Motor Mount

Mounting the motor to the leg has become a much more in depth problem than we first imagined. Our first idea for the motor mount was to build a plate which would be held in place using the velcro straps on the knee brace. Another velcro strap would then wrap perpendicular to the first, around both the motor and through slits on the plate, securing the motor in place. Unfortunately, this plan did not work for several reasons:

sufficiently aluminum scrap was not available
wood was not strong enough to support the necessary load from the strap tension
mounting location on top of the thigh was found to be too flexible

For these reasons, a new motor mount and location was needed. Our team realized from the first failure, that the motor would have to be mounted on to a rigid structure; the options were to either build a rigid structure between the plastic hinges, or to mount the motor onto the link. We chose to the mount the motor onto the link in order to use the most solid mount possible and reduce the amount of design and machine time necessary to build a "bridge" between hinges. We were also unsure of whether the "bridge between the hinges would be rigid enough given that they are simply held in place with velcro.

The main risk in mounting the motor to the link is that the motor is no longer mounted to "ground" but to a moving link. We feel the risk of mounting to the link instead of ground is minimal given that the upper leg would actually move more than the link, and the link really only moves a small distance during actuation (seen in a numerical model of the system).

The main complication of mounting the motor to the link instead of the upper leg is that the axis of rotation of the motor must now be turned 90 degrees in order to interface with the link. This issue will be addressed in a separate page.

In order to mount the motor to the actual link, we designed the piece shown below.

In this mount, the motor rests upon the top groove machined into the aluminum while the link is attached via the two countersunk holes. The groove gives two lines on contact so that the motor cannot twist off center of the mount. The three grooves on the bottom allow for two different functions. The first two grooves allow for worm gear hose clamps to slide between the link and the mount. These hose clamps apply a significant force on the motor and thus onto its two lines of contact such that the motor will not be allowed to twist along the axis of the motor. Additionally, the hose clamps ensure that the motor will stay firmly fixed to the link (three sliding DoF are removed as well as the last rotational DoF). The final groove in the bottom of the mount is to allow the wires from the motor to leave the motor casing and attach to the arduino microcontroller. Finally, the overall length of the mount was precision machined to be the exact distance between the encoder and gearhead on the motor. This tight fit helps to more securely attach the motor, as well as orient the output shaft of the gear head directly along the axis of the link so that it may mesh with other components.

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