16channel battery powered wifi data acquisition system - Page 1
Forum section: electronics / projects

author: Bootalito
author_rank: Unknown
timestamp: September 15, 2018, 04:55:00 am
content: I'm creating a battery powered 16 channel 8000 samples per second per channel WiFi data acquisition system for use in a pharmaceutical environment.The requirements:1. Extremely compact: Fit in a very small 2" x 2" x 2" (5cm x 5cm x 5cm) envelope2. Small signals: Amplify 14 half-bridge load cells @ 1mV/V3. Lots of channels: 14 half bridge load cells and 2 accelerometers for rotational position4. Vibration: Device will be hermetically sealed in a small roller the shape of a wheel (6" diameter, 2" across). This roller will be placed in a bag and ultrasonically cleaned about once a day.5. The roller rotates at a maximum of 30RPM and compacts a drug compound. The pressure exerted by this roller on the product is measured. All 14 load cells are mounted transversely such that they all flex at the same time during a 30degree arc. Expected frequency is 6hz: 30RPM wheel creating 14 pulses at the same time within a 30 degree6. Battery Powered: 2h minimum charge, 8h desirable. We plan on having an external battery pack that will attach magnetically and make contact with exposed copper pads, similar to how walkie-talkie battery packs connect, except larger as they need to be cleanable with hand wipes. My design will use about 1.5W, so a single 18650 battery enclosed in a custom made battery pack should do the trick7. Relatively High Speed: 1000 samples per second per channel minimum, 2000 nice to have8. High accuracy: 1% maximum overall measurement tolerance. Currently measurements are made through a slip ring to a 16bit ADC (unregulated power goes in, gets regulated inside the spinning roller, then like 16 wires come out(through the slip ring), amplified 300X by INA114 then measured by a high speed 16bit ADC. I haven't measured it but I think they are lucky if they get 10 bits of noise free resolution. Hence the desire to NOT go through a slip ring to measure millivolt level signals!9. Configuration and Data interface will be socket connections via a Labview.10. One side of this hermetically sealed stainless steel wheel shaped roll will be some sort of plastic in order to not completely shield the wifiThe current design is to create a stack of three PCBs with the power and micro controller on the bottom, and an 8Ch ADC with 8 instrumentation amps on each of the other two layers. And then mount this PCB sandwich inside the wheel shaped roll on vibrational mounts.So a couple questions:1. With the set-up described above, can the electronics survive being rotated at 30RPM all day then being ultrasonically cleaned once a day? (Note that they will still be inside this hermetically sealed wheel during the cleaning).2. Any suggestions on how to limit RFI noise from the WiFi controller to the sensitive AFE? Do you think I'll need to place a can over each of the AFE PCBs? The controller is already canned.https://easyeda.com/terryjmyers/16ChanDAQ

author: iMo
author_rank: Super Contributor
timestamp: September 15, 2018, 07:03:04 am
content: QuoteThe most sensitive parts would be the crystal(s), large none-smd parts (ie the 1000uF capacitor), trimmers(!) and connectors. Otherwise it should survive.

author: Ian.M
author_rank: Super Contributor
timestamp: September 15, 2018, 07:29:12 am
content: If the PCBs are conformally coated (for vibration damping) and resiliently mounted inside the roller, and it is air or inert gas filled, there should be fairly poor coupling from the ultrasound in the cleaner to the PCBs.I'd be concerned about the 30 RPM rotation of the WiFi antenna possibly affecting the stability of the WiFi link. You won't know for certain if this is going to be an issue till you try it in situ with the actual access point, other nearby devices and metal objects.Personally I'd look at inductive coupling for power and data, with a relatively dumb MCU in the roller, and the smarts for the WiFi interface mounted somewhere where it doesn't have to withstand daily ultrasonic cleaning.If you *MUST* go battery powered, consider using a much dumber lower power RF data link with a loop antenna parallel to the face of the wheel so the polarisation doesn't change as the wheel rotates, + inductive charging. If you cant do that, then your charging contacts need to be either stainless steel or heavily gold plated.

author: Bootalito
author_rank: Unknown
timestamp: September 15, 2018, 05:31:21 pm
content: Quote from: imo on September 15, 2018, 07:03:04 amThanks. I consider getting rid of the large cap or useing copious amounts of silastic as its only purpose is to keep DVDD more stable as it takes big wifi gulps and isn't strictly necessesary. The trimmers are needed to null the tolerance of each strain guage, but again perhaps just a lot of silastic..Quote from: Ian.M on September 15, 2018, 07:29:12 amWe are still considering inductive charging, however I'd rather keep the incoming voltage clean for the AFE. The WiFi link is a consideration that we've already determined will need to be a bit of trial and error due to alla of the other metal in the machinery. We are going to inert the inside with a positive pressure.

author: jbb
author_rank: Super Contributor
timestamp: September 15, 2018, 07:41:54 pm
content: OK, thank you for the clear description of your requirements. There are a couple of things in there that are concerning:- hermetically sealed + rechargeable battery- vibration & ultrasonic cleaning + trimpotsFirst up, pretty much any rechargeable battery needs some kind of vent so you don’t make a bomb. I suggest you look at inductive power transfer (aka wireless power) and skip the internal battery. Imagine how grumpy people will be if this roller goes pop and sprays some small chunks of fibreglass into the production environment, and everything has to be cleaned up. Or worse injures someone.There is a temptation to try to squeeze power and data through the same magnetic system. Unfortunately it turns out to be a pain in the backside (I saw several well-informed PhD students suffer on this altar), so a simple RF link as imo suggests is probably a good plan.Secondly, those trimpots are a bit concerning. They seem likely to drift over time. As the device is hermetically sealed, I suggest you use software trimming which can be remotely configured. In the interests of size, I suggest you consider using 24b ADCs (instead of 16b) which measure the gauges directly. Can you use full bridge load cells to get more output voltage? It would help with noise.