Experiment #2 SENSORS AND DATA ACQUISITOIN

 Objective:

1. Theory Overview

1. Strain Sensors

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Figure 1. Strain Sensor

A strain gauge’s electrical resistance varies in proportion to the amount of strain in the device. A fundamental parameter of the strain gage is its sensitivity to strain, expressed quantitatively as the gauge factor (GF). GF is the ratio of the fractional change in electrical resistance to the fractional change in length, or strain:

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The GF for metallic strain gauges is usually around 2. You can obtain the actual GF of a particular strain gage from the sensor vendor or sensor documentation.

In practice, strain measurements are not larger than a few millistrain (ex10^-3). Therefore, to measure the strain, you have to accurately measure very small changes in resistance. For example, suppose a test specimen undergoes a strain of 500 mε. A strain gage with a GF of 2 exhibits a change in electrical resistance of only 2 (500 x 10^-6) = 0.1%. For a 120 Ω gage, this is a change of only 0.12 Ω.

Figure 2. Quarter-bridge circuit for strain sensing

A bridge circuit consists of 4 resistors connected as 2 parallel voltage dividers. R₁ and R₂ compose one voltage divider circuit, and R₄ and R₃ compose the second voltage divider circuit. The output of a Wheatstone bridge, Vo, is measured between the middle nodes of the two voltage dividers. 

You can use this formula to help you calculate the Vo:

For reading out strain response, strain gauge is placed in R₄ in quarter bridge case.

2. Load Cells

The basic concept behind force is that force is generated by pushing and pulling. A force-sensing resistor is one whose resistances changes whenever pressure or force is applied to it. Force has both magnitude and direction. Because of this, force is classified as a vector quantity. Force is measured in the SI unit of Newtons. Newton’s second law states that force applied to a mass will change its momentum.

Real-world applications of force measurement can be found in the calculations of thrust, drag, and torque. Thrust increases the velocity of an object, drag decreases velocity, and torque changes any rotation of an object.

A load cell is a transducer that converts mechanical force into electrical signals. There are many different types of load cells that operate in different ways, but the most commonly used load cell today is the strain gage (or strain gauge) load cell. As their name implies, strain gauge load cells use an array of strain gauges to measure the deformation of a structural member and convert it into an electrical signal. This configuration is usually a pre-configured bridge. Different shapes of these strain gauge load cells are:

• Shear beam load cells

• Bending beam

• S-type load cells.

• Pancake, low profile compression

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Figure 4: Pancake, compression load cell

2. Pre-Lab Quiz

• Quiz Date: xxxxxx

• XXXXXXXXXXXXXXXXXXXXXXXXX

3. Experimental Procedure

Before programming with the NI ELVIS III, you must first create an NI ELVIS III project. With an NI ELVIS III project, you can group together all the files relevant to your application and run VIs on the NI ELVIS III.

Safety

Be aware of powering the NI board and be aware of not creating any short circuit on the board (connecting ground and power levels (+5V, 3.3V) is not recommended, and may create some damage on the electronic components, be careful about that.)

Hardware/Software Equipment Check

Prior to starting the lab, make sure the equipment is working by conducting the following steps:

Step 1:   Make sure that the NI setup is open from the ON/OFF switch as it found at the back of the device.

Step 2:   Make sure that you press the open button on the device.

3. Data Collection

Part 1 Strain Sensors

Complete the following steps to create an NI ELVIS III project by using the NI ELVIS III Project template.

·       On the desktop, open the folder named MeasurementsAndInstrumentationSoftware

·       From this folder, open the folder named Strain

·       Select QuarterBridge

·       Open the project named Strain Quarter Bridge

·       From the project window, configure the NI ELVIS III IP address to reflect the IP address of the actual NI ELVIS III your computer is connected to.

·       You can find the IP address of your NI ELVIS III by clicking and holding the button on the left-hand side until the IP address is displayed on the LED screen.

·       To configure the NI ELVIS III from your project window,

o   right-click NI ELVIS III (0.0.0.0)[Unconfigured IP Address]

o   click General in the window prompt you get.

·       In the IP address section enter the IP address of the NI ELVIS III connected to your computer.

·       Connect the strain gauge to the quarter bridge input of the Elvis Ni Automated Measurement Board.

·       Connect two lines of the strain gauge red cable to top and white cable to bottom of the connector J1.

·       Connect A/AI0 to socket 1 and connect A/AI4 to socket 2. Note that B/AI1 and B/AI5 are preconnected to the excitation of the quarter bridge.

·       Configure Gauge Factor and Strain Gauge Resistance based on the sensor type.

·        Open the RT Main VI and run it 

·       When you run the project, you will see strain value in the chart. Gently bend the sensor and see the changes in the graph. (Be careful! When the sensor is bent hard, it might get damaged. If you won’t be able to finish the experiment by breaking your sensor lab report will not be graded)

Part 2 Load Cell Measurement with NI Elvis III (35 points)

·       On the desktop, open the folder named MeasurementsAndInstrumentationSoftware

·       From this folder, open the folder named Force

·       Select Load Cell

·       Open the project named Force Load Cell

·       From the project window, configure the NI ELVIS III IP address to reflect the IP address of the actual NI ELVIS III your computer is connected to.

·       You can find the IP address of your NI ELVIS III by clicking and holding the button on the left-hand side until the IP address is displayed on the LED screen.

·       To configure the NI ELVIS III from your project window,

o   right-click NI ELVIS III (0.0.0.0)[Unconfigured IP Address]

o   click General in the window prompt you get.

·       In the IP address section enter the IP address of the NI ELVIS III connected to your computer.

·        Connect the load cell to the full-bridge input of the Elvis NI Automated Measurement Board.

o   Connect V+ to pin 1 of J2, OUT+ to pin 2 of J2, OUT- to pin 1 of J5, and V- to pin 2 of J5.

o   Connect A/AI0 to socket 3 and connect A/AI4 to socket 4.

Part 3

1. Application: Force Indication with LEDs (35)

·       On the left side of NI ELVIS board, there are LEDs numbered 0,1,2,3.

·       From the block diagram, you may insert this LED block from Academic IO / Control IO/ LED. This LED IO takes Boolean inputs. When the input is 1, LED emits light. When the input is 0, LED will not light.

·       By manipulating the block diagram as you wish, perform this operation and receive check marks from your Tas

o   LED #0 lights when applied force is 0-10 N

o   LED #0 and #1 lights when applied force is 10-20 N

o   LED #0, #1 and #2 lights when applied force is 20-30 N

o   All leds light when applied force is greater than 30N.

Responsible TAs:

• Deniz Aktaş, denizaktas20@ku.edu.tr

• Emine Bardakçı, ebardakci20@ku.edu.tr