GB2087596A

GB2087596A - Apparatus and Method for Controlling a Variable in Accordance with a Schedule

Info

Publication number: GB2087596A
Application number: GB8131417A
Authority: GB
Original assignee: BOC Ltd
Current assignee: BOC Ltd
Priority date: 1981-10-19
Filing date: 1981-10-19
Publication date: 1982-05-26

Abstract

A memory (15) is programmed (14) to store a desired schedule of gas pressure to be attained at regularly spaced intervals of time. Signal generators (18, 20) generate respectively, a control signal and reference signal representative of the pressure to be attained at a next time and the given time reference. The gas pressure is sensed by a transducer (8) and compared (26) with the reference signal to form an error signal (30). The control signal is then adjusted (22) in response to the previously generated error signal to provide an adjusted signal for controlling the gas pressure (16). By this means the gas pressure can be controlled in accordance with a schedule calling for rapid and substantial variation of magnitude. The apparatus is stated to be applicable to spray casting processes. <IMAGE>

Description

SPECIFICATION Apparatus and Method for Controlling a Variable in Accordance with a Schedule This invention relates to apparatus and methods for controlling the magnitude of a process variable, e.g. a fluid pressure, in accordance with a predetermined schedule of magnitude variation with time.

In spray casting processes, for example, it has been proposed to atomize a vertically descending stream of molten metal by directing a jet of gas at the stream. The need has arisen for equipment capable of varying the pressure of the atomizing gas in accordance with a predetermined schedule.

The schedule may call for a range of pressure variations that is both rapid and substantial, and existing control apparatus has not met this need.

According to the present invention there is provided apparatus for controlling the magnitude of a process variable in accordance with a 'predetermined schedule of magnitude variation with time, said apparatus comprising: means for generating at selected times first signals each representative of the magnitude the variable is scheduled to assume at the next said time; means for generating second signals representative of the actual magnitude of said variable at said selected times; means for comparing each said second signal as associated with a given said time with that said first signal as generated at a said time preceding said given time and generating an error signal representative of any difference therebetween; and means for adjusting each said generated first signal in response to a previously generated said error signal to provide an adjusted first signal for controlling the magnitude of said process variable until a next said adjusted first signal is provided.

In another aspect the invention provides a method for controlling the magnitude of a process variable in accordance with a predetermined schedule of magnitude variation with time said method comprising.

generating at selected times first signals each representative of the magnitude the variable is scheduled to assume at the next said time; generating second signals representative of the actual magnitude of said variable at said selected times; comparing each said second signal as associated with a given said time with that said first signal as generated at a said time preceding said given time and generating an error signal representative of any difference therebetween; and adjusting each said generated first signal in response to a previously generated said error signal to provide an adjusted first signal for controlling the magnitude of said process variable until a next said adjusted first signal is provided.

The process variable at issue may be the magnitude of fluid pressure in a conduit downstream of a controllable valve, the adjusted first signals being applied to control said valve.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a block diagram of control apparatus according to the invention; and Figure 2 is a more detailed schematic diagram of the apparatus of Figure 1.

Figure 1 shows a conduit 2 connecting a source of pressurized gas (not shown) to the left through a controllable valve 4 to equipment (not shown) to the right for utilizing the gas. The equipment may be equipment for atomizing a stream of molten metal by directing a jet of gas at the stream, and it may be required to vary the gas pressure in accordance with a schedule such as the following.

Time (sects) Pressure (psig) 0 30 1 50 2 100 3 110 4 100 5 90 6 80 7 30 8 50 etc.

It may be required to repeat the cycle (0 to 7 seconds) over a prolonged period, e.g. 30 minutes or more.

The valve 4 is operated pneumatically, its setting depending on the magnitude of a control pressure. The gas pressure downstream of the valve is sensed through a line 6 by a transducer 8 which generates an electrical signal representative of the actual magnitude of the sensed gas pressure. The signal is amplified by amplifier 10, which may be integral with transducer 8. The amplified signal is supplied to a control unit 12 which may be programmed by a programmer 14. The control unit 12 generates a control signal which is converted by transducer 1 2 to the control pressure to adjust the valve 4.

