Configuration/Tuning (Mecanum Rover)

This topic provides a step-by-step guide for setting up your Mecanum rover. Successive steps enable drive modes with more autopilot support and features.

WARNING

Each step is dependent on the previous steps having been completed. Modes will only work properly if the preceding modes have been configured.

Basic Setup

To start using the mecanum rover:

1. Enable Rover support by flashing the PX4 rover build onto your flight controller. Note that this is a special build that contains rover-specific modules.

2. In the Airframe configuration select Generic Rover Mecanum frame:

   

   Select the Apply and Restart button.

   INFO

   If this airframe does not show up in the UI, it can alternatively be selected by setting the SYS_AUTOSTART parameter to 52000.

3. Use Actuators Configuration & Testing to map the motor functions to flight controller outputs.

Manual Mode

WARNING

For this mode to work properly the Basic Setup must've already been completed!

The basic setup already covers the minimum setup required to use the rover in Manual mode.

This mode is also affected by (optional) acceleration/deceleration limits. As configuration of these limits becomes mandatory for subsequent modes, we do this setup here.

Navigate to Parameters in QGroundControl and set the following parameters:

1. RM_WHEEL_TRACK [m]: Measure the distance from the centre of the right wheel to the centre of the left wheel.

   

2. RO_MAX_THR_SPEED [m/s]: Drive the rover at full throttle and set this parameter to the observed value of the ground speed.

   INFO

   This parameter is also used for the feed-forward term of the speed control. It will be further tuned in the configuration of Position mode.

3. (Optional) RO_ACCEL_LIM [m/s^2]: Maximum acceleration you want to allow for your rover.

   TIP

   Your rover has a maximum possible acceleration which is determined by the maximum torque the motor can supply. This may or may not be appropriate for your vehicle and use case.

   One approach to determine an appropriate value is:

   1. From a standstill, give the rover full throttle until it reaches the maximum speed.
   2. Disarm the rover and plot the measured_speed_body_x from RoverVelocityStatus.
   3. Divide the maximum speed by the time it took to reach it and set this as the value for RO_ACCEL_LIM.

   Some RC rovers have enough torque to lift up if the maximum acceleration is not limited. If that is the case:

   4. Set RO_ACCEL_LIM to a low value, give the rover full throttle from a standstill and observe its behaviour.
   5. Increase RO_ACCEL_LIM until the rover starts to lift up during the acceleration.
   6. Set RO_ACCEL_LIM to the highest value that does not cause the rover to lift up.

4. (Optional) RO_DECEL_LIM [m/s^2]: Maximum deceleration you want to allow for your rover.

   TIP

   The same considerations as in the configuration of RO_ACCEL_LIM apply.

   INFO

   This parameter is also used for the calculation of the speed setpoint during Auto modes.

Acro Mode

WARNING

For this mode to work properly Manual mode must've already been configured!

To set up Acro mode navigate to Parameters in QGroundControl and set the following parameters:

1. RO_YAW_RATE_LIM [deg/s]: This is the maximum yaw rate you want to allow for your rover. This will define the stick-to-yaw-rate mapping for all manual modes using closed loop yaw control and set an upper limit for the yaw rate setpoint for all auto modes.

2. RM_MAX_THR_YAW_R [m/s]: This parameter is used to calculate the feed-forward term of the closed loop yaw rate control. The controller calculates the required speed difference between the left and right motor to achieve the desired yaw rate. This desired speed difference is then linearly mapped to normalized motor commands. To get a good starting value for this parameter drive the rover in manual mode forwards at full throttle and note the ground speed of the vehicle. Then enter half this value for the parameter.

   TIP

   To further tune this parameter, first make sure you set RO_YAW_RATE_P and RO_YAW_RATE_I to zero. This way the yaw rate is only controlled by the feed-forward term, which makes it easier to tune. Now put the rover in Acro mode and then move the right-stick of your controller to the right and/or left and hold it at a few different levels for a couple of seconds each. Disarm the rover and from the flight log plot the adjusted_yaw_rate_setpoint from RoverRateStatus and the measured_yaw_rate from RoverRateStatus over each other. If the actual yaw rate of the rover is higher than the yaw rate setpoint, increase RM_MAX_THR_YAW_R. If it is the other way around decrease the parameter and repeat until you are satisfied with the setpoint tracking.

