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One of the simplest types of closed-loop servo systems is an analog positioning system. ill. 9-1 (below) shows a diagram for this type of circuit. The input sensor for this system is a linear potentiometer. The resistance for the potentiometer is 1.5 kΩ, and its movable arm is connected to a tensioning arm that's used to apply pressure to a sealing bar for closing plastic bags in a packaging process. The sealing bar provides heat to the bag to cause the plastic to melt and seal. The amount of pressure the sealing bar applies to the plastic bag is set by an operator by adjusting the SP potentiometer. If more pressure is required, the SP is adjusted for more voltage, and if the pressure is too much the potentiometer is set to provide less voltage. The output of the op amp is sent to the coil of a magnetic pull bar that moves the sealing bar. The electromagnet in the pull bar applies more pressure to the sealing bar when the voltage becomes stronger. A spring provides a means to loosen pressure on the sealing bar when the magnetic strength is reduced. The arm of the linear pot is connected to the sealing bar and it's used to determine its position. This system is overly simplified so that you can understand each part.
Above: ill. 1: Example of an analog closed- loop servo system that indicates
the position of a sealing bar used to close plastic bags in a packaging application.
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