What Is A 4-wire Servo? It’s Easy To Understand What The Extra Wire Is For.

Have you ever encountered this situation: When you bought aservo, you were confused when you saw the 4 wires. You didn’t know how to connect it, and you couldn’t figure out the difference between it and an ordinary 3-wireservo? Don't worry, I was totally confused when I first got in touch with it. Today I'm going to talk to you about what this 4-wireservois all about.

What is the difference between 4-wire servos and 3-wire servos?

The ordinary servos we usually see usually have three wires, one of which is the positive pole of the power supply, one is the negative pole of the power supply, and the other is the signal wire. As for the 4-wire servo, simply speaking, it has an extra wire on top of the 3-wire servo. This extra line usually has a special function. It is mainly used to feedback the angle of the servo. You can imagine that the 3-wire servo is like the kind of existence that only obeys commands but does not report the situation. The 4-wire servo is different. It will not only follow the commands and perform actions, but also tell you where it is at this moment.

This feedback line is particularly useful in practical applications. For example, if you are making a robotic arm and have a 3-wire servo, you can only give instructions to make it turn 90 degrees, but you are not sure whether it turns or not. The 4-wire servo can tell you the current angle in real time through this feedback line, so that a closed-loop control can be formed, and the accuracy and reliability are instantly improved.

Where should the 4 wires be connected?

The colors of these four wires are usually distinguished as follows: the red wire is used to connect the positive pole of the power supply, the brown or black wire is responsible for connecting the negative pole of the power supply, the orange or yellow wire is used as the signal wire, and the other white or blue wire is the angle feedback wire. In terms of voltage, most 4-wire servos can support power supply from 5V to 7.4V, but the specific power supply range depends on the servo model. It is best to check the parameter table before performing wiring operations.

There is a little trick when wiring. First connect the power cable and signal cable. After testing that the servo can rotate normally, connect the feedback cable. This makes it easier to troubleshoot if there is a problem with the wiring. Never plug or unplug wires while the power is on, as this can easily burn out the servo or control board. I have learned this lesson myself.

How to control the rotation of the 4-wire servo

The principle of controlling the rotation of the 4-wire servo is the same as that of an ordinary servo, relying on the PWM signal. The signal line is connected to the PWM port of the development board and gives a pulse with a period of 20ms. The high level time is between 0.5ms and 2.5ms, corresponding to 0 degrees to 180 degrees or 180 degrees to 0 degrees. The specific direction depends on the servo model.

You may ask, how to use the feedback line? The feedback line outputs a voltage signal corresponding to the current angle of the servo. For example, the feedback voltage is 0V at 0 degrees, 3.3V at 180 degrees, and the middle is a linear relationship. Use the ADC port of the development board to read this voltage value and convert it to know the current angle.

What practical functions can be done with feedback lines?

With this feedback line, there's a lot that can be done. The simplest is to do angle calibration. Due to mechanical errors in many servos, if you give a command to turn 90 degrees, it may actually only turn 85 degrees. By reading the feedback value, you can know the deviation and then correct it in the program to make the movement more accurate.

A little more advanced, you can use anti-pinch protection. For example, if you make a flip lid for a smart trash can and encounter resistance when closing the lid, and the angle detected by the feedback line changes abnormally, the program will immediately stop rotating or reverse backward to avoid pinching your fingers. This kind of function is strictly needed in consumer products.

What should you pay attention to when choosing a 4-wire servo?

When choosing a servo, first check whether the thing you want to make requires high accuracy. If you are just doing a simple remote control car steering, an ordinary 3-wire servo is enough, and there is no need to spend more money. But if you want to make a robotic arm, robot joint, or smart home automatic device and require precise position control, then a 4-wire servo is worth investing in.

Torque and size are also critical. The steering gear housing is usually marked with a model number, such as MG995, etc. You only need to search for the relevant parameters to know the specific torque value. The high-torque servo is larger and consumes power very rapidly, so the power supply current must match it, otherwise it cannot be driven at all. In addition, there is also the important point of mounting hole location. It is best to measure the size before purchasing to avoid being unable to install it after buying it.

Furthermore, when considering a servo, the factors of torque and size cannot be underestimated. The steering gear housing is often marked with model numbers, such as MG995, etc. You can determine the torque by searching for the corresponding parameters. High-torque servos tend to be larger and consume more power. The power supply must be sufficient, otherwise it will be difficult to drive. Regarding the installation hole positions, it is best to measure the size before purchasing to prevent the problem of not being able to install it after buying it.

How to use it well in actual projects

When working on a project, I suggest you first build a simple circuit to test the feedback characteristics of the servo. Use or ESP32, connect the wires, write a program to make the servo rotate back and forth slowly, and at the same time print out the feedback value through the serial port. In this way, you can intuitively see the corresponding relationship between angle and voltage, and you will have a good idea for subsequent programming.

If you are making batch products, don’t forget to add a protection circuit. A large capacitor is connected to the power input end to prevent the servo from pulling down the voltage instantly and causing the control board to restart. It is best to use shielded wires for feedback lines, especially when the servo is far away from the control board, which can reduce interference and ensure stable feedback data.

In fact, once you understand the 4-wire servo, you will find that it is not complicated, and the extra feedback wire can help you make a more reliable product. Are there any projects you are currently working on that require 4-wire servos? Chat about your thoughts in the comment area, and give a like so more people can see this article~