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171 lines
12 KiB
Markdown
171 lines
12 KiB
Markdown
# Ackermann Rover
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<Badge type="tip" text="main (PX4 v1.16+)" /> <Badge type="warning" text="Experimental" />
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An _Ackermann rover_ controls its direction by pointing the front wheels in the direction of travel — the [Ackermann steering geometry](https://en.wikipedia.org/wiki/Ackermann_steering_geometry) compensates for the fact that wheels on the inside and outside of the turn move at different rates.
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This kind of steering is used on most commercial vehicles, including cars, trucks etc.
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:::info
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PX4 does not require that the vehicle uses the Ackermann geometry and will work with any front-steering rover.
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:::
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## Basic Setup
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To start using the ackermann rover:
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1. Enable the module by flashing the [PX4 rover build](../frames_rover/index.md#flashing-the-rover-build) onto your flight controller.
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2. In the [Airframe](../config/airframe.md) configuration select the _Generic Rover Ackermann_:
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::: warning
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Do not use the _Generic Ground Vehicle (Ackermann)_ airframe as that will load the [(Deprecated) Rover Position Control](../frames_rover/rover_position_control.md) module.
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:::
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Select the **Apply and Restart** button.
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::: info
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If this airframe is not displayed and you have checked that you are using rover firmware (not the default), you can alternatively enable this frame by setting the [SYS_AUTOSTART](../advanced_config/parameter_reference.md#SYS_AUTOSTART) parameter to `51000`.
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:::
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3. Open the [Actuators Configuration & Testing](../config/actuators.md) to map the steering and throttle functions to flight controller outputs.
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This is sufficient to drive the the rover in [manual mode](../flight_modes_rover/index.md#manual-mode) (see [Drive modes](../flight_modes_rover/index.md)).
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:::info
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Many features of this module are disabled by default, and are only enabled by setting certain parameters.
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The [Tuning (basic)](#tuning-basic) section goes through the minimum setup required to start driving missions
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and the [Tuning (advanced)](#tuning-advanced) section outlines the remaining features and tuning variables of the module.
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:::
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## Tuning (Basic)
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To start driving missions navigate to [Parameters](../advanced_config/parameters.md) in QGroundControl and set the following parameters:
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| Parameter | Description | Unit |
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| ----------------------------------------------------------------------------------------------------------------------------------------------------------------------- | -------------------------------------------------------------------------- | ---- |
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| <a id="RA_WHEEL_BASE"></a>[RA_WHEEL_BASE](../advanced_config/parameter_reference.md#RA_WHEEL_BASE) | Wheel-base of the rover which is measured from the back to the front wheel | m |
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| <a id="RA_MAX_STR_ANG"></a>[RA_MAX_STR_ANG](../advanced_config/parameter_reference.md#RA_MAX_STR_ANG) | Maximum steering angle of the rover | deg |
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| <a id="RA_MISS_VEL_DEF"></a>[RA_MISS_VEL_DEF](../advanced_config/parameter_reference.md#RA_MISS_VEL_DEF) | Default velocity the rover will drive during the mission | m/s |
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This is enough to start driving missions, but depending on the rover might not yet lead to satisfactory performance .
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If that is the case further tuning is required which is outlined in [Mission parameters](#mission-parameters).
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## Tuning (Advanced)
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To get an overview of all parameters that are related to the Ackermann rover module navigate to the _Rover Ackermann_ group in the _Parameters_ section of QGroundControl.
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### General Parameters
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These parameters affect the general behaviour of the rover. This will influence both auto and manual modes.
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| Parameter | Description | Unit |
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| ----------------------------------------------------------------------------------------------------------------------------------------- | ------------------------------------------ | ---- |
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| <a id="RA_MAX_SPEED"></a>[RA_MAX_SPEED](../advanced_config/parameter_reference.md#RA_MAX_SPEED) | Speed the rover drives at maximum throttle | m/s |
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This is used for a feed-forward term on the speed controller in mission mode and necessary for the [acceleration slew rate](#slew-rates).
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#### Slew Rates
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Slew rates limit how fast the signal that is sent to the motors is allowed to change:
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| Parameter | Description | Unit |
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| ----------------------------------------------------------------------------------------------------------------------------------------------------------------------- | -------------------------------------- | ----- |
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| <a id="RA_MAX_ACCEL"></a>[RA_MAX_ACCEL](../advanced_config/parameter_reference.md#RA_MAX_ACCEL) | Limit on the acceleration of the rover | m/s^2 |
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| <a id="RA_MAX_STR_RATE"></a>[RA_MAX_STR_RATE](../advanced_config/parameter_reference.md#RA_MAX_STR_RATE) | Limit on the steering rate | deg/s |
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:::warning
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The slew rates are not based on measurements but on assumed linear relation between the throttle input and [RA_MAX_SPEED](#RA_MAX_SPEED) or steering input and [RA_MAX_STR_ANG](#RA_MAX_STR_ANG) respectively.
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Therefore these two parameters have to be set for the slew rates to work!
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:::
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## Mission Parameters
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These parameters only affect vehicle in [Mission Mode](../flight_modes_rover/index.md#mission-mode).
