Patent GB-1225119-A (GB)
IPC: F15C1/00
Filed: 1967-08-31
Abstract: 1,225,119. Axial-flow compressors. PLESSEY TELECOMMUNICATIONS RESEARCH Ltd. Nov.28, 1968 [Aug.31, 1967], No.39945/67. Heading F1C. [Also in Division G3] Apparatus for exercising a control in dependence on a function comprising the product of a frequency and the reciprocal of the square root of a temperature, comprises means for generating a first fluid pulse at a time determined by the frequency, a fluid delay line for transmitting the first pulse to a comparator and arranged to be supplied with fluid at the temperature, means for generating a second fluid pulse whose timing is determined by the frequency, the second pulse being supplied to the comparator which is arranged to produce an output signal for operating actuator means for adjusting a variable affecting the frequency, the temperature, or the geometry of a system where the compared pulses can.be brought into time coincidence. The frequency may be a rotational speed expressed in revolutions per unit time. The apparatus may be used to control the inlet guide vanes of the compressor of a gas turbine engine. Fig. 1 shows an arrangement not in accordance with the invention, in which a rotating slotted disc SD driven by the compressor generates a signal pulse by allowing momentary access of compressed air through the slot to the end A of a delay line DL. After a time delay inversely proportional to the square root of the temperature T of the compressed air, the pulse arrives at a comparator C. After a time delay inversely proportional to the rotational speed N, the slot has passed through the angle # from A to B and allows a reference pulse of compressed air to pass to the other input of the comparator C. When the two time delays are equal, the angle # is in a predetermined proportional relationship to the ratio N/#T, and the comparator C gives no output. When the two time delays are unequal, one pulse arrives before the other and produces an output from the comparator C. This output causes an actuator to move the inlet guide vanes in the appropriate direction, and a mechanical feedback linkage correspondingly changes the angle #, e.g., by moving the inlet A or B, or alters the length of the delay line, the output ceasing when the time delays have been equalized. In an arrangement, Fig.2, in accordance with the invention, the delay line DL is flushed with compressor delivery air at temperature T 2 , exhausting at A through a choked nozzle. The signal pulse is generated by a lobe on the disc SD which momentarily blocks the exhaust, the pulse being transmitted against the flushing air stream to the comparator C. Instead of being in accordance with N/#T 2 , the control may be in accordance with N/#T 1 , where T 1  is the engine intake total temperature, the delay line DL being flushed with engine intake air instead of compressor delivery air. A suction pump may then be needed to provide a low enough pressure to choke the exhaust nozzle. In the arrangement shown in Fig.7, the comparator has two positions C 1 , C 2 . The tendency for noise in the compressor delivery air to produce spurious operation of the comparator is prevented by the use of a dummy delay line DML to cancel the noise. During normal operation, the leading edge of the signal pulse lies between the two positions C 1 , C 2  when the reference signal is applied to the comparator, and the output of both halves of the comparator is then connected to vent. If the leading edge of the signal pulse does not lie between the positions C 1 , C 2  when the reference pulse is applied, the output of one half of the comparator goes to the corresponding monostable fluid switch M 1  or M 2 , which in turn operates a corresponding fluid amplifier A 1  or A 2  to apply fluid pressure to the appropriate side of a diaphragm AV controlling the valve of an actuator which turns the inlet guide vanes in the appropriate direction. When not giving an output to the corresponding amplifier A 1  or A 2 , each switch M 1  or M 2  is vented to a corresponding monostable fluid switch S 1  or S 2 . The pressure difference across the diaphragm AV is applied as a bias to the switches S 1,  S 2 . Thus if the amplifier A 1  is switched on, the switch S 1  is biased on, although it has no supply pressure from the switch M 1  until the amplifier A 1  is switched off. When the actuator has established the correct inlet guide vane angle, the amplifier A 1  is switched off and power is restored to the switch S 1  . Since there is a delay in exhausting the pressure applied to the diaphragm AV through the vent of the amplifier A 1 , the switch S 1  remains biased on by this pressure and it consequently provides an output to switch on the opposing amplifier A 2  to compensate for the tendency of the actuator to overrun. At very low values of N/#VT 2 , the signal pulse passes completely through the delay line DL before the reference pulse is applied, so that the comparator produces an output corresponding to that for high N/#VT 2 . To prevent false operation under these conditions, an override comparator OC is provided. The signal pulse occupies a sufficient length of the delay line DL to overlap the comparator OC when its leading edge lies between the positions C 1  and C 2 , so that the output of the comparator OC is vented when the reference pulse is applied. At low values of N/#VT 2 , the comparator OC sees no signal pulse when the reference pulse is applied, and its output is then switched to provide an input signal to the switches M 1  and M 2  to override any signal from the main comparator and produce an operation consistent with the low value of N/#T 2 . The logic arrangements may be modified by using inverse signals throughout, both sides of the diaphragm AV then being pressurized when the system is at rest.