- Email: [email protected]
A method for measuring the wire tensions in proportional and drift chambers
Nuclear Instruments and Methods 211 (1983) 369-370 North-Holland Publishing Company
A METHOD FOR MEASURING AND DRIFT CHAMBERS
369
THE WIRE TENSIONS
IN PROPORTIONAL
Malcolm COUPLAND University College, University of London, London, England * Received 4 October 1982
A device to measure the resonant frequency of a proportional chamber wire is described. From this frequency the wire tension can be calculated. The device is self-tuning, and in some instances may be used without opening the chamber.
The precise control to better than 1% of the wire tensions in multiwire proportional and drift chambers is crucial to their stable operation. Usually the tensions are checked when the chamber is first assembled by exciting them to oscillate at their resonant frequency with a strong oscillating electric field - the "electrostatic" method [ 1]. This method is efficient and precise when all the wires have very nearly the same resonant frequency and the chamber is open for easy access. However there are several situations where the simple electrostatic technique is inconvenient or impossible, and where the method described in this paper is much more appropriate: the wires may not all have the same length; there may be wires of different mass, such as guard wires; it may be inconvenient to open the chamber; or in an ageing chamber the tensions may have shifted so much that their resonance curves no longer overlap. Chase has described an improved version of the electrostatic method which uses the signal induced on the cathode planes to control the frequency of the oscillator with a phase detector [2]. The device described here relies on electromagnetic induction, and the circuit is much less elaborate than the Chase device. No direct comparison between the two devices has been made, though in principle they should perform similarly. The wire is excited by a current passed along it in the presence of a static transverse magnetic field. The device is self-adjusting, so one can move rapidly from one wire to the next. An ordinary permanent magnet can provide a useful field strength at least a few centimetres from the pole faces, so if both ends of the wire are accessible the measurements may be made with the chamber unopened, or even in situ.
* Now at Birkbeck College, University of London, London, England. 0167-5087/83/0000-0000/$03.00 © 1983 North-Holland
When the wire is in a transverse magnetic field its electrical impedance at the resonant frequency is greater than the dc resistance owing to the emf induced by the motion. Including the wire as one arm of an impedance bridge therefore provides a means of determining the resonant frequency, and hence the tension. The increase in impedance over the dc value is typically~ only a fraction of a percent and so a self-balancing bridge oscillator is to be preferred. This also makes for a much greater ease and speed of use than methods requiring manual tuning [3] - an important consideration for large multiwire proportional chambers. The circuit described here is based on an original design due to Muratori [4] which used a two-stage vacuum triode amplifier and was ac coupled. This latter feature necessitated manual adjustment very close to the resonant condition before oscillation could start spontaneously, and a double bridge circuit was employed. The present circuit uses an integrated circuit operational amplifier in a single bridge, and is dc coupled, so that the bridge adjusts itself quickly to the dc balance condition, when the loop gain becomes very large, and oscillation rapidly builds up. In practice, no manual retuning is needed when measuring many wires of the same nominal length and mass. The basic circuit is shown in fig. 1. With the component values shown the device will operate for wires in the resistance range l ~ to 1 k ~ . The filament lamp L acts as a positive temperature coefficient resistance, increasing the amount of negative feedback as the amplitude of oscillation increases and so stabilizing the balance condition. Various "thermistor" type negative temperature coefficient resistors were tried in the R I position instead of using a lamp, but none were found to have the required combination of thermal time constant and electrical operating point. The only disadvantage of a filament lamp is that they are slightly microphonic, though no spurious oscillation was ever
M, Coupland / Measuring wire tension
370
100n II
R, Elk
R2
1470
,n__-+-
100n
II
L
~ Is hoping ~
frequency counter I
24V ( 20mA
wire
input
Fig, 1. Schematic diagram of the self-balancing bridge oscillator.
