Using VOXL ESCs with PX4

Table of contents

1. Overview
2. PX4 1.14 and Newer (eg. VOXL2) using Voxl ESC
   1. Actuator Setup using QGC
      1. Example Screen Shot with Labels
      2. Instructions using QGC
   2. Actuator Setup Using Command Line
   3. Actuator Testing Using Command Line
   4. PX4 Params
3. PX4 1.13 and Older
   1. Overview
   2. Video Tutorial
   3. Hardware Connections
   4. PX4 Setup (1.13 and older)
      1. Enable UART ESC
         1. TELEM3
   5. PX4 Params (1.13 and older)
   6. RGB LED Codes
4. Debugging Tips
   1. Setting Up PX4 Motor ID to UART_ESC_MOTORX
5. ESC Telemetry and Battery Status
   1. Which channels measure current
   2. battery_status from the ESC
   3. RPM limits: PX4 and ESC params must match
   4. Motor mapping and direction
   5. Reviewing ESC data in flight logs
6. Development

Overview

The VOXL 2, VOXL Flight and Flight Core systems provides the ability to easily and quickly interface with UART ESCs. The following guide provides you the necessary details on doing so.

PX4 1.14 and Newer (eg. VOXL2) using Voxl ESC

The modal_io driver was renamed to voxl_esc starting in SDK 1.1.1 and voxl-px4 version 1.14.0-2.0.58 or newer, additional information can be found in the release notes. The voxl_esc driver is used to interface with the VOXL ESC and can be configured using the QGroundControl Actuators tab or manually using the command line on target.

The driver is located at src/drivers/actuators/voxl_esc.

Actuator Setup using QGC

Example Screen Shot with Labels

• we’ve not yet seen the “Identify & Assign Motors” work with any flight controller at the time of this writing so please do not rely on using this.

Instructions using QGC

• Click on the Actuators tab
• In the Configure drop down (#1 in graphic above), select VOXL ESC
  • reboot the vehicle
• Inspect the wiring of your ESC to motors, e.g.:

• Use this for configuring the ESC Function (#2 in graphic). Note the silkscreen is 0 based… In the following example we have:
  • silksreen ID0 –> ESC 1 –> wired to PX4 motor 1 –> ESC 1 = Motor 1
  • silksreen ID1 –> ESC 2 –> wired to PX4 motor 4 –> ESC 2 = Motor 4
  • silksreen ID2 –> ESC 3 –> wired to PX4 motor 2 –> ESC 3 = Motor 2
  • silksreen ID3 –> ESC 4 –> wired to PX4 motor 3 –> ESC 4 = Motor 3
• Do not use “Rev Range” option
• Configure the Bitrate if needed (#3). For VOXL2, the default is 2000000, for FCv1/v2, use 250000.
• Configure the RPM Min and RPM Max based on your motor selection
• The ESC Command Type and ESC Minimum Power entries correspond to the VOXL_ESC_CMD and VOXL_ESC_PWR_MIN params below
• Using the Actuator Testing section, you can enable using a slider (#4) and use the motor sliders (#5) to spin the motors manually
  • Note: currently VOXL2 needs to use command line for this
• If needed, change the ESC Spin Direction for a given ESC ID (#6)

Actuator Setup Using Command Line

• Configure mapping using VOXL_ESC_FUNCX params, where:
  • Motor 1 = 101
  • Motor 2 = 102
  • Motor 3 = 103
  • Motor 4 = 104
• In the above example, we’d have:

  VOXL_ESC_CONFIG = 1
  VOXL_ESC_MODE = 0
  VOXL_ESC_FUNC1 = 101
  VOXL_ESC_FUNC2 = 104
  VOXL_ESC_FUNC3 = 102
  VOXL_ESC_FUNC4 = 103
  VOXL_ESC_SDIR1 = 0
  VOXL_ESC_SDIR2 = 0
  VOXL_ESC_SDIR3 = 1
  VOXL_ESC_SDIR4 = 0
  VOXL_ESC_RPM_MAX = 8000
  VOXL_ESC_RPM_MIN = 3000
  

Actuator Testing Using Command Line

• To spin motor 1 at 10% for 1 second, the following command can be used, in which we’d expect the front right motor to spin:

  actuator_test set -m 1 -v 0.1 -t 1

  (on VOXL 2, the command is prefixed: px4-actuator_test set -m 1 -v 0.1 -t 1)

• You can also drive an ESC directly through the driver, for example 1000 RPM for 64 command cycles on ESC ID 0:

  voxl_esc rpm -i 0 -r 1000 -n 64

PX4 Params

The following are the PX4 parameters that are used by the voxl_esc driver:

The following are used to control “Turtle Mode” (flip over after crash, which is currently in test):

PX4 1.13 and Older

Overview

This feature is currently in developer preview and requires loading the PX4 firmware from a branch.

