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: First, measure the actual output voltage of the power supply. In most cases, the voltage drops sharply due to insufficient current. In this case, just replace the high current source.。

Q: Why can’t it be powered by a mobile phone charger?

A: The nominal 5V/2A of the charger is the peak state. Its actual continuous output capability is insufficient, and the voltage ripple is very large.

Q: What type of parallel capacitor should I choose?

If situation A exists, it means low ESR aluminum electrolytic capacitors or polymer capacitors. Tantalum capacitors are not recommended due to their weak ability to withstand impact.

Q: What should I do if the battery is fully charged and starts shaking after being used for a while?

A: The discharge capacity of the battery has declined. Either replace the battery with a high discharge rate, or add a voltage stabilizing module behind the battery.

Q: Can the servo cable and control cable share the same power supply?

A: A voltage monitor is an absolute no-no. The power cord needs to be directly connected to the servo with a thick wire. The control wire is only used to transmit signals and does not provide power supply.

Looking back, the issue of insufficient servo voltage is actually a "matching contest". If you regard the servo as a ferocious beast, then you must provide sufficient power; if you feed the wolf with fodder, but still complain that the wolf does not do anything, does this make sense? This principle is applicable to any brand of steering gear, includingkpowerProfessional products like Servo. The product manual states that the voltage is 6 to 8.4V, but you insist on giving it 5V. If it vibrates, it is the mildest protest against you.

Finally, I give you three life-saving suggestions:

① For every new project, calculate the current general ledger first. Don’t wait for the wiring to be done before making up the lessons.

② The shorter and thicker the power cord, the better. Line loss is much greater than you think.

③ Prepare several types of capacitors, namely 470μF, 1000μF, and 2200μF, and connect them in parallel wherever jitter occurs.

Remember this sentence, the servo does not lie, it shakes because of hunger, and it stops because of paralysis. The next time your voltage monitor shows that your robot starts moving like a demon again, don't accuse the servos of poor quality. Take a multimeter and measure the voltage at the power supply - if the number is flashing, it's not that the meter is faulty, but that you need to add energy-replenishing food to this "Hercules" thing.