vb.net barcode reader from image MECHANICAL PRESSURE SENSORS in Software

Maker QR in Software MECHANICAL PRESSURE SENSORS

29.4 MECHANICAL PRESSURE SENSORS
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Conductive Foam
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FIGURE 29-8 Construction detail for a pressure sensor using conductive foam. The leads are soldered or attached to foil (copper works best). Choose a foam that has a good spring to it.
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(when using aluminum foil, the wire is wound around one end). Mylar plastic, like the kind used to make heavy-duty garbage bags, is glued on the outside of the sensor to provide electrical insulation. If the sensor is small and the sense of touch does not need to be too great, you can encase the foam and foil in heat-shrink tubing. There are many sizes and thicknesses of tubing; experiment with a few types until you find one that meets your requirements. The resistance of the conductive foam pressure transducers changes abruptly when they are compressed. The output may not return to its original resistance value (see Fig. 29-9). So in the control software, you should always reset the transducer just prior to grasping an object. For example, the transducer may first register an output of 30K (the exact value depends on the foam, the dimensions of the piece, and the distance between wire terminals). The software reads this value and uses it as the set point for a normal (nongrasping) level to 30K. When an object is grasped, the output drops to 5K. The difference 25K is the amount of pressure. Keep in mind that the resistance value is relative, and you must experiment to find out how much pressure is represented by each 1K of resistance change. The transducer may not go back to 30K when the object is released. It may spring up to 40K or go only as far as 25K. The software uses this new value as the new set point for the next occasion when the gripper grasps an object.
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THE SENSE OF TOUCH
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30 25 20 Resistance 15 (k ohms) 10 5 0 0 1 2 3 4 5 6 Pressure (ounces) 7 8 9 Press Release
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FIGURE 29-9 The response curve for the conductive foam pressure sensor. Note that the resistance varies depending on whether the foam is being pressed or released.
29.4.2 STRAIN GAUGES
Obviously, the home-built pressure sensors described so far leave a lot to be desired in terms of accuracy. If you need greater accuracy, you should consider commercially available strain gauges. These work by registering the amount of strain (the same as pressure) exerted on various points along the surface of the gauge. Strain gauges are somewhat pricey about $10 and over in quantities of 5 or 10. The cost may be offset by the increased accuracy the gauges offer. You want a gauge that s as small as possible, and preferably one mounted on a flexible membrane. See Appendix B, Sources, for a list of companies offering such gauges.
29.4.3 CONVERTING PRESSURE DATA TO COMPUTER DATA
The output of both the homemade conductive foam pressure transducers and the strain gauges is analog a resistance or voltage. Neither can be directly used by a computer, so the output of these devices must be converted into digital form first. Both types of sensors are perfect for use with an analog-to-digital converter. You can use one ADC0808 chip (under $5) with up to eight sensors. You select which sensor output you want to convert into digital form. The converted output of the ADC0808 chip is an eightbit word, which can be fed directly to a microprocessor or computer port. Fig. 29-10a shows the basic wiring diagram for the ADC0808 chip, which can be used with conductive foam transducer; Fig. 29-10b shows how to connect a conductive foam transducer to one of the analog inputs of the ADC0808. Notice the 10K resistor in Fig. 29-10, placed in series between the pressure sensor and ground. This converts the output of the sensor from resistance to voltage. You can change the value of this resistor to alter the sensitivity of the circuit. For more information on ADCs, see 14, Computer Peripherals.
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