WO1995034758A1

WO1995034758A1 - Automatic pressure controller

Info

Publication number: WO1995034758A1
Application number: PCT/GB1995/001376
Authority: WO
Inventors: Graham Westerman
Original assignee: Haxey Engineering Limited
Priority date: 1994-06-15
Filing date: 1995-06-14
Publication date: 1995-12-21
Also published as: GB2291510A; GB9411944D0

Abstract

A pressure controller (10) forms part of a pressure circuit (20) comprising a pump (21) whose delivery is controlled by valve (22), first accumulator (23) and second accumulator (24) having associated therewith accumulator valve (25). The circuit further comprises a pressure transducer (26), a tank (27) controlled by a tank valve (28). The controller (10) operates valves (22, 25 and 28) responsive to pressure readings taken from pressure transducer (26).

Description

AUTOMATIC PRESSURE CONTROLLER

This invention relates to an automatic pressure controller of the type used to control static fluid pressure or hydraulic pressure within a pressurised fluid system.

There are many occasions when it is necessary to accurately control static fluid pressure or hydraulic pressure. In many applications, it is necessary not only to control the pressure at a constant level, but to be able to vary the pressure according to the application to which the system is to be applied.

One application which requires accurate control of fluid pressure is in the fusion joining of thermoplastic pipes. When fusion joining thermoplastic pipes, two pipes made of a thermoplastics material are conventionally joined by means of an apparatus having two clamps which are engageable with the pipes. One or more hydraulic rams are used to apply pressure to force the clamps towards one another. The hydraulic rams are typically pressurised by a manually operable or electrically driven pump.

During the fusion process the pipe ends are first heated by being forced against a heater plate and then are separated by retraction of the hydraulic rams. This allows the plate to be retracted and the pipe ends are forced together to form a joint. Whilst the pipes are forced against the heater plate, beads are formed at the ends of the pipes (bead-up) , and these are further formed when the pipe ends are forced together.

In manually operated systems, it is necessary for the operator to judge the length of time the pipe ends must abut against the heater plate in order to control the size of the beading formed. It is also necessary for the operator to control the time during which the pipe ends are heated up and forced together.

It is known to automatically control pressure in, for example, a system designed to fuse thermoplastic pipes, using proportional pressure control valves controlled by other valves and external detection and/or sensing means to detect the position of the pipes or parts of the system during the process. By automatically controlling pressure, it is believed that operator error and interference is reduced.

All known automatic control methods and systems rely on the use of external sensors which sense pressure and/or position of components in the system at different times. In the case of thermal fusion, the known systems sense pressure and/or position of components at different stages of the fusion process.

It is known that proportional pressure control valves cannot be used to accurately control low pressures if it is also required to control higher pressures in the same system. This is often the case with systems having multi-function purposes which require both high and low pressures.

Proportional pressure control valves necessarily operate with pulsating electrical currents which cause them to dither. This means that their valve seatings partially open instantly in response to pulsating current signals which cause the valve to dither in order for them to control pressure without the valve sticking. This action allows several bars of pressure through the valve.

When a proportional pressure control valve is used in systems requiring multi-pressure range control it is necessary for the system to be calibrated with respect to a current controlling device used to operate the valve.

A problem related to the use of proportional pressure control valves is that for pressures between the usual opening pressure (two to three bars) , up to about eight bars there exists a non¬ linear relationship between the current produced by the current control device and the pressure in the system. It is usually only possible to obtain linearity above about eight bars, and even within this pressure range the valve and current controller require calibration to ensure accuracy within the linear part of the scale.

Whilst it is possible to use proportional pressure control valves at low pressures (ie pressures below about eight bars) , it is necessary to re-calibrate the valve and current controlling device for the pressures required within this low pressure range. Once the system has been calibrated in the low pressure range, the accuracy of the high pressure range is lost.

Calibration in the lower pressure range (between three and eight bars) whilst still possible, results in an inaccurate control between three to eight bars. In practice this means that in order to achieve multi-range pressure control, proportionate pressure control valves are calibrated for use between eight bars and their maximum working pressure control in order to avoid the need for re-calibration. This clearly means that such a system is not able to accurately measure pressures below eight bars.

