An electropneumatic teststand for teaching control to engineering undergraduates

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Virginia Tech


Agricultural operations such as harvesting and sorting typify controlled processes. Engineering students who wish to specialize in automatic control often do not get sufficient experience in the integration of the three areas of a complete control system: sensors, computing, and actuation. A teststand was designed and built for laboratory exercises which illustrate the inter-relationship of sensors, computing, and actuation in a sorting operation.

Black, gray, and white blocks were placed on a conveyor and moved past two sensors which provided color and position information to a control program. Based upon the information provided by the sensors, the control algorithm activated pneumatic cylinders that sorted the blocks. Two algorithms, identified as open-loop and closed-loop, were tested. For the open-loop tests, the conveyor moved at constant velocity, and for the closed-loop tests, conveyor velocity was cycled. The algorithms calculated the delay between the time a block passed the position sensor and the point where the block was struck by a cylinder, and adjusted the actuation time accordingly.

Tests were run at conveyor speeds between 10 and 80 cm/s. Using the open-loop algorithm, 99.7% of the blocks were identified and struck by the correct cylinder, and for the closed-loop algorithm the accuracy was 98.6%. The control program was written in C and executed on an IBM PC. Three laboratory exercises at various levels of difficulty were developed for the teststand.