RTV AND RUN OUT DISC BRAKE MEASUREMENT SYSTEM
GENERAL
DESCRIPTION
Release 02c
CAPAAB 22 / 32 Sentier des Torques BAT 8 92290 Chatenay Malabry Tel / Fax : 01 71 22 73 72
jul. 2010
www.capaab.fr
RTV & RUN OUT MEASUREMENTS
Copyright CHATENAY 2001 CAPAAB
2/11
RTV AND RUN OUT DISC BRAKE MEASUREMENT SYSTEM GENERAL
DESCRIPTION
CONTENTS CAPAAB : THE COMPANY RELIABLE NO CONTACT DISTANCE MEASUREMENTS PRINCIPLE OF OPERATION
A UNIQUE SOLUTION : FROM SENSOR TO ACQUISITION FROM CALIBRATION TO REAL TIME DISTANCE DISPLAY FROM DEVELOPMENTS TO ONBOARD SYSTEMS
CAPAAB 22 / 32 Sentier des Torques BAT 8 92290 Chatenay Malabry Tel / Fax : 01 71 22 73 72
www.capaab.fr
RTV & RUN OUT MEASUREMENTS
Copyright CHATENAY 2001 CAPAAB
3/11
CAPAAB : THE COMPANY The Company provides products and CAPAAB original solutions mainly around technical implementing capacitive sensors and conditioners. Indeed, both the capacitive principle is known and has long been applied in the field of measurement, as it has never been easy to draw all the possibilities. Indeed, the representation of a capacitive sensor is not as simple as a capacitor banal: there are the parasitic capacitances that can have values millions times greater than the value to be measured! This is where the expertise comes in: through conditioning techniques used by CAPAAB, the parasitic elements are eliminated. This opens the way for a variety of applications ranging from distance measurements nanoscale distance measurement or vibrations to hundreds of kilohertz, for example. Besides the technical capacitive CAPAAB has expertise in the techniques inductive / resistive as well as acquisition and data processing.
CAPAAB 22 / 32 Sentier des Torques BAT 8 92290 Chatenay Malabry Tel / Fax : 01 71 22 73 72
www.capaab.fr
RTV & RUN OUT MEASUREMENTS
Copyright CHATENAY 2001 CAPAAB
4/11
RELIABLE NO CONTACT DISTANCE MEASUREMENTS Reliable medium or high temperature no contact distance measurement involves for instance compressor and turbine blade tip clearance, rotor shaft, disk displacement measurements in aeronautical industry. RTV and Run Out disk measurements are typical application in car industry. For medium temperature range, the CCHT capacitive probe from CAPAAB suit to a wide series of application like disc brake Rotation Thickness Variation and run out measurements. The co-operation between CAPAAB and the world wide known THERMOCOAX company, whose business is the manufacture of mineral insulated cable and derived products, including heating elements, thermocouples and capacitive probes, results in a wide range of solutions. So, from medium temperature probes up to 1300 °C probes (without cooling), we can provide a complete multi-channel solution (up to 8) with real time processing and display of data in the well known LabView software environment. CCHT CAPACITIVE PROBE
800 °C THERMOCOAX CAPACITIVE PROBE
CAPAAB 22 / 32 Sentier des Torques BAT 8 92290 Chatenay Malabry Tel / Fax : 01 71 22 73 72
www.capaab.fr
RTV & RUN OUT MEASUREMENTS
Copyright CHATENAY 2001 CAPAAB
5/11
PRINCIPLE OF OPERATION The principle of no contact distance measurement is given below : The system includes mainly two capacitive probes fitted on each side of the disc to be characterised. So, the capacitance changing between each probe inner electrode and the facing disc is related to the distance. The instant value the capacitance reaches is the direct result of variation in distance. Disc CAPACITIVE Max lenght = 50 m PROBE Out signal ACQUISITION Display PROBE 1 CONDITIONING & DIGITAL PROCESSING Storage PROBE CABLE
PROBE 2 CONDITIONING
Typical lenght 5 meters
The rough signal coming from the probe conditioning unit (A output) looks like the sketch below : as the probe is facing a almost constant distance, the signal is close to a constant and its small variation is due to RTV and run out. Actual values are related to the distance Output 1
1 disc rotation time The 1 signal is an example of real result. The max value is due to run out.
Output 2
time
The two signals are digitized and processed to display the RTV and mean runout values in engineering units.
RTV = Disc thickness variation during a revolution CAPAAB 22 / 32 Sentier des Torques BAT 8 92290 Chatenay Malabry Tel / Fax : 01 71 22 73 72
The output 2 signal looks like the graph on the left picture.
