[Zookes]

Hello CNCZoners, I’m very new to all this and though my project is more automation than CNC, stepper motors and other hardware will need to be numerically controlled, so I’m sure this community can assist me in that regard.

I am looking to control a number of stepper motors, some numerically controlled valves and a numerically controlled air decompresser from a PC running linuxCNC, which I have been assured can do the job.

Unfortunately I’m not 100% clear on what the logistics will be to deliver my coded commands to the mechanical hardware and where to find such hardware in the first place.

In short, where can I find:

- A numerically controlled air decompresser (vacuum pump),
- Some numerically controlled valves (preferably either cheap or easy maintenance),
- Some stepper motors (general purpose),
- Whatever controllers, drivers or boards I might need to connect it all to a PC?

All in the UK preferably, though I don’t mind ordering overseas through Ebay and the like.

I realise this is perhaps outside the purview of a CNC forum, but I had hoped that there might be those among you with varied enough interests to shed some light on it, or at least point me in the right direction.

Thanks in advance answerers!

[Markwind]

I have never heard of an "air decompresser" can you give me more detail, is this a vacuum pump?

[Zookes]

Oh, sorry, an air pump that can suck the air out of a vacuum chamber. Isn't that decompression? I might have used the wrong name; I'll edit the original post.

[chuck_hobby]

Zookes,
While more description of what you are automating will help, what you describe so far sounds like standard industrial automation, or in smaller scale laboratory automation. Regardless, the valves and pumps that you will likely use will not be "numerically controlled" in and of themselves...google "PLC" or programmable logic controller for something that might be useful in your application.
As for your motion needs, steppers are great and there are many options including 'smart' steppers with the controller and driver integrated...just connect to a USB-serial port and go (IMS mDrive, Linistepper).
I'll be interested in seeing what you come up with, so post back on what you're doing and where it all goes...
Cheers!
Chuck

[Zookes]

Yes, it's pretty much industrial automation, but I couldn't find a decent forum dedicated to the subject (any directions would be much appreciated). LinuxCNC apparently has some PLC functionality so I assume I'd be able to have some looping program with the valves, vacuum pump and steppers all moving in sequence according to the time signatures of a single program?

Is there a good on-line retailer in the UK that sells PLC hardware, and the sundry junk mentioned in the first post?

Thanks for setting me right about PLC and such chuck!

[Zookes]

Aha! I think 'actuated valves' are what I'm looking for.

Edit:

Something like this perhaps:

http://www.coleparmer.com/Product/So...at/EW-08617-96

Cheap and cheerful. Would I be able to use LinuxCNC and a PLC controller to send commands to this unit?

Edit the second:

Or this even:

http://www.espares.co.uk/part/dishwa...gn=google+base

Even cheaper and more cheerful.

[Al_The_Man]

I think you have to decide if you want any servo actuated devices? via either servo, stepper motors or servo valve?
So far the devices you have linked to appear to be simple on/off not requiring any servo action, If so I don't think you need the complexity of a numerical controlled system.
A PLC would probabally do all you want to do, most even have a simple servo or stepper option anyway.
If an operator interface is needed, there are several simple two or three line displays with keypad that offer simple input to the PLC.
A closed loop servo system, whether motor or valve operated, implies feedback, PID loop for e.g., which again PLC can do.
There are many PLC options available and probably much depends on your present level of expertise and the requirement of the application?
Al.

[john-100]

Hi Zookes ,

I'd use a solenoid valve like the one from coleparmer
as it has a safe 24VDC coil


the hotpoint solenoid valve has a mains powered 240V AC coil

john

[Zookes]

To Al: I do need some movement in the system of a pair of large plates on some bi-directional rails such as this:

Bi-directional ball screws move nuts in opposite directions

The plates must move to a certain point, and, hopefully the steppers should power down, releasing the plates.

Which brings me on to another question, can stepper or servo motors work in conjunction with springs, i.e. temporarily resist the force of a spring for a few seconds, without causing damage to the unit? Say if I programmed them to power down at a certain point where the movement caused by the force of the spring would be minimal (a few millimetres)?

Can a PLC system be relied upon to rotate steppers to a certain specific point and back again at a given time signature?


To John: Ah, I see, I wouldn't have noticed that. Are your concerns based on safety alone or are there other things to consider in terms of voltage?

Edit:
Hah, I just saw the name display of the link. I think I'll leave it be.

[john-100]

Hi Zookes ,

not only will the 24V V solenoid valve will be easier and safer interface with an electronic controller
but its also designed to use with standard industrial standard pipe connectors

about 15 years ago I repaired a PCB used in an old EMI-MEC sprint
plug-board capstan lathe
they used similar looking 50V solenoid valves to control the pilot air to the pneumatic valves
that in turn controlled combined double acting pneumatic and hydraulic cylinders to move the slides and turret
the pneumatic cylinder providing the power and the hydraulic cylinder
the speed control

John

[Zookes]

To John: Ah I see. In my case I'll be working with liquids, does the spring always come into contact (as per the diagram on the wikipedia page for solenoid valves) with the transported liquid or can it be kept separate? I have need for the locomotion of three different liquids, one of which cures into a solid after time.

Will I need a different valve solution for a curing liquid, such as a rubber pipe with a pinch or something of that nature? Or perhaps a sequence that flushes the curing liquid out of the valve before it has time to cure (the cure time is long, so it needn't be swift)?

