NL2009200C2 - Grasper embodied with a bi-stable element. - Google Patents

Description

Grasper embodied with a bi-stable element

The invention relates to a grasper embodied with a bistable element and grasper legs, which legs have a stable 5 first position and a stable second position, wherein convert ing between the first position and the second position and vice versa is supported by the bi-stable element.

An elastic yet not bi-stable grasper is proposed on the Internet on July 9, 2010 on the website 10 www.compliantmechanisms.3me.tudelft.nl/.../MSc_Bi-stable element Accord ing to this website conventional graspers transfer force and motion by utilizing multiple rigid links and kinematic joints. A compliant grasper on the other hand is a mechanism that gains some or all of its motion from the relative flexi-15 bility of its members rather than from rigid-body joints, which provides numerous advantages such as single-piece production, absence of coulomb friction, no need for lubrication, compactness, etc. Further, bi-stability has been an important topic in compliant mechanisms such as MEMS. These 20 mechanisms only have two stable positions and the energy which is needed to achieve the same displacement compared to conventional compliant mechanisms is less. The website proposes to develop a high performance compliant grasper which is bi-stable. The design according to his research proposal 25 should provide a fully compliant grasper which has two stable positions, i.e. "open" and "close". The bi-stability should allow the grasper to work in a wider range of displacement with less input energy (force) required than available designs and grasp all size of subject-matter within the legs of 30 the grasper.

The invention is not limited to a compliant grasper but may also relate to the grasper that is embodied with rigid parts that are connected through joints.

One of the problems with conventional graspers is that 35 when the grasper holds something between the legs, a continu ous grasping force is required to maintain the grabbed subject matter between the legs of the grasper. Specifically with surgical graspers, this may cause fatigue in the sur- 2 geons muscles which is detrimental, particularly when the grasper is used in laparoscopic surgery.

Another problem with graspers, specifically with surgical graspers, is that generally speaking there is insuffi-5 cient force feedback which is detrimental for the accuracy of the operation that is performed with the grasper.

The invention aims to provide a solution to the above mentioned problems and to provide an alternative for existing solutions known from the prior art.

10 The grasper of the invention is to that end provided with the features of one or more of the appended claims. It is expressly pointed out that the features that are provided in any one of the appended claims may be applied independent from the features of any of the other claims. This applies 15 also when such features are mentioned in a claim that is de pendent from one or more other claims.

In a first aspect, the grasper of the invention that is embodied with a bi-stable element and grasper legs, wherein the legs have a stable first position (the first po-20 sition being preferably although not necessarily the open po sition) and a stable second position (the second position being preferably although not necessarily the closed position), and wherein conversion between the first position and the second position and vice versa is supported by the bi-stable 25 element, has the features that when the legs are disconnected from the bi-stable element, said legs are preferably in the first position and exhibit positive stiffness when converting from the first position to the second position, and that with the bi-stable element connected to the grasper legs and con-30 verting the legs from the first position to the second posi tion, the bi-stable element is arranged with negative stiffness that is higher in magnitude and counteracts the positive stiffness of the grasper legs. This arranges that the single or mere combination of the grasper legs with the bi-stable 35 element exhibits negative stiffness and bi-stable behavior and provides an effective solution to avoid the situation that continuously a force must be applied to the grasper in order to hold subject matter that is grabbed with the legs of 3 the grasper. The bi-stable element provides a prestressed grasper, which alters the total amount of its potential energy. This enables the (surgical) grasper to provide the necessary force to hold for instance tissue or a needle for an 5 endured period of time and without effort by the surgeon.

Preferably the grasper legs are embodied with flexible beams that merge between the legs into an actuation part. The actuation part can then be used for manipulation of the grasper legs. Beneficially then the actuation part is con-10 nectable to the bi-stable element.

There are of course many variations feasible to the grasper of the invention with the above mentioned features. One of the possible embodiments that is preferred for effectively linking the actuation part and the bi-stable element, 15 has the features that the actuation part is embodied with a screw threaded protrusion extending from the actuation part through a hole in the bi-stable element. Preferably the screw threaded protrusion is provided with fixing means that are movable along the screw threaded protrusion to enable adjust-20 ing the location where the bi-stable element connects to the screw threaded protrusion. This enables preloading of the bistable element so as to cause that there is a well-defined first position of the grasper legs.

In another aspect of the invention the grasper is em-25 bodied with a bi-stable element and grasper legs, which legs have a stable first position and a stable second position, wherein converting between the first position and the second position and vice versa is supported by the bi-stable element, wherein when the grasper legs are disconnected from the 30 bi-stable element said legs have preference for the first po sition and exhibit positive stiffness when converting from the first position to the second position, and that the bistable element is arranged with negative stiffness so as to arrange that with the bi-stable element connected to the 35 grasper legs and converting the legs from the first position to the second position, the negative stiffness of the bistable element compensates the grasper's positive stiffness so as to provide that the legs are statically balanced. By 4 this arrangement it is possible not only to merely improve the grasper but to provide effective force feedback to a user of the grasper, and make working with the grasper of the invention a pleasure.