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RTV & RUN OUT MEASUREMENTS
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Copyright CHATENAY 2001 CAPAAB
Run out = Inner and outer side of the disc distance variation The processing unit is used to convert the digitized voltage into mm or inches thanks to the polynomial representation of the calibration curve, stored in the application program. The typical aspect of the capacitive probe calibration curve is given below. CC3 HTa D ISTAN CE CALIB R ATION 4
3
2
Dis tan ce = 2,1108E-06x - 1,5395E-04x + 6,8971E-04x + 3,1124E-01x - 5,4706E-02
3,5 Vout 3 C onditioner
DISTANCE (mm)
2,5 Probe
2
distance Target (grounded)
1,5
1
0,5 Vout (Volt) 0 0
2
4
6
8
10
To get this calibration curve, no tricky device is needed. Static calibration is used by fitting a flat metallic target on a simple mechanical unit facing the probe. This target can be the disc itself or made with any metallic flat sheet, even if the disc tested is carbon made. A driving screw with a reading device allows the measurement of the actual distance. The corresponding voltage is a DC value, read on a voltmeter. Thanks to a spreadsheet, the polynomial expression of this calibration curve is obtained. The constant and the coefficients are loaded in the system software in order to display the clearance in mm or inches in real time. In the ideal case, the capacitance C is related to the distance d which separates two plates (S surface) surrounded by a media of dielectric constant ε , by the expression : C = S ε / d
CAPAAB 22 / 32 Sentier des Torques BAT 8 92290 Chatenay Malabry Tel / Fax : 01 71 22 73 72
www.capaab.fr
RTV & RUN OUT MEASUREMENTS
Copyright CHATENAY 2001 CAPAAB
7/11
The block diagram of the capacitive conditioning unit is given below : AMPLITUDE MODULATION
C
PROBE BIAIS SYSTEM R
GUARD CONDUCTOR
k Vout Cx
Ipol
Ie
AMPLIFICATION +
Iref
SYNCHRONOUS DEMODULATION
Vout
PROBE
Cref REFERENCE CAPACITY CAPACITANC
E
Vref
OSCILLATOR
The system is built around a carrier frequency bridge. The main bridge functions are used : oscillator, amplifier, synchronous demodulator . The originality of this bridge is to be servo balanced by the use of a loop built around a "perfect" comparator (active output RC integrator) and an amplitude modulator. The summarised description of the system is given in inverse of capacitance mode that is to say linear vs distance. The servo loop is used to adjust the Ipol current level in order to make it equal to Iref current. The Ie current is then equal to 0. The modulator drives the probe with a voltage (k.Vout) whose amplitude is the image of the output voltage Vout. When equilibrium is reached, Ipol = jω k .Vout . Cx . and Iref = jω . Vref . Cref . (where ω = oscillator pulsation). As Ipol = Iref, then : Vout = Vref . Cref / k . Cx This relation corresponds to an inverse of capacitance mode. The output voltage is linear versus 1/Cx. So, if Cx is design to follow the behaviour of a "parallel plate capacitor", the Vout voltage is linear versus the distance between the electrodes. In order to have Vout linear versus Cx , it is simply necessary to exchange the way the Cx and Cref are connected. For this configuration we get : Vout = Vref . Cx / k . Cref Vout voltage is then proportional to Cx. CAPAAB 22 / 32 Sentier des Torques BAT 8 92290 Chatenay Malabry Tel / Fax : 01 71 22 73 72
www.capaab.fr
RTV & RUN OUT MEASUREMENTS
Copyright CHATENAY 2001 CAPAAB
8/11
NOTICE : For the two working modes (linear vs distance or linear vs capacitance), two independent generators deliver the same voltage to the probe measuring electrode and to the probe guard ring electrode (though the tri axial connection cable between probe and electronics). This arrangement, very common in quality capacitive measurements, allows an optimal electric field configuration existing between the measuring electrode and the target. Without guard ring electrode, linear distance measurement would be very difficult (in fact the measuring range would be much reduced). Moreover, this guard ring, feed from the cable, allows quality measurements even if the probe is located at the end of this cable, several meters away from the electronics. The guard ring "eliminates" the effect of parasitic capacitance of the cable.
