Understanding Optical Tachometers
An optical tachometer is a non-contact speed-measurement device that uses light — a visible LED, a laser, or infrared — together with a photodetector to sense rotation, either by detecting reflections from a shaft marked with reflective tape or by sensing the interruption of a light beam. It does two jobs at once: it reports rotational speed in RPM, and it delivers a once-per-revolution timing pulse used as the phase reference in vibration analysis, field balancing, and order tracking. The term covers both handheld laser units — the most common type — and permanently installed optical sensors built around various light sources. Optical tachometers are closely related to laser tachometers, but the optical category is broader, taking in non-laser light sources as well.
1. Types of Optical Tachometer
1.1 Reflective Type (Most Common)
- Light source and detector share a single housing.
- The unit detects light reflected from a strip of reflective tape on the shaft.
- It works across a range of standoff distances, typically 50–500 mm.
- Handheld laser tachometers use this method.
- Simple, convenient, and portable — ideal for walk-up checks.
1.2 Through-Beam Type
- Light source and detector are separate units facing one another.
- The rotating object interrupts the beam as it turns.
- Each blade, spoke, or feature that crosses the beam creates a pulse.
- This allows multi-pulse-per-revolution measurement when desired.
- Common in permanently installed systems.
1.3 Fibre-Optic Type
- Light is transmitted and received through fibre-optic cables.
- The electronics sit remote from the measurement point.
- Useful in confined spaces, high electromagnetic interference, or explosive atmospheres.
- Intrinsically safe versions are available for hazardous areas.
2. Light Sources
The choice of emitter sets the working distance, spot size, and immunity to ambient light.
- Laser (red or IR): a coherent, focused beam giving a long working distance and a small spot for precise positioning — the best performance and the usual choice in handheld units.
- LED (visible or IR): incoherent light with a shorter working distance and larger spot, but lower cost; common in permanently installed sensors.
- Infrared (IR): invisible to the eye, less affected by ambient light, and therefore better in bright environments, with the safety advantage of no visible laser beam.
3. Applications
3.1 Speed Measurement
- Quick RPM checks during condition-monitoring surveys.
- Verifying nameplate running speed.
- Detecting speed variation under load.
- Calculating slip frequency in induction motors.
3.2 Vibration-Analysis Phase Reference
This is the role that makes the optical tachometer indispensable to a vibration specialist. By timing the vibration waveform against the once-per-revolution pulse, the analyser converts a delay into a phase angle:
- It provides the trigger for phase-locked measurements.
- It is essential for теңгеру, where phase determines the angular position of each correction weight.
- It enables order tracking on variable-speed equipment.
- It supports Bode plot generation during startup and coastdown.
3.3 Synchronous Measurements
- Triggering a stroboscope so a rotating mark appears frozen.
- Synchronising time-domain averaging to reject non-synchronous noise.
- Once-per-revolution sampling for order-based processing.
4. Advantages
The optical approach earns its place through three qualities:
- Non-contact operation: nothing touches the rotating part, so there is no friction or loading on the shaft, no speed limit imposed by the sensor, and no wear of a sensing element.
- Ease of use: apply a strip of tape, point, and measure — results are instant and the instrument is fully portable.
- Versatility: it works on virtually any rotating object across a wide speed range, with an adjustable working distance, and suits both temporary surveys and permanent installations.
5. Installation and Environmental Factors
For a permanent installation, mount the sensor at the recommended standoff, align its optical axis perpendicular to the shaft, apply the reflective tape at an accessible spot, protect the optics from contamination with a window if necessary, and provide adjustment for distance and aim. Several environmental factors degrade performance and deserve attention:
- Ambient light: bright sunlight can swamp the detector — use an IR source or shield the target.
- Contamination: oil mist and dust on the optics weaken the signal.
- Vibration: mount the sensor securely so it does not vibrate relative to the shaft.
- Temperature: stay within the sensor’s rating, typically −20 to +60 °C.
6. Best Practices and Troubleshooting
For reliable handheld readings, brace against a stable surface, aim at the centre of the reflective tape, hold the manufacturer’s recommended distance, shield from bright light, and take several readings to confirm consistency. When the pulse is used as a phase reference, treat the tape position as the 0° mark — note and document it — ensure a stable, clean signal, verify a single pulse per revolution, and check the waveform on an oscilloscope if anything looks doubtful. The common faults map to simple fixes:
- No signal: check standoff distance, clean the optics, confirm the tape is present, and check the battery.
- Unstable reading: reduce the distance, improve the tape, and shield from stray light.
- Multiple pulses (doubled RPM): remove extra tape pieces, keyway glints, or other reflective marks that fire a second pulse per turn.
On a portable balancer the optical tachometer is not an accessory but the timing backbone of the whole workflow. The Балансет-1А, for example, ships with an optical laser tachometer that triggers from a small piece of reflective tape, working at 50–500 mm standoff across a 250–90,000 rpm range; its once-per-revolution pulse supplies the phase reference the software needs to compute each balance weight and to verify the residual unbalance afterwards. Conceptually it behaves like a non-contact photoelectric sensor and serves the same function as a permanently installed keyphasor.
Optical tachometers — laser types in particular — have become indispensable in vibration analysis and balancing. Their non-contact operation, ease of use, accuracy, and dual function as both speed sensor and phase reference make them essential tools for vibration specialists, reliability engineers, and maintenance technicians working on rotating equipment in the field.
