TM Page 40
6.1
NON-OPERATION
If the system does not function the problem is almost certainly due to insufficient supply voltage to the transmitter, caused
by over large air-gap, proximity of metal close to one or both loops, or a seriously de-tuned condition. A simple way to
check is to apply a 6-9VDC supply (battery, for example) to the transmitter DC power supply pins (not the bridge excitation
pins). If the system then works, the problem is confirmed as being insufficient volts from the inductive power supply
Voltages across the power in and power pickup loops may be checked with an oscilloscope (high impedance probe):
1) Across smallest (65mm diameter) IL2 loop when correctly tuned: 12V P/P
2) Across largest IL2 loop when correctly tuned: 70V P/P
3) Across shaft mounted power pickup loop: 2-3V P/P
4) Across IP2 module output: 4V P/P
If voltages are less than the above either loop efficiency is being affected by the proximity of metal or the pickup loop
requires partial tuning - see section 4.5 below.
6.2
IMPROVING EFFICIENCY OF INDUCTIVE POWER SUPPLY
Apart from the air-gap being too large, there are 4 main reasons for inefficiency in the inductive power supply:
1) Self-inductance in the power pickup loop, which limits power supplied to the transmitter rectifier circuitry. Inductance is
directly proportional to loop diameter and the problem becomes more serious for loop diameters greater than about 150mm.
2) Poor flux linkages between power transmitting and power pickup components, particularly a problem in the case of the
inductive head when used with small loop diameters. The loop form of power input is much more efficient in this respect.
3) Poor quality factor "Q" of the loop due to insufficient insulation from the shaft and/or the proximity of metal components.
This causes a dissipation of electrical energy, reducing that available to the transmitter.
4) De-tuning of the inductive head caused by close proximity to metal. This reduces head efficiency.
Assuming that no improvements can be made by reducing air-gap, re-locating any from metal components or improving the
power in head/loop tuning, there several methods of improving performance:
1) Incorporate a capacitor in series with the power pickup loop to partially cancel out the loop self-inductance. The
transmitter input circuitry is optimised for a loop diameter of 90mm, but for larger loop diameters, the required capacitor
value (within approximately
±
10%) may be calculated from:
C (in Pfd) = 29 x 10
5
Loop Diameter in mm - 90
To obtain the greatest possible improvement, the exact capacitor value may have to be determined experimentall. It is
important that the capacitor be a high quality type (COG, NPO or silvered-mica dielectric), suitable for RF operation.
Tantalum or electrolytic capacitors, for example, are quite unsuitable. The output power improvement can be in the order
50-200%, dependant somewhat upon loop diameter (for large loop diameters, the improvement is not as great, since stray
capacitance effects tend to swamp the tuning capacitance).
For loop diameters less than about 60mm, increase the number of turns to two, three for less than 30mm diameter.
This may give sufficient improvement, if not a series tunig capacitor may be added. Calculate the value from:
C (in Pfd) =
29 x 10
5
(Loop Diameter in mm x n
2
) - 90
n = number of turns
Increasing the power pickup loop turns increases the flux linkages mentioned in (2), but, by spreading the turns slightly,
also has the advantage of improving power pickup when the shaft moves in the axial direction - due to vehicle suspension
movement, for example. Spacing between the turns may be increased to about 4mm.
2) Insert a band of "Mu-Metal" (high permeability nickel alloy) underneath the power pickup. Firstly insulate the shaft
surface using 0.5mm thick glass-fibre tape and then wind 1 turn of 0.1- 0.2mm thickness x 40mm wide Mu-Metal on top of
the glass-fibre. The ends of the Mu-Metal band must stop approximately 2mm before making contact with each other. The
metal then acts as a form of armature and concentrates the magnetic field into the loop. The recommended method of
supporting the metal band and loop on the shaft surface is to use a bandage of glass-fibre tape, impregnated with epoxy
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