THE MAIN ADVANTAGES OF THIS TECHNIQUE MAY BE RESUME : With comparison to conventional capacitive system, this technics allows to : - use a grounded target - use very small probes (down to 0.1 mm for the diameter of the measuring electrode) - use long probe to electronics cable without reduced performances - measure capacitive, differential capacitive, inverse of capacitive signal - have extended probe range due to the extremely low leakage equivalent input capacitance - have a wide frequency range ( up to 250 khz -for High frequency model- for the output signal) - have a very low drift and a high accuracy due to the servo loop technique - work in harsh environment due to the full guard ring probe and cable - accommodate high temperature probes (up to 1300 °C) - be uninfluenced by magnetic field, gas ionisation and ionising radiation
CAPAAB 22 / 32 Sentier des Torques BAT 8 92290 Chatenay Malabry Tel / Fax : 01 71 22 73 72
www.capaab.fr
RTV & RUN OUT MEASUREMENTS
Copyright CHATENAY 2001 CAPAAB
9/11
A UNIQUE SOLUTION : FROM SENSOR TO ACQUISITION FROM CALIBRATION TO REAL TIME DISTANCE DISPLAY FROM DEVELOPMENTS TO ONBOARD SYSTEMS Two different capacitive conditioning electronics are available: the fist one is dedicated to test stand field, the second one for on board applications. The main features are given below.
CAPACITIVE CONDITIONNING UNIT FOR TEST STAND SYSTEMS : • MEASURING RANGE : FROM 0→ 0.5 mm to 0 → 12 mm according to the inner electrode diameter of the sensor used • LINEARITY : < +/- 0.5 % (F.S.O) OF THE PROBE RANGE • TEMPERATURE DRIFT (/ °C) < 0.05 % (F.S.O) OF PROBE RANGE • FREQUENCY RANGE (-3dB) : 0 to 20 kHz typical • DYNAMIC RESOLUTION AT MAXIMUM PROBE DISTANCE : 5 µm @ 1 kHz bandwidth • MEASUREMENT OUTPUT : 0 / +10V LINEAR VARIATION vs PROBE CAPACITANCE • OFFSET MEASUREMENT OUTPUT: FROM 0 to -10V & 0 to - 1V • ADJUSTABLE OUTPUT GAIN : RATIO FROM 1 to 5 • PACKAGING : 3U 7F MODULE FOR 19' RACK ; +/-15 V supplies • COMPATIBLE WITH CAPAAB & THERMOCOAX TRIAXIAL CAPACITIVE SENSORS (temperature range : from -270°C to +1300°C)
CAPAAB 22 / 32 Sentier des Torques BAT 8 92290 Chatenay Malabry Tel / Fax : 01 71 22 73 72
www.capaab.fr
RTV & RUN OUT MEASUREMENTS
10/11
Copyright CHATENAY 2001 CAPAAB
CAPACITIVE CONDITIONNING UNIT FOR ONBOARD SYSTEM : This unit is calibrated for each probe and its output can be connected to the digitizing unit. Usually, the module is located inside the car, and is 12V DC powered. The general metrological features are the same than the CAP ones. The size (mm) is 220*140*70.
The same digitizing and processing unit can be used with the two previous conditionning unit. Build around a Pentium PC, running under Win98, up to 4 units can be connected to digitizing channels with simultaneous sampling. The application software is developed under LabView from National Instrument. An example of the M.M.I associated to post processing is given below. M.M.I EXAMPLE ( RTV AND RUN OUT APPLICATION) Zoom Tools
Zoom Users
Print
FILE NAME
SELECTION
SAMPLING FREQUENCY
Nb OF SAMPLES
ACQUIS. DURAT. (S)
MANUAL
Processing Thickness (mean)
MEAN
Sampling freq. Hz
Cut off freq.
FILTER order
Thickness
NO filter
Thickness
DISC BRAKE TEST : EXAMPLE OF RESULTS ended
PRINTING CAPAAB yes 22 / 32 Sentier des Torques BAT 8 92290 Chatenay Malabry Tel / Fax : 01 71 22 73 72
Filter type
www.capaab.fr
RPM Thickness (mean)
RTV & RUN OUT MEASUREMENTS
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Copyright CHATENAY 2001 CAPAAB
Results can be displayed in real time on the PC monitor, but also printed very simply because data are also saved in ASCII format.
RTV & RUNOUT mm
CAPAAB RTV & RUNOUT MEASUREMENTS COMPLETE SOLUTION DIGITIZING UNIT PROBES CONDITIONNING UNIT
PC + DISPLAY
IME
V
TIME
CAPAAB 22 / 32 Sentier des Torques BAT 8 92290 Chatenay Malabry Tel / Fax : 01 71 22 73 72
www.capaab.fr
