Servo Drive Circuit Schematic Diagram And Pit Avoidance Guide: Solve The Problem Of Jitter And Weakness And Build A Stable Circuit

Seeing that you are makingservo-related products, you must have encountered such a situation: the program is obviously written without any problems, but theservokeeps shaking, or it gets stuck while turning. Don't rush to doubt the code. Nine times out of ten, there is something wrong with the drive circuit. When many people make products, they spend all their energy on algorithms and structures, and end up stumbling on seemingly simple circuits. Today we will talk about the pitfalls of theservodrive circuit and how to build a reliable circuit.

Why does the steering gear need a special drive circuit?

You may think that it is enough to directly connect the three wires of the servo (power, ground, signal) to the microcontroller? However, the reality is not that simple. There is a DC motor inside the steering gear. During its operation, the current is not small. Especially at the moment of starting and stalling, the current can soar to one or two amps. The IO port of the microcontroller is like a small arm or leg. It can output a signal of a few milliamps, but it cannot withstand such a large current. If the connection is forcibly connected, the servo will be unable to operate at best, and in severe cases, the microcontroller will be burned directly. Therefore, the drive circuit is like an amplifier, which can amplify the control signal of the microcontroller into a current strong enough to drive the motor.

What to do if the power supply is unstable

Many servos appear to be shaking and weak. The source of the problem is traced back to the power supply. If you think about it carefully, once the servo moves, it will instantly "draw" a large current from the power source. If the power supply fails to respond in time, the voltage will be pulled down in an instant. As this voltage decreases, the control chip inside the servo may restart or cause logical confusion, and its external manifestation is the servo shaking. What's even worse is that if the microcontroller and the steering gear share the same power supply, voltage fluctuations may cause the microcontroller to malfunction. Therefore, it is very necessary to provide a separate power supply for the servo, or to connect a large-capacity electrolytic capacitor in parallel to the power input end, just like a water tower, to temporarily stabilize the voltage.

Judging from the actual situation, the source of many servo vibration and weakness problems lies in the power supply. When the servo moves, it will instantly draw a large amount of current from the power supply. Once the power supply becomes sluggish, the voltage will drop rapidly. This voltage drop will cause the internal control chip of the servo to restart or cause logic disorder, which will then cause the servo to vibrate. What's more serious is that if the microcontroller and the servo share the same power supply, voltage fluctuations may cause the microcontroller to malfunction. Therefore, it is very necessary to provide a separate power supply to the servo, or to connect a large-capacity electrolytic capacitor in parallel to the power input end to temporarily stabilize the voltage.

How to choose a reliable driver chip

Choosing a driver chip is like choosing a partner, pay attention to the right match. The first step is to check the current. You need to know the specific value of the stall current of the servo you choose. It is best to leave a margin of 1.5 to 2 times for the continuous output current of the driver chip. For example, if the maximum current of the servo is 1A, it would be safer to choose a chip that can continuously output 1.5A or 2A. The second thing is to pay attention to the voltage. The voltage of the driver chip must be able to cover the working voltage range of your servo. Common chips like L293D are usually used for small servos. If your servo is very powerful, you may have to consider using an H-bridge circuit built with a higher-power MOS tube.

Is it necessary to isolate the circuit?

It depends on how complex your system is. If the servo is the only "high-power" component in your product, and the power supply is properly designed, then you can directly pull a line from the IO port of the microcontroller and string a resistor of several hundred ohms in the middle for current limiting. Usually, the problem will not be too big.

But if there are sources of strong interference such as motors and electromagnets in your system, or the servo is far away from the control board, it is best to isolate them. The most common way is to use an optocoupler to "translate" the control signal of the microcontroller into an optical signal and then transmit it to the electrical signal on the other side. In this way, the electrical signal is completely isolated and interference cannot pass through.

Do I need to add a resistor to the signal line?

It's a good habit, and while it's not always necessary, it can help you sleep more soundly. On the signal line output from the microcontroller to the servo, a small resistor of 100Ω to 300Ω can be connected in series to play two roles. The first is to limit the current to prevent the IO port of the microcontroller from being configured incorrectly and causing the output to short-circuit and burn out immediately. Second, it can form a low-pass filter with the distributed capacitance on the line to absorb some high-frequency noise interference, making the waveform transmitted to the steering gear cleaner and more stable. The cost of this small operation is very low, but the profit is very high.

What are some tips for circuit layout?

When drawing the circuit board, some "special care" can be given to the driving part of the servo.

First of all, in terms of wiring, the power cord and ground wire should be as thick and short as possible when laying them. This is because during the operation of the circuit, the driving part of the servo needs to pass a large current. If the power wire and ground wire are too thin, heat and voltage drop will easily occur, which will affect the normal operation of the circuit. Secondly, regarding the ground of the drive circuit and the signal ground of the microcontroller, it is best to merge at a single point, such as at the root of the power supply filter capacitor. This can effectively prevent large currents from forming a potential difference on the ground, thereby preventing interference to the microcontroller. Another point is that the driver chip needs to be placed close to the interface of the servo. This can minimize the path taken by the large current, thereby controlling the interference radiation to the minimum range and ensuring the stability and reliability of the entire circuit system.

Seeing this, you should have a good idea of the servo drive circuit. Next time you encounter a servo that is disobedient, you can first check from the aspects of power supply and signal isolation. I wonder if you have encountered any particularly weird failures when debugging the steering gear? You are welcome to share it in the comment area and communicate with us. Maybe your experience can help another engineer who is scratching his head. If you find the article useful, don’t forget to like and share it so that more people can see it.