2015-09-04 (F) Weekly Summary

Getting this thing to move has been literally and figuratively difficult. Control and mechanisms have started to come together but there was not a ready-made solution to copy verbatim. Moving a four bar linkage in a parallelogram formation isn't particularly difficult in concept but attaching inexpensive servo motors to an oddly shaped keyboard and making it sit flush is far from a concept. It's possible.

Four straight levers were modeled and printed last week. Two of the levers were meant for acting as a hinge at each end while the other two were meant to attach to a servo at one end. Servo ends were revised from last week to attach in a more logical way. These simple parts were able to prove the desired motion was possible but the range range was not enough.

Screenshot of servo levers

Demonstration of parallelogram four bar linkage movement

To get the desired range a pair of servos were taken apart and converted to continuous rotation so they could reach a fully extended and a fully retracted position. Unfortunately the ability to sense position is lost during the process so limit switches will be necessary.

Servos in the process of being converted to continuous rotation

Trying to move something as oddly shaped as the keyboard would be easy if I just attached wide supports but that would be bulky and awkward to type on. Wide supports would also look awful and I would like this project to look cool. To make things work the shape of the levers was changed. Length was not changed since the hinges must still act like a parallelogram.

Offset servo levers

Making offset servo levers seems to be the right path although the first attempt had a few problems. The first problem was the offset size was way off in the x and y dimensions. The second problem was one end was supposed to attach to a servo. Thirdly, two levers were printed but only one was necessary. Lastly, there was unanticipated interference from a key pad.

This lever cannot reach the bolt

With all the previously mentioned problems a second lever was modeled and tested but this time it worked well. The lever was attached at the back of the thumbboard and all the clearances agreed with the model. All the printed parts were assembled with long #6 bolts. Although the servo controller has not been programmed and installed it was possible to manually move the parts and see the motion of the linkage which looked good. Making clearances for a fully retracted model may be difficult and more troublesome than worthwhile but a fully extended model which rests in the user's hand will be necessary.

Double offset servo lever

Partially assembled wrist mounted keyboard in the extended position

The retracted position hasn't been fully realized

Next week construction of the levers will continue. Limit switches and a controller need to be installed and a method of deploying the keys will be necessary. Enclosures still need to be made which will hold all the electronics. My dear hope is that next week there will come a moment when I hold this to my wrist and press a button causing these cobbled pieces of metal and plastic to move some switches into my palm. If this happens I will have such a smile.

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