GB1587067A

GB1587067A - Level control systems

Info

Publication number: GB1587067A
Application number: GB3129976A
Authority: GB
Original assignee: Horstmann Gear Group Ltd
Current assignee: Horstmann Gear Group Ltd
Priority date: 1976-07-27
Filing date: 1976-07-27
Publication date: 1981-03-25
Also published as: FR2360125A1; IT1085468B; FR2360125B3; DE2733662A1

Description

(54) IMPROVEMENTS IN OR RELATING TO LEVEL CONTROL SYSTEMS (71) We, HORSTMANN GEAR GROUP LIMITED formery known as THE HORSTMANN GEAR COMPANY LI MITED, a British Company, of Newbridge Works, Bath BA1 3EF, Avon, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a level control system.

It is sometimes necessary to provide level control of a gated tone signal, for example for a ripple control system in which the control signals are in the form of a gated tone control signal. Level control circuits usually measure tone amplitude by means of a diode detector circuit. The detector time constant is a compromise between response time (which requires a short time constant) and cyclic level variation (which requires a long time constant). However, for a gated tone system, the duration of each tone burst may be too short to allow a satisfactory diode level detection.

According to the invention, there is provided a level control system comprising a variable attenuator of step form for digital operation, a control unit for said attenuator, an amplitude sampler including means for presetting a desired level, and a comparator for comparing the desired level with an actual level to be controlled, the control unit being responsive to an output signal from the comparator to effect variation of the attenuator, means being provided for resetting the attenuator to one of the limit values of the attenuator and the control unit varying the value of the attenuator in steps, one step for each cycle of a gated tone signal, from the reset limit value towards the other limit value over a period of time until the output of the comparator indicates that the actual level to be controlled approximates to the desired level.

Since the attenuator is stepped and operated digitally, it is ensured that the detection and adjustment of the attenuation is immediately effective.

In order that the invention shall be clearly understood, various exemplary embodiments will now be described with reference to the accompanying drawings, in which: Figure I is a block diagram of the control system; Figure 2 is a more detailed diagram of the control system; and Figure 3 is a circuit diagram of a modified version of a control system, built for testing at laboratory level.

In Figure 1, a gated tone signal is supplied on line 10 to a switched attenuator 11, the output of which forms a drive signal on line 12 to an external load. A feedback signal on line 13 is derived from the external load and gives an exact measure of the drive signal which reaches the load. This feedback signal is passed to an amplitude sampler 14, which also receives the gated tone signal on line 15 for a purpose which will be described in connection with Figure 2. An output signal from the amplitude sampler is passed to a control logic 16 which, dependent upon the outcome of the sampling process passes signals on line 17 to control the attenuator 11. The attenuator 11 may be reset by means of an attenuator reset signal 31.

In general terms, the amplitude sampler 14 compares the peak amplitude of the feedback signal voltage with one or more reference voltages. If the peak amplitude exceeds an upper reference voltage, a pulse is sent to switch extra attenuation into the signal path; if the peak amplitude of the feedback signal does not exceed a lower reference voltage, a pulse is sent to switch attenuation out of the signal path. The attenuator is switched in sections and during a tone burst, the control logic 16 switches in or out one stage of attenuation each cycle until the peak amplitude of the feedback signal voltage has reached a desired value.

The response time depends upon the number of attenuator stages and the required accuracy.

Figure 2 show.s the same system in a little more detail. The amplitude sampler 14 is seen to have two comparators 20, 21, which compare the feedback signal with an upper and a lower preset d.c. voltage respectively.

The upper and lower voltages are set on resistors 22, 23.

