Hello,
You may have noticed that your Alfred robot mower sometimes does not follow the path precisely (depending on slope, ground disturbance, etc. it is 10cm apart the planned mowing line). The reason for this might be the position of the GPS antenna. This is how to improve this.
In an ideal world, the GPS antenna is located directly at the center point between the wheels axle (aka 'steering point' of the robot, point Q in the image below).
Unfortunately, this is not the case with Alfred robot mower (actually, it's located a few cm's behind the correct position). Now we have to two choices to improve this:
1. Transform the physical GPS position (not exactly between the wheels) to a virtual position (exactly between the wheels) like it is done here ( https://forum.ardumower.de/threads/sunray-ce-a-simple-collection-of-fw-mods.25444/ ). This also requires that we have a precise orientation of the robot (yaw, pitch ,roll), so we know the exact orientation of the robot coordinate frame in the GPS world frame. This can be done, and I will try this out too later (however, because we have slope here, I doubt the IMU will be sufficient precise for such a transformation...).
2. We can move the GPS antenna to the correct position. I have done this today. The position estimation is improved and due to this, one can set the line tracking controller more 'aggressive' (you could try this without a changed GPS antenna, however then the robot will start to oscillate on the mowing line).
As you can see the GPS antenna position is now directly between the wheels axle (and not few cm's behind them):
The Stanley parameters I used for more 'aggressive' mowing line tracking (the sliders will appear if you toggle on the DEBUG button):
These parameters can be changed permanently in your config.h ( you can find out more about how to custom compile your firmware here: https://github.com/Ardumower/Sunray?tab=readme-ov-file#sunray_alfred ) :
And because the robot position estimation is more precise now, we can reduce the target threshold (in config.h) that informs the robot if it has reached a target position:
With these changes (GPS antenna position, Stanley parameters, target threshold) the mowing line tracking precision has improved here greatly.
Cheers,
Alexander
You may have noticed that your Alfred robot mower sometimes does not follow the path precisely (depending on slope, ground disturbance, etc. it is 10cm apart the planned mowing line). The reason for this might be the position of the GPS antenna. This is how to improve this.
In an ideal world, the GPS antenna is located directly at the center point between the wheels axle (aka 'steering point' of the robot, point Q in the image below).
Unfortunately, this is not the case with Alfred robot mower (actually, it's located a few cm's behind the correct position). Now we have to two choices to improve this:
1. Transform the physical GPS position (not exactly between the wheels) to a virtual position (exactly between the wheels) like it is done here ( https://forum.ardumower.de/threads/sunray-ce-a-simple-collection-of-fw-mods.25444/ ). This also requires that we have a precise orientation of the robot (yaw, pitch ,roll), so we know the exact orientation of the robot coordinate frame in the GPS world frame. This can be done, and I will try this out too later (however, because we have slope here, I doubt the IMU will be sufficient precise for such a transformation...).
2. We can move the GPS antenna to the correct position. I have done this today. The position estimation is improved and due to this, one can set the line tracking controller more 'aggressive' (you could try this without a changed GPS antenna, however then the robot will start to oscillate on the mowing line).
As you can see the GPS antenna position is now directly between the wheels axle (and not few cm's behind them):
The Stanley parameters I used for more 'aggressive' mowing line tracking (the sliders will appear if you toggle on the DEBUG button):
These parameters can be changed permanently in your config.h ( you can find out more about how to custom compile your firmware here: https://github.com/Ardumower/Sunray?tab=readme-ov-file#sunray_alfred ) :
Code:
// stanley control for path tracking - determines gain how fast to correct for lateral path errors
#define STANLEY_CONTROL_P_NORMAL 2.0 // 3.0 for path tracking control (angular gain) when mowing
#define STANLEY_CONTROL_K_NORMAL 0.5 // 1.0 for path tracking control (lateral gain) when mowingAnd because the robot position estimation is more precise now, we can reduce the target threshold (in config.h) that informs the robot if it has reached a target position:
Code:
// below this robot-to-target distance (m) a target is considered as reached
#define TARGET_REACHED_TOLERANCE 0.05 // 5 cm toleranceWith these changes (GPS antenna position, Stanley parameters, target threshold) the mowing line tracking precision has improved here greatly.
Cheers,
Alexander
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