Evaluation

The coupling centre was initially manufactured from aluminium to reduce the weight of the assembly, however, due to the loading required from the springs to ensure the rigidity, aluminium was a poor choice, as it was too soft, and experienced plastic deformation under load. This issue was rectified by replacing the original aluminium centre piece with a case hardened steel centre piece that didn’t deform, but added additional weight to the assembly, with the aluminium centre piece weighing 9g and the hardened steel centre piece weighing 29g. However, this weight difference would have little effect on the overall system, when taking into consideration the variance in arm weight of 1.33kg found in Table 3.1.

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Figure 5.30 - Break joint assembled with robot

The coupling, shown in Figure 5.29 & Figure 5.30, was tested for suitability of application by experimental means. Initially, the coupling was tested using a spring balance to apply and measure the load. With the coupling mounted on the robot for convenience, the coupling was positioned with its central axis horizontally aligned with one of the three spring detent mechanisms aligned vertically. The spring balance was suspended above the couplings central axis, 80mm from the central axis of the spring detent mechanism, as this was selected arbitrarily as the location for the central axis of the patients arm during the design process.

The spring balance was connected to the coupling via a turn buckle arrangement for fine adjustment. As previously discussed in section 3.1, and used as the maximum loading in the design calculations, the loading at which the coupling should disconnect was determined to be 34N to minimise risk of injury to the patient while still being capable of completing rehabilitation exercises.

The robot was used to apply a loading of approximately 30N, with the final loading via adjustment of the turnbuckle to achieve a loading of 34N. The spring tension of the

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detent mechanism was adjusted to the point where the mechanism would release the coupling, thereby setting the break load.

Using a spring balance was proven to not be the best apparatus for setting the break loading, as it was discovered during testing that as the mechanism began to release, the splint end of the coupling to move in the direction of the loading force, thereby reducing this force. As the force released, the coupling remained engaged in a semi flexible state. This meant that the coupling mechanism displays a degree of passive compliance that was not expected. This is advantageous to the operation of the system, allowing the system to remain engaged while still reducing the load in the case of marginal overload conditions which may otherwise disconnect the coupling and stop the rehabilitation exercises. This behaviour would be dependent upon the depth and contact angle of the ball detent mechanism. However this will require further investigation in future developments of the system.

Due to the unexpected results of testing the coupling with the spring balance, the procedure was repeated using weights for the loading force to ensure a constant force throughout the disconnect event of the coupling to ensure the correct break loading is set. With weights used in place of the spring balance, the coupling operated as expected, disconnecting cleanly at the set loading of 34N.

To test the operation of the coupling with regard to rotation about the central axis, a bar was bolted to the centre piece in place of the splint mount, using a cap screw. The head of the cap screw was supported to remove any cantilever loading on the coupling, ensuring a pure torque loading about the axis. Considering the splint selected for use in this project (Figure 5.22), it was considered that the risk of injury would come from the arm end of the splint, where it is likely to press against the flesh of the forearm of the patient, as

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opposed to the wrist and hand that were fully supported, and held firm by the splint, with minimal risk of injury. The distance from the end point of the splint to the central axis of the coupling was approximately 110mm. Therefore, the 34N loading was suspended from the bar attached to the coupling at 110mm form the central axis to simulate the loading conditions with the splint in place. With the coupling previously set to break at 34N, it was found that in a pure torque loading, the centre piece of the coupling rotated easily, and with further investigation, reducing the loading to 29N, and gradually increasing the loading by increments of 0.5N, the break loading was 33N, or 3.63Nm, which is less than the calculated value, but still acceptable for the operation of the coupling.

The difference in calculated torque loading to the actual loading measured is likely due to the accuracy of the machining of the dimple and groove of the coupling centre piece. Although the centre piece was machined to the tight tolerances (-0.02mm) of the design drawings this still allowed for an angular variance of 1.3º which would equate to a loading variance of approximately 1N as observed during testing.

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