Design of the frame and arms of a Master for robotic surgery

Transcription

1 Design of the frame and arms of a Master for robotic surgery P.W. Poels DCT Traineeship report Coach(es): dr. ir. P.C.J.N. Rosielle ir. R. Hendrix Technische Universiteit Eindhoven Department Mechanical Engineering Control Systems Technology Group Eindhoven, July, 2007

2 Contents Introduction 3 1 Frame and arms of the master Frame construction The arms Center of rotation Building of the wooden model 9 3 Axle bearings for the arm rotation Choice of bearings Placement of the bearings in the arms Placement of the bearings for rotation of the upper arm Placement of the bearings for rotation of the forearm Brakes Brake of the forearm Brake of the upper arm Cable guidance 20 Conclusion 21 Bibliography 22

3 Introduction Nowadays surgical operations can be executed using Minimal Invasive Surgery. There is a robotic part at the operating table, the slave. This slave consists of three robotic arms. Two of these arms handle instruments, the third one handles an endoscope. The surgeon operates sitting in a chair behind a master. The master is a frame on which two arms are mounted. With each of these arms, the surgeon can reach and fix a point in space which corresponds to a trocar (entry point of the instrument on the patients body). From the fixed point the control handles are suspended which provide setpoint generation for the slave robot arms. A design for the frame and the arms of the master is made. From this design a wooden model is made. The axle bearings of the arms are designed. Movement of the arms during the operation isn t allowed, therefore some concept ideas for brakes are discussed. Also an idea for the guidance of the electrical cabling is proposed.

4 Chapter 1 Frame and arms of the master The master is the part from where the surgeon controls the operation. It should be a compact, movable system. This to be able to move the master to a corner in the operation room after the operation. The ceiling should also be left free. A screen should be placed to view the image of the endoscope. In this chapter the designs of the frame and the arms are discussed. 1.1 Frame construction The front view, the right side-view and the bottom view of the master are shown in figure 1.1. The bottom will be made of three hollow rectangular beams, two beams of 1150 x 200 x 100 mm and one with the dimensions 720 x 283 x 100 mm. All beams have a wall thickness of 6 mm. In the ends of the two outer beams (A) a caster can be mounted. The four casters should fold in and out with the movement of a handle. Honeycomb walls should be mounted on top of the beams. There is a distance provided between these mounted walls and the walls of the hospital, whereby the beams (A) serve as a bumper. The surgeon sits between those two outer beams. The crossbeam (B) serves besides the constructional purpose, as a place where the surgeon can put his feet. Also some control pedals are installed on this beam. On top of the crossbeam a vertical tube (C) is mounted. The diagonal of this vertical tube, with the dimension 200 x 200 mm, must fit with its ends on the sides of the crossbeam. The two outer beams and the vertical tube are standard dimensions from MCB [2], the crossbeam has to be fabricated from plate. The frame is welded. In the hollow beams some partitions are placed to give the construction more stiffness. These partitions are placed in the crossbeam under the outer points of the diagonal of the vertical tube. Also partitions are placed in the two outer beams where the crossbeam is welded. At the top of the vertical tube there are two intersecting plates (D). Between these plates which are on the front a bit longer, the arms are placed. On top of the upper horizontal plate the screen is placed. The construction has a mass of about 120 kg. Simple calculations show that the stiffness of the constructions lies in de the order of 10 6 N/m. The displacement of the frame with (figure 1.2) and without (figure 1.3) partitions is calculated in Unigraphics. The ends of the two outer beams are fixed in all degrees of freedom. A force of 300 Newton is applied to the upper intersecting plate in a direction as can be seen in the figures. Without the partitions the stiffness is 2.3e6N/m and with 3.9e6N/m.

5 1.1 Frame construction 5 Figure 1.1: Front view, the right side-view and the bottom view of the frame; A: Outer beams; B: Crossbeam; C: Vertical tube; D: Horizontal intersecting plates

6 1.1 Frame construction 6 Figure 1.2: Displacement of the frame with partitions with an applied force of 300 Newton Figure 1.3: Displacement of the frame without partitions with an applied force of 300 Newton

