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Rotational speed fluctuation
A measurement at a single position examines how angular velocity or angular position changes over time.
This can be used to determine rotational irregularity, angular velocity, angular acceleration and order components.
FUNDAMENTALS
What is torsional vibration?
Torsional vibration involves periodic or transient changes in the angular velocity of rotating components.
Real machines and drive systems are subject to time-varying torque, stiffness, inertia and external excitation.
ω(t) = ω₀ + Δω(t)
Analysing these fluctuations provides insight into the dynamic behaviour of engines, gearboxes, couplings, shafts, generators and complete powertrains.
UNDERSTANDING THE TERMINOLOGY
Speed fluctuation or relative torsional motion?
The terminology is often used interchangeably. For measurement, it is useful to distinguish what is observed at one position from the relative motion between positions.
Relative torsional motion
Measurements at two or more positions along a powertrain compare angular motion between those positions.
Δφ(t) = φ₁(t) − φ₂(t)
This makes it possible to investigate dynamic twist in shafts, couplings, gearboxes and complete powertrains.
MEASUREMENT PRINCIPLE
How is torsional vibration measured?
High-resolution torsional vibration measurement is based on determining the angular position of a rotating component over time as accurately as possible.
Gears, toothed discs, incremental encoders, magnetic encoders or optical targets produce defined angular events.
A sensor detects the signal edges. The measurement system determines their timing and uses this to calculate instantaneous angular velocity.
Rotation
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Sensor
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Signal edge
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Time measurement
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Analysis
TIME RESOLUTION
Precision starts with each signal edge.
Mean rotational speed is not the only factor in torsional vibration measurement. The timing precision of each individual signal edge also matters.
Higher time resolution in counter-based measurement can, in principle, resolve smaller changes in the timing of the rotational speed signal.
Time
resolution
Counter-based measurement
Measurement quality is not determined by the counter timebase alone. Sensor quality, signal-to-noise ratio, edge steepness, triggering, target geometry, run-out, manufacturing tolerances and the analysis method also affect the result.
THE COMPLETE MEASUREMENT CHAIN
The measurement chain is part of the result.
Reliable results depend on sensors, signal conditioning, data acquisition, analysis and an understanding of the mechanical system working together.
Sensors
The starting point for a reliable measurement signal.Suitable sensors and careful installation provide the basis for dependable measurement data and analysis.
Signal conditioning
Turning sensor outputs into usable measurement signals.Appropriate signal conditioning reduces interference and prepares the relevant information for data acquisition.
RASdelta data acquisition
Synchronised measurement data acquisition.RASdelta acquires rotational speed, analogue, CAN and other supported measurement quantities synchronously on a common timebase.
RAS software
Understand the data. Identify the relationships.RAS software visualises and analyses measurement data in the time, frequency and order domains.
Engineering insights
From measurement data to informed engineering decisions.Engineering expertise combines measurement data, system knowledge and experience to support technical assessments and recommendations.
MEASUREMENT SYSTEM & ANALYSIS
RASdelta and RAS software – a connected measurement workflow.
High-resolution rotational speed acquisition, synchronous analogue measurement and specialist analysis for rotating machinery and powertrains.
DATA ACQUISITION
RASdelta
High-resolution rotational speed, angle and torsional vibration measurement with synchronous acquisition of additional measurement quantities.
- High-resolution counter-based acquisition
- Analogue acquisition to suit the measurement task
- Synchronous rotational speed and angle measurement
- CAN integration
- Strain gauges and other supported measurement quantities
ANALYSIS
RAS software
Specialist analysis of dynamic measurement data for rotating systems and powertrains.
- Order spectrum
- Order tracking
- Campbell diagrams
- Torsional angle
- Transmission error
APPLICATIONS
Torsional vibration analysis in development and operation.
From combustion engines, electric drives and gearboxes to marine, wind energy and motorsport applications.
Combustion engines
Crankshaft dynamics, firing orders, resonances, auxiliary drives, couplings and dampers.
Electric drives
Analyse rotational speed, current, voltage, torque and vibration signals together.
Gearboxes
Gear mesh orders, transmission error, load dependence and input/output dynamics.
Marine
Consider the engine, coupling, gearbox, shaft line and propeller as a complete dynamic system.
Wind energy
Investigate gearbox dynamics, transmission error and load-dependent gear meshing phenomena.
Motorsport
Dynamic analysis at high rotational speeds and during rapid load changes.
ENGINEERING SERVICES
From the measurement task to the technical assessment.
Not every organisation needs its own torsional vibration measurement system. ROTEC can carry out complete measurement projects, from defining the measurement positions to interpreting and documenting the results.
Measurement concept
Mechanical integration
Sensor installation
Data acquisition
Analysis
Interpretation
Reporting
Discuss an engineering project
FAQ
Frequently asked questions about torsional vibration measurement.
How do you measure torsional vibration?
Torsional vibration is determined by measuring angular position, or the timing of defined angular events, at high resolution. The time intervals allow angular velocity and its fluctuations to be calculated.
Which sensors are suitable?
Depending on the application, rotational speed, available space and target, suitable options may include magnetoresistive sensors, GMR sensors, encoders, optical sensors or laser tachometers.
What is an order?
An order expresses frequency relative to the rotational frequency. The first order is the rotational frequency itself; the second order is twice that frequency.
What is order tracking?
Order tracking examines signal components relative to rotational speed. This allows speed-synchronous phenomena to be followed as rotational speed changes.
Can rotational speed and analogue signals be measured synchronously?
Yes. Synchronous acquisition allows rotational dynamics to be correlated with acceleration, torque, force, pressure or other supported measurement quantities.
What is the difference between speed fluctuation and relative torsional motion?
A single-position measurement describes fluctuations in angular velocity at that position. A multi-position torsional measurement examines the relative angular motion between at least two positions.
YOUR MEASUREMENT TASK
Planning a torsional vibration measurement?
Our measurement and engineering specialists can work with you to assess a suitable measurement setup for your application.
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