Understanding Optical Tachometers
Definition: What is an Optical Tachometer?
Optical tachometer is a non-contact speed measurement device that uses light (visible LED, laser, or infrared) and a photodetector to sense rotation by detecting reflections from a rotating surface marked with reflective tape or by sensing interruptions of a light beam. Optical tachometers provide both rotational speed (RPM) measurements and once-per-revolution timing pulses used as phase reference in vibration analysis, field balancing, and order tracking.
The term “optical tachometer” encompasses both handheld laser tachometers (the most common type) and permanently-installed optical sensors using various light sources. They are closely related to laser tachometers but the optical category includes non-laser light sources as well.
Types of Optical Tachometers
1. Reflective Type (Most Common)
- Light source and detector in same housing
- Detects reflected light from reflective tape on shaft
- Works at various distances (typically 50-500 mm)
- Handheld laser tachometers use this method
- Simple, convenient, portable
2. Through-Beam Type
- Light source and detector in separate units facing each other
- Rotating object interrupts beam
- Each blade/spoke/feature creates pulse
- Can measure multi-pulse-per-revolution
- Used in permanently installed systems
3. Fiber Optic Type
- Light transmitted and received via fiber optic cables
- Electronics remote from measurement point
- Useful in confined spaces, high EMI, explosive atmospheres
- Intrinsically safe versions available
Light Sources
Laser (Red or IR)
- Coherent, focused beam
- Long working distance
- Small spot size (precise positioning)
- Best performance
- Most common in handheld units
LED (Visible or IR)
- Incoherent light
- Shorter working distance
- Larger spot size
- Lower cost
- Common in permanently installed sensors
Infrared (IR)
- Invisible to humans
- Less affected by ambient light
- Better in bright environments
- Safety advantages (no visible laser)
Applications
Speed Measurement
- Quick RPM checks during surveys
- Verify nameplate speeds
- Detect speed variations
- Calculate slip frequency in motors
Vibration Analysis Phase Reference
- Provides trigger for phase-locked measurements
- Essential for balancing (determines correction weight angle)
- Order tracking in variable-speed equipment
- Bode plot generation during startup/coastdown
Synchronous Measurements
- Trigger for stroboscope synchronization
- Time-domain averaging synchronization
- Once-per-revolution sampling
Advantages
Non-Contact Operation
- Safe (no contact with rotating parts)
- No friction or loading on measured shaft
- Works at any speed (no mechanical limitations)
- No wear or maintenance of sensing element
Ease of Use
- Simple tape application
- Point and measure
- Instant results
- Portable and handheld
Versatility
- Works on any rotating object
- Wide speed range
- Adjustable working distance
- Suitable for temporary and permanent installations
Installation Considerations
Permanent Installation
- Mount sensor at appropriate distance from shaft
- Align optical axis perpendicular to shaft
- Apply reflective tape at accessible location
- Protect optics from contamination (use protective window if needed)
- Provide adjustment for alignment and distance
Environmental Factors
- Ambient Light: Bright sunlight can interfere (use IR or shield)
- Contamination: Oil mist, dust on optics degrades signal
- Vibration: Secure mounting to prevent sensor vibration
- Temperature: Within sensor rating (typically -20 to +60°C)
Best Practices
For Handheld Use
- Hold steady, brace against stable surface
- Aim at center of reflective tape
- Maintain proper distance per manufacturer spec
- Shield from bright lights if possible
- Take multiple readings for verification
For Phase Reference
- Tape position becomes 0° reference—mark and document
- Ensure stable, clean tachometer signal
- Verify single pulse per revolution
- Check signal quality on oscilloscope if problems
Troubleshooting
- No Signal: Check distance, clean optics, verify tape, check battery
- Unstable Reading: Reduce distance, improve tape, shield from lights
- Multiple Pulses: Remove extra tape pieces or marks
Optical tachometers, particularly laser types, have become indispensable tools in vibration analysis and balancing. Their non-contact operation, ease of use, accuracy, and dual function (speed measurement and phase reference) make them essential instruments for vibration specialists, reliability engineers, and maintenance technicians performing field diagnostics and balancing work on rotating equipment.
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