The signal from the comparator 20 is in the form of a series of pulses at the tone frequency, which are applied to the "shift right" input of the shift register so as to produce an increase in the attenuation in the injection signal line. In the event that the feedback signal is lower than the preset minimum voltage, the output from comparator 21 will be a continuous d.c. voltage. It is necessary to convert this into a pulsed signal to produce stepping of the shift register, and this is achieved by the application of the gated tone signal on line 15. The latter is applied first to a phase shift component 22' which aligns its phase with that of the feedback signal, and then to a peak pulse generator 23', which produces a signal centered accurately on the maximum voltage of each cycle of the signal (in effect, a pulse shaper).The output of this generator 23' and of the comparator 21 are passed to an AND gate 24 which therefore produces a pulsed output when there is a continuous signal from the comparator. This pulsed output is applied to the "shift left" input of the shift register 16 to reduce the attenuation in the line. In either case, as soon as one stage of attenuation has been inserted or removed from the path of the gated tone signal, there will be a change in the feedback signal. Eventually, a sufficient change will have been made in the attenuation so that the feedback signal complies with the conditions preset in the registers 22, 23 and neither comparator 20 nor comparator 21 will produce a signal in a sense to increase or reduce the attenuation.

The attenuator 11 may consist of a number of series connected resistors 27, each shunted by a switch 26 controlled by the shift register. The outputs 25 of the shift register are seen to control in turn the switches 26 of resistors 27.

Figure 3 shows a practical circuit evolved for testing in the laboratory, although the circuitry involved is applicable on a larger scale. The circuit is modified as compared with Figure 2. The attenuator is controlled by a series of analogue switches driven from the outputs of an eight-stage shift register.

The gated 600 Hz signal passes through the attenuator stages to the power amplifier and the injection transformer. The injected signal appearing on the laboratory ring main (treated as the load) is detected by a monitor receiver, and passed to a single comparator 30.

The shift register stages are normally held reset by the inverse output of a monostable circuit controlling the -transmission of the tone signal. The reset is removed when a command signal is transmitted and the attenuator requires to be controlled.

The output of the monitor receiver is compared with the single preset reference level. If that level is exceeded, an inhibit signal is removed from an oscillator and clock pulses are supplied to the shift register. As the clock pulses are supplied, the shift register stages are set in turn to logical 1. Each stage of the shift register is connected to an analogue switch in the attenuator. If all stages of the shift register are set to a logical 0, all analogue switches are open and the gated tone signal is not attenuated. If the first stage of the shift register is set to a logical 1, an attenuation of 0.9 is applied; the second stage increases the attenuation to 0.8, and so forth. The maximum attenuation when all stages of the shift register are set is 0.2.

Thus, if the feedback signal level exceeds the preset value, clock pulses are supplied to the shift register until the feedback level is less than the preset value. The full range of attenuation is switched in in steps within eight cycles of the 600 Hz signal at the rate of one step for each cycle of the signal.

In an alternative system, the attenuation can be set initially to a maximum and reduced until the transmitted signal exceeds a preset minimum level. In this case also, only a single comparator is needed.

Both these circuits involving only a single comparator require an attenuator reset signal 31 as shown in Figure 3.

Figure 3 also embodies a further useful feature in the form of a short circuit switch 32 in the primary of the injection transformer. This switch 32 is controlled by a relay 33 actuated again by the normal output of the monostable controlling the transmission.

WHAT WE CLAIM IS: 1. A level control system comprising a variable attenuator of step form for digital operation, a control unit for said attenuator.

an amplitude sampler including means for presetting a desired level, and a comparator for comparing the desired level with an actual level to be controlled, the control unit being responsive to an output signal from the comparator to effect variation of the attenuator, means being provided for resetting the attenuator to one of the limit values of the attenuator and the control unit varying the value of the attenuator in steps,

