Zephyr Servo Controller

This repository contains a Zephyr RTOS based application for controlling servo motors via UART commands.

A companion app providing a web GUI is available under pkoscik/servo-ctrl-zephyr-gui-flask.

Quickstart

Ensure that the Zephyr SDK and west tool are installed. For setup instructions, refer to the Zephyr Getting Started Guide.

Cloning the project

west init -m https://github.com/pkoscik/servo-ctrl-zephyr servo-ctrl-zephyr
cd servo-ctrl-zephyr
west update

Building and flashing the Zephyr application

Note

This payload expects the servo{1,2,3} devicetree nodes to be configured. An example overlay file can be found in the nucleo_f767zi.overlay file. The following instructions will assume this board as a target.

west build -b nucleo_f767zi app

Flashing the Board

west flash --runner openocd

Usage

Warning

As part of the servo bootstrapping routine, the binary will set the servos to their mid-point upon reboot. Please be aware of the robot's dynamics and working envelope during reboots.

Send commands via UART to control the servos. The expected command format is:

S{id}:{position}\n

where:

• id is the servo ID (0, 1, or 2)
• position is the desired position percentage (0-100)

For example, sending S0:50 will set servo 0 to 50% of its range.

UART output

The (optional) UART output can be used for diagnosing issues with the board during initialization and operation. Below is an example of the output from a properly functioning board:

*** Booting Zephyr OS build v4.0.0 ***
[INFO] set_servo_pulse_safe: Setting channel: 1 to width: 1440000
[INFO] set_servo_pulse_safe: Setting channel: 2 to width: 1600000
[INFO] set_servo_pulse_safe: Setting channel: 3 to width: 1600000

After inputting a position, a confirmation message will be displayed:

# S2:0
[INFO] set_servo_pulse_safe: Setting channel: 3 to width: 700000

Incorrect inputs that could cause hardware damage will result in an error message being displayed, followed by a kernel panic:

# S10:0
[ERROR] set_servo_pulse_safe: Servo ID out of range: 10 (max 3)
[00:00:04.173,000] <err> os: r0/a1:  0x00000004  r1/a2:  0x35bb5418  r2/a3:  0x00000000
[00:00:04.173,000] <err> os: r3/a4:  0x00000004 r12/ip:  0x00005460 r14/lr:  0x080007b9
[00:00:04.173,000] <err> os:  xpsr:  0x01000000
[00:00:04.173,000] <err> os: Faulting instruction address (r15/pc): 0x080007c8
[00:00:04.173,000] <err> os: >>> ZEPHYR FATAL ERROR 4: Kernel panic on CPU 0
[00:00:04.173,000] <err> os: Current thread: 0x20020670 (unknown)
[00:00:04.224,000] <err> os: Halting system

# S1:150
[ERROR] set_servo_pulse_safe: Requested pulse out of reach: 3400000 (range: 700000 - 2500000)
[00:00:28.394,000] <err> os: r0/a1:  0x00000004  r1/a2:  0x6d904075  r2/a3:  0x00000001
[00:00:28.394,000] <err> os: r3/a4:  0x00000004 r12/ip:  0x00005460 r14/lr:  0x0800052f
[00:00:28.394,000] <err> os:  xpsr:  0x01000000
[00:00:28.394,000] <err> os: Faulting instruction address (r15/pc): 0x0800053e
[00:00:28.394,000] <err> os: >>> ZEPHYR FATAL ERROR 4: Kernel panic on CPU 0
[00:00:28.394,000] <err> os: Current thread: 0x20020670 (unknown)
[00:00:28.447,000] <err> os: Halting system

The board will need to be reset following such an incident.

Parse errors (and, in the future, other non-critical errors) will return an error message:

# foo:bar
e001: Invalid input format.

In this case, the payload will continue to operate without the need for a reset.

Hardware compability

This project has been tested with the Nucleo F767ZI board.

The servos use PWM channels 1-3 derived from the TIMER2 timer. The signals are pin-muxed to the following pins:

• Channel 1: pa5 (CN7, top right)
• Channel 2: pb3 (CN7, top left)
• Channel 3: pb10 (CN10, bottom right)