7 1.2 The arms The arms There are two arms, each arm consists of two parts. The design of one arm is shown in figure 1.4, the other arm is identical. The joint on the z-axis should cover an area of about 500 mm wide, 350 mm long and 200 mm high. To cover this area a long arm (forearm) has been mounted on top of a short arm (upper arm). There is chosen for this configuration because points close to the vertical tube are in reach with the least conflicts between the upper and forearm. Other configurations need longer arms to cover the same working area. The position of the joint in de working area is the same as that of the trocar in the patient s skin. The handle is the part of the master which symbolizes the instrument inside the human body, therefore it should be haptic, i.e. with force feedback. Figure 1.4: Top view and side-view of one arm with z-axis, joint and handle In the top view the arms can be seen to be tapered. The basic width of 110 mm is dependent on the bearings, see section 3.1. The forearm is mounted on top of the upper arm to have the mean working point, in z-direction, in the middle between the two horizontal plates of the frame. This provides optimal stiffness from the bearings. An additional advantage is the high stiffness of the upper arm due to the height of 200 mm near the frame. The lowest reachable point of the joint lies 800 mm above the ground (see also figure 1.1). This is to avoid collisions between the legs of the surgeon and the handles, also this is a reasonable working height. The forearm can be reduced in height on the distal end, but this depends on the z-mechanism which has to be built in. The arms are fabricated from 3 mm plate, so they are hollow.

8 1.3 Center of rotation Center of rotation The position of the center of rotation of the arms in relation to the vertical tube of the frame also determines the working area. The two extremes are shown in figure 1.5. Besides these two cases, all the intermediate positions are possible. In the first case (left figure) the rotation points are chosen as close as possible to each other and the vertical tube to make the construction as small as possible. A disadvantage is that the upper arms can make an angle of 135 degrees max. This limits the area covered lateral, but the working area is still in reach. In the second case (right figure) the arms can make an angle of 180 degrees. In this variant the area coverage is maximized. The arms can reach in each others working area. The center of rotation on the prototype of the master is mainly determined by ergonomic reasons. This is one of the reasons a wooden model is built. Figure 1.5: Side view and section B-B of the two ultimate center of rotation positions of the upper arms, left case 1, right case 2

9 Chapter 2 Building of the wooden model For the construction of the frame scrapmultiplex of 15 and 22 mm thick is used. After sawing the wood in all the necessary parts, the frame beams are glued together with construction glue. The parts are clamped with handscrews, which are removed after 4.5 hours. The frame from the bottom side with the partitions can be seen in figure 2.1. Figure 2.1: Bottom side of the wooden model without the floor plate

10 10 The outer beams and the crossbeam are joined by bolting near the upper edge, figure 2.2. The bottom plate is glued to the bottom side of the joined beams. The fabricated frame can be seen in figure 2.3. Figure 2.2: The coupling of one horizontal beam and the crossbeam The arms are made of 3 mm thick plywood. A fabricated arm can be seen in figure 2.4. In the corners are square cross section stiffeners glued. For the rotation axis steel tubes with in the ends thread are used. At the rotation points multiplex is glued on the plywood to avoid wear out of the turning points. The outer ends of the arms are closed with half PVC tubes. When the arms are assembled between the two horizontal plates of the frame, the wooden model is complete. This can be seen in figure 2.5. In figure 2.6 some different configurations of the arms are shown.

11 11 Figure 2.3: Wooden model of the frame Figure 2.4: A fabricated arm, without the PVC end pieces

12 12 Figure 2.5: The wooden model of the master Figure 2.6: Two possible configurations of the arms

13 Chapter 3 Axle bearings for the arm rotation To reach various positions in the working area, the arms have to perform a rotational movement. This is to be done without great force. Also the suspension of the arms should be stiff. Because of these reasons axle bearings for the arms are necessary. 3.1 Choice of bearings Single row deep groove ball bearing are chosen for the axle bearing of the arms. These bearings are low cost and sufficient for this purpose. Through the bore of the bearing the cables for the electronics of the z-mechanism should be guided. This prevents the cables from being trapped during arm movement. For the upper arm SKF [1] bearing 6011 is chosen. The outer diameter is 90 mm, the diameter of the bore is 55 mm. Bearing 6009 is chosen for the forearm. This bearing has an outer diameter of 75 mm and the diameter of the bore is 45 mm. 3.2 Placement of the bearings in the arms The placement of the bearings in the arms can be seen in figure 3.1. First the placement of the bearings for the rotation of the upper arm is discussed, then the placement of the bearings for the rotation of the forearm Placement of the bearings for rotation of the upper arm See figure 3.1. A tube (T1) is fixed by welding in the upper arm. Both ends of this tube should be machined on a lathe to receive the outer race of the ball bearings (B6011). The vertical distance between the bearings is maximal. This provides the highest stiffness. A tubular spacer (TS1) is placed between the top and bottom bearing inner race through which a hollow axle (HA) is inserted. Between the horizontal plates and the bearing inner race, a spacer (S1 and S2) is placed to create a small distance between the upper arm and the horizontal plates of the frame (P1 and P2). The hollow axle has a groove in which a circular cilinder fits (CC). The bottom horizontal plate (P2) of the frame receives this circular cilinder. On the top side the hollow axle has thread on the inside. In this thread a hollow screw (SC) is turned, which is received by the top horizontal plate (P1) of the frame. With this screw the different parts are compressed and the bearings are prestressed. This configuration allows mounting of the arms after assembly of the frame Placement of the bearings for rotation of the forearm See figure 3.1. The bearings of the forearm are placed in the upper arm. A tube (T2) is welded in the upper arm. The ends of the tube should be machined to receive the outer race of the ball bearings

