Underwater solenoid valve from Nuoling Pneumatic

The valves were a lot bigger and heavier than I expected.  You can see it’s basically a big chunk of brass connected to the waterproof electrical components.  Fortunately it’s easy to take apart:

Inner workings of this solenoid valve.

The valve assembly consists of three parts: the brass base; a rubber gasket connected to the spring-mounted iron solenoid core; and a metal sheath that encloses the core, bonded to the upper brass component.  (The solenoid is mounted on the metal sheath.)

Inside the solenoid valve's brass base.

The above photo shows the structure inside the brass base.  Notice the arrow indicating the expected direction of flow through the valve.  The rubber gasket seals the inner aperture, and the pressure of the incoming fluid is dispersed around the perimeter of the large outer chamber so that it doesn’t push the gasket up.  When the solenoid is activated it pulls the iron core up, which pulls the gasket away from the inner aperture.  Fluid flows under the lifted gasket into the aperture until the solenoid is de-energized, at which time the iron core is released and the spring return pushes the gasket against the aperture’s lip to seal it back up.

Solenoid valve specification.

This label shows the valve’s specification, including the desired 12 V power supply.  In fact, I tested with a fully charged 7.2 V Li-ion battery and it still actuated in air.  The operating pressure label is a little disconcerting, as the datasheet the supplier sent me specified a working pressure of up to 500 kPa.  A 0.10 kgf/cm^2 pressure corresponds to less than 10 kPa.

The valve's waterproof solenoid.

The solenoid is made of a copper winding connected to some nice, solid leads.  The winding is coated in an encapsulant, which is a pretty thin layer of some kind of urethane-like material, and wrapped in a fabric mesh to hold everything together tightly.  I haven’t tried it underwater yet, but this part (housed in the black casing in the photo at the top of this post) is exposed to the environment so I assume it’s actually waterproof.

This may turn out to be the most useful part of the valve.  I don’t think I can justify putting all that brass and steel in my underwater robot, it’s just too heavy.  By a happy coincidence, the valve I made previously fits perfectly inside this structure, so hopefully I can just mount this solenoid on my existing valve (or maybe an elongated version of the current valve.  I tried, and my spring return is too strong–the compression spring on the solenoid’s iron core is surprisingly febrile–but this gives me hope that I can cut it down further and reduce the amount of force needed to pull the valve open.

One problem I foresee is that in my configuration, the transvalve pressure will be pushing the valve closed, so the deeper the robot, presumably the more force is needed to open the valve.  It’s not a big deal, it will just limit the robot’s depth, which is better than making the robot unrecoverable.  I’ll be happy if the stupid valve opens at all.

I bought a rail-to-rail push-pull comparator, the MCP6541, and tried it with the receiver circuit, and sure enough it increased the maximum range significantly.  Unfortunately it also increased the minimum range significantly.

Here’s the circuit diagram for the modified receiver circuit:

Infrared receiver with comparator for amplification.

The 1 kΩ and 10 kΩ resistors are in a voltage divider configuration to generate a 3.0 V reference against which the input is compared.  If the input is higher than 3.0 V, the comparator outputs 3.3 V, and otherwise the comparator outputs 0 V.

I guess there are a bunch of transistors inside the comparator, and putting them in a chain with the pair of external transistors messes things up.  I don’t know why, but it might be a gain-bandwidth thing (too much gain, lowers the maximum frequency the circuit can operate at).  This is what the output looks like at about 6.5 cm:

'U' character sent at 19200 bps over infrared with comparator amplifier.

The jagged bits aren’t there without the comparator.  I used the U character to test because its binary pattern is 01010101.  As you can see, the pattern is there but it’s kind of messed up and it’s dangerously close to 3.0 V.  (Now that I look at this again I see maybe reducing the 3.0 V threshold would improve the minimum range.)

I figured I could put an OPA2134 op-amp voltage follower in between the external transistors and the comparator.  This would de-couple the two sets of transistors, so the weird transistor chaining effect would disappear.  This worked perfectly, once, for no apparent reason at all.  Eventually, after spending a day trying to reproduce my success, I read the op-amp’s datasheet more closely and discovered that it’s not rail-to-rail and should never have worked (it can’t output the full 3.3 V signal, so the comparator’s input was never reaching 3.0 V).

I switched to a JFET op-amp that had rail-to-rail output, and it was able to transmit the ‘U’ character at a good set of ranges, both small and large.  Unfortunately this is the signal it was outputting, at 7.5 cm and at 2 cm:

'U' sent at 19200 bps over infrared with JFET voltage follower at 7.5 cm.

'U' sent at 19200 bps over infrared with JFET voltage follower at 2 cm.

The observant among you, my imaginary audience, will notice two strange things from this screenshot:

1. The signal is peaking at almost 4 V.
2. It’s sending 9 bits at 7.5 cm and 19 bits at 2 cm.

Hoo boy, it didn’t work at all, and it was pure luck that the ‘U’ transmitted correctly at short range (followed, presumably, by a framing error that my system silently ignored).  No other character worked.

I found out that you can use an op-amp as a comparator, so I tried that with my OPA2134:

'U' sent at 19200 bps over infrared with op-amp comparator.

Okay!  You can see what it means that the amplifier is not rail-to-rail, it’s only going up to 2.6 V (and that only briefly), but if I remember correctly it received.  Unfortunately the high bits got narrower for some reason as the range decreased, so it didn’t actually do anything to solve the range problem.

I tried using the JFET amp as a comparator, but it didn’t output anything at all so I gave up and ignored the project for a few weeks.  My current solution is to pretend there isn’t a problem, and if I run into the  minimum range issue then I can just bend the receiver askew so that the signal is damped enough to receive.  In the meantime, the transmitter now has a wider field of view, which ought to make it easier to place several receivers in range of the control unit.

