Are load cell signal conditioners limited to sample speeds?

[belae1ka]

Hi,

I need to buy a bridge amplifier for my load cell. I need to be able to take at least 350SPS (samples per second). The only load cell amplifier I can find within my budget states that it has a selectable 10 SPS and 80 SPS output data rate. Heres the link to the amplifier; https://www.sparkfun.com/products/13879 .

If I buy this amplifier, does this mean that I am only limited to a max sample rate of 80SPS?

I was told my one of my resources that load cell signal conditioners are not limited to sample speeds but I wanted to get another opinion on this. As mentioned before, I need to take at least 350 SPS.

**In case it helps, I am using a NI USB-6002 data acquisition device to read in the analog value**

Thank you

 

[Baluncore]

If you use a delta-sigma converter you will be limited by sample rate. Other converters have different data rates to the HX711.
Load cells have a low frequency response while accelerometers are used with faster signals.

I do not know why you need such a high data rate from a DS converter, but if subjected to impulses the output will not necessarily follow the signal but the internal digital filter may ring as an impulse response to the excitation.

How accurately do you need to digitise the load, how many bits?
Why do you need such a high conversion speed?

 

[belae1ka]

If you use a delta-sigma converter you will be limited by sample rate. Other converters have different data rates to the HX711.
Load cells have a low frequency response while accelerometers are used with faster signals.

I do not know why you need such a high data rate from a DS converter, but if subjected to impulses the output will not necessarily follow the signal but the internal digital filter may ring as an impulse response to the excitation.

How accurately do you need to digitise the load, how many bits?
Why do you need such a high conversion speed?

Hi Baluncore,

The reason I would like at least 300 samples per second (I said 350 earlier but I meant 300) is because I would like acquire an accurate plot for my project. The end goal of my project is to achieve a Force vs Displacement chart. The force comes from the load cell while the displacement will come from a potentiometer analog output. Every 0.2 seconds I would like at least 60 samples taken for both force and displacement so that is where the 300SPS comes from.

If I would like each sample for both force and displacement to be taken at the same exact time, I would want both of my outputs to be analog right?

If this is true, this leads me to believe that the amplifier I was looking at may not be what I need for my application. Theoretically I should be fine if I read in the analog output from my load cell sensor right? If I'm not mistaken this should solve my sample rate issue.

The DAQ I am using is a NI USB-6002. Here is the reference manual if needed; http://www.ni.com/pdf/manuals/374259a.pdf

Thank you for your help Baluncore, it is highly appreciated!

 

[Baluncore]

The HX711 amplifier has an analogue amplifier and a 24 bit converter in the same package. Take a look at the data sheet.
https://cdn.sparkfun.com/datasheets/Sensors/ForceFlex/hx711_english.pdf
The advantage of integrating the A to D converter with the bridge amplifier is that it is ratiometric so bridge voltage variation do not effect the accuracy.

But you appear to be planning on using the 16 bit converters in the NI USB-6002. That means you need a stable voltage to drive your load cell and a stable gain amplifier to give a big enough signal to get enough bits from the 16 bit converters.

You need to find the maximum output voltage expected from your load cell and the full range input of the 16 bit converters so you can work out what analogue gain you require.

 

[belae1ka]

The HX711 amplifier has an analogue amplifier and a 24 bit converter in the same package. Take a look at the data sheet.
https://cdn.sparkfun.com/datasheets/Sensors/ForceFlex/hx711_english.pdf
The advantage of integrating the A to D converter with the bridge amplifier is that it is ratiometric so bridge voltage variation do not effect the accuracy.

But you appear to be planning on using the 16 bit converters in the NI USB-6002. That means you need a stable voltage to drive your load cell and a stable gain amplifier to give a big enough signal to get enough bits from the 16 bit converters.

You need to find the maximum output voltage expected from your load cell and the full range input of the 16 bit converters so you can work out what analogue gain you require.

Thank you for the response Baluncore,

The voltage source I am using is a regulated 12V DC power supply. Here is the link if needed;
https://www.amazon.com/gp/product/B00K93AYTK/?tag=pfamazon01-20

Is it safe to assume that "regulated" means stable?

If I'm not mistaken the maximum output voltage depends on;
1) The power supply
2) The maximum force that will be applied to the load cell (I asked the mechanical engineers in my group to find this out for me).

Just to clarify, by full input range of the 16 bit converter you are talking about the analog input voltage range right?
In the manual it states that the input range and working voltage is -10V to 10V.

Thank you for your help, I am so close to finishing!

 

[Baluncore]

Is it safe to assume that "regulated" means stable?

No. It could be temperature and load dependent and vary over about half a volt.

For a load cell you need an accurate voltage supply to the load cell bridge and an accurate stable gain amplifier. The bridge output signal will be very small so you will need a high gain amplifier to get enough bits to be accurate.

If you do not have a temperature stable supply then you must also convert the bridge supply voltage and compute the ratio to correct for changes to the bridge reference.

 

[belae1ka]

I found this;
http://www.mechtechplace.net/mech-t...g-a-low-cost-strain-gage-load-cell-amplifier/

All my problems are solved! (hopefully)

 

[Baluncore]

belae1ka.
It looks like it should work well with your NI USB-6002.
http://www.ti.com/lit/ds/symlink/ina125.pdf
It also comes in a DIP package with pins and is reasonably priced.
https://www.digikey.com/product-detail/en/texas-instruments/INA125P/INA125P-ND/254670

For those who find this later.
If you have a serial data interface, the Sparkfun HX711 board still appears to be the lowest cost and simplest integrated system for load cells. The ratiometric conversion with 24 bit resolution and internal amplifiers removes the requirement for external gain.