What should I do if the servo voltage is insufficient? 3 tips to solve the problem of jitter and weakness

While you were playing, the robot suddenly developed a "Parkinson's" condition, and its arms kept shaking. And the wheel becomes soft after turning for a while. Even the simplest reset movements began to cause twitching. Have you ever encountered such a weird thing? Is the first reaction of the voltage monitor that "the steering gear is broken"? Wait, don't throw it away yet. In most cases, this thing is not damaged at all, it is just shouting with its own life: "I'm hungry! The voltage is not enough!".

Believe it or not, let’s take the extremely common 7.4V servo as an example. How many people have absentmindedly plugged in a 5V USB power bank for power supply? The data doesn't lie: when the input voltage is 15% lower than the rated voltage, the output torque of the servo will drop sharply by more than 40%. In other words, the robotic arm that was originally able to lift two bottles of mineral water is now unable to even hold a chopstick. This situation is not a malfunction, but a slap in the face caused by the laws of physics.

Here comes the question:Why does insufficient voltage make the servo go "crazy"?

Looking at the disassembly, the inside of the servo is actually a small motor plus a reduction gear and a control board.The control board needs a stable voltage to correctly process the PWM signal, and the motor is an "electric tiger". The current at the start-up moment is 3 to 5 times that of normal operation.。If the power supply cannot keep up and the voltage is instantly pulled down, the control board will have to constantly reset and try to reposition itself.. So what you see is: shaking, weakness, rocking back and forth, or even no movement at all. The voltage monitor makes it clear that this is a typical "power hunger syndrome", which is the same as the crazy pop-up window when the battery of a mobile phone is 1% left.

Don't think this is some high-end technical issue. Take a look at some common rollover cases:

In Project A, when the six-legged robot was directly connected to the power bank through a USB cable, the robot stood unstable. The output of the power bank was 5V/1A, and the six-legged robot needed to drive 18 servos at the same time. The instantaneous current exceeded 6A, causing the voltage to drop directly to 3.8V. As a result, the insect collapsed into a piece of cake.

• Project B: The 3S lithium battery used for the 12V servo has a nominal voltage of 11.1V, which feels pretty good in actual use - when the battery is fully charged and the voltage is 12.6V, the entire operation is normal. However, once the power drops to 11V, twitching will begin. With just a voltage difference of 1.5V, the torque is reduced by 35%, so that the robotic arm cannot even bear its own weight.

• Project C: The low-quality battery wire is connected to the servo wire. There are cases where the wire is long and thin. Specifically, the 1-meter-long 22AWG wire has a voltage drop of up to 0.3V at 2A current. For a 3.7V micro servo, this 10% voltage loss is enough to double the response speed.

Then, here comes the pure solution.

The first step is to settle accounts.You need to take out your calculator and count the servos that operate at the same time in the project. How many are there at most? You also need to know what the locked-rotor current of each servo is.. The voltage monitor cannot be judged by feeling. The current of a standard torque servo is usually between 0.5 and 1.5A, and the current of a large servo can reach 3 to 5A. Multiply the total current requirement by a safety factor of 1.5, and the result is the lower current limit that the power supply must reach.

The second move: exchange blood. Give the servo a "well fed" power supply solution.

2-5 small servos: directly powered by 2S or 3S lithium batteries, pay attention to voltage matching

Prepare six to twelve medium-sized servos and install BEC (voltage stabilizing module) on them to stabilize the high-voltage battery to the precise voltage required by the servos.

More than 12 or large servos: directly connect the adjustable voltage stabilized power supply and abandon the battery solution

The third trick: achieve muscle gain. On the power line of each servo, connect a capacitor with a larger capacity in parallel. The capacity of this capacitor is between 470μF and 2200μF. These capacitors are like micro reservoirs. At the moment when the steering gear is started, the peak value of the current is "temporarily discharged" to top up the peak value of the current, thereby preventing the voltage from collapsing. Experimental data shows that when a 1000μF capacitor is added, the voltage fluctuation can be reduced from ±30% to within ±8%.

There is another fatal misunderstanding that needs to be made clear. High voltage does not mean that the servo is burned, but low voltage can kill the servo. Many people do not dare to increase the voltage because they are afraid that the servo will be burned. In the end, being in an under-voltage state for a long time causes the servo to vibrate repeatedly and become hot, which makes it easier to burn the control board. The withstand voltage range of most servos is plus or minus 20% of the rated voltage. Just like a six-volt servo can withstand 7.2 volts, but once it is lower than 4.8 volts, problems begin to occur.

Q/A time

Q: The servo vibrates as soon as the power is turned on. Is it broken?

A：首先去测量电源实际输出的电压，在大多数的情形之下，是因为电流不足从而致使电压急剧下降，此时更换大电流源就行。

Q：用手机充电器供电为什么不行？

A：充电器所标称的5V/2A是峰值状态，其实际能够持续具备的输出能力较为不足，并且电压纹波的幅度很大。

Q：并联电容应该选什么型号？

若存在A情况，即属于低ESR铝电解电容或者聚合物电容这类，而钽电容由于自身抵抗冲击的能力较弱，所以不建议使用。

Q：电池满电正常，用一会就抖怎么办？

A：电池的放电能力出现了下降的情况，要么更换具有高放电倍率的电池，要么在电池的后面添加稳压模块。

Q：舵机线和控制线能共用电源吗？

A：电压监控器是绝对不可以的。电源线需要直接用粗线连接到舵机，控制线仅仅是用来传输信号而不提供电力供应。

往后瞧，舵机电压不够这桩事，实际上就是一场“匹配较量”。要是你把舵机视作凶猛野兽，那就得给予充足电量；要是你用草料去喂狼，却还埋怨狼不做事，这说得通吗？这样的道理，放置在任何品牌的舵机上都是适用的，其中涵盖kpower Servo这类专业产品。产品说明书标明电压为6至8.4V，你却非要给到5V，如此一来，它产生抖动，便是对你最为温和的一种抗议了。

最后给你三条保命建议：

① 每个新项目先算电流总账，别等接线了再补课

② 电源线越短越粗越好，线损比你想的要大得多

③ 准备几种电容，分别是470μF的，1000μF的，2200μF的，哪里出现抖动就和哪里并联。

记得这么一句话，舵机不会说谎，它抖动是由于饥饿，它停止是源于瘫痪。当电压监控器显示下一次你的机器人再度开始像群魔乱舞般乱动时，别去指责舵机质量不好。要拿起万用表，去测量一下供电端电压——要是数字处于闪烁状态，那并非是表出现故障了，而是你该给这位好似“大力士”的玩意添加能补充能量的食物了。