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117 lines
7.8 KiB
Markdown
117 lines
7.8 KiB
Markdown
# Differential-steering Rovers
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<Badge type="tip" text="main (PX4 v1.16+)" /> <Badge type="warning" text="Experimental" />
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:::warning
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Support for rover is [experimental](../airframes/index.md#experimental-vehicles).
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Maintainer volunteers, [contribution](../contribute/index.md) of new features, new frame configurations, or other improvements would all be very welcome!
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:::
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A differential-steering rover's motion is controlled using a differential drive mechanism, where the left and right wheel speeds are adjusted independently to achieve the desired forward speed and yaw rate.
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Forward motion is achieved by driving both wheels at the same speed in the same direction.
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Rotation is achieved by driving the wheels at different speeds in opposite directions, allowing the rover to turn on the spot.
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## Basic Setup
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To start using the differential-steering 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 Differential_:
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Select the **Apply and Restart** button.
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::: info
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If this airframe does not show up in the UI, it can alternatively be selected by setting the [SYS_AUTOSTART](../advanced_config/parameter_reference.md#SYS_AUTOSTART) parameter to `50000`.
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:::
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3. Open the [Actuators Configuration & Testing](../config/actuators.md) to map the motor 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 [Flight modes](../flight_modes_rover/index.md)).
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:::info
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The parameter [RD_MAN_YAW_SCALE](../advanced_config/parameter_reference.md#RD_MAN_YAW_SCALE) can be used to scale the manual input for the yaw rate.
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:::
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## Tuning (Basic)
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This section goes through the basic parameters that need to be set to use all other features for the differential-steering rover.
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Navigate to [Parameters](../advanced_config/parameters.md) in QGroundControl and set the following parameters:
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1. [RD_WHEEL_TRACK](../advanced_config/parameter_reference.md#RD_WHEEL_TRACK) [m]: Measure the distance from the center of the right wheel to the center of the left wheel.
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2. [RD_MAX_SPEED](../advanced_config/parameter_reference.md#RD_MAX_SPEED) [m/s]: In manual mode, drive the rover with full throttle and enter the observed speed as the parameter.
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3. [RD_MAX_YAW_RATE](../advanced_config/parameter_reference.md#RD_MAX_YAW_RATE) [deg/s]: This is the maximum yaw rate you want to allow for your rover.
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This will define the stick-to-yaw-rate mapping in acro mode as well as setting an upper limit for the yaw rate in mission mode.
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4. [RD_YAW_RATE_P](../advanced_config/parameter_reference.md#RD_YAW_RATE_P) and [RD_YAW_RATE_I](../advanced_config/parameter_reference.md#RD_YAW_RATE_I) [-]: Tuning parameters for the closed-loop yaw rate controller.
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::: info
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This can be tuned by setting all previous parameters and then setting the rover to _acro mode_.
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Use the right stick to yaw the rover on the spot and then observe the desired and actual yaw rate in the flight log.
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Change parameters and iterate.
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Suggestion: Start the tuning process with [RD_YAW_RATE_I](../advanced_config/parameter_reference.md#RD_YAW_RATE_I) equal to zero and only set if necessary.
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:::
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This is enough to start using the rover in acro mode.
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To start driving mission the parameters in [Tuning (Mission)](#tuning-mission) also must be set.
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## Tuning (Mission)
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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 [Mission Mode](../flight_modes_rover/index.md#mission-mode) for the basic tuning process.
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The additional parameters are separated into the following sections:
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### Mission Velocity
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These parameters tune velocity control in missions:
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- [RD_MISS_SPD_DEF](#RD_MISS_SPD_DEF): Sets the default velocity ($m/s$) for the rover during the mission.
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- [RD_MAX_ACCEL](#RD_MAX_ACCEL) ($m/s^2$) and [RD_MAX_JERK](#RD_MAX_JERK) ($m/s^3$) are used to calculate a velocity trajectory such that the rover comes to a smooth stop as it reaches a waypoint.
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- [RD_SPEED_P](#RD_SPEED_P) and [RD_SPEED_I](#RD_SPEED_I) are used to tune the closed-loop velocity controller during missions.
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### Yaw Rate
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The yaw rate setpoint is calculated by using the heading error calculated by the pure pursuit algorithm for a PID controller that can be tuned with [RD_HEADING_P](#RD_HEADING_P) and [RD_HEADING_I](#RD_HEADING_I).
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:::info
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There is some degree of overlap between this tuning and the pure pursuit controller gain set in [Mission Mode](../flight_modes_rover/index.md#mission-mode) as they both have an influence on how aggressive the rover will steer.
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:::
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### State Machine
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The module employs the following state machine to make full use of a differential-steering rovers ability to turn on the spot:
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These transition thresholds can be set with [RD_TRANS_DRV_TRN](#RD_TRANS_DRV_TRN) and [RD_TRANS_TRN_DRV](#RD_TRANS_TRN_DRV).
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### Parameters
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The following parameters affect the differential-steering rover in mission mode (overview):
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| Parameter | Description | Unit |
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| -------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | -------------------------------------------------------------- | ------- |
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| <a id="RD_MISS_SPD_DEF"></a>[RD_MISS_SPD_DEF](../advanced_config/parameter_reference.md#RD_MISS_SPD_DEF) | Mission speed for the rover | $m/s$ |
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| <a id="RD_MAX_ACCEL"></a>[RD_MAX_ACCEL](../advanced_config/parameter_reference.md#RD_MAX_ACCEL) | Maximum acceleration for the rover | $m/s^2$ |
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| <a id="RD_MAX_JERK"></a>[RD_MAX_JERK](../advanced_config/parameter_reference.md#RD_MAX_JERK) | Maximum jerk for the rover | $m/s^3$ |
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| <a id="RD_SPEED_P"></a>[RD_SPEED_P](../advanced_config/parameter_reference.md#RD_SPEED_P) | Proportional gain for speed controller | - |
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| <a id="RD_SPEED_I"></a>[RD_SPEED_I](../advanced_config/parameter_reference.md#RD_SPEED_I) | Integral gain for speed controller | * |
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| <a id="RD_HEADING_P"></a>[RD_HEADING_P](../advanced_config/parameter_reference.md#RD_HEADING_P) | Proportional gain for heading controller | - |
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| <a id="RD_HEADING_I"></a>[RD_HEADING_I](../advanced_config/parameter_reference.md#RD_HEADING_I) | Integral gain for heading controller | * |
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| <a id="RD_TRANS_DRV_TRN"></a>[RD_TRANS_DRV_TRN](../advanced_config/parameter_reference.md#RD_TRANS_DRV_TRN) | Heading error threshold to switch from driving to spot turning | deg |
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| <a id="RD_TRANS_TRN_DRV"></a>[RD_TRANS_TRN_DRV](../advanced_config/parameter_reference.md#RD_TRANS_TRN_DRV) | Heading error threshold to switch from spot turning to driving | deg |
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