Strain Gauge Load Cell Questions

In summary: The LTC6943 can provide a differential to single-ended converter. This will allow you to sum the outputs from the load cells without having to individually connect each one to a microcontroller. This will save time and wiring. Thanks for the suggestion!
  • #1
FinanclEngr
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I am not a (real) engineer, but have undertaken an ambitious project which contains some technical intricacies I have to learn on the fly. My first question (of many to come no doubt) is about strain gauge load cell technology - specifically powering the device.

My project requires a battery powered weighing system, load range 0 to 500lbs, split between two contact points with required accuracy of .2% (1lb). I need to calculate the power draw of prospective load cell types so that I can select the best battery (cost vs. capacity) for my application.

Question 1: is the load cell's output resistance figure the best proxy for total device resistance?
Question 2: if yes to above, do I derive total Amp usage by dividing excitation voltage by output resistance?
Question 3: Is the listed excitation voltage an absolute requirement or can I power the device with a lower voltage battery? (ex if specs. state 15v, can I power with a 5v or 10v, perhaps this is dependent on the MV/V rating?)

Here is one particular cell I am considering: http://www.flintec.com/type_pb_planar_beam_load_cell.html

Hope EE is the right place for this question, if not please advise. Thanks in advance!
 
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  • #2
Welcome to PF.
Yes, this is the right place.
A1. The symmetrical bridge will have similar input and output resistances. Use the input resistance.
A2. Current consumption will be excitation voltage divided by input resistance.
A3. Output is the product of excitation voltage and bridge distortion due to load.
If you lower excitation voltage then you proportionally reduce sensitivity and lower the resolution.
You will then need more gain in the amplifier to reach FSD. Noise may be proportionally greater.
 
  • #3
Thanks Baluncore

With the aforementioned sensitivity in mind, is there any way to increase resistance of the sensor to lower total current draw? It looks like most single point sensors in my weight capacity range are ~350-400 ohms. I need this type of sensor but with >1000ohm resistance...any ideas? If I drive 2+ sensors with the same battery does overall circuit resistance increase? Will excitation voltage subsequently drop/vary between sensors? Can I weigh a load with a series of sensors connected in a single series or parallel circuit?
 
  • #4
How you minimise current will be determined by the electronics you use with the sensor.
You only need to power the sensors when a reading is needed.

An accuracy of 0.2% is 1 part in 500 so it will require at least a 9 bit converter.
The FSD output from the load cell bridge is probably one part in 1000, = 1mV per 1V of excitation. With 3V excitation you will only get 3mV signal. You could possibly use the two sensors in series, probably running on between 3V and 5V, but then you will have only half that signal.

Strain gauges are ideally suited to sigma-delta converters. They convert to between 16 and 24 bits.

I would consider ground referencing and summing the outputs by using an LTC6943.
http://cds.linear.com/docs/en/datasheet/6943f.pdf
Figure 1. Differential to Single-Ended Converter.
 
  • #5


I am familiar with strain gauge load cell technology and can provide some insights on your questions.

To answer your first question, the load cell's output resistance can be a good proxy for total device resistance, but it may not be the only factor to consider. The total device resistance also depends on the wiring and connections, so it is important to take those into account as well.

For your second question, yes, you can calculate the total Amp usage by dividing the excitation voltage by the output resistance. This will give you the current draw of the load cell.

In regards to your third question, the listed excitation voltage is usually a recommended value for optimal performance. However, it is possible to power the load cell with a lower voltage, but this may affect its accuracy and performance. The MV/V rating of the load cell will also play a role in determining the minimum excitation voltage required for accurate readings.

Regarding the specific load cell you are considering, it is important to carefully review its specifications and consult with the manufacturer to ensure that it meets your project requirements. Additionally, it is always a good idea to test the load cell in your specific application before fully committing to it.

Overall, it is important to carefully consider all factors, including power draw, in selecting the best load cell for your project. I hope this helps and wish you success in your project.
 
  • #6


I can provide some information and advice on strain gauge load cell technology and power requirements.

Firstly, the output resistance of a load cell is not necessarily the best proxy for total device resistance. The output resistance only reflects the resistance of the strain gauge itself, and does not take into account the resistance of other components in the circuit such as wires and connectors. It is important to consider the total circuit resistance when calculating power draw.

Secondly, you can calculate the total Amp usage by dividing the excitation voltage by the total circuit resistance. This will give you the current draw of the load cell.

Thirdly, the listed excitation voltage is typically an absolute requirement for the load cell to function properly. The load cell is designed to operate at a specific voltage, and using a lower voltage may result in inaccurate readings or damage to the load cell. It is important to use the specified excitation voltage for the best performance.

Regarding the specific load cell you mentioned, it is important to note that the excitation voltage for this particular model is 5-15 VDC, which means that it can operate within a range of 5 to 15 volts. However, it is always best to use the recommended voltage for optimal performance.

In terms of selecting the best battery for your application, it is important to consider not only the voltage and capacity, but also the discharge rate and overall power consumption of the load cell. You may want to consult with an engineer or specialist in load cell technology to ensure that you choose the most suitable battery for your specific needs.

I hope this information is helpful in your project. Good luck!
 

1. What is a strain gauge load cell?

A strain gauge load cell is a sensor that is used to measure force, weight, or pressure by converting the applied force into an electrical signal. It consists of a metal structure with embedded strain gauges that change resistance when the structure is under stress. This change in resistance is then converted into an electrical signal that can be measured and interpreted.

2. How does a strain gauge load cell work?

A strain gauge load cell works by using the principle of piezoelectricity. When a force is applied to the load cell, it causes the metal structure to deform, which in turn changes the resistance of the strain gauges. This change in resistance is then converted into an electrical signal, which can be measured and interpreted to determine the applied force.

3. What are the advantages of using a strain gauge load cell?

There are several advantages of using a strain gauge load cell. These include high accuracy and precision, wide range of measurement capabilities, and robustness and durability. Additionally, strain gauge load cells are relatively inexpensive and easy to install, making them a popular choice for many industrial and scientific applications.

4. What are some common applications of strain gauge load cells?

Strain gauge load cells are used in a variety of applications, including weighing scales, force measurement in mechanical testing, and industrial process control. They are also commonly used in medical and research settings for measuring physiological forces, such as muscle contractions and blood pressure, and in aerospace and automotive industries for measuring the performance and stress on vehicles and aircraft.

5. How do you calibrate a strain gauge load cell?

The calibration process for a strain gauge load cell involves applying known forces to the load cell and recording the corresponding electrical signals. The data is then used to create a calibration curve, which is used to convert the electrical signals into force measurements. The calibration process should be performed regularly to ensure accurate and precise measurements.

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