14 3.2 Placement of the bearings in the arms 14 (B6009). Between the inner race of the bearings a tubular spacer (TS2) is placed. On top of the top bearing inner race a spacer (S3) is placed. From the top a tube (T3), which is welded in the forearm, is inserted which is made to fit the bore of the bearing inner race. On the bottom side of the tube a nut (N) is screwed. Between this nut and the inner race of the bottom bearing a spacer (S4) is placed. The nut compresses all parts and the bearings are prestressed.

15 3.2 Placement of the bearings in the arms 15 Figure 3.1: Top view and section A-A of one arm, with axle bearings, placed between the two horizontal plates (P1 and P2) of the frame

16 Chapter 4 Brakes During an operation, the position of the trocars on the patient doesn t change, so the position of the joint should be fixed. Therefore, brakes for the forearm and upper arm need to be designed. The forearm has to make a large rotation angle. Preferred are a brake system which is easy to imply in the existing design and with as least as possible lose parts due to the use in an operation room. Therefore disc brakes are chosen. For the upper arm disc brakes are also the most obvious. The disc can be clamped with a caliper. The schematic placement of the caliper is discussed. The exact realization and fixation of the calipers isn t part of this report. 4.1 Brake of the forearm The disc brakes can be integrated in the forearm, as can be seen in figure 4.1. It is possible that some partitions are needed to have enough stiffness. The caliper can be placed on top of the upper arm. Another option is to make a nut on which the disc is fixed. This nut is turned on the circular tube which is welded in the forearm, figure 4.2. The nut should be locked with glue or a screw. The caliper can be placed inside the upper arm. If necessary a hatch can be made in the upper arm for service and replacement. Disadvantage of this option is the lower stiffness of the upper arm due to the hole needed for clamping the disc. In the first option, the caliper limits the rotation angle of the fore arm. In the second option this is avoided. Also the caliper is concealed. Therefore the second option is the best.

17 4.1 Brake of the forearm 17 Figure 4.1: Top view and section D-D of the disc brake integrated in the forearm and the placement position of the caliper

18 4.1 Brake of the forearm 18 Figure 4.2: Top view and section D-D of the nut with the disc brake screwed in the forearm and the placement position of the caliper

19 4.2 Brake of the upper arm Brake of the upper arm The disc brake of the upper arm can be mounted on the outside as can be seen in figure 4.3 where the disc brakes are colored gray. The caliper can be placed in the vertical tube (A) or between the two arms outside the vertical tube (B). In the case of placement in the vertical tube, a hatch is needed for (re)placement and service. Disadvantage of this option is that a hole is needed in the vertical tube to be able to clamp the disc. In the second case (B) the caliper for both arms can be integrated in one part. The caliper can be fixed to the vertical tube and to the two horizontal plates of the frame. Disadvantage is that the caliper limits the rotation angle of the upper arm. Also the caliper is a place for dirt. The best option of these two is the first (A), but a redesign of the brake should be considered. Figure 4.3: Top view, side-view and section A-A of the disc brakes (colored gray) of the upper arm and the possible placement positions of the caliper

20 Chapter 5 Cable guidance For the movement and position measurement of the z-mechanism, the joint and the handle several motors and encoders are necessary. Therefore cables need to be guided from the z-axis to the frame. One option is to go from the forearm to the frame with a cableslap. A disadvantage is the high friction. Also it gives the master an unfinished, technical look and it is a place for dirt. The best option is to guide the cables through the circular tubes in the bearings. This could be done in a way as shown in figure 5.1. For the guidance of the cables over the arms some rectangular tubes can mounted on the arms. For easy service and assembly connectors are necessary. In the circular tubes, which are mounted in the bore of the bearings, windings should be made to avoid strangling of the cables during the rotational movement of the arms. Figure 5.1: Schematic view of the guidance of the cables from the z-axis to the frame

21 Conclusion A design is made for the frame and arms of the master. From this design a wooden model is made. For the rotational movement of the arms, the axle bearing is designed. This design enables mounting of the arms after assembly of the frame. To block the rotation of the arms during the operation, some concept ideas for the brakes are designed. There is chosen for disc brakes. For the fore arm two possibilities are designed, an integrated brake and a brake mounted on a screw. The last option is the best. A design for the brake of the upper arm with two possible placement positions for the caliper is proposed. The concept of this design should be reconsidered. Finally an idea for the guidance of the cables is proposed.

22 Bibliography [1] [2]