How to control pneumatics with a MCU? - Page 1
Forum section: electronics / beginners

author: aah136
author_rank: Newbie
timestamp: October 27, 2018, 12:52:12 am
content: I am a part of a liquid fueled rocket design team at my university and we have a nitra pneumatic valve bank we are trying to use in our plumbing system. Each solenoid in the system turns on with 24 VDC +/- 10%. I am trying to work out a circuit where I could send a signal from our Tiva MCU to trigger individual valves. My first thought was to use enhancement MOSFETs in some kind of high/low side configuration. How can I achieve this though? It seems to me that however I set it up I have some Vds drop across the FET when it is in saturation that pulls down the 24V supplied. Should I just try to get the FET operating in the linear region to have a lower Vds? I'm pretty new to electronics design and any advice is appreciated.

author: tpowell1830
author_rank: Frequent Contributor
timestamp: October 27, 2018, 12:59:59 am
content: Hi aah136, welcome to the forum. Some preliminary schematics would be helpful, in order to help direct you. A system block diagram would also be helpful ( you can draw on a napkin and send jpeg if you want), but if you want help, send info that we can work with.

author: Smokey
author_rank: Super Contributor
timestamp: October 27, 2018, 01:22:23 am
content: If you are working as part of a university project, you might want to go talk to pretty much anyone from the EE department. It sounds like they would be able to hook you up, and they would probably like to get a credit on the project if it works out.

author: Rerouter
author_rank: Super Contributor
timestamp: October 27, 2018, 01:35:09 am
content: My main comment would be to make sure your using logic level mosfets, e.g. your tiva board i am guessing is a 3.3V system, so you would want a mosfet that is fully turned on at atleast 2.5VThen are you measuring across the mosfet when switching to say its dropping voltage, not just across the solenoid?as depending on your power supply a beefy solenoid valve could be pulling the supply lower just from its load.

author: Marco
author_rank: Super Contributor
timestamp: October 27, 2018, 01:52:37 am
content: Quote from: aah136 on October 27, 2018, 12:52:12 amYou're always designing to be in the linear region when you are using it as a switch. Why would you intentionally lower the Vgs?

author: vk6zgo
author_rank: Super Contributor
timestamp: October 27, 2018, 03:15:06 am
content: Quote from: Marco on October 27, 2018, 01:52:37 amDon't fart about----use relays!

author: Gregg
author_rank: Super Contributor
timestamp: October 27, 2018, 04:35:11 am
content: QuoteAlso consider opto isolators to keep your MCU away from those nasty inductive spikes solenoid valves can produce.

author: Ian.M
author_rank: Super Contributor
timestamp: October 27, 2018, 07:38:40 am
content: Nobody in their right mind would gratuitously use a mechanical relay in a rocket motor control system if a solid state alternative was feasible. Mechanical relays are just too heavy and too sensitive to high amplitude vibration.Search Google for:automotive high side switchand you'll find a large number of devices designed for switching difficult loads (e.g solonoids, inductive loads, motors, filament bulbs etc.) from logic level control signals, with various load status and fault monitoring capabilities. N.B. you need to check their voltage ratings as although many will be suitable for 24V (nom.) systems, some may only be intended for 12V systems.If you want more specific advice, you'll need to tell us the DC resistance of the solonoid (or nominal operating current as we have the operating voltage of 24V), and how fast you need to turn it on and off.

author: max_torque
author_rank: Super Contributor
timestamp: October 27, 2018, 11:55:36 am
content: Given that rocketry, even on a good day is intrinsically risky, you'd be mad to roll your own drivers!As mentioned, simply use an off-the-shelf automotive high side driver (infineon have a large range) which will include a load of protections as standard and are certified to a high level!You will also i hope, be providing an completely separate, failsafe and foolproof "SAFE and ARM" override system i presume??

author: aah136
author_rank: Newbie
timestamp: October 29, 2018, 01:10:33 am
content: Other members of the team have suggested relays but I have been on the side of using digital because of vibrations being an issue. When we hooked up the valves to a power supply in the lab, the supply was giving it 0.035 A at 24 V, so I guess its effective resistance is about 700 ohms. Using a dedicated IC to do the switching sounds like a good idea. The only thing that concerns me is testing surface mount chip circuits because I don't have much design experience using IC's. As you can't really breadboard it, are there any good surface mount prototyping techniques? I suppose you could solder it to a perf board and mess with it that way?

author: Brumby
author_rank: Unknown
timestamp: October 29, 2018, 04:38:53 am
content: Quote from: aah136 on October 29, 2018, 01:10:33 amLook for "SMD breakout boards". These match SMD devices to breadboard environments - as well as hand soldered lash-ups.

author: max_torque
author_rank: Super Contributor
timestamp: October 30, 2018, 05:10:05 pm
content: You can prototype with SMC components, especially the bigger pitched stuff, you just need to get creative!That's ^^^ an Infineon smart H bright (TLE5206) with some of it's pins bent up, soldered to a bit of copper clad, with another bit of strip board acting as another "layer" to connect to the logic pins........

author: aah136
author_rank: Newbie
timestamp: November 22, 2018, 04:58:28 pm
content: I have found some SMD breakout boards on adafruit to try out for prototyping. Here is the high side switch I found:https://www.digikey.com/product-detail/en/texas-instruments/TPS1H000AQDGNRQ1/296-47601-2-ND/8020866.My concern now is how to ensure we have the right 24V output to the valves. As any mosfet switch is a VCCS so the 24V supplied will have a big drop across the FET switch, should I instead look to some kind of opamp switch circuit maybe that will act as a VCVS? Are there any standard, simple ways to achieve voltage controlled voltage source behavior?