Real-Time Feedback and Response with Closed-Loop Motor Control
Closed-loop motor control provides direct feedback about how a motor is behaving in real-world conditions, not just how it should behave according to the system, and uses this feedback to improve performance, safety, and efficiency.
Motion control without feedback from the motor assumes that motors always behave as expected. In reality, various aspects of a mechanical system can influence a motor. Even the user itself can affect motor behavior, for example by accidentally hitting a robotic arm or picking up a 3D printer while it’s printing. For stepper motors under common conditions, the position of the motor is deterministic and predictable. To cover uncommon conditions, motor feedback systems can be implemented to create powerful servo drive systems for stepper motors or to add positioning functionality to BLDC drives.
What Makes a Closed-Loop System in Motor Control?
Contrary to open-loop systems, closed-loop motor control is designed to automatically achieve the target output condition and maintain it by feeding back the actual state of the motor, such as velocity or position. Whenever motor feedback is processed, it is a closed-loop system. That’s why it’s also called feedback control.
There are various ways to close the loop in a motor drive system, like using sensors or encoders. Another option is to sense the motor’s electrical behavior since the power consumption is directly related to the axis’ load. Analog Devices, Inc. (ADI) provides a variety of sensors and encoders that can be used to create a safer, more efficient closed-loop motor control system.
Hall Sensors: Analog vs. Digital
An analog Hall sensor measures the strength of a magnetic field and provides an output voltage proportional to it. To protect the Hall sensor from being damaged by large magnetic fields, the amplifier will saturate before reaching the limits of the power supply.
Contrary to analog Hall sensors, a digital Hall sensor only provides two outputs: ON or OFF. The digital output is compared to a preset reference and when this reference is exceeded, the output will turn on. As long as the output of the Hall sensor is below the reference point, the output is off.
Magnetic Encoder ICs
Placing a magnet at the back of a rotor and a magnetic sensor IC close to that magnet creates a feedback system showing the rotor’s absolute position without taking up much space. While not suited for higher velocities or applications demanding a high resolution, magnetic encoder ICs are a cost-effective closed-loop solution with acceptable feedback for many systems. To avoid any alignment errors, it’s important to mount both the sensor IC and magnet accurately. ADI Trinamic™ technology offers a plug-and-play solution in its SensOstep™ technology and PANdrive™ smart motors.
Incremental Encoders
Incremental encoders can be used to keep track of a motor’s position and to determine its velocity. Using an encoder disc and optical sensors, incremental encoders provide motor feedback by generating an output signal for each incremental step. That’s why they’re also known as optical encoders.
Incremental encoders typically have two output signals, A and B. These are mounted at a 90-degree offset to each other so that rotation and direction of the encoder can be detected (A before B, or B before A). A third output signal can be added as a reference point that only appears once per full rotation.
Current Sensing
Current sensing detects current and converts it, generating an output voltage directly proportional to the current in the desired direction. A shunt or Hall effect current sensor can be used to sense the current at either the high-side, inline, or bottom. The detected values can also be stored for later use, including monitoring system behavior. This makes current sensing a cost-effective and reliable method for monitoring equipment status, increasing safety, and detecting unusual behavior.