About Serial BusServo Motor

Abby Brown's picture

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Joined: Mar 19, 2021

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About Serial BusServo Motor
May 15, 2021 - 3:42pm

There are many kinds of servos, traditional PWM servo and bus servo, it is not recommended to use PWM servo here.
The reasons are as follows:
1. PWM is the most widely used angle rotation module on aircraft models. The advantage is that it can complete the angle change in an instant. It requires a good explosive force and a strong lock-rotor ability in an instant. It does not emphasize the life and continuous force (so the servo specification is clear Is the locked-rotor torque, not the rated torque). The robotic arm pays attention to the process and the result, that is, whether the force applied during the motion process is consistent with the final result.
2. The accuracy is poor. The PWM servo only requires the rotation from the initial position to the target position, even if there is a certain error from the executed position to the target position, it is basically no problem. However, as the joint parts of the robotic arm, the precision requirements are very high, which will affect the end application of the robotic arm. Of course, there are servos with high structural precision.
3. PWM servo is basically the control method of aeromodelling. If it is used as a robot arm, each servo needs to be connected to the main board to send pulse control. The lines are complex and easy to cause problems and difficult to troubleshoot.
4. Lifespan will be a difficult problem to deal with. After someone bought the servo, the debugging was very accurate and powerful at the beginning. Later, due to the long load time, a false position was generated between the gears, the servo current increased, heat generation, and the demagnetization torque decreased, and the false position basically increased later. If the gear is deformed and broken.
to sum up:
The bus servo is recommended here. Bus servo is an alternative servo derived from the steering gear that is not used in the field of model aircraft but is more used in other fields. For example, FEETECH servo, their bus servo has been researched for eight or nine years. At present, the effect is good on my equipment. In order to be able to answer comprehensively, I deliberately studied it again.
Let me talk about some reasons why I recommend his bus servo:
1. Due to the wider range of use of the bus servo, the structural hardware requirements are very high. It needs to use a stronger shell such as CNC cut aluminum metal, and it needs to use stronger metal teeth such as steel teeth. It needs to be designed by experienced structural engineers. The gear combination ensures concentricity and reasonable speed ratio. In addition to high structural hardware requirements, there are also control aspects.
2. In terms of control, the bus servo uses a serial port to send instructions, so that the servo executes work at a predetermined speed target position. The bus servo can be controlled in series, that is, one servo is connected to the control board. Each server has an assigned ID, which is similar to an identification, and will execute it only after receiving the corresponding ID number and command. If the command with ID:1 is sent, only the servo with ID:1 will respond and give an execution command. Of course, if the bus servo simply receives commands and executes them, there is no recommended meaning.
3. The bus servo has a closed-loop feedback function, and its built-in control board gathers sensors for voltage, speed, temperature, position, current, and load. These data can be fed back to the control board for monitoring in real time. When the torque exceeds the set percentage or the input voltage exceeds V, the temperature exceeds how many degrees, or the current exceeds how many A, the servo will stop running or unload the force in the set mode. wait. All parameters can be set accordingly on the host computer. In addition, the servo can also set the operating angle, baud rate, and working mode (such as motor mode, continuous rotation).
4. Another very critical aspect of the servo is accuracy, which is emphasized again. Ordinary servo may still use traditional potentiometer sensing, using 1024-step algorithm. The angle of the potentiometer is limited. At present, the maximum angle of the potentiometer is 320 degrees. Except for the insensitive areas at both ends, it is actually only 300 degrees. That is, 300 degrees is divided into 1024 steps to control. The accuracy is only 0.29, which does not count as the accuracy error caused by the structure, imaginary position and other factors. The high-end steering gear adopts the magnetic encoding sensor method for control, which can be controlled in 360 degrees. With more powerful algorithms, it can achieve 360-degree control in 4096 steps, and the accuracy is increased to 0.088. Of course, the selection of high-end materials must also use high-end structural parts, all-steel teeth, aluminum metal shells and so on. These combinations can improve the servo to a great degree. From the accuracy, torque, sound, heat dissipation, smoothness of operation and other aspects have been qualitatively improved.
5. In order to become a highly applicable bus servo, it is also necessary to make changes in the operation effect, mass production installation and other links. The traditional operation effect is that the "W" type reaches the target position at a constant speed, that is, it reaches the target position at a constant speed after sending the pulse. The moment it starts is a kick in fuel, and the moment it stops, it is also a kick in the brake. With the addition of the "acceleration" function, the steering gear will operate more smoothly, just like a small car with continuously variable transmission at high speed, slowly accelerating and then slowly stopping. There is no sense of "mechanics" at all.
6. In mass production, when installing each servo, always consider debugging to an angle and then install the rudder disc. The left and right rotation angle is reasonable to prevent excessive risks such as gear blockage and current increase, but this efficiency Is it slow and error-prone? But using the "one-key set current position as neutral" function in the bus servo, there is no need to debug to a reasonable angle. This kind of function can only be realized on the hardware basis of magnetic encoding at present. That is, any angle can be set in 360°, and it can run on the left and right.
7. In addition to the above functions, there are PDI parameter debugging, motor mode, multi-turn controllable, power-on free mode, etc. These functions may also be used. PDI can adjust the rigidity and flexibility coefficient, overshoot coefficient and static error of the motor free of charge. Generally speaking, the P coefficient is the toughness of the steering gear, the larger the tougher. The D parameter is a state that allows the steering gear to reach the target position. The larger the parameter value, it is similar to the earlier the car brakes, and the smaller it is, it may exceed the position to be braked. The I parameter means that there is an error between the executed position and the target position. The error can be reduced or eliminated by adjusting the I parameter, but the larger the set value, the jitter may also occur. This is one of the reasons why some servos tremble by themselves when there is no load. Vibration may also be caused by gaps in gear errors, poor variable resistors, and problems with potentiometer position feedback.
8. The last recommendation is that the price is more favorable and more suitable for commercial mass production. A Dynamixel X series steering gear is more than three hundred dollars. A Feetech SM40BL is only more than one hundred dollars. The price difference is three or four times. The effect is already comparable.

Sourav Gupta's picture

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Joined: Feb 12, 2018

Replies: 628

I think this dependes on the Application perspective.

Abby Brown's picture

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Joined: Mar 19, 2021

Replies: 3

It's really a scenario that needs to be looked at.
But the serial bus servo has more advantages than the PWM servo in some aspects, more conducive to our control.