Open-Loop Control
Open-loop refers to a control technique that does not measure and act upon the output of the system. Most piezoelectric systems and inexpensive micrometer-replacement actuators are open-loop devices.
Open-loop positioners are useful when remote control is desired for improved accessibility or to avoid disturbing critical components by touching them.
Stepper and ministepper motors often use open-loop as well. The count of pulses is a good indicator of position but can be unpredictable unless loads, accelerations, and velocities are well known. Skipped or extra steps are frequent problems if the system is not properly designed.
Open-loop motion control has become very popular. Advances in ministepping technology and incorporation of viscous motor-damping mechanisms have greatly improved the positioning dependability and reduced vibration levels of today’s highest quality stepper devices.
Open-loop is by no means a synonym for crude. Even inexpensive open-loop devices can achieve very fine incremental motions. Nanometer-scale incremental motions are achievable by open-loop piezo-type devices.
Open-loop systems infer the approximate position of a motion device without using an encoder. In the case of a piezo device, the applied voltage is an indicator of position. However, the relationship is imprecise due to hysteresis and non-linearities inherent in commonplace piezo materials.
Closed-Loop Control
Closed-loop refers to a control technique that measures the output of the system compared to the desired input and takes corrective action to achieve the desired result. Electronic feedback mechanisms in closed-loop systems enhance the ability to correctly place and move loads.
Closed-Loop Control Techniques
Depending upon how the feedback signals are processed by the controller, different levels of performance can be achieved. The simplest type of feedback is called proportional control.
Other types are called derivative and integral control. Combining all three techniques into what is called PID control provides the best results.