Three Tips To Help You Solve The Problem Of Power Outage Due To High Startup Current Of The Servo

When we make products or engage in projects, whenever we use high-torqueservos, we will most likely run into an annoying problem: at the moment of power-on, the current when theservos are started is extremely strong, directly "pulling out" the power supply. In serious cases, the entire system automatically resets, or even burns the fuse. Many friends have asked me about this. In fact, it's not that theservois broken, or that the power supply is too watery, but that the starting current peak value does not match the power supply plan well. Today we will talk about how to fill this hole.

Why does the steering gear starting current suddenly increase sharply?

Many people are surprised when they measure the servo current for the first time. For a servo with a nominal locked-rotor current of 2A, the peak value at the moment of startup can reach 4A or even higher. This is because the inside of the steering gear is a DC motor acceleration and deceleration gear structure. The moment the motor goes from rest to rotation, the rotor needs to overcome the static friction and load inertia. At this time, the counter electromotive force has not yet been established. The coil is equivalent to a short circuit state, and the current naturally rushes to the maximum value.

The duration of this transient spike is actually very short, only tens to hundreds of milliseconds, but it is precisely in this short moment that the overcurrent protection of the power chip is triggered. If you are using a switching power supply or a lithium battery protection board, their tolerance to instantaneous overload is often very low. Once it detects that the current exceeds the threshold, even if it is only 10 milliseconds, the output will be cut off directly.

How to determine the real reason why the power supply is blocked by the servo

If you get a circuit board that is stuck by the servo, don't rush to replace it with a larger power supply. I am used to using an oscilloscope to capture the power output waveform in the first step to see the amplitude and duration of the drop. If the voltage drops below the chip reset threshold, the problem lies in the power supply response speed; if the power supply does not drop voltage at all but the output is turned off, then the protection circuit is most likely too sensitive.

The second step is to calculate the general ledger. Multiply the number of servos that may be started simultaneously by the single starting peak current, and leave a 30% margin. This is the peak power supply capacity you need. Many people only look at the average current or locked-rotor current and ignore the key scenario of "simultaneous starting". For example, if all four legs of a quadruped robot are powered on at the same time, the total starting current will definitely not be the peak value of a single leg multiplied by four, but will have a superposition effect.

Can a soft-start resistor connected in series solve overcurrent?

This is a very classic old method. A power resistor is connected in series with the output end of the power supply, and the resistor is used to limit the current. After the servo is turned up, a relay or MOS tube is used to short-circuit the resistor. The advantage is that the cost is extremely low and the solution is simple. It only costs a few cents for a cement resistor and a relay. The disadvantages are also obvious: the resistor heats up seriously, and there will still be a current shock at the moment of short circuit.

This solution is more suitable for scenarios where the power output capability is just stuck at the critical point. For example, a 12V 5A adapter with a peak 6A servo, and a 0.5 ohm resistor in series can reduce the current to less than 5A. But if your servo load is frequently started and stopped, and rotates forward and reverse frequently, the life of the relay will be a headache. It is recommended to switch to an all-solid-state solution.

How to choose capacity and type of capacitor energy storage solution

Capacitors are the most direct helper in dealing with instantaneous large currents. The principle is not difficult to understand: the power supply charges the capacitor when the power supply is normal, and the capacitor releases the stored electric energy the moment the servo is started, helping the power supply to withstand the peak of several hundred milliseconds. The key is how to choose this capacitor.

There is a rough formula for capacity: try 1000 microfarads per ampere of peak current. For example, if the peak value is 5A, first solder 4700 microfarads and measure the voltage drop. If it is not enough, add more. In terms of type, low-ESR solid capacitors or high-frequency low-resistance electrolytic capacitors are preferred. Ordinary electrolytic capacitors have large internal resistance and poor instantaneous discharge capability, so if they are installed, they are useless. The location is also very particular. The capacitor must be close to the servo power input terminal. The shorter the lead, the better. The PCB trace should be wider and no via holes should be used.

Are there any differences in the starting characteristics of digital servos and analog servos?

This issue is critical when selecting a model. The analog servo relies on a comparator to directly drive the motor and does not have a microprocessor. It responds quickly but the starting current is very hard. There is an MCU inside the digital servo, which can be programmed to control the PWM duty cycle and slow start. Many high-end digital servos even have their own current limiting function.

So if your project is still in the design stage, it may be less troublesome to directly switch to a digital servo that supports slow start than to fiddle with the power supply. For example, some brands support setting the startup slope through the serial port, allowing the current to rise gently within 200 milliseconds, and the peak value can be suppressed by more than half. Of course, the price is that it is more expensive and the control method is more complicated.

What current smoothing processing can be done at the software level?

If the hardware is already dead, don’t panic, the software can still make up for it. The most effective way is to start at a staggered peak, so that all servos are not powered on at the same time. For example, when the robot is powered on, the servos of each leg are initialized sequentially at intervals of 50 milliseconds, and the peak current is immediately dispersed.

Another trick is PWM frequency adjustment. Some servos support external PWM signal control position. You can first send a narrower pulse width to let the servo move to a small angle position. The current will naturally be smaller than directly driving it to a large angle. This is particularly useful in the zero return action of the robotic arm. Let the arm hang down first, and then slowly lift it up. The current curve will be much flatter.

In the final analysis, the essence of the problem of steering gear starting current is the game between instantaneous power and average power. When we do projects, we don’t have to use tank-level power supplies to power the roads. A smarter way is to “pay in installments” the energy. Friends who read this, you may wish to think back: when you encountered similar problems before, did you immediately change to a higher-power power supply, or did you first find a breakthrough in these software and hardware details? Welcome to share your practical experience in the comment area. If you find it useful, please give it a like and forward it to more friends who have been frustrated by servos.