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:::warning
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The parameters in [Tuning (basic)](#tuning-basic) must also be set to drive missions!
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:::
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The module uses a control algorithm called pure pursuit, see [Pure Pursuit Guidance Logic](../flight_modes_rover/index.md#pure-pursuit-guidance-logic) for the basic tuning process.
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:::info
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Increasing [PP_LOOKAHD_MIN](../advanced_config/parameter_reference.md#PP_LOOKAHD_MIN) can help to make the steering less aggressive at slow speeds.
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This can be useful especially if the [corner slow down effect](#corner-slow-down) is enabled.
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:::
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### Cornering Parameters
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#### Corner cutting
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The module employs a special cornering logic causing the rover to "cut corners" to achieve a smooth trajectory.
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This is done by scaling the acceptance radius based on the corner the rover has to drive (for geometric explanation see [Cornering logic](#mission-cornering-logic-info-only)).
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The degree to which corner cutting is allowed can be tuned, or disabled, with the following parameters:
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| Parameter | Description | Unit |
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| ----------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ----------------------------------------------------- | ---- |
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| <a id="NAV_ACC_RAD"></a>[NAV_ACC_RAD](../advanced_config/parameter_reference.md#NAV_ACC_RAD) | Default acceptance radius | m |
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| <a id="RA_ACC_RAD_MAX"></a>[RA_ACC_RAD_MAX](../advanced_config/parameter_reference.md#RA_ACC_RAD_MAX) | Maximum radius the acceptance radius can be scaled to | m |
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| <a id="RA_ACC_RAD_GAIN"></a>[RA_ACC_RAD_GAIN](../advanced_config/parameter_reference.md#RA_ACC_RAD_GAIN) | Tuning parameter | - |
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The tuning parameter is a multiplicand on the calculated ideal acceptance radius to account for dynamic effects.
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#### Corner slow down
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To smoothen the trajectory further and reduce the risk of the rover rolling over, the mission speed is reduced as the rover gets closer to a waypoint:
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- During cornering the rover drives at a speed that is equal to the the inverse of the acceptance radius (calculated using the [corner cutting logic](#corner-cutting)) multiplied with a tuning parameter called [RA_MISS_VEL_GAIN](#RA_MISS_VEL_GAIN).
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- In between waypoints (straight line) the rover speed is regulated such that it will arrive at the acceptance radius of the waypoint with the desired cornering speed.
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This requires [RA_MAX_ACCEL](#RA_MAX_ACCEL) and [RA_MAX_JERK](#RA_MAX_JERK) to be set.
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The mission speed is constrained between a minimum allowed speed [RA_MISS_VEL_MIN](#RA_MISS_VEL_MIN) and the default mission speed [RA_MISS_VEL_DEF](#RA_MISS_VEL_DEF).
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| Parameter | Description | Unit |
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| -------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ----------------------------------------------------------- | ------- |
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| <a id="RA_MISS_VEL_DEF"></a>[RA_MISS_VEL_DEF](../advanced_config/parameter_reference.md#RA_MISS_VEL_DEF) | Default mission speed | $m/s$ |
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| <a id="RA_MISS_VEL_MIN"></a>[RA_MISS_VEL_MIN](../advanced_config/parameter_reference.md#RA_MISS_VEL_MIN) | Minimum the speed can be reduced to during cornering | $m/s$ |
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| <a id="RA_MISS_VEL_GAIN"></a>[RA_MISS_VEL_GAIN](../advanced_config/parameter_reference.md#RA_MISS_VEL_GAIN) | Tuning parameter for the velocity reduction | - |
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| <a id="RA_MAX_JERK"></a>[RA_MAX_JERK](../advanced_config/parameter_reference.md#RA_MAX_JERK) | Limit for forwards acc/deceleration change. | $m/s^3$ |
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### Mission Cornering Logic (Info only)
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To enable a smooth trajectory, the acceptance radius of waypoints is scaled based on the angle between a line segment from the current-to-previous and current-to-next waypoints.
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The ideal trajectory would be to arrive at the next line segment with the heading pointing towards the next waypoint.
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For this purpose the minimum turning circle of the rover is inscribed tangentially to both line segments.
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The acceptance radius of the waypoint is set to the distance from the waypoint to the tangential points between the circle and the line segments:
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$$
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\begin{align*}
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r_{min} &= \frac{L}{\sin\left( \delta_{max}\right) } \\
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\theta &= \frac{1}{2}\arccos\left( \frac{\vec{a}*\vec{b}}{|\vec{a}||\vec{b}|}\right) \\
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r_{acc} &= \frac{r_{min}}{\tan\left( \theta\right) }
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\end{align*}
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$$
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| Symbol | Description | Unit |
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| ----------------------------------- | ---------------------------------- | ---- |
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| $\vec{a}$ | Vector from current to previous WP | m |
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| $\vec{b}$ | Vector from current to next WP | m |
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| $r_{min}$ | Minimum turn radius | m |
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| $\delta_{max}$ | Maximum steer angle | m |
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| $r_{acc}$ | Acceptance radius | m |
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<!--
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## Parameters
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The following parameters affect the ackermann-steering rover:
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-->
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