5 It is further possible to implement the grasper of the invention such that the actuation part is spring-loaded to provide additional positive stiffness to the positive stiffness of the grasper legs in order to substantially match the negative stiffness of the bi-stable element when the legs 10 convert between the first position and the second position.

This effectively enables fine-tuning of the static balancing of the grasper of the invention.

Another embodiment of the grasper of the invention which can be applied independent from the other features as 15 discussed herein or in addition thereto, has the features that the grasper is embodied with a bi-stable element and grasper legs, which legs have a stable first position and a stable second position, wherein converting between the first position and the second position and vice versa is supported 20 by the bi-stable element, and wherein when the grasper legs are disconnected from the bi-stable element said legs have preference for the first position and exhibit positive stiffness when converting from the first position to the second position, and that the bi-stable element is arranged with 25 negative stiffness, and that further an actuation part con nects to the grasper legs which is spring-loaded to provide additional positive stiffness to the positive stiffness of the grasper legs in order to substantially match the negative stiffness of the bi-stable element when the legs convert be-30 tween the first position and the second position, and that said actuation part is convertible between said spring-loaded condition and a non-spring-loaded condition so as to arrange that in the non-spring-loaded condition the grasper legs with the connected bi-stable element exhibit negative 35 stiffness when the legs convert between the first position and the second position, and that in the spring-loaded condition the grasper legs with the connected bi-stable element exhibit zero or near to zero stiffness when the legs convert 5 between the first position and the second position. In this manner the grasper becomes switchable between a bi-stable operation in the non-spring-loaded condition, and a statically balanced operation in the spring-loaded condition. This can 5 be effectively arranged when a screw threaded protrusion ex tends from an actuation part of the grasper legs, and by mounting a spring on said screw threaded protrusion wherein the simultaneous movement of the spring with the screw threaded protrusion is selectable between a limited range and 10 an unlimited range. Suitably therefore a movable stop is pro vided which can be placed in a position to limit the excursion of the spring when said spring moves together with the screw threaded protrusion.

The invention will hereinafter be further elucidated 15 with reference to the drawing.

In the drawing: -figure 1 shows a first exemplary embodiment of a grasper in accordance with the invention; -figure 2 shows an exploded view of a second exemplary 20 embodiment of a grasper in accordance with the invention; -figure 3 shows in detail the connection between a bistable element and an actuation rod that form part of the grasper shown in figure 2; -figure 4 shows a schematic drawing of the grasper of 25 figure 1, having an actuation rod that is switchable between the spring-loaded and non-spring-loaded condition.

Whenever in the figures the same reference numerals are applied, these numerals refer to the same parts.

Figure 1 shows in accordance with the invention a 30 typical example of a grasper 1 embodied with a bi-stable ele ment 2 and grasper legs 3. The legs 3 have a stable first position and a stable second position, and converting between the first position and the second position or vice versa is supported by the bi-stable element 2. When disconnected from 35 the bi-stable element 2 the grasper legs 3 are structured to have preference for the first position, for instance the open position. The grasper legs 3 further exhibit positive stiffness when converting from the first (preferably open) posi- 6 tion to the second (preferably closed) position. When the bistable element 2 is connected to the grasper legs 3, as is shown in figure 1, during converting the legs 3 from the first position to the second position, the bi-stable element 5 exhibits negative stiffness that is higher in magnitude and counteracts the positive stiffness of the grasper legs 3. The combination of the grasper legs 3 with the bi-stable element 2 thus Exhibits negative stiffness and bi-stable behavior.

As is shown in the typical grasper 1 of figure 1, yet 10 also present in the grasper 1 shown in figure 2 and figure 4, the grasper legs 3 are embodied with flexible beams 4 that merge between the legs 3 into an actuation part 5. The actuation part 5 is connected to the bi-stable element 2.

With reference to figure 4 is shown that the actuation 15 part 5 has a protrusion 6 which is preferably screw threaded and extends from the actuation part 5. This (preferably screw threaded) protrusion 6 extends through a hole 7 in the bistable element 2 (see the exploded view of figure 2). The protrusion 6 is provided with fixing means, for instance nuts 20 8, that are movable along the screw threaded protrusion 6 to enable adjusting the location where the bi-stable element 2 connects to the protrusion 6. Thus when the legs 3 convert between the first position and the second position the negative stiffness of the bi-stable element 2 is capable to com-25 pensate the positive stiffness of the legs 3 so as to provide that the legs 3 are statically balanced.