Another problem known to exist with proportional pressure control valves is that whenever there is pressure acting against the valve, the valve, when energised in a normally closed position, will allow leakage of pressure of at least two to three bars when pressure acts against an inlet port of the valve.

A further disadvantage with proportional pressure control valves is that they are very expensive. In addition, it is normally necessary to use a proportional pressure control valve in conjunction with several other valves within a pressure circuit.

According to a first aspect of the present invention there is provided a system for controlling hydraulic pressure within a pressure circuit, the system comprising: a pump for supplying fluid at a predetermined pressure into the circuit; a pump valve for controlling the amount of pressurised fluid supplied to the circuit by the pump; a first accumulator for storing pressurised fluid; a tank for removal of excess fluid; a pressure detector for detecting pressure within the circuit; and control means for effecting release of fluid from the accumulator into the circuit when pressure falls below a predetermined level, and for effecting release of fluid from the circuit into the tank when pressure rises above a predetermined level.

According to a second aspect of the present invention there is provided a method of controlling movement of a component forming part of an hydraulically controlled system, using a system comprising: a pump valve for supplying fluid at a predetermined pressure into the circuit; a pump valve for controlling the amount of pressurised fluid supplied to the circuit by the pump; a first accumulator for storing pressurised fluid; a tank for removal of excess fluid; a pressure detector for detecting pressure within the circuit; and control means for effecting release of fluid from the accumulator into the circuit when pressure falls below a predetermined level, and for effecting release of fluid from the circuit into the tank when pressure rises above a predetermined level.

The system according to the present invention may thus be used to accurately control hydraulic pressure within a circuit and to control the pressure within a first predetermined pressure range. Advantageously, the system further comprises a second accumulator for use in a second pressure range and an accumulator valve used to direct fluid within the system to either the first or second accumulator as appropriate.

Advantageously, the system further comprises a tank valve for controlling flow of excess fluid into the tank.

In use therefore if the system is being used in a first pressure range, for example, a high pressure range the first accumulator will be used, and the accumulator valve will be switched into a position which directs fluid to the first accumulator. If however it is then required to operate the system within a second pressure range, for example, a lower pressure range, the accumulator valve will switch to a second position which directs fluid to the second accumulator. Any excess fluid producing high pressure will be dumped to the tank.

By means of the present invention it is thus possible to operate the system in both high and low pressure ranges without having to re-calibrate the system to operate in, for example, the low pressure range.

Preferably, the controller comprises an electronic pressure controller having a memory adapted for data logging and retrieval.

The applicant has made the discovery that in a system designed to carry out a process in which pressure must be controlled, for example, the process of fusion joining thermoplastic pipes, the pressure within the system at a given time and place within the process is substantially the same each time to process is carried out under the same conditions. This means that in order to determine the position of, for example, a clamp in a fusion process, it is not necessary to have a sensor mounted within the vicinity of the clamp. It is merely necessary to be able to accurately measure the pressure with respect to time within the system. Emperical experiments have been carried out on such systems which show that the pressure will change with time during a pressure controlled process in a predictable way. Through knowledge gained from emperical experimentation, pressure readings accurately taken by means of the present invention may be used to accurately determine, for example, the position of an hydraulic ram within a butt fusion process system.

Pressure readings with respect to time may also be used to determine whether any aspect of the process requires correction, for example, in the case when the clamps are slipping relative to pipes being joined.

In addition, it is possible by controlling the pressure within the system to produce the required conditions within the process.

Preferably, the pump valve, accumulator valve and tank valve each comprise a poppet valve. Poppet valves are commonly used valves and are therefore very cheap, particularly relative to the price of a proportional pressure valve. This means that the system according to the present invention is economical.

In addition, the inclusion of the poppet valves in the system means that the system can react very quickly to changes in pressure within the system. Typically, the time taken for the system to react to changes in pressure is of the order of ili- seconds.

Although poppet valves are preferable, many other types of valve may also be used in the system according to the present invention.

The pressure controller according to the present invention is capable of accurately controlling static fluid pressures in that it is able to apply very small step pressure changes from 0 gauge pressure to any pressure required (up to the component ratings within the equipment being controlled) , without the use of specialised proportional pressure control valves or the need to recalibrate between low and high pressure control.

