The Load Cell

The Load Cell

The load cell component is used to measure the weight of the passengers. Another name for the load cell is the strain gauge which as its name suggests, is a device used to measure the strain of an object. When a mass is placed on the load end of the strain gauge the flexible metal is deformed and causes a change in the electrical resistance of the gauge or resistor attached to the flexible metal.

Operation

When the load is attached to the strain gauge, figure 4.2.2 clearly shows that the ∆L will change and will affect the resistance from point 1 to point 2 in the device. The longitudinal strain is the ratio of the length and the change of length of the metal.

(Eq. 1)

Where

ɛL is the Longitudinal Strain

∆ L is the Change in Length

L is the original Length



The Gauge factor can be calculated by dividing the change ratio of the resistance by the change ratio of the length.



(Eq. 2)

Where

∆R is the change in resistance

R is the original resistance

ɛL is the longitudinal strain



The modulus of elasticity is a mathematical way of describing an objects tendency to be temporarily deformed when a force is applied to it.



(Eq. 3)

Where

λ is the modulus of elasticity [N/m2]

σ is the stress of the system [N/m2]

ɛ is the strain of the system



From the electrical resistance that is measured from the strain gauge, the amount of applied stress can be determined. The resistance output from the gauge is proportional to the amount of force that is being applied to the device, though not linearly proportional. As long as the force is within the physical limits of the device, the value of the force can be determined by observing the resistance generated by the load cell and through simple calculations.

External Effects

Temperature can have different effects on the accuracy and operation of the load cell. This is due to thermal expansion, which will be detected as a strain by the gauge and will affect the resistance output of the device. However, most gauges have self-temperature compensation (STC). This allows the thermal effects to be cancelled out and is done by selecting a gauge made of a certain alloy that will react differently than that of the load. By alloy-matching, the resistance changes due to the temperature are negated. Gauges that are not STC will require a dummy gauged that is wired in a particular way to a Wheatstone bridge so that the temperature effects on the dummy gauge will cancel with the effects on the active gauge. 

Different Types of Load Cells

There are a few different type of strain gauges that have been developed and are important for certain applications. Semiconductor strain gauges are used for measurements of a small strain as they usually have a larger gauge factor than the traditional foil gauge. These strain gauges are called piezoresistors. Despite the benefits of the larger gauge factor, they are more sensitive to temperature change, are more fragile and are usually more expensive. Another type of strain gauge is called a mercury-in-rubber-strain gauge. This type is valuable in biological measurements such as blood flow and tissue swelling. It uses a small amount of liquid mercury that is enclosed in a rubber tube. The tube is applied around a body part and the swelling of the body part will result in the stretching of the tube, increasing electrical resistance. Capacitive strain gauges can indicate the level of mechanical deformation by use of a variable capacitor. There are also vibrating wire strain gauges which are used mostly in civil engineering applications. It consists of a vibrating tensioned wire and the strain is calculated by measuring the resonant frequency of the wire. Any change in the tension would result in a change of the resonant frequency.

Measuring Force

The strain gauge is not a linear device; its output is more of an exponential function. However, implementing an exact exponential function that matches that of the strain gauge cannot be easily done in c programming. Using a simple linear function to replace the exponential would not be sufficient, the values being obtained would differ too greatly from the actual values and a large error would be seen.


The graph depicts that using a simple linear function that has the same zero and max value would have an error that is constantly changing. To remedy this, multiple linear functions are to be generated at specific points where an error is found. The more linear functions that are created the more accurate the system would become.



In order to have a high resolution, the ATD controller will be set to have a high resolution with auto conversion. To create the multiple linear functions, the load cell will need to be calibrated by using a series of test weights to formulate an equation. By using the trend line function in an excel plot, the data can be entered and a linear function will be generated.

To read analogue values into the Microcontroller, the signal from the strain gauge is fed into the A/D port of the PIC. For this design, Port RD0 will be used. Values read from the A/D controller is stored in the buffer and assigned to a variable for the remainder of the program.

The code is an infinite loop that will calculate the load in kilograms and transmit this value through the wireless transmitter.

Application

Two Load Cells will be placed at each end of the seat to gather information on how the combined weight of the riders is distributed. If there are three patrons on the ride the two load cells will be sufficient since according to ASTM F2291 the standard mass of an amusement ride adult patron is 170 lb, and the standard mass of a child patron is 90 lb. The load cells are not used to gather an exact weight per rider but just to determine if they are not sitting in the seat. If one rider stands up, the load cells will detect this change in force on the seat.

Implementation of The Load Cell

The main application of the load cell in this device is to detect when one or more riders have risen from their seat while the ride is still in motion. In order to do this, the microcontroller has to be configured in order to read the input analog signal.



To properly configure the microcontroller, the ADC control registers need to be initialized. The ADC control consists of 6 registers; AD1PCFG, AD1CON1, AD1CSSL, AD1CON1, AD1CSSL, AD1CON2, AD1CON3. The functions of each register is described below:



AD1PCFG is a mask that is used to determine which pins will be configured to be analog or digital. By setting the channel to 0 it will be configured as an analog input channel, when the channel is set to 1, it is configured as a digital input.



AD1CON1 is a register that determines if to set the conversion to start automatically.



AD1CSSL determines if a scanning function is required.



AD1CON2 uses MUXA and connects AVdd and AVss pins to the ADC reference inputs.



AD1CON3 is a register that is used to configure the conversion clock.



After all these pins have been configured to suit the application, the ADON control bit is set to 1 to turn on the ADC controller. Below is the initialization function required for this project.



AD1PCFG = 0xFFFD;

AD1CON1 = 0x80E4;

AD1CSSL = 0;

AD1CON3 = 0x1F05;

AD1CON2 = 0;

AD1CON1bits.ADON = 1;





In order process an analog input signal, a few simple steps are followed:



1. Select the analog input channel (AD1CHS)

2. Start the sampling and the conversion (AD1CON1bits.SAMP)

3. Wait for the conversion to complete

4. read the value from the buffer (ADC1BUF0)



The ADC controller is then turned off and the ADC control registers are reconfigured for the second load cell. The process outlined above is repeated and the 2 values of the load cells are stored as data.



The warning system designed to determine when a rider is no longer seated follows a simple algorithm where the initial weight of the riders are stored and compared to the current weight of the riders during each cycle of the code. In order to compensate for the rider's body shifting and changing positions on the seat, the code has been developed with a very large safety margin so that the warning will not falsely trigger during normal operation. Once the warning is triggered, the rider must return to his/her seat for the warning to clear or the ride must come to a complete stop. The entire code is available in the Appendix of this report for reference.

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