3. RO_YAW_RATE_P [-]: Proportional gain of the closed loop yaw rate controller. Unlike the feed-forward part of the controller, the closed loop yaw rate control will compare the yaw rate setpoint with the measured yaw rate and adapt to motor commands based on the error between them. The proportional gain is multiplied with this error and that value is added to the motor command. This compensates for disturbances such as uneven ground and external forces.

   TIP

   This parameter can be tuned the same way as RM_MAX_THR_YAW_R. If you tuned RM_MAX_THR_YAW_R well, you might only need a very small value.

4. RO_YAW_RATE_I [-]: Integral gain of the closed loop yaw controller. The integral gain accumulates the error between the desired and actual yaw rate scaled by the integral gain over time and that value is added to the motor command.

   TIP

   An integrator might not be neccessary at this stage, but it will become important for subsequent modes.

5. (Optional) RO_YAW_ACCEL_LIM and RO_YAW_DECEL_LIM [deg/s^2]: This is the maximum yaw acceleration and deceleration you want to allow for your rover. This can be used to smooth the yaw_rate setpoints and make their trajectory feasible based on the physical limitation on the rover to improve tracking and avoid integrator build up.

   TIP

   Your rover has a maximum possible yaw acceleration/deceleration which is determined by the maximum torque the motor can supply. This may or may not be appropriate for your vehicle and use case.

   One approach to determine an appropriate value is:

   1. Put the rover into Manual mode.
   2. From a standstill, move the right stick all the way to the right or left until the rover reaches the maximum yaw rate then return the right stick to the middle.
   3. Disarm the rover and plot the measured_yaw_rate from RoverRateStatus.
   4. Divide the maximum yaw rate by the time it took to reach it and set this as the value for RO_YAW_ACCEL_LIM.
   5. Divide the maximum yaw rate by the time it took to return to a standstill and set this as the value for RO_YAW_ACCEL_LIM.

The rover is now ready to drive in Acro mode.

Stabilized Mode

WARNING

For this mode to work properly Acro mode must've already been configured!

For Stabilized mode the controller utilizes a closed loop yaw controller, which creates a yaw rate setpoint to control the yaw when it is active:

Unlike the closed loop yaw rate, this controller has no feed-forward term. Therefore you only need to tune the closed loop gains:

1. RO_YAW_P [-]: Proportional gain for the closed loop yaw controller.

   TIP

   In stabilized mode the closed loop yaw control is only active when driving a straight line (no yaw rate input). Start with a value of 1 for RO_YAW_P. Put the rover into stabilized mode and move the left stick of your controller up and/or down to drive forwards/backwards. Disarm the rover and from the flight log plot the measured_yaw and the adjusted_yaw_setpoint from the RoverAttitudeStatus message over each other. Increase/Decrease the parameter until you are satisfied with the setpoint tracking.

INFO

For the closed loop yaw control an integrator gain is useful because this setpoint is often constant for a while and an integrator eliminates steady state errors that can cause the rover to never reach the setpoint. Since the yaw and yaw rate controllers are cascaded, there only needs to be one integrator which is in the yaw rate controller. If you observe a steady state error in the yaw setpoint increase the RO_YAW_RATE_I parameter.

The rover is now ready to drive in Stabilized mode.

Position Mode

WARNING

For this mode to work properly Stabilized mode must already be configured!

Position mode is the most advanced manual mode, utilizing closed loop yaw rate, yaw and speed control and leveraging position estimates.

To configure set the following parameters:

INFO

The speed control in longitudinal and lateral direction use the same tuning parameters.

1. RO_SPEED_LIM [m/s]: This is the maximum speed you want to allow for your rover. This will define the stick-to-speed mapping for position mode and set an upper limit for the speed setpoint for all auto modes. For mecanum rovers the speed is defined in the direction of travel (magnitude of the velocity vector consisting of the longitudinal and lateral speed).

2. RO_MAX_THR_SPEED [m/s]: This parameter is used to calculate the feed-forward term of the closed loop speed control which linearly maps desired speeds to normalized motor commands. A good starting point is the observed ground speed when the rover drives at maximum throttle in Manual mode.