In addition, controlled hydraulic swept volumes or load displacements, for example, those produced by hydraulic rams or diaphragms, can be measured, and load removal may be detected, all without the use of external displacement sensors being fitted to the equipment being controlled.

The pressure controller according to the present invention is also capable of locking loads at predetermined pressures and also allows for the assessment of frictional pressures required to overcome "drag" which will exist in, for example, the movement of hydraulic rams. Further, the pressure controller is able to assess leakage rates and to measure, check and confirm effective cylinder areas from known lengths of travel or swept volumes, all without the need for any external detection, displacement sensing or confirmation from the equipment being controlled.

The pressure controller is able to achieve the above mentioned features because of the arrangement of valves in conjunction with the electronic controller which allows for extremely rapid measurement of pressure, and also allows for extremely accurate measurement of change in pressure.

Another advantage of the present invention is that because no external sensors are required to be positioned around equipment in which pressure is being controlled or measured, the invention may be easily and simply attached to an existing equipment. Many of the existing automatic pressure controllers require the use of external sensors to be positioned around the equipment in which the pressure is being applied, and often it is not possible to retrofit these sensors. An important feature of the present invention is that it is possible to input into the memory of the electronic controller, empirical data collected from known systems. In particular, data relating to the pressure and volume characteristics of the or each accumulator may be input into the memory of the electronic controller. Data relating to industry standards may also be input into the memory.

The invention will now be further described by way of example only with reference to the accompanying drawings in which:

Figure l is a schematic representation of a pressure controlled circuit according to the present invention; and

Figure 2 is a more detailed representation of a part of the pressure controller of Figure 1.

Referring to the Figures, a pressure controller according to the present invention is designated generally by the reference numeral 10.

The essential features of the pressure controller 10 are shown in more detail in Figure 2 which shows the basic pressure circuit which is designated by the reference numeral 20. The circuit 20 comprises a pump 21 whose delivery is controlled by a valve 22, first accumulator 23 and second accumulator 24 having associated therewith accumulator valve 25. The circuit further comprises a pressure transducer 26, and a tank 27 controlled by a tank valve 28. Pressure in the system is applied to a hydraulic ram 27 which in this illustrated example is used in the process of butt welding thermoplastic pipes for use in, for example, the supply of gas or water.

All the valves 22, 25 and 28 are in the illustrated example poppet valves although any convenient type of valve could be used. In use, pressure is supplied to the circuit 20 by means of pump 21. Fluid applied by pump 21 is prevented from entering the circuit whilst valve 22 is in its closed position. Whenever it is required to apply a load to hydraulic ram 27, the electronic pressure controller 30 sequentially operates valves 22, 25 and 28 responsive to pressure readings taken from pressure transducer 26. Any immediate excess pressure results in fluid being diverted into tank 27 through valve 28.

In order to control and supply either low or high pressure, for example in the range of 0 to 70 bars which are typical butt fusion requirements, the controller initially calculates the pressure required using the database which has been previously programmed into it for the particular use to which the pressure controller is being put. The controller will effect closure of valve 25 if a pressure greater than ten bars is required. In the illustrated example therefore the pressure range of ten to seventy bars is termed a high pressure range. The closure of valve 25 allows accumulator 23 which is the high pressure accumulator to charge up to the predetermined pressure if more than ten bars controlled pressure is required.

Whilst this is happening, pressure transducer 26 will read the pressure within the system and confirm the pressure level to the controller. If the pressure level within the circuit is greater than required, the controller 30 will open valve 28 momentarily in order to allow fluid to exit the circuit into tank 27 thus relieving excess pressure from the system.

Once the pressure transducer 26 indicates the required pressure within the system, the controller detects the pressure and causes valves 22 and 28 to close allowing the desired stored pressure level in the high pressure accumulator 23 to control the equipment.

