Eyes Closed is Open-Loop, Eyes Open is Closed-Loop — Servo

1. Open-Loop and Closed-Loop

As everyone knows, on a regular soccer field, walking from one side to the opposite goalpost: normal people can do it with eyes open; with eyes closed and no assistance, nine out of ten won’t walk straight.

The reason: the stride length of both feet won’t be absolutely consistent. With eyes closed, you cannot know the actual situation or adjust stride length and direction — this is called open-loop in control engineering; with eyes open, you can continuously compare the actual situation with expectations and adjust direction — this is called closed-loop.

This example involves manual control: the brain is the controller, the feet are the actuators, and the eyes are the feedback device.

Eyes closed = open-loop: the brain only issues commands based on expectations, and because it doesn’t understand the actual situation, it cannot correct the commands;

Eyes open = closed-loop: the actual situation is continuously fed back to the brain, which adjusts commands and controls the actuators after comparing with expectations.

To implement automated closed-loop control with instruments, this structure is needed: use sensors as feedback devices to transmit the actual situation to the controller; the controller compares the difference between “actual situation” and “expectation,” continuously issuing corrective commands to achieve the goal.

Feedback can reduce disturbances (external/internal, unexpected/uncertain resistance). Early controllers issued corrective commands based on the difference of “desired value – feedback value” (because the feedback was taken as negative, feedback was once called “negative feedback”); modern (digital, AI) controllers can make complex judgments and calculations based on feedback before issuing commands.

“Servo” comes from the Latin “Servo” (same root as “Servant”), with the core being “following + feedback” — like a servant observing and reflecting the actual situation, therefore only with feedback can it be called servo, which is the core of industrial automation.

2. Closed-Loop Control of Electro-Proportional Valves

Under open-loop control, the spool of an ordinary electro-proportional valve is susceptible to disturbances, making it difficult to precisely achieve the desired displacement (opening). When closed-loop control is adopted, the displacement sensor transmits the actual displacement of the spool to the controller, which adjusts the current of the proportional solenoid to achieve the target displacement.

If the controller adopts PID (Proportional-Integral-Derivative) correction, the response speed of the valve can be significantly improved, with performance approaching traditional servo valves. Therefore, this type of closed-loop controlled electro-proportional valve is also called an “industrial servo valve.”

3. Closed-Loop Control of Hydraulic Systems

Hydraulic systems have disturbances such as load fluctuations and changes in control element characteristics, making it difficult for open-loop control to ensure that actuators achieve the desired motion.

The core of improving the motion accuracy of hydraulic systems is adopting closed-loop control: add position sensors to feed back the actual position of the load to the controller; the controller compares the difference between “desired position value” and “actual position,” continuously correcting commands.

Factors for Selecting Components of Closed-Loop Systems

When designing closed-loop systems, the core characteristics of the following components need to be considered (different application requirements have differences):

(1) Actuators

Must simultaneously satisfy:

• Strong driving force: sufficient to overcome maximum load force;
• Flexible fine adjustment: avoid execution accuracy problems such as overshoot.

(2) Feedback Devices

Theoretically, electrical feedback, hydraulic feedback, or mechanical feedback can be adopted. The applicability of different types needs to be evaluated from three aspects:

Accuracy: The accuracy of execution results will not be higher than the accuracy of the feedback device (similar to “shooter’s accuracy cannot be higher than vision”), therefore the feedback device accuracy needs to be higher than execution requirements.

Timeliness: Before the actual situation is fed back, the controller/actuator will blindly continue using old commands; error ≈ execution speed × feedback delay time, therefore the higher the execution speed, the more timely feedback is needed:

• Electrical sensors: feedback transmission speed is easily fast;
• Mechanical sensors: clearance delays feedback;
• Hydraulic feedback: oil pressure transmission speed approaches sound speed, but cavity elasticity slows transmission; in load-sensing circuits, selecting the highest load pressure through multiple shuttle valves may cause delays that trigger oscillation.

Load influence: The disturbance of the feedback device on the actuator should be as small as possible (similar to “a blind person exploring the road doesn’t use a thick iron rod to avoid affecting movement”).

Core principle: feed back what you need to control — control position, feed back position; control pressure, feed back pressure; if controlling flow (flow sensors respond slowly), feed back the pressure difference across the throttle orifice.

(3) Controllers

Analog control: Before the popularization of digital components, composed of analog electronic components with limited signal processing capability, only able to perform PID processing on the “actual – desired” difference;

Digital control: Starting four or five decades ago, computers/PLCs gradually replaced analog controllers; some practitioners, for ease of understanding, call the computer’s signal processing “digital PID.”