Patent GB-1422572-A (GB)
Title: Speed sensitive control arrangements
IPC: G01P21/00
Filed: 1973-01-05
Abstract: 1422572 Automatic control of gas turbines ROBERT BOSCH GmbH 5 Jan 1973 [8 Jan 1972] 823/73 Heading G3N [Also in Division H3]  In a shaft rotation measuring system suitable for measuring the speed of a turbine a pulse generator and a frequency to voltage converter are used to derive a first measurement of the speed; a further frequency to voltage converter is connected to the same or a further pulse generator driven by the shaft to give a second speed measurement, the two measurements being compared so that a failure in the measuring system may be detected. The basic system is shown in Fig. 1 where the fuel pump 12 of a turbine delivers fuel from tank 11 to jet 17 via a throttle valve 13 and a quick acting valve 15. A pulse generator 20 is driven by the shaft of the turbine and is connected to a frequency to voltage converter 21 comprising monostable trigger 22 and filter 23. The converter signal passes through a variable gain amplifier 24 where it is amplified in dependance on other parameters such as temperature which are fed to gain control terminals 25, 26; the signal is also supplied to a monitor stage 28 which includes the second converter (which is connected to generator 20 or to a second generator) and a signal comparator. The output from the variable gain amplifier 24 is further amplified at 27 to control fuel throttle 13 according to the turbine speed. The output of the comparator in monitor stage 28 is fed to amplifier 27 and can cause the throttle valve 13 to be closed; since throttle 13 is relatively slow acting this output is also connected to electrically operated valve 15. In the event of a fault condition in the system valve 14 is rapidly closed and the fuel is pumped back to the tank via line 18. Various arrangements are described for monitor stage 28. In the following figures the pump and associated valves of Fig. 1 are not shown. In Fig. 2, two generators 20, 30 are used, their signals being compared at 34. A differentiator 35 is also provided, connected to threshold device 36. If the difference between the generator signals is large, indicating a fault in one of them, comparator 34 actuates gate 37 and valve 15. Likewise if both generators fail, so that both their outputs are faulty (but equal) the differentiator actuates the gate and valve 15. In Fig. 3 (not shown) only one generator is used, and the differentiator is omitted; the threshold device is retained, so that the circuit can again respond if there is a difference between the two converter outputs or if one of them gives a signal outside the predetermined threshold range. In Fig. 4 a single generator 20 is again used, and in monitor stage 28 two low pass filters are used, one driving comparator 34 and the other connected to threshold device 41. Gate 37 is thus controlled as before if a fault occurs. Normally a small difference in the two speed measurements is produced due to component tolerance and this is allowed for in comparator 34. A further amplifier stage 39 is provided which responds to whichever of the speed signals is the smaller, so that throttle 13 meters fuel according to the smaller signal. Circuitry: Fig. 5 shows a threshold circuit which may form the device 41 of Fig. 4. The input pulses from monostable 32 are rectified at 44 and produce a negative voltage with respect to earth line 52. This keeps transistor 50 switched off. If the pulse rate drops below a pre-set value of negative voltage will be too small to maintain the off condition and gate 37 will be activated. This circuit increases the safety margin since the rectifier 44 is the only source of negative voltage in the system; no short circuit can produce a negative voltage to keep transistor 50 off. The comparator stage is also described, Fig. 7, and comprises two operational amplifiers whose outputs are gated by NOR gate 62. If the input voltages from filters 23, 33 are equal the amplifiers 60, 61 have their outputs at earth potential regardless of the input magnitude and the gate gives a '1' output. When the inputs differ the outputs also differ and the gate switches to the 'O' condition. The difference between the inputs necessary for switching depends on the gate threshold and on the amplifier gain determined by feedback resistors 67, 69.