PLC Motion Control & Drive Systems 2 — Questions and Answers
Question 1: What is the primary function of an encoder in a servo motor system?
- To regulate voltage supplied to the motor windings
- To provide real-time position and velocity feedback to the servo controller (Correct answer)
- To limit peak motor current during startup
- To convert AC supply power to DC bus voltage
Correct answer: To provide real-time position and velocity feedback to the servo controller
An encoder provides continuous position and velocity feedback to the servo controller, enabling the closed-loop control required for precise positioning applications.
Question 2: What fundamentally distinguishes a servo drive from a standard VFD?
- Servo drives only operate with AC induction motors
- Servo drives use encoder feedback for precise closed-loop position and torque control (Correct answer)
- Servo drives operate exclusively at voltages below 24V DC
- Servo drives cannot communicate digitally with PLCs
Correct answer: Servo drives use encoder feedback for precise closed-loop position and torque control
Servo drives utilize encoder or resolver feedback for tight closed-loop control of position, velocity, and torque, while VFDs are primarily used for open-loop or simple closed-loop speed regulation.
Question 3: In PLC motion control, what does performing a 'homing' routine accomplish?
- It returns the PLC program to its default factory settings
- It establishes a known reference position so the controller knows the axis's absolute location (Correct answer)
- It sets the maximum travel speed limit for the axis
- It transfers the motion program from PLC memory to the drive
Correct answer: It establishes a known reference position so the controller knows the axis's absolute location
A homing routine moves the axis to a defined reference point (home position) so the motion controller establishes a coordinate origin for all subsequent absolute position moves.
Question 4: A quadrature incremental encoder outputs two channels (A and B) that are 90° out of phase. What information does the phase relationship between A and B provide?
- Absolute shaft position from a fixed reference
- Direction of rotation (clockwise vs. counterclockwise) (Correct answer)
- Motor winding temperature
- Supply voltage level
Correct answer: Direction of rotation (clockwise vs. counterclockwise)
The 90° phase offset between channels A and B allows the controller to determine the direction of rotation — if A leads B the motor turns one way; if B leads A it turns the other way.
Question 5: What is the purpose of a 'jog' command in a PLC motion control application?
- Command the axis to move to a programmed absolute position at full velocity
- Move the axis continuously at a slow speed only while the command signal is held active (Correct answer)
- Execute the automatic homing sequence at startup
- Stop the servo drive immediately with zero deceleration
Correct answer: Move the axis continuously at a slow speed only while the command signal is held active
A jog command moves the axis at a controlled, low speed only while the discrete input or button is held active, allowing operators to manually position equipment during setup or maintenance.
Question 6: What is the function of a 'torque limit' parameter in a servo drive configuration?
- It increases the motor's maximum operating temperature threshold
- It caps the maximum force the servo can apply, protecting mechanical components from overload (Correct answer)
- It sets the maximum allowable shaft speed in RPM
- It defines the number of encoder pulses per revolution
Correct answer: It caps the maximum force the servo can apply, protecting mechanical components from overload
A torque limit prevents the servo drive from commanding more torque than the mechanical system can safely handle, protecting gearboxes, tooling, and workpieces from damage during unexpected obstructions.
Question 7: According to PLCopen motion control standards, which function block commands an axis to move to a specified absolute position?
- MC_MoveVelocity
- MC_Stop
- MC_MoveAbsolute (Correct answer)
- MC_Home
Correct answer: MC_MoveAbsolute
MC_MoveAbsolute is the PLCopen-standard function block that commands an axis to travel to a specified absolute coordinate at a defined velocity and acceleration.
PLC Motion Control & Drive Systems 2
What is the primary function of an encoder in a servo motor system?