0086-574-86627307
Jan 03, 2025

How Do Servo Mechanisms or Motors Work?




 

Servo mechanisms or motors are integral to automatic control systems, which generally fall into two categories:

  1. Open-Loop Designs: These are ON-OFF systems, such as heaters and air conditioners, that operate based on a controlled difference (deadband) between the turn-on and turn-off points.

  2. Closed-Loop Designs: These systems operate continuously, like automotive cruise controls, and include sensors that provide real-time information to a control unit. This feedback minimizes the difference between the target command (setpoint) and the actual output.

Simplifying Servo Mechanisms

A servo mechanism (a type of closed-loop system) can be seen as an ON-OFF control system with a greatly reduced deadband. This minimized deadband enables continuous adjustments or corrections, resulting in real-time comparisons of feedback data (e.g., actual temperature) against the desired setpoint (e.g., target temperature). These comparisons optimize the process.

In each servo-loop cycle, the controller compares real-time data to the commanded target, making systematic corrections to refine the process. These corrections are typically enacted through controlled actuators, such as pneumatic or hydraulic systems, electric motors, or other mechanisms. These actuators are often referred to as servos in common usage.

Components and Operation of a Servo Controller

A closed-loop servo controller continuously minimizes the error between the commanded setpoint and the measured output using feedback sensors. For example, in a motion system such as a linear actuator, the velocity command changes dynamically due to accelerations, decelerations, load disturbances, and other factors. To achieve optimal performance, the controller employs proportional-integral-derivative (PID) control within each loop of the circuit.

Practical Example: Automotive Cruise Control

Consider a simplified closed-loop velocity control system, such as the cruise control in a car. The driver sets a target velocity (setpoint), and the system works as follows:

  1. When the Car Slows: A positive error (difference between the setpoint and actual velocity) is detected. The servo controller commands the engine to increase power, raising the car's speed until the error is eliminated.

  2. When the Car Speeds Up: A negative error (actual velocity exceeds the setpoint) prompts the controller to reduce engine power, slowing the car until the error disappears.

This system uses continuous feedback to adjust the car’s speed, maintaining the desired velocity. With additional sensors, the system can adapt to external conditions, such as maintaining a safe distance from a vehicle ahead (adaptive cruise control). Further advancements integrate position monitoring via cameras or GPS, enabling lane assist and potentially autonomous driving.

Characteristics of Servo Systems

Servomechanisms are known for their ability to provide precise control of position, velocity, or torque (or force). Key characteristics include:

  1. Real-Time Feedback: Servo systems use feedback to automatically adjust for disturbances, load changes, and command modifications.

  2. Energy Efficiency: They consume only the energy required to complete the commanded action.

  3. Operational Modes: Servos can operate in current/torque mode (current-loop closure), velocity mode (velocity-loop closure), or position mode (position-loop closure). Each mode incorporates inner-loop closures for more refined control.

  4. Continuous Adjustments: Servo systems make small adjustments around setpoints, such as back-and-forth motions in velocity control or minor position adjustments in position control.

  5. Disturbance Correction: Within their operational limits, servo-mechanisms correct for disturbances and maintain system stability.

Operational Limits and Special Conditions

When a servo mechanism operates at or beyond its physical limits, it may lose its ability to function as a true servo system. For example:

  • Operating in a saturated state restricts its ability to make necessary corrections.

  • Non-normal operations, such as holding position under load, require careful sizing of the servosystem (including the motor and drive).

Understanding and designing within these limits ensures that servo mechanisms perform reliably and achieve the desired outcomes in demanding applications.

 
OstBridge is based on the market in China and Europe to provide our clients with original HEIDENHAIN, PRECIZIKA, SIEMENS, B & R, BOSCH REXROTH, EATON VICKERS, PARKER, Large stock, accept MOQ, fast delivery, If you are looking for the fields of precise motion control, electronic control, automation and hydraulic system, the OstBridge team must be your first choice to bring you the right solution.