Motor speed control by Identify mixer and actuator_control

PX4 Autopilot Flight Controllers đź’»
1

Hi, dear colleges.

Now I am trying to control the motor speed by sending messages to the actuator_control topic with PX4 1.7.3.

I found the control flow in PX4 is like this:

actuator_control------------>mixer------------>normalised PWM------------>PWM----------->ESC(motor)

  1. I identify the relation between the PWM sent to ESC and the motor speed: omega = g(PWM)

  2. the relation PWM = f(normalised PWM) can be found

effective_pwm[i] = control_value * (max_pwm[i] - min_pwm[i]) / 2 + (max_pwm[i] + min_pwm[i]) / 2;

from PX4-Autopilot/src/modules/systemlib/pwm_limit/pwm_limit.c at v1.7.3 · PX4/PX4-Autopilot · GitHub.

  1. An identity mixer is defined such that normalised PWM = actuator_control. The mixer is defined as
    I followed the steps Modifying mixer file without reflashing
M: 1
O:      10000  10000      0 -10000  10000
S: 0 0  10000  10000      0 -10000  10000

M: 1
O:      10000  10000      0 -10000  10000
S: 0 1  10000  10000      0 -10000  10000

M: 1
O:      10000  10000      0 -10000  10000
S: 0 2  10000  10000      0 -10000  10000

M: 1
O:      10000  10000      0 -10000  10000
S: 0 3  10000  10000      0 -10000  10000

Finally, the control messages publish to actuator_control can be computed from motor speed as
actuator_control = f^-1(g^-1(omega)).

However, I have meet some problems:

  1. with the mixer in (auto.land mode), the /rc/out shozs that PWM sent to the motors are the same. However, the motor speeds are quite different. In fact, when PWM = 1500, the motor 4 are around 9000 rpm while motor 1,2,3 are like 14000. So, is there some other setting that I need to change, or my mixer is not defined correctly?

  2. when switch to the OFFBOARD mode, /target_actuator_control is defined to copy the messages from /actuator_control and sent to PX4 in mavros. Howevcer, /target_actuator_control change to [0,0,0,0,0,0,0,-1] periodically. As a consequence, when I publish [-0.7,-0.7,-0.7,-0.7,0,0,0,0] to /actuator_control, /rc/out vibrates between [1512, 1512, 1512, 1512] and [1590,1590,1590,1590]. Is this a bug or I misunderstood something?

2

Thanks for your question.
But I’m a new develop of px4, I dont know how to get the motor speed(motor rpm). The way you choose to get motor speed is by using PWM?
Thanks! I’ll appreciate it.

3

Yes, I did tests to control motor speed by sending normalised PWM. It seems like controlling a single motor was fine, but when I tried integrating motor speed control in a flight control system, it failed.

4

please have a look at my answer at RE: Motor speed control by Identify mixer and actuator_control

5

Appreciate your answer!
But the page you show which I can’t visit.

6

Now my method is to get the motor speed in gazebo, which can be get indirectly. And then pub it to mavlink.

7

Hi, this is the answer in my link.

If I remember correctly, we cannot get rpm directly from mavros. In PX4, there is a reference motor speed signal, target_actuator_control, and a motor speed state actuator_control. Both them have the same unit ranging from 0-1.

Their units are not rpm, but I would like to call them normalised rpm.

What I did is to find a (linear) relation between a normalised rpm and the real rpm for a given motor.

It is kind of identification: I measured the real rpm with an external sensor and recorded the normalised rpm. With the data, I tried linear approximation.

8

In simulation, that can be a solution. But if we want to implement motor speed control in real world experiment, I think it could be difficult.

9

Yes you’re right, there just one way to get the motor speed in the real world where we need to use the "dshot " ESC , which is expensive to buy.

10

Thanks your replies, But I found that just a linear formulation relation between motor speed and PWM is not a accuracy way . If we wanna get more accurate motor rpm in SITL, I find the only way is to get motor speed in gazebo-simulator (the number of motor rpm is in “gazebo-motor-plugin”).

11

This can be a good idea.