Sunday, 21 September 2014

Progress without actually doing much

Tonight we managed to get a new vacuum head laser cut. It was only a small job, but seemed to take ages to get just right.

Since the last one had problems with a single elastic band being too tight, and two bands being too slack, the new design incorporates an adjustable height mechanism. This way, we can add just enough tension in the bands to act like springs, to get the vacuum pen to return, without overwhelming the puny little 9g servo we're using.

Straight off the laser cutter, and straight onto our cnc pick-n-place machine. Now, by slackening off the top nuts, we can adjust the height of the top piece by rotating the nuts (screwing them either up or down the M3 bolt connected to the bottom piece) immediately beneath it. This worked surprisingly well.

We also adjusted the size of the holes in these pieces - dropping them from 10mm to 9.4mm. We measured the barrel of the pen with some digital calipers and it reported that the width was around 9.4mm (they were only cheap calipers, and tended to vary between 9.35mm and 9.48mm along the length of the pen!)

With the new, tigher-fit holes, the pieces lock around the pen barrel using just the friction between the mdf and the pen (is this what is often called an interference fit?). Anyway, the result is quite pleasing - as the servo arm lifts, the entire pen lifts also. When the servo arm goes down, the pen snaps back into exactly the same place, every time.

This did get us thinking about how it would actually work in operation.
So far, we've not given much thought about rotating pieces when placing them on the PCB. In fact, since this machine was only every designed to be semi-automated (it will pick up a piece and place it over the correct place on the board, then wait for an operator to confirm the position is correct - jogging the head up/down/left/right if necessary - then hitting a button to actually drop the piece down) we thought that simply rotating the pen by hand would suffice to begin with.

But now things have got a bit tricky.
When the pen was a very loose fit in our pen-raising mechanism, this would work fine. But with a tight, snug fit, it's not really possible to rotate the pen at all now - as it would require the attached elastic-band-and-servo assembly to be rotated also.

The obvious answer is, of course, to ensure that the component to be placed is facing the correct way around before the pen picks it up. Which in turn means that our automated parts feeder - which worked so well in earlier trials - is effectively redundant. So we're actually making progress by removing parts of the machine. This is the kind of progress we like - improve things by simplifying them!

It's not unreasonable (although it feels like a bit of a cop-out) to have the vacuum head simply move to a "ready" location, and present it with a component (already turned around to the correct orientation) before switching on the vacuum pump, to draw the piece onto the travelling head.

How the component is prepared can be developed as a separate, independent part of the project. To begin with, we could even just place the component on a piece of board and move it - by hand - into position, under the head, before starting the pump-on-head-up-move-to-position-wait-head-down routine. In future, we could develop a "component preparation" module for the pick-n-place machine, which would automatically wind the next component from the reel, and rotate the component (rather than rotating the head) ready to be picked up.

But for now, we can actually make further progress with our machine by taking that bit out and worrying about it later.

In fact, were it not so late, we'd probably be ready to give the machine it's first test run: we've got x/y axis moving nicely now, the entire pen moves up and down with the servo, the vacuum pump can be triggered manually when required, and the components can be presented to the head, rather than have them pulled out of an automatically fed tape.

The firmware needs a bit of revision (it's currently using a function-blocking serial-poll method of getting data, which is just plain shonky) but that's all that's really stopping us from giving it a spin. So maybe tomorrow night we'll get the firmware updated and perhaps after that look at building a nice application to run it from a desktop computer.

Hardware based, interrupt-driven serial/uart data parsing here we come.............

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