Wheel Encoders

Rover supports up to two “AB” wheel encoders like those in this Pololu motor which can be used to improve position estimation especially when no GPS is used.

Connection and Setup

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  • connect motor encoder’s A and B outputs to the autopilot (i.e. Pixhawk’s) AUX OUT 3,4,5 and 6 pins. Normally 3,4 should be used for the left motor’s encoder, 5,6 for the right’s.
  • set BRD_PWM_COUNT to 2 to allow AUX OUT 3 and 4 to be used as inputs

Note

in firmware versions 4.2 and later, the method for setting a PWM/SERVO/MOTOR outputs to be a GPIO function is changed. Instead of BRD_PWM_COUNT being used, the individual SERVOx_FUNCTION parameter is merely set to “-1”. If set to “0”, it remains a PWM output, unassigned to a function, and outputs that output’s trim value when board safety is not active. If the servo function is being “mirrored” to a remote device, as in the case of a DroneCAN or KDECAN ESC, then in order to change the autopilot board’s corresponding output pin to be a GPIO, but allow the SERVOx_FUNCTION to still be assigned to the remote device, the SERVO_GPIO_MASK parameter can be used to assign the board pin to be a GPIO without affecting the SERVOx_FUNCTION assignment for the remote device.

To enable position estimation using the wheel encoders the EKF3 must be used and configured as follows:

After making the above changes the autopilot should be rebooted

Ground Testing

The WHEEL_DISTANCE MAVLink message shows the total distance travelled by each wheel in real time. Mission Planner’s MAVLink Inspector can be used which can be opened by pressing Ctrl-F and then pushing the “MAVLink Inspector” button.

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DataFlash logging

The wheel encoder data including total distance travelled by each wheel appears in the WENC dataflash log messages.