What is a Tracking Filter? Order-Based Filtering • Portable balancer, vibration analyzer "Balanset" for dynamic balancing crushers, fans, mulchers, augers on combines, shafts, centrifuges, turbines, and many others rotors What is a Tracking Filter? Order-Based Filtering • Portable balancer, vibration analyzer "Balanset" for dynamic balancing crushers, fans, mulchers, augers on combines, shafts, centrifuges, turbines, and many others rotors

What is a Tracking Filter? Order-Based Filtering

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Understanding Tracking Filters

Definition: What is a Tracking Filter?

Tracking filter (also called order tracking filter or synchronous filter) is a narrow bandpass filter in vibration analysis instruments that automatically adjusts its center frequency to follow a multiple (order) of the machine’s rotational speed. For example, a “1× tracking filter” continuously tracks the running speed frequency, filtering out all other frequencies and passing only the fundamental 1× component. Similarly, 2× and 3× tracking filters follow twice and three times the running speed.

Tracking filters are essential tools for analyzing variable-speed equipment, startup/coastdown transients, and for isolating specific order components in order analysis. They enable measurement of amplitude and phase of synchronous components even as machine speed changes.

How Tracking Filters Work

Basic Principle

  1. Speed Reference: Tachometer or keyphasor provides once-per-revolution pulse
  2. Frequency Calculation: Instrument calculates instantaneous rotational frequency from tachometer
  3. Order Multiplication: Multiplies rotational frequency by order number (1, 2, 3, etc.)
  4. Filter Centering: Narrow bandpass filter centered at calculated frequency
  5. Continuous Adjustment: As speed changes, filter frequency tracks continuously
  6. Output: Filtered signal containing only the selected order component

Filter Characteristics

  • Bandwidth: Typically ±2-10% of center frequency
  • Narrowness: Rejects nearby frequencies effectively
  • Tracking Rate: Can follow rapidly changing speeds
  • Multiple Filters: Modern instruments provide simultaneous multiple order tracking

Applications

1. Startup and Coastdown Analysis

Primary application for tracking filters:

  • Track 1× amplitude and phase vs. speed during transients
  • Generate Bode plots (amplitude and phase vs. speed)
  • Identify critical speeds from amplitude peaks
  • Measure damping from resonance peak width
  • Track 2×, 3× simultaneously to identify multiple modes

2. Variable Speed Equipment Analysis

  • Maintain order-based measurements despite speed variations
  • VFD-driven motors with continuously varying speed
  • Wind turbines with varying wind speeds
  • Process equipment with load-dependent speed changes
  • Enables consistent trending regardless of speed fluctuations

3. Balancing

  • Track 1× component during balancing procedure
  • Filter out non-1× components for cleaner measurement
  • Phase measurement at 1× frequency only
  • Improves accuracy by rejecting other vibration sources

4. Order-Specific Analysis

  • Isolate specific orders for detailed study
  • Example: Track 2× to monitor misalignment progression
  • Track blade passing order in fans/pumps
  • Separate overlapping frequency components

Advantages of Tracking Filters

Speed Independence

  • Measurements meaningful regardless of speed variations
  • Compare data from different speeds on same basis (orders)
  • Essential for equipment without constant speed

Component Isolation

  • Separates specific order from all other frequencies
  • Cleaner signals than full-spectrum FFT
  • Better signal-to-noise ratio for order components
  • Enables precise amplitude and phase measurement

Transient Analysis

  • Track components through speed changes
  • Continuous measurement during acceleration/deceleration
  • No need for steady-state conditions
  • Reveals speed-dependent behavior

Limitations and Considerations

Requires Tachometer

  • Accurate speed reference essential
  • Tachometer signal quality affects filter performance
  • Cannot use on equipment without speed reference
  • Once-per-revolution pulse must be reliable

Only Tracks Synchronous Components

  • Non-synchronous faults not captured (most bearing defects)
  • Electrical frequencies not tracked
  • Random vibration filtered out
  • Must use complementary analysis for complete diagnosis

Filter Bandwidth Trade-offs

  • Narrow Filter: Better rejection of adjacent frequencies but slower response to speed changes
  • Wide Filter: Faster tracking but may include nearby components
  • Optimal: Typically 5-10% bandwidth for most applications

Tracking Filter vs. FFT

Feature FFT Analysis Tracking Filter
Speed Requirement Works at any speed Requires tachometer
Speed Variation Requires steady speed Handles varying speed
Information Full spectrum, all frequencies Single order only
Non-Synchronous Faults Detects all faults Misses non-synchronous
Transient Analysis Difficult Excellent
Best For General diagnostics, steady-state Critical speed analysis, variable speed

Modern Implementations

Digital Tracking Filters

  • Software-based filters in modern analyzers
  • Multiple simultaneous orders (1×, 2×, 3× concurrently)
  • Adjustable bandwidth
  • Real-time display during transients

Order Analysis Integration

  • Tracking filters as foundation of comprehensive order analysis
  • Full order spectrum extracted (all orders simultaneously)
  • Color maps showing order vs. speed
  • Automated critical speed detection from order tracking data

Tracking filters are specialized but powerful tools in vibration analysis, particularly for rotor dynamics and variable-speed equipment. By maintaining focus on specific orders despite speed changes, tracking filters enable transient analysis and speed-independent component monitoring that would be impossible with standard FFT techniques, making them essential for critical speed identification and advanced machinery diagnostics.

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