Resistive-sensor Measurement using Current Source

We show you how to excite a force-sensitive resistor using a current source, instead of the traditional Wheatstone bridge-based approach.

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Resistive-sensor Measurement using Current Source

Things used in this project

Hardware components

Raspberry Pi 3 Model B

ProtoCentral Electronics ADS1220 24-bit, 4-channel Low Noise ADC Breakout Board

ProtoCentral Electronics PROTOPI-T V2 GPIO BREAKOUT WITH USB SERIAL ACCESS FOR RASPBERRY PI 3 / 2

Story

The traditional way to read a resistance-based sensor - such as a force-sensitive resistor (FSR) - is to connect the sensor as one of the arms of a Wheatstone bridge. This approach is shown in the following figure. Here Rx is the unknown resistance, which can be calculated using the values of the other three resistors.

Traditional Wheatstone bridge

Why this didn't work for us:

We ran into an issue with the conventional setup when we had to measure very precise resistance which varies in a few milli-ohms. The traditional did not work for us because:

1. The output voltage from the bridge would depend completely on the excitation voltage and the quality of the voltage source used. Even if a high-precision ADC is used, it still depends on the characteristics of the excitation voltage source.

2. The temperature variation in resistance of the three bridge resistors means that the thermal noise in them could skew the sensor readings.

3. High component count (take into account the bandgap reference used for excitation)

What we did:

We wanted to use a constant current source for excitation of the sensor, rather than a voltage source. We used the TI's ADS1220 24-bit ADC (on a board from ProtoCentral) which is a 24-bit high-precision ADC with a low-noise front-end as well as built-in current source!

ADS1220 Block Diagram (Source: Texas Instruments)

We immediately got to work trying to put together a setup to read the voltage using the ADC with a current source powering the resistor. This is a simple relation of Ohm's law (V=IR).

We used a Raspberry Pi to read the data using Python, using SPI to communicate with the ADS1220 breakout board. We used the ProtoCentral ProtoPi-T with USB access to control the Raspberry Pi without connecting to a monitor. Our setup looked something like this (apologies for bad photography):

ADS1220, ProtoPi-T and an FSR on a breadboard

Connecting a USB cable to access the Pi's serial console

In the software running on the Raspberry Pi, we have setup the ADS1220 to communicate with the Pi through SPI. The internal multiplexer on the chip is used to read the differential voltage drop across the sensor and two channels are routed internally for the current source output.

What we saw:

We noticed that we were able to sense even very small changes in resistance. Here we have demonstrated using the FSR, but this was originally done for a medical pressure sensor. While for an FSR, a precision reading may not be required (for most applications), for an application actually requiring high precision, this approach worked out really well.

Setting up current sources and achieving precision current control can get complicated and also expensive, which explains the lack of projects using this technique for measurement. However, the ADS1220 simplifies all this by putting together everything on the same chip.

Check out the Python script below to try it out on your own.

#!/usr/bin/env python
print "test"

import spidev
import time
import sys
import RPi.GPIO as GPIO
from array import *

#ads1220 registers
RREG  =   0x20
WREG  =   0x40
START =   0x08
STOP  =   0x0A
RDATAC=   0x10 
SDATAC	=	0x11	
RDATA	=	0x12

GPIO.setmode(GPIO.BOARD)

# Pin declararion
DRDY_PIN  = 13
START_PIN = 15
CS_PIN    = 18
RESET_PIN = 16

GPIO.setwarnings(False);

#Set pin direction
GPIO.setup(CS_PIN,GPIO.OUT)
GPIO.setup(DRDY_PIN,GPIO.IN)

spi = spidev.SpiDev()				# create spi object
spi.open(0,1)					# open spi port 0, device (CS)1
spi.max_speed_hz = (500000)
spi.mode = (1)

print "test"
count = 0

def Reg_Write(address,data):    # write Fibonacci series up to 
	print ("Writing %s to Register %s" %( hex(data),hex(address)))
	print(data);
    opcode1 = (address<<2) | 0x40

	GPIO.output(CS_PIN, False)
	time.sleep(0.002)               # sleep for 0.1 seconds
	GPIO.output(CS_PIN, True)
	time.sleep(0.002)               # sleep for 0.1 seconds

	GPIO.output(CS_PIN, False)
	time.sleep(0.002)               # sleep for 0.1 seconds
	resp = spi.xfer2([opcode1])        # transfer one byte
	resp = spi.xfer2([data])        # transfer one byte
	time.sleep(0.002)               # sleep for 0.1 seconds
	GPIO.output(CS_PIN, True)
	return;

def Reg_read(address):    # write Fibonacci series up to
	print ("Reading from Register %s" %(hex(address)))
        opcode1 = address | 0x20

        GPIO.output(CS_PIN, False)
        time.sleep(0.002)               # sleep for 0.1 seconds
        GPIO.output(CS_PIN, True)
        time.sleep(0.002)               # sleep for 0.1 seconds

        GPIO.output(CS_PIN, False)
        time.sleep(0.002)               # sleep for 0.1 seconds
        resp = spi.xfer2([opcode1])        # transfer one byte
        resp = spi.xfer2([0xff])        # transfer one byt
        time.sleep(0.002)               # sleep for 0.1 seconds
        GPIO.output(CS_PIN, True)