The control unit is programmed so as to change the control pressure applied to the valve so that the gas pressure downstream of the valve changes in accordance with the predetermined schedule of magnitude variation with time, e.g. as set out above.

Referring now to Figure 2 the control unit 12 comprises a memory 15 programmed by programmer 14 to store a desired schedule in the form of a finite set of signal values respectively representative of the magnitude the gas pressure is scheduled to assume at regularly spaced times in said schedule.

Signal generating means comprises a control signal generator 1 8 and a reference signal generator 20 each connected to the memory. The generators 1 8 and 20 are operable, in response to clock signals from a master clock (not shown) to generate at each said time in the schedule a first reference signal and a first control signal. The first reference signals are each representative of the magnitude the pressure is scheduled to assume at that given time, and the first control signals are each representative of the magnitude the variable is scheduled to assume at the next said time.

The control signal generator 18 is connected through an adjusting circuit 22 to the transducer 1 6. The reference signal generator 20 is connected to one input 24 of a comparator 26, and the amplifier 10 is connected to another input 28 of the comparator. An output 30 of the comparator is connected to the adjusting circuit 22.

In use, the times in said schedule occur at intervals dt selected in view of the response characteristics of the valve 4 and in relation to the rates of magnitude change with time called for by a particular schedule. In one embodiment the times are at intervals dt of one-tenth of a second.

Assume the time is t and the valve is set to provide a scheduled downstream gas pressure of p. At that time t the control signal generator 1 8 generates a first control signal representative of a pressure (p+dp) in the sense that this first control signal would, if applied unadjusted through circuit 22 and transducer 16 to the valve 4, alter the setting of the valve 4 from a position intended to provide a downstream pressure of p to a position intended to provide a downstream pressure of (p+dp). The pressure (p+dp) is the pressure the gas is scheduled to assume at the time (t+dt). At the given time t the reference signal generator 20 generates a first reference signal still representative of the pressure p scheduled for time t, and supplies that signal to the input 24 of comparator 26.

At the same given time t the amplifier 10 is supplying to input 28 of comparator 26 a second signal representative of the actual magnitude of the pressure at time T. The comparator compares the signals present at its inputs 24 and 28, and generates an error signal representative in magnitude and sign of any difference therebetween. It will be appreciated that the comparator is comparing each actual value signal as associated with a given said time t to the value (as represented by the current reference signal) of that said control signal as actually first generated at the time (t-dt) preceding said given time t.

The error signal, positive or negative, is supplied to the adjusting circuit 22 to adjust the current first control signal to provide an adjusted first control signal. It will be appreciated that, at each time, the current first control signal is being adjusted in response to the immediately previously generated error signal. In this way the control apparatus can handle a schedule calling for a range of pressure variation that is both rapid and substantial.

For example, assume that the schedule requires the pressure at time t to be 30 psig and at time (t+0.1) to be 32 psig. At time t a control signal is therefore generated to call for such increase in the valve operating pressure as to change the setting of the valve 4 to give the desired downstream pressure of 32 psig. At time t, however, suppose that the actual gas pressure sensed by transducer 8 and supplied to comparator 26 is 30.2 psig. At the time t the reference signal supplied by generator 20 to comparator input 24 represents 30 psig. The comparator therefore provides an error signal representing -0.2 psig. The control signal is therefore adjusted by circuit 22 to actually call for such increase in the valve operating pressure as to change the setting of valve 4 to give a gas pressure increase of 2-0.1 psig =1.8 psig.This valve setting change should be accomplished within one-tenth of a second.

In the described gas atomization equipment, other process variables depend on the gas pressure downstream of the valve 4. One of these other variables may therefore be sensed in place of direct sensing of the gas pressure by transducer 8, and moreover it may be desirable to provide a schedule adapted to control the valve 4 so as to control said other variable in accordance with a predetermined schedule. The provision of a programmer 14 for the memory 1 5 gives flexibility in permitting rapid change of schedule when desired.

