Absolute Encoder (ABS)

Reads a magnetic absolute encoder on a joint and publishes its position and velocity on the CAN bus. Firmware lives in src/abs.

Absolute encoders keep their reading across a power cycle, so a joint knows where it is at boot without a homing sequence. This is what the arm’s differential and other joints use for position feedback.

Hardware

Interface	Peripheral	Notes
AS5047U magnetic encoder	SPI (DMA)	14-bit absolute rotary position
CAN	FDCAN1
Logging	LPUART1 (DMA)	see esw.stlink

Behavior

Two independent timers, both configured from flash at startup:

Timer	Rate	Purpose
htim16	poll_frequency	read the encoder
htim17	publish_frequency	send ABSEncoderState

Both default to 30 Hz in the .ioc and are overwritten by the configured values during init(). Decoupling them means you can poll fast for a smooth velocity estimate while publishing slowly to keep bus load down.

The raw encoder reading is transformed before publishing, using the configured options:

• invert flips the direction of travel.
• position_offset is subtracted, defining where zero is.
• output_scalar scales the result, which is how a gear ratio is applied.
• continuous_mode selects whether the reading wraps at a full turn or accumulates across turns.
• bounded_mode clamps the output to min_bound and max_bound.

CAN Interface

Receives

Message	Base ID	DLC	Signals
ABSResetCmd	0x80210000	1	reset, clear_faults
ABSZeroCmd	0x80220000	4	offset
ESWConfigCmd	0x80F00000	6	address, value, apply
ESWProbe	0x80F10000	4	data

Sends

Message	Base ID	DLC	Signals
ABSEncoderState	0x80200000	8	position (32, signed), velocity (32, signed)
ESWAck	0x80F20000	4	data

Zeroing

ABSZeroCmd takes the encoder’s current raw reading, writes it to the position_offset register and applies it immediately. The joint is therefore zeroed wherever it is standing when the command arrives, so move it to the reference position first.

Because position_offset is a configuration register, the new zero is written to flash and survives a power cycle.

tools/scripts/abs_zero.py does this for the arm’s differential pitch and roll encoders, but with hardcoded CAN IDs (0x37, 0x38) on can1. Read it before running it.

Configuration

Definition: config/abs.yaml. Device values live under rover/ra/, for example rover/ra/abs_de_pitch.yaml.

Register	Type	Meaning
can_id	uint8	this board’s CAN node ID
host_can_id	uint8	where to address published state, normally 0x10
sys_cfg	uint16	bit field: continuous_mode (0), bounded_mode (1), invert (2)
output_scalar	float32	applied to the raw reading, e.g. a gear ratio
position_offset	float32	subtracted from the raw reading; set by ABSZeroCmd
poll_frequency	float32	encoder read rate, Hz
publish_frequency	float32	CAN publish rate, Hz
min_bound	float32	lower clamp, used when bounded_mode is set
max_bound	float32	upper clamp, used when bounded_mode is set

28 bytes total. See Register Definitions and CAN Configuration Interface.

Building

./scripts/build.sh --src src/abs --preset Debug
./scripts/build.sh --src src/abs --preset Debug --flash

src/abs is in ci.json, so CI builds it at both presets.