	#print("Data read is %s" %(hex(resp)))
	print(resp);
	return;

def Spi_command(command):    # write Fibonacci series up to
        GPIO.output(CS_PIN, False)
        time.sleep(0.002)               # sleep for 0.1 seconds
        GPIO.output(CS_PIN, True)
        time.sleep(0.002)               # sleep for 0.1 seconds

        GPIO.output(CS_PIN, False)
        time.sleep(0.002)               # sleep for 0.1 seconds
        resp = spi.xfer2([command])        # transfer one byte
        time.sleep(0.002)               # sleep for 0.1 seconds
        GPIO.output(CS_PIN, True)

        return;


def initialisation1():  
	print ("INITIALISATION");
	#GPIO.output(CS_PIN, False)


	Spi_command(START);				
	time.sleep(0.1);
	Spi_command(STOP);				
	time.sleep(0.1);

	Spi_command(SDATAC);				
	time.sleep(0.3);

	Reg_Write(0x00, 0x01); 		
	time.sleep(0.01); 
	Reg_Write(0x01, 0x04);	
	time.sleep(0.01); 
	Reg_Write(0x02, 0x10);		
	time.sleep(0.01); 
	Reg_Write(0x03, 0x00);	
	time.sleep(0.01); 
	
	Reg_read(0x00);
	time.sleep(0.1);
	Reg_read(0x04);
	time.sleep(0.1);
	Reg_read(0x08);
	time.sleep(0.1);
	Reg_read(0x0c);
	


        
	Spi_command(RDATAC);				
	time.sleep(0.3); 

	Spi_command(START);
	time.sleep(0.1);
	 

	GPIO.output(CS_PIN, True)
	return;


def toHex(dec):
	x = (dec % 16)
	digits = "0123456789ABCDEF"
	rest = dec / 16
	if (rest == 0):
		return digits[x]
	return toHex(rest) + digits[x]

def initialisation2():

	time.sleep(0.2)
	print "STARTED"

	#GPIO.output(CS_PIN, False)
	resp = spi.xfer2([0x11])        # transfer one byte
	time.sleep(0.3)	
	return;

def Read_Data():
	buff = [0,0,0,0,0,0,0,0,0];
	GPIO.output(CS_PIN, False)
	buff = spi.xfer2([0xff,0xff,0xff])
	GPIO.output(CS_PIN, True)
	
	print("0x %x %x %x\t"%(buff[0],buff[1],buff[2]))
	value = buff[0]<<16 | buff[1]<<8 | buff[2]
	print(value)
	volt = (float(value) * .000244140625)
        print  volt,"mV"
	return buff;
	#return r

def Read_adcvoltage():
	result = Read_Data()
	return result
	
	
voltage = 0

def ADCRead():
	#global voltage
	while(GPIO.input(DRDY_PIN) == True):
		time.sleep(0.001)		

	if (GPIO.input(DRDY_PIN) == False):
		#if 1:
		data = Read_adcvoltage();
		#val = ((data[5]&0xff) << 16) | ((data[6]&0xff) << 8) | (data[7]&0xff)
		val = ((data[6]&0xff) << 16) + ((data[7]&0xff) << 8) + (data[8]&0xff)
		#val = (data&0xff0000) + (data&0xff00) + (data&0xff)
		val = int(val)

		if( (val&(1<<(24-1))) != 0 ):
			value = val - 0x1000000			
		else:
			value = val
			
		voltage = (float(value)*100/8388607)
	return voltage	


while True:

	Read_Data()
	time.sleep(0.5)

Install Python Libraries

To install Python packages: 
Open the terminal type:
sudo apt-get update

sudo apt-get install python-dev
git clone https://github.com/Gadgetoid/py-spidev
cd py-spidev
sudo make install 

To install raspberry spi module
mkdir python-spi	
cd python-spi
wget https://raw.github.com/doceme/py-spidev/master/setup.py
wget https://raw.github.com/doceme/py-spidev/master/spidev_module.c
sudo python setup.py install

To install raspberry gpio module
sudo apt-get install python-dev
sudo apt-get install python-rpi.gpio

Install time package
pip install time

Note:  Make Sure is serial interface is SPI is enabled.
Check Menu-> Preferences-> Raspberry Pi configuration -> SPI-> Enable
Connections

Raspberry ------> ads1220 breakout
Pin1 (3.3V) -----> VDD
Pin9 (GND) -----> GND
Pin13 -------> DRDY  
Pin 15 -------> START
Pin16 -------> RESET
Pin 18 ------->CS
Pin19 -------> MOSI
Pin 21 -------> MISO
Pin23 -------> SCK  
Running script
Copy the example code “example.py” to “python-spi “ created.

Run the script using “sudo python example.py”

Code explanation

ADCRead()
Reads the ADC value and convert it to corresponding voltage to be plotted

initialisation1()
Intialises the ads1292 to be in a Continuous conversion mode, 125SPS .

Reg_Write(address,data)
Writes one register to an address

Reg_read(address)
Reads  value from an address.

Spi_command(command)
Sends a SPI command to the ads1292

generator()
Copies the ADC valus to an array, and send it to the plotter to be plotted