With reference further to figure 4 it is shown that the actuation part 5 can be implemented as spring-loaded by spring 9 to provide additional positive stiffness to the 30 positive stiffness of the grasper legs 3 in order to substan tially match the negative stiffness of the bi-stable element 2 when the legs 3 convert between the first position and the second position. This figure 4 also shows that the actuation part 5 can be arranged to be convertible between a spring-35 loaded condition and a non-spring-loaded condition. For this purpose the spring 9 can be switched between loading and not loading the actuation part 5. In the non-spring-loaded condition the grasper legs 3 with the connected bi-stable element 7 2 exhibit negative stiffness when the legs 3 convert between the first position and the second position, whereas in the spring-loaded condition the grasper legs 3 with the connected bi-stable element 2 can be arranged to exhibit zero or near 5 to zero stiffness when the legs 3 convert between the first position and the second position.

For the purpose of switching the spring 9 and arranging the changeover between the spring-loaded condition and the non-spring-loaded condition of the actuation part 5 of 10 the grasper 1 of the invention, the spring 9 can be suitably mounted on the screw threaded protrusion 6. It can then be arranged that simultaneous movement of the spring 9 with the screw threaded protrusion 6 is possible in the non-spring-loaded condition by selecting an unlimited excursion range of 15 the spring 9. If on the other hand the excursion range of the spring 9 is limited, the actuation part 5 connected to the screw threaded protrusion 6 is placed in the spring-loaded condition. This can be effectively implemented by arranging that a movable stop 10 is provided which can be placed in a 20 position to limit the excursion of the spring 9 by engaging it when said spring 9 moves together with the screw threaded protrusion 6. This condition is shown in figure 4. Movement of the stop 10 in accordance with the arrows arranges for the switchover between the spring-loaded condition and the non-25 spring-loaded condition of the actuation part 5 of the grasper 1 of the invention. In this non-spring-loaded condition the grasper number 1 of the invention exhibits bi-stable behavior of its legs 3.

It is specifically pointed out that the foregoing de-30 scription with reference to examples of graspers in accor dance with the invention serves to explain the principles of the invention, without intent to limit the appended claims to what is shown in the drawings. On the contrary, the scope of protection that merits the invention is solely determined by 35 the appended claims.

Claims (8)

A gripper (1) provided with a bistable element (2) and gripper legs (3), which legs have a stable first position and a stable second position, in which the transition between the first position and the second position and vice-versa is supported by the bistable element (2), characterized in that when they are detached from the bistable element (2) the gripper legs (3) are preferred for the first position and have a positive stiffness when moving from the first position to the second position, and when the bistable element (2) is connected to the gripper legs (3) and the legs (3) move from the first position to the second position, the bistable element (2) is arranged with negative rigidity that is larger in size and counteracts the positive stiffness of the gripper legs (3) in order to cause the single combination of the gripper legs (3) with the bistable element (2) to have negative stiffness and bistable behavior of shows. 2. Gripper (1) according to claim 1, characterized in that the gripper legs (3) are provided with flexible beams (4) which merge into an actuating part (5) between the legs (3), and in that the actuating part (5) ) is connectable to the bistable element (2). 3. Gripper (1) according to claim 2, characterized in that the actuating part (5) has a threaded protrusion (6) which extends from the actuating part (5) through a hole (7) in the bistable element (2) ), wherein the threaded protrusion (6) is provided with fixing means (8) movable along the threaded protrusion (6) in order to make it possible to adjust the location where the bistable element (2) is connected stands with the threaded protrusion (6). 4. Gripper (1) as claimed in any of the foregoing claims, characterized in that when the legs (3) transition between the first position and the second position, the negative stiffness of the bistable element (2) compensates for the gripper's positive stiffness to provide that the legs (3) are statically balanced. Gripper (1) as claimed in any of the foregoing claims, characterized in that the actuating part (5) is spring-loaded for providing additional positive stiffness to the positive stiffness of the gripper legs (3), in order to substantially reduce the negative stiffness of match the bistable element (2) when the legs (3) convert between the first position and the second position. Gripper (1) according to claim 5, characterized in that the actuating member (5) is convertible between a spring-loaded condition and a non-spring-loaded condition, in order to ensure that in the non-spring-loaded condition the gripper legs (3) with the connected bistable element (2) having negative stiffness when the legs (3) convert between the first position and the second position, and that in the spring-loaded condition the gripper legs (3) with the connected bistable element (2) ) have zero stiffness or substantially zero stiffness when the legs (3) convert between the first position and the second position. Gripper (1) according to claim 6, wherein a threaded protrusion (6) extends from an actuating part (5) of the gripper legs (3), characterized in that a spring (9) is mounted on said threaded front - the projected projection (6) and the simultaneous movement of the spring (9) with the threaded projection (6) can be selected between a limited displacement range and an unlimited displacement range. 8. Gripper (1) according to claim 7, characterized in that a movable stop (10) is provided which can be placed in a position to engage the spring (9) when said spring (9) moves simultaneously with the threaded protrusion (6).