Which device attached to a servo motor shaft helps to track its position?

The correct response identifies the encoder as the device that tracks the position of a servo motor shaft. An encoder is a crucial component in servo systems, providing feedback about the actual position of the motor. This feedback allows for precise control of the motor's movements, ensuring accuracy in applications where the exact position is critical.

Encoders work by converting the mechanical position of the motor into electrical signals, which the control system can interpret. This feedback loop is vital for maintaining the desired position, enabling the servo to make adjustments in real-time based on its current location compared to the target position.

In contrast, the other devices listed have different functions. An amplifier primarily increases the strength of the control signal but does not provide position feedback. A speed controller helps manage how fast the motor operates but does not track its position. The drivetrain refers to the mechanism that transmits the motor's power to the output, which does not involve tracking position either. Therefore, the encoder's role in position tracking makes it the correct answer in this context.