Load Cell Connection

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Trensicourt

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I plan on using these force sensors.
https://www.farnell.com/datasheets/3626263.pdf

Right now I am considering this variant because it is much cheaper.
https://www.newark.com/te-connectivity-sensors/fx292x-100a-0100-l/load-cell-sensor-20mv-v-100lb/dp/40AH0483

However, I have a concern that a Teensy 4.1 might not be able to pick the small mV analog signal. Preferably I don't want to use amplifiers but will do so if needed. Right now I have HX711 amplifiers. I'm not immediately sure how to use these with the amplifiers so far and may have to use a typical one with my own resistors. My question in the end, is will the Teensy support readings of 20mV intervals for this load cell?
 
You have to use instrumentation amps with loadcells, the signals are very tiny (microvolts) and you need the gain and the CMRR
of an instrumentation amp. I don't understand the problem with the HX711 its commonly used for loadcells and
there's lots of info out there on them I believe.

Note that the datasheet you link describes a sensor that comes both as a bare straingauge and as digital version(s)
with amp and ADC built in, though its rather confusing about this.
 
Hi,

The load cell you say you are considering has options with 0.5-4.5V amplified output or I2C digital output. With a Teensy 4.1, if you choose the I2C output then you won't need an amplifier. With the 0.5-4.5V output, you'll need an op-amp to reduce the voltage.

All the best,

Alan
 
@MarkT, the output type is in millivolts, not microvolts. Would you connect V+ to E+, V- to E-, O+ to A+, and O- to A-?

@AlanK, I would love to get the digital version but it costs about $5 to $20 per unit more depending on which version and distributer you get it from. I plan to get at least 8 of these. Also, I think you mean 'op-amp to "increase" the voltage'.
 
Trensicourt said:
@AlanK, I would love to get the digital version but it costs about $5 to $20 per unit more depending on which version and distributer you get it from. I plan to get at least 8 of these. Also, I think you mean 'op-amp to "increase" the voltage'.
Click to expand...

I'm not AlanK but he did mean reduce. The maximum voltage you can apply to a T4.1 pin is 3.3V -- so you will need to do something to make sure the 4.5V maximum never reaches the Teensy
 
Hi Trensicourt, I grabbed the PDF you linked. The "0.5 to 4.5V output" version and the I2C version are shown in block diagrams as supporting 2.7 to 3.3V power supply and signalling. That does not agree with the "Electrical Specifications (Analog)" table. It's as if they want it to be a natural fit with today's increasingly common 3V3 power environment.

AlanK is reading the Electrical Specifications for the 0.5-4.5V analog part correctly: It appears to be a 5V part. That said, the "Electrical Specifications (Digital)" at the bottom of page 5 shows 2.7-5.5V supply voltage.

It would be great if the device were really 3.3V tolerant which would make it directly compatible with the Teensy 4x.

$25 is expensive for a load cell but comfortably priced for either the amplified or I2C version. The company I work for builds heavy duty data acquisition systems based on strain gauges. My role is firmware on the processor (NXP K60, like a Teensy 3.6), including ADC controls. The analog signal chain is subtle and surprisingly easy to get wrong. So unless you really want to get practice and learn lessons in analog design, the amplified or I2C version of this device would probably save you valuable time and effort.

If I were building something depending on measuring compressive force and accuracy were important, I would go with a packaged solution.... Unless my company was expecting to build and sell the product in large quantities. :)
 
@LenSamuelson

What load cell would you recommend for an I2C version? I plan to use either two sets of a pair for a foot plate to represent the heal and toe. Preferably, the weight would be anywhere between 50kg-100kg. I don't mind slightly lower like 100lbf or 500N. I am not an expert and the load cell series FX29 appeared to be best I can find for my application. I've narrowed it down to button type compact compressive load cells, preferably to threaded or protruded sensors to make for easy attachment (unfortunately not the case for the FX29 series).
 
One of my favorite shops is SparkFun Electronics at https://www.sparkfun.com/ who offer many products including a variety of load and force measurement components. They offer load sensors (single strain gauge), load cells (full wheatstone bridge configuration) and multiple levels of electronic solutions, from the HX711 amplifier (low bandwidth good accuracy and noise immunity) to breakout boards that handle many of the details.

A nicely packaged breakout that connects a load cell (full wheatstone bridge) to a microcontroller using an I2C interface is the "SparkFun Qwiic Scale - NAU7802" ($14.95). Sparkfun sells the load sensors and load cells separately. For hobby and experimentation, they sell an inexpensive load sensor rated to 50 kg. It is possible to arrange two load sensors and a pair of completion resistors, or four load sensors to complete a wheatstone bridge that would provide input to the Qwiic Scale board.

In case you have not already done so, it would be worth your time to do a bit of study of Wheatstone bridge load measurement. Search terms "wheatstone bridge load cell" show plenty of links and images to inform your research. Sparkfun also has several nice tutorials.

I bought a set of 4 of the strain gauge load sensors for work a few years ago, and they worked well to guide further research into our industrial products. AT $3.95 each, they are affordable and could be helpful in understanding how these things work.

Edit: It's easy to forget that load cells work upside down... If the "bottom" of a load cell is flat and your physical design allows some flexibility in how the base is constructed, flip a disc load cell over, with the center post pointing downward.
 
@LenSamuelson

I've tried the 50kg strain load cells, but I get creep consistently with the 4 load cell configuration. I'm not sure how to fix that without constant recalibration.
 
Trensicourt said:
@LenSamuelson

I've tried the 50kg strain load cells, but I get creep or ever increasing values consistently with the 4 load cell configuration. I'm not sure how to fix that without constant recalibration.

https://hackaday.io/project/182680-testing-out-50kg-load-cells I found this blog that suggests you could solder the analog ground and ground but I'm not sure where the analog ground is on the hx711 chip.
Click to expand...

I'm not surprised by their variability. In our project, the original plan for them was to build a desk-sized strain gauge system while our project hardware being built, since the real sytem is very large and very expensive. As part of that process we knew they were cheap but our goal was simply to test ideas about the front-end analog signal chain.

I'm guessing that I would need to calibrate each sensor and to recalibrate regularly. That limits the utility of these sensors for long term use unless recalibration can be done inexpensively or automatically.

It's been years since I've used these little sensors, but I know where they are. I'll measure a few of them, and report back what I learn ("recall", I should know but I am very very preoccupied with the rest of the project these days and forget the details).
 
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