• VOXL Flight or Flight Core with this branch
  • Note: this is scheduled to release with PX4 v1.12
• A ModalAI UART ESC VOXL ESC v2, PN: MDK-M0049-1-01 - 4-in-1 UART ESC (gen2)
• UART ESC Serial Cable MCBL-00013-1

Video Tutorial

For those of you who prefer videos, we have a video tutorial here that covers the software configuration and usage. Note, you’ll want to look at the hardware connections below as that part is skipped in this video.

TL;DR

Hardware Connections

The guide here describes the connectivity: VOXL ESC v2 Manual

PX4 Setup (1.13 and older)

Enable UART ESC

To enable the UART ESC feature, set the UART_ESC_CONFIG parameter to a value of 1 and power cycle the vehicle.

You can use QGroundControl’s Parameter feature to query and modify the UART_ESC params.

The remainder of the settings can be left as default if you followed the hardware connections described in the previous section.

TELEM3

The UART ESC uses /dev/ttyS1, same as TELEM3. On bootup, if any MAVLink instance is configured to use TELEM3, it will be disabled via boards/modalai/fc-v1/init/rc.board_defaults.

PX4 Params (1.13 and older)

See the params section above, replacing the VOXL_ESC prefix with UART_ESC — for example VOXL_ESC_CONFIG in 1.13 and older is UART_ESC_CONFIG.

RGB LED Codes

This is currently a work in progress. The standard LED codes should output to the all the LED channels in a similar way as other PX4 RGB LED peripherals with the exception that the breathing pattern is replaced by a fast blink.

Debugging Tips

Setting Up PX4 Motor ID to UART_ESC_MOTORX

Say you have the following:

M0049 silkscreen ID0 channel –> wired to a motor at PX4 Motor ID 2 M0049 silkscreen ID1 channel –> wired to a motor at PX4 Motor ID 3 M0049 silkscreen ID2 channel –> wired to a motor at PX4 Motor ID 1 M0049 silkscreen ID3 channel –> wired to a motor at PX4 Motor ID 4

Then you would have the following params set:

UART_ESC_MOTOR1 = 2 UART_ESC_MOTOR2 = 3 UART_ESC_MOTOR3 = 1 UART_ESC_MOTOR4 = 4

And you can test by spinning, for example, 1000 RPM for 64 command cycles:

# spin PX4 motor #1 (legacy 1.13 modalai_esc driver syntax)
modalai_esc -i 1 -r 1000 -n 64 rpm

# on current PX4, the equivalent through the voxl_esc driver is:
voxl_esc rpm -i 0 -r 1000 -n 64

ESC Telemetry and Battery Status

Each ESC channel streams telemetry (RPM, voltage, current, temperature, applied power) back over the shared UART. The four channels report round-robin at one quarter of the PX4 update rate — on VOXL 2, PX4 runs at 800Hz, so each channel reports at 200Hz. The voxl_esc driver publishes this as esc_status (note that PX4 logs it at a lower rate than it arrives).

Which channels measure current

This is by design, not a fault — on M0129/M0138 the per-channel current fields are simply not populated. On VOXL 2 with the FPV ESC, the total also includes the VOXL 2’s own current draw.

battery_status from the ESC

On boards with total-current sensing, the reporting channel emits an extra telemetry message carrying battery voltage and total current. When VOXL_ESC_PUB_BST is 1, the driver publishes it as battery_status — use this when the ESC also powers the system and there is no separate power module. When a power module (APM) is used, set VOXL_ESC_PUB_BST to 0 and let the voxlpm driver own battery reporting (POWER_MANAGER=VOXLPM in /etc/modalai/voxl-px4.conf; the Mini-ESC configuration is the exception since the ESC’s regulator powers the system). See the ESC FAQ for the full no-battery_status checklist.

A quick bench check that battery telemetry is flowing (props off, PX4 stopped):

cd /usr/share/modalai/voxl-esc-tools
./voxl-esc-spin.py --id 255 --power 0

--id 255 addresses all channels — watch for BOARD_VOLTAGE and BOARD_CURRENT in the output.

RPM limits: PX4 and ESC params must match

VOXL_ESC_RPM_MIN / VOXL_ESC_RPM_MAX in PX4 must equal the min_rpm / max_rpm in the ESC params. The PX4 values do not override the ESC — the ESC caps incoming commands itself — but a mismatch silently mis-scales PX4’s 0-100% thrust mapping.

Motor mapping and direction

The ESC’s channel numbering differs from PX4’s motor numbering. Map channels with VOXL_ESC_FUNC1-FUNC4 and reverse individual directions with VOXL_ESC_SDIR1-SDIR4, then confirm with an actuator test, props off.

Reviewing ESC data in flight logs

The standard PX4 Flight Review does not plot the VOXL ESC’s custom telemetry. ModalAI maintains a fork of flight-review that adds plots for commanded vs actual RPM, ESC temperatures, per-motor currents (where measured), and total battery current — use it when debugging ESC behavior from logs.

Development

The PX4 Driver source code can be found on github