invention is shown as in fig6 . in this figure , there is a stump attachment 601 , a principal knee axis 602 , a chassis 603 , an auxiliary axis 604 , a foot portion 606 , a first resilient member 611 , a second resilient member 605 , a member 608 fixed to the foot portion pivotally linked to chassis 603 . thus movement by extension is resisted by resilient member 605 , and the resilient member 605 allows a range of movement 607 that is input to the corresponding total displacement 610 . further input into total displacement 610 arises from the deflection of resilient member 611 when the chassis pivotally extends about pivot 602 relative to 601 . by the use of suitable materials , it will be apparent to the man skilled in the art that a degree of inherent resilience of the chassis may provide a sufficient degree of resilience whereby the specific use of first and second resilient members is not necessary . the device is manufactured such that when there is only one input signal — of two required signals , then such signal is not sufficient to enable displacement , or a signal 610 large enough to affect the state of the controller 614 , that provides a resistance to a variation in its telescopic length , and which is pivotally connected to members 610 and 603 . advantageously the controller 614 reduces the resistance to compression significantly when signal 610 achieves a required minimum value . the controller may comprise further means that acts as a memory and will maintain the controller in a low resistance mode ( in lieu of signal 610 ) until the controller extends as a result of the joint extension , which will null the memory and bring the controller in a state of readiness of providing high resistance against compression , unless signal 610 is reapplied . in terms of gait analysis , 611 can only provide a signal 613 when the knee hyper extends , and that is only possible with a ground reaction force passing anterior to the knee joint , providing inherent stability to the knee joint . further , again in terms of gait analysis , 605 can only provide signal 607 with a ground reaction force passing in front of pivot 604 . in order to explain the induced signal 613 , reference shall be made to the first embodiment in fig7 , where there is indicated a distance or radius 701 representing a minimum required hyperextension torque 701 about pivot 602 . there is shown a number of possible ground reaction force vectors that arise from a number of possible positions that the foot can create input 613 . the minimum hyperextension torque 701 acts as a filter to screen out any force that passes too close or posterior to the auxiliary knee centre 604 to effect signal 607 . in order to detail the induced signal 607 , fig8 shows a similar radius of minimum hyperextension torque 801 about pivot 604 . an array of force vectors 802 is indicated that would meet the direction and magnitude and origin to meet the requirement to make input 607 . vector 803 is a sample of a possible vector , but non - occurring in the normal ground reaction space envelope , or butterfly diagram . to produce a vector like 803 the heel would need to be slapped backwards onto the ground in a hammer - back - slap like action . the minimum hyperextension torque 801 acts as a filter to screen out any force that passes too close or posterior to the knee centre 602 to effect signal 613 . fig9 shows a range of ground reaction vectors that would meet the requirements to create compound input 610 . the geometrical limitations to the signal strength of 607 and 613 make these signals digital rather than analogue , and suitable for processing through a logical port , a logical and - gate , which in our preferred embodiment is purely hydro - mechanical . when the available force vectors as illustrated in fig1 are superimposed onto the knee joint it becomes apparent that only the force vectors indicated within bracket 102 are of sufficient magnitude and direction to pass both filters 701 and 801 to produce compound signal 610 . it follows all realistic vectors that can create compound input 610 also naturally stabilize the knee joint against collapse . bracket 103 in fig1 indicates force vectors that are of insufficient magnitude to produce both signals / inputs 607 and 613 . the second aspect of the invention is the provision of a memory mode that can maintain the energizing power of 610 to release controller 614 into a low resistance mode , so that the ground force vector can be directed in a posterior sense with respect to the knee joint axis 602 by movement of the amputation stump 601 upon hip flexure effort . for clarity , the overlapping of the parallel members that form 601 only serve to indicate that movement , and show a possibility of displacement measure , and does not form the representation of an actual construction . in fact 601 could represent be a closing gap ( suitable for electrical contact making ), an overlap ( suitable for a variable resistance contact making ), a resilient member ( suitable for strain gauging ), or a means to open a hydraulic valve to permit a hydraulic flow . fig6 is shown as one of a large number of types of possible construction of prosthetic . since connection 604 allows only a minor degree of pivot action , it can be provided as a resilient member , a hinge or otherwise , provided that a signal at 610 can be determined . it is also possible that an element with a known resilience so as to combine properties 604 