Block diagram and components suggestions for load cell amplifier - Page 1
Forum section: electronics / projects

author: rvalente
author_rank: Frequent Contributor
timestamp: August 19, 2015, 02:02:12 am
content: Dear colleagues,I'm looking to develop two boards for load cell measurements.One will be battery operated, probably single cell Li-ion with internal charger, some wifi connection, gyro for angle positioning and some hard accelerometer (if some bastard drops on the floor, I need to log it).Charger MCP7381?http://ww1.microchip.com/downloads/en/DeviceDoc/21823D.pdfFor the load cell PGA+AD I've narrowed to Analog Devices solutions (expensive, but top quality) or maybe Cirrus.I will read only one load cellAD7190http://www.analog.com/media/en/technical-documentation/data-sheets/AD7190.pdfAD7191http://www.analog.com/media/en/technical-documentation/data-sheets/AD7191.pdfAD7192http://www.analog.com/media/en/technical-documentation/data-sheets/AD7192.pdfAD7193http://www.analog.com/media/en/technical-documentation/data-sheets/AD7193.pdfCS5524http://www.cirrus.com/cn/pubs/proDatasheet/CS5521-22-23-24-28_F8.pdfHX711https://cdn.sparkfun.com/datasheets/Sensors/ForceFlex/hx711_english.pdfFor the supply, I'm totally open to suggestions in components and topology, I think it requires a buck-boost for the micro (3V3) and maybe some voltage doubler + LDO for the AD reference and load cell excitation (thought would be less noisy)For the micro, probably STM32-F0 or F3 (since I've the discovery boards), its low cost micro and I'm just done with those PIC16/PIC24 ( don't get me wrong, the micro is great, the debugger sucks, can't stand).For the gyro? I believe 2000 degrees/second is more than adequate. Suggestions for long time production?For the accel, want to use for drop on floor logging, 50g full-scale?WiFi chipset? ESP8266? Something more commercial? Looked up some ST stuff, but disliked. Microchip/rooving networks looks expensive.Graphic display, small 128*64, 96*48, 60*30mm with backlight, good sources (Arrow, avnet, future preferred hered)For the 24V poweredthis unit will read two to four load cells.Suggestions for the power supply? No low power required here, buck to 6V and 5V(load cell) + 3V3 (micro) LDO? Dual buck?For the micro, STM32 F4 this time, due to the internal MACPHY LAN8710/8720?Well, this design will stress me a lot, any suggestions are very very welcome!

author: Kleinstein
author_rank: Super Contributor
timestamp: August 19, 2015, 08:22:43 am
content: I would definitely avoid the charge pump converters: they are horribly noisy - should sell as spike source too. Normal inductive switchers are usually lower noise.For Wifi or similar I would only consider a ready made module - every thing else is way to difficult. This, the dispaly and maybe the load cell set the requires voltages. So in many rspects the supply comes after it's clear what volatges are needed. There is a real option to use just 3 V from a LDO and limit the lower voltage used form the battery.With the load cell interface it depends on the requirements: the ready made chips for laod cells are a good idea, if they are good enough. Using less than 5 V excitation may save quite some power. More excitation also means more self heating of the DMS and thus more thermal EMF. SO it really depends if more is better. Making the Loads cell supply switchable is definitely a good idea. If low drift is needed even using AC excitation (reversible sign) might be an option.

author: max_torque
author_rank: Super Contributor
timestamp: August 19, 2015, 02:40:32 pm
content: This seems like a awful lot of effort, cost and compenents for a single channel load cell amplifier? Unless you are planning on doing some very high bandwidth measurements or something processor heavy, i really can't think why you'd need that level of complexity?

author: rvalente
author_rank: Frequent Contributor
timestamp: August 19, 2015, 02:58:16 pm
content: KleinsteinThanks by your reply, will totally avoid the charge-pump.These new STM32 seems to work fine from 2.0 to 3.6V, must check the gyro, accelerometer and WiFi solutions, but with the li-ion must stop at 2.8V anyway. But I'll be considering a buck converter from 4.1 to 3.0V, just to get more juice in the high energy phase from the battery.Any suggestion for the buck from 4.1 to 3.0V? Suggestion for the buck from 24V to 3V3/5V?WiFi Chipset suggestion? Display brands?max_torque, this will be an industrial equipment, I need to be battery operated, some connectivity and user proof!Would you mind suggesting some topology?

author: poorchava
author_rank: Super Contributor
timestamp: August 19, 2015, 03:55:21 pm
content: For the AFE I'd consider also LMP90100 (and Lowe versions if it) from TI (designed by NS) and ADSxxxx also from TI (designed by TI).

author: rvalente
author_rank: Frequent Contributor
timestamp: August 19, 2015, 07:09:59 pm
content: Quote from: poorchava on August 19, 2015, 03:55:21 pmLiked this LMP90100 (a little pain to solder, exposed pad) but has a very good catalog price!How would you power this guy from a Li-ion cell? (5v bridge excitation)

author: max_torque
author_rank: Super Contributor
timestamp: August 19, 2015, 07:14:25 pm
content: 1) Specify the requirements, like accuracy, bandwidth, resolution, etc2) Consider how you are going to interface the device to the real world3) Consider cost. In reality, if you are doing it just as a hobby project you can ignore this one, but for anything you want to sell, this part IS the most critical part. Generally, you want an electrical architecture that juuuust meets your requirements set in 1). In particular you need to know your expected volumesIn every project i've ever done on a commercial basis, a significant of the "Nice to have" specification has been nullified by cost implications! Saving £2 on the BOM might not sound like much, as might saving 5min on the assy time, but make anything more than about 100 of them and it starts to make a BIG difference!Also, for industrial usage, generally, getting minimum power consumption isn't as important as keeping costs down (unless you have some very specific energy supply constraints). Especially true for low power devices like a load cell amplifier, generally a basic, cheap (and quiet) linear power supply section, which is admittedly wasteful, is often the correct choice (you need to do the power budget and thermal calcs to validate this decision)For example, you might find the 12b ADC on the STM32 is actually sufficient, especially as if you have a low bandwidth requirement you can do significant oversampling as the ADC is fairly fast on that chip.IMO, spend the money on decent analogue front end components and pcb design, and minimise the digital side of things using the KISS principal.Of course the other critical bit of research is to investigate what is already in the market, how much it costs, and what specifications it has. Use this information to taylor your device, either to offer a better functionality or accuracy, or a similar capability but at a lower price.

author: max_torque
author_rank: Super Contributor
timestamp: August 19, 2015, 07:18:12 pm
content: oh yeah i forgot to add:DON'T DEVELOP TWO DEVICES!!!Develop ONE device that can be configured to do both jobs! At the absolute worst, make the Analogue input on a configerable daughter board, so you can build either a 1 or 4 channel device, using the SAME digital and PSU stages. Ideally, the only difference between the single and 4 channel unit will be that 3 channels won't be populated on the single channel version! (and a firmware flag will be set to "single channel")

author: lgbeno
author_rank: Frequent Contributor
timestamp: August 20, 2015, 07:49:14 am
content: HX711 is ridiculously easy to use and overall a pretty impressive chip for the cost:https://hackaday.io/project/4648-analogio/log/23345-measuring-the-weight-of-beehivesI run it off two AA's no problem. (3.3v) I just changed one bias resistor.I completely agree about power management, keep it simple, if there is a way to keep the converter in Buck only mode that is best. It sounds to me like you could run straight from battery.Is this a commercial project? If yes, I don't know if ESP8266 is quite there yet, if for a hobby, hell yes use ESP8266. You can also think about CC3200 for either case.Sent from my iPhone using Tapatalk