Pymavlink and rc override for autonomous guidance in ACRO mode

Blue Robotics Software
1

Thanks in advance!

Goal: to arm the vehicle and put it in a mode, say ACRO, and while in that mode have my laptop running a pymavlink script process camera images, calculate corresponding pitch, roll, yaw and thrust to send to the Pixhawk in order autonously steer the vehicle towards a target.

Approach: Using the set pwm rc override commands to transmit 1100-1900 pwm’s on rc channels 1-4.

Problem: the motors only activate for a few seconds and then stop rotating.

The following script I’m using is a compilation of scripts found here.

################################################################################

from pymavlink import mavutil

############################################################## function to send RC values
def set_rc_channel_pwm(id, pwm=1500):
    """ Set RC channel pwm value
    Args:
        id (TYPE): Channel ID
        pwm (int, optional): Channel pwm value 1100-1900
    """
    if id < 1:
        print("Channel does not exist.")
        return

    # We only have 8 channels
    #http://mavlink.org/messages/common#RC_CHANNELS_OVERRIDE
    if id < 9:
        rc_channel_values = [65535 for _ in range(8)]
        rc_channel_values[id - 1] = pwm
        master.mav.rc_channels_override_send(
            master.target_system,                # target_system
            master.target_component,             # target_component
            *rc_channel_values)                  # RC channel list, in microseconds.

# Create the connection
master = mavutil.mavlink_connection('udpin:192.168.2.1:14550')
# Wait a heartbeat before sending commands
master.wait_heartbeat()

############################################################## Arm
master.mav.command_long_send(
    master.target_system,
    master.target_component,
    mavutil.mavlink.MAV_CMD_COMPONENT_ARM_DISARM,
    0,
    1, 0, 0, 0, 0, 0, 0)

#############################################################set mode :
# Choose a mode
mode = 'ACRO'

# Check if mode is available
if mode not in master.mode_mapping():
    print('Unknown mode : {}'.format(mode))
    print('Try:', list(master.mode_mapping().keys()))
    exit(1)

# Get mode ID: return value should be 1 for acro mode
mode_id = master.mode_mapping()[mode]
master.mav.set_mode_send(
    master.target_system,
    mavutil.mavlink.MAV_MODE_FLAG_CUSTOM_MODE_ENABLED,
    mode_id)


############################################################# Set some ptich
set_rc_channel_pwm(1, 1600)
2

Hi Tim,

You need to send the channel value periodically.
Like:

while True:
    set_rc_channel_pwm(1, 1600)
    time.sleep(0.2)
3

Thanks Patrick! That works. However, is there a command to set pitch rate, roll rate, yaw rate and throttle that doesn’t require periodically updating?

4

No, this is a safety mechanism. You can disable the failsafe for pilot input or gcs failsafe to override this. Be careful with that route though; it’s set up this way for a reason.

5

Thank you. I will be careful!

I did disable those two failsafes using QGC, however i still need to periodically set the channel at a quite fast rate in order to keep the thrusters continuously spinning. Is there something else I need to disable in order to be able to set and forget?

6

Is there a way to set the rc channel without having to periodically update it? I have disabled pilot input and gcs in QGround Control to no avail however.

7

What version of ardusub are you using? What is the problem with updating the values at a regular interval? Can you share your code?

8

Hi Jacob: I have firmware version 3.5.2 installed. The interval method is computationally cumbersome for application. Is there a way to be able to bypass that requirement? As I mentioned, I disabled pilot input and gcs heartbeat.

9

If you disabled the failsafes it should work. Please share your code.

10

Jacob, here’s the code:

from pymavlink import mavutil

mavutil.set_dialect("ardupilotmega")
# Create the connection
master = mavutil.mavlink_connection('udpin:192.168.2.1:14550')
# Wait a heartbeat before sending commands
master.wait_heartbeat()

# Set all rc chans pwm to 1500 before arming:
data = [ 1500 ] * 8
master.mav.rc_channels_override_send(
	master.target_system, master.target_component, *data)

#############################################################set mode:
# Choose a mode
mode = 'MANUAL'
# Get mode ID: return value should be 1 for acro mode
mode_id = master.mode_mapping()[mode]
# Set new mode
master.mav.set_mode_send(
master.target_system,
mavutil.mavlink.MAV_MODE_FLAG_CUSTOM_MODE_ENABLED,
mode_id)

############################################################## Arm
master.mav.command_long_send(
master.target_system,
master.target_component,
mavutil.mavlink.MAV_CMD_COMPONENT_ARM_DISARM,
1,
1, 0, 0, 0, 0, 0, 0)
data[2] = 1600 # set some throttle 
master.mav.rc_channels_override_send(master.target_system, master.target_component, *data)
11

Are you connected to QGC (with a joystick) while you are doing this?

12

No. Running QGC without Joystick.

13

The relevant code in the firmware is here and here.

Please verify that you have disabled the failsafes.

14

Also, in QGC, try looking at Widgets->Mavlink Inspector->RC_CHANNELS_RAW to see if the values match what you send, or if they change unexpectedly.

16

I thought QGC was allowing me to disable all failsafes however after I rebooted it shows this. The “Unknown” tabs do not have a Disabled option.

29%20PM
29%20PM1840×1122 278 KB

17

What version of QGC are you using?

18

Ok after another reboot it appears all failsafes are reported disabled. However, the rc channel override still requires periodic update.

26%20PM
26%20PM2632×856 168 KB

19

I’ve tried your code, I had to change the link to udpin:0.0.0.0:14550. Everything is working here as I’ve indicated it should.

ArduSub 3.5.2
QGC 3.2.4 R4

20

Thank you Jacob! That works.

21

This should not be much overhead, you only have to send the message around ~1Hz. This will ensure your ROV doesn’t run away from you during your experiments :).