and 605 could be provided so as to enable the provision of a contribution to the compound signal 610 . signal 610 could also be embodied within damping control means 614 when the signal 607 ( arising from a pivotal motion ) contained in member 609 is passed to the damping control means 614 , and when signal 613 is passed through into controller 614 when member 613 is part of the piston rod of controller 614 . in this way the signals are preserved , but neatly brought into the centre of the controller for hydraulic processing . electrical switches could be activated to determine a digital switching mechanism : a variable resistor could be arranged to provide an analogue state — in either of these alternatives , an electronic capacitor could be permitted to gain charge , and on maintenance of that charge the controller 614 could be made free by maintaining a solenoid valve state . equally mechanical memory the same as or similar to fig5 could be employed . the second embodiment as shown in fig1 , comprises a hydraulic solution wherein a valve 1001 / 1002 is disposed in controller 614 which valve , in an open state , permits a low resistance state of the controller . the reference numerals of his figure correspond with those of fig6 - 9 . two valve bodies are constructed such that the valve is closed when these bodies meet and the valve is open if the valve bodies are parted . the valve is constructed such that with a minimum input 613 the top valve body is raised , and with a minimum of input 607 the lower valve body is drawn down . due to the fact that valve body 1001 can be raised up by member 612 against resistance of resilient member 611 , and valve body 1002 can be pulled down by member 606 / 608 a gap 610 can form . accordingly , signal 613 is produced in such a way that member 612 can produce the signal by over extending the knee joint , but cannot remove be removed solely upon flexion of the knee joint . head 1006 can therefore move freely in space 1005 . on depression of piston 612 hydraulic fluid flow commences through passage 1007 , which by virtue of pressure drop 1003 between valve members 1001 and 1002 will maintain these valve members in an open condition — and keep the valve open during knee flexion motion — although , for clarity hydraulic fluid drain paths necessary for this are not shown . this creates the required memory to permit low torque resistance to the knee joint flexion . on extension , the pressure drop 1003 is lost ( due to a different return flow from accumulator 1004 by means of one way return valves , also not shown , and the valve bodies 1001 and 1002 close again preparing the joint for high torque resistance use , such as required for knee stability under the bearing of weight . whereas this means of providing a memory is the preferred means within this disclosure , a means such as described in fig2 in gb2464620 ( prior art ) would also be employable . the present invention benefits from the use of known components that are widely employed yet arranged in a distinct fashion whereby to create a logical and - gate , as per example made by the sufficient lifting of valve member 1001 and sufficient depression of valve member 1002 , which conditions must both be true to permit a flow of fluid to pass through the valve which causes a low state of resistance against bending the joint about axis 602 . for the avoidance of doubt , the hydraulic fluid under piston 612 causes a pressure on valve member 1001 to enforce closure once closed . fig1 and 11 a show the operational logic of the present invention . box 1101 delimited by dashed lines , comprises the logic step comprising a second signal as a part requirement to enable a low resistance functional mode of the joint by evaluating an and logic condition 1103 and 1103 a , such that the force vectors arising from normal gait conditionally ( depending on origin , direction and magnitude ) create signals that need to be summed to a required value to energize a valve 1102 and associated memory 1104 . this added requirement makes a distinct difference to any known mechanical joint on the market , as it removes the effects of weight bearing on the heel and hyperextension through muscular effort as a mode to release swing phase ; indeed , this is a different mode to known computer controlled systems that permit swing initiation on weight bearing on the toe combined with angular measurement i . e . : ‘ continuously monitoring akp knee angle ’ ( u . s . pat . no . 5 , 383 , 939 ) ‘ electronic sensing means for continuously monitoring position of the center of gravity of the user &# 39 ; s body relative to the akp foot and emitting signals representative thereof ’ ( u . s . pat . no . 5 , 383 , 939 ), of the knee being straight using ‘ programmed computer means for receiving the emitted signals ’ ( u . s . pat . no . 