The controlling of the pressure to the desired level will happen within a few milli-seconds. If pressure transducer 26 indicates that the pressure discharge from the high pressure accumulator 23 has fallen by more than a tolerance level of, for example, 10% from the required pressure, valve 22 will again be opened by the controller 30 and the above described sequence will be repeated until the appropriate control pressure has been achieved. The predetermined pressure required in the system will be applied for a predetermined length of time calculated from information that has been previously programmed into the controller's memory. If the equipment to which the pressure is being applied requires a low pressure control, for example, down to two bars, the controller will cause the valve 25 to open thus switching in the low pressure accumulator 24 into the circuit and allowing the low pressure accumulator 24 to charge to the predetermined pressure. Valve 28 will be momentarily opened to relieve excess pressure to the tank 27. Once the correct pressure level has been achieved which happens within milli¬ seconds typically, valves 22 and 28 are closed to allow the low pressure accumulator 24 to deliver its low pressure level to the equipment. If the pressure transducer 26 signals to the controller 30 a drop in the pressure greater than the allowed tolerance which is typically 10%, valve 22 will be opened by the controller 30 to allow more fluid into the system thus increasing the pressure back to the predetermined level. In other words, the pressure within the system is maintained within 10% of the pressure level required.

By means of the present invention therefore, it is possible to operate the pressure control system within a low pressure range without having to recalibrate the system.

When the pressure controller of the present invention is applied to control pressure in butt fusion equipment, it is necessary to control the pressure applied to a hydraulic ram in order to achieve the appropriate welding force which will be applied to abutting ends of adjacent thermoplastic pipes. The weight of the pipe and the frictional forces of the machine clamping the pipes require a certain amount of pressure to overcome the forces due to weight and friction. The level of pressure required to overcome these forces is known as the static drag pressure and is inherent for all load pressure conditions. In the case of the butt fusion of thermoplastic pipes, in order to determine the appropriate pressure to apply for efficient fusion to occur, the drag pressure must be added to the fusion pressure to calculate the appropriate pressure.

In order to overcome frictional forces a higher drag pressure is required in comparison with the drag pressure level needed for maintaining motion of the equipment once the equipment has started to move (ie dynamic drag) .

In known automatic pressure control machines, sensors are fitted to the butt fusion apparatus in order to make several detection readings. It is common for the sensors to be positioned on the butt fusion apparatus and the same sensors may be used to indicate when a pipe clamp is either in an open or closed position.

Before commencing an automatic fusion cycle it is necessary to ensure an open condition for sensing when the pipe clamp has moved forward in order to signal to the controller that this movement has occurred.

Known controllers apply stepped pressure changes and the pressure level arrived at when the sensor signal has been received is taken as the drag value. However it is believed that this measurement may measure several times higher than the actual dynamic drag pressure value, and therefore a higher pressure than is required is being applied to the fusion apparatus.

In one known system, it is necessary to sense a second pressure reading and to take an average of the two readings. This however requires an external sensor on the butt fusion machine which sensor is used for checking other conditions as well.

Drag pressure, whether static or dynamic may be lower than eight bars. This means that in conventional systems using proportional pressure control valves, the value for drag falls outside the area of linear calibration. The drag could be, for example, three to four bars, and unless calibrated to a low pressure range, the proportional pressure control valve will instantly apply eight bars to the system resulting in sensors confirming this level of drag once movement has occurred.

The pressure controller of the present invention is able to accurately detect definitive drag pressure without the need of an external sensor. The pressure controller of the present invention is able to accurately measure and assess drag because by means of the arrangements of valves and type of valves used, fast sequential measurements and fast operation of valves may take place.

It is possible to rapidly take measurements from the pressure transducer 26 such that several readings are taken within one second thus establishing changes in pressure which happen very quickly with respect to time.

Experiments have shown that when a specific oil flow rate passes through a valve opened at a specific speed, a specific pressure wave form is consistently achieved. High level peak pressure of the wave form denotes the pressure required to overcome static drag, and a lower level reading on the pressure wave form indicates dynamic drag pressure.

It is also possible to programme the controller to initiate a required amount of linear movement to one or more of the hydraulic rams by rapidly opening and closing valve 22. The movement of the one or more rams can be monitored by calculating the expected swept volume from known systems and checking the actual reading taken.

This method of drag assessment is both rapid and accurate, and allows the pressure controller of the present invention to select the drag pressure necessary for the particular load application required.

