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FOURIER MEASUREMENT SYSTEM

The CLAVIS Fourier Measurement System monitors the tension of the drive belt using an optical sensor head.  It has two forms, one for Electric Power Assisted Steering (EPAS) belts and the other for Front End Auxiliary Drive (FEAD) belts.

Electric Power Assisted Steering (EPAS)

Electric power assisted steering (EPS/EPAS) or motor-driven power steering (MDPS) uses an electric motor to assist the driver of a vehicle.  Sensors detect the position and torque of the steering column, and a computer module applies assistive torque via the motor, which connects to either the steering gear or steering column.  This allows varying amounts of assistance to be applied depending on driving conditions.

CLAVIS sets the belt tension on all EPAS units produced by the four volume manufacturers, TRW, Nexteer (Delphi), SKF and Hitachi.

Front-End Auxiliary Drive (FEAD)

Front-End Auxiliary Drive (FEAD) or accessory drive systems in motor vehicles are used to drive auxiliary equipment, such as alternator, coolant pump, power steering pump or A/C compressor.

Accessories are driven by a poly V-belt or Serpentine belt, the tension of which is precisely adjusted to the required loads using a mechanical or hydraulic tensioning system.  Guide pullies are used to create the required wrap angle around the front-end accessories.  Poly V-belts are designed to perform at high loads transmitting the engine torque – up to 350 Nm is not unusual in modern cars, without slip from the crankshaft to all its accessories.

CLAVIS Belt Adjustment System

The mass production of belt driven EPAS units has only been possible by the ability to set the belt tension accurately.  Due to packaging constraints the belt is very short with belt spans less than 100mm.  Early attempts at setting belt tension by either the use of load cells or torque measurement proved to be unsuccessful which has led to some of the world's leading EPAS manufacturers to come to CLAVIS for belt tension measurement equipment.  EPAS belts provide a unique technical challenge.

The CLAVIS Fourier Measurement System measures the natural frequency of vibration of a belt span.  This frequency is directly related to the tension in the belt.  As the tension in the belt is increased the frequency of vibration also increases.  The belt is forced into vibration by gently tapping or plucking it.  The vibration is usually not visible and also inaudible.

The relationship between the measured frequency and the tension of the belt should be determined from a calibration test on the belt span.  The relationship between belt tension (T) and frequency of vibration (f) may be calculated from knowing the mass per unit length of the belt (m), and the belt span (l), using the expression;

  • T = Newtons
  • f = Hertz
  • l = Meters
  • m =Kg/meter

The CLAVIS system monitors the tension of the drive belt using the gauge.  The gauge unit contains the optical sensor which recovers the belt vibration signal and the hammer assembly which forces the belt into vibration.

The gauge contains a solenoid activated hammer which is used to vibrate the belt.  An infra red optical sensor is employed to convert the belt vibration into an electrical signal which is processed by the control cabinet.  The gauge unit is connected to the control cabinet via a 2m cable.  A PC may be connected to the control cabinet to initiate measurements and record the subsequent belt vibration frequency.  The resulting belt frequency is shown on a LED display.  This may be particularly useful when first setting up the system.

The Optical Vibration Recovery System

The tension of the drive belt is determined by tapping the belt with an electro-mechanical hammer and monitoring the subsequent vibration using an infra-red optical sensor and associated electronics.

The hammer is pushed against the belt by a small solenoid within the gauge sensor head which is excited by a 10 ms 48V pulse generated within the control cabinet under microprocessor control.

The gauge sensor head contains a photo diode and a LED which are used to monitor the belt vibration.  The LED is driven with a 20 KHz square wave producing around 25 mW of modulated infra red light.  The light is reflected by the belt and detected by the photo diode.  The signal from the photo diode is electronically processed in the control cabinet by a technique known as synchronous demodulation which reduces unwanted environmental noise and recovers the signal modulation.

As the belt vibrates the amplitude of the modulated light entering the photo diode changes producing a subsequent change in the recovered signal.  The signal is then amplified and filtered.  The signal is fed into a Schmidt trigger to produce a square wave which is then fed into a microprocessor.  The microprocessor determines the frequency of oscillation of the belt by observing several vibration cycles.  In addition prior to the Schmidt trigger the analogue signal is sampled at a rate of 1.024 kHz in order to calculate the Fourier transform of the sample to determine the frequency spectrum.

Time-Domain Measurement and Frequency-Domain Measurement

The CLAVIS system analyses the belt signal in two different ways.  An interval based measurement in the time-domain and a Fourier based measurement in the frequency-domain.

The ideal belt would vibrate at a pure sinusoidal frequency.  The interval based measurement is ideal for determining the frequency of such a belt.  Not all belts vibrate in this ideal way - a belt may vibrate at several frequencies simultaneously which can be problematic for interval based frequency determination.

Fourier analysis can solve this problem, it allows a single waveform to be broken down into a series of single frequency sinusoids that can be combined together to reform the original waveform.

Using the Fourier transforms the belt signal into a spectrum showing all the frequency components in that waveform and the relative amplitude of those frequencies.  The CLAVIS system can perform this process live and show the spectrum of the belt signal and identify the peak frequency.

Good Spectrum Image

Belts vibrating at multiple frequencies have been noted as a particular problem for EPAS units that have a composite pulley as opposed to a metal pulley.  This combination of multiple sinusoids can result in a waveform that is problematic for interval-based analysis to determine the belt frequency and thus the tension.  In a situation when a pure frequency cannot be achieved through good positioning of the gauge, the Fast Fourier Transform (FFT) can be used to determine alternative belt tension setting criteria.

In a situation when a pure frequency cannot be achieved through good positioning of the gauge the FFT can be used to determine alternative belt tension setting criteria.

The EPAS Gauge Unit

Optical Sensor Unit

The sensor unit contains a high power infra red LED and a spectrally matched photo diode.

The unit should be kept clean and free from grease or oil. If the unit becomes damaged it may be replaced by removing the sensor head top cover.  The sensor wires should be unsoldered from the sensor PCB.  The unit may then be removed.  The new unit may then be fitted taking care to solder the wires correctly to the sensor PCB.

An air feed connector is present on the sensor head to prevent belt dust from gathering inside the sensor head.  Dry air should be fed to the sensor at 5 – 10 PSI.

Hammer Assembly

A small solenoid is used to throw a brass hammer against the belt.  The hammer is supported by two bearings and a restoring force is applied using a small spring.

The Fourier Control Cabinet

The control cabinet contains the system power supplies, interfaces with a PC/PLC to control measurements, operates the gauge unit, recovers the belt vibration signal and displays the resulting measurements on various displays and indicators.

  • Frequency LED Display - This displays the frequency measurement on a 7-segment LED display.
  • LCD Display - This displays either the scope output or the FFT output.  Illustrates the vibration signal both in the frequency and time domain.
  • Quality Factor Bar Graph Display
  • Belt 'Tight-O.K.-Slack' indicators
  • USB Port - Used to save an image of the view on LCD display.
  • Switched mode (universal input voltage) 5V PSU - Provides 5V for the analogue circuitry in the control cabinet and gauge unit.
  • Switched mode (universal input voltage) 48V PSU - The hammer power supply PCB provides 48V for the solenoid unit in the gauge unit.  The output is fused by a 1 amp slow blow fuse.
  • RS232 Serial connection (PL7) and Ethernet TCP client connection (PL8) allows control and result reporting to a PC/PLC.




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