Driver

Finally my stepper driver arrived. Now I don't need to use my own home-grown driver circuit.

The benefits are: DIP-switch controllable current (up to 4.2A, my own circuit only does up to 1.5A) and sub-stepping up to ludicrous levels (using Allegro A3955 I get only down to 1/8 steps). And it's very easy to use. Just plug it in and it does its work. I just have two pins to worry about - direction and pulse. It steps the stepper itself. The downside of course the price.

Unfortunately the speed I hoped for did not really materialize. If only slightly. Currently I'm unsure if it can keep up with what the HP control circuit expects. If not - need to get a better stepper motor. But that's easy - just need to plug it in, driver takes care of the rest.

And on another note I found out why my printer got paper jam errors. The contraption to move the sensor was just not fast enough. I down-geared it by around ~1:4.45 because the encoder wheel has 1200 dashes while my stepper does 400 steps. Instead I should have relied on sub-stepping the motor and attached the encoder directly. If I overdrive the motor I can almost get it to work. It even started scrolling out the paper when the paper sensor was on. But its checking systems move it forw-back very sharply and the stepper tends to jam on that.

I don't feel like rebuilding the stepper thingie right now so am instead reverting to reverse-engineering the sensor. I've already attached wires to it and it seems to indeed be a two-channel system. Though I haven't really got the sense of all the pins, only two of them seem to have any sensible life in them as can be seen on the following picture:

As you can see the green leads the yellow - two channels being covered by the encoder strip as it moves.

Remote sensing

The printer's paper motor runs on 32V. To get the signal from that I'm going to use an optoisolator. But the motor can run both ways and so I need to protect the LEDs inside the optoisolator from the huge reverse voltage.

Now I'm not absolutely certain this is the way, but I went with protecting them using another diode plus adding a big resistor to take care of the reverse current still leaking through.

And this is the resulting circuit. Seems to work fine so far. The optoisolator I'm using is HCPL 2631.

Unfortunately I can't report success yet. I get paper jam errors no matter what I do at the moment. Have to investigate more. Perhaps the stepper is too jumpy. Or too fast. Or too slow. I think I need to get another printer of this kind to get a solid reference and make a comparison.

The fifth wheel

I have a problem. The paper roller of the printer has an encoder wheel attached to it. So the printer uses that to determine how much it has scrolled the paper on. It's all driven by a simple DC motor.

Now as I don't have the roller anymore but I move the printer itself instead. How do I tell the printer board how much it has moved. I basically have three options:

• Attach a roller to the side of my table and have it roll on something. While it seems simple and straightforward I see several problems. For one making such a mechanical system is actually not that simple. Secondly I want to move the printer around myself, as it needs to move backwards. If the printer board sees it - it'll detect a malfunction and panic. Also the printer logic has some redundant roller moving on itself - its head cleaning cycle for one - I don't want to move for that.
• The second idea would be to reverse engineer the sensor and feed the printer with fake electrical signals. The problem here is that I have not managed to find a specification for this sensor. And as I'm not familiar with them I don't really know where to start. It seems to have 6 leads. Two are probably for the emitter led, and the others are probably two pairs for two phototransistors. So it's probably a two-channel sensor.
• The third and the silliest option is to use a separate stepper motor to drive the encoder wheel myself. So I can fake it for the printer properly. And that's what I've done for now. Later on I should try reverse engineering the sensor itself using an oscilloscope. But for now, I went with the silly solution.

Here I used the timing belt of the actual printer's paper motor. But I reassembled it between two aluminium pieces. The stepper motor is the same I got off eBay.

I'm watching you

I need a sensor to determine when the assembly has moved to the beginning. I was thinking of using an optical sensor to do it. Once I've determined the start position, I can just count stepper steps to know where exactly I am in reference to it. To the right is the schematic of the circuit I'm building. I'm using Osram's SFH9315 slotted sensor. It consists of a LED as emitter on one side and a phototransistor as sensor on the other side.

Here's the completed circuit:

Now I just need to figure out how and where to mount it on my table.

And here it is:

The Force is all around us

I finally got everything machined and connected. Behold, it moves!

It is a bit slow at the moment. Though I can live with this. The motor is specked as having 0.36Nm holding torque. However my drive circuit is pretty primitive and does not squeeze the maximum out of it. I've ordered a driver board that hopefully will get more speed and power. Here's the table in a high resolution photo:

Driving a stepper

So I managed to build a circuit to test-run the stepper motor. I used two Allegro A3953 IC-s to drive the two coils of the motor.

The motor is actually a 6-wire motor, ideally driven with a unipolar driver. But as I don't have that I just used halves of the coils and ran it as bipolar.

I got this motor off ebay. It's a 400-stepper, very small, cost me 8€ including shipping.

Vises and vices

I finally got my cross vice (or cross vise for American english). Amazing how these two languages are diverging.

Also I got a few stepper motors I can start playing with now. As I don't really have any experience with them.

I also have a ball screw attached to the rig. I got it a few weeks ago and had the ends machined. Also I got some oldham couplings - that is a way to connect the shaft of the stepper to the shaft of the screw. The black belt to the right is the energy chain.