experienced from this cause. In use the potential divider R 3 is set to give approximately one third maximum dc output from the operational amplifier when contact is first made with the wire, indicating that the bridge is in a self-balancing condition. The loop gain is then given by Vo/u o, where V0 is the dc output voltage and u 0 is the input offset voltage. When setting up the device u 0 is made as small as possible to give the maximum sensitivity. The rate at which a useful amplitude builds up will depend on, among other things, the mechanical damping and the strength of the magnetic field, and it is possible for the damping to be so large that oscillation cannot occur with easily available magnetic fields. However in most cases a single permanent U-magnet of about 3 cm pole width is sufficient at distances of up to 2 cm. This enables the magnet to be placed outside most multiwire proportional chambers, and if both ends of the wire are externally accessible the chamber need not be opened. Several magnets may be ganged side-by-side if a greater sensitivity is needed. A simple calculation yields the following expressions for the ac impedance Z and the time constant T for the onset of oscillation: Z= 1 T
lB2,c
References
2# ' lB2Vo A 2~uoAo(r+Ri)
constant for natural damped vibration of the wire, A 0 is the dc gain of the amplifier, and A is the amplifier gain at the resonant frequency. These formulae should be sufficient to permit design alterations to suit particular circumstances. A few operational points are worth noting. Although a certain amount of electrical noise probably assists the onset of oscillation, too much can drive down the loop gain and so prevent oscillation. For this reason a screened cable should be used to connect to the wire, and any other instrument (such as an oscilloscope) connected to the input of the device should be connected at the device end of this cable to avoid ground currents. When measuring long or thick wires it may be useful to operate at a harmonic of the resonant frequency, which is easily achieved by placing the magnet at the corresponding antinode. The inclusion of an audio amplifier and loudspeaker is strongly recommended, since operators with only an average musical ear can easily spot out of tolerance wires without looking at the frequency meter, which again speeds up the measurements considerably.
1 ~'
where B is the magnetic field strength, / is the length of wire over which the field acts, ~ is the linear density of the wire, r is the resistance of the wire, ~- is the time
[1] K.B Burns, B.R. Gunman and T.A. Nunamaker, Nucl. Instr. and Meth. 106 (1973) 171. [2] R.L. Chase, Nucl. Instr. and Meth. 113 (1973) 395. [3] H. Hultschig and A. Ladage, DESY 81-065 (1981). [4] G. Muratori, CERN internal report CERN EP INT 76-14 (1976).
A METHOD FOR MEASURING AND DRIFT CHAMBERS
369
THE WIRE TENSIONS
IN PROPORTIONAL
Malcolm COUPLAND University College, University of London, London, England * Received 4 October 1982
A device to measure the resonant frequency of a proportional chamber wire is described. From this frequency the wire tension can be calculated. The device is self-tuning, and in some instances may be used without opening the chamber.
The precise control to better than 1% of the wire tensions in multiwire proportional and drift chambers is crucial to their stable operation. Usually the tensions are checked when the chamber is first assembled by exciting them to oscillate at their resonant frequency with a strong oscillating electric field - the "electrostatic" method [ 1]. This method is efficient and precise when all the wires have very nearly the same resonant frequency and the chamber is open for easy access. However there are several situations where the simple electrostatic technique is inconvenient or impossible, and where the method described in this paper is much more appropriate: the wires may not all have the same length; there may be wires of different mass, such as guard wires; it may be inconvenient to open the chamber; or in an ageing chamber the tensions may have shifted so much that their resonance curves no longer overlap. Chase has described an improved version of the electrostatic method which uses the signal induced on the cathode planes to control the frequency of the oscillator with a phase detector [2]. The device described here relies on electromagnetic induction, and the circuit is much less elaborate than the Chase device. No direct comparison between the two devices has been made, though in principle they should perform similarly. The wire is excited by a current passed along it in the presence of a static transverse magnetic field. The device is self-adjusting, so one can move rapidly from one wire to the next. An ordinary permanent magnet can provide a useful field strength at least a few centimetres from the pole faces, so if both ends of the wire are accessible the measurements may be made with the chamber unopened, or even in situ.
