Once-Per-Revolution Reference

A once-per-revolution reference is a timing pulse or angular mark associated with one known shaft position per turn. It allows measured vibration, proximity-probe displacement, orbit shape, order content or balance vector phase to be expressed relative to shaft angle instead of only relative to clock time.

If the reference pulse occurs at times t_k, then one revolution is the interval:

and the rotational frequency is:

The reference also defines an angular zero. A vibration phase angle \phi is only meaningful when the reference mark, rotation direction, sensor direction and phase convention are stated.

Engineering Role

A once-per-revolution reference is essential when an engineer needs repeatable phase, not just speed magnitude. It is used for:

• order tracking during run-up, coastdown and variable-speed operation;
• 1x amplitude and phase trends near critical speed;
• orbit orientation and precession interpretation;
• influence-coefficient field balancing;
• slow-roll runout or electrical-runout compensation;
• comparison of repeat tests at the same shaft angle convention;
• distinguishing synchronous shaft-order content from fixed-frequency content.

Without a stable reference, a spectrum can still show frequency peaks, but the engineer loses angular meaning. That makes balance weights, phase-lag trends, orbit key marks and compensation maps unreliable.

Phase from Time Delay

For a shaft rotating at angular speed \Omega, a time offset \Delta t corresponds to phase:

Using rotational frequency f_{rot}:

This relation is simple but important. A timing error that seems small in milliseconds can become a large phase error at high speed.

Delay, Filtering and Phase Bias

Reference timing must be aligned with the vibration or displacement channels that use it. If the reference channel and vibration channel have different delays, the measured phase contains a bias:

where t_{ref} is reference-channel delay and t_{sig} is signal-channel delay. The same problem can appear when the reference pulse is filtered differently from the vibration channel, when different acquisition devices are synchronized poorly, or when software timestamps are assigned after buffering.

This bias is different from jitter. Jitter varies from pulse to pulse and broadens uncertainty. A fixed delay offset can be repeatable and still wrong if the baseline and current tests use different filters, trigger thresholds, cable paths or acquisition modules.

For balancing, orbit orientation, Bode plots and slow-roll compensation, the acceptance record should state whether phase has been corrected for channel delay and whether the same convention is used across all repeat tests.

Worked Example: Convert Reference Jitter to Phase Error

A turbine vibration system uses a once-per-revolution optical reference. At a test speed of:

the rotational frequency is:

The reference pulse has measured timing jitter of:

The equivalent 1x phase uncertainty is:

If the engineer is balancing with a 1x vector of:

then the small-angle tangential vector uncertainty associated with phase jitter is approximately:

Engineering comment: the reference may be adequate for general trending, but a few degrees of jitter can matter for precision balancing or for comparing phase across a critical speed. The engineer should also check missing pulses, trigger threshold stability, target runout, signal-to-noise ratio, cable shielding, filtering and acquisition latency.

Distinction from Related Terms

Once-per-revolution reference is not a tachometer. A tachometer measures speed; it may provide the reference pulse, but the engineering function here is angular synchronization and phase.

Once-per-revolution reference is not an encoder. An encoder can provide many pulses per revolution and sometimes an index pulse. The once-per-revolution reference is the single angular zero used for phase convention.

Once-per-revolution reference is not order tracking. Order tracking is the analysis method. The reference is one of the inputs that lets the method follow shaft angle and speed.

Once-per-revolution reference is not an orbit plot. An orbit plot may include a reference mark to show angular position or precession direction, but the orbit remains the X-Y displacement display.

Once-per-revolution reference is not a balance correction. Balancing uses amplitude and phase referenced to shaft angle; the reference does not by itself determine the correction mass.

Validation and Common Mistakes

A defensible once-per-revolution reference states sensing principle, target geometry, pulses per revolution, reference angular location, rotation direction, trigger threshold, speed range, timing resolution, jitter, latency, missed-pulse handling, filtering, grounding, cable routing and how phase is reported.

Common mistakes include:

• changing the reference mark between baseline and repeat tests;
• mixing phase conventions, such as probe angle, rotation direction, lag angle and trial-weight angle;
• using an unstable or noisy pulse as if it were an exact angular zero;
• assuming a multi-tooth tachometer target automatically provides a unique once-per-revolution reference;
• filtering the reference and vibration channels with different delays;
• ignoring target runout, eccentricity, trigger hysteresis or weak signal-to-noise ratio;
• reporting a phase angle without stating the reference, sensor location and sign convention.