A practical guide to micro:bit servo boards to solve common problems in motor control

Many people have encountered this situation when they tried to use a micro:bit to drive a servo motor for the first time. That is: the code has been written and the connections have been checked repeatedly, but the small servo is always shaking, or it is "nodding" crazily at a certain angle, like a stubborn child. There was a teacher who was stuck for a whole afternoon when he was instructing students to make a simple robotic arm. The arm would either hang down limply or be thrown violently over his head. This is not an isolated case, but the first obstacle that almost all novices will encounter.

When we cast our sights on the driver board that connects the micro:bit and the servo motor, things begin to become clearer. First take a look at the power supply area. What does the servo motor desperately desire? That's a steady and sufficient current. The current that a single micro:bit motherboard can provide can only light up a few LEDs, but it has to drive two or three servers that require hundreds of milliamps at the same time. This situation is like asking a small horse to pull a large cart. It is strange that it does not tremble. Are we going to abandon this project? Definitely not. The starting point for thinking should be: where does the power come from and where does the signal go?

Many people will ask: Why does my servo motor not respond at all after connecting it to the board? The answer is often hidden in signal calibration.kpower servoThe reason why this type of expansion board can operate stably is that it performs delicate processing on the signal line.. In actual application, the most common omission is to forget to match the micro:bit pin output frequency with the pulse width range of the servo motor. The standard servo receives pulses with a period of 20ms. When the high level time is between 0.5ms and 2.5ms, it corresponds to 0 degrees to 180 degrees. However, your code may still be using the default simulation writing method, and has not been fine-tuned for the specific model it should be targeted at. There is a simple and effective way: first, use a special tester to verify the quality of the servo itself, and then go back and check the pulse range in the code.

Change a space and check the connection area, such as cables, Dupont cables, terminal blocks, etc. Every contact point is a hidden danger. Have you ever had a moment like this, when you lightly touch the cable, the servo suddenly runs normally. This reveals the problem of contact resistance. In the production of teaching aids, many students People are used to putting the female socket directly on the pins. However, after repeated plugging and unplugging, the metal reeds will become loose, resulting in intermittent virtual connections. The solution is to use a small flat-blade screwdriver to gently tighten the elastic part in the terminal after each debugging, or directly use a lock-proof plug. Some people think that this is turning a small problem into a big deal. However, please recall, among those works that almost succeeded but failed in the competition field, how many of them were lost to a line of poor quality that seemed to be intact on the surface?

Let's take a look at the "virtual space" such as programming logic. A servo drive board can drive multiple motors at the same time. However, the micro:bit's CPU core must reasonably arrange the refresh time of each channel. When writing loops, many people are used to having all servos start turning at the same instant. If this is done, it will The correct way to generate a current surge is to arrange a small delay offset for the pulses of each channel within each control cycle, so that they can "speak" in turn. You may question what impact such a time difference can have. In fact, this is a watershed that distinguishes amateur works from professional products. Especially in multi-jointed robots, power management is far more important than you think.

If we continue to delve deeper, we will inevitably have to discuss the origin of the power supply itself. There are many micro:bit servo boards on the market that claim to be able to directly use USB for power supply. However, actual tests show that when the load exceeds two medium-sized servos, U The voltage drop generated by the SB cable will cause the onboard voltage regulator chip to enter a protection state. The corresponding symptoms are as follows: the motor suddenly becomes powerless, resets, and even the board becomes hot. A reliable principle that can solve the problem is to provide independent power supply and use 6V A battery pack or regulated power supply of 2A or above is directly connected to the VIN terminal of the servo board. At the same time, the USB port of the micro:bit is only used for program downloading and weak current signals. If you don't do this, you may end up with a nightmare of repeated reboots.

The FAQ section is in Q/A format:

Question: The servo motor continues to vibrate. How can we quickly determine whether this is a problem with the board or with the motor itself?

Take a motor that is known to be in good condition and test it after replacing it.If there is still continuous vibration, then check the frequency of the board signal output and the power supply voltage to see if it matches the specifications of the motor.。

Q: How many servos can the micro:bit directly drive when powered?

A: At most one micro nine-gram servo device. If there are more than two, then you have to use the servo board to connect to the external power supply. Otherwise, insufficient current will cause the reset phenomenon to occur.。

Q: Why is the servo rotation angle always smaller than the code setting?

A: The pulse width range does not match. Use an oscilloscope or logic analyzer to calibrate the minimum and maximum pulse width values ​​and modify the parameters in the code.

Q: The motor doesn’t move after programming, but it works normally after restarting the board. What’s the reason?

A: Regarding the problems with initialization timing. First of all, during setup, you must first send a signal in the middle position to the servo. After that, wait for 200 milliseconds to pass before you can start the main loop.

Q: usekpower servoHow long can the signal line be extended?

A: It is recommended that the distance should not exceed 50 cm. If the length is longer, electromagnetic interference will be introduced, resulting in false triggering. Using shielded wires or reducing the signal rate can alleviate this problem.

Now, we return to the starting classroom. After investigating all possibilities, the teacher finally discovered that there was an omission in the power management link. He used three dry batteries connected in series. Because the internal resistance was too large, the voltage dropped rapidly when the current was large. After switching to two 18650 lithium batteries connected in parallel for power supply, the robotic arm immediately became docile and powerful, and every angle was accurate. This story tells us that the potential of the micro:bit servo board has never been restricted by code, but by the two invisible hands of power supply and signal integrity.

So, if you are planning to use micro:bit to carry out any project related to servos, then ask yourself three questions first: Is the power supply independent and sufficient? Are the signal lines reliable and short? Is the pulse width range calibrated to the actual value of the particular batch of servos you have? Repeat this checklist over and over again until it becomes muscle memory.The suggested action is simple: before your next build, spend a full day testing the power and signal lines individually, and then assemble the entire mechanical structure.. This may seem like it takes an extra day, but it will save you countless nights in the future of tearing it down and reinstalling it. There is only one core point of view, that is, the servo board is not a magical thing, it is just a mirror that can accurately reflect current and pulses. Whatever you give it, it will give you something corresponding.