Sensors and op-amps

Thread Starter

Colin6502

Joined May 17, 2017
11
Hi
I’ve built an RC experimental water craft. It has 4 Force Sensitive Resistor Thin Film Pressure Sensors that I want to connect to a STM32F103C8T6. I have 2 adjustable voltage regulators and a 22.2V battery supply. One of the supplies will be adjusted to 3.2V to supply the STM32F103C8T6 the other will supply the sensors (12v I don’t know?). The mounts for the sensors have screws that can preload the sensors to say a maximum resistance 500K. Rather than simple voltage dividers I assume it would be best to use op-amps? I could wire up a board using a quad chip but I would much prefer to buy one but I’m not sure what to order. On Amazon I see pre-amp boards for microphones etc. But I really don’t know what I’m looking for. Advice please.
 

LowQCab

Joined Nov 6, 2012
4,305
Resistors don't have a Voltage "Rating", ( but they do have a very high Safe-Voltage-Limit ).
The Resistors should be operated on the same Power-Supply as the Micro-Controller,
there is no need to use a higher-Voltage if it is actually just a variable-Resistor.

Please supply a Spec-Sheet for the Strain-Sensors.

On the surface, this appears to be a project that is doomed to failure.
What are You trying to accomplish ?
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MisterBill2

Joined Jan 23, 2018
19,461
First we need a description of what the goal is, and then a more detailed description of what the sensors do, and finally we need a description of a STM32F103C8T6. and what you intend for it to do. Consider hat unless I have used that specific device recently, or look up the data sheet and study that, i have no idea as to what the device is.
AND, to provide any suggestions that are not random guesses, I need to know what you are trying to do. A bunch of folks here can provide very excellent advice, but none are mind readers, and many are poor guessers.
 

Thread Starter

Colin6502

Joined May 17, 2017
11
Ok new version
The water craft is driven by an air propeller, the mount shown below is equipped with a resistive pressure sensor (shown in red). The motor can slide forward and pull against the plunger at the back which presses against the sensor. The sensor can be preloaded using the hex screws ether side of the plunger for a starting resistance around 500k. The sensors are just pressure sensitive resistors from Amazon:
1711560020486.png
In the past (using PIC’s and not needing high resolution) I would simply add the sensor to a voltage divider and input into an A/D port. I would increase the supply voltage to the sensor to meet the minimum amperage requirements to make the A/D work. This would not be required if I replace the voltage divider with an op-amp. But do I really need to?
I only need a resolution of about 120 increments from min./max. pressure.
I will write a lookup table to the controller to convert the readings to p.s.i.
1711560076881.png
 

MisterBill2

Joined Jan 23, 2018
19,461
Certainly an op-amp can provide a full scale and more linear reading for the system than just using the force sensor as a single variable resistor. And if there are four sensors it would make setting the responses to be equal a lot simpler.
How repeatable are the transducers, and is the measurement for data or for control?

So the question is how accurate does the system need to be?? Creating a set of specifications when the requirement is unknown is not a very good plan.
 

LowQCab

Joined Nov 6, 2012
4,305
Getting the mechanical arrangement figured-out is going to be the challenge.

The setup shown will not produce a practically usable measurement of anything,
just a whole bunch of weird-noises and oscillations.

Start-out by stating the purpose of the project.
What is it supposed to do ?
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Thread Starter

Colin6502

Joined May 17, 2017
11
I’m a retired programmer not an analog guy. The model has quite a few data collecting devices including a GPS board that updates every 1/4 second which triggers a write to an SD card (timestamp, velocity plus the other sensory data). The magic of programming is I can sample all the sensors (motor wattage, rpm, thrust and the pressure sensors) on an interrupt timing of my choosing (e.g. every ms) and average the readings.
Anyway Mr. Bill2 I will go with the op-amps (4) which brings me back to my original question.
Do I build it or can I buy something on Amazon like a 4 channel microphone amp?
 

LowQCab

Joined Nov 6, 2012
4,305
It would seem that you've got it all figured-out.

Well, I suppose that everybody has their own way of learning.
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MrChips

Joined Oct 2, 2009
31,088
Start by drawing a block diagram showing all the essential major components. The details are not required at this stage.

Augment the block diagram with a list of functions that the STM32F103 MCU has to perform.
 

MrChips

Joined Oct 2, 2009
31,088
Couldn't have done it without your wise and scholarly advice.
All that's left now is an answer to my original question.
Once you have your master plan laid out, you can then focus on actual details. For that you need a complete detailed circuit diagram.

You can focus on the sensor portion by drawing the sensor voltage supply, sensor, and signal conditioning. For that, we need the technical details of the sensor. For best resolution, you need the ADC input voltage to stay within the range of the ADC reference voltage.

What was your question again?
 

MrChips

Joined Oct 2, 2009
31,088
Resistors can be used to attenuate a signal.
An op amp is used to amplify and shift the signal to a suitable range.

These are generalizations. We still need the technical details.
 

LowQCab

Joined Nov 6, 2012
4,305
And our questions are,
what is the Part-Number of the Sensors that You intend to use, ( or provide a Spec-Sheet ),
and how will You mount them in such a manner that is most likely to produce a useful Signal.
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MrChips

Joined Oct 2, 2009
31,088
Assuming that you are using a pressure sensor, the likes of FSR402, how much amplification required will depend on the maximum force applied on the sensor. What is the resistance reading of the sensor from minimum to maximum force?
You don't need 12V power supply on the sensor. You need +3V or lower to be compatible with the STM32F103 supply voltage. You can use an ADC but is not necessary. You can measure resistance with a simple RC circuit connected to the MCU.

1711635146305.png
 

Thread Starter

Colin6502

Joined May 17, 2017
11
Yes it’s a FSR402 series maximum 10 kg.
0.5 Inch Dia FSR402 Resistive Thin Film Pressure Sensor Force Sensing Resistor 0-10kg for Arduino
1711642821339.png

My maximum thrust is going to be about 4.5 kg.
I couldn’t find any minimum amperage requirements for the A/D (12 bit) on a STM32F103C8T6 on my pic16f1829 it was 0.5 ma.
512 increments of resolution is more than enough.
I will be preloading the sensor down to 10k resistance and whatever the A/D value is that will represent zero thrust. At maximum thrust it looks like the resistance of the sensor is going to be maybe 200Ω.
As far as converting the readings to grams I can simply pull on the motor with my digital fish scale until the A/D reading matches what I wrote to the SD card.
Not sure what you mean about using an RC circuit in a DC application?
My latest thought was a 1 transistor voltage follower and replacing the RM resistor with a pot to dial in the maximum voltage if I use a higher voltage on the voltage divider.
 

MisterBill2

Joined Jan 23, 2018
19,461
After the blocks are defined, then you need to define the signals that enter and leave the blocks., and the magnitude of those signals.
As for buy or build the four amplifier module, given that you are defining the size, shape, and power consumption of the system, my advice is to build it yourself. Consider that there are some excellent design talents that frequent these threads, once you can define the input and output signals, creating an amplifier package will not be that difficult for them. And advice on rugged construction as well.
For commercial modules, consider that when "one size fits all" it fits none of them very well.
 

MrChips

Joined Oct 2, 2009
31,088
Not sure what you mean about using an RC circuit in a DC application?
Put R and C in parallel across a GPIO pin.
Turn the pin to HIGH output.
Turn the pin to input and measure the time it takes to discharge below the LOW input threshold.
You can achieve 16-bit resolution over a very wide range of resistance values.
Then you can use your lookup table for whatever function you desire.
 
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