Eel3111 force sensor paper july 2010  (Page 2/3)

Note the term pressure which means force per unit area. While advertised as a force sensor, in reality, the device actually is pressure sensitive. Testing has shown that the same 100 gram weight will cause a greater output voltage swing if the force is concentrated on a smaller area. Based on the above description it should be clear why. Localized high pressure on a given area of the sensor creates a very low resistance bridge between the enmeshed conductors at that point. Spreading the force over a larger area will decrease the resistance of the sensor as a whole but will not result in the very low resistance circuit created under high pressure.

Operation

The 10 kilo-ohm resistor allows the circuit to act as a voltage divider. It also restricts the buildup of excessive current flow when pressure is exerted on the sensor. The equation for V _out is shown below:

V _out =

R _FSR represents the sensor’s resistance under a given load. In this configuration the output voltage is very small when no load is applied. The voltage increases in a nonlinear fashion with respect to the applied load. This is due to the manner in which the resistance of the FSR changes with the applied load (See figure 4). Note that only as a substantial force is applied does the rate of change of resistance with respect to applied load move towards a constant value. Switching the positions of the two resistors would cause voltage to be at a maximum under no-load conditions.

Ugb

As discussed above, V _out is the voltage pickoff from the FSR network. This voltage is sent next to an op-amp that functions as a UGB. This is accomplished by connecting the output to the inverting input terminal [4]. No feedback resistor is used just a copper wire. This connection drives the gain of the amp towards unity and no amplification of the signal occurs. As discussed in class the purpose of this is to isolate the input to the microprocessor from the FSR circuit (fig. 5).

The op amp we used has the part number TLV2765. It was soldered directly on to the board. The rail voltages are 3.3 volts to the positive and ground (reference voltage) which is applied to the negative. This op amp is a rail-to rail op amp meaning that high fidelity can be achieved even when the input signal (V _out ) is very close to one of the rail voltages [3]. This function is essential because under no-load conditions the input signal to the op-amp is very close to the reference voltage which acts as the negative rail voltage. In fact, during construction of the device we had to hook up multiple op amps before we could find one that was faithful under no load conditions. We could have dedicated a separate -3.3 volt supply to use as the negative rail voltage but selection of this op amp made that unnecessary. Full credit should be given to Mike Toth for solving this design problem.

Msp430f449 and lcd

Up until this point in the circuit the signal representing the force applied was analog. The real magic takes place inside the microprocessor where the signal is converted in to a digital signal which is suitable for display on the LCD. Again we want to give credit to Mike Toth in the lab for doing most of the programming that was required. The MSP430F449 is a microcontroller unit complete with timers, analog to digital converters, and a LCD driver. It also contains memory which can be used to program the device to perform specific functions. The primary use for such a device is to capture analog signals, convert them to digital, and output the digital signal to an LCD or to some other device [2]. They can be purchased or procured as student samples at the Texas Instrument website. Figure six is a flow chart provided by Mike Toth that illustrates the flow of the program we used in this device. Notice the infinite loop (wait forever) and the one second interrupt. The infinite loop tells the program to execute a command with no predefined condition (that is not always true). Technically speaking this is like telling a program to run without ever giving it instructions on when to stop. This means that the only way to halt operation is to power down the device using the sliding switch on the front of the package. In C, the programming language used by engineers, there are different ways to exit an infinite loop. You can use the break command to end the cycle when a specific condition is encountered. Another method is to include in the program what is called an interrupt.