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.