Valves, lasers and regulation (with pictures!)

TL;DR because this is the longest post to date:

I have solenoid valves, but they're not big enough. I designed around that, and learned how to use a laser cutting/engraving machine. Amazon sent me a regulator even though it was out of stock, and their CSRs offer conflicting information.

Valves

After the previous solenoids proved to be unusable for my needs, I found another option. These valves are cheap enough that they will not be the majority of the project cost. The problem with these (there's always something!) is that they do not allow a sufficient volume of air through to allow most sizes of horns to sound. Because of this, I've been working on a way to use them to drive a larger valve to allow higher throughput. I think I've got a solution, but testing is still needed. The current design is a flow-driven, spring-return gate valve. What this means is that the valve is held closed by a spring until it is opened by an air flow applied to the control port. Using air flow means that a) as long as I limit how much air is actually needed to hold the valve open, I don't have to worry about getting a perfect seal, and b) I don't have to worry about releasing the air pressure holding the valve open the way I would with a sealed piston, because it bleeds away by design. All I need to worry about is preventing pressure from reaching the output port and making sure that I don't overtax the air supply. The layout of the design looks like this, with ports in the front for line and control and a rear output port:

 The green part is the gate, and it slides up and down in the shell (darker transparent part) to open and close the valve. The top port is the control, so if air pressure is applied there, it will push the gate down, moving the hole in the gate in line with the main port and allowing air to flow through. The funky teeth below the gate are there to keep the return spring in place, so that it can't get out of place. If the valves are to be permanently fused closed, I don't want to have to worry about needing to get into them again if something goes wrong.

Anyways, that top port gets a 1/4" (6mm) air tube jammed into it, which will come from the solenoid valve. The main port in the front gets a 3/8" (9.5mm) air tube which comes directly from the supply, and the back side goes to the horn. Since the smaller horn sizes don't have room for a 3/8" input (and don't really need one), I decided to make the design work with multiple sizes of air tube, and because I have a student license of Autodesk Inventor which has lots of fun tools to play with, I decided to make the design fully parametric and customizable from a nice menu dialog. I wound up reconstructing three of the parts completely in order to set up the parameters so that they would propagate correctly and update everything, and in the end it took several hours longer than it should have, but I figured out how to make it work, and now I can change a number in a box and the whole design will update as soon as I hit the 'apply' button (which took quite a while to sort out; the angle setting wasn't updating everything right away, for some reason).

The form in all its glory, and the valve in 'open' position.

 What the design doesn't show (aside from the spring) is that there will be a bit of space between the sides of the gate and the shell, which will allow a small amount of air to pass by the gate; and there will also be a vent at the bottom of the spring notch to allow a similarly small amount of air to exit the lower chamber of the valve. These small gaps are critical to the valve's operation. If there were no way for air to escape the lower chamber, the air pressure would build up and prevent the valve from opening all the way, and if there were no gap for the air to escape from the top to the bottom, the valve would never close because the air pressure at the top would come up to working pressure and then stay there when the solenoid valve closed.

Lasers!

The current valve design is made of three layers of acrylic plastic which can be cut into shape using a laser cutting/engraving machine. If everything works perfectly, production should be a 2-step process: cut out the valve layers on the laser, then assemble the layers and fix them together with cement or fasteners. I'm gonna start with cyanoacrylate (superglue) and see what happens. One concern is that the glue will spread inward and affix the gate as well, so I will have to be careful about that. I may try leaching the glue in from the outside edges, if I can keep my fingers from getting glued as well. I'm concerned that if I try to solvent-weld the layers together, I'll wind up clamping the valve gate into position and it won't return (and might not move at all). Another worry is that whatever adhesive I use might be wicked into contact with the gate, which would fix it in place even more permanently. I have ideas about how to prevent this, but I'll deal with that when I need to.

Working with the laser cutter/engraver is actually not much more complicated than running a standard home printer; the main differences are that a) your 'paper' isn't always the same thickness or size or shape, and b) instead of ink or toner, you're using a 50-watt laser that can cut through a wide variety of materials.

Example output, to be sent to the laser cutter.

In order to send a file to the laser, you need to use a format that it can work with. Inventor can't output a compatible format directly, but I can output a .dxf file from Inventor that I can open in Inkscape (a free vector-graphics program) and set up for printing. Once it's ready, I can save an .svg file that contains all of the paths that the laser will cut and all of the areas that will be engraved, and that file gets sent to the laser, which happily burns through pretty much whatever you put underneath that's not metal.

At the moment, I've got everything set up for producing single valves that can be put wherever they need to be, but I am planning on at least setting up for making ganged valves that are cut in sets of 8 to correspond with the sets of 8 solenoid valves coming from the 8-output control PCBs. I should be able to do that fairly easily with some of the Inventor design tools.

Regulations

I ordered a low-pressure regulator from Amazon after poring over the datasheets for a couple of days doing back-of-the-envelope calculations to try to determine what I actually needed. I estimated that my pressure requirements were likely to be on the low extreme of what a standard regulator could handle, based on the fact that the average person can't put out more than about 10 kPa of pressure (about 1.5 psi). I also took a wild guess calculated that I would need somewhere on the order of a kL/min of airflow at working pressure, which meant that using a compressed air tank would significantly limit my run time, but also meant that I would need to be sure that there were no choke points that would limit flow rate at lower source pressure.

When I first looked at the Amazon listing, it said that there was 1 item remaining in stock, and more would be on the way soon. I had seen this before and often wondered if this meant that the item was out of stock, but they thought more would be coming soon enough that they could hide it, so I decided to talk an Amazon service representative and see if they could tell me. I was assured that more would be stocked within the next day, so I could buy it without concern for a backorder. The next night, I decided to go through with it on the off-chance that more stock would come in by the time the order was processed. The next day (a Sunday), the order was still showing as preparing for shipment, so I called to check on the order and see if the regulator would be included in the delivery.

I was surprised to hear that not only was the item no longer in stock, the CSRs do not, in fact, have access to information about incoming stock ETAs or in fact anything aside from that which can be found on the website; as such, the previous rep could not have had the information that I was provided, and the veracity thereof remained in question. I was informed that no further such information would be forthcoming, and he apologized for the confusion. I was certainly confused, but I was more confused when I received a notification the following day informing me that my order had been sent to the shipping company, all items included.

The order arrived, and it did, in fact, contain the regulator (it's bigger than I expected, although I should have expected its size). Some testing proved that it would provide sufficiently low pressure and was relatively controllable in spite of the fact that its range exceeded my maximum operating pressure by a factor of about six.

Closing remarks

Stay tuned for an update in the upcoming days with results of valve testing! I'll also post an update with an overview of some of the other small parts involved in valves, regulation and air supply. Check the parts list, as it will be kept relatively up-to-date as work continues.

The project yet lives!