EXAMPLE: SERVO APPLICATIONS

Closed-loop servo systems have changed during the past 25 years due to the advent of the op amp and the microprocessor chip. In the 1960s and 1970s when op amp integrated circuits were introduced into control circuits on a mass scale, they provided a means to use analog sensors in systems that controlled analog output signals. E.g., a load cell could be used to control a proportioning valve to provide an accurate filling and weighing system. A setpoint voltage that represented the desired weight could be set as one input to an op amp and the voltage from the load cell feedback signal could provide the voltage for the other input. The output signal is sent to a proportioning valve that opens from 0-100% to allow the proper amount of material to drop into the weighing hopper.

In the 1980s and 1990s when microprocessor chips became available, they were first integrated with the op amps. This type of control required analog-to-digital converters and digital-to-analog converters. Today a wide variety of sensors is available to provide a true digital input signal, and a wide variety of output devices is available that's controlled by digital signals. This means that a control system today may be all analog, or have analog inputs and outputs with digital microprocessor control, and still others may be a complete digital system where the input and output signals and the control are all digital.

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