syringe pump struggles

[anmo...@colorado.edu]

The last couple of weeks I've encountered a new issue: my syringe pump keeps blowing out. I've been using the turbidostat off and on for a year now with no issues to the syringe pump. However in the last week and a half I've blown three syringe pumps. The first lasted me a year (of occasional use), the second lasted me a week of continuous use, and the third lasted me 3 days. Anyone experience this issue before?

My best hypothesis right now is that the media bottle filter is getting clogged and having a harder time passing air. I'm not sure how I can test this hypothesis though... I could always start a new run but ideally I figure out the issue before I waste time and money on another blown syringe pump.

Something to note is that I'm using a 5mL syringe rather than a 3mL syringe (speaking of which, if anyone also wants to change to a 5mL syringe, I've designed some custom 3D printed parts to accommodate the bigger syringe. I've attached the files here). But I've been using the 5mL syringe for half a year now with no issues until now. But I suppose this could be causing added strain on the motor?

Any suggestions would be appreciated.

-Andrew

[Chris Takahashi]

Hi Andrew,

This has been a known issue for a while.

The 3d printed pump uses a hobby servo.  Some tend to die very fast.  I had others that lasted hundreds to a couple thousand hours.  The root of the issue though is heat and the motor's feedback circuit.  If the motor stops before it reaches it's destination then it will continue to apply a small force but not move (this is the humming you may hear).  This creates a lot of excess heat which will cause the motor to fail eventually.  Some motors are better about not doing this than others.  Sometimes lubrication on the sliding part helps too.  if you're really gung-ho you could cut a hole in the motor case and add heatsinking to the driver IC. 

I have a software fix for this as well that shuts off the pump motor after 3 seconds of no motion, but it needs testing and I don't have an extra mainboard right now (I'm working on a complete rework of the mainboard right now, so the current design isn't a priority).  I'll see if I can dig up a test board this week though.  

Chris

[Chris Takahashi]

One other thing.  The syringe mount top and bottom shouldn't be screwed too tightly or it'll add a lot of friction and break motors.  I typically don't even use the top.

On Monday, November 13, 2017 at 10:21:38 AM UTC-8, anmo...@colorado.edu wrote:

[Andrew Morgenthaler]

Interesting. Glad that this is a known issue (as opposed to something wrong with my main board causing the motor to short or something). And that is awesome that you've been working on a software fix for it. I definitely would be interested in that once it comes online. Let me know if you'd like me to test anything out on my set up if you can't get access to one.

You say that if the motor stops before it reaches its set position, it continues to apply a small force without moving, eventually causing overheating and failure... why would it ever not reach its set position?

I've been using the syringe mount top so I'll stop using that from now on. Are you saying it adds friction to the plunger or to the syringe body?

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[Chris Takahashi]

Hobby servos use proportional control, so if they get close but there's too much friction(or back pressure) they'll just stall.  it's usually only a tiny bit of error but because of the very simple control electronics it can be a problem.  Some motors seem to do better than others and I'm not 100% sure why.

over tightening the top of the syringe body holder will squeeze the syringe around the plunger adding to the friction.

[Chris Takahashi]

Try this mainboard firmware.  You should see the pump move as normal and hum like normal.  it should then stop humming after 3 seconds.

[Andrew Morgenthaler]

Awesome! I'll give it a shot next week!

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[Andrew Morgenthaler]

Looks like the new firmware works. The motor stops straining after ~2s. I just tested this by applying manual force against it, but I'll see how it works on a real run next week.

Thanks so much for the firmware update Chris.