Fortunately the signal output by the Darlington transistor pair on the receiver is a pretty clean digital signal.  At full power it ranges from (a little above) 0 V to (a little below) 3.3 V.  As the transmitter gets farther away the digital signal remains but the low voltage increases beyond the UART receiver’s ability to read a 0.  For example, at a large distance the UART signal might range from 2.5 V (logical 0) to 3.3 V (logical 1).

There are several options I’ve considered:

The handy old Schmitt trigger is the most straightforward, but it’s touchy to design and it will add a bunch of parts (an op-amp and a bundle of resistors).  I tried using a comparator chip, which is like the Schmitt trigger but simpler, but I found that it had a big limitation.  It worked, but the inputs can’t exceed Vcc – 1.5 V (the “common mode input voltage range” on the datasheet).  It can’t read a digital signal ranging between 1.8 (or more) and 3.3 V, so it would provide some amplification but not a lot.  I’m targeting 90% of Vcc as my threshold, so it should read anything below 3.0 V as low.  I also considered using another PNP transistor to amplify the difference between Vcc and the input voltage, but: that much amplification is risky noisewise, an extra transistor reduces the bandwidth too much, and the PNP transistor will produce a logically inverted signal.

Another option is to amplify the signal using crazy op-amp magic, but aaaaaaah no.

So I tried thinking like I’m supposed to for once, and I’m going to try a software solution.  AVR processors include an analog comparator, which I can (hopefully) use to amplify the digital signal coming from the infrared optotransistor.  The downside to doing it this way is that it bypasses the UART module, which means that the application needs to receive data manually (I can still use UART to transmit the signal).  Receiving data will tie up the processor, but as long as there’s enough time to run the receiver code it should be okay, I don’t foresee much data traffic.  Here’s a potential flow chart for the receive procedure:

Software receiver flow chart.

Some notes:

• The wait process is a euphemism for “disable the timer, store the data, blah blah blah, then go back to the program until the next start bit.”  This process takes place during the stop bit, so if there’s another byte incoming then it should finish in time to receive the next start bit.
• Kevin Rosenberg’s awesome tool AVRCalc calculates that the 8-bit timer running at 8 MHz can be used to generate a 57554 Hz interrupt rate using an OCR value of 0x8A.
• I might need to mess around with the timing so that the AC reads the right bit, but probably not.  There should be plenty of interrupt overhead delay, and the sampling rate is a little slow and will drift toward the end of the bit.
• I’m not sure how long the analog comparator takes to produce a result but I’m guessing it’s pretty fast because it doesn’t have to do much.  The AVR datasheet doesn’t have much information on it.  I also don’t know if the comparator has a large common mode input range.

Failing that, I can look for an external comparator with a large common mode input range, or just suck it up and use an op-amp.

The puzzle I’m working on right now is how best to actuate the transducer to generate a 40 kHz pulse.  The SRF04 does it using a chip that’s intended to convert 5 V logic signals into the ±12 V signals used for the RS-232 serial protocol.  Unfortunately most RS-232 converter chips aren’t made to power an acoustic transducer, and they aren’t able to provide enough current to generate a strong signal.

I bought a couple ADM208EANZ RS-232 converters, which seem like they should be able to provide up to 40 mA.  I didn’t read the datasheet carefully enough and failed to notice that it needs a bunch of 0.1 μF polarized (i.e. aluminum electrolytic) capacitors.  I have 0.1 μF ceramic caps, and I have various electrolytic caps, but I don’t have 0.1 μF electrolytic caps.  The chip will generate +9 V with a 10 μF cap, but it won’t generate the -9 V rail.  I have no idea why some circuits require capacitors with polarity, but if the diagram has a plus sign next to a capacitor you really need to pay attention.  Ceramic capacitors, which don’t have polarity, might not work.

(Most RS-232 converter chips generate ±9 V instead of ±12 V, presumably because ±9 V is easier.  The input is 5 V, then they put that through a voltage doubler to generate 10 V and put that through an inverter to generate -10 V.  Then I guess they put both through something that drops 1 V (a regulator or something to stabilize the output) and you end up with ±9 V.)

If the ADM208EANZ can power the transducer, I think it will be the best solution.  I see three downsides to using an RS-232 converter:

1. You’re limited to an 18 V swing (or perhaps even less) instead of the full 20 V.  This isn’t a major problem because the transducer output is roughly a logarithmic function of the voltage input, so the output difference between 18 V and 20 V isn’t very high.  But if you want to upgrade your transducer to something like the Maxbotix MaxSonar-UT transducer, which takes up to 60 V, then you’re stuck at 20 V.
2. The receiver’s amplifier is a little more complex because you have to operate it on a single rail (i.e. 0 V to 5 V instead of -10 V to +10 V).  This downside is overwhelmed by the upside of not having to generate a  ±10 V power supply.
3. The SRF04 documentation notes that they had to turn the RS-232 chip off while receiving to reduce noise.  The noise is probably from the step-up converter and inverter that generate the positive and negative voltages.  This is pretty annoying, and now that I mention it I recall that the ADM208EANZ doesn’t have a disable feature.  I might be able to filter the noise.

The upside is that the RS-232 converter can be powered from the same 5V supply as the rest of the electronics.  It doesn’t need a complicated battery assembly or external step-up converter, or a bunch of regulators to generate reliable voltage rails.  It just needs a battery and one 5 V regulator, which is more power efficient and space efficient.

One final comment: ADM208EANZ looks like Adam 20 Beanz.