FSR Sensors Circuitry

[am1]

Hi all,

I am looking to connect my FSR (flexiforce A502) sensors to my PLC using a +/-10V analog input module (B&R X204222). I just want simple circuitry right now to do initial integration with the sensor in my controls. I am using circuitry similar to the attached Arduino example (R2 is the FSR sensor), but making the following changes:  the 5V pin would go to the +5V on the power supply, A0 would go to the B&R analog input +, GND would go to the GND (-) of the power supply AND the B&R analog input -.

As to not short the power supply, I will be adding a series resistor,  but I'm unsure of how to determine my series resistor size. I attached an example of recommended circuitry from the sensor datasheet where a 100kOhm resistor is used, but since I am not using this op amp circuitry and only a series resistor how do I determine resistance I should use? (I will be using a +5V supply).

Thanks in advance!

[Terry Bites]

https://esp32io.com/tutorials/esp32-force-sensor

[VRomanov]

This brings back some memories; you'll probably need to include that op-amp circuit, not just the resistor. It looks like they're referencing the op-amp which is a microchip I'd assume doesn't cost over a $1. Any reason as to why you wouldn't include it?
On a side note, what are you planning on doing with this sensor? It doesn't seem like a typical plc application.
All the best...

[am1]

This brings back some memories; you'll probably need to include that op-amp circuit, not just the resistor. It looks like they're referencing the op-amp which is a microchip I'd assume doesn't cost over a $1. Any reason as to why you wouldn't include it?
On a side note, what are you planning on doing with this sensor? It doesn't seem like a typical plc application.
All the best...

Thanks VRomanov. I did initial Arduino testing with a series resistor and wanted to do some initial testing with my current components, but I will try it using an op-amp as well. After prototyping, I hope to get custom sensors with circuitry ready for me to integrate into my plc to have to avoid in-house calibration for production devices, but I want to do testing with it in the current system.

The application is for a medical device with footplates similar to an elliptical. I want to indicate when weight shift has occurred from the right to left foot, but only need relative loads, not precise measurements. This is why I am leaning towards FSRs over load cells, strain gauges, etc. due to cost, size, and complexity. It is definitley not a common PLC application, so input is really appreciated!

[edavid]

1. I don't see any reason to use the op amp circuit.  I find it convenient to put the FSR between the ADC pin and ground though.

2. The minimum resistance of the square type of FSR ("RP-S40-ST") is about 10K, so you might start with a 10-20K series resistor.

3. For foot sensing, I prefer the 100mm strip FSRs ("ZD10-100") to the square ones.  They have much lower resistance, minimum about 500 ohms.

4. The Tekscan price seems extremely high.  I think these FSRs are now a generic item.  I have bought them from Amazon and Temu and the quality was fine.