5 , 383 , 939 ). it also makes a distinct difference to the known weight activated joints , as the release of swing by weight bearing on the toe is facilitated instead of resisted as in the known weight activated joints , that need springs to overcome that problem , with the same springs resisting the efficacy of the joint stabilising with weight bearing on the heel . the present invention is further distinct to the known hyperextension release mechanisms , as such hyperextension effort alone is insufficient to permit low resistance knee flexion . to avoid doubt , fig1 shows and gate 1103 using delimited analogue signals or inputs x and y , and in order to energise the switchover 1102 and subsequent memory 1104 it requires the total input to be greater than x + y + c , wherein c is constant to represent a margin of safety . this analogue signal processor describes the hydraulic solution as drawn in fig1 . fig1 a shows essentially the same and gate 1103 a using logical input signals or inputs x = true and y = true , and in order to energise the switchover 1102 and subsequent memory 1104 it requires the inputs x and y both to be logically true . the logical gate is the preferred description if for instance normally open electrical switches x and y are put into a series to form an and gate ( 1103 a ). alternatively the signals x and y could be derived from signals from strain gauges that produce signals from two height positions within the limb prosthesis . the advantage of using strain gauges is that these can be placed in various positions in leg prosthesis as illustrated in fig1 b , with stump receptacle 1110 , knee 1111 , ankle 1114 , foot 1112 . the requirement in accordance to the invention is that these are located in suitable places to sense an hyperextension moment in two locations in the leg prosthesis such as to only permit force vectors anterior to knee 1111 and ankle 1113 joints to produce positive signals . alternative to such strain gauges mechanical deflections or electrical switches could be used to suitable advantage . the addition of a second signal in the operational logic allows the exclusion of some common force vector stimuli that are able to energise one or the other signal on their own , but not both together . this permits to narrow the range of force vectors as illustrated in fig1 that can energise the opening of a valve to the extent that , within normal use patterns ( such as walking , walking down stairs , walking down slope , sudden stopping , tripping , wading through water and long grass , etc . ), this only happens under voluntary control , and to the exclusion of inadvertent collapse . reference is again made to kaufman et al . with respect to the particular normal abnormal uses of an artificial leg , and the failures noted in different knee designs . whereas in the embodiment in fig1 the and gate ( the parting of valve member 1001 and 1002 ) and the memory function 1102 ( the distance of the parted members 1001 and 1002 being maintained by the flow through the open valve ) are combined in one , an electrical evaluation of the same mechanical inputs is a less preferred alternative due to the fact that batteries are required or an onboard electricity generator , that all needlessly add to complexity . the present invention thus provides an and gate functionality , which provides a logical evaluation instead of an on / off evaluation , and provides new logic in this field of medical devices whereby safety is ensured independently of patient experience / skill / ability . for further clarification of the workings of the and gate , fig1 shows piston 612 that , on extension of the joint i . e . being pulled upwards , compresses resilient member 1201 to cause a delimited displacement 613 which on maximum level has a signal strength = 1 . this will be less than distance 610 , so that a zero gap 613 ( signal strength = 1 ) is insufficient to part valve bodies 1001 and 1002 . it also shows resilient member 605 ( be it in a different location as in fig1 ) that will permit a delimited displacement 607 . when distance 607 is a zero gap its signal strength is 1 such that this displacement is insufficient to cause gap 610 to close fully . indeed both signals 613 and 607 are required to cause gap 610 to come to zero so that valve bodies 1001 and 1002 must part despite the effort of biasing valve - closing element 1205 . this is the and - gate : the requirement of both signal 613 and 607 to occur for valve bodies 1002 and 1002 to part . once piston 612 depresses the fluid in space 1206 through passage 1207 into valve space 1208 ( when the bodies 1001 and 1002 are parted ) meeting a resistance in 1209 and flowing to accumulator space 1210 . a pressure difference dp between those pressures in valve spaces 1208 and 1210 a keeps valve body 1001 lifted against the force of valve closing biasing element 1205 . this forms a memory effect . when piston 612 extends again , the said pressure differential dp disappears and valve body 1001 is depressed by biasing element 1205 onto body 1002 . other flow paths are not shown .
the present invention relates to artificial limbs generally and joints for same . the present invention provides hydraulic functional units whereby enabling movement of artificial joints to closely correspond with natural human movement . in the provision of realistic joints , as used in prosthetic limbs , an important aspect in achieving realistic movement is providing a different operating characteristic to the joint when under load . one important characteristic of an artificial leg for achieving a natural - looking walking gait corresponding with those of a stabilised knee , i . e . a knee resisting flexion when under load , is when it bears at least some of the weight of the amputee . properties of resilient mechanical members are utilized to enable a hydro - mechanical system to be controlled so that it releases a low joint resistance mode relative to a default high resistance mode . the invention also permits alternative embodiments such as electronic , electro fluidic or electromechanical means .