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. or out one stage of attenuation each cycle until the peak amplitude of the feedback signal voltage has reached a desired value. The response time depends upon the number of attenuator stages and the required accuracy. Figure 2 show.s the same system in a little more detail. The amplitude sampler 14 is seen to have two comparators 20, 21, which compare the feedback signal with an upper and a lower preset d.c. voltage respectively. The upper and lower voltages are set on resistors 22, 23. The signal from the comparator 20 is in the form of a series of pulses at the tone frequency, which are applied to the "shift right" input of the shift register so as to produce an increase in the attenuation in the injection signal line. In the event that the feedback signal is lower than the preset minimum voltage, the output from comparator 21 will be a continuous d.c. voltage. It is necessary to convert this into a pulsed signal to produce stepping of the shift register, and this is achieved by the application of the gated tone signal on line 15. The latter is applied first to a phase shift component 22' which aligns its phase with that of the feedback signal, and then to a peak pulse generator 23', which produces a signal centered accurately on the maximum voltage of each cycle of the signal (in effect, a pulse shaper).The output of this generator 23' and of the comparator 21 are passed to an AND gate 24 which therefore produces a pulsed output when there is a continuous signal from the comparator. This pulsed output is applied to the "shift left" input of the shift register 16 to reduce the attenuation in the line. In either case, as soon as one stage of attenuation has been inserted or removed from the path of the gated tone signal, there will be a change in the feedback signal. Eventually, a sufficient change will have been made in the attenuation so that the feedback signal complies with the conditions preset in the registers 22, 23 and neither comparator 20 nor comparator 21 will produce a signal in a sense to increase or reduce the attenuation. The attenuator 11 may consist of a number of series connected resistors 27, each shunted by a switch 26 controlled by the shift register. The outputs 25 of the shift register are seen to control in turn the switches 26 of resistors 27. Figure 3 shows a practical circuit evolved for testing in the laboratory, although the circuitry involved is applicable on a larger scale. The circuit is modified as compared with Figure 2. The attenuator is controlled by a series of analogue switches driven from the outputs of an eight-stage shift register. The gated 600 Hz signal passes through the attenuator stages to the power amplifier and the injection transformer. The injected signal appearing on the laboratory ring main (treated as the load) is detected by a monitor receiver, and passed to a single comparator 30. The shift register stages are normally held reset by the inverse output of a monostable circuit controlling the -transmission of the tone signal. The reset is removed when a command signal is transmitted and the attenuator requires to be controlled. The output of the monitor receiver is compared with the single preset reference level. If that level is exceeded, an inhibit signal is removed from an oscillator and clock pulses are supplied to the shift register. As the clock pulses are supplied, the shift register stages are set in turn to logical 1. Each stage of the shift register is connected to an analogue switch in the attenuator. If all stages of the shift register are set to a logical 0, all analogue switches are open and the gated tone signal is not attenuated. If the first stage of the shift register is set to a logical 1, an attenuation of 0.9 is applied; the second stage increases the attenuation to 0.8, and so forth. The maximum attenuation when all stages of the shift register are set is 0.2. Thus, if the feedback signal level exceeds the preset value, clock pulses are supplied to the shift register until the feedback level is less than the preset value. The full range of attenuation is switched in in steps within eight cycles of the 600 Hz signal at the rate of one step for each cycle of the signal. In an alternative system, the attenuation can be set initially to a maximum and reduced until the transmitted signal exceeds a preset minimum level. In this case also, only a single comparator is needed. Both these circuits involving only a single comparator require an attenuator reset signal 31 as shown in Figure 3. Figure 3 also embodies a further useful feature in the form of a short circuit switch 32 in the primary of the injection transformer. This switch 32 is controlled by a relay 33 actuated again by the normal output of the monostable controlling the transmission. WHAT WE CLAIM IS:

1. A level control system comprising a variable attenuator of step form for digital operation, a control unit for said attenuator.

one step for each cycle of a gated tone signal, from the reset limit value towards the other limit value over a period of time until the output of the comparator indicates that the actual level to be controlled approximates to the desired level.

2. A system as claimed in claim 1, wherein two comparators are provided, one of which is set to produce an output when the actual value is above an upper level threshold and the other of which is set to produce an output when the actual value is below a lower level threshold and wherein the control unit varies the value of the attenuator upwards or downwards depending upon which comparator is producing an output signal.

3. A system as claimed in claim 2, wherein the output of the comparator set to the lower level threshold has an output passing through an AND-gate fed also with the signal whose level is to be controlled, whereby the AND-gate produces no output in the absence of the signal.

4. A system as claimed in any one of claims 1 to 3, wherein the attenuator comprises a multi-stage switched arrangement and the control unit comprises a shift register, the individual outputs of the shift register being connected to individual stages of the attenuator.

5. A level control system substantially as described herein with reference to the drawings.