Pressure Sensor

I selected a pressure sensor from Freescale Semiconductor wide range. I picked up one with the following specifications: integrated, absolute measure, 0 to 250 kPa. 

It comes in various form-factors, either small outline / surface mount package or unibody package, with or without a nose.

The manufacturer provides an excellent application note Understanding Pressure and Pressure Measurement. 

The sensor provides a voltage linear to the pressure measured. So implementation is rather easy. 

After extensive testing with the Arduino board, I went into packing the sensor into a fischertechnik block so I could use it with my industrial models. 

Hardware

The sensor is powered by a fischertechnik battery. Signal is sent to A0 analog input. 

When used, SD card is connected to TX → 1 pin.

I guess the hardware has a consequence on the measures: 
  • the Arduino processor runs at 16 MHz, 
  • the SD card is linked through a serial connection at 19200 bps,  
  • the USB connection speed to the PC is 57600 bps. 
Not to mention the not-optimised code. 

Software

Here are the measures with the SD-card and USB connection: 

  • Autonomous SD OpenLog
The SD card is connected to TX → 1 pin.
Serial speed to SD card = 19200 bps
Measure rate = up to 115 Hz
Reactivity = 9,8 kPa / two consecutive measures

  • USB Connection to PC
double speed=19600; is replaced by double speed=57800;
USB serial speed to PC = 57600 bps
Measure rate = up to 350 Hz
Reactivity = 3,6 kPa / two consecutive measures

Measures rate and reactivity are good enough for my fischertechnik industrial models!
int sensorPin = A0;
float sensorValue = 0; 
double speed=19600; 


void setup() { 
  Serial.begin(speed); 
  Serial.print("\n\n\n\n***\n"); 
  Serial.print("serial speed\t"); 
  Serial.print(speed, 0); 
  Serial.print("\n"); 
  Serial.print("start us\t"); 
  Serial.print(micros()); 
  Serial.print("\n");    
  Serial.print("us\tV\tkPa\n"); 
} 

void loop() { 
  sensorValue = float(analogRead(sensorPin))*5.0/1023.0; 
  Serial.print(micros()); 
  Serial.print("\t"); 
  Serial.print(sensorValue); 
  Serial.print("\t");          
  Serial.print((sensorValue/5.1+0.04)/0.004, 1); 
  Serial.print("\n"); 
}

Pressure Sensor Mathematical Model

I've built a very basic model based on the ideal gas law (reference).

I considered R and T as constants and took 1 atm close to 100 kPa. Actually, 1 atm = 101,33 kPa instead of 1 atm ≈ 100 kPA.

I measured the volumes of the tank and the piston rods, with and without spring.
PV = nRT

with
  • P = Pressure (in atm)
  • V = Volume (in L)
  • n = moles
  • R = universal gas constant 8.314 J·K−1·mol−1
  • T = Temperature (in K) 

Measures

I conducted two series of measures:
  • 1b— one tank and two piston rods
  • 2b— two tanks and two piston rods

Then I filled the model in with the measures I've recorded.

As always, the model and the measures are provided below. It's always nice to see that Nature obeys physical laws!
Picture

Results

The findings are:
  • 20 s to fill either one or two tanks, from 100 kPa to 160 kPa
  • steps appear around 135 kPa
  • activating a piston rod requires either 10 kPa with 1 tank or 5 kPa with 2 tanks, the same
  • at least 127 kPa of pressure is needed to activate a piston rod, on either one or two tanks
  • one tank can deal with up to three or four piston rods
  • two tanks can deal with four or more and up to 7 piston rods 

Downloads

Please find the RoboPro project and Excel sheets discussed in this article.
smart_pressure-b.rpp.zip
File Size: 25 kb
File Type: zip
Download File
smart_pressure_-b1.xls.zip
File Size: 13 kb
File Type: zip
Download File
smart_pressure_-b2.xls.zip
File Size: 13 kb
File Type: zip
Download File