Thanks a metric ton for all the help so far chaps, it's been edifying.

[john-100]

Hi Zookes ,

I've had a quick look to see if the return spring in a hydraulic valve is in the fluid and found this info :-

http://docs-europe.electrocomponents...6b80033847.pdf

how are the three liquids mixed ?

may be you could use three metered pumps
either something like a double acting piston as in some beer pumps or
the peristaltic pump in dialysis machines

John

[Zookes]

Hmm, I'll admit that document is mostly Greek to me, but it seems as if the solenoid itself never comes into contact with the liquid, correct?

The three liquids are polyester resin pre-catalyst, the catalyst itself and the final resin+catalyst mixture which has begun to cure (Edit: oh, and some pigment, so four liquids including the resultant mixture).

My current system design has the resin and catalyst mixed in a vacuum chamber to prevent air bubbles in the resultant mixture.

A vacuum can remove most air pockets of consequence in the mixture so either they are mixed while under vacuum; or before, whichever gives better results.

I think that solenoids should be fine for the component solutions pre-mix, as they are more or less inert, but perhaps for the final exit valve, I might need something more robust, like what you have described.

[Al_The_Man]

The plates must move to a certain point, and, hopefully the steppers should power down, releasing the plates.

Which brings me on to another question, can stepper or servo motors work in conjunction with springs, i.e. temporarily resist the force of a spring for a few seconds, without causing damage to the unit? Say if I programmed them to power down at a certain point where the movement caused by the force of the spring would be minimal (a few millimetres)?

Can a PLC system be relied upon to rotate steppers to a certain specific point and back again at a given time signature?

There is not enough information as to the demands of the system to give anything in detail, but if you are moving steppers to fixed points every time then you may not need a sophisticated/operator interface?
Steppers are open loop servo's, IOW they will travel to the same position or point to point position and return every time, they also run at max torque at all times, even when stationary (holding torque), they can also be disengaged when stationary, there will be a small amount of resistance to movement (indent torque).
If dispensing a liquid of some kind there are valves available for dispensing adhesives.
http://www.techconsystems.com/fluid-dispensing-valves
There is also a tech paper on the site.
http://www.techconsystems.com/images...UR_PROCESS.pdf
It sounds as if you are designing a simple point to point actuator or gantry?
Al.

[Zookes]

To Al: The design is basically as follows:

A pair of plates will be under compression (quite light compression, no more than a few Kg in force, perhaps even just a few hundred grams) from some springs.
A(some) stepper motor(s) will be used in conjunction with a bi-directional screw to briefly pull the plates apart a specified width.
Some other steppers will do some stuff.
Then the stepper(s) will return to the original location and disengage, allowing to springs to provide the compression force once again.

Does that make things a little clearer? If not, please ask me to explain some specific details more clearly.

As to the interface, I have been told that my program will be more or less achievable by utilising simple G-Code; so it's really rather straightforward, I just need to be able to relay a looping program to a bunch of components.
I wrote some pseudo-code on the LinuxCNC forums, would you like me to post it here?

Edit: I can post a simple schematic if you like?

[Al_The_Man]

s, I just need to be able to relay a looping program to a bunch of components.
I wrote some pseudo-code on the LinuxCNC forums, would you like me to post it here?

It may be worth looking as to whether the system can be designed as a point to point system, i.e. repetitive position by limit switch etc?
If so the use of a PC with Linux and G code, may be a bit of overkill?

Edit: I can post a simple schematic if you like?

That would help!
Al.

[Zookes]

Hmm. I hope you weren't expecting too much when I said schematic. : |

Edit: Obviously the actual design is more complicated, but the basis of the function is these two plates, there will probably be more rails for stability and whatnot, but probably only two actuators on either side.

[john-100]

Hi Zookes ,

the cutaway diagram of hydraulic valve on page 3 of the info
not only shows the return spring is in the oil ,but there is also a narrow passage way for the oil to flow to the solenoid

in your application I'd expect the resin to stop the valve working

Al's link to the dispensing valves to provide a better solution

John

[Zookes]

Hmm, what about solenoids that are food-safe? I can't imagine ketchup mingling with the innards of a solenoid without dire consequences.

Al's suggestions look good, but a little low volume for my purposes, I also think that I'll attempt to move the mixed solution with gravity alone, to prevent air disturbance. Are there versions of the same action on larger valves, or might the cost be prohibitive?

Edit:
To Al:

If so the use of a PC with Linux and G code, may be a bit of overkill?

I realise this, but PCs are home territory for me so if I can customise programs and monitor controller-PCs from off-site, that'd be grand. Write logs to files that I can read from an off-site PC, set up sensors and alarms later on, all that jazz.

[Al_The_Man]

If I am clear on the motion side of it, and you want to apply some pressure determined by the spring etc, rather than positioning to a pre-programmed value, allow the steppers to bottom out and verify that the position is reached via L.S. etc.
If this method is used, there is no need for a programmed position that may inherit error over time, as position is not going to be confirmed.
This is the advantage to steppers, they can be stopped in motion without any excessive current happening as with servo's.
If I read you correctly on the dispensing side of it, if using a medium that hardens after evacuation then you want a form of dispenser that clears the medium each time, which is probabally not going to happen with a normal fluid or air valve?
Al.