Sunday 24 August 2008

Alternative Z-endstop

I prefer my machine to home the z-axis away from the workpiece so that homing is always a safe operation, regardless of what state the machine is in. The standard endstop bracket allows the opto to be mounted on the horizontal rails but for a bottom endstop it needs to be mounted off one of the vertical posts. I designed a new bracket and tab for this: -

The tab enters the opto from the side which gives the best resolution. Here it is installed :-

It can be mounted anywhere on the vertical post so could be used as a top endstop as well but I don't see the point of stops at both ends.

The way I calibrate HydraRaptor's z-axis for FFF is that I get the head somewhere near the table and measure how far away it is with a rule to get rough calibration. I then instruct it to go to 3mm above the table. I roll a 3mm bright steel rod under the nozzle and jog the axis up and down in s/w until it just touches. This has to be done with the nozzle fully warmed up to the working temperature because the PTFE expands about 0.5mm.

The calibration drifts on HydraRaptor because the frame is made from wood so the weather affects it, something that Darwin should not suffer from.

The files are here:

Friday 22 August 2008

Shaft encoder, second attempt

I made a right mess of the first design, not only did I get the angle wrong but the second opto is also at the wrong radial distance, which is why its amplitude was less. So I had to redesign the bracket, here is my second attempt: -

The correct angle for quadrature needs to be (K + 0.5)(180 / n) where n is the number of slots in the wheel and K is an arbitrary integer. The first convenient angle which straddles the bolt through the motor is 45°.

A single coat of BBQ paint seems adequate to block the IR beam.

A bit of a pain to wire up! If it proves to be a useful encoder I will design a pair of PCBs.

The waveforms are now both full amplitude and in quadrature: -

The edges are slow because I am using 4 pin optos, Zach seems to have bought up the world's supply of the five pin ones for the RRRF! The five pin versions have a built in Schmitt trigger to square up the waveform before it goes down the cable. Most micros have Schmitt trigger inputs these days but the disadvantage is that any noise in the cable will advance or delay the edge slightly, giving a timing error. Given the imprecise nature of this encoder I don't think it will make much difference.

Monday 11 August 2008

RepRapped Shaft Encoder

I have started making an extruder for my Darwin but I am running out of ABS. I bought some more from Tempatron but it is very oval, up to 3.5mm, so it wont fit through HydraRaptor's extruder. So it is a race against plastic to make a new one with a bigger bore! I will try making the filament guide out of HDPE as that is the most slippery of the four plastics I have.

One thing I definitely wanted to try in ABS before it ran out was to make a shaft encoder. The latest RepRap V1.1 design has one with a single opto but it needs support material and gears. I am happy with the older design now that I have got it to run reliably, so I needed an encoder that mounts directly on the motor. I also prefer two optos in quadrature to avoid errors from backlash and stopping exactly on the edge of a slot. I want to experiment with backing up the filament as well, so I will make this version reversible.

I knocked up a design which uses a pair of slotted optos in CoCreate : -

The wheel is the same as Ed's design except that I have added a boss which mates with the top shaft of the GM3 gearmotor. It has 18 teeth which gives 72 steps per rev with quadrature encoding. One turn of the extruder feeds 0.8mm of plastic with an M5 drive screw. That will extrude about 0.4mm of filament per step, so not great resolution.

When making the opto flags for my Darwin I realised that quite thin walled objects come out OK and are still reasonably strong. I think the bracket is the thinnest thing I have designed so far, its walls are only 2.4mm thick. Its shape really requires support material for the slots and holes but it came out fine without any. It was particularly hairy though when it came off the machine: -

A bit of whittling with a penknife soon cleaned it up :-

Here it is installed on the motor :-

I wired up to an oscilloscope to test it: -

As you can it is very noisy because I have not made a new suppressor for the GM3 yet. Also the top trace does not go high properly. This is because quite a lot of IR light gets through the ABS when it is so thin. Not surprising as you can see visible light through it. I painted the top surface black with BBQ paint and that improved it a lot.

I think a second coat on the underside will improve it further. The waves are not in quadrature because somehow I managed to get the angle wrong, so I will have to make another bracket. It will function fine at half the resolution so I might press on with the extruder first.

So a cheap and cheerful shaft encoder, but not very high resolution. Since slotted optos are just LEDs and photo transistors in a bit of plastic, I think I will make another one with the raw components that will be even cheaper. I could put some more teeth on but that would make it harder for people to make and I want people to be able to upgrade their machine with their machine. Another way to do it is by printing onto film with a laser printer. There is a good site about that here.