Файл: Industrial Control (Students guide, 1999, v1.1 ).pdf

Experiment #3: Digital Output Signal Conditioning

Industrial Control Version 1.1 •Page 79

Experiment #3: Digital Output Signal Conditioning

PAUSE 100

Plot_data:

DEBUG IBIN IN1,BIN IN2,BIN OUT3,BIN OUT4, BIN OUT5,CR

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Experiment #3: Digital Output Signal Conditioning

Figure 3.5: Screen Shot of the Sequential Machining Process using StampPlot Lite

Note that the traces appear from top to bottom in the order which they were listed in the Debug digital plot command. Therefore, the top two traces are of the active high pushbuttons IN1 (product in position) and IN2 (depth switch). The next three traces are outputs OUT3 (conveyor), OUT4 (clamp), and OUT5 (drill). Remember that the outputs are wired in the current sink mode. A High is OFF and a Low is ON.

Notice that in the initial setting for the StampPlot Lite interface, “Save data to file” (!SAVD) is ON. During the production run, the data at each sample point is saved into a text file, stampdat.txt. The data includes the time of day and program time that the sample was taken, the sample number, and the analog and digital values at the time of each sample. The data are comma delimited (separated by commas), and therefore, ready to be brought into a variety of spreadsheet or database software packages. Once the data is in the package, it is available for analysis and manipulation. Figure 3.6 represents a portion of the production run data, as it would appear in a Microsoft Excel spreadsheet. The complete file contains 500 samples (rows of data). Figure 3.7 is an Excel graph constructed from the data file.

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Experiment #3: Digital Output Signal Conditioning

Figure 3.6: Sequential Control Production Run (samples only)

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Experiment #3: Digital Output Signal Conditioning

Figure 3.7: Graph of Sequential Control Production Run

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Experiment #3: Digital Output Signal Conditioning

Programming Challenge: Sequential Mixing Operation

A mixing sequence is pictured in Figure 3.8. In this process, an operator momentarily presses a switch to open a valve and begin filling a vat. A mechanical float rises with the liquid level and closes a switch when the vat is full. At this time, the “fill” solenoid is turned off, and a mixer blends the vat contents for 15 seconds. After the mixing period, a solenoid at the bottom of the vat is opened to empty the tank. The mechanical float lowers, opening its switch when the vat is empty. At this point, the “empty” solenoid is turned off and the valve closes. The process is ready for the operator to start another batch.

Figure 3.8: Mixing Sequential Control Process

Assign the following to the BASIC Stamp inputs and outputs to simulate the operation.

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Construct a flowchart and program the operation.

Exercise #2: Current Boosting the BASIC Stamp

The BASIC Stamp’s output current and/or voltage capability can be increased with the addition of an output transistor. Either the bipolar transistor shown in Figure 3.9a or the power MOSFET transistor in Figure 3.9b can be effective when loads need more power than the BASIC Stamp’s output can deliver. Understanding each of these circuits will be important in future industrial applications.

For this exercise, and upcoming experiments, we have two loads that we wish to drive in this manner. They are a brushless DC fan and a 47-ohm, half-watt resistor. The brushless fan specifications include a full line voltage of +12 V and line current of 100 mA. The resistor will draw approximately 190 mA when powered by the +9 V Vin power supply.

Let’s consider the design of the biplar transistor for driving the 47-ohm resistor. The circuit values should be designed such that a high (+5V) output of the BASIC Stamp drives Q1 into saturation without drawing more current than the BASIC Stamp can source.

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