An absolute encoder reports its angle immediately at power-on without needing to move to a reference. This is invaluable for mechanisms that must know their position before they move — swerve steering, arms, and wrists.
How absolute output is encoded#
Two common methods:
- Analog: the sensor outputs a voltage proportional to angle (0V-5V mapped across one rotation). You read it with WPILib's
AnalogEncoder. - Duty cycle (PWM): the sensor outputs a steady square wave whose pulse width encodes the angle. You read it on a single DIO channel with
DutyCycleEncoder.
The REV Through Bore Encoder#
The REV Through Bore Encoder (V1: part REV-11-1271; an updated V2 exists, REV-11-3174) is one of the most popular absolute encoders in FRC because it provides both outputs in one device (built around a Broadcom AEAT-8800 chip):
- Quadrature output (A, B, Index): 2048 cycles per revolution = 8192 counts per revolution at 4x decoding (incremental, high resolution for velocity and fine position).
- Absolute duty-cycle output: a 1025 microsecond period (~975.6 Hz) pulse, with pulse width from 1 us (0 degrees) to 1024 us (360 degrees), giving 10-bit resolution — read on one DIO line.
Its 1/2-inch hex through-bore (with inserts down to 3/8" hex, 5 mm hex, or 1/4" round) lets you slide it directly onto a shaft.
A common pattern: use the absolute channel once at startup to seed position, then use the high-resolution quadrature (or an integrated motor encoder) for fast feedback.
Other FRC absolute encoders#
WPILib documents several magnetic absolute encoders: the Thrifty Absolute Magnetic Encoder, Team 221 Lamprey2, US Digital MA3, AndyMark Mag Encoder, and the CTRE Mag Encoder (a duty-cycle/quadrature encoder, distinct from the CAN-based CTRE CANcoder, covered next).
Why absolute matters for swerve#
In a swerve module the steering (azimuth) angle must be known at boot so the wheels can snap to a known heading. Teams mount an absolute encoder (CANcoder or Through Bore) on each module and record a calibration offset so 'forward' reads zero.
the part worth keeping
Key takeaways
- Absolute encoders know their angle at power-on — essential for swerve steering, arms, and wrists.
- The REV Through Bore gives 8192-count quadrature AND a 10-bit duty-cycle absolute output in one device.
- A common pattern is to seed position from the absolute channel at boot, then rely on a high-rate incremental encoder.
Programming, Controls & SensorsEncoders: Measuring Position and Velocitylesson 2 of 3
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where this came from
Sources and corrections
This lesson is AI-assisted: drafted from primary sources, then reviewed and edited by hand. Errors still get through. When one is reported we fix it and write down what changed, in public, in the corrections log.
sources and further reading
- revrobotics.comREV Through Bore Encoder V1 (REV-11-1271)
- docs.revrobotics.comREV Through Bore Encoder specifications
- docs.wpilib.orgWPILib: Encoders (hardware)
clipped to this lesson
Articles that go further on this
The lesson gets you through the topic. These go wider on it, and they read in one sitting.
- 17 min readFRC Sensors Explained: Encoders, Gyros, Beam Breaks, and Limit SwitchesA practical FRC guide to encoders (quadrature, absolute, CANcoder, through-bore), gyros (NavX2, Pigeon 2.0), limit switches, beam breaks, and current sensing./blogread it
- 13 min readFRC Swerve Module Offsets: Calibration & Backwards WheelsZero your FRC swerve module offsets correctly, and fix wheels that spin backwards, modules that fight each other, and field-relative drive that feels rotated./blogread it
- 20 min readREV Robotics for FRC: NEO Motors, Spark MAX/Flex, PDH & MAXSwerveA practical guide to the REV Robotics FRC ecosystem: NEO, NEO Vortex and NEO 550 motors, Spark MAX and Spark Flex controllers, the PDH, and MAXSwerve./blogread it
answer sheet
Lesson quiz
All 3 right completes the lesson. Miss one and only that question comes back, anything you already answered correctly stays banked.
0 of 3 answered
01A duty-cycle (PWM) absolute encoder like the REV Through Bore in absolute mode encodes the shaft angle in what way on its signal wire?
02An analog absolute encoder reports the shaft's absolute position to the roboRIO using what kind of signal and port?
03Compared to an incremental quadrature encoder, what is the main advantage of an absolute encoder such as the REV Through Bore in its absolute mode?
Answer every question to submit.
All 51 lessons in Programming, Controls & Sensorsopenclose
01 / prerequisites
02 / foundations-tools-and-first-program
03 / robot-program-and-command-based
04 / motors-and-control
05 / autonomous-trajectories-simulation
06 / sensing-fundamentals
07 / encoders
08 / gyros-imus-orientation
09 / closed-loop-control
10 / vision-pose-estimation
11 / worked-examples-mini-projects
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12 / common-mistakes-troubleshooting
13 / advanced-techniques-case-studies
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