FSR Sensors with PLCs

[afm]

Hi all, I am working to integrate multiple FSR sensors with my B&R X20 PLC. My project requires me to know relative load, so the accuracy of a load cell is not needed. I have done simple external bench testing using Arduinos, but want to try integrating it to my PLC. I haven't had much luck finding information online for that integration, does anyone have experience with these sensors? Thanks.

 

[IO_Rack]

Do you have a PLC brand / model in mind?

An Arduino typically uses 0 - 5V analog input. Most PLCs will have an option for this.

An Omron CP1L has a built in 1/256 resolution analog input this may be your least expensive option.

 

[afm]

Do you have a PLC brand / model in mind?

An Arduino typically uses 0 - 5V analog input. Most PLCs will have an option for this.

An Omron CP1L has a built in 1/256 resolution analog input this may be your least expensive option.

Yes, I need to integrate it into a B&R X20 system. I am not sure how to approach the circuitry integration with the PLC. Most FSR circuits use capacitors, inverting/non-inverting op-amps, and/or voltage dividers. I have heard of people using Wheatstone bridge style modules for load cells and force sensors to take care of the circuitry, or making custom PCBs. I can look into IO-Link modules or analog output modules for reading in the values, but any additional feedback is appreciated

 

[IO_Rack]

Yes, I need to integrate it into a B&R X20 system.

Sorry, I guess I read past your first sentence.

I'm not familiar B&R but if you can get a voltage analog input card, the circuit will be the same as your Arduino. You'll need to scale the data differently and calibrate it to your application. You'll also need an external 5V power supply.

The force sensor is essentially a variable resistor (a potentiometer for all practical purposes). Make sure to use a series resistor so you don't short the power supply.

You could use 10V or even 24V but you need to be mindful of the power ratings on the sensor. I don't know what the low end resistance value is on the sensor. Since you already have it working safely on the Arduino, 5V will be easiest.

Below was a quick search I did. R2 is the FSR. R1 is the series resistor. In this example, 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 -.

 

[Jeev]

+1 for voltage divider and analogue input. If your FSR is just a straight resistive with no onboard processing then a standard 0-10V analogue would also work. Check your measurement span and then offset and scale in the PLC as required.

 

[afm]

Thank for the replies. I have my simple voltage divider working well with my Arduino, so I will carry over the same circuitry for the AI card and see how my outputs look. I am using Flexiforce A201 sensors and one recommendation the manufacturer makes is a 1-1.1 V supply for a life cycle of 3 million + actuations; larger 2-5V supplies can be used also, but square waves are recommended in these scenarios for more off times to preserve that life cycle of the sensor. How could I generate this from my plc?

I also have concerns with calibration for the sensors. I am using these sensors in a medical application, I have two footplates that simulate walking. The goal for these sensors is to determine relative load in each footplate (Right foot = 46% load bearing, Left = 54%) and to determine relative load % in the users heel vs toe. So I will be suing 4 sensors per machine. Each sensor need to be calibrated for repeatable outputs for the relative loads. I am unsure how to do that calibration since I cannot create a new software and calibration value for each sensor for each machine built. Any recommendations on how to approach this? thanks!

 

[afm]

Sorry, I guess I read past your first sentence.

I'm not familiar B&R but if you can get a voltage analog input card, the circuit will be the same as your Arduino. You'll need to scale the data differently and calibrate it to your application. You'll also need an external 5V power supply.

The force sensor is essentially a variable resistor (a potentiometer for all practical purposes). Make sure to use a series resistor so you don't short the power supply.

You could use 10V or even 24V but you need to be mindful of the power ratings on the sensor. I don't know what the low end resistance value is on the sensor. Since you already have it working safely on the Arduino, 5V will be easiest.

Below was a quick search I did. R2 is the FSR. R1 is the series resistor. In this example, 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 -.

View attachment 67765

I have been unable to get a sensor output reading in my controls. My circuitry is the same for my Arduino (using 100KOhm series resistor for R2) and I have shifted it over and configured it to my PLC.

I have the +5V on a motor module and the ground and A0 on an IO link module that I have used for other analog input sensors. I do not have a AI+ and AI- pin of this module. I used a multimeter to check that the +5V supply is received across the circuit. I attached the pinouts of the 2 modules in using. Any advice on if I can make this module work for this application?

IO module datasheet: https://download.br-automation.com/...fe30241694774cd01d1897b55a&px-time=1706200613

Motor Module: X20MM4455

 

[IO_Rack]

I don't believe you have the right equipment. You need an analog input module.

For example, your Arduino reads 0 to 5V and converts it to 0 to 1023 decimal value. For reference, this is 10 Bit resolution.

I don't know B&R equipment but likely you'll have options. Possibly 0 to 10V, -10 to +10V, 4 to 20mA, etc... that will convert to some resolution. 10 Bit resolution is not common the PLC world. It will likely be 12 Bit or 16 Bit.

 

[afm]

I don't believe you have the right equipment. You need an analog input module.

A few weeks of lead time and I am back with a +/-10V AI module (12 bit).

Below was a quick search I did. R2 is the FSR. R1 is the series resistor. In this example, 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 -.

I have my circuitry as described above similar to the Arduino example, but I'm unsure of how to determine my series resistor size. This is 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).

 

[IO_Rack]

In my opinion, a series resistor will not be very important. Arduinos are very sensitive devices where the input to the Arduino directly drives the input to the microcontroller. Electrical engineers and Arduino pros generally use series resistors to help protect the input from an over current situation. I haven't seen the specs on the B&R analog input card you have, but I'm willing to bet the input is very tough. Most PLC hardware is.

On another aspect, the series resistor will not effectively change the input voltage you generate to it. The input impedance of the analog input is probably very high. This means, whatever voltage you produce, from your voltage divider, will be the voltage that the analog input card reads. << I hope that makes sense.

I would create the simple voltage divider shown in the picture I posted above, then use a multimeter to measure the voltage from A0 (note, it doesn't need to be connected yet. Just measure from that point to GND), test the force sensor and monitor the output voltage. Play around with the voltage divider resistor value until you get the response you are looking for.

Once you have good measurements between Load and No Load, then you can connect to the analog input card and tackle that portion.

 

[afm]

Thank you for your explanation and breakdown. I have the sensor and module working well now and will play around with resistor to adjust the outputs. I will be getting custom sensors with signal conditioning already done in the future, but for initial testing this should work well. I appreciate the help!