What is a Laser Tachometer? Non-Contact Speed Measurement • Portable balancer, vibration analyzer "Balanset" for dynamic balancing crushers, fans, mulchers, augers on combines, shafts, centrifuges, turbines, and many others rotors What is a Laser Tachometer? Non-Contact Speed Measurement • Portable balancer, vibration analyzer "Balanset" for dynamic balancing crushers, fans, mulchers, augers on combines, shafts, centrifuges, turbines, and many others rotors

What is a Laser Tachometer? Non-Contact Speed Measurement

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Understanding Laser Tachometers

Definition: What is a Laser Tachometer?

Laser tachometer is a non-contact optical speed measurement device that uses a laser beam reflected from a rotating surface to measure rotational speed (RPM) and provide once-per-revolution timing pulses for phase reference in vibration analysis and balancing. Typically, reflective tape is applied to the shaft or rotating component, and the laser tachometer detects the reflection pulse each revolution, calculating speed from the pulse rate and providing a trigger signal for phase-locked vibration measurements.

Laser tachometers have largely replaced contact tachometers and magnetic pickups for vibration work due to their convenience (no shaft preparation beyond applying tape), safety (no contact with rotating parts), and accuracy. They are essential tools for field balancing, order analysis, and any vibration measurement requiring speed and phase information.

Operating Principle

Reflective Tape Method (Most Common)

  1. Tape Application: Small piece of reflective tape applied to shaft
  2. Laser Emission: Tachometer emits visible laser beam (typically red, 650 nm)
  3. Reflection Detection: Photodetector senses reflected light intensity
  4. Pulse Generation: When tape passes, strong reflection creates pulse
  5. Speed Calculation: Time between pulses = rotation period; RPM = 60 / period
  6. Phase Reference: Rising edge of pulse marks 0° reference position

Surface Contrast Method

  • Detects natural surface features (keyways, marks, color changes)
  • No tape required if sufficient contrast
  • Less reliable than reflective tape
  • Useful for quick checks

Key Features and Specifications

Speed Measurement

  • Range: Typically 10-250,000 RPM
  • Accuracy: ±0.01-0.05% of reading
  • Update Rate: Real-time display (multiple times per second)
  • Resolution: 0.1 RPM typical

Distance (Working Range)

  • Typical: 50-500 mm (2-20 inches) from target
  • Depends on laser power and reflective tape quality
  • Too close: spot size too small
  • Too far: insufficient reflected light

Output Signals

  • Digital Display: RPM readout on screen
  • Analog Output: Voltage proportional to speed (0-10V typical)
  • Pulse Output: TTL or logic pulse once per revolution
  • Direction: Some models detect rotation direction

Applications in Vibration Analysis

Field Balancing

  • Provides once-per-revolution phase reference
  • Tape position marks 0° for phase measurements
  • Speed verification during balancing runs
  • Essential for influence coefficient method

Order Analysis

  • Speed signal enables order tracking
  • Tracking filters use tachometer for synchronization
  • Variable-speed equipment analysis
  • Startup/coastdown testing

Phase Measurement

  • Tachometer pulse triggers phase measurement
  • Determines timing of peak vibration
  • Critical for balancing and diagnostics
  • Phase accuracy depends on stable tachometer signal

Speed Verification

  • Quick RPM checks during surveys
  • Verify nameplate speed
  • Detect speed variations
  • Measure actual vs. synchronous speed for slip calculation

Reflective Tape

Types and Selection

  • Retroreflective Tape: Reflects light back to source, most effective
  • Aluminum Tape: Good reflection, economical
  • White Tape: Adequate for many applications
  • Size: 10-25 mm (0.5-1 inch) typical

Application Best Practices

  • Clean surface before applying
  • Apply on smooth, cylindrical shaft section
  • Avoid location where tape could contact stationary parts
  • Single piece per revolution (multiple pieces confuse instrument)
  • Secure edges to prevent peeling at high speed
  • Mark angular position if using for balancing reference

Advantages Over Other Tachometers

vs. Contact Tachometer

  • Laser: No contact, safer, no shaft damage, works at any speed
  • Contact: Physical contact, friction, limited speeds, potential damage

vs. Magnetic Pickup

  • Laser: Works on any material, simple tape application, precise positioning
  • Magnetic: Requires ferrous target, permanent installation, less positioning flexibility

vs. Strobe Light

  • Laser: Direct measurement, quantitative, phase reference output
  • Strobe: Visual observation only, matching not measuring, no phase signal

Common Issues and Solutions

Unstable or Missing Signal

  • Causes: Dirty optics, wrong distance, poor tape, ambient light interference
  • Solutions: Clean lens, adjust distance, replace tape, shield from bright lights

Incorrect Speed Reading

  • Multiple Tape Pieces: Reading multiples of actual speed
  • Reflective Surface: Detecting features other than tape
  • Solution: Ensure only one reference mark per revolution

Phase Measurement Errors

  • Tape position shifted from original reference
  • Tape peeling or moving during operation
  • Solution: Secure tape properly, verify position, reapply if needed

Laser tachometers are essential tools for modern vibration analysis and balancing, providing safe, accurate, non-contact speed and phase measurements. Their combination of convenience, precision, and versatility has made them the standard for field vibration work, replacing older contact and magnetic tachometer technologies in most industrial applications.

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