TECHNICAL SUPPORT
Published 2026-03-27
Have you ever encountered such an embarrassment: you want to make a robotic arm, a smart car, or a cool robot, but theservoalways shakes badly, or gets stuck halfway through the rotation? In fact, controlling theservois much simpler than you think. Today we will talk about how to perfectly coordinate this powerful "heart" with the small "muscle" of theservo.
Many friends use the servo for the first time, and the biggest headache is the vibration of the servo. This is usually not because the servo is broken, but because the control signal is unstable. The steering gear relies on a 20ms period PWM wave, in which the high level time varies between 0.5ms and 2.5ms, corresponding to 0 to 180 degrees. It comes with an advanced timer that can output extremely precise PWM waves, which is much more stable than using delay function simulation. You can imagine that the servo is equipped with a professional conductor, and every beat is steady.
Another common cause is insufficient power supply. The instantaneous starting current of the servo can reach more than 1A. If the power is taken directly from the 3.3V of the development board, the signal will be messed up when the voltage drops. ️ The correct approach is to provide external independent power supply, separate the power supply of the development board and the servo, and only share the ground. This is like using a dedicated line for high-power appliances at home, otherwise the light bulb will flash twice when the air conditioner is turned on.
The timer resources are very rich, TIM1 to TIM14, but not all of them are suitable for direct control of the servo. For ordinary angle servos, it is enough to use general timers TIM2, TIM3, TIM4, and TIM5. They can easily generate a 20ms period PWM. If you control many servos at the same time, such as more than 8, then you should consider using advanced timers TIM1 or TIM8. They have complementary output and braking functions and are more suitable for multi-channel control.
How to choose? First look at the pins, and secondly look at the resources. Connect the servo signal line to the pin with the timer output channel. For example, PA0 is channel 1 of TIM2, and PB6 is channel 1 of TIM4. It is recommended that you open the pin definition table in the reference manual and look up the pins like a map to find those pins that are convenient for wiring and do not conflict with other functions. Remember, a good engineer's wiring starts with selecting the right pins.
There are actually three steps to configure PWM: turn on the clock, set parameters, and output waveform. Taking TIM2 as an example, if you want the period to be 20ms, which is 50Hz, set the auto-reload value ARR to 2000 and the prescaler PSC to 839. In this way, the timer clock is 84MHz/(839+1)=, and 2000 pulses are exactly 20ms. Here comes the key, the duty cycle control angle: 0 degrees corresponds to a comparison value CCR of 50, 90 degrees is 150, and 180 degrees is 250. This calculation formula is not difficult, right?
If you use library functions to develop, the waveform will come out as soon as () is called. But be careful, never use Delay to control the rotation of the servo in the program, as it will cause the CPU to freeze and the servo to freeze. ️ The correct approach is to change the CCR value and let the hardware automatically output a new waveform, just like changing gears without turning off the engine, silky and smooth. If you are using the standard library or register operation, the principle is the same. If you understand this one thoroughly, you will know everything about other timers.
Controlling one servo is simple, but many people fall into misunderstandings when controlling multiple servos. A timer can only use one cycle, but can output multiple channels. For example, CH1, CH2, CH3, and CH4 of TIM2 can output PWM with different duty cycles at the same time. This means that one timer can drive 4 servos without interfering with each other. You only need to set the CCR value of each channel separately to allow multiple servos to assume different postures at the same time.
If you want to build a robot with 18 degrees of freedom, you need multiple timers to work together. Group the servos into groups, such as TIM2 for the arms, TIM3 for the body, and TIM4 for the legs. Each timer manages several servos of its own. This not only makes the logic clear, but also prevents confusion caused by too many interrupts from one timer. It's like a production line in a factory. Each line performs its own duties so that the overall efficiency can be maximized.
Many novices like to write directly in the main loop to control the rotation speed of the servo. As a result, once the servo rotates, the response of other buttons will be slow. In fact, it is very simple to achieve smooth rotation, using the idea of "state machine". For example, if you want the servo to turn from 0 degrees to 90 degrees, instead of changing the CCR from 50 to 150 all at once, you should increase it every short period of time and make steps in the timer interrupt or interrupt. In this way, the CPU can be busy with other things and the servo can move evenly.
You can also use DMA to assist. If you have a series of predefined action sequences, such as the continuous actions of a robot arm grabbing items, you can save these CCR values into an array and let the DMA automatically move to the comparison register of the timer. In this way, there is almost no load on the CPU, and the servo movement can be precise at the microsecond level. Although it seems complicated, the DMA configuration is actually very simple. Once you try it once, you will know its power.
No matter how perfectly written the program is, hardware problems can easily drive people crazy. The first problem is the common ground problem. The GND of the development board and the GND of the servo power supply must be connected together. Otherwise, the control signal will be suspended and the servo will either not move or rotate randomly. Many people use a multimeter to measure the voltage correctly, but the signal is just wrong. Nine times out of ten, the ground wire is not connected properly. Secondly, if the servo signal line is too long, it is susceptible to interference. It is best to use a twisted pair or a small 100 ohm resistor in series.
Another artifact is the logic analyzer. A small device that costs tens of dollars, just clip it to the signal line, and you can see with your own eyes whether the PWM waveform is correct and whether the high-level time is what you want. Compared with blind debugging, this kind of visual debugging can help you quickly locate whether it is a code problem or a hardware problem. Don't bother, take 10 minutes to check it out, it will save you an afternoon of guessing.
After reading this, do you think that controlling the steering gear is actually quite systematic? Next time you develop a robot project, you might as well think about the points we talked about today, from power supply to timer selection, from single servo to multi-channel linkage, every step is important. What interesting works are you planning to use this technique to create? Is it a bionic hand, a six-legged robot, or an omnidirectional car? Welcome to share your creativity in the comment area, and don’t forget to like and save it so that more partners can play with servo control!
Update Time：2026-03-27
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