There are several stages in the overall control of the butt fusion process by the controller of the present invention, namely pressure control, drag assessment, detection of the machine being closed, the machine being open, clamp slip, clamp removal, linear movement. For each of these stages, the controller is programmed to carry out checks at appropriate stages of each controlled function. The function commands cause the electronic controller 30 to accurately and rapidly operate the valves such that changes in pressure level with respect to time may be measured within milli-seconds. Empirically known pressure wave forms with respect to time establish the required time breaks between each set of readings for each functional check. The sequence of checking is programmed into a database within the controller 30 and readings of pressure response are checked for the correct functional level within 10%. This means that the pressure reading taken may be used to detect the position of a component within the system being controlled and obviates the need to have sensors positioned to determine the position of the component.

In order to measure swept volumes or linear displacement of one or more of the hydraulic rams, values for the effective cylinder area are initially input into the controller's database. The controller is then programmed to sequentially open and close valve 22 in conjunction with the other valves in the circuit such that the pressure response of the circuit may be measured via transducer 26. From these measurements, it is possible to derive by calculation the amount of cylinder travel or volume of oil moved (ie swept volume) . It has been found that if valve 22 is rapidly opened and closed over a longer period of time, progressively increasing pressure pulses are produced until the frictional forces have been overcome. This results in more accurate measurement of static and dynamic drag pressures.

Similarly the pressure controller of the present invention is able to detect when the clamp of the butt fusion apparatus is in an "open position" by assessing pressure levels at certain stages of a machine cycle which be assessed several times within a split second.

Typically, during a butt fusion sequence, it is necessary to open the machine for inserting and retracting a hotplate. Upon closing, it is also possible for the automatic electronic pressure controller according to the present invention to detect from a pressure wave form when jointing has occurred thus allowing a time cooling down period to commence exactly on time.

The pressure controller of the present invention when used to control the pressure of a butt fusion apparatus is able to detect if the clamp used to hold one or both of the two pipes has slipped from its appropriate position, or if a pipe has been prematurely removed from the machine before jointing is complete. This is possible due to the fact that the pressure controller is able to accurately measure and assess pressure response with time. If slippage has occurred, the pressure readings will indicate this condition. This therefore allows such situations to be detected without the need for external sensors on the machine.

The pressure controller according to the present invention has been described predominantly in connection with the control of pressure in a butt fusion machine used for fusing thermoplastics pipes. However, it is to be understood that the pressure controller of the present invention may be used in any application in which it is required to control pressure, and the invention is not to be taken as being limited to use in connection with a butt fusion machine.

Claims

1. A system for controlling hydraulic pressure within a pressure circuit, the system comprising:

a pump for supplying fluid at a predetermined pressure into the circuit; a pump valve for controlling the amount of pressurised fluid supplied to the circuit by the pump; a first accumulator for storing pressurised fluids; a tank for removal of excess fluid; a pressure detector for detecting pressure within the circuit; and control means for effecting release of fluid from the accumulator into the circuit when pressure falls below a predetermined level, and for effecting release of fluid from the circuit into the tank when pressure rises above a predetermined level.

2. A system according to claim 1 further comprising a second accumulator for use in a second pressure range and an accumulator valve used to direct fluid within the system to either the first or second accumulator as appropriate.

3. A system according to claim 1 or claim 2 further comprising a tank valve for controlling flow of excess fluid into the tank.

4. A system according to any one of the preceding claims comprising an electronic pressure controller having a memory adapted for data logging and retrieval.

5. A system according to claim 3 or claim 4 wherein the pump valve, accumulator valve and tank valve each comprise a poppet valve.

6. A method of controlling movement of a component forming part of a hydraulically controlled system, using a system comprising:

a pump for supplying fluid at a predetermined pressure into the circuit; a pump valve for controlling the amount of pressurised fluid supplied to the circuit by the pump; a first accumulator for storing pressurised fluid; a tank for removal of excess fluid; a pressure detector for detecting pressure within the circuit; and control means for effecting release of fluid from the accumulator into the circuit when pressure falls below a predetermined level and for effecting release of fluid from the circuit into the tank when pressure rises above a predetermined level.