* Now at Birkbeck College, University of London, London, England. 0167-5087/83/0000-0000/$03.00 © 1983 North-Holland
When the wire is in a transverse magnetic field its electrical impedance at the resonant frequency is greater than the dc resistance owing to the emf induced by the motion. Including the wire as one arm of an impedance bridge therefore provides a means of determining the resonant frequency, and hence the tension. The increase in impedance over the dc value is typically~ only a fraction of a percent and so a self-balancing bridge oscillator is to be preferred. This also makes for a much greater ease and speed of use than methods requiring manual tuning [3] - an important consideration for large multiwire proportional chambers. The circuit described here is based on an original design due to Muratori [4] which used a two-stage vacuum triode amplifier and was ac coupled. This latter feature necessitated manual adjustment very close to the resonant condition before oscillation could start spontaneously, and a double bridge circuit was employed. The present circuit uses an integrated circuit operational amplifier in a single bridge, and is dc coupled, so that the bridge adjusts itself quickly to the dc balance condition, when the loop gain becomes very large, and oscillation rapidly builds up. In practice, no manual retuning is needed when measuring many wires of the same nominal length and mass. The basic circuit is shown in fig. 1. With the component values shown the device will operate for wires in the resistance range l ~ to 1 k ~ . The filament lamp L acts as a positive temperature coefficient resistance, increasing the amount of negative feedback as the amplitude of oscillation increases and so stabilizing the balance condition. Various "thermistor" type negative temperature coefficient resistors were tried in the R I position instead of using a lamp, but none were found to have the required combination of thermal time constant and electrical operating point. The only disadvantage of a filament lamp is that they are slightly microphonic, though no spurious oscillation was ever
M, Coupland / Measuring wire tension
370
100n II
R, Elk
R2
1470
,n__-+-
100n
II
L
~ Is hoping ~
frequency counter I
24V ( 20mA
wire
input
Fig, 1. Schematic diagram of the self-balancing bridge oscillator.
experienced from this cause. In use the potential divider R 3 is set to give approximately one third maximum dc output from the operational amplifier when contact is first made with the wire, indicating that the bridge is in a self-balancing condition. The loop gain is then given by Vo/u o, where V0 is the dc output voltage and u 0 is the input offset voltage. When setting up the device u 0 is made as small as possible to give the maximum sensitivity. The rate at which a useful amplitude builds up will depend on, among other things, the mechanical damping and the strength of the magnetic field, and it is possible for the damping to be so large that oscillation cannot occur with easily available magnetic fields. However in most cases a single permanent U-magnet of about 3 cm pole width is sufficient at distances of up to 2 cm. This enables the magnet to be placed outside most multiwire proportional chambers, and if both ends of the wire are externally accessible the chamber need not be opened. Several magnets may be ganged side-by-side if a greater sensitivity is needed. A simple calculation yields the following expressions for the ac impedance Z and the time constant T for the onset of oscillation: Z= 1 T
lB2,c
References
2# ' lB2Vo A 2~uoAo(r+Ri)
constant for natural damped vibration of the wire, A 0 is the dc gain of the amplifier, and A is the amplifier gain at the resonant frequency. These formulae should be sufficient to permit design alterations to suit particular circumstances. A few operational points are worth noting. Although a certain amount of electrical noise probably assists the onset of oscillation, too much can drive down the loop gain and so prevent oscillation. For this reason a screened cable should be used to connect to the wire, and any other instrument (such as an oscilloscope) connected to the input of the device should be connected at the device end of this cable to avoid ground currents. When measuring long or thick wires it may be useful to operate at a harmonic of the resonant frequency, which is easily achieved by placing the magnet at the corresponding antinode. The inclusion of an audio amplifier and loudspeaker is strongly recommended, since operators with only an average musical ear can easily spot out of tolerance wires without looking at the frequency meter, which again speeds up the measurements considerably.
1 ~'
where B is the magnetic field strength, / is the length of wire over which the field acts, ~ is the linear density of the wire, r is the resistance of the wire, ~- is the time
[1] K.B Burns, B.R. Gunman and T.A. Nunamaker, Nucl. Instr. and Meth. 106 (1973) 171. [2] R.L. Chase, Nucl. Instr. and Meth. 113 (1973) 395. [3] H. Hultschig and A. Ladage, DESY 81-065 (1981). [4] G. Muratori, CERN internal report CERN EP INT 76-14 (1976).