Pneumatic pressure switch using current parts

[ord]

Has anyone built such a thing?

All pressure switch designs I can find online are similar to this one by @mocbuild101 and use the old PF polarity switch, which hasn't been produced for 10 years now:

I have been thinking that maybe a switch built using the current system could use a variable sensor and be fine-tuned, turning the pump on/off at whatever pressures are desired.

Here's a quick proof of concept I made. I don't have a manometer or storage tank to properly test it but if somebody else did that would be great. It uses the pump speed itself to detect when the pressure has risen and turn off, then every 5 seconds does a short pump to check if the pressure has since dropped. Maybe this is too ambitious and something else should be used as the sensor...

If anyone has designed something similar using current parts I would be interested to see it! Alternatively, feel free to improve on my concept or use this as a discussion topic.

Edited September 10, 2023 by ord

[kolbjha]

Perhaps use the PU force sensor (45606) combined with a pneumatic cylinder.

[2GodBDGlory]

Ooh, that's an interesting concept to indirectly detect pressure based on motor speed! I guess the big question is just how accurate it is, as you pointed out

[ord]

20 minutes ago, kolbjha said:

Perhaps use the PU force sensor (45606) combined with a pneumatic cylinder.

That could work...

From the Lego website:

The LEGO^® Technic^™ Force Sensor measures pressures of up to 10 Newtons (~1kg) for accurate, repeatable results.

From what I've seen, a maximum pressure of 2 bars seems adequate for a Lego compressor, so, if using a small cylinder to actuate the sensor, the maximum force it would exert would be:

2 bars * area of cylinder

= 0.2N/mm² * pi*3mm² (assuming piston diameter of 6mm)

= 5.65N

On paper then, this is within the realm of possibility, and could be as simple as connecting a cylinder directly to a force sensor and getting accurate pressure readings! If anyone has the necessary parts to try this out that would be great. Otherwise, I think I'll be ordering a manometer, a small cylinder, and a force sensor :).

[Mr Jos]

Yes I have recently made one for my GBC.

It shuts off the compressor at exactly 2bar, and turns back on around 1.25 bar.

It works very reliable and simple.

Edited September 10, 2023 by Mr Jos

[dhc6twinotter]

I’ve built pressure sensors using the PF switches, but I haven’t tried it with PU.  I think the PU forces sensor connected to a pneumatic cylinder would work well for this. 

[ord]

@Mr Jos nice! I did skim through your GBC videos to look for a compressor but I didn't notice it. 

Good to hear that it's reliable and working in (what seems like) a good pressure range. 

8 minutes ago, dhc6twinotter said:

I think the PU forces sensor connected to a pneumatic cylinder would work well for this. 

I think I will give this a try.

[Mr Jos]

47 minutes ago, ord said:

@Mr Jos nice! I did skim through your GBC videos to look for a compressor but I didn't notice it. 

Good to hear that it's reliable and working in (what seems like) a good pressure range. 

I think I will give this a try.

My compressor is a bit hidden, it's on the backside of the large EV3 motor, valve selector. It can pump in between 2 valves by constantly turning, and operate 1 of the 5 valves by just turning like 30°. The compressor is a modified version from my multivalve MOC on Rebrickable (I use 4x v2 reinforced pumps instead of 1)

Using only 2 motors to get regulated pressure (1.25 - 2bar) and operating 5 individual cylinders.

This is the compressor/valve selector without the pressure switch yet.

[vascolp]

Hmmmmmmm very nice!

Maybe a bit crazy, but that battery box has a built in gyro. Couldn't we use it to measure pressure? The battery weight against a cylinder?

[kolbjha]

1 hour ago, vascolp said:

Hmmmmmmm very nice!

Maybe a bit crazy, but that battery box has a built in gyro. Couldn't we use it to measure pressure? The battery weight against a cylinder?

Excellent idea! I also like the idea of @ord, especially since it doesn't need any extra parts.

[ord]

6 hours ago, Mr Jos said:

Using only 2 motors to get regulated pressure (1.25 - 2bar) and operating 5 individual cylinders.

Thanks for sharing. It's a smart solution you have come up with to operate many cylinders with just 2 motors.

5 hours ago, vascolp said:

Maybe a bit crazy, but that battery box has a built in gyro. Couldn't we use it to measure pressure? The battery weight against a cylinder?

Someone has used the battery weight mechanically with the old switch:

https://rebrickable.com/mocs/MOC-113088/olivierz/pneumatic-pressure-controller/#details

I don't think it's crazy to use the gyro. In fact, I think it could give accurate readings. The downside I think is that it would always have to be on a flat surface (due to gravity), whereas a force sensor could be put anywhere.

[ord]

On 9/10/2023 at 11:30 PM, ord said:

...could be as simple as connecting a cylinder directly to a force sensor and getting accurate pressure readings

Sadly, things are not this simple. I believe the internal friction of Lego cylinders prevents accurate readings, at least in both directions. 

With this setup, while increasing the pressure, I could obtain fairly accurate readings. However, once I then decreased the pressure, it would take a drop of about 0.8 bar before any change was detected by the force sensor. I think this is simply due to the static pressure (EDIT: static friction) that has to be overcome to move the cylinder.

I also tried using a battery box to gauge the pressure. It works, in an on-off fashion, but needs a similar drop in pressure before it falls back down (which isn't necessarily a bad thing for a simple compressor). 

My hopes of building a fine-tunable compressor are dashed. I probably won't revisit the OP concept, as I don't like the idea of constantly having to run a motor to check the pressure. At least, the SPIKE force sensor can be used as an on-off pressure switch (with on and off points ~0.8bar apart), so this is what I'll use going forward.

Edited October 27, 2023 by ord

[AutoBacon]

Would it help to have a spring in the opposite direction of the small cylinder?

[aeh5040]

I wonder if a large cylinder might work better for the hub-as-weight. The small ones tend to stick easily - i.e. the friction is large compared with the force they produce.

FWIW I used a spring + cylinder+ PF switch setup (not visible in the video unfortunately) here for the opposite reason: it was important that the mechanical system did not start until the pneumatic system was up to pressure. 

 

[ord]

Apologies for the late reply. 

On 9/29/2023 at 6:27 AM, AutoBacon said:

Would it help to have a spring in the opposite direction of the small cylinder?

I don't think so. I think a spring just creates a constant opposing force that does nothing to change the friction that must be overcome. The internal friction of the cylinder is what I think is the problem.

On 9/29/2023 at 8:49 AM, aeh5040 said:

The small ones tend to stick easily - i.e. the friction is large compared with the force they produce.

True, I have changed to a large cylinder (levered onto the force sensor) and it seems slightly better.

Still, there is a large drop in pressure before a change is detected. This is the best I could come up with:

 

On 9/29/2023 at 8:49 AM, aeh5040 said:

FWIW...

Interesting, thanks for sharing! 

[Mr Jos]

Oh .. my ... god.. !!! That force sensor looks so great. This could be the one and only reason to get me using something else than the EV3. Really great how you can adjust the desired pressure by just changing a parameter instead of having to change springs etc.

[ord]

Indeed, it's a pretty nice sensor! I had initially thought it was just a touch sensor. 

I wonder what other novel uses it may have... 🤔 

Edited October 27, 2023 by ord