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. 

Pressure Sensor Tests

I carried out some tests with the pressure sensor on an Arduino board. 

How many measures per second? What is the reactivity?

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. 

Measures rate and reactivity are good enough for my fischertechnik industrial models!
 

    









					
									

		
	







Picture







Pressure Sensor Mathematical Model




	

		
			

				
					
						


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



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) 



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.

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!



					
										
					
						




					
							

		
	





Results and Interpretation




	

		
			

				
					
						


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 

Now, how to build a smart pressure management?



					
										
					
						





					
									

		
	





Downloads




	

		
			

				
					
						


Please find the RoboPro project and the Excel sheets discussed in this article. 



					
										
					
						




 smart_pressure-b.rpp.zip
File Size:  25 kb
File Type:   zip
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 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