Sunday, 7 September 2014

More pick-n-place feeder testing

Our earlier trial for a feeder worked ok. But was a bit wonky at times. The tape could easily jump off the sprocket, and as the motor span, the tape wouldn't necessarily advance properly.

So this afternoon, it was down to the Nerd Unit to carve out some new designs!
We stuck with the same principles - it seemed to work, after all - and just made a few slight changes to improve performance.

Firstly, we reduced the number of teeth down to 20. This reduced the overall size of the cog, which we weren't thrilled about - but it did mean that the mounting bolts for the motor were no longer in danger of catching on the teeth/tape as it passed through.


The height of the channel that the tape travelled through was reduced from 3mm to 1.3mm, so that the tape would  be pushed closer to the teeth of the cog as it rotated (and it had nowhere to move to, veritcally, so reducing the chance of it skipping off the teeth).
We also removed part of the top of the feeder, so that we could push the sides of the device closer together, but still gain access to the surface-mount component (in this case a 1206 resistor).


A small shim was placed in the top channel of the feeder, to reduce the amount of room the tape had to move left-to-right.



Now the tape couldn't move left-to-right, nor up-and-down as the sprocket rotated, it was time to try the new design out....


The result was a much smoother, more accurate tape advance feeder. The teeth engaged with the tape every time, and the tape advanced with no slipping or missed feeds. Except....

We forgot that with our smaller sprocket, with fewer teeth, we needed fewer steps to make the tape advance by the 4mm between each component. A quick bit of maths told us:

The stepper requires 4096 steps for a full revolution (according to the datasheet) but we're using single-stepping, not half-stepping, so this is 2048.

2048 / 20 = 102.4 steps per 4mm distance.
After a few tries, we got it almost right. But every now and again, the tape would advance too far. Even allowing for the rounding error (we would make the motor move 102 steps twice, then 103 steps every third time) the tape wasn't advancing quite correctly.

We soon discovered that our stepper motor wasn't quite performing according to the datasheet. Maybe it's because we're driving it directly from a 12v supply (the same as the steppers on the x-y chassis) but 2048 steps seemed to take the motor just beyond one complete revolution.

We coloured one of the cog teeth (so we could see where it started, and where it ended up) and made the motor spin 2048 steps. After a number of trials, the coloured tooth definitely appeared to be further round that from where it originally started.

With a bit of Steve-coding (trying different numbers until something appears to work properly) we discovered that 2036 was the more likely value to get a true single revolution. Now 2036/20 is 101.8 - which means that some times we should be using the value 102 and sometimes 101. For a small number of components (up to, say, 100) the difference is probably negligible. But over time, that cumulative error could make the alignment of the components and the picking head go all to cock.


We'll hack in some nasty Stevecode to get around it for now. And maybe a more permanent solution might be some simple jog buttons on the feeder - so after we've filled one board with it's resistors, we can jog the feeder forwards/backwards to get it exactly in position before starting the next run?

Given the quality of the components we're using, we're not going to worry about getting it exactly right - our stepper motor is supposed to have 2048 steps per revolution, but it's more like 2036. There's also quite a bit of backlash in the gears (it shouldn't be an issue if you're only ever driving forwards, but gives an indication about the build quality of the motor). We're also using an mdf container, which may, or may not, create additional friction between the tape and the sides of the feeder, which the cheap, low-quality motor has to overcome.

Rather than fix the problem properly, with decent quality components and an aluminium milled body for the feeder, we'll just acknowledge the potential problem, and provide a means of correcting any cumulative errors quickly and easily (a method which may still be necessary, even with expensive, top of the range parts too!)