Tachometer/generators and optical encoders are two types of devices generally used to provide speed reference signals in which speed control system?

• A. Open-loop
• B. Closed-loop
• C. Semi-closed
• D. Manual

Answer: (B)

Feedback sensing is what makes speed control automatic and stable. Tachometer/generators and optical encoders provide a measurement of how fast the machine is actually turning. This measurement is fed back to the controller so it can compare it with the commanded speed and adjust the drive to minimize any difference. A tachometer gives a voltage proportional to speed, while an optical encoder provides a pulse stream whose rate translates to speed. Because the system continuously uses this feedback to correct the output, the setup operates in a closed-loop fashion. Without feedback, as in open-loop, there’s no automatic correction for disturbances like load changes or friction. Manual or semi-closed arrangements don’t fit the standard automatic feedback speed-control scenario, so the best description for these devices is a closed-loop speed control system.