Friday, 19 September 2014

Vacuum head fail

It's been a little while since we managed to do any work on our CNC pick-n-place system. With real life and work and the Scottish Referendum getting in the way (ok, maybe not the referendum) there's not been much activity on this little project for over a week.

And - given we've got some screens prepared for printing multiple copies of our board game PCB - it's time this was finished so we can start using it to populate circuit boards with SMT components!

So far we've got our x/y axis moving and an SMT parts feeder running. Now we just need to connect up our vacuum pen and lift it up and down, and we're ready to try the whole thing out!

For our z-axis (pen up/pen down) we're planning on using a servo. This worked well for us on our CNC drilling machine of a few years back, but it's going to need a little modification. For the drill, we wanted to force the head of the drill beyond the surface of the piece being cut (so that the drill would in fact drill a hole in the piece). For our pick-n-place, we want to put our SMT component on the top of the board, but not try to ram it all the way through!

Because we don't know at this stage exactly how thick each of our components is likely to be (IC chips are likely to be thicker than 1206 resistors for example) we can't be sure, beforehand, how far down we want to drive our servo, to place the piece on the board. What we were looking for is a spring-loaded system, so that we can raise the pen/head up, but when released, it returns (downwards) under spring tension. When the component is on the surface of the PCB, the spring can't ram the piece any further down into the board.

This simple modification should mean that we can easily accommodate SMT components of varying thickness.

Of course, being cheap, and looking for an easy win, we replaced springs with elastic bands, mounted the servo onto a pen-holder, and dropped the vacuum pen into place:

When the servo is activated, it lifts the pen upwards, against the tension in the elastic bands. When the servo goes the other way, it does not drive the pen downwards - it simply moves out of the way, and allows the elastic bands (springs) to pull the pen back to the surface. If the tip of the pen plus the component it is holding is resting on the PCB surface, the elastic bands can't actively drive the pen tip any further downwards (potentially damaging the tip of the pen).

Despite it being a historic night for British Democracy (the Scottish/UK referendum was taking place) a few of us turned up at BuildBrighton last night and tried the vacuum head assembly on the actual CNC frame.

The results were less than impressive.
It turns out that the tension in the elastic is too great for the servo to overcome. So when trying to drive the head upwards, you can hear the little 9g servo straining - but there is no movement.
Remove the elastic bands, and it can flip the pen holder attachment up easily (though without the bands to offer some resistance, the pen sometimes jumps out of the holder!)

We tried using two rubber bands (making them longer, and thus putting less strain on the servo). This worked, but two bands tied together were simply too "floppy". So we've decided to try making an adjustable height mechanism on the pen holder. By adjusting the height of piece that the elastic bands connect to, we should be able to adjust the amount of tension in the bands (and thus, make it easier/more difficult for the servo to lift the head).

We also need to change the way we drive the servo, in our firmware.
At present, the servo does not "lock" into place. We issue our 20ms pulses to get the servo to move into position, only long enough for the servo to move into place. During this time, the firmware does nothing else. This means that once the servo is in position, it is not longer being actively driven into place (and can be moved, by hand - or by the tension in the elastic bands - into a new position). We don't want to use a timer on the PIC to re-issue the servo position every 20ms, as this might interfere with our (non-buffered) serial read instructions, and result in garbled data.

So we're going to create a dedicated driver board, just for our servo. It will be a cheap PIC based board, with it's own dedicated timer, driving the servo to a new position every 20mS. This will cause the servo to lock into place, and will also mean we don't have to worry about running everything off one chip, and worry about interrupt routines colliding and potential data loss.

Last night felt light a we'd failed in a lot of respects. But, given that we came away with a whole load of ideas on how to get the head working, it was actually quite a productive night. That doesn't quite seem right - getting something done on a Thursday evening... Maybe we can convince the Scottish Parliament to hold a referendum every Thursday, and see if it makes our other Thursday nights a bit more productive too!

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