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[Reply #15 on:]

but idk about that pair of bellows, how to "drive" it and about its "fixed" volume with its flexible parts.

To drive it you would just need to push or pull the two sides together, easily done with a mechanical linkage operated by a servo.

I'd expect the displacement to be consistent, if not very predictable. If consistency does prove to be a problem I expect you could improve it by enclosing the bellows. I'd have thought this was much easier to build than something capable of generating enough force to operate a syringe - I'm envisaging something that takes significant force in both directions, and needs to be positioned accurately.

[Reply #16 on:]

Bellows might have issues such as being made of flexable materials, and finding a bellows with a small area end so a lot of force isn't required to generate pressure.

[Reply #17 on:]

Not find; make. It just needs a couple of paddles hinged together with a plastic bag trapped between them. I see this as being literally a five minute job to get something working.

[Reply #18 on:]

Not find; make. It just needs a couple of paddles hinged together with a plastic bag trapped between them. I see this as being literally a five minute job to get something working.

Kind of like a zip lock sandwich bag between two pieces of wood? 

[Reply #19 on:]

That's the sort of thing I had in mind. Large quantities of tape or elastic bands securing the bag onto a small hose, and you've got the connection to the measuring chamber. The nice thing about this approach is it's all low tech and doesn't need much construction. Not as refined as motors driving nuts spinning around threaded bars all mounted in an assembly driving a syringe and all that, but I much prefer to get something simple working and then improve on it as necessary.

[Reply #20 on:]

What is important is repeatabilty, so it should always be those 40ml/ 4cm pushing and pulling.

If there is some degree of precision required, then a total low tech might not meet the specs. Just looking at the Martin P. jones parts catalog, they have a 60cc "irrigation" syringe for $.95 (with $9.95 s&h  ). Sometimes the big syringes are used to feed infants and animals too.

[Reply #21 on:]

You could just use threaded rod from a hardware store.

[Reply #22 on:]

hi,

even if it works with bellows, isnt this still the easiest approach with some guarantuee about precision/repeatability compared to bellows?

[Reply #23 on:]

even if it works with bellows, isnt this still the easiest approach with some guarantuee about precision/repeatability compared to bellows?

So how is that going to know how much fluid has been moved? You would probably need an encoder on the threaded shaft for rotation determination. You also need to think about the possible need for thrust bearings and such.

[Reply #24 on:]

i havent used a motor with arduino before but this is how id imagine it works:

using a stepper motor, testing how many 360° turns i need to get the syringe down to 0ml, then reversing the turn direction and testing how many turns i need to get to the 40ml position. then programming that into arduino to control the stepper.
This way with stepper i have  every time exactly the same amount the syringe gets pulled/pushed.

Does it work the way i imagine it?

[Reply #25 on:]

Does it work the way i imagine it?

In an ideal world, yes. But what happens if the stepper misses one or more steps (or oversteps)? Then where the Arduino thinks the syringe plunger is at versus where the plunger -really- is at are two different things. Thus the need for some sort of feedback mechanism (a linear-taper, linear-slide potentiometer could work in this manner, for instance). You would want at a minimum limit switches at each end, so that if for some reason there is a problem or error, you don't over-travel in either direction - which could break the mechanism, motor, drive electronics, etc (if you didn't want switches, you might try monitoring motor current instead - though that is more complex, especially with a stepper, depending on your driver electronics).

[Reply #26 on:]

Low tech, but at the bottom of the below page is a ~simple setup where a single SPSD relay and limit switches controls the direction and travel limits for a motor. Energize the relay and a motor runs until a limit switch is actuated. Deenergize the relay and the motor runs in the opposite direction until another limit switch is actuated. With the appropriate limit switch adjustment setup, reasonable accuraccy might be achieved.

http://web.comporium.net/~shb/switch.htm

[Reply #27 on:]

thx for the link and replies.

But what happens if the stepper misses one or more steps (or oversteps)?

I feared this would be a reply. yesterday evening a read a datasheet of a stepper and it said "stepping accuracy 5%" and i was like omg pleaaase dont -why cant this be just plain simple. 

Is the "forward" and "reverse" movement of steppers/gearmotors usually done with relays?

[Reply #28 on:]

Is the "forward" and "reverse" movement of steppers/gearmotors usually done with relays

No it is normally done with a h-bridge circuit that is the normal part of the stepping motor driver. It is just the software sending a differant pattern of pulses.

[Reply #29 on:]

An electrical or mechanical end stop would remove any concerns about stepper motor slip.