Question

Which of the following devices is not needed in measurement of velocity of rolled bars in a rolling mill?

• A. PD controller
• B. Tachometer
• C. Ward Leonard controller
• D. ON-OFF controller

Hint:

Contact tachometers are completely unsuited for hot, linearly moving products like steel bars in rolling mills.
Non-contact optical methods are used instead.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
We need to identify which of the given control and measurement devices is not needed or cannot be used for the direct measurement of the linear velocity of rolled metal bars in an industrial rolling mill.

Step 2: Detailed Explanation:

• In hot rolling mills, metal bars are processed at extremely high temperatures and high speeds. To measure the linear speed of these moving bars, non-contact measurement methods (such as laser Doppler velocimeters or optical sensors) are strictly required to avoid damaging the sensors or marking the hot metal.

• Let us analyze the options:

• PD Controller, Ward Leonard Controller, ON-OFF Controller: These are control devices and methods used in regulating the electrical drive systems, motors, and speed control loops of the rollers to keep the mill operating at the desired speed.

• Tachometer: A tachometer is a contact-type or proximity instrument designed to measure the rotational speed (RPM) of a rotating shaft. Because the rolled bars are translating linearly and are extremely hot, a tachometer cannot be placed in direct mechanical contact with the moving bars to measure their velocity.

• Therefore, a tachometer is not needed or suited for direct velocity measurement of the rolled bars themselves.

Step 3: Final Answer:
The device not needed in the direct measurement of velocity of rolled bars in a rolling mill is the Tachometer.