In motion-control applications, which statements about a DC servo motor are typically correct regarding feedback, control input, and achievable control mode?

Introduction / Context:
DC servo motors are core actuators in robotics, machine tools, and instrumentation. Their popularity stems from excellent controllability, fast dynamics, and straightforward interfacing with drive electronics and feedback sensors. This question checks common properties across speed and position control modes.

Given Data / Assumptions:

• Servo implies closed-loop control using feedback devices.
• Typical sensors include encoders (position) and tachometers (speed).
• Drives accept voltage/current commands proportional to desired torque/speed.

Concept / Approach:
A DC servo system comprises motor + power stage + controller + feedback. Speed loops often use tachometer or estimator feedback; position loops use encoder or resolver feedback with cascaded speed/position control. Commanding a variable DC level (or PWM equivalent) adjusts the armature voltage and hence speed/torque relationship.

Step-by-Step Solution:
1) Speed feedback: Tachometers generate a voltage proportional to speed; many legacy and some modern systems use them. 2) Command modality: Variable DC or PWM after filtering acts as an effective voltage command for speed control. 3) Position control: With encoders and appropriate control loops, DC servos achieve precise positioning, including contouring and indexing. 4) Hence, all listed statements are valid.

Verification / Alternative check:
Drive manuals show cascaded loops: inner current/torque, middle speed (often with tach or estimator), outer position with encoder feedback, confirming the claims.

Why Other Options Are Wrong:
Option E: Invalid because each statement A–C is standard practice.

Common Pitfalls:
Confusing open-loop DC motors with servo systems; the “servo” designation implies feedback and closed-loop control for precision.

Final Answer:
All of the above