The valve 4 has been described as a pneumatically actuated valve. It may be otherwise actuated, e.g. hydraulically or electrically, and may for example be a solenoid valve. The valve is also preferably of the kind that closes unless it is receiving a signal to the contrary. The valve may have linear or non-linear characteristics. The electrical circuits within control unit 12 are preferably integrated circuits, and are preferably digital. The transducers 8 and 1 6 would include appropriate analogue/digital conversion circuits.

It will be appreciated that a control apparatus and method as particularly described herein is applicable to control of process variables other than gas pressure.

In one embodiment of the apparatus shown in Figure 2 of the drawings the programmer 14, memory 15, control signal generator 18 and reference signal generator were a single unit, being a Turnbull Control Systems Programmer 6850, and the comparator 26 and adjusting circuit were another single unit being a Turnbull Control Systems Process Controller 6350.

Claims

1. Apparatus for controlling the magnitude of a process variable in accordance with a predetermined schedule of magnitude variation with time, said apparatus comprising: means for generating at selected times first signals each representative of the magnitude the variable is scheduled to assume at the next said time; means for generating second signals representative of the actual magnitude of said variable at said selected times; means for comparing each said second signal as associated with a given said time with that said first signal as generated at a said time preceding said given time and generating an error signal representative of any difference therebetween; and means for adjusting each said generated first signal in response to a previously generated said error signal to provide an adjusted first signal for controlling the magnitude of said process variable until a next said adjusted first signal is provided.

2. Apparatus as claimed in claim 1 wherein said comparing means is operable to compare each said second signal with that said first signal as generated at that said time immediately preceding said given time, and wherein said adjusting means is operable to adjust each said generated first signal in response to the immediately previously generated said error signal.

3. Apparatus as claimed in claim 1 or claim 2 wherein said means for generating first signals is operable to generate first reference signals and first control signals respectively representative of the scheduled magnitude at that time and at the next said time, wherein said comparing means is operable to compare each said second signal with the pertinent said first reference signal, and wherein said adjusting means is operable to adjust each said generated first control signal in response to the pertinent said error signal.

4. Apparatus as claimed in any one of claims 1 to 3 wherein said means for generating first signals is operable to generate said first signals at regularly spaced times.

5. Apparatus as claimed in any one of claims 1 to 4 including a memory adapted to retain a said schedule of magnitude variation with time in the form of a finite set of signal values respectively representative of the magnitude the variable is scheduled to assume at said selected times.

6. Apparatus as claimed in claim 5 wherein said memory is programmable.

7. Apparatus as claimed in any one of claims 1 to 6 adapted to control the magnitude of fluid pressure in a conduit downstream of a controllable valve, wherein said means for generating second signals includes means responsive directly or indirectly to the fluid pressure downstream of said valve, and including means for applying said adjusted first signals to control said valve.

8. Apparatus as claimed in claim 7 wherein said means responsive to the fluid pressure includes a transducer for sensing said fluid pressure downstream of said valve.

9. Apparatus for controlling the magnitude of a process variable in accordance with a predetermined schedule of magnitude variation with time substantially as described herein with reference to the accompanying drawings.

1 0. A method for controlling the magnitude of a process variable in accordance with a predetermined schedule of magnitude variation with time said method compr;sing:- generating at selected times first signals each representative of the magnitude the variable is scheduled to assume at the next said time; generating second signals representative of the actual magnitude of said variable at said selected times; comparing each said second signal as associated with a given said time with that said first signal as generated at a said time preceding said given time and generating an error signal representative of any difference therebetween; and adjusting each said generated first signal in response to a previously generated said error signal to provide an adjusted first signal for controlling the magnitude of said process variable until a next said adjusted first signal is provided.

11. A method as claimed in claim 10 including successively altering said predetermined schedule and controlling said variable in accordance with the altered schedule until said altered schedule of variation yields a predetermined process performance.

12. A method as claimed in claim 10 and substantially as described herein with reference to the accompanying drawings.