Wire Tension Measurements
The Region 2 drift chambers are designed to reach a spatial accuracy of
about 0.15 mm.
It is therefore important to control the sagging of each wire due to
gravity to within a
considerably smaller tolerance. This in turn requires us to measure
(and adjust, if
necessary) the tension on each wire to about 25% within its nominal
value. Two different methods are used, but both share the same basic
physics principles.
Wires being tested for tension
Principle of operation:
The tension is determined by measuring the eigenfrequency of each wire.
Since both
the mass per unit length and the overall length of each wire are well
known, the
tension can be directly determined from the frequency. The frequency is
proportional
to the square root of the tension; this sets an upper limit of 12%
frequency deviation
from the nominal value. The wire is set into oscillation by running an
alternating
current of specified frequency through it and applying an external
magnetostatic
field. The current frequency is varied until resonance is found.
In the following figure one can see the large air coils that produce
the magnetic field.
Magnetic coils for wire tension measurements
"Ringing" Method:
This method has been used at CEBAF for Region 3 drift chambers. The
resonance is found
by exciting the wire with an AC current for a short while, and then
looking for the
induced EMF signal from the oscillations in the magnetic field (with
the AC current
switched off). Both the relative phase and the amplitude of these
oscillations are
indicators whether one is on resonance. The frequency is tuned by hand
and the
resonance frequency typed into the computer.
Automatic Scan:
This alternative method is fully computerized. Eight wires are tested
simultaneously.
We use a PC with LabView to control the frequency generator and to read
back the
induced EMF in each wire (8 ADC channels). A peak finding algorithm
determines
the resonance frequency, which is written to a file automatically.
Screenshot of LabView Virtual Instrument after finding the wire
resonance
frequency
Back to Region 2 Drift Chambers.