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DIY Inkjet Servo Macro Rail

 
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nucleobyte



Joined: 22 Aug 2010
Posts: 51
Location: Virginia, USA

PostPosted: Fri Nov 10, 2017 5:03 pm    Post subject: DIY Inkjet Servo Macro Rail Reply with quote

I've been experimenting with a homemade stacking rail using parts from a discarded InkJet printer. These cheap printers use servo motors with encoders rather than stepping motors, so I wasn't sure what kind of precision could be achieved. At any rate, I got it working, and the results were acceptable, so I though some folks might be interested. I have not yet implemented any kind of PID control, which should be possible with the encoder feedback and a better motor controller. Here is a link to a (slightly shaky phone camera) video of the device in operation: https://www.youtube.com/watch?v=wFLTmwMCWn4
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rjlittlefield
Site Admin


Joined: 01 Aug 2006
Posts: 18248
Location: Richland, Washington State, USA

PostPosted: Fri Nov 10, 2017 6:37 pm    Post subject: Reply with quote

Very nice!

The last couple of seconds show an interesting graph, but I can't figure out what step size it corresponds to.

What is the minimum step size that you consider acceptably repeatable?

--Rik
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Saul



Joined: 31 Jan 2011
Posts: 914
Location: Naperville, IL USA

PostPosted: Fri Nov 10, 2017 6:47 pm    Post subject: Reply with quote

Great & effective design ! I like these couplers from the tubing - in most cases they work better than machined from aluminum
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Saul
Studio, horizontal and field setups
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nucleobyte



Joined: 22 Aug 2010
Posts: 51
Location: Virginia, USA

PostPosted: Sat Nov 11, 2017 12:10 am    Post subject: Reply with quote

Rik,

Here is the graph:



This was from a test run - I wanted to establish a baseline to compare with possible improvements. The run was 1000 sequential steps resulting from activating the motor for 7 ms with a 12 V motor supply. This resulted in movement with each step measured by the number of encoder counts.

To calculate the step size, one rotation on a 16 tpi thread would be 1.5875 mm. The encoder is counting 4800 counts per revolution. Estimating 140 counts from the graph, 140/4800 * 1.5875 = 46.3 um per step (high of about 170 count/56 um and low of about 100 count/33 um.)

The question then becomes, how precise (or consistent) is enough? You could probably answer that better than me! I'm not sure at what point variation in the step size causes problems. Turning down the motor-on duration shortens the step but increases the variation. Inverting the logic, I can tell the motor to move and stop after, for example, one encoder count -- but this results in 10 counts or more due to inertia. Motor voltage has a large effect on starting torque and effects this as well.

I guessing, but with a better motor controller and possibly a PID system, it might be possible to get about 20 um consistently enough for stacking, I think that would be good enough for about 5X magnification.

- Greg
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mawyatt



Joined: 22 Aug 2013
Posts: 985

PostPosted: Sat Nov 11, 2017 6:59 am    Post subject: Reply with quote

Greg,

Nice work!! Cool idea using an ink jet printer carriage.

Since you have the drive screw coupled with a plastic tube coupler, why not use a 200 or 400 step stepper motor instead of the DC motor and encoder? Steppers have a pretty good reputation working as the drive force for focus rails and the error isn't accumulative. I believe typical stepper motors (NEMA 17) have less than 10% step errors and the real error source ends up being the backlash from the thread/nut interface, and the screw thread error which I would assume is very small and repeatable. There are all sorts of programmable controllers for steppers available for DIU efforts too.

Best,

Mike
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nucleobyte



Joined: 22 Aug 2010
Posts: 51
Location: Virginia, USA

PostPosted: Sat Nov 11, 2017 11:54 am    Post subject: Reply with quote

Thanks Mike,

I should have mentioned in the OP, one of my goals for this project was to see how much of the inkjet printer I could reuse. I was also simply curious as to how accurate it could be. Clearly, the printer makers in this case hp, get very high precision -- this drive train feeds the paper line by line for image priniting without banding. The printer contains a custom motor driver chip which I could not find documentation for, probably secret. They may also may be using the sine-wave encoder fully, I am converting it to square for much simpler handling.

The inkjet carriage I thought was a cool idea, but is not really a success, it is stable only in the hoizontal position. In the printer it is intended only for a very light load and weakly attached to the 8mm rod through a plastic bushing. I plan to switch to 8mm brass bushings on 2 rods. These bushing are available cheaply through the usual online markets.

- Greg
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