   TIP

   To further tune this parameter:

   1. Set RO_SPEED_P and RO_SPEED_I to zero. This way the speed is only controlled by the feed-forward term, which makes it easier to tune.
   2. Put the rover in Position mode and then move the left stick of your controller up and/or down and hold it at a few different levels for a couple of seconds each.
   3. Disarm the rover and from the flight log plot the adjusted_speed_body_x_setpoint and the measured_speed_body_x from the RoverVelocityStatus message over each other.
   4. If the actual speed of the rover is higher than the speed setpoint, increase RO_MAX_THR_SPEED. If it is the other way around decrease the parameter and repeat until you are satisfied with the setpoint tracking.

   INFO

   If your rover oscillates when driving a straight line in Position mode just set this parameter to the observed ground speed at maximum throttle in Manual mode and complete steps 5-7 first before continuing the tuning of the closed loop speed control (Steps 2-4).

3. RO_SPEED_P [-]: Proportional gain of the closed loop speed controller.

   TIP

   This parameter can be tuned the same way as RO_MAX_THR_SPEED. If you tuned RO_MAX_THR_SPEED well, you might only need a very small value.

4. RO_SPEED_I [-]: Integral gain for the closed loop speed controller.

   TIP

   For the closed loop speed control an integrator gain is useful because this setpoint is often constant for a while and an integrator eliminates steady state errors that can cause the rover to never reach the setpoint.

5. PP_LOOKAHD_GAIN: When driving in a straight line (no yaw rate input) position mode leverages the same path following algorithm used in auto modes called pure pursuit to achieve the best possible straight line driving behaviour (Illustration of control architecture). This parameter determines how aggressive the controller will steer towards the path.

   TIP

   Decreasing the parameter makes it more aggressive but can lead to oscillations.

   To tune this:

   1. Start with a value of 1 for PP_LOOKAHD_GAIN
   2. Put the rover in Position mode and while driving a straight line at approximately half the maximum speed observe its behaviour.
   3. If the rover does not drive in a straight line, reduce the value of the parameter, if it oscillates around the path increase the value.
   4. Repeat until you are satisfied with the behaviour.

6. PP_LOOKAHD_MIN: Minimum threshold for the lookahead distance used by the pure pursuit algorithm.

   TIP

   Put the rover in Position mode and drive at very low speeds, if the rover starts to oscillate even though the tuning of PP_LOOKAHD_GAIN was good for medium speeds, then increase the value of PP_LOOKAHD_MIN.

7. PP_LOOKAHD_MAX: Maximum threshold for the lookahead distance used by pure pursuit.

   TIP

   Put the rover in Position mode and drive at very high speeds, if the rover does not drive in a straight line even though the tuning of PP_LOOKAHD_GAIN was good for medium speeds, then decrease the value of PP_LOOKAHD_MAX.

The rover is now ready to drive in Position mode.

Auto Modes

WARNING

For this mode to work properly Position mode must already be configured!

In auto modes the autopilot takes over navigation tasks using the following control architecture:

The mecanum module fully leverages the omnidirectionality of this type of rover by maintaining the initial heading of the rover during the entire mission (see Pure Pursuit Guidance Logic).

The required parameters are separated into the following sections:

Speed

These parameters are used to calculate the velocity setpoint in auto modes:

1. RO_SPEED_LIM: Sets the default speed ($m/s$) for the rover during the mission (as well as the maximum speed). For mecanum rovers the speed is defined in the direction of travel (magnitude of the velocity vector consisting of the longitudinal and lateral speed).

2. RO_DECEL_LIM ($m/s^2$) and RO_JERK_LIM ($m/s^3$) are used to calculate a velocity trajectory such that the rover comes to a smooth stop as it reaches a waypoint.

   TIP

   Plan a mission for the rover to drive a square and observe how it slows down when approaching a waypoint:

   • If the rover decelerates too quickly decrease the RO_DECEL_LIM parameter, if it starts slowing down too early increase the parameter.
   • If you observe a jerking motion as the rover slows down, decrease the RO_JERK_LIM parameter otherwise increase it as much as possible as it can interfere with the tuning of RO_DECEL_LIM.

   These two parameters have to be tuned as a pair, repeat until you are satisfied with the behaviour.

