Sensor

Method on interfacing and use the Flexiforce Sensors(A201)

by cl.chin
August 11, 2012

Arduino, BBFUINO, Flex sensor, Flexiforce, PIC

The FlexiForce single element force sensor acts as a force sensing resistor in an electrical circuit. When the force sensor is unloaded, its resistance is very high. When a force is applied to the sensor, this resistance decreases. The resistance can be read by connecting a multimeter to the outer two pins, then applying a force to the sensing area. In this tutorial, you will going to learn on how to display the voltage level on 2x8 LCD and light up the LED based on the voltage level as the different force exerted to the Flexiforce Sensors(A201).

Introduction

The FlexiForce A201 force sensor is an ultra-thin, flexible printed circuit. The force sensors are constructed of two layers of substrate (polyester/polyimide) film. On each layer, a conductive material (silver) is applied, followed by a layer of pressure-sensitive ink. Adhesive is then used to laminate the two layers of substrate together to form the force sensor. The active sensing area is defined by the silver circle on top of the pressure sensitive ink. Silver extends from the sensing area to the connectors at the other end of the sensor, forming the conductive leads. A201 sensors are terminated with male square pins, allowing them to be easily incorporated into a circuit.

The FlexiForce single element force sensor acts as a force sensing resistor in an electrical circuit. When the force sensor is unloaded, its resistance is very high. When a force is applied to the sensor, this resistance decreases. The resistance can be read by connecting a multimeter to the outer two pins, then applying a force to the sensing area. In the image below, the plot shows both the Force vs. resistance and Force vs. conductance (1/R). Note that the conductance curve is linear, and therefore useful in calibration.

Specifications:

Thickness: 0.208mm
Length: 203mm Width: 14mm
Sensing Area: 9.53mm (diameter)
Response Time < 5 microseconds
Operating Temperatures: -9oC to 60oC
Force Range: 0-25lbs
Sensor resistance at no load > 5M ohm

In this tutorial, you will going to learn on how to display the voltage level on 2×8 LCD and light up the LED based on the voltage level as the different force exerted to the Flexiforce Sensors(A201). You can refer the data sheet(attachment) before starting this tutorial.

Hardware and Accessories

1 x BBFuino 1 x SK28A 1 x Flexiforce Sensors(A201)

1 x LCD (8×2) 1 x Breadboard GL-12 (C&D) 4 x LED Super Bright 5mm Blue

1 x 100k ohm resistor Male to Male Jumper Wires 1 x Atten 830L Digital Multimeter

1 x Straight Female Header 2×40 Ways Straight Pin Header (Male) 2×40 Ways Resistor 0.25W 5% 220R

Getting Started

Part 1: BBFuino with Flexiforce Sensors(A201)

1. A cutter used to cut the 2×40 header pin (Male) into 2×8 ways. The pins was soldered to the 2×8 LCD as shown below.

Before (Front view) After (Back view)

Figure 1: Soldering the header pins onto the 2×8 LCD

2. A 2×40 header pins (Female) was cutting into 2×8 ways and being soldered onto the LCD pad (DS3) of BBFuino board as shown below.

Front view Back view

Figure 2: Soldering the header pin onto the BBFuino Board

3. Soldered the JP8 on the BBFuino as shown below in order the LCD backlight can be function properly.

Before After

Figure 3: Soldering the JP8 on the BBFuino board

4. The 2×8 LCD with 2×8 ways of header pin was plug in into the BBFuino board and fit them to a breadboard as shown below.

Figure 4: 2×8 LCD plug to the BBFuino board and fit them to a breadboard

5. Measured the resistance for the flexiforce sensors as it is unloaded and exert a large force to the sensor for the purpose of choosing a suitable resistor for voltage divider.

a) The resistance of sensors is very b) The resistance of sensors ~ 85kΩ

large(> 5MΩ ) as no loaded

Note: The resistance (~85kΩ) showed as the above was the max forced that I can exerted on the sensors. Different people may obtain different lowest resistance. From the resistance obtained, the resistor of 100kΩ was chosen for the voltage divider purpose.

Figure 5: The resistance of Flexiforce Sensors(A201) as different forced exerted on it.

6. Connect the circuit as the schematic provided as below.

a) The circuit schematic

b) Circuit connection

Note: The Op Amp such as LM324 can be used in here in order to amplify the output voltage. If the max output was low, LM324 will amplify the voltage level of the the output voltage to 5V by adjusting the Op Amp gain.The gain of the Op Amp can be calculated by A = 1 + (Rf/Rs).

7. Plug in the power supply. Exert force to the Flexiforce sensor and observed the voltage change with the force exerted by using digital multimeter.

a) Voltage as Flexiforce sensor unloaded ~ 0.1V b) Voltage as Flexiforce sensor unloaded ~ 2.47V

Figure 6: Measure the resistance of Flexiforce Sensors(A201) as different forced exerted on it by multimeter

8. The following show the some part of the sample source code..

Arduino Ide version 0021 for BBFuino can be download from here

main function: It will obtain the analog value and display it as decimal on the LCD. The Led will light up according to the voltage level reached.

Note: The sample source file can be downloaded from the attachment below.

9. Load the source code the the BBFuino. Exert force to the Flexiforce sensor and observed the result.

a) Led will not light up for range of b) Only Led1(left) will light up for c) Only Led1 and LED2(from left)

0 ~ <1V range of 1 ~ <2V will light up for range of 2-3V

Figure 7: The LEDs were turn on based on the voltage level

From the theoretical calculation,

~~ unloaded voltage ~= 0V

~~ 85kΩ of resistance as max force(shown at the top) acting on Flexiforce sensor = [100 / (100+85)] * 5 ~= 2.7V

**Note: The LED1, LED2, LED3 will be light up if the voltage is in range of 3-4V

The LED1, LED2, LED3 and LED4 will light up if the voltage is in range of 4-4.8V

The LED1, LED2, LED3 and LED4 will blinking if the voltage exceed 4.8V

Part 2: SK28A with Flexiforce Sensors(A201)

1. Repeat the step 1 to step 4 in section Part 1.

2 Next, connect the circuit as the schematic provided.

a) The circuit schematic

b) Circuit connection

3) The following show the some part of the sample source code..

The micro-controller used for this tutorial is PIC16F876A.

The MPlab X version 1.20 and XC8 compiler version 1.0 can be downloaded from here

Please refer to here for MPlab X tutorial

main function: It will obtain the analog value and display it as decimal on the LCD. At the same time, Led will be turn on according to the voltage level.