3. RM_MISS_SPD_GAIN: The rover slows down when approaching a waypoint based on the angle between the line segment from the previous to current waypoint and current to next waypoint. How much the rover slows down can be tuned with this parameter:

   $$v_{transition}=v_{max}\cdot (1-{\theta }_{normalized}\ast k)$$

   with:

   • $v_{transition}:$ Transition speed
   • $v_{max}:$ Maximum speed (RO_SPEED_LIM)
   • ${\theta }_{normalized}:$ Angle between the line segment of the prev-current and current-next waypoints, normalized from $[0\mathrm{°},180\mathrm{°}]$ to $[0,1]$
   • $k:$ Tuning parameter (RM_MISS_SPD_GAIN).

4. Plot the adjusted_speed_body_x_setpoint and measured_speed_body_x from the RoverVelocityStatus message over each other. If the tracking of these setpoints is not satisfactory adjust the values for RO_SPEED_P and RO_SPEED_I.

Path Following

The pure pursuit algorithm is used to calculate a desired bearing for the vehicle that is then close loop controlled. For mecanum rovers the path following is achieved by directly controlling the velocity of the rover in direction of the desired bearing. During any auto navigation task observe the behaviour of the rover:

1. If you are unsatisfied with the path following you need to further tune the pure pursuit parameters.
2. If the rover does not maintain its initial heading well enough, adjust the value for RO_YAW_P and potentially further tune RO_YAW_RATE_I.

Pure Pursuit Guidance Logic

The desired bearing setpoints are generated using a pure pursuit algorithm: The controller takes the intersection point between a circle around the vehicle and a line segment. In mission mode this line is usually constructed by connecting the previous and current waypoint:

The radius of the circle around the vehicle is used to tune the controller and is often referred to as look-ahead distance.

The look ahead distance sets how aggressive the controller behaves and is defined as $l_d=v\cdot k$. It depends on the velocity $v$ of the rover and a tuning parameter $k$ that can be set with the parameter PP_LOOKAHD_GAIN.

INFO

A lower value of PP_LOOKAHD_GAIN makes the controller more aggressive but can lead to oscillations!

The lookahead is constrained between PP_LOOKAHD_MAX and PP_LOOKAHD_MIN.

If the distance from the path to the rover is bigger than the lookahead distance, the rover will target the point on the path that is closest to the rover.

To summarize, the following parameters can be used to tune the controller:

Parameter	Description	Unit
PP_LOOKAHD_GAIN	Main tuning parameter	-
PP_LOOKAHD_MAX	Maximum value for the look ahead radius	m
PP_LOOKAHD_MIN	Minimum value for the look ahead radius	m

Parameter Overview

List of all parameters of the mecanum rover module:

Parameter	Description	Unit
RM_WHEEL_TRACK	Wheel track	m
RM_MAX_THR_YAW_R	Yaw rate turning left/right wheels at max speed in opposite directions	m/s
RM_MISS_SPD_GAIN	Tuning parameter for the velocity reduction during waypoint transition	-
RM_COURSE_CTL_TH	Threshold to update course control in manual position mode	rad
RO_YAW_RATE_LIM	Maximum allowed yaw rate for the rover	deg/s
RO_YAW_RATE_P	Proportional gain for yaw rate controller	-
RO_YAW_RATE_I	Integral gain for yaw rate controller	-
RO_YAW_P	Proportional gain for yaw controller	-
RO_SPEED_LIM	Maximum allowed speed for the rover (and default mission speed).	m/s
RO_MAX_THR_SPEED	Speed the rover drives at maximum throttle	m/s
RO_SPEED_P	Proportional gain for speed controller	-
RO_SPEED_I	Integral gain for speed controller	-
PP_LOOKAHD_GAIN	Main tuning parameter for pure pursuit	-
PP_LOOKAHD_MAX	Maximum value for the look ahead radius of the pure pursuit algorithm	m
PP_LOOKAHD_MIN	Minimum value for the look ahead radius of the pure pursuit algorithm	m
RO_ACCEL_LIM	(Optional) Maximum allowed acceleration	m/s^2
RO_DECEL_LIM	(Optional) Maximum allowed deceleration	m/s^2
RO_JERK_LIM	(Optional) Maximum allowed jerk	$m/s^3$
RO_YAW_ACCEL_LIM	(Optional) Maximum allowed yaw acceleration	m/s^2
RO_YAW_DECEL_LIM	(Optional) Maximum allowed yaw deceleration	m/s^2